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Dipolar Bent and Linear Acetylenes Substituted by Cationic Quinolinium and Anionic Benzoates. Formation of Mesomeric Betaines
Abstract
3-Ethynylquinoline was subjected to a Sonogashira-Hagihara reaction with methyl 2-, 3-, and 4-bromobenzoates, respectively, and then N-methylated to give 3-[(-(methoxycarbonyl)phenyl)ethynyl]-1-methylquinolinium salts (two X-ray analyses). On saponification of the 3- and 4-substituted benzoates, the mesomeric betaines 3- and 4-[(1-methylquinolinium-3-yl)ethynyl] benzoates were formed. By contrast, the 2-benzoate derivative gave either the corresponding (1-oxo-1H-isochromen-3-yl)quinolinium derivative, or the mesomeric betaine 2-(1-methylquinolinium-3-yl)-1,3-dioxo-2,3-dihydro-1H-inden-2-ide depending on the reaction conditions. A DFT calculation predicts a transoid conformation of the acetylene bond in the intermediate 2-[(1-methylquinolinium-3-yl)ethynyl]benzoate which is due to a strong hydrogen bond between the carboxylate group and 2-H of the quinolinium ring, in addition to a 1,5-interaction between the carboxylate group and the CC triple bond. The bond angles of the transoid CC triple bond were calculated to be 211.6 degrees and -175.1 degrees in vacuo. The corresponding linear triple bond is 50.4 kJ/mol less stable in vacuo according to the calculation, and the N-heterocyclic carbene quinoline-2-ylidene is not formed as a tautomer.
... For the synthesis of diquinoline ethynyls under Sonogashira-Hagihara conditions different approaches were used (Scheme 6). In analogy to the previously reported syntheses of 3,4-(3 b) and 3,3-ethynyl interconnected quinolines (3 c), [21] the 2,3-corresponding isomer 3 a was obtained in 38 % yield starting from 3-ethynylquinoline 2 b (accessible in two steps [22] ) and 2-chloroquinoline 1 a. In analogy to the compounds 3 a-c, the 2,4-interconnected isomer 3 f was formed in the reaction of 4-bromoquinoline 1 c and 2-ethynylquinoline 2 a in 20 % yield, whereas the reverse reaction of 4-ethynylquinoline 2 c with the chloro derivative 1 a gave compound 3 f only in 7 % yield. ...
Dicationic bis‐quinolinium salts possessing different types of conjugation are presented in which the hetareniums are separated by acetylenic or 1,4‐diethynylbenzene spacers. Cross‐conjugation is induced by interconnections via the 3‐positions of the heteroareniums, and conjugation is realized by 2,2/2,4/4,4‐interconnections. We also synthesized mixed cross‐conjugated/conjugated 2,3/4,3‐interconnected species. The different types of conjugation determine the charge distribution according to the rules of resonance which translate into characteristic ¹³C NMR as well as UV‐Vis‐spectroscopic properties, LUMO geometries, and chemical properties. We found that the triple bond's resonance frequencies are sensitive ¹³C NMR spectroscopic indicators for the type of conjugation. Combining cross‐conjugated and conjugated structure elements of dicationic bis‐quinolinium dyes via triple bonds without additional benzene spacer results in considerable bathochromic shifts of the UV‐Vis absorption maxima [Δλmax≈100 nm]. A charge transfer was calculated from the HOMO of the conjugated into the LUMO of the cross‐conjugated partial structure.
... Treating a salt 1d with 1,3-indandione gave 28% yield of 8dg (Fig. 4). Previously, 8dg was prepared by 6 step-synthesis, and at least 2 steps used Pd metal for Sonogashira-Hagihara coupling [30]. The Fig. 4. Synthesis of 8df, 8 dg and 8dfM. ...
During functionalization at C2, C3, and C4 of quinoline, we came across a one-step metal free synthesis of C3 quinolinium derivatives of mesomeric betaine dyes. Some mesomeric betaine dyes had solvatochromic properties, and were sensitive to pH and solvates in the water. Certain betaine dyes had different properties in acidic and basic environments; the reversible interconversion between yellow (neutral or basic condition) and colorless (acidic condition) forms of the dye occurs by pH change. The changes in UV absorbance intensities of the dyes in the presence of cyclodextrins indicated the inclusion complex formation between cyclodextrins and dyes. The betaine dyes were partially dissolved in water due to the presence of positive and negative charges in the molecule. These betaine dyes did not exhibit cytotoxic activity, hence may be used in biological research.
... In the course of our projects dealing with the types of conjugation 24,25 we studied systematically planar dicationic systems such as 6, 7, and 8 in which the positive charges are in conjugation, cross-conjugation, or a combination of both, and which have a considerable influence on their properties 26 ...
4-[(Quinolin-4-yl)ethynyl]benzoic acid underwent a cycloreaction with tetraphenylcyclopentadienone to give the hexa(hetero)arylbenzene- 1-(quinolin-4-yl)-2-(4-benzoic acid)-3,4,5,6-tetraphenylbenzene which was N-methylated and finally deprotonated to give the title compound. The molecule adopts a propeller-shaped configuration in which the benzoate ring as well as the phenyl ring adjacent to the quinolinium substituent display non-isochronous resonance frequencies, respectively, due to a hindered rotation. Although the molecule formally belongs to the class of pseudo-cross-conjugated heterocyclic mesomeric betaines, the propeller-shape causes a charge distribution of cross-conjugated systems. DFT-calculations (frontier orbital profiles, geometries) have been carried out.
Mesoionic polarization allows access to electron‐rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3‐methylpyridinium‐derived mesoionic olefins (py‐mNHOs). We used a DFT‐supported design concept, which showed that the introduction of aryl groups in the 1‐, 2‐, 4‐, and 6‐positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py‐mNHOs are remarkably strong σ‐donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py‐mNHOs with electron‐poor π‐systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3‐dipolar cycloadditions was observed experimentally and analyzed by quantum‐chemical calculations.
Mesoionic polarization allows access to electron‐rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3‐methylpyridinium‐derived mesoionic olefins (py‐mNHOs). We used a DFT‐supported design concept, which showed that the introduction of aryl groups in the 1‐, 2‐, 4‐, and 6‐positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py‐mNHOs are remarkably strong σ‐donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py‐mNHOs with electron‐poor π‐systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3‐dipolar cycloadditions was observed experimentally and analyzed by quantum‐chemical calculations.
Mit einer einzigen Ausnahme waren Sydnonmethide bisher unbekannt. Hier werden Synthesen beschrieben, die zu zehn stabilen und intensiv farbigen, leicht fluoreszenten Sydnonmethiden mit Z=CN, COOMe und SO2Me führten. Durch Deprotonierung ergeben sich einzigartige anionische π-elektronenreiche N-heterocyclische Carbene, die zu Thioethern und Selenethern, Quecksilber-, Gold- und Rhodiumkomplexen sowie mit Acylchloriden zu 4-Acylsydnonmethiden reagieren.
Abstract
Wir beschreiben Sydnonmethide, von denen bisher nur ein einziges Beispiel in der Literatur erwähnt worden war. Sydnonmethide können zu Anionen deprotoniert werden, die als π-elektronenreiche anionische N-heterocyclische Carbene formulierbar sind. Schwefel- und Selenaddukte konnten wir als ihre Methylether stabilisieren sowie Quecksilber-, Gold- und Rhodiumkomplexe von Sydnonmethiden herstellen. Die Anionen von Sydnonmethiden gehen zudem C-C-Kupplungsreaktionen mit 1-Fluor-4-iodbenzol unter Pd(PPh3)4- und CuBr-Katalyse ein. Zur Abschätzung der elektronischen Eigenschaften dieser anionischen N-heterocyclischen Carbene bestimmten wir die ⁷⁷Se-NMR-Resonanzfrequenzen und die ¹JC4-Se-Kopplungskonstanten der Selen-Addukte sowie die ¹JC4-H-Kopplungskonstanten der mesoionischen Vorläufermoleküle. Im Vergleich zu klassischen N-heterocyclischen Carbenen erscheint das Carben-Kohlenstoffatom des Sydnonmethid-Anions 3 j in den bei −40 °C gemessenen ¹³C-NMR-Spektren mit δ=155.2 ppm bei extrem hohem Feld.
Sydnone methides are described from which only one single example has been mentioned in the literature so far. Their deprotonation gave anions which can be formulated as π‐electron rich anionic N‐heterocyclic carbenes. Sulfur and selenium adducts were stabilized as their methyl ethers, and mercury, gold as well as rhodium complexes of the sydnone methide carbenes were prepared. Sydnone methide anions also undergo C−C coupling reactions with 1‐fluoro‐4‐iodobenzene under Pd(PPh3)4 and CuBr catalysis. ⁷⁷Se NMR resonance frequencies and ¹JC4‐Se as well as ¹JC4‐H coupling constants have been determined to gain knowledge about the electronic properties of the anionic N‐heterocyclic carbenes. The carbene carbon atom of the sydnone methide anion 3 j resonates at δ=155.2 ppm in ¹³C NMR spectroscopy at −40 °C which is extremely shifted upfield in comparison to classical N‐heterocyclic carbenes.
The natural product punicine (Punica granatum) exists in two tautomeric forms, the cross-conjugated mesomeric betaine 1-(pyridinium-1-yl)-2-hydroxy-phenyl-5-olate and the conjugated mesomeric betaine 1-(pyridinium-1-yl)-5-hydroxy-phenyl-2-olate. Punicine as well as its picoline derivatives reacted with tris(pentafluorophenyl)borane exclusively at the 2′-olate group to form zwitterionic borates. Correspondingly, the 5′-dehydroxy derivate of punicine, the conjugated heterocyclic mesomeric betaine 1-(pyridinium-1-yl)-phenyl-2-olate and its picoline derivatives also gave borates, whereas analogous reactions of the cross-conjugated isomer 2′-dehydroxypunicine [1-(pyridinium-1-yl)-phenyl-3-olate] did not result in the formation of stable adducts.
Alkene activation by an internal nucleophile that enables ‘incompetent’ alkenes to undergo halocyclization, via enforcing a concerted AdE3 mechanism, has been thoroughly investigated. Detailed analyses of starting material spectroscopy, addition stereopreferences, isotope effects, and nucleophile–alkene interactions have thus enabled the modulation of the alkene reactivity and halocyclization rates by nucleophile proximity and basicity. A computational study of the regio‐ and stereo‐selectivity of diene hydroboration has addressed the deviation from the anti‐Markovnikov rule. Atomic charges of cyclic and acyclic dienes were correlated with the calculated activation energy barriers, and intriguingly, 1,3‐cyclohexadiene was identified as the main example that does not follow the anti‐Markovnikov rule. Density functional theory calculations revealed that the catalytic effect of iodine in nucleophilic additions to aα,β‐unsaturated carbonyls and nitrostyrenes originates from the halogen bonding to the Michael acceptor, which results in the reduction of activation energies ΔG≠ by 1.8–7.6 kcal mol‐1.
2‐Bromo‐2‐chloro‐3‐arylpropanenitriles can be prepared by Meerwein reaction from 2‐chloroacrylonitrile and various aryldiazonium salts under copper(II) bromide catalysis. They proved to be stable compounds which form 2‐chlorocinnnamonitriles upon treatment with bases. Reaction of the title compounds with substituted thioglycolates gave substituted 3‐aminothiophenes which have not yet been accessible by other routes. Three‐component reactions with the title compound, sodium sulfide and bromonitromethane, chloroacetonitrile, or ethyl 4‐chloroacetoacetate gave 2‐nitro‐ and 2‐cyano‐substituted 3‐aminothiophenes, and thienopyridinediones in high yields and in one single step, respectively.
1,4‐Diethynylbenzene was used as conjugated all‐carbon and rigid spacer between the 2‐, 3‐ and 4‐positions of two 1‐methylquinolinium rings. Thus, for a systematic study, a series of dicationic salts with 2,2‐, 3,3‐, 4,4‐, 3,2‐, and 3,4‐interconnections of the two positive charges was prepared, in which all even‐numbered substitution patterns are conjugated, and all odd‐numbered substitution patterns are cross‐conjugated. As a consequence, conjugated/conjugated, cross‐conjugated/cross‐conjugated, and conjugated/cross‐conjugated dications have been prepared. The different combinations result in considerably different charge distributions of the positive charges within the π‐electron systems according to the rules of resonance which translate into different DFT‐calculated frontier orbital profiles and spectroscopic properties such as ¹³C NMR chemical shifts, IR and Raman absorptions, and the measured as well as calculated UV/Vis spectra.
Diels-Alder reaction of 2-, 3- and 4-(phenylethynyl)quinolines and tetraphenylcyclopentadienone gave three regioisomeric 2,3,4,5,6-pentaphenyl-1-(quinolin-2-yl, -3-yl, and -4-yl)benzenes. Restricted rotation of the 3-yl and 4-yl substituted derivatives is observed between the central core and the substituents, resulting in propeller-shaped molecules. Likewise, 1,2-diquinolinyl-3,4,5,6-tetraphenylbenzenes with 3-yl,3-yl and 3-yl,4-yl connectivity were prepared. As evidenced by NMR spectroscopy, they form two diasteromers due to their restricted rotation. A cobalt-catalyzed [2 + 2 + 2]-cyclotrimerization of 2-(phenylethynyl)quinoline resulted in the formation of triphenyl-2,4,6- and -3,5,6-tri(quinolin-2-yl)benzenes. The same reaction was applied to 3,3′-ethyne-1,2-diyldiquinoline which formed hexa(quinolin-3-yl)benzene. N-Methylation gave the title compounds. Among those, the hexacationic hexa(N-methylquinolinio-3-yl)benzene is described. Stereochemical aspects are predominantly discussed by means of results of NMR experiments. DFT-calculations on the most stable conformations and the frontier orbital profiles of the hexacation as well as of its neutral precursor have been carried out.
This work examines the prospect of making stable mesoionic compounds of the type mesomeric betaine R ⁺ -CO 2⁻ from direct oxidative additions of carbon dioxide to suitably-delocalized singlet carbene moieties, with bold objectives of carbon sequestration and overall energy storage. A set of possible candidates for such mesoionic compounds is theoretically explored through DFT calculations, inspecting coupling paths, thermodynamic and kinetic stabilities, and geometric and electronic structural features. Among others, the addressed cationic parts include aromatic rings in their broader sense, phenalene systems, and odd linear polyenic chains. Various structurally-close neutral alternatives such as oxiranones or carbene-acid forms are also considered. In the linear polyenic chain family, there is stark contrast between 4N + 1 and 4N - 1 lengths, with ensuing substantial consequences for stabilities and structures. Amino substitutions can favor mesoionic arrangements through their cation-stabilizing π-donor properties, further supported by possible strong intramolecular hydrogen bonds, but they can also contribute to weaken their kinetic stability through the existence of stable neutral imino alternatives. All in all, mesoionics including tropylium, phenalene, or 4N + 1 odd polyene frames as cationic parts could be reasonable targets.
1‐Methylquinolinium salts substituted at the C3 position with phenyl, naphthalen‐1‐yl, phenanthren‐9‐yl, and pentaphenyl phenyl as well as aryl ethynyl substituents were prepared. The phenyl, naphthalen‐1‐yl, and phenanthren‐9‐yl substituents are in conjugation with the quinolinium ring, and the sterically crowded pentaphenyl phenyl residue causes a propeller‐type molecule possessing a calculated dihedral angle between aryl substituent and the quinolinium ring of approximately 54°. The different influences of the substituents including the aryl ethynyl residues on the π‐architecture is well reflected in their frontier orbital profiles, their spectroscopic properties, and in their ability to form N‐heterocyclic carbenes. The latter were generated from the C3‐aryl substituted quinolinium salts and trapped as sulfur and selenium adducts, whereas the aryl ethynyl substituted derivatives failed to undergo the NHC generation under the conditions applied. ⁷⁷Se NMR measurements of the corresponding selenones reveal that quinolin‐2‐ylidenes are electron poor carbenes with strong π‐acceptor character.
The three isomers 1-methylquinolinium-2-, 3-, and 4-ethynyl(phenyl-4-carboxylates) belong to two distinct types of heterocyclic mesomeric betaines. The quinolinium substituted in position 3 is a cross-conjugated mesomeric betaine (CCMB), whereas the quinolinium derivatives substituted in positions 2 and 4 are members of the class of pseudo-cross-conjugated mesomeric betaines (PCCMBs). While the charges are strictly separated within the common π-electron system of the CCMB according to the canonical formulae, the charges are effectively but not exclusively delocalized in the PCCMBs because cumulenoid resonance forms including electron sextet structures without external octet stabilization can be formed in accordance with the definition of PCCMBs. As a consequence, despite being closely related structures, the three isomers differ in their chemical and spectroscopic behaviors. Thus, on trying to hydrolyze the ester group of the methyl quinolinium-2-ethynyl-benzoate into the corresponding acid by subsequent treatment with sodium hydroxide in methanol and aqueous hydrochloric acid at pH 3, the acetal methyl 1,1-dimethoxy-2-(quinolinium-ylidene)ethyl]benzoate and the corresponding β -enamino carbonyl compound were formed, respectively. The corresponding acids of the 2- and 4-substituted quinolinium-ethynyl-benzoates were obtained by a modified procedure. On deprotonation, the resulting cross-conjugated quinolinium-3-ethynyl-benzoate betaine proved to be stable, whereas the corresponding pseudo-cross-conjugated quinolinium-2- and -4-ethynyl-benzoate betaines decomposed. Frontier orbital profiles were calculated, and IR and Raman spectra of the starting materials were measured and calculated to analyze the differences of CCMBs and PCCMBs of mesomeric betaines possessing triple bonds. A higher contribution of the cumulenoid resonance forms to the overall structure of the PCCMBs was determined.
The sydnone imines Molsidomine and 5-(benzoylimino)-3-(2-methoxyphenyl)-1,2,3-oxadiazolium-2-ide gave anionic N-heterocyclic carbenes on deprotonation at C4 which were trapped as methylated selenium adducts, gold complexes (X-ray analysis) as well as palladium complexes (X-ray analysis). The ¹³C NMR resonance frequencies of the carbene carbon atom are extremely shifted upfield and appear at δ = 142.1 ppm (Molsidomine carbene) and δ = 159.8 ppm (sydnone imine carbene). The Pd complexes were applied as catalysts in Suzuki-Miyaura and Sonogashira-Hagihara cross-coupling reactions.
4- and 5-((1-Methylquinolinium-3-yl)ethynyl)thiophene-2-carboxylates were prepared starting from 3-ethynylquinoline which was subjected to a Sonogashira-Hagihara coupling with ethyl 4- and 5-bromothiophenecarboxylates, respectively. Methylation to give the corresponding quinolinium salts, saponification under basic conditions, acidification to give the carboxylic acids, and basification finally yielded the title compounds. Calculations of the frontier orbital profiles confirm the classification of these mesomeric betaines as cross-conjugated systems, as the HOMO is essentially located in the carboxylate group which is connected to the spacer by a nodal position. Fluorescence spectroscopic investigations were performed.
2-Amino-6-chloropyrimidin-4(3H)-one (6-chloroisocytosine) reacted with methyl-, vinyl-, phenyl-, and benzylimidazole, respectively, to give isocytosinate-imidazolium salts which were deprotonated with an anion exchange resin to yield the corresponding cross-conjugated mesomeric betaines (imidazolium-isocytosinates). The vinyl derivative, however, polymerized under these conditions. The betaines are in equilibrium with their N-heterocyclic carbenes, isocytosine-imidazol-2-ylidenes. Results of DFT calculations reveal that the betaine tautomer is the most stable tautomer in polar solvents, whereas the carbene is more stable in vacuo. Trapping reactions of the carbene tautomers, including the vinyl derivative, were performed with sulfur and selenium which resulted in the formation of the corresponding thiones and selenones, respectively. The in situ generation of the betaine/carbene tautomers in the presence of triethylborane in a Schlenk tube at elevated temperatures and pressures gave novel zwitterionic 8-amino-10,10-diethyl-6-oxo-6,10-dihydro-1H-imidazo[2′,1′:3,4][1,4,2]diaza-borolo[1,5-c]pyrimidin-4-ium-10-ides, in which the boron atom bears the negative charge whereas the positive charge is delocalized in the imidazolium ring. This ring system is a formal trapping product of the carbene as well as the betaine tautomer.
Acta Crystallographica Section C is continuing its transition to a journal that publishes exciting science with structural content, in particular important results relating to the chemical sciences.
The conjugated mesomeric betaines 2-(1-phenyl-4H-1,2,4-triazolium-4-yl)phenolates are masked N-heterocyclic carbenes of 1,2,4-triazole which can be trapped as thiones and selenones. Reaction with triethylborane and triphenylborane resulted in the formation of first representatives of a new zwitterionic heterocyclic ring system, benzo[e]1,2,4-triazolo[3,4-c][1,4,2]oxazaborinium-4-ide, as a formal trapping product of an anionic N-heterocyclic carbene.
The synthesis of a pyrazol-3-ylidene palladium complex is described, and results of a single-crystal X-ray diffraction study are presented. The properties of pyrazolium salts as well as of two types of heterocyclic mesomeric betaines, the pseudo-cross-conjugated mesomeric betaine pyrazolium-3- carboxylate and its cross-conjugated analog pyrazolium-4-carboxylate, are compared.
The title compound, C24H26O2Si2, has C2 crystallographic symmetry. The dihedral angle between the aromatic rings is 84.5 (2)°. The acetylene group is slightly non-linear, with angles at the acetylene C atoms of 175.7 (2) and 177.0 (2)°. In the crystal structure, only van de Waals interactions occur.
The synthesis of a heterocyclic mesomeric betaine by quaternization reaction of 1-butylimidazole and tetrabromo-1,4-benzoquinone is presented. The structure was verified by means of X-ray single-crystal analysis, NMR and IR spectroscopy. Inclusion complexes of the heterocyclic mesomeric betaine with randomly methylated (1.8) β-cyclodextrin were investigated by UV-vis spectroscopy. Furthermore, the reaction conditions were applied to poly(vinylimidazole) and 1,4-bis(1H-imidazol-1-yl)butane to obtain functionalized polymer networks and condensate polymers, respectively.
Anion metathesis of imidazol(in)ium chlorides with KHCO(3) afforded an easy one step access to air stable imidazol(in)ium hydrogen carbonates, denoted as [NHC(H)][HCO(3)]. In solution, these compounds were found to be in equilibrium with their corresponding imidazol(in)ium carboxylates, referred to as N-heterocyclic carbene (NHC)-CO(2) adducts. The [NHC(H)][HCO(3)] salts were next shown to behave as masked NHCs, allowing for the NHC moiety to be readily transferred to both organic and organometallic substrates, without the need for dry and oxygen-free conditions. In addition, such [NHC(H)][HCO(3)] precursors were successfully investigated as precatalysts in two selected organocatalyzed reactions of molecular chemistry and polymer synthesis, namely, the benzoin condensation reaction and the ring-opening polymerization of d,l-lactide, respectively. The generation of NHCs from [NHC(H)][HCO(3)] precursors occurred via the formal loss of H(2)CO(3)via a concerted low energy pathway, as substantiated by Density Functional Theory (DFT) calculations.
The series 'The Chemistry of Functional Groups' is planned to cover in each volume all aspects of thc chemistry of one of the important functional groups in organic chemistry. The emphasis is laid on the functional group treated and on the effects which it exerts on the chemical and physical properties, primarily in the immediate vicinity of the group in question, and secondarily on the behaviour of the whole molecule. For instance, the volume The Chemistry of the Ether Linkage deals with reactions in which the C-O-C group is involved, as well as with the effects of the C-O-C group on the reactions of alkyl or aryl groups connected to the ether oxygen. It is the purpose of the voluine to give a complete coverage of all properties and reactions of ethcrs in as far as these depend on the presence of the ether group but the primary subject matter is not the whole molecule, but the C-O-C functional group.
This chapter reviews meso-ionic heterocycles. It discusses the currently known types of meso-ionic heterocycles, which can then be considered in relation to the total number of possibilities associated with certain structural conditions. A compound may be appropriately called meso-ionic if it is a five or possibly a six-membered heterocyclic compound that cannot be represented satisfactorily by any one covalent or polar structure and possesses a sextet of electrons in association with the atoms comprising the ring. The ring bears a fractional positive charge balanced by a corresponding negative charge located on a covalently attached atom or group of atoms. The adoption of the term meso-ionic and its frequent use has demonstrated its usefulness. The five-membered mesoionic heterocycles can be divided into two general classes: type A and type B. This chapter is concerned with the currently known 44 members of type A and eight members of type B.
This chapter reviews the distinct classes of heterocyclic mesomeric betaines and analogs in natural product chemistry. The chemistry of mesoionic heterocycles of type A, type B, heteropentalenes, and conjugated mesomeric betaines isoconjugate with alternant hydrocarbon atoms are also reviewed. Mesoion is restricted to five-membered conjugated heterocyclic mesomeric betaines and includes the well-known sydnones, münchnones, and derivatives. Betaines with an odd number of charges form a distinct class of compounds. They are divided into four major classes: (1) conjugated (CMB, (2) cross-conjugated (CCMB), (3) pseudo-cross-conjugated (PCCMB) heterocyclic mesomeric betaines, and (4) conjugated heterocyclic N-ylides. The four major classes can be subdivided into four additional categories on the basis of their isoconjugate relationships to odd/even, alternant/nonalternant hydrocarbon, and anions/dianions. The representatives of all four major classes of heterocyclic mesomeric betaines are isolated from natural sources. The chapter also reviews the isolation, characterization, known spectroscopic and chemical properties, and total syntheses of alkaloids and nucleobases, which are heterocyclic mesomeric betaines. Relatively few heteroaromatic N-oxides occur in nature. The chemistry of compounds that contain the oxidized peptide bond and their role in iron metabolism is also discussed, and the natural occurrence of N-oxide is reviewed.
The N-heterocyclic carbenes of indazole, generated by deprotonation of indazolium salts, proved to be versatile starting materials for organic synthesis. They undergo ring-opening reactions to generate ketenimines which readily add thiols thus affording what are, to the best of our knowledge, the first examples of 2-anilinobenzothioimidates. Water converts the ring-opened intermediates into functionalized 2-anilinobenzamides which can be thionated with Lawesson's reagent and subsequently cyclized with formaldehyde and propionaldehyde, respectively, to give benzo[d][1,3]thiazines and quinazoline-4-thiones. The outcome of the cyclization depends upon the thiobenzamide substitution pattern.
Rhodium(I) dicarbonyl complexes 1–4 containing chelating N-donor ligands bis(pyrazolyl)methane (bpm), bis(imidazolyl)methane (bim), tris(pyrazolyl)toluidine (tpt) or tris(imidazolyl)methanol (tim) were investigated as catalysts for the hydroalkoxylation of alkynyl benzoic acids (5a–g). The regioselectivity of the reaction was shown to be highly dependent on the nature of the terminal alkyne substituent (R) of the alkynol substrate. It was also determined that the presence of a third uncoordinated N-donor group in complexes 3 and 4 suppressed the catalytic efficiency of these complexes, and that the selectivity of the reaction for forming either endocyclic (6) or exocyclic (7) hydroalkoxylation products was influenced by the pendant hydroxyl group present in complex 4. We used 13C NMR spectroscopy to quantify the polarity of the alkynyl benzoic acid CC bond and our efforts to correlate this measure of bond polarity to the observed regioselectivity of the reaction are discussed.
Some mesoionic compounds, i.e. five-membered representatives of the class of conjugated mesomeric betaines (CMB), are in equilibrium with their tautomeric normal N-heterocyclic carbenes (nNHC). In addition, anionic N-heterocyclic carbenes, generated by deprotonation of mesoionic compounds, have been described. The first examples of conversions of crossconjugated mesomeric betaines (CCMB), 6-oxopyrimidinium-4-olates, into normal Nheterocyclic carbenes have been reported as well. CCMB such as imidazolium-4-carboxylate and pyrazolium-4-carboxylate can decarboxylate to form abnormal (aNHC) or remote N-heterocyclic carbenes (rNHC). Most conversions of betaines into N-heterocyclic carbenes start from pseudocross-conjugated mesomeric betaines (PCCMB) which can be regarded as heterocumulene adducts of nNHC. Thus, decarboxylations of imidazolium-2-carboxylates, 1,2,4-triazole-3carboxylates, pyrazolium-3-carboxylates or indazolium-3-carboxylates yield N-heterocyclic carbenes which have been used in catalysis, complex chemistry, heterocyclic synthesis, and organocatalysis.
The properties of type A and type B mesoionic diazolium olates, their fully covalent isomers and their aza derivatives have been investigated using the B3LYP/6-311++G(d,p) and MP2/aug-cc-pVDZ methods. The results, together with those for six-membered heterocyclic mesomeric betaines (HMBs), are in agreement with a revised classification of HMBs based on connectivity-matrix analysis. The calculated properties of the type B mesoionic rings are shown to be consistent with their classification as semi-conjugated HMBs; they have properties comparable to six-membered semi-conjugated HMBs including high classical aromaticity. The type A mesoionic rings are conjugated HMBs and have quite different properties including lower classical aromaticity. Magnetic aromaticity of both type A and type B mesoionic rings, measured by NICS indices, is low compared to imidazole and pyrazole. The results support the view that semi-conjugated HMBs should be recognized as a discrete class of dipolar heterocycle.
N-Heterocyclic carbenes of indazole (indazol-3-ylidenes), which are substituted at N1 with aromatics were generated in situ from the corresponding indazolium salts. At 60 °C the indazol-3-ylidenes underwent a ring-opening under N–N bond cleavage to intermediary N-(6-methylenecyclohexa-2,4-dien-1-ylidene)anilines. Trapping of these intermediates by alcohols proved to be a convenient method for the preparation of 2-anilinobenzimidates, which have scarcely been described in the literature. The reaction temperature avoids carbene dimerization, which occurs at −80 °C or rearrangement of the ring-opened intermediate to acridines, which affords 100 °C. Water converted the ring-opened products into anthranilamides, which were cyclized on reaction with formaldehyde to 2,3-dihydroquinazolin-4(1H)-ones.
A mixture of potassium hydroxide and potassium phosphate was found to be an active reagent mixture for the cleavage of 2-hydroxypropyl-protected acetylenes. The reaction was performed in toluene at reflux temperature and gave terminal acetylenes in good to excellent yields within very short periods of time. Numerous other functional groups are tolerated. Supplementary materials are available for this article. Go to the publisher's online edition of Synthetic Communications® for full experimental and spectral details.
The hydration of o-ethynylbenzoic acid derivatives, such as ethyl o-ethynylbenzoate, o-ethynylbenzonitrile, and o-ethynylbenzamide, in sulfuric acid in the presence of marcuric sulfate, afforded 3-substituted isocoumarins. The palladium-catalyzed reaction of o-halobenzoic acid derivatives with terminal acetylenes to obstain the starting materials for the cyclization is also described.
A detailed investigation on the thermodynamic and kinetic stability of four carbenic tautomers of quinoline 1, including quinoline-2-ylidene 2, quinoline-3-ylidene 3, quinoline-4-ylidene 4, and 3,4-dihydroquinoline-4-ylidene 5, reveals that singlet planar six-membered ring N-heterocyclic carbenes (NHCs) 2 and 4 have less stability than Arduengo type NHC but seems to have enough conceivably for reaching at B3LYP/aug-cc-pVTZ//B3LYP/6–31+G* and B3LYP/6–311++G**//B3LYP/6–31+G* levels. All these six-membered NHCs are extremely ambiphilic with the more nucleophilic and electrophilic characters compared to the Arduengo type one. The aromaticity of singlet 2 and 4 is a significant contributor to their stability which is confirmed through their Nucleus-independent chemical shift(1)zz values. Finally, among 2–5, the normal NHC 2 is thermodynamically preferred but the remote NHC 4 is kinetically proffered over the other isomeric carbenes. The effects of different N- or C-substituted NHCs of 2 are studied using appropriate isodesmic reactions. The trimethylsilyl substituent exhibits slightly larger carbene stabilization in quinoline-derived NHCs than the pyridine analogue. © 2014 Wiley Periodicals, Inc.
Indol-2-yl-imidazolium salts were deprotonated at N1 of the indole ring to give ylides. Their tautomeric N-heterocyclic carbenes (NHCs) were trapped by sulfur to give imidazole-2-thiones. Treatment of the ylides with triethylborane resulted in the formation of zwitterionic borane adducts. An analogous sequence of reactions was performed with 8-azaindol-2-yl-imidazolium salts, which served as precursor to prepare first representatives of a new heterocyclic ring system on reaction of their NHC-tautomers with triethylborane. Similarly, an indol-2-yl-1,2,4-triazolium salt was examined with respect to ylide–NHC tautomerism and trapping reactions. A nucleophilic ring transformation of indol-3-amine with a 1,3,4-oxadiazolium salt gave an indol-3-yl-triazolium salt, which was converted into a triazolethione by trapping of the tautomeric N-heterocyclic carbene of its ylide.
The structures and electronic properties of seven isomeric six-membered heterocyclic mesomeric betaines (HMBs) and four fully-covalent isomers have been calculated using B3LYP/6-311++G(p,d) methodology. The aromaticity indexes Bird I6, HOMA, NICS(1) and aromatic stabilisation energy (ASE) have been calculated and compared. In agreement with an earlier study employing connectivity-matrix analysis, the previously unrecognised and little known semi-conjugated HMBs (Class 3) have calculated properties that are quite distinct from the isomeric conjugated HMBs (Class 1) and cross-conjugated HMBs (Class 2). In particular semi-conjugated systems have a higher degree of both classical and magnetic aromaticity than the isomers. In agreement with the calculated vertical ionisation potentials and electron affinities, the six-membered semi-conjugated species have high energy HOMOs and low energy LUMOs and these observations are consistent with the limited experimental data available. The results support the view that semi-conjugated rings should be recognised as a discrete class of dipolar heterocycle.
A multipole model of the charge density distribution in the di-substituted alkyne title compound containing weak intramolecular interactions as defined by the topology of the charge density, has been obtained from high-resolution single crystal diffraction data. Bond paths are located between formally non-bonded C and O atom pairs and also between a pair of O atoms. This observation of weak intramolecular interactions in an experimentally derived charge density proves that the multipole modeling formalism is sufficiently sensitive to detect such features which are small on an absolute scale. Evidence is presented which suggests that the cis conformation of the molecule in this particular crystalline state is due to strong intermolecular hydrogen bonding.
The relationship between heterocyclic mesomeric betaines (HMBs) isoconjugate with hydrocarbon dianions and pairs of alternant hydrocarbon fragments is recognised. An analysis of the interactions of these alternant hydrocarbon fragments with the pairs of associated 2π heteroatoms (NR, O, S) leads to the definition of seven connectivity matrices, which in turn identify five discrete classes of HMB (Classes 1–5). Two of these classes, which we describe as semi-conjugated and pseudo-semi-conjugated HMBs, have not previously been recognised. Further examination of the connectivity matrices leads to the recognition of three sub-classes of conjugated (Class 1A–C), cross-conjugated (Class 2A–C) and pseudo-cross-conjugated (Class 4A–C) HMBs. Using this approach similarities and differences in a vast and diverse area of heterocyclic chemistry can be identified and new areas of potential interest identified.
The structure of 1-methylquinolinium-3-carboxy chloride monohydrate, 3QBWHCl, has been studied by X-ray diffraction, B3LYP/6-31G(d,p) calculations, NMR, FTIR and Raman spectra. The crystals are triclinic, space group P1¯. The unit cell comprises six symmetry-independent complexes denoted by letters A–F. Each of those 1-methylquinolinium-3-carboxylate is hydrogen-bonded to a water molecule (in four complexes) or a hydronium ion (in two complexes), O–H···O(W) = 2.556–2.585 Å. The water molecules and hydronium ions are hydrogen bonded to two Cl− anions, O(W)–H···Cl− = 3.093–3.119 Å. The Cl− anions and water molecules additionally interact electrostatically with the positively charged nitrogen atom, N+···Cl− = 3.375–3.394 Å, N+···O(W) = 4.027–4.081 Å. Structures of eight most stable complexes (G–N) have been optimized by the B3LYP/6-31G(d,p) level of theory and the results have been compared with the X-ray data. Linear correlations between the experimental 1H and 13C NMR chemical shifts (δexp) of 3QBHCl in D2O and DMSO-d6, and the GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding tensors (σcalc) using the screening solvation model (COSMO), δexp = a + bσcalc, are reported. The probable assignments of the anharmonic experimental solid state vibrational frequencies of anhydrous complexes L (3QBHCl and 3QBDCl) based on the calculated B3LYP/6-31G(d,p) harmonic frequencies have been proposed.
There are now a large number of papers in the spectroscopic literature which make use of multiple-valued (frequently double-valued) coordinate systems and the associated multiple-groups of the permutation-inversion group to deal with the symmetry properties of large-amplitude motions in molecules of high symmetry. The use of multiple-valued coordinate systems, and the resultant appearance of more minima on the potential surface than would be found on the surface for a single-valued coordinate system, can lead to conceptual confusion and questions of mathematical legitimacy. In the present paper we demonstrate that treatments using multiple-valued coordinate systems simply represent one scheme for applying the appropriate quantum mechanical boundary conditions to Schrödinger’s partial differential equation defined in a single-valued coordinate system. The demonstration is not general, but rather focuses on the specific example of the S1 electronic state of C2H2, which has local minima only for nonlinear configurations, and on the twofold and eightfold extended permutation–inversion groups recently introduced to simultaneously treat symmetry questions in trans-bent and cis-bent acetylene. Some discussion of the mathematical convenience lost by using a single-valued coordinate system is also presented.
The electronic ground state potential surface of acetylene (HCCH) has a minimum at the linear conformation, but the excited electronic states may have potential minima at a variety of nonlinear equilibrium shapes. This work is concerned with the group theoretical ideas necessary to treat simultaneously the symmetry properties of rovibronic states associated with three different planar acetylene equilibrium configurations, namely trans bent acetylene, cis bent acetylene, and vinylidene (H2CC). We make use of three different kinds of groups: (i) point groups, (ii) permutation-inversion (PI) groups, and (iii) extended PI groups. The PI group is G4 or G8, depending on whether CH bond breaking is impossible (no bent acetylene vinylidene interconversion) or possible. The extended PI groups are G4(2) and G8(2), respectively, when the only large amplitude motions are the CCH bends at each end of the molecule, and G4(8) and G8(8), respectively, when internal rotation is added as a third large amplitude motion. Applied to acetylene, the results indicate that there will be no splittings of the rovibronic levels unless CH bond breaking occurs. Even without bond breaking, however, states of the cis and trans isomers just below their interconversion barrier will show "staggerings" in their K-structures, i.e., a given vibrational level will have three tunneling components at slightly different energies: one component will have levels with K = 4n only (where n is an integer), a second component will have levels with K = 4n + 2 only, and the third will have only odd-K levels. New experimental results for the S1-cis electronic state of acetylene [21] are reviewed, and are found to be consistent with the group theory in so far as comparison is possible.
Treatment of perchlorobuta-1,3-diene with 4-(N,N-dimethylamino)pyridine resulted in the formation of 1,1',1 '',1'''-(2,3-dichlorobuta-1,3-diene-1,1,4,4-tetrayl)tetrakis[4-(dimethylamino)pyridinium] tetrachloride (structure confirmed by X-ray analysis), which was converted into persulfurated butatrienes ([3]cumulenes), pentakis(sulfanyl)-substituted buta-1,3-dienes, or tetrakis(sulfanyl)-substituted 2,3-dichlorobuta-1,3-dienes by reactions with thiolates or a sulfinate. The outcome of the reaction depends on the reaction conditions and the substitution pattern of the starting sulfur nucleophiles.
Treatment of the o-ethynylbenzoic acids 1A with a catalytic amount of Pd(II) and triethylamine resulted in the 6-endo-dig cyclization to give the 3-substituted isocoumarins 3 as major products in moderate to good yields. Similarly, the palladium-catalyzed cyclization of o-ethynylbenzamides 1B also proceeded but afforded only the isoquinolin-1-ones 3B.
Accurate structures have been determined for dicyanomethane, H2C(CN)2, vinyl dicyanide, H2CC(CN)2, carbonyl cyanide, OC(CN)2, and sulfur dicyanide, S(CN)2, by combining the results of electron diffraction, microwave spectroscopy and ab initio calculations. The different methods are compared. The influence of the correlations between the rotational constants when calculating the rρm structure and the weighting scheme in the least-squares fits (rϵ,I and r0 methods) are analyzed. The possibility of obtaining the true equilibrium CO distance from ab initio calculations is also discussed.
This study deals with the synthesis of a polyphenol bearing photosensitive groups, and its photochemical behavior upon UV irradiation. Mesoionic 5‐(4‐hydroxybenzyl)‐6‐oxo‐1,2,3‐triphenyl‐1,6‐dihydropyrimidin‐3‐ium‐4‐olate was prepared by condensation of 4‐benzyloxybenzylmalonic acid with N , N ′‐diphenylbenzamidine using dicyclohexylcarbodiimide as condensing agent, followed by deprotection of the benzyl group. The monomer was polymerized by an iron‐salen complex to give a polymer with a mesoionic 6‐oxo‐1,6‐dihydropyrimidin‐3‐ium‐4‐olate moiety as the pendant group. Spin coated polymer films were prepared and characterized. It was shown by IR spectroscopy that irradiation with UV light converted the mesoionic structure to a bis( β ‐lactam) structure. Waveguide spectroscopy showed a large decrease in the film thickness without refractive index changes during irradiation.
Time development of the refractive index and the film thickness measured with p‐polarized light.
magnified image Time development of the refractive index and the film thickness measured with p‐polarized light.
The structure of 1-methylquinolinium-3-carboxylate (benzotrigonelline) monohydrate has been studied by X-ray diffraction, B3LYP/6-31G(d,p) calculations, NMR, FTIR and Raman spectra. The crystals are monoclinic, space group P21/c, with a=6.6716(2), b=12.8422(4), c=11.3638(5)Å, β=99.925(4)°, V=959.06(6)Å3, and Z=4. Two 1-methylquinolinium-3-carboxylate molecules are linked by a pair of water molecules into a centro-symmetric dimer via two O(W)–H⋯O(1) bifurcated hydrogen bonds of lengths 2.867(2) and 3.046(2)Å. Structures of two of the most stable conformers of dimer, two of hydrated monomer and one anhydrous molecule have been analyzed by the B3LYP/6-31G(d,p) level of theory and the results have been compared with the X-ray data. The probable assignments of the anharmonic experimental solid state vibrational frequencies of the investigated compound, based on the calculated harmonic frequencies in vacuum at the same level of theory for the optimized structure, have been made. Correlations between experimental chemical shifts and GIAO/B3LYP/6-31G(d,p) calculated magnetic isotropic shielding constants, δexp=a+bσcalc, are reported.
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Irradn. of Group VIA carbonyl complexes in the presence of 1,3-dimethylimidazol-2-ylideneborane (IMe·BH3) afforded the new borane complexes [CpMn(CO)2(η1-H3B-IMe)] and [M(CO)5(η1-H3B-IMe)] (M = Cr, Mo, W). With the exception of [Mo(CO)5(η1-H3B-IMe)], which was detected only in soln., the solid-state structures of all species were validated by single-crystal x-ray diffraction. The NHC-BH3 moiety is coordinated to the metal center through a B-H-M three-center two-electron bond presumably without any B-M interaction. π-Back-donation from the metal center to the B-contg. ligand is most likely negligible. [on SciFinder(R)]
Effects of substituents on transformations of 4-(phenylethynyl)benzaldehydes and related
benzaldehydes were analyzed in aldol and thiazolidine formation reactions. The aldol reaction
of 4-cyanobenzaldehyde was 54-fold faster than that of 4-methoxybenzaldehyde. In contrast,
the aldol reaction of 4-(4-cyanophenylethynyl)benzaldehyde was only 1.4-fold faster than that of4-(4-methoxyphenylethynyl)benzaldehyde. Electronic features of substituents are significantly less influential in a diphenylacetylene system than in a simple benzene system.
Mesoionic pyrimidinium-olate derivatives have been known to undergo photoreactions upon irradiation with UV light to form
bis(beta-lactame) structures for about two decades. Here, new mono-, poly- and macrocyclic mesoions were prepared and their
photochemical rearrangement behavior was investigated. The synthesized compounds were characterized by NMR, IR, UV/Vis
spectroscopy, elemental analysis and mass spectrometry. The extension of the aromatic core leads to a significant red-shift of the absorption
maximum fromw380 tow430 nm, indicating a strong electronic coupling of the mesoionic base chromophore with the annellated aromatic
subunit. The rigidity of the extended aromatic system prevents polycyclic mesoions from shifting to their bis(beta-lactame) isomers, while
the alkyl bridge of the 16-membered macrocycle in an ansa-mesoion does not inhibit photochemical rearrangement.
q 2004 Elsevier Ltd. All rights reserved.
In the presence of NBS 3-methylindole reacted with various imidazoles to (indol-2-yl)imidazolium salts 21a-f which were converted in aqueous solution into the 2-(imidazolium-3-yl)-3-methylindolates 22a-f by base. These conjugated ylides - which represent a subclass of mesomeric betaines - are the exclusively detectable form in the NMR spectra taken in DMSO-d6. A DFT calculation revealed that the betaine 22a is -9.3 kJ/mol more stable than the tautomeric N-heterocyclic carbene 23a, and that the energy for the betaine - carbene interconversion is DeltaG# = 66.4 kJ/mol. The N-heterocyclic carbenes (3-methyl-indol-2-yl)imidazol-2-ylidenes, however, can be trapped by sulfur, triethylborane, and triphenylborane, respectively. Whereas the former trapping reaction yielded the expected imidazolethiones, the borates gave first representatives of new zwitterionic borane adducts, imidazo[2',1':3,4][1,4,2]diazaborolo[1,5-a]indolium-11-ides 26a-h. We performed DFT-calculations on the structures of mesomeric betaine 22a, the carbene 23a, and the mechanisms of the borane adduct formation to 26a-h, NMR spectroscopic investigations including 15N, 7Li and 11B NMR spectroscopy, and an X-ray single crystal analysis of one of the borane adducts.
Versatile precursor to metal complexes: The lithiated N-heterocyclic carbene derivative [Li(imino-imidide)(tmeda)] (imino-imidide=4-(2,6-diisopropylphenylimino)imidazol-2-ylidenide), obtained by deprotonation and lithiation of a mesoionic 4-aryliminoimidazolium, serves as a versatile precursor to metal complexes with the anionic 4-aryliminoimidazolide by salt metathesis. DFT calculations demonstrate that tuneable electronic and steric factors of the substituent at the N(exo) imino function of the heterocycle dictate the preferred site of the metallation (C(NHC) vs. N(exo) ).
This review article summarizes progress in N-alkylations, N-alkenylations, N-and C-arylations and hetarylations, halogenations, as well as organometallic chemistry of pyrazoles mainly achieved during the last decade. In addition, the generation and chemistry of N-heterocyclic carbenes of pyrazole (pyrazol-3-ylidenes) and remote N-heterocyclic carbenes of pyrazole (pyrazol-4-ylidenes) are described.
We have developed a gold(I)-catalyzed intramolecular cyclization of γ- and δ-alkyne acids in mild conditions yielding various alkylidene lactones. Whereas a slight electronic effect of the R group was observed on the regioselectivity, bulky substituents on the R group bearing the alkyne strongly modify the reactivity. The cycloisomerization of o-alkynylbenzoic methyl esters was achieved rather with AuCl3 as catalyst owing the presence of water affording exclusively the isocoumarins.Graphical abstract
The present report discloses a modular synthetic route to a new type of anionic N-heterocyclic carbene ligand incorporating an enolate group as a reactive backbone component of its heterocyclic framework. The presence of such a reactive unit facilitates further tailoring of the ligand, even after its complexation to transition metals, with concomitant tuning of the electronic donor properties of the carbene center. Simple acylation of the formamidine Ar-NH-CH═NAr (1a,b) (a: Ar = mesityl (Mes); b: Ar = 2,6-diisopropylphenyl (DIPP)) by chloroacetyl chloride gives an acylated formamidine (2a,b), which undergoes a thermally induced cyclization to afford the 4-hydroxyimidazolium salt 3a,b. Single deprotonation by NEt3 gives the zwitterionic imidazolium-4-olate 4, whereas double deprotonation by 2 equiv of LiHMDS gives the imidazolin-2-ylidene-4-olate ligand 5−, which reacts with 0.5 equiv of [RhCl(1,5-COD)]2 to give, after acidification with HCl, the complex [(5H)RhCl(COD)] (7), in which the “acidified” carbene ligand noted “5H” adopts the keto form. By contrast, treatment of 4 with electrophilic reagents E-X (E1 = tBuCO; E2 = Me; E3 = Tf) results in the O-functionalized imidazolium derivatives [4E1−3]+X−, which can be subsequently complexed under the form of the carbene 5E1−3 to give [(5E1−3)RhCl(COD)] (7E1−3). Alternatively, the carbene ligand 5 can be post-functionalized from its complex [(5H)RhCl(COD)] (7) by a simple sequence involving (i) base-induced deprotonation and (ii) addition of E-X, exemplified by the generation of [(5E4−5)RhCl(COD)] (7E4−5) (with E4 = Ph2PO; E5 = tBuSiMe2 (TBDMS)). In parallel, C-functionalization of the metal-bound ligand can be achieved from 7 by deprotonation with LiHMDS, leading to [(5−)RhCl(COD)]Li(thf)n+ followed by a classical aldolization/elimination sequence leading to [(5CH2)RhCl(COD)] (8), in which the C5 carbon of the NHC bears a methylene group. Parallel direct transformations can be achieved after ligand complexation to copper, as exemplified by the generation of [(5H)CuCl] (9) or [(5E5)CuCl] (9E5) (E5 = TBDMS). The donor properties of all the above NHC ligands were evaluated from the standard complexes (NHC)RhCl(CO)2 against the classical IR ν(CO) scale. The resulting order of nucleophilicities for the modulated carbenes is 5CH2 < 5H 5E4 < 5E1 < 5E2 < 5E5 5− . The X-ray structures analyses of 7E2 and 7E4 are reported.
We report a thorough investigation of a series of isomeric complexes with the general formula trans-(pyridylidene)M(PPh3)2Cl (M = Pd, Ni). For the first time, a systematic comparison of normal, abnormal, and remote bonding modes is presented. X-ray structural and 13C NMR data indicate the importance of carbenoid mesomeric contributions in their compound class. The catalytic performance of the palladium complexes trans-(pyridylidene)Pd(PPh3)2Cl as precursors in Suzuki−Miyaura-type cross-coupling suggests a correlation of remote bonding with catalyst robustness and effectivity. When metal precursors M(PPh3)4 are reacted with 2,4-dichloropyridinium salts, preferential formation of remote carbene complexes occurs and indications are that electronic parameters rather than steric influences are responsible for the observed selectivity. Calculations at the BP86/TZ2P level of theory support interpretation of the results.
Rotational transitions for four isotopomers of diethynyl sulfide, S(CCH)2, were observed by using a pulsed nozzle Fourier transform microwave spectrometer. Vibration−rotation interaction constants obtained from ab initio calculated force fields were used to convert the ground-state rotational constants to equilibrium values. The latter were used to derive structural parameters, several of which are not in good agreement with those obtained from high level ab initio calculations. The structure has planar C2v symmetry with the following parameters: d(SC) = 1.708(20) Å, d(CC) = 1.211(10) Å, d(CH) = 1.061(10) Å, (CSC) = 100.5(5)°, (SCC)outside angle = 174.4(15)°, (CCH)inside angle = 177.0(25)°. The structure of bis(phenylethynyl) sulfide, S(CCC6H5)2, was determined by X-ray crystallography. The unit cell contains two inequivalent molecules. The structural parameters are (average of both species; uncertainties represent 1σ) d(SC) = 1.695(4) Å, d(CC) = 1.198(2) Å, d(CCphenyl) = 1.440(4) Å, (CSC) = 101.4(9)°, (SCC) = 176.4(19)°, and (CCCphenyl) = 178.2(8)°. The phenyl groups are twisted out of the C−S−C plane from 20 to 85° such that each phenyl ring is oriented nearly 90° to its intramolecular mate. The structural parameters are compared with several other sulfides and with hydrocarbons containing acetylenic linkages. The nonlinearity about the acetylenic bond follows a pattern seen in other similar species, although it is more pronounced in S(CCH)2 than in hydrocarbons. These data constitute the first experimental structural studies on uncomplexed diethynyl sulfides.
A theoretical and experimental study of cross-conjugated (CCMB) and pseudo-cross-conjugated (PCCMB) mesomeric betaines shows that they are best represented as containing distinct cationic and anionic segments which comprise a common π-electron system. The PCCMB differ from the CCMB in that they all contain a 2-oxyallyl cation 1,3-dipole. The behavior of these betaines under electrophilic and nucleophilic reaction conditions has been established.
Reaction of pyrazoles, 1,2,3-triazoles, and 1,2,4-triazoles with aryl(chlorocarbonyl)ketenes, alkylmalonyl dichlorides, or carbon suboxide results in a series of cross-conjugated mesomeric betaines, characterized by the presence of distinct cationic and anionic segments. 1-Substituted imidazoles, suitably substituted 1,2,4-triazoles, and pyridine with the above reagents give rise to pseudo-cross-conjugated mesomeric betaines which, in addition to charge separation, are characterized by the presence of the 2-oxyallyl cation 1,3-dipole. Alternative syntheses of cross-conjugated mesomeric betaines which allow the introduction of more diverse substituents are also described.
(Z)-3-(1-Alkylidene)phthalides and 3-substituted isocoumarins, which include compounds bearing a substituent on their benzene ring, have been selectively and efficiently synthesized by a new protocol which involves: (i) the conversion of methyl 2-hydroxybenzoates into the corresponding nonaflates; (ii) Pd-catalyzed alkynylation reactions of these derivatives; (iii) the conversion of the so obtained methyl 2-(1-alkynyl)benzoates into the corresponding carboxylic acids followed by a transition metal-catalyzed heteroannulation reaction. This procedure has been used to prepare either natural products such as senkyunolide B, senkyunolide C, 3-propylisocoumarin and artemidin, or the MEM-ether of senkyunolide E. The regioselectivity of the transition metal-catalyzed cyclization reactions of 2-(1-alkynyl)benzoic acids has proven to be affected either by the catalyst used or the type of 1-alkynyl group present in these carboxylic acids. (C) 2000 Elsevier Science Ltd. All rights reserved.
Ein Pyrazolderivat, das Dyker, Bertrand et al. für ein stabiles fünfgliedriges cyclisches Allen 1 a hielten, wird als Zwitterion 1 b reinterpretiert. Es wird dargelegt, dass die für Allene charakteristischen Bindungsverhältnisse für die Struktur 1 a nicht möglich sind. Ar = 2,6-Dimethylphenyl.
Two new second generation ruthenium(11) dichloride-indenylidene complexes [RuCl2(9-isobutylphosphabicyclo[3.3.1]nonane)(NHC)(3-phenyl-1-indenylidene)1, where NHC,3-bis(2,4,6-tri-methylphenyl)imidazolin-2-ylidene (SIMes) or its unsaturated imidazol-2-ylidene analogue (1Mes). were isolated in high yields upon heating a tetrahydrofuran (THF) solution of the diphosphane complex [RuCl2(isobutylphobane)2(3-phenyl-1-indenylidene)] with a two-fold excess of the corresponding imidazol(in)ium-2-carboxylate zwitterions. Both products were characterized by H-1, C-13, and P-33 NMR spectroscopy, and the molecular structure of [RuCl2(isobutylphobane)(SIMes)(3-phenyl-1-indenylidene)] was determined by X-ray diffraction analysis. A close inspection of the packing structure revealed the presence of different types of intra- and intermolecular interactions that enhanced the global stability of the crystals, while low temperature NMR experiments showed the existence of two distinct rotational isomers due to the unsymmetrical nature of the phobane ligand The catalytic activity of both compounds was assessed in olefin metathesis using
Five well-known ruthenium-N-heterocyclic carbene (NHC) catalysts for olefin metathesis and related reactions, including the second generation Grubbs and Hoveyda–Grubbs catalysts, were prepared by phosphane exchange between first generation ruthenium-benzylidene or indenylidene complexes and NHCs generated in situ upon thermolysis of imidazol(in)ium-2-carboxylates. Because N-heterocyclic carbene carbon dioxide adducts (NHC⋅CO2) are stable zwitterionic compounds that can be stored and handled with no particular precautions, this procedure is particularly attractive from a practical point of view. Reaction courses were conveniently monitored by 31P NMR spectroscopy and the experimental conditions were carefully adjusted to obtain high yields of pure products within short periods of time.