Article

Origin of Regiochemical Control in Rh(III)/Rh(V)-Catalyzed Reactions of Unsaturated Oximes and Alkenes to Form Pyrdines

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Abstract

The Rh(III)-catalyzed reactions of α,β-unsaturated oximes with alkenes are versatile methods for the synthesis of pyri-dines. Density functional theory (DFT) calculations reported here reveal the detailed mechanism and origins of selec-tivity in this reaction. The Rh(III)/Rh(V)/Rh(I) catalytic cycle was found to be more favorable than the previously pro-posed Rh(III)/Rh(I)/Rh(III) catalytic cycle. The Rh(III)/Rh(V)/Rh(I) catalytic cycle involves C–H activation, alkene in-sertion, deprotonation, oxime migratory oxidative addition, nitrene insertion, 1,5-hydrogen shift, and β-hydride elimi-nation to give the pyridine product and form a Rh(I) species. Subsequent oxidation by Ag+ regenerates the Rh(III) cata-lyst. Reductive elimination from a alkyl-Rh(III) species is predicted to be difficult, so that the Rh(III)/Rh(I)/Rh(III) cata-lytic cycle can be excluded. The reactivities of oxime ethers and oxime esters are compared. The oxime ester acts as both a directing group and an internal oxidant. In this reaction, the N–O bond is activated by the pivalate, and migratory oxidative addition onto the Rh(III) species generates the corresponding Rh(V) nitrene complex. However, in the ab-sence of the pivalate on the oxime ether, the activation energy for oxidative addition is much higher. The reactivity was analyzed by NPA charge calculations, comparison of the N–O bond orders, and the bond dissociation energies. The calculations also explain the regioselectivity of alkene insertion, which is shown to be an electronic effect rather than a steric effect.

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A review of the computational studies of Pd-, Ni-, Rh-, and Ir-mediated transformations that have been conducted between 2008 and 2014 and been of significance for organic synthesis is presented. The review is meant to provide the reader with a summary of the recent computational studies that have been conducted in relation to synthetically relevant Pd-, Ni-, Rh-, and Ir-catalyzed transformations, the choice and performance of computational methodology used therein, and the numerous mechanistic insights gained. The review also states that energy calculations are important steps for accurate and reliable studies of Pd, Ni, Rh, and Ir complexes and related systems.
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The divergence between Rh(III)-catalyzed C-H activation/cycloaddition of phenyl- and 2-furanyl-containing benzamides with methylenecyclopropanes (MCP) was studied by DFT calculations. Calculations found that the C-H activation via a CMD mechanism is the most difficult step of the reaction involving phenyl. In contrast, the C-H activation of the 2-furanyl-containing substrate is kinetically easier but the formed five-membered rhodacycle is relatively unstable, making the following MCP insertion more difficult. Thus, different KIE data was obtained in experiments. The MCP insertion forms a seven-membered-ring rhodacycle intermediate, from which the chemoselectivity of the whole reaction is determined by the competitive pivalate migration (path I) and β-C elimination (path II). While the β-C elimination is lower in energy when a furanylene is contained in the intermediate, a reversed preference of pivalate migration was predicted for the phenylene counterpart. Structural analysis suggested that the unfavorable β-C elimination in the phenylene case should be attributed to the obviously increased ring strain in the corresponding transition state, instead of the difference in electronic properties between the aryl groups. This accounts for why aryl-dependent chemoselectivity was observed. In addition, the results indicated that for both paths I and II the generation of a Rh(V)-nitrenoid intermediate from pivalate migration is crucial for the final C-N bond formation. This explains why no reaction occurred when the N-OPiv moiety was replaced with an N-OMe group, as no Rh(V) intermediate could be formed in this system.
Article
Metal-catalyzed CH activation not only offers important strategies to construct new bonds, it also allows the merge of important research areas. When quinoline N-oxide is used as an arene source in CH activation studies, the NO bond can act as a directing group as well as an O-atom donor. The newly reported density functional theory method, M11L, has been used to elucidate the mechanistic details of the coupling between quinoline NO bond and alkynes, which results in CH activation and O-atom transfer. The computational results indicated that the most favorable pathway involves an electrophilic deprotonation, an insertion of an acetylene group into a RhC bond, a reductive elimination to form an oxazinoquinolinium-coordinated Rh(I) intermediate, an oxidative addition to break the NO bond, and a protonation reaction to regenerate the active catalyst. The regioselectivity of the reaction has also been studied by using prop-1-yn-1-ylbenzene as a model unsymmetrical substrate. Theoretical calculations suggested that 1-phenyl-2-quinolinylpropanone would be the major product because of better conjugation between the phenyl group and enolate moiety in the corresponding transition state of the regioselectivity-determining step. These calculated data are consistent with the experimental observations. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Article
A mechanistic study of the substituent-dependent ring formations in Rh(III) -catalyzed CH activation/cycloaddition of benzamide and diazo compounds was carried out by using DFT calculations. The results indicated that the decomposition of the diazo is facilitated upon the formation of the five-membered rhodacycle, in which the Rh(III) center is more electrophilic. The insertion of carbenoid into RhC(phenyl) bond occurs readily and forms a 6-membered rhodacycle, however, the following CN bond formation is difficult both kinetically and thermodynamically by reductive elimination from the Rh(III) species. Instead, the Rh(V) -nitrenoid intermediate could be formed by migration of the pivalate from N to Rh, which undergoes the heterocyclization much more easily and complementary ring-formations could be modulated by the nature of the substituent at the α-carbon. When a vinyl is attached, the stepwise 1,3-allylic migration occurs prior to the pivalate migration and the 8-membered ring product will be formed. On the other hand, the pivalate migration becomes more favorable for the phenyl-contained intermediate because of the difficult 1,3-allylic migration accompanied by dearomatization, thus the 5-membered ring product was formed selectively. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Article
The first example of Rh(III)-catalyzed cyclization of arylnitrones to indolines under external oxidant-free conditions is presented. An intermolecular coupling of arylnitrones with internal alkynes is made possible by the dual role of Cp*Rh(III) catalyst mediating both the C-H bond activation and oxygen atom transfer. Synthetically important and pharmacologically privileged indoline derivatives were obtained in good yields with high diastereoselectivity.
Article
Transition metal-catalyzed direct C-H functionalization has drawn great attention in the past several decades owing to its advantages compared to conventional organic transformations, including higher atom-, step- and cost-economy and the avoidance of tedious prefunctionalization and waste emission. At the current stage, to make the C-H functionalization more applicable, chemists have devoted themselves to expanding the substrate and reaction scope. In the past decade, we exerted ourselves to develop new transformations based on direct C-H functionalization. In this minireview we report on our recent achievements on the addition of C-H bonds to carbonyls and imines. The addition of organometallic reagents, such as Grignard reagents, toward carbon-heteroatom double bonds is one of the most powerful reactions in organic synthesis to produce secondary and tertiary alcohols and amines. This chemistry is broadly used in both laboratory and industry. However, this powerful transformation suffers from some drawbacks: (1) the preparation of initial organohalides from easily available fossil feedstocks is tedious and sluggish; (2) substantial amounts of metal halide salts are emitted as waste; (3) last but not least, the manipulation of organometallic reagents is complicated due to their sensitivity to air and moisture. In contrast, direct insertion of polar double bonds to C-H bonds via transition-metal catalysis is ideal from the viewpoint of atom-, step- and cost-economy and the avoidance of the waste emission, as well as of the complex manipulation of sensitive reagents. Starting from this point, we made a commitment to this project years ago and have made credible achievements in this field. We first carried out Ir-catalyzed addition of pyridinyl C-H bonds to aldehydes promoted by silane, showing an unusual C-3 selectivity. Later on, we developed Rh-catalyzed addition of aryl C-H bonds with aldimines in the absence of any additives with directing strategy with highest atom- and step-economy. The mechanism was investigated in depth by the isolation of key intermediates and systematic thermodynamic and kinetic studies. Such a concept was expanded to the coupling of aryl/alkenyl C-H bonds with aldehydes and imines. Notably, a tandem process of relayed C-H activation/alkyne insertion/cyclization between benzoates/benzimide and alkynes was developed, indicating the potential of the direct coupling of esters and amides with C-H bonds. Ideally, this strategy opens a new window to approach the ideal reactions to produce amines and alcohols from hydrocarbons.
Article
DFT calculations have been carried out to study the detailed mechanism of Rh(III)-catalyzed C–H activation/cyclization of 2-acetyl-1-arylhydrazines with alkynes leading to the formation of indoles, in which the hydrazine moiety is used as the internal oxidant. The energy profiles associated with the catalytic cycle, involving N–H deprotonation, C–H activation (a concerted metalation–deprotonation (CMD) process), alkyne insertion, ring rearrangement/isomerization, and finally N–N bond cleavage/reductive elimination to regenerate the active species, are presented and analyzed. Through analysis of the calculation results, we found that the combined processes of the CMD and alkyne insertion contribute to the overall rate-determining step. The N–N bond cleavage step was examined in detail to understand how the internal oxidant interacts with the metal center to facilitate the catalytic reactions. The factor influencing regioselectivity was also investigated. How different types of substrates (alkynes versus alkenes) and internal oxidants (−NH(NHAc) versus −NH(OAc)) influence the reaction mechanisms, Rh(III)/Rh(I) versus Rh(III)/Rh(V) catalytic cycles, was discussed.
Article
Rh(III)-catalyzed C-H activation assisted by an oxidizing directing group (DG) has evolved to a mild and redox-economic strategy for the construction of heterocycles. Despite the success, these coupling systems are currently limited to cleavage of an oxidizing N-O or N-N bond. Cleavage of an oxidizing C-N bond, which allows for complementary carbocycle synthesis, is unprecedented. In this article, -ammonium acetophenones with an oxidizing C-N bond have been designed as substrates for Rh(III)-catalyzed C-H activation under redox-neutral conditions. The coupling with -diazo esters afforded benzocyclopentanones and the coupling with unactivated alkenes such as styrenes and aliphatic olefins gave ortho olefinated acetophenoes. In both systems the reactions proceeded with a broad scope, high efficiency, and functional group tolerance. Moreover, efficient one-pot coupling of diazo esters has been realized starting from -bromoacetophenones and triethylamine. The reaction mechanism for the coupling with diazo esters has been studied by a combination of experi-mental and theoretical methods. In particular, three distinct mechanistic pathways have been scrutinized by DFT studies which revealed that the C-H activation occurs via a C-bound enolate-assisted concerted metalation-deprotonation me-chanism and is rate-limiting. In subsequent C-C formation steps, the lowest energy pathway involves two rhodium carbene species as key intermediates.
Article
Quinoline N-oxides were found to undergo Cp*Rh(III)-catalyzed coupling with internal diarylalkynes to provide 8-functionalized quinolines through a cascade process that involves remote C–H bond activation, alkyne insertion, and intramolecular oxygen atom transfer. In this reaction, the N-oxide plays a dual role, acting as a traceless directing group as well as a source of oxygen atom, as confirmed by an 18O-labeling experiment.
Article
Oxidative cross-coupling reactions between two nucleophiles are a powerful synthetic strategy to synthesize various kinds of functional molecules. Along with the development of transition-metal-catalyzed oxidative cross-coupling reactions, chemists are applying more and more first-row transition metal salts (Fe, Co, etc.) as catalysts. Since first-row transition metals often can go through multiple chemical valence changes, those oxidative cross-couplings can involve single electron transfer processes.
Article
[Cp*RhIII]-catalyzed CH activation of arenes assisted by an oxidizing NO or NN directing group has allowed the construction of a number of hetercycles. In contrast, a polar NO bond is well-known to undergo O-atom transfer (OAT) to alkynes. Despite the liability of NO bonds in both CH activation and OAT, these two important areas evolved separately. In this report, [Cp*RhIII] catalysts integrate both areas in an efficient redox-neutral coupling of quinoline N-oxides with alkynes to afford α-(8-quinolyl)acetophenones. In this process the NO bond acts as both a directing group for CH activation and as an O-atom donor.
Article
The Csp3–Csp2 vs Csp3–Csp3 site selectivity in the C–C bond activation in Rh-catalyzed ring opening of benzocyclobutenol was systematically investigated using density functional theory (DFT). The catalytic cycle includes three elementary steps: the proton transfer from the substrate to a rhodium hydroxide, the C–C cleavage, and the proton transfer from water onto a carbon forming the final product with regeneration of the rhodium hydroxide. The site selectivity is determined by the C–C cleavage step; the Csp3–Csp2 cleavage is favored over the Csp3–Csp3 cleavage because the former transition state is stabilized by an interaction between the benzene ring of the substrate and Rh. DMSO, a more polar solvent, reduces the site selectivity as the more polar Csp3–Csp3 transition state (TS) is stabilized more than the Csp3–Csp2 TS and decreases the advantage of the latter TS. DPPF ligand is bulky, and the steric repulsion on the tighter Csp3–Csp2 TS causes the loss of the site selectivity. For the even more crowded Rh(P(t-Bu)3)2 catalyst, one phosphine has to dissociate before the C–C cleavage reaction takes place, and the advantage of the Csp3–Csp2 TS is regained for the less crowded RhP(t-Bu)3 active catalyst.
Article
An experimentally simple additive-free Rh(iii)-catalysed direct alkynylation of alkenes has been developed. This protocol employs commercially available TIPS-EBX as the alkyne source, giving access to conjugated terminal enynes following a simple silyl-deprotection. This method has also been applied to arenes.
Article
α,β-Unsaturated carboxylic acids undergo Rh(III)-catalyzed decarboxylative coupling with α,β-unsaturated O-pivaloyl oximes to provide substituted pyridines in good yield. The carboxylic acid, which is removed by decarboxylation, serves as a traceless activating group, giving 5-substituted pyridines with very high levels of regioselectivity. Mechanistic studies rule out a picolinic acid intermediate, and an isolable rhodium complex sheds further light on the reaction mechanism.
Article
A Rh(I)-catalyzed formal carbene insertion into C-C bond of benzocyclobutenols has been realized by employing diazoesters as carbene precursors. The product indanol derivatives were obtained in good yields and in diastereoselective manner under mild reaction conditions. All-carbon quaternary center is constructed at the carbenic carbon. This catalytic reaction involves selective cleavage of C-C bond, Rh(I) carbene insertion and intramolecular aldol reaction.
Article
The rhodium(III)-catalyzed ortho C-H alkynylation of non-electronically activated arenes is disclosed. This process features a straightforward and highly effective protocol for the synthesis of functionalized alkynes and represents the first example of merging a hypervalent iodine reagent with rhodium(III) catalysis. Notably, this reaction proceeds at room temperature, tolerates a variety of functional groups, and more importantly, exhibits high selectivity for monoalkynylation. Hot rhod: A rhodium-catalyzed, electronically reversed Sonogashira reaction between unbiased arenes and the hypervalent iodine reagent 1 proceeds through C-H activation. This reaction displays excellent functional-group tolerance and high efficiency, and thus opens a new synthetic pathway to access functionalized alkynes. Cp*=C5Me5, DCE=1,2-dichloroethane, Piv=pivaloyl, TIPS=triisopropylsilyl.
Article
A Rh(III)-catalyzed aryl C-H bond insertion into cyclopropenones via a C-H activation pathway has been reported. A series of arenes bearing directing groups such as 2-pyridyl, 2-pyrimidyl, N-pyrazyl, and oxime can be applicable, providing chalcones in excellent yields under mild conditions. Several possible Rh(III) intermediates in this reaction were investigated.
Article
Direct C-H amination of arenes offers a straightforward route to aniline compounds without necessitating aryl (pseudo)halides as the starting materials. The recent development in this area, in particular in the metal-mediated transformations, is significant with regard to substrate scope and reaction conditions. Described herein are the mechanistic details on the Rh-catalyzed direct C-H amination reaction using organic azides as the amino source. The most important two stages were investigated especially in detail: i) the formation of metal nitrenoid species and its subsequent insertion into a rhodacycle intermediate, and ii) the regeneration of catalyst with concomitant release of products. It was revealed that a stepwise pathway involving a key Rh(V)-nitrenoid species that subsequently undergoes amido insertion is favored over a concerted C-N bond formation pathway. DFT calculations and kinetic studies suggest that the rate-limiting step in the current C-H amination reaction is more closely related to the formation of Rh-nitrenoid intermediate rather than the pre-supposed C-H activation process. The present study provides mechanistic details of the direct C-H amination reaction which bears both aspects of the inner- and outer-sphere path within a catalytic cycle.
Article
An extended basis set of atomic functions expressed as fixed linear combinations of Gaussian functions is presented for hydrogen and the first‐row atoms carbon to fluorine. In this set, described as 4–31 G, each inner shell is represented by a single basis function taken as a sum of four Gaussians and each valence orbital is split into inner and outer parts described by three and one Gaussian function, respectively. The expansion coefficients and Gaussian exponents are determined by minimizing the total calculated energy of the atomic ground state. This basis set is then used in single‐determinant molecular‐orbital studies of a group of small polyatomic molecules. Optimization of valence‐shell scaling factors shows that considerable rescaling of atomic functions occurs in molecules, the largest effects being observed for hydrogen and carbon. However, the range of optimum scale factors for each atom is small enough to allow the selection of a standard molecular set. The use of this standard basis gives theoretical equilibrium geometries in reasonable agreement with experiment.
Article
A contracted Gaussian basis set (6‐311G∗∗) is developed by optimizing exponents and coefficients at the Møller–Plesset (MP) second‐order level for the ground states of first‐row atoms. This has a triple split in the valence s and p shells together with a single set of uncontracted polarization functions on each atom. The basis is tested by computing structures and energies for some simple molecules at various levels of MP theory and comparing with experiment.
Article
A Set of seven-component f-type polarization functions has been optimized for use with the pseudo-potentials of Hay and Wadt at the CISD level of theory for the transition metals ScCu, YAg, LaAu in the energetically lowest-lying s1 dn electronic state.
Article
Multisubstituted isoquinoline and pyridine N-oxides have been prepared by Rh(III)-catalyzed cyclization of oximes and diazo compounds via aryl and vinylic C-H activation. This intermolecular annulation procedure involving tandem C-H activation, cyclization and condensation steps, proceeds under mild conditions, obviates the use of oxidants, releases N2 and H2O as the byproducts, and displays a broad scope with respect to the substituents.
Article
Summary Nonrelativistic and quasirelativisticab initio pseudopotentials substituting the M(Z-28)+-core orbitals of the second row transition elements and the M(Z-60)+-core orbitals of the third row transition elements, respectively, and optimized (8s7p6d)/[6s5p3d]-GTO valence basis sets for use in molecular calculations have been generated. Additionally, corresponding spin-orbit operators have also been derived. Atomic excitation and ionization energies from numerical HF as well as from SCF pseudopotential calculations using the derived basis sets differ in most cases by less than 0.1 eV from corresponding numerical all-electron results. Spin-orbit splittings for lowlying states are in reasonable agreement with corresponding all-electron Dirac-Fock (DF) results.
Article
Give it a tweak: A novel oxidizing directing group was developed for a rhodium(III)-catalyzed CH functionalization of N-phenoxyacetamides with alkynes. A small change in the reaction conditions leads to either ortho-hydroxyphenyl-substituted enamides or cyclization to deliver benzofurans with high selectivity (see scheme; Cp*=C5 Me5 ).
Article
The Rh(III)-catalyzed oxidative coupling of alkenes with arenes provides a greener alternative to the classical Heck reaction for the synthesis of arene-functionalized alkenes. The present mechanistic study gives insights for the rational development of this key transformation. The catalyst resting states and the rate law of the reaction have been identified. The reaction rate is solely dependent on the catalyst and alkene concentrations, and the turnover-limiting step is the migratory insertion of the alkene into a Rh-C(aryl) bond.
Article
The ab initio method at the Hartree-Fock level has been used on two zirconocene systems, the unbridged Cp2ZrCH3+ and the bridged (Si(CH3)2)Cp2ZrCH3+ compounds. For each complex the insertion of an ethylene molecule has been followed from reactant to product through the corresponding transition states. The calculated activation energies are of the order of magnitude of those obtained experimentally, explaining the higher activity found for the unbridged catalytic complex compared to the bridged. The geometries are in agreement with other studies and comparable with those derived by crystallographic methods. The α-agostic interaction observed in the reactants as well as in the transition states and resulting products confirms that the Brookhart-Green mechanism is followed for the polymerization reaction.
Article
Nonrelativistic and quasirelativistic ab‐initio pseudopotentials representing the Ne‐like X(Z−10)+ cores (X=Sc–Zn) of the first row transition metals and optimized (8s7p6d1f )/[6s5p3d1f ]‐GTO valence basis sets for use in molecular calculations have been generated. Excitation and ionization energies of the low lying states of Sc through Zn from numerical HF‐ as well as SCF‐ and CI(SD)‐pseudopotential calculations using the derived basis sets differ by less than 0.1 eV from corresponding all‐electron results.
Article
α,β-Unsaturated O-pivaloyl oximes are coupled to alkenes by Rh(III) catalysis to afford substituted pyridines. The reaction with activated alkenes is exceptionally regioselective and high-yielding. Mechanistic studies suggest that heterocycle formation proceeds via reversible C-H activation, alkene insertion, and a C-N bond formation/N-O bond cleavage process.
Article
We suggest a new contraction of the basis sets associated with the Hay-Wadt relativistic effective core potentials (RECPs) for the main group and transition metal atoms. These bases are more suitable for density functional theory investigations than the previous ‘double-ζ’ contractions based upon Hartree−Fock atomic results. The original Hay-Wadt primitives are now contracted [5s5p3d], [4s4p3d], and [4s4p3d] for the first, second, and third transition series, respectively, and denoted as LANL2TZ basis sets. For the main group atoms, we advocate using a completely uncontracted basis denoted LANL08. While modestly extending the size of the basis, the resulting sets should be suitable for both DFT and wave function based approaches. The valence bases for the transition metal atoms can be supplemented with the polarization functions determined by Frenking.
Article
Polarization functions are added in two steps to a split-valence extended gaussian basis set: d-type gaussians on the first row atoms C. N, O and F and p-type gaussians on hydrogen. The same d-exponent of 0.8 is found to be satisfactory for these four atoms and the hydrogen p-exponent of 1.1 is adequate in their hydrides. The energy lowering due to d functions is found to depend on the local symmetry around the heavy atom. For the particular basis used, the energy lowerings due to d functions for various environments around the heavy atom are tabulated. These bases are then applied to a set of molecules containing up to two heavy atoms to obtain their LCAO-MO-SCF energies. The mean absolute deviation between theory and experiment (where available) for heats of hydrogenation of closed shell species with two non-hydrogen atoms is 4 kcal/mole for the basis set with full polarization. Estimates of hydrogenation energy errors at the Hartree-Fock limit, based on available calculations, are given.
Article
Two extended basis sets (termed 5–31G and 6–31G) consisting of atomic orbitals expressed as fixed linear combinations of Gaussian functions are presented for the first row atoms carbon to fluorine. These basis functions are similar to the 4–31G set [J. Chem. Phys. 54, 724 (1971)] in that each valence shell is split into inner and outer parts described by three and one Gaussian function, respectively. Inner shells are represented by a single basis function taken as a sum of five (5–31G) or six (6–31G) Gaussians. Studies with a number of polyatomic molecules indicate a substantial lowering of calculated total energies over the 4–31G set. Calculated relative energies and equilibrium geometries do not appear to be altered significantly.
Article
Despite the remarkable thermochemical accuracy of Kohn–Sham density-functional theories with gradient corrections for exchange-correlation [see, for example, A. D. Becke, J. Chem. Phys. 96, 2155 (1992)], we believe that further improvements are unlikely unless exact-exchange information is considered. Arguments to support this view are presented, and a semiempirical exchange-correlation functional containing local-spin-density, gradient, and exact-exchange terms is tested on 56 atomization energies, 42 ionization potentials, 8 proton affinities, and 10 total atomic energies of first- and second-row systems. This functional performs significantly better than previous functionals with gradient corrections only, and fits experimental atomization energies with an impressively small average absolute deviation of 2.4 kcal/mol.
Article
The Rh(III)-catalyzed C-H functionalizations of benzamide derivatives with olefin were studied by DFT calculations to elucidate the divergent pathways controlled by the N-OR internal oxidants. For substrates of N-OMe and N-OPiv internal oxidants, the energy profiles for consecutive N-H deprotonation/C-H activation/olefin insertion sequences were similar, and different properties and reactivities of the generated 7-membered rhodacycles were predicted. When N-OMe is involved, this intermediate is generally unstable, and the olefination occurs easily via a β-H elimination/reductive elimination (RE) sequence to generate the Rh(I) intermediate, which is then oxidized to the active Rh(III) via MeOH elimination from the N-OMe reduction in the presence of a HOAc. However, for a 7-membered rhodacycle containing a N-OPiv moiety, the coordination of the acyloxy carbonyl oxygen stabilizes this intermediate and increases the barrier of the olefination pathway. Instead, the migration of the acyloxy from N to Rh(III) via a 5-membered ring TS to form a cyclic Rh(V) nitrene intermediate is more kinetically favorable, then the facile RE of this Rh(V) species forms the heterocycle product and regenerates Rh(III). Notably, for both reactions, the direct C-N formation from intermediates containing a C(sp(3))-Rh(III)-N(sp(3)) unit would be very difficult with barriers over 40 kcal/mol.
Article
Over the last several decades, researchers have achieved remarkable progress in the field of organometallic chemistry. The development of metal-catalyzed cross-coupling reactions represents a paradigm shift in chemical synthesis, and today synthetic chemists can readily access carbon–carbon and carbon–heteroatom bonds from a vast array of starting compounds. Although we cannot understate the importance of these methods, the required prefunctionalization to carry out these reactions adds cost and reduces the availability of the starting reagents.
Article
A synthetic method for azaheterocycles from aryl ketone O-acetyl oximes and internal alkynes has been developed by using the Cu(OAc)(2)-[Cp*RhCl(2)](2) bimetallic catalytic system. The reactions proceeded with both of anti- and syn-isomers of oximes with a wide scope of substituents. The Cu-Rh bimetallic system could be applied for the synthesis of isoquinolines as well as β-carboline, furo[2.3-c]pyridine, pyrrolo[2,3-c]pyridine, and thieno[2,3-c]pyridine derivatives.
Article
C-H amination of N-aryl benzamides with O-benzoyl hydroxylamines has been achieved with either Pd(II) or Pd(0) catalysts. Furthermore, we demonstrate that secondary amines can be directly used with benzoyl peroxide in a one-pot procedure that proceeds via the in situ generation of the appropriate O-benzoyl hydroxylamines. This catalytic reaction provides a new disconnection for the convergent synthesis of tertiary and secondary arylalkyl amines starting from benzoic acids.
Article
Directing groups that can act as internal oxidants have recently been shown to be beneficial in metal-catalyzed heterocycle syntheses that undergo C-H functionalization. Pursuant to the rhodium(III)-catalyzed redox-neutral isoquinolone synthesis that we recently reported, we present in this article the development of a more reactive internal oxidant/directing group that can promote the formation of a wide variety of isoquinolones at room temperature while employing low catalyst loadings (0.5 mol %). In contrast to previously reported oxidative rhodium(III)-catalyzed heterocycle syntheses, the new conditions allow for the first time the use of terminal alkynes. Also, it is shown that the use of alkenes, including ethylene, instead of alkynes leads to the room temperature formation of 3,4-dihydroisoquinolones. Mechanistic investigations of this new system point to a change in the turnover limiting step of the catalytic cycle relative to the previously reported conditions. Concerted metalation-deprotonation (CMD) is now proposed to be the turnover limiting step. In addition, DFT calculations conducted on this system agree with a stepwise C-N bond reductive elimination/N-O bond oxidative addition mechanism to afford the desired heterocycle. Concepts highlighted by the calculations were found to be consistent with experimental results.