Eva Hevia

University of Strathclyde, Glasgow, Scotland, United Kingdom

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Publications (103)652.34 Total impact

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    ABSTRACT: Advancing the rational design of main-group N-heterocyclic carbene complexes, this study reports the synthesis, X-ray crystallographic and NMR spectroscopic characterisation of a novel series of Ga complexes containing neutral or anionic NHC ligands using the unsaturated carbene IPr (IPr= 1,3-bis-(2,6-diisopropylphenyl)imidazole-2-ylidene). Starting from normal adduct GaR3∙IPr (1) (R = CH2SiMe3), the addition of polar organolithium reagent LiR led to the formation of NHC-stabilised gallate species IPr∙LiGaR4 (2), resulting from co-complexation of the single-metal species. Contrastingly, reversing the order of addition of these organometallic reagents, by treating unsaturated free IPr, first with LiR followed by GaR3, furnished novel heteroleptic gallate (THF)2Li[:C{[N(2,6-iPr2C6H3)]2CHCGa(CH2SiMe3)3}] (3), which contains an anionic NHC ligand acting as an unsymmetrical bridge between the two metals, coordinating through its abnormal C4 position to Ga and through its normal C2 position to Li. Electrophilic interception studies of 3 using methyl triflate (MeOTf), methanol and imidazolium salt (IMes∙HCl) led to the isolation and structural elucidation of the two novel neutral abnormal NHC (aNHC) complexes [CH3C{[N(2,6-iPr2C6H3)]2CHCGa(CH2SiMe3)3}] (4) and aIPr∙GaR3 (5) (aIPr = HC{[N(2,6-iPr2C6H3)]2CHC). These studies disclose the preference of the anionic IPr ligand present in 3 to react with electrophiles via its C2 position, leaving its Ga-C4 bond intact. Compounds 1, 3 and 5 constitute a rare example of a series of normal, anionic and abnormal complexes incorporating the same metal-coligand partnership. Abnormal complex 5 can also be accessed by a thermally induced rearrangement of its normal isomer 1. Combining NMR spectroscopic and kinetic studies with DFT calculations, new light has been shed on this intriguing transformation, which suggests that it occurs via a dissociative mechanism, highlighting the importance of the donor ability of the solvent used in these thermal isomerisations as well as the steric bulk of the substituents on the NHC and the Ga reagent. These findings intimate that relief of the steric hindrance around Ga by forming an abnormal complex is a key driving force behind these rearrangements
    Chemical Science 07/2015; DOI:10.1039/C5SC02086G · 8.60 Impact Factor
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    ABSTRACT: This study explores the synthetic utility of homo(aryl) lithum zincate reagents [LiZnPh3] (2) and [Li2ZnPh4] (3), made by cocomplexation of variable amounts of their monometallic components LiPh and ZnPh2 (1), as chemoselective nucleophilic arylating reagents. Lithium zincates 2 and 3 were both characterized by multinuclear (1H, 13C, and 7Li) NMR spectroscopy, and in the case of 2, a classical reagent in heterobimetallic chemistry, the molecular structure of its OnBu2 solvate [LiZnPh3(OnBu2)2] 2·2OBu2 has been established by X-ray crystallography. Using the synthetically relevant N-heterocyclic molecule acridine (acr, NC13H9), a new zincate-mediated arylating approach is demonstrated which allows the chemoselective arylation of acr at its C9 position, affording 9,10-dihydro-9-phenylacridine (4) in 95% yield using microwave irradiation (125 °C, 20 min). These conditions are in contrast with previous transition-metal-catalyzed methodologies using ZnPh2 as an arylating reagent, which require significantly longer reaction times (130 °C, 20 h). Oxidation of 4 with DDQ furnished 9-phenylacridine (5) in a 71% yield. New insights into the constitution of the intermediate organometallic species involved in these reactions prior to the hydrolysis step have been gained by trapping homometallic [(THF)3Li(NC13H9-Ph)] (6). Interestingly the reaction of acr with 3 equiv of PhLi/TMEDA led to the isolation of a different product, namely the novel paramagnetic [(THF)(TMEDA)Li{NC13H8-Ph}•–] (7), which contains a radical anion of 9-phenylacridine. The structure of the donor–acceptor complex [(acr)ZnPh2] (8) has also been included as a result of the reaction of 1 with acr.
    Organometallics 06/2015; 34(11):2614-2623. DOI:10.1021/om501251q · 4.25 Impact Factor
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    ABSTRACT: Using bis(amide) Zn(HMDS)2 (HMDS= 1,1,1,3,3,3-hexamethyldisilazide) as a precursor, this study explores the synthesis of N-heterocyclic carbene stabilized mixed amido-hydride zinc complexes using two alternative hydride sources, namely dimethylamine borane (DMAB) and phenylsilane PhSiH3. Hydride-rich zinc cluster Zn4(HMDS)2H6•2IPr (1) (IPr= 1,3-bis(2,6-di-isopropylphenylimidazol-2-ylidene), which can be envisaged as a co-complex of IPr•ZnH2 and (HMDS)ZnH, is obtained when DMAB is employed, with the concomitant formation of heteroleptic bis(amido)borane [HB(NMe2)(HMDS)] and H2 evolution. NMR studies in d8-THF show that although the bulky carbene IPr does not bind to the zinc bis(amide), its presence in the reaction media is required in order to stabilise hemiamide (apropos zinc), 1. Reactions using the slightly less sterically demanding NHC IXy (IXy = 1,3-bis-(2,6-dimethylphenyl)-imidazol-2-ylidene) led to the isolation and structural elucidation of the carbene adduct Zn(HMDS)2•IXy (2). Contrastingly, mixtures of equimolar amounts of PhSiH3 and the bis(amide) (60oC, 3 h, hexane) led to the isolation of monomeric heteroleptic hydride (HMDS)ZnH•IPr (3). NMR studies, including DOSY experiments, revealed that while the integrity of 3 is retained in polar d8-THF solutions, in lower polarity C6D6 it displays a much more complex solution behaviour, being in equilibrium with the homoleptic species ZnH2•IPr, Zn(HMDS)2 and IPr
    Dalton Transactions 03/2015; 44(17). DOI:10.1039/C5DT00312A · 4.10 Impact Factor
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    ABSTRACT: Reaction of ferrocene with 1 or 2 molar equiv of the synergistic-operative bimetallic sodium zincate base TMEDA·Na(μ-TMP)(μ-tBu)Zn(tBu) yields mainly mono- or dizincated complexes TMEDA·Na(μ-TMP)[μ-(C5H4)Fe(C5H5)]ZntBu (1) and [TMEDA·Na(μ-TMP)Zn(tBu)]2(C5H4)2Fe (2). Likewise, the separated pairing of Li(TMP) and (TMP)AliBu2 in the presence of THF can mono- or dimetalate ferrocene in a synergistic two-step lithiation/trans-metal-trapping protocol to give THF·Li(μ-TMP)[μ-(C5H4)Fe(C5H5)]Al(iBu)2 (4) or [THF·Li(μ-TMP)Al(iBu)2]2(C5H4)2Fe (5). In the absence of Lewis donating cosolvents, a 4-fold excess of the sodium zincate appears to produce an unprecedented 4-fold zincated ferrocene of formula Na4(TMP)4Zn4(tBu)4[(C5H3)2Fe] (3), whereas when donor solvent is withheld from the lithium/aluminum pairing, only dimetalation of ferrocene is possible. Tetrametalation seems to be inhibited by the in situ generation of TMP(H) via amido basicity, which then acts as a Lewis donor toward lithium, preventing inverse-crown formation and preferentially forming the Lewis acid-Lewis base adduct [TMP(H)·Li(μ-TMP)Al(iBu)2]2(C5H4)2Fe (6). With the exception of 3, all aforementioned complexes have been characterized by X-ray crystallography, while 1-6 have also been studied by solution NMR spectroscopic studies.
    Organometallics 02/2015; 34(11):150204142258000. DOI:10.1021/om5012352 · 4.25 Impact Factor
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    ABSTRACT: Using structurally defined potassium- tetra(alkyl)magnesiates, a new straightforward methodology to promote regioselective Mg-H exchange reactions of a wide range of aromatic and heteroaromatic substrates is disclosed. Contacted ion pair intermediates are likely to be involved, with K being the key to facilitate the magnesiation processes.
    Chemical Communications 09/2014; 50(85). DOI:10.1039/C4CC05305B · 6.72 Impact Factor
  • Eva Hevia
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    ABSTRACT: A graphical abstract is available for this content
    Dalton Transactions 09/2014; 43(38). DOI:10.1039/c4dt90136c · 4.10 Impact Factor
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    ABSTRACT: Expanding the synthetic potential of Mg–Zn hybrid organyl reagents (generated via transmetallation reactions), this study uncovers a versatile approach, involving a sequence of direct Zn–I exchange and Pd catalysed cross-coupling reactions which grants access to a wide range of asymmetric bis(aryls). By combining X-ray crystallography with ESI-MS and non-deuterium NMR spectroscopic studies, new light is shed on the heterobimetallic constitution of the intriguing organometallic species [(THF)4MgCl2Zn(tBu)Cl] (1) and [{Mg2Cl3(THF)6}+{ZntBu3}−] (2), formed through transmetallation of tBuMgCl with n equivalent amounts of ZnCl2 (n = 1 and 3 respectively). Operating by cooperative effects, alkyl-rich hybrid 2 can effectively promote direct Zn–I exchange reactions with aromatic halides in short periods of time at room temperature in THF solution. The structural elucidation of key organometallic intermediates involved in some of these Zn–I exchanges, provides new reactivity insights into how these bimetallic systems operate. Thus, while the reaction of 2 with 3 equivalents of 2-iodoanisole (3b) gives magnesium dizincate [{Mg(THF)6}2+{Zn(o-C6H4-OMe)3}2−] (4) which demonstrates the 3-fold activation of the tBu groups attached to Zn in 2, using 2-iodobenzonitrile (3i), only two tBu groups react with the substrate, affording [(THF)4MgCl(NC-o-C6H4)ZnI(o-C6H4-CN)(THF)] (7). In 7 Mg and Zn are connected by an aryl bridge, suggesting that the formation of contacted ion-pair hybrids may have a deactivating effect on the outcome of the Zn–I exchange process. A wide range of homoleptic tris(aryl) zincate intermediates have been prepared in situ and used as precursors in Pd catalysed cross-coupling reactions, affording bis(aryls) 6a–s in excellent yields under mild reaction conditions without the need of any additive or polar cosolvent such as NMP or DMI.
    Chemical Science 06/2014; 5(9). DOI:10.1039/C4SC01263A · 8.60 Impact Factor
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    ABSTRACT: Using an interlocking co-complexation approach, a homologous series of unsolvated alkali-metal zincates [MZn(CH2SiMe3)3] (M = Li , Na , K ) was prepared by reacting equimolar amounts of Zn(CH2SiMe3)2 with the relevant alkali-metal alkyl M(CH2SiMe3) employing non-coordinating hexane as a solvent. X-ray crystallographic studies reveal that these heterobimetallic compounds exhibit unprecedented supramolecular assemblies made up exclusively of a three-fold combination of M-CH2, Zn-CH2 and MMe interactions. Revealing an important alkali-metal effect, displays a linear chain structure; whereas and form much more intricate 3D and 2D coordination networks respectively. Shedding new light into the formation of these solvent-free zincates, DFT calculations indicate that the infinite degree of aggregation observed in plays a major role in thermodynamically driving the co-complexation reactions of their homometallic precursors. NMR spectroscopic studies suggest that in C6D6 solution exist as discrete contacted ion-pair species, where the alkali-metal is partially solvated by molecules of deuterated solvent. The supramolecular assemblies of can be easily deaggregated by adding the polydentate N-donors PMDETA (N,N,N',N'',N''-pentamethyldiethylenetriamine) or TMEDA (N,N,N',N'-tetramethylethylenediamine), affording monomeric [(PMDETA)LiZn(CH2SiMe3)3] () and [(TMEDA)2NaZn(CH2SiMe3)3] ().
    Dalton Transactions 06/2014; 43(38). DOI:10.1039/c4dt01131g · 4.10 Impact Factor
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    ABSTRACT: Despite their enormous synthetic relevance, the use of polar organolithium and Grignard reagents is greatly limited by their requirements of low temperatures in order to control their reactivity as well as the need of dry organic solvents and inert atmosphere protocols to avoid their fast decomposition. Breaking new ground on the applications of these commodity organometallics in synthesis under more environmentally friendly conditions, this work introduces deep eutetic solvents (DESs) as a green alternative media to carry out chemoselective additions of ketones in air at room temperature. Comparing their reactivities in DES with those observed in pure water suggest that a kinetic activation of the alkylating reagents is taking place, favoring nucleophilic addition over the competitive hydrolysis, which can be rationalized through formation of halide-rich magnesiate or lithiate species.
    Angewandte Chemie International Edition 06/2014; 53(23). DOI:10.1002/anie.201400889 · 11.34 Impact Factor
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    ABSTRACT: Despite their enormous synthetic relevance, the use of polar organolithium and Grignard reagents is greatly limited by their requirements of low temperatures in order to control their reactivity as well as the need of dry organic solvents and inert atmosphere protocols to avoid their fast decomposition. Breaking new ground on the applications of these commodity organometallics in synthesis under more environmentally friendly conditions, this work introduces deep eutetic solvents (DESs) as a green alternative media to carry out chemoselective additions of ketones in air at room temperature. Comparing their reactivities in DES with those observed in pure water suggest that a kinetic activation of the alkylating reagents is taking place, favoring nucleophilic addition over the competitive hydrolysis, which can be rationalized through formation of halide-rich magnesiate or lithiate species.
    Angewandte Chemie 06/2014; 126(23). DOI:10.1002/ange.201400889
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    ABSTRACT: The pivalates RZnOPiv⋅Mg(OPiv)X⋅n LiCl (OPiv=pivalate; R=aryl; X=Cl, Br, I) stand out amongst salt-supported organometallic reagents, because apart from their effectiveness in Negishi cross-coupling reactions, they show more resistance to attack by moist air than conventional organometallic compounds. Herein a combination of synthesis, coupling applications, X-ray crystallographic studies, NMR (including DOSY) studies, and ESI mass spectrometric studies provide details of these pivalate reagents in their own right. A p-tolyl case system shows that in [D8 ]THF solution these reagents exist as separated Me(p-C6 H4 )ZnCl and Mg(OPiv)2 species. Air exposure tests and X-ray crystallographic studies indicate that Mg(OPiv)2 enhances the air stability of aryl zinc species by sequestering H2 O contaminants. Coupling reactions of Me(p-C6 H4 )ZnX (where X=different salts) with 4-bromoanisole highlight the importance of the presence of Mg(OPiv)2 . Insight into the role of LiCl in these multicomponent mixtures is provided by the molecular structure of [(THF)2 Li2 (Cl)2 (OPiv)2 Zn].
    Angewandte Chemie International Edition 03/2014; 126(10). DOI:10.1002/anie.201309841 · 11.34 Impact Factor
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    ABSTRACT: The lithium TMP-aluminate bases “LiTMP·Al(iBu)3” 1 and “LiTMP·Al(TMP)(iBu)2” 2, where TMP is 2,2,6,6-tetramethylpiperidide, have recently come under the spotlight as “aluminating” reagents in that they can perform aluminium-hydrogen exchange on a wide variety of aromatic substrates. Previous studies have intimated that 1 existed as a single species in THF solution formulated as [(THF)·Li(μ-TMP)(μ-iBu)Al(iBu)2] 1·THF, having a contacted ion pair structure as evidenced by an X-ray crystallographic study of isolated crystals. But here using anisole as a case substrate it is revealed that pre-crystallised 1·THF cannot deprotonate anisole at all whether in hexane or THF solution contradicting earlier in situ applications of 1 which revealed near quantitative metallation of anisole. NMR spectroscopic studies of 1 made in situ in THF solution ascribe this reactivity distinction from 1·THF to complex equilibria involving five major species in LiTMP·THF, Al(iBu)3·THF, [{Li(THF)4}+{Al(TMP)(iBu)3}−] 1·(THF)4, [(THF)·Li(μ-TMP)(μ-OC4H7)Al(iBu)2], 4, and (TMP)Al(iBu)2·THF. Reagent 2 in contrast is found to exist as only two separated homometallic species in LiTMP·THF and (TMP)Al(iBu)2·THF in THF solution. The constitutions of 1 and 2 in non-polar hexane solution are also revealed. With the aid of DFT calculations, discussion focuses on the fact that none of the aluminate species present in THF solutions of 1 or 2 can deprotonate/metallate anisole, instead the metallation processes appear to be LiTMP lithiations followed immediately by trapping by an alkylaluminium complex, in a metal exchange which drives the reaction to the product (arylaluminated) side.
    Chemical Science 01/2014; 5(8):3031. DOI:10.1039/c4sc01108b · 8.60 Impact Factor
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    ABSTRACT: Exploring the reactivity of sodium butylmagnesiate reagent [{Na(THF)6}(+){(Ph2Si(NAr*)2)Mg(Bu)(THF)}(-)] () supported by the bulky chelating silyl(bisamido) ligand {Ph2Si(NAr*)2}(2-) (Ar* = 2,6-iPr2-C6H3) towards N-methylbenzimidazole (bIm(Me)), pyrrole and 2,6-diisopropylaniline (NH2Ar*), this study provides new insights into the ability of this bimetallic base to facilitate direct Mg-H exchange reactions as well as to exhibit polybasicity. Thus effectively promotes the deprotomagnesiation of bIm(Me) under mild reaction conditions to give the α-metallated intermediate [{Na(THF)5}2(+){(Ph2Si(NAr*)2)Mg(bIm(Me)*)}2(-)] () (bIm(Me)* = 2-N-methylbenzimidazolyl). Analysis of crystallographic and NMR data of combined with DFT calculations show that the metallated C in the bIm(Me)* ligands possesses a significant carbenic character. Contrasting with previous studies of benzothiazole (btz), does not react with an excess of bIm(Me) even under forcing refluxing conditions. Contrastingly, the amination reactions of equimolar amounts of with pyrrole and 2,6-diisopropylaniline allowed the isolation of [{(Ph2Si(NAr*)(NHAr*))Mg(NC4H4)2(THF)Na(THF)2}] () and [{Na(THF)6}(+){(Ph2Si(NAr*)(NHAr*))Mg(NHAr*)2(THF)}(-)] () respectively as crystalline solids. Highlighting the ability of to act as a polybasic reagent, and are formed as the result of the deprotonation of two molecules of the relevant amine via its butyl group and one amido arm of the silyl(bisamido) ligand. Similarly, the reactions of with 3 molar equivalents of the relevant amine yielded homoleptic tris(amido) compounds [(THF)2NaMg(NC4H4)3] () and [{Na(THF)6}(+){Mg(NHAr*)3}(-)] (), with the concomitant formation of bis(amine) Ph2Si(NHAr)2, as a result of the complete amination of using its three basic sites. The structures in the solid state of and were elucidated by X-ray crystallography. Despite their similar constitution, these heteroleptic tris(amido)magnesiates adopt contrasting structures, with the former displaying a contacted ion-pair structure, where Na and Mg are connected by two bridging pyrrolyl anions, whereas the latter gives rise to a solvent-separated ion pair motif. To the best of our knowledge represents the first crystallographically characterized magnesium compound containing an anionic non-substituted form of pyrrole. Noticeably, Mg interacts exclusively with the N atoms of the pyrrolyl ligands, forming strong σ-bonds, whereas Na prefers to engage with the π-systems of both NC4-rings.
    Dalton Transactions 11/2013; 43(11). DOI:10.1039/c3dt52639a · 4.10 Impact Factor
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    ABSTRACT: Lithium 2,2,6,6-tetramethylpiperidide (LiTMP), one of the most important polar organometallic reagents both in its own right and as a key component of ate compositions, has long been known for its classic cyclotetrameric (LiTMP)4 solid-state structure. Made by a new approach through transmetalation of Zn(TMP)2 with tBuLi in n-hexane solution, a crystalline polymorph of LiTMP has been uncovered. X-ray crystallographic studies at 123(2) K revealed this polymorph crystallises in the hexagonal space group P63 /m and exhibited a discrete cyclotrimeric (C3h ) structure with a strictly planar (LiN)3 ring containing three symmetrically equivalent TMP chair-shaped ligands. The molecular structure of (LiTMP)4 was redetermined at 123(2) K, because its original crystallographic characterisation was done at ambient temperature. This improved redetermination confirmed a monoclinic C2/c space group with the planar (LiN)4 ring possessing pseudo (non-crystallographic) C4h symmetry. Investigation of both metalation and transmetalation routes to LiTMP under different conditions established that polymorph formation did not depend on the route employed but rather the temperature of crystallisation. Low-temperature (freezer at -35 °C) cooling of the reaction solution favoured (LiTMP)3 ; whereas high-temperature (bench) storage favoured (LiTMP)4 . Routine (1) H and (13) C NMR spectroscopic studies in a variety of solvents showed that (LiTMP)3 and (LiTMP)4 exist in equilibrium, whereas (1) H DOSY NMR studies gave diffusion coefficient results consistent with their relative sizes.
    Chemistry - A European Journal 10/2013; 19(42). DOI:10.1002/chem.201302709 · 5.70 Impact Factor
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    ABSTRACT: Aiming to shed new light on the stability and constitution of the organometallic intermediates involved in direct ortho-metalation processes, using trifluoromethylbenzene (1) as a case study, this paper investigates the deprotonation of 1 using group 1 alkyl bases tBuLi and nBuNa in the presence of the Lewis donors TMEDA (N,N,N′,N′-tetramethylethylenediamine), THF, and PMDETA (N,N,N′,N″,N″-pentamethyldiethylenetriamine). A systematic and comprehensive study combining structural, spectroscopic, and theoretical studies reveals that these donors strongly influence the final outcome of the reactions, not only by activating the alkali-metal bases and facilitating deprotonation of 1 but also by tuning the regioselectivity of the reaction. Thus, while using tBuLi/TMEDA, ortho-metalation of 1 is preferred, switching to THF gives a complex mixture of products with the meta-regioisomer being the major species crystallizing from hexane solution. This donor effect is significantly reduced when nBuNa is employed, as ortho-regioselectivity is observed almost exclusively using THF, TMEDA, or PMDETA. DFT calculations computing the relative energies of the ortho-, meta-, and para-regioisomers obtained from these metalating systems have also been carried out. Reinforcing the experimental findings, these theoretical studies show that although in all cases the product of ortho-metalation is the most thermodynamically preferred, the energy difference between the three possible modeled regioisomers is much larger for the Na systems than for the Li ones. The structures of key reaction intermediates [(TMEDA)·Li(C6H4-CF3)]2 (2), [(TMEDA)·Na(C6H4-CF3)]2 (3), and [(PMDETA)·Na(C6H4-CF3)]2 (4) have been elucidated by X-ray crystallographic studies. All compounds exhibit a similar dimeric arrangement with a four-atom core constituting a {MCMC} ring. Interestingly for Na derivatives 3 and 4 unusual Na···F dative interactions are found, which appear to contribute to the overall stability of these compounds, therefore favoring ortho-metalation of 1, as the meta or para structures do not contain these additional interactions.
    Organometallics 09/2013; 32(19). DOI:10.1021/om4007664 · 4.25 Impact Factor
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    ABSTRACT: Mixed-metal reagents [NaMg(CH2SiMe3)3] () and [(THF)NaMg(NPh2)3(THF)] [] can act as precatalysts to selectively promote the hydroamination/trimerization of isocyanates (RNCO) depending on the steric bulk of the R substituent.
    Chemical Communications 08/2013; 49(77). DOI:10.1039/c3cc45167d · 6.72 Impact Factor
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    ABSTRACT: Merging two evolving areas in synthesis, namely cooperative bimetallics and N-heterocyclic carbenes (NHCs), this study reports the isolation of the first intermediates of alkali-metal-mediated zincation (AMMZn) of a free NHC and a Zn–NHC complex using sodium zincate [(TMEDA)NaZn(TMP)(tBu) 2 ] (1) as a metallating reagent. The structural authentication of (THF) 3 Na[:C{[N(2,6-iPr 2 C 6 H 3)] 2 CHCZn(tBu 2)}] (2) and [Na(THF) 6 ] + [tBu 2 Zn:C{[N(2,6-iPr 2 C 6 H 3)] 2 CHCZn(tBu 2)}] À (4), resulting from the reactions of 1 with unsaturated free NHC IPr (IPr ¼ 1,3-bis(2,6-di-isopropylphenylimidazole-2-ylidene) and NHC complex ZntBu 2 IPr (3) respectively demonstrates that in both cases, this mixed-metal approach can easily facilitate the selective C4 zincation of the unsaturated backbone of the NHC ligand. Furthermore, the generation of anionic NHC fragments enables dual coordination through their normal (C2) and abnormal (C4) positions to the bimetallic system, stabilising the kinetic AMMZn intermediates which normally go undetected and provides new mechanistic insights in to how these mixed-metal reagents operate. In stark contrast to this bimetallic approach when NHC-complex 3 is reacted with a more conventional single-metal base such as tBuLi, the deprotonation of the coordinated carbene is inhibited, favouring instead, co-complexation to give NHC-stabilised [IPr$LiZntBu 3 ] (5). Showing the potential of 2 to act as a transfer agent of its anionic NHC unit to transition metal complexes, this intermediate reacts with two molar equivalents of [ClAu(PPh 3)] to afford the novel digold species [ClAu:C{[N(2,6-iPr 2 C 6 H 3)] 2 CHCAu(PPh 3)}] (6) resulting from an unprecedented double transmetallation reaction which involves the simultaneous exchange of both cationic (Na +) and neutral (ZntBu 2) entities on the NHC framework.
    Chemical Science 08/2013; 4(11):4259. DOI:10.1039/c3sc52101j · 8.60 Impact Factor
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    ABSTRACT: Opening-up new synthetic applications of β-diketiminate stabilised magnesium complexes, this case study compares the ability of the alkyl [{Ar*NC(Me)CHC(Me)NAr*}Mg(Bu)(THF)] (1) and the amido reagent [{Ar*NC(Me)CHC(Me)NAr*}Mg(TMP)] (2) (Ar* = 2,6-i-Pr2-C6H3) to promote direct Mg–H exchange towards the series of 1,3-benzoazoles: benzoxazole (boz), benzothiazole (btz) and N-methyl benzimidazole (bImMe). Both reagents deprotonate boz at room temperature to yield [{Ar*NC(Me)CHC(Me)NAr*}Mg{O(o-C6H4)NC}(THF)] (3) via the C–O bond cleavage of a putative C2-magnesiated-benzoxazolyl intermediate. Structurally tracking the reactivity of 1 and 2 towards less acidic btz and bImMe showed that the behaviour of reagents 1 and 2 diverged dramatically. Kinetically activated TMP-reagent 2 effectively promotes the deprotonative magnesiation of btz and bImMe under mild reaction conditions, giving the alpha-metallated intermediates [{Ar*NC(Me)CHC(Me)NAr*}2Mg2{btz*}2] (4) and [{Ar*NC(Me)CHC(Me)NAr*}2Mg2{bImMe*}2] (7) (btz* = 2-benzothiazolyl; bImMe* = 2-N-methylbenzimidazolyl). Analysis of crystallographic and NMR data revealed that in 4 and 7 the metallated carbon atoms display a markedly carbenic character and that in solution these species exist at room temperature solely as the ring-closed products, without any observable equilibration to the acyclic isomers. Contrastingly, alkyl reagent 1 decreases the magnesiation rate of btz facilitating an intriguing new cascade activation process of two molecules of substrate involving a sequence of deprotonation/coordination/C–C coupling and ring-opening reactions to yield [{Ar*NC(Me)CHC(Me)NAr*}Mg{(btz*)C(H)N(o-C6H4)S}] (5). Hydrolysis of 5 followed by addition of the radical oxidant TEMPO ultimately produces the homocoupled product bis(benzothiazole) 6 in a 72% isolated yield. Thus, this establishes a novel transition-metal-free method to prepare homocoupled thiazoles. More straightforwardly, the coordination product [{Ar*NC(Me)CHC(Me)NAr*}Mg(Bu)(bImMe)] (8) was obtained when equimolar amounts of bImMe and 1 were reacted, illustrating the kinetic stubbornness of the Mg–C bond in butyl derivative 1. Complex 8 can be envisaged as a valuable guide to the constitution of a premetallation complex (in relation to the complex-induced proximity effect, CIPE).
    Chemical Science 03/2013; 4(4):1895-1905. DOI:10.1039/C3SC22326D · 8.60 Impact Factor
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    ABSTRACT: Exploring the co-complexation reactions between the gallium alkyl Ga(CH2SiMe3)3 and alkali-metal alkyl MCH2SiMe3 (M = Li, Na, or K) using an arene/hexane solvent mixture has allowed the isolation of solvent-free alkali-metal tetraorganogallates [{MGa(CH2SiMe3)4}∞] (M = Li, 1; Na, 2) and related benzene adduct [{(C6H6)2KGa(CH2SiMe3)4}∞] (3). By combining X-ray crystallography, NMR spectroscopy, and DFT calculations, this study sheds new light on the constitution of these mixed-metal species. X-ray crystallographic studies reveal that all gallates exhibit novel polymeric arrangements, with 1 and 2 sharing the same linear chain structure, made up exclusively of M–C and Ga–C bonds, whereas 3 displays a significantly more open structural motif, where the K and Ga atoms are connected by a single alkyl bridge and propagation occurs via weaker K···Me electrostatic interactions of a methyl from a SiMe3 group of an alkyl ligand from one monomer to the potassium from a neighboring monomeric unit. Multinuclear NMR spectroscopic studies suggest that in deuterated benzene solutions 1–3 exist as discrete solvent-separated ion-pair species where the alkali-metal is solvated by the arene solvent. DFT calculations show that while the infinite aggregation of these polymeric structures is key for thermodynamically favoring the formation of 1 and 2, in the case of 3 the solvation of unsaturated potassium by two molecules of benzene, via π-electrostatic interactions, appears to be the major contributor to its overall stability.
    Organometallics 01/2013; 32(2):480–489. DOI:10.1021/om3009469 · 4.25 Impact Factor
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    ABSTRACT: This paper presents the synthesis and characterization of new homoleptic lithium magnesiate reagents incorporating the silyl-substituted alkyl ligand CH2SiMe3 in the presence of a variety of Lewis base donors, namely tetrahydrofuran (THF), 1,4-dioxane, N,N,N',N'-tetramethylethylenediamine (TMEDA), and N,N,N',N '',N ''-pentamethyldiethylenetriamine (PMDETA). The constitution of these bimetallic compounds has been assessed in both the solid state and solution using a combination of X-ray crystallographic studies and multinuclear NMR spectroscopy, including H-1 diffusion-ordered (H-1-DOSY) NMR experiments. These studies highlight the major role played by the donor molecule in controlling the structure of the complexes as well as the wide structural diversity available for these mixed-metal species ranging from discrete molecules, as found for [(PMDETA)LiMg(CH2SiMe3)(3)] (6), to more complex supramolecular arrangements, as in the 1D-polymeric chain [{(THF)LiMg(CH2SiMe3)(3)}(infinity)] (2) or in the stoichiometrically distinct dioxane solvates [{(dioxane)(2)LiMgR3}(infinity)] (3) and [{(dioxane)Li2Mg2R6}(infinity)] (4). Furthermore, these studies have also revealed that in some cases the donor molecule can promote a redistribution process, as shown for the reaction of triorganomagnesiate [LiMg(CH2SiMe3)(3)] (1) with 1 molar equiv of TMEDA, which led to the formation of lithium-rich tetraorganomagnesiate [(TMEDA)Li2Mg(CH2SiMe3)(4)] (5) along with Mg(CH2SiMe3)(2). The formation of the unprecedented cationic lithium magnesiate [{(PMDETA)(2)Li2Mg(CH2SiMe3)(3)}(+){Mg-3(CH2SiMe3)(6)(OCH2SiMe3)}(-)] (7) is also described, by the controlled exposure to oxygen of the monomeric compound [(PMDETA)LiMg(CH2SiMe3)(3)] (6).
    Organometallics 07/2012; 31(14):5131-5142. DOI:10.1021/om300477m · 4.25 Impact Factor