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Organometallic Chemistry: Volume 26

Organometallic Chemistry: Volume 26

by M Green (Editor)
Organometallic chemistry is an interdisciplinary science which continues to grow at a rapid pace. Although there is continued interest in synthetic and structural studies the last decade has seen a growing interest in the potential of organometallic chemistry to provide answers to problems in catalysis synthetic organic chemistry and also in the development of new


Organometallic chemistry is an interdisciplinary science which continues to grow at a rapid pace. Although there is continued interest in synthetic and structural studies the last decade has seen a growing interest in the potential of organometallic chemistry to provide answers to problems in catalysis synthetic organic chemistry and also in the development of new materials. This Specialist Periodical Report aims to reflect these current interests reviewing progress in theoretical organometallic chemistry, main group chemistry, the lanthanides and all aspects of transition metal chemistry. Specialist Periodical Reports provide systematic and detailed review coverage of progress in the major areas of chemical research. Written by experts in their specialist fields the series creates a unique service for the active research chemist, supplying regular critical in-depth accounts of progress in particular areas of chemistry. For over 80 years the Royal Society of Chemistry and its predecessor, the Chemical Society, have been publishing reports charting developments in chemistry, which originally took the form of Annual Reports. However, by 1967 the whole spectrum of chemistry could no longer be contained within one volume and the series Specialist Periodical Reports was born. The Annual Reports themselves still existed but were divided into two, and subsequently three, volumes covering Inorganic, Organic and Physical Chemistry. For more general coverage of the highlights in chemistry they remain a 'must'. Since that time the SPR series has altered according to the fluctuating degree of activity in various fields of chemistry. Some titles have remained unchanged, while others have altered their emphasis along with their titles; some have been combined under a new name whereas others have had to be discontinued. The current list of Specialist Periodical Reports can be seen on the inside flap of this volume.

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Organometallic Chemistry Volume 26

A Review of the Literature Published during 1996

By M. Green

The Royal Society of Chemistry

Copyright © 1998 The Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84755-415-4


Group 1: The Alkali and Coinage Metals


1 Alkali Metals

1.1 Introduction: Organisation and Major Advances – In line with previous years, this part of the report is organised in sections which are defined primarily by the type of organic anion (R-) found with the alkali metal cation (M+) in a given species. Within each section, the order of treatment is (i) synthetic uses of the alkali metal derivatives, chiefly in organic syntheses but also in the syntheses of compounds of other metals; included also are mechanistic aspects of such syntheses, probed chiefly by kinetic measurements and by molecular orbital (MO) calculations, (ii) structural studies, by X-ray crystallography in the solid state, by NMR (and other) spectroscopy in solution, and by MO optimisations.

Organo-alkali metal chemistry continued to advance rapidly in 1996, as it had done in the ten or so years previous. Lithium derivatives remain to the fore, although there are signs of increased interest in sodium and potassium species, and indeed in caesium ones: the year saw in essence a doubling of the number of known organocaesium structures. However, rubidium chemistry – in all its aspects – remains largely unexplored. The expansion in organo-Na+, K+, Cs+ chemistry can be attributed firstly to improved synthetic and manipulative skills (since these species are an order of magnitude more air- and moisture-sensitive than their Li+ analogues) and secondly to improved crystal mounting techniques and X-ray data collection/refinement (since these larger cations tend to cause aggregation to polymeric materials whose detailed structures frequently contain interactions between M+ cations and C atoms or groups within the anion R- as well as the formal bonds between M+ and the actual carbanionic centres of R-).

The above general points apart, 1996 saw notable activity and developments in the following areas of organo-alkali metal chemistry: (i) investigation of the active species present in mixed-metal/mixed-anion reagents ('superbases', e.g., of type RLi/R'OK); (ii) mechanistic and other studies on organic syntheses which employ R-M+ reagents, and with these an increased recognition that M+ plays a crucial role, i.e., that the reagent is not merely a source of the carbanion/ nucleophile R-; (iii) increased application of ever more sophisticated (and quicker) ab initio MO calculations to probe mechanistic and structural problems; and (iv) X-ray diffraction studies on species whose structures exhibit M+···CH3 or M+····Ph interactions, and on π-bonded 'sandwich-type' derivatives where R- is such as Cp- or a heteroatom-substituted Cp- (e.g., with B or P).

1.2 Alkyl Derivatives – Alkyllithium species, whether commercially available ones (e.g., MeLi, BuLi) or ones generated from them (or by other means, e.g., LDA, Li metal), remain probably the most widely used organometallic reagents/ intermediates in modern synthetic organic chemistry. The use of Li metal and a catalytic amount of 4,4'-di-tert-butylbiphenyl has been extended to produce HOCH2- and MeNHCH2- synthons, whose treatment with electrophiles affords functionalised carbamates, and to reductively open various N-containing heterocycles to give C,N-Li2 intermediates whose work-up leads to functionalised amines. The surprisingly stable halocarbenoid (3-pyridinylchloromethyl)lithium can be trapped with various electrophiles at low temperature to give functionalised pyridines. Bis(lithiomethyl)silanes, of general type R1R2Si(CH2Li)2, have proved to be useful building blocks towards organosilanes and disilacyclobutanes. Sulphur-containing alkyllithiums have been the subjects of three interesting studies: thiophilic addition of MeLi to sulphines RC(SMe)=S=0 and then treatment with electrophiles (E+X-) gives specifically dithioacetal oxides RC(SMe)(E)S(Me)=O which can then be converted into aldehydes or ketones; the reagent bis(lithiomethyl)sulphide, LiCH2SCH2Li, has been obtained by Li-Sn or Li-Ge exchange and then used to prepare both inorganic and organic compounds containing the -CH2-S-CH2- unit; and the adduct Me3NSO3 reacts with alkyllithiums by inserting SO3 into the Li-C bond, affording RSO3Li and thereby sulphonic acids. Another insertion reaction, this time of CO into lithiated trimethylsilyldiazomethane Me3SiC(Li)N2, forms the basis of new ketenylation reactions. Two especially interesting uses of lithium reagents in inorganic chemistry have been published. Firstly, reaction of YCl3 with Cp*K and [PhC(NSiMe3)2]Li OEt2 affords {Cp*[PhC(NSiMe3)2]Y(μ-Cl)}2 whose treatment with MeLi/TMEDA results in Cp*[PhC(NSiMe3)2]Y(μ-Me)2Li TMEDA, a useful precursor for other Cp*-Y-benzamidinate derivatives. Secondly, the highly unusual organolithium polymer [(Me2NMe2Si)3CLi]∞, whose structure reveals a 'free' carbanionic centre with no C-Li attachments, has been used to transfer the bulky anion to other metals such as Sn and Hg.

Regio-, diastereo- and enantio-selective lithiation/substitution syntheses continue to be of great interest, in particular regarding the mechanistic reasons for such selectivities. The area has been reviewed. Organocuprate adducts R2CuLi BF3 and alkyllithiums RLi give anti and syn 1,2-aminoalcohols respectively when added to an α-siloxyaldimine, the stereoselectivity for RLi being explicable by a 'chelation model' (often referred to as the 'complex-induced proximity effect', CIPE) in which the reagent is first complexed by the O centre of the organic precursor. Stereo-defined organofluorine compounds have been obtained by treatment of RCH(OR')CFBr2 precursors with "BuLi to give the carbenoids RCH(OR')CFBrLi which then add organic carbonyls diastereoselectivity, an outcome again explained in terms of chelation between Li+ and the alkoxy O atoms. Support for CIP effects has been found in the crystal structure of a tetrameric lithium carbamate complex gained by lithiation of 2,3-dimethylindole followed by CO2 insertion into the N-Li bond; certain O atoms are near to the 2-Me groups, suggesting a reason for second lithiation occurring specifically at this position. Asymmetric lithiation-substitution of an N-methylamide by sBuLi in the presence of (-)-sparteine has been shown to proceed via a pathway in which asymmetry is induced in a post-deprotonation step.

Lithiation mechanisms have also been probed by kinetic measurements and by MO calculations. α-Deuterium kinetic isotope effects in reactions of (MeLi)n in Et2O may imply a pre-equilibrium between an aggregate and a reactive monomer. Measurements of rates and extents of directed ortho metallations of anisoles using nBuLi in THF/hexane mixtures have shown, rather surprisingly, that the reactions work best in a 14% solution of THF in the hydrocarbon. The different diastereoselective behaviours of dilithio compounds obtained from cinnamyl alcohol and cinnamylamines on carbolithiation have been examined by 1H, 6Li NMR spectroscopy and by PM3 semi-empirical calculations, the results showing that the species assume different aggregation states which in turn allow two different modes of intramolecular chelation. Finally, the structures and reactivities of 'superbase' mixtures (RLi + R'OM) have been studied by ab initio MO calculations. For MeLi:MeOK, a 1:1 complex shows a reactivity towards MeH similar to that of MeLi alone; however a 1:3 tetramer is more promising since in its optimised structure Me- is separated from Li+ by three K+ and three O- centres. Mixed aggregates of RLi and R'OM(M = Na-Cs) have also been evaluated by MP2 calculations, dimers usually being more stable than other combinations.

Several interesting solid-state structures of alkyllithium compounds appeared in 1996. The lithiated phosphane imine [Me3SiN(PMe2)CH2Li]4 forms a Li4C4 cubane whose Li+ ions are each coordinated by one N centre, while the related dimer [Me3SiN(PiPr2)CMe2Li]2 has a ladder-type structure. The dilithiated di(trimethylsilylmethyl)-bipyridine complex [C5H3NCH(SiMe3)]2·[Li(TMEDA)]2 is monomeric, with each Li+ being chelated by the C atom of one pyridine unit and the N atom of the other. An unusual cyclic lithiate anion [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], isolated as its Li(TMEDA)2+ salt, contains a slightly bent C-Li-C linkage. Of particular interest have been solid-state structures of alkali metal species which, while not being alkyl derivatives in the formal sense, show M+····alkyl interactions. The mixed adduct Me3Al·(PhCH2)2NLi·HN(CH2Ph)2 contains a central Al(Me)LiN ring whose Li····CH3 attachment stabilises the monomeric lithium amide unit. In the alkoxide [(c-CH2CH2CH)2C(Me)OLi]6, Li+ cations coordinate to cyclopropane ring edges and cause elongation of the C-C bonds, an electrostatic effect probed by high-level calculations for M+(Li+-Cs+) in general. The dilithium phosphandiide [RPLi2(F-R)]2 {R = (2,4,6-iPr3C6H2)SiiPr2} has a P2Li4F2 ladder framework whose Li+ centres are stabilised electronically by Li···CH3 or by η2-Li···aryl interactions. Perhaps expectedly, such interactions become yet more important within Na+ and K+ derivatives. The bis(amido)sodate salt Na+{Na[N(Si-Me3)2AlMe3]2}- consists of an infinite array of anions and Na+ cations, the latter being 'complexed' by Na+···CH3 interactions alone. In a potassium aluminate, K2(Me3AlOBut)2·PMDETA units are interconnected by K···CH3(Al) interactions to give a polymer.

Solution structural studies have included an NMR investigation (7Li, 31P) of the dynamics of solvent/complexant (HMPA, Me2O) exchange within chiral alkyllithium reagents. Finally, and regarding MO structural optimisations, the results of calculations on hyperlithiated molecules (e.g., CLi6, Li2CN) have been reviewed. An important paper has reported the results of high-powered calculations on (MeLi)n, n = 1, 2, 4, the key finding being that covalent components in polar C-Li bonds play a significant role.

1.3 Alkenyl, Allyl, Vinyl, Alkynyl and Related Derivatives – Lithium species containing acyclic unsaturated anions continue to be important within organic synthetic methodologies. Chiral lithio enol ethers of type R*OCH=CHLi have been made and reacted with haloalkanes to give specifically new chiral Z-enol ethers. The vinyllithium (EtO)2C=CHLi can be synthesised and stored in THF at -25 °C, and has been used as a masked ethyl acetate synthon. α, β-unsaturated carbonyl compounds have been obtained by reacting carboxylic acid derivatives with Me3SiC(Li)=CH2. Dilithiated vinyl species of type RN(Li)CH2CLi=CH2, with R being an aliphatic group, undergo on heating either a dimerisation or a regio- and stereo-selective cyclodimerisation, the product obtained depending upon the reaction time. A solvent dependency has been observed for reactions of N,C-dilithiated 2-allylpyrrole with electrophiles, Z-isomers being obtained in THF but E-ones in Et2O. 3,4-dilithio-1,2-butadienes, R2C=C=C(Li)C(Li)R2, can be generated by treating substituted butatrienes with Li metal and then derivatised to give substituted 1,2- or 1,3-butadienes, or 2-butynes. The (phosphaalkenyl)lithium carbenoid (Z)-Mes*P=CClLi can be transmetallated with MHal2 (M = Mg, Zn, Hg) to give carbenoids of other metals; the Li and Mg derivatives undergo 1,2-addition with carbonyl compounds, leading to β-phosphaallyl alcohols.

Several interesting solid-state structures have appeared. The cyclopropenyl-lithium [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] has a central (αCLi)2 ring with tetrahedral αC centres; the cyclopropenyl ring is distorted due to rehybridisation at the lithiated carbon. The structures of three novel η3-allyl-Li and 1-azapentadienyl-Li or -K compounds have been reported; the K+ species, for example, is a polymer in which two cations lie above and below each η4-azapentadienyl anion. In the interesting titanium 'tweezer' complex [(C5HMe4)2Ti(η1-C [equivalent to] CC [equivalent to] CSiMe3)2]-·[Li(THF)2]+, the Li+ ion is embedded between the inner triple bonds of the diynyl ligands.

Regarding structural studies in solution, a detailed NMR investigation into silyl-substituted allenyl/propargyllithium species has given insight into the thermodynamics and kinetics of allenyl-propargyl ligand interconversion, and into how these are affected by complexating solvents. Ion-pair acidities (M+ = Li+, Cs+) have been determined for some terminal acetylenes in THF over various concentrations, the results then being converted into average aggregation numbers for (A-M+)n ion pairs. Two structural studies by computation are also noteworthy. For singlet 1,1-dilithioethene, earlier optimised structures with H2C=CLi2 formulations were found to be transition states; a Cs monobridged structure with a planar tetracoordinate C is now found to be the lowest lying true minimum. Eight local minima have been found on the potential energy surface of C3Li4 calculated at high level: all show either multiple bridging lithiums or clusters of lithium atoms bound to the carbon moiety.

1.4 Aryl Derivatives – Several synthetic applications of aryllithium derivatives have been reported. On warming in the presence of TMEDA, 2-(2-propenoxy) phenyllithium rearranges and undergoes an unusual cyclisation-elimination sequence to give, after methanolysis, 2-(cyclopropyl)phenol. The C6H4 unit of the crown ether 1,3-phenylene-16-crown-5 can be lithiated selectively at the intraanular 2-position without affecting the remainder of the crown, and the resulting aryllithium can then be derivatised with various electrophiles or with metal (Sn, Hg, Mg) halides. So-called ortho -directed lithiations of aryl groups have been used to generate selectively functionalised benzothiazoles from (haloaryl)thioamides containing the -NHC(=S)R directing group. The mechanism of this type of reaction has been explored by ab initio MO calculations on the lithiations of ortho-substituted toluenes, o-CH3C6H4X (X = OH, NH2, F); the key finding is that the regiospecificity of the reaction is not due to 'complex-induced proximity effects' in an initial complex but rather to stabilising interactions in the transition state.


Excerpted from Organometallic Chemistry Volume 26 by M. Green. Copyright © 1998 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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