Organolithium carbanion chemistry

W. Bauer, P. von Rague Schleyer, Recent Results in NMR Spectroscopy of Organolithium Compounds, in Advances in Carbanion Chemistry (V Snieckus, Ed.), 1992, 1, JAI Press, Greenwich, CT. 

This reaction has a very important application in carbanion chemistry and is one of the rare and general methods to prepare organolithium species. ... 

The alkyllithium-initiated, anionic polymerization of vinyl and diene monomers can often be performed without the incursion of spontaneous termination or chain transfer reactions (1). The non-terminating nature of these reactions has provided methods for the synthesis of polymers with predictable molecular weights and narrow molecular weight distributions (2). In addition, these polymerizations generate polymer chains with stable, carbanionic chain ends which, in principle, can be converted into a diverse array of functional end groups using the rich and varied chemistry of organolithium compounds (3). 

In this section, we will concentrate on the chemistry of the Brook isomerization mediated by organolithium compounds and on their unusual routes to potentially useful carbanions. 

This volume, which complements the earlier one, contains 9 chapters written by experts from 7 countries. These include a chapter on the dynamic behavior of organolithium compounds, written by one of the pioneers in the field, and a specific chapter on the structure and dynamics of chiral lithium amides in particular. The use of such amides in asymmetric synthesis is covered in another chapter, and other synthetic aspects are covered in chapters on acyllithium derivatives, on the carbolithiation reaction and on organolithi-ums as synthetic intermediates for tandem reactions. Other topics include the chemistry of ketone dilithio compounds, the chemistry of lithium enolates and homoenolates, and polycyclic and fullerene lithium carbanions. 

Organoselenium compounds are easily attacked by a nucleophile. Reaction with organolithium reagents allows a convenient synthesis of carbanionic species. C. Paulmier and S. Ponthieux describe this chemistry in Chap. 5. 

While most of the chemistry discussed in this chapter has been developed in the past decade, several important methods have withstood the test of time and have made important contributions in areas such as natural product synthesis. Methods such as cuprate acylation and the addition of organolithiums to carboxylic acids have continued to enjoy widespread use in organic synthesis, whereas older methods including the reaction of organocadmium reagents with acid halides, once virtually the only method available for acylation, has not seen extensive utilization recently. In the following discussion, we shall be interested in cases where selective monoacylation of nonstabilized carbanion equivalents has been achieved. Especially of concern here are carbanion equivalents or more properly organometallics which possess no source of resonance stabilization other than the covalent carbon-metal bond. Other sources of carbanions that are intrinsically stabilized, such as enolates, will be covered in Chapter 3.6, Volume 2. 

Alkylation chemistry has also been reported for alkylphosphine oxides and dialkyl phosphonates. The phosphonate carbanions are often preferred as they offer many advantages over the phosphonium ylides. In particular, the phosphonate group is highly acidifying and the resulting organolithium can be smoothly alkylated. 

Another type of reaction relying on the same fundamental mechanism involves organolithium compounds <85JA4700>. The general scheme is depicted in Equation (29). The vinyl carbanion product isomerizes rapidly even at — 70 °C, hence a mixture of ( ) and (Z) isomers is obtained upon reaction with electrophiles. This kind of chemistry has been used to prepare some unusual phos-phino-l,2,3-butatrienes (Equation (30)) <92SL635>. Formally involving an hydride ion and related to the same scheme is the reaction of a phosphirene imine with a borane complex (Equation (31)) <94CB313>. 

Carbon nucleophiles play a central role in organic chemistry, as they form the basis of carbon-carbon bond formation. A few are shown in Figure 1.2, including such carbanionic species as organolithiums (RLi), Grignard reagents (typically written as RMgBr), and the cyanide (CN ) and acetylide (R-C=C ) anions. Other examples such as enolates, enols, and enamines will be briefly discussed in Section 1.15. 

One of the most surprising and useful aspects of the organolithium chemistry of 1,1-diphenylethylene is that reactions with simple and polymeric organolithiums form the corresponding 1,1-diphenylalkyllithiums which are effective initiators for the anionic polymerization of styrene and diene monomers [64]. As described in Sect. 2.2, the pKa (DMSO) value of diphenylmethane (32.2) is much lower than the estimated pKaS (DMSO) of toluene (43) and propene (44) (see Table 2) [43]. These pKa differences correspond to an energy difference of > 64.5 kj/mol thus, the diphenylmethyl carbanion is 64.5 kj/mol more stable than either the benzyl carbanion or the allyl carbanion. Since these latter car-... 

Many interesting and important synthetic applications of 1,1-diphenylethylene and its derivatives in polymer chemistry are based on the addition reactions of polymeric organolithium compounds with 1,1-diphenylethylenes. Therefore, it is important to understand the scope and limitations of this chemistry. In contrast to the factors discussed with respect to the ability of 1,1-dipheny-lalkylcarbanions to initiate polymerization of styrenes and dienes, the additions of poly(styryl)lithium and poly(dienyl)lithium to 1,1-diphenylethylene should be very favorable reactions since it can be estimated that the corresponding 1,1-diphenylalkyllithium is approximately 64.5kJ/mol more stable than allylic and benzylic carbanions as discussed in Sect. 2.2 (see Table 2). Furthermore, the exothermicity of this addition reaction is also enhanced by the conversion of a tt-bond to a more stable a-bond [51]. However, the rate of an addition reaction cannot be deduced from thermodynamic (equilibrium) data an accessible kinetic pathway must also exist [3]. In the following sections, the importance of these kinetic considerations will be apparent. 

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