Spectroscopic Studies of Carbanions

The electronic spectra of carbanionic polymers have been exploited for many years as a means of determining the concentration of the absorbing species. More recently, careful study has shown that other information may be obtained — notably concerning the conformation of allylic anions. Even more powerful in this respect has been the application of NMR techniques, greatly helped by the development of instrumentation capable of examining nuclei (such as 13C) at the natural abundance level. 

The broken curves show increasing absorption at 320 nm in the order---------—, 1.9 x 10 3 -------, 

Somewhat similar spectral changes are observed 191) when ether solutions of (CH3)3CCH2CH = C(CH3)CH2Li - a model for poIy(isoprenyl)lithium - are 

Morton et al.135,141) were the first to study the poly(butadienyl)lithium anionic chain end using (b). They found no evidence of 1,2-chain ends and concluded that only 1,4-structures having the lithium cr-bonded to the terminal carbon were present. A later study by Bywater et al.196), employing 1,1,3,4-tetradeuterobutadiene to minimize the complexity of the spectrum that arises from proton-proton coupling, found that the 1 1 adduct with d-9 fert-butyllithium in benzene exists as a mixture of the cis and trans conformers in the ratio 2.6 1. Glaze et al. 36) obtained a highly resolved spectrum of neopentylallyllithium in toluene and found a cis trans ratio of about 3 1. 

Monomer Solvent Temp. Isomer 8. Counter- ion % transisomer 

At equilibrium, the proportion of the cis form increases with decreasing temperature. However, the spectra indicate that the trans conformation is preferentially produced by propagation, the more so the lower the temperature (kinetic and not thermodynamic control). At about 0 °C, the rate of conformational relaxation becomes sufficiently rapid that the spectrum is independent of the occurrence of propagation. 

Morton et al. were the first to study the poly(butadienyl)lithium anionic 

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