Diphenylallyl anions

FIGURE 3. The proposed interconversion scheme for substituted 1,3-diphenylallyl anions... 

Exposure of a solution of loose ion pairs of the diphenylallyl anion to white light results in partial isomerisation of the solute into the cis,trans conformation (scheme 1). Whereas the trans,trans ion is planar, the cis,trans ion is non-planar as a consequence of steric interaction between the hydrogen... 

The diphenylallyl carbanion is conveniently formed by the abstraction of an allylic proton from the corresponding diphenyl propene. Under certain conditions it was found that the initial product formed from trans 1,3 diphenyl-2-methyl propene was the trans,trans anion which isomerised completely into the cis,trans conformation within a few minutes [3]. Clearly, the abstraction reaction proceeds under kinetic and-not thermodynamic control. 

In the triarylallyl carbanion study and in the absence of accepting stilbene, EjZ photoisomerization of the irradiated carbanion was observed and the effect of changing the substituent at position 2 was examined. Starting from the zero coefficient at carbon-2 in the non bonding MO of the allyllic system, Tolbert considers that the substituent at the 2 position does not affect the energy of the MO. If an electron transfer mechanism governs the reactivity, the substituent on this position will not modify the process in a significant way. The experimental results were very dependent on the C-2 substitution and this reaction was therefore considered as relevant of the intrinsic photochemistry of the anion [145]. This view has been confirmed in other studies on 1,3-diphenylallyl carbanion the kinetic parameters of the photoisomerizations were found to be inconsistent with an electron transfer mechanism [146, 147]. 

Ion-pair effects, as in the case of the unsubstituted allyl alkali metal compounds 23a-d, do not markedly influence the rotational barriers of the 1,3-diphenylallyl-lithium species 27a-f, although the reason is different 27a-f are solvent-separated ion pairs 36). Addition of HMPT to the THF solution of 27a raises the AG3 c value by 0.9 kcal/mol which corresponds to a rate retardation of 5-6 times. In the case of the methyl-substituted allyl anion 27b HMPT slows down the rate by a factor of only 2-3. With the 2-cyano anion 27c the AG values of the Li+ compound in THF and the Li +, Na+ andK+ species in dimethyl sulfoxide (DMSO) are the same18). In the case of 27e and 27f the rate of the rearrangement is not affected if HMPT or TMEDA are added to the THF solutions. The observations of some solvent dependence in the case of the sterically less hindered 27a and b, but of no effect with the more crowded 27e and f are in line with the general observation that solvent-separated ion pairs are favored with respect to contact ions pairs by increasing steric hindrance 25). Hence, these experimental results could be interpreted to mean that in the case of 27a and b contact ion pairs participate in the allyl anion rearrangement reaction. 

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