Solvation and Steric Effects on Electrophilic Reactivity of Ethylenic Compounds. 1. Stereochemistry and Bromination of Congested Adamantylidenealkanes

Article abstract of DOI:10.1021/jo00295a013

In order to evaluate the dependence of the steric effects of alkyl groups on the crowding of the double bond, bromination rate constants of adamantylidenealkanes 1, Ad=CRR' with R = H or Me and R'= H, Me, i-Pr, t-Bu, or neo-Pe, and similarly substituted isopropylidenealkanes 2, Me2C=CRR', are compared.Since the bromination rate of 1a (R = R'= H) is that expected by considering only the polar effect of two gem-isopropyls, the adamantyl group in 1, like the gem-methyls in 2, clearly does not exhibit anyl intrinsic steric effect.However, branched substituents R'slow thereaction of 1 twice as much as that of 2.This difference between the effects on 1 and 2 does not arise from differences in the stereoarrangement of R and R'since, according to MM2 calculations, they adopt exactyl the same conformation in both alkene series.Comparision of the bromination rates of 1 in methanol with those measured in acetic acid reveals that the solvent effect (k_MeOH/k_AcOH about 4) is markedly smaller than that (k_MeOH/k_AcOH = 25) on linear alkenes, which suggests that greater steric retardation in adamantylidenealkanes can be attributed to mechanistic changes: inhibition of nucleophilic solvent assistance in the ionization step and/or return resulting from a slow product-forming step.