SOLVATION AND STERIC EFFECTS ON ELECTROPHILIC REACTIVITY OF ETHYLENIC COMPOUNDS .1. STEREOCHEMISTRY AND BROMINATION OF CONGESTED ADAMANTYLIDENEALKANES

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 i, 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 la (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 any intrinsic steric effect. However, branched substituents R’ slow the reaction of 1 twice as much as that of 2. This difference between the effectson 1 and 2 does not arise from differences in the stereoarrangement of R and R’ since, according to MM2 calculations, they adopt exactly the same conformation in both alkene series. Comparison of the bromination rates of 1 in methanol with those measured in acetic acid reveals that the solvent effect (kMeOH/kAcOH about 4) is markedly smaller than that (kMeOH/kAcOH =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. © 1990, American Chemical Society. All rights reserved.