CONCERTED AND STEPWISE SOLVOLYTIC ELIMINATION AND SUBSTITUTION-REACTIONS - STEREOCHEMISTRY AND SUBSTITUENT EFFECTS

Solvolysis of the R,R and R,S isomers 2a-X and 2b-X, respectively, (X = I, Br, OBs) in 25 vol % acetonitrile in water gives the elimination products 4, 5a, and 5b along with the substitution products 2a-OH, 2b- OH, 2a-NHCOMe, and 2b-NHCOMe. The rates of elimination (k(E)) increase with increasing acidity of the beta-hydrogen of the substrate as expr essed by a Bronsted parameter of alpha = 0.08 and 0.07 for the iodides and the bromides, respectively. In contrast, the brosylates, which gi ve much smaller fractions of the stable alkenes, exhibit a negative Br onsted parameter for the elimination, alpha = -0.10. This shows that t he brosylates, in contrast to the halides, do not give elimination exc lusively by a solvent-promoted E1cB reaction or an E2 reaction with a large component of hydron transfer in the transition state and indicat es a large amount of reaction through carbocation intermediates to giv e stable alkenes.