Solvolysis of substituted benzyl azoxyarenesulfonates: characterisation ofthe transition state and the selectivity of benzylic intermediates in 50% aqueous 2,2,2-trifluoroethanol

Thirteen substituted benzyl azoxyarenesulfonates have been prepared and rat e constants for solvolysis of twelve have been measured in 50% (v/v) aqueou s 2,2,2-trifluoroethanol over a range of temperatures from which activation parameters and rate constants at 25 degreesC have been determined. A six-p oint Hammett correlation for substituents in the benzylic electrofuge, with azoxytoluene-p-sulfonate as the common nucleofuge, gives p(sigma (+)) = -3 .27. With 4-methylbenzyl as a common electrofuge, four substituents in the azoxybenzenesulfonate nucleofuge give p(sigma) = 1.07, Comparison with mode l reactions indicates a synchronous concerted rate-limiting fragmentation f or these substrates. With the less reactive 3-chlorobenzyl as electrofuge, p for substituents in the nucleofuge is only ca. 0.7 which indicates a lowe r degree of charge development in the arenesulfonate in the transition stat e for these reactions. Product analyses have been carried out for compounds of high, intermediate, and low reactivity. These indicate a reactivity-sel ectivity relationship for capture of the substituted benzylic electrophiles by water and 2,2,2-trifluoroethanol. Substituted benzaldehydes are formed from the less reactive substrates, indicating trapping of the first-formed benzylic carbenium ion by nitrous oxide and subsequent elimination of nitro gen and a proton. but yields are low and decrease as the substituted benzyl cation becomes increasingly stable.