THE BETA-EFFECT WITH VINYL CATIONS - KINETIC-STUDY OF THE PROTIODEMETALLATION OF SILYLALKYNES, GERMYLALKYNES, AND STANNYLALKYNES

The relative magnitude of the hyperconjugative stabilization of vinyl cations by adjacent C-M bonds (M = Si, Ge, Sn; the beta-effect) has been examined by measuring the rate constants for the protonation and subsequent protiodemetalation of group 14 metalated (trimethylsilyl)-acetylenes (R3MC=CSiMe3). The relative beta-effect arising from the second-order rate constants Sn much greater than Ge > Si (maximum k(M)/k/(si) = 10(8), 5 x 10(2), 1, respectively) follows the same order as that reported for simple carbenium ions. The product ratio from the protonation of Ph3GeC=CSiMe3 was found to be particularly sensitive to acid concentration and strength, leading to loss of Ph3Ge preferentially with weaker acids. With tin groups, the rate of destannylation decreased with increasing steric bulk, unlike the corresponding situation with silyl groups. The origins of both these observations may be attributed to nucleophilic interaction at the metal center during protonation.