Theoretical calculations concerning the fragmentation of organotin and organosilicon radical cations

Calculations of the fragmentation reaction [Me3MR]* . --> Me3M+ + R . (M = Sn, Si; R = Me, i-Pr, allyl, PhCH2, Ph) were performed with the semiempirical AMI, PM3 and MNDO methods. Results of ab initio calculations are reported for M = Sn, Si; R = Me, allyl. Most methods calculate the reaction as endothermic for all Rs and its feasibility follows the order i-Pr greater than or equal to Me > allyl > PhCH2 > Ph. This is the order of the one electron Sn-C bond length in the corresponding radical cations, which is related to the stability of the radicals released, with the exception of the allyl and benzyl derivatives that are stabilised to a greater extent by sigma - pi conjugation. The experimentally determined alkylating capability of the organotin compounds in photochemical reactions, where the allyl and benzyl derivatives are known to be the best alkylating agents, is proposed to be explained by the fragmentation feasibility of the radical cations and the ionisation potential of the neutral precursors. According to PM3 and ab initio studies, the fragmentation reaction for M = Si is less facile, thus explaining the diminished experimental reactivity of silicon derivatives as alkylating agents. (C) 1999 Elsevier Science B.V. All rights reserved.