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 semiempiric al AMI, PM3 and MNDO methods. Results of ab initio calculations are reporte d for M = Sn, Si; R = Me, allyl. Most methods calculate the reaction as end othermic 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 an d benzyl derivatives that are stabilised to a greater extent by sigma - pi conjugation. The experimentally determined alkylating capability of the org anotin compounds in photochemical reactions, where the allyl and benzyl der ivatives are known to be the best alkylating agents, is proposed to be expl ained by the fragmentation feasibility of the radical cations and the ionis ation potential of the neutral precursors. According to PM3 and ab initio s tudies, the fragmentation reaction for M = Si is less facile, thus explaini ng the diminished experimental reactivity of silicon derivatives as alkylat ing agents. (C) 1999 Elsevier Science B.V. All rights reserved.