Reaction of thianthrene cation radical with grignard reagents : Evidence for electron transfer and trapping of alkyl radicals by the thianthrene cation radical

Abstract

Reactions are reported between RMgCl and thianthrene cation radical perchlorate (Th .+ClO - 4) suspended in ether and tetrahydrofuran (THF). In ether solution reactions R = Bu, s-Bu, t-Bu, 5-hexenyl, and cyclopentylmethyl. Major products were the alkane, the alkene R(-H) in some cases, and, in the cases of R = Bu, 5-hexenyl, and cyclopentylmethyl, the 5-alkylthianthrenium perchlorate (ThR +ClO - 4). When 5-hexenylMgCl was used a mixture of 5-(5-hexenyl)- and 5-(cyclopentylmethyl)thianthrenium per-chlorates in the ratio of approximately 2 was obtained. Since the ratio of 5-hexenyl/cyclopentylmethyl in the Grignard reagent was 10.4, it is concluded that the C 6 sulfonium ions were formed by radical trapping by Th .+ after single electron transfer from Grignard to cation radical had occurred, thus allowing for cyclization of 5-hexenyl radical. Formation of ThBu +ClO - 4 is attributed to the trapping of butyl radical by Th ·+, while formation of RH and R(-H) is in all cases also attributed to alkyl radical reactions. Reactions in THF(R = Me, i-Pr, Bu, s-Bu, t-Bu, Ph) led almost exclusively to RH and Th. Polymerization of THF was also initiated and took place slowly giving rise to low molecular weight poly(THF). By using THF- d 8, as solvent for reaction between BuMgCl and Th .+, it was possible to find Bu groups ( 1H-NMR) in the poly(THF- d 8). Polymerization of THF is attributed, in some cases (R = Me, Bu), to alkyl-cation transfer from ThR + to THF. In other cases initiation of polymerization by R + and THF(-H) + is considered.