Solvent and Counterion Effects on the Stereochemistry and the Competition between Electron-Transfer and SN2 Mechanisms in the Reaction of (Trimethylstannyl)alkalies with Bromides

Article abstract of DOI:10.1021/ja00288a023

Recations of (trimethylstannyl)alkalies (Me3SnM, M=Li, Na, K) with bromides have been studied in solvents including tetraglyme and tetrahydrofuran, in mixtures of tetrahydrofuran with ether and with benzene, and with added crown ether, 18-C-6.Product distributions and stereochemistry have been examined.Dicyclohexylphosphine (DCPH) was used as a trap for intermediate free radicals to detect participation of an electron-transfer (ET) process which occurs in competition with the S_N2 mechanism.The effect of the nature of the cation on the course of the reaction depends upon the medium.The effects is not usually in simple relation to the size of the cation.The S_N2 mechanism competes most effectively in a good coordinating medium but is not the exclusive one with 2-bromooctane even in THF containing 18-C-6.In the poorly coordinating mixed solvents, 2-bromooctane reacts virtually exclusively by an ET process.Even the primary 1-bromooctane and 6-bromo-1-hexene show ET contributions in the mixed solvents of low cation coordinating ability.In the latter case the ET component was established both by DCPH trapping experiments and by formation of the cyclic substitution product, (cyclopentylmethyl)trimethylstannane.The mechanistic implications of these and other observations are examined.