Reaction of 1-aryl-3-chloropropenes with Grignard reagents. Nucleophilic substitution versus single-electron transfer

Abstract

The reactions of ambident 1-aryl-3-chloropropenes (1a–;e) with a series of Grignard reagents, R′MgY (R′= Me, Pr, Ph, Prⁱ, Bu^t; Y = Br, I), were carried out in diethyl ether (EE) and tetrahydrofuran (THF). The products were a mixture of two alkylation products (2) and (3), and three dimerization products (4)–;(6). The alkylation: dimerization ratio and the composition of the two alkylation products were a marked function of substitutent electronic effects in the chlorides (1a–;e), R′ or Y of R′MgY, and solvent. On the basis of the stereochemistry of alkylation, cyclizable probe experiments, and the effect of the addition of FeCl₃ on product composition, the following conclusions were obtained. First, dimers (4)–;(6) are most likely to be produced by a mechanism involving single-electron transfer (SET). Second, for the formation of alkylation products (2) and (3), three alternative pathways contribute depending on the nature of R′MgY and solvent, (a) competitive S_N2–S_N2′ pathways in the reaction of R′MgBr in EE, (b) a process involving SET in the reaction with R′Mgl in EE, and (c)S_N2 pathways in the reaction of R′MgBr in THF.