Grignard reagent formation from aryl halides. There is no aryl radical intermediate along the dominant reaction channel

For Grignard reagent formation from magnesium and an aliphatic halide RX in an ether solvent, a route through R-. is the major pathway. Part of the ev idence is that by-products of side reactions of R-. are formed in substanti al yields. Similar reactions of phenyl and o-(3-butenyl)phenyl halides give very low (sometimes trace) yields of by-products derived from side reactio ns of R-., despite the fact that: aryl R-. are much more reactive than alky l in both solvent attack and cyclization [o-(3-butenyl)phenyl case]. Grigna rd reactions of aryl halides appear to proceed largely through a pathway al ong which R-. is not an intermediate. This is probably a dianion pathway, t hat is, one along which RX2- is an intermediate or transition stale. (C) 19 99 Elsevier Science S.A. All rights reserved.