Electron transfer at the solid liquid interface; new insights on the mechanism of formation of the grignard reagent

Abstract

The strong inhibiting effect of trace quantities of selected compounds on the reaction between ultra fine particles of magnesium and alkyl halides in THF strongly suggests that the generally accepted linear mechanism should be replaced by a chain mechanism.The reactivity of batteries of free radical clocks centered on an aliphatic skeleton displays unexpected leaving group effects. Such effects could be rationalized according to a variety of mechanistic schemes.The main ones are: 1) coexistence of polar and electron transfer routes, 2) participation of solvated electrons in the mechanism, 3) gradient of reactivity starting from the magnesium surface to the bulk solution. A series of experiments aiming at a choice between these possibilities is described.

Formation of Grignard Reagents from Aryl Halides: Effective Radical Probes Hint at a Nonparticipation of Dianions in the Mechanism

“…In a recent paper, we described the behavior of 2-(3-butenyl)phenyl halides 1 with active Mg* in THF. Despite the fast radical cyclization ( k cycl = 5.3 × 10 8 s -1 , 30 °C), only the uncyclized Grignard reagent was formed (Scheme ).…”

Structural Effects in Radical Clocks and Mechanisms of Grignard Reagent Formation: Special Effect of a Phenyl Substituent in a Radical Clock when the Crossroads of Selectivity is at a Metal/Solution Interface

“…Two teams independently used this type of tool and observed the same surprising result: apparently, and in contrast with their alkyl counterparts, aryl halides were reacting with magnesium metal with almost no participation of radical species (here σ-type sp 2 -C-centred radicals). [32,33] The unexpectedness of this observation was further accentuated by the fact that the rate constants of intramolecular addition to unsaturated systems were known to be higher for aryl radicals than for alkyl radicals (5 ϫ 10 8 s -1 vs. 2 ϫ 10 5 s -1 , room temperature). In his quantitative D model of reactivity, Garst was predicting that 64 % of cyclised aryl Grignard should be formed, whereas less than 1 % was experimentally observed in THF.…”

“…However, being much more reactive intermolecularly than alkyl radicals, the aryl radicals were far less readily trapped intramolecularly by the double bond than the alkyl radicals. [32,35] The logical conclusion of such a hypothesis, then, was to synthesise aryl halides in which the side chain containing the unsaturated system is specifically designed to increase the rate constant of cyclisation. Radical clocks with higher cyclisation rates should display higher yields of rearranged products.…”

Clear-cut difference in the rearrangement of 1-bromo-2-(2-phenyl-3-butenyl)benzene under anionic or radical conditions

“…Our interest in the mechanism of formation of Grignard reagents and homogeneous versus heterogeneous electron transfers focused our attention on 1-bromo-2-(3-butenyl)benzene 1. [1][2][3][4] Cyclization of o- (3-butenyl)phenyl radical 2r (kcy = 5 x 10 8 s -1 , 30 °C) to 1-methylindanyl radical 3r and related rearrangements have been used to detect radical intermediates in several reactions. [5][6][7][8] However, cyclization of the o- (3-butenyl)phenyl anion 2c has to be considered (Scheme 1).…”