The Reactions of Grignard Reagents with Transition Metal Halides: Coupling, Disproportionation, and Exchange with Olefins

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Abstract
The reactions of transition metal halides (Mn, Fe, Co, Ni, Pd, Cu, and Ag) with the low-molecular-weight alkyl Grignard reagents in tetrahydrofuran and diethyl ether, especially in the presence of styrene were reinvestigated. An alkyl transition metal species formed in situ by metathesis decomposed to a dialkyl (oxidative dimerization) or to an alkene and an alkane (oxidative disproportionation). Silver(I) and copper(II) were particularly effective in oxidative dimerization of primary alkyl groups. Alkyl groups which contain no β-hydrogen were also coupled by iron, cobalt, nickel, palladium, and copper(I) halides with varying degrees of efficiency. Oxidative disproportionation was generally the more common route to decomposition for alkyl groups which have β-hydrogens. It seemed to proceed directly via a bimolecular interaction of alkylmetals or indirectly by elimination of a hydrido-metal species. If the latter added to an alkene reversibly, exchange was observed between Grignard reagent and alkene. Styrene was reduced to ethylbenzene during the reaction both in tetrahydrofuran and in diethyl ether in varying yield with transition metal halide and Grignard reagent. In addition to alkyl exchange, dehydrogenation of the ethereal solvent by active (reduced) metal species complicated the stoichiometry of decomposition as measured by the value of the empirical parameter, Q,(R). The selection of transition metal in addition to the temperature, solvent and triphenylphosphine was effective variables in promoting the efficiency of the exchange process.