Solvent effects in the Grignard reaction with alkynes

Kinetic studies were carried out on the reaction of phenylmagnesium bromide with hex-1-yne in diethyl ether, and in binary mixtures of diethyl ether with toluene, chlorobenzene and dichloromethane. The reaction was accelerated by addition of non-donating solvents. The replacement of a coordinated solvent molecule by the alkyne is necessary for the reaction to proceed, according to density functional theory (DFT) calculations with B3LYP/6-31 + G* method. The non-donating solvents accelerate the reaction by shifting the replacement equilibrium in favour of the complex formation. An analysis in terms of the Koppel-Palm equation revealed a rate decrease with increase in solvent polarity and polarizability. Copyright (C) 2002 John Wiley Sons, Ltd.