MAGNESIUM BROMIDE IN GRIGNARD-REAGENT FORMATION

The progress of the reaction of magnesium with bromocyclopropane in diethyl ether at reflux varies with time in a sigmoid fashion, reflecting an initial induction period during which autocatalysis is evident. The initial addition of MgBr2 to the medium greatly reduces or eliminates the autocatalytic induction period, suggesting that the autocatalysis in the initial absence of MgBr2 is due to its formation. Magnesium does not react with either 1-bromo-2,2,3,3-tetramethylcyclopropane or bromopentamethylbenzene in pure diethyl ether. It reacts smoothly with either in 2.6 M magnesium bromide in diethyl ether, giving the corresponding Grignard reagents (2,2,3,3-tetramethylcyclopropylmagnesium bromide, 28%; pentamethylphenylmagnesium bromide, 49-80%). In the case of 1-bromo-2,2,3,3-tetramethylcyclopropane, by-products representing > 44% solvent attack are formed. Magnesium bromide also has more subtle effects. In reactions of magnesium with bromocyclopropane in diethyl ether, the product distribution varies significantly with the initial concentration of the substrate. Initially-added MgBr2 emulates the effect of higher initial concentrations of the substrate, suggesting that the substrate concentration effects are responses to the buildup of polar solutes (MgBr2, RMgBr) as the reaction proceeds. The bromocyclopropane reaction contrasts with that of 5-hexenyl bromide, for which the extent of cyclization is not very sensitive to added magnesium bromide. The early turbidity that usually forms in reactions of magnesium with organic halides in pure diethyl ether is absent for reactions in 2.6 M MgBr2. Turbidity not due to a precipitate (MgX2, RMgX, or other) may be due to a separation of dilute and concentrated liquid phases of MgBr2 solutions. There are many parallels between Grignard reagent formation and metallic corrosion in contact with aqueous solutions, suggesting that Grignard reagent formation, like aqueous corrosion, involves local galvanic cells.