Stereoselective electrochemical and chemical reductive silylation of bicyclic geminal dihalocyclopropanes. Effects of ultrasound

Electrochemistry of Organomagnesium Compounds

“…During electrochemical reduction of bicyclic 1,1-dibromocyclopropanes (69a-69c) in DMF or THF in the presence of chlorotrimethylsilane (70), using a stainless steel cathode and a sacrificial magnesium anode in an undivided cell, the replacement of one or both bromine atoms (depending on the amount of current passed) by a trimethylsilyl group was observed 127 . The reaction (equation 39) was stereoselective giving 71, the exo-silyl bromide with the trialkylsilyl group in the cis position to hydrogen atoms at the ring junction, as the major product (and the endo-silyl compound 72 as the second product), in contrast to the reaction of 69 with n-BuLi and 70 which yield 72 as the major product.…”

“…A detailed mechanism was proposed for 69a as an example 127 . The overall reaction (equation 40) finally produced the disilyl compound 74a with a yield not much higher than 50%.…”

“…Thus, the nature of the ultrasonic effect was explained assuming that beside the electrochemical silylation at the cathode, a parallel silylation process occurs at a magnesium anode, namely the silylation by 70 of an intermediate Grignard reagent produced from dibromide 69. It appears as a rare example of the 'anodic reduction' 127 . However, the increase in the current density during electrolysis caused a decrease in the apparent current efficiency.…”

“…This observation indicates a chemical nature of the anodic process. Of course, the ultrasonic irradiation facilitates the formation of the organomagnesium intermediate at the sacrificial anode 127 reaction in THF in the presence of a bulk magnesium rod; the latter reaction had given even higher yields and stereoselectivity.…”