Target: Monovalent magnesium

10 September 2012 - Grignards

GrignardMechanism  Tomasz Kruczynski et al. have news to report on the Grignard reaction and more specifically the SET mechanism believed to be responsible for the formation of the Grignard reagent (DOI). As a quick reminder, in SET the organohalide R-X reacts with magnesium metal to form the organic radical (R.) and Mg.+X-. In a second SET step the organic group gains an electron opening the way for formation of R-Mg-X as the Grignard reagent. The isolation of obscure MgBr as opposed to common MgBr2 would then be a nice addition to Grignard know-how and this is what the Kruczynski team set out to accomplish.

One established route is the gas-phase reaction of magnesium diboride with hydrogen bromide at 700°C followed by a dilute low-temperature quench in toluene. Main disadvantage: raising the temperature to above -40°C results in disproportionation to elemental magnesium and MgBr2 with loss of the EPR signal. A large counterion for magnesium should improve stability but addition of the lithium salt of tert-butylthiol (replacing bromine) did not yield the expected MgS(tBu) but rather the (tBuMg-S-tBu)4 cage compound again with loss of the EPR signal. According to the report a

"redox reaction has occurred in which the tBu+ unit in the StBu- moiety is reduced to tBu-, which then is directly connected to the oxidized Mg2+ cation; that is, the SR anion could react like the isoelectronic RCl species in a Grignard-like formation reaction with elemental magnesium".