On the Preparation and Determination of Configurational Stability of Chiral Thio- and Bromo[D1]methyllithiums

ABSTRACT: Thio- and bromo[D1]methyllithiums (ee 99%) were generated from the respective stannanes by tin−lithium exchange at temperatures ranging from 0 to −95 °C. Thio[D1]methyllithiums 6 were found to be microscopically configurationally labile on the time scale of the thiophosphate-α-mercaptophosphonate rearrangement even at −95 °C. Thio[D1]methyllithiums 13a and 13b underwent a thia-[2,3]-Wittig rearrangement down to −95 °C and 13b only down to −50 °C. The former were microscopically configurationally stable below −95 °C, and the latter racemized completely at −50 °C. Chiral bromo[D1]methyllithiums are chemically unstable at −78 °C but microscopically configurationally stable at the time scale of their addition to benzaldehyde and acetophenone. In previous papers, we have shown that heteroatom-substituted chiral [D1]methyllithiums, the smallest organometallic com-pounds, can be prepared in homochiral form by tin−lithium exchange from the respective tributylstannyl derivatives at lo