CHIRAL ORGANOMETALLIC REAGENTS .9. ON THE ORIGINS OF THE DIASTEREOSELECTIVITY IN THE BROMINE LITHIUM EXCHANGE OF 3-ALKOXY-1,1-DIBROMOALKANES

The 3-alkoxy-1,1-dibromopentanes 5 are compounds, which according to MM2 calculations populate one conformation to >80% at -110-degrees-C. On reaction with n-butyllithium the bromine atom which resides in the extended position is exchanged with selectivities around 80:20, irrespective of the is solely to bias the conformational equilibria in favor of the conformer 17. Coordination of the butyllithium to the oxygen functionality plays an important role only in the carbamoyl derivative 5f, for which the sense of the asymmetric induction is reversed.