Chiral Organometallic Reagents, IX. 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°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 nature of the oxy residue at C–3. The role of this substituent 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.