Chiral Synthesis via Organoboranes. 11. Hydroboration. 82. Asymmetric Hydroboration of 1-Heteroarylcycloalkenes with Monoisopinocampheylborane. Synthesis of trans -2-Heteroarylcycloalkyl Boronates and Derived Alcohols of Very High Enantiomeric Purity

Abstract

The hydroboration of representative 1-heteroarylcycloalkenes with monoisopinocampheylborane (IpcBH2) was investigated systematically to establish the degree of asymmetric induction during hydroboration. The hydroboration of 1-heteroarylcyclopentene with IpcBH2 at −25 °C proceeded cleanly to afford the corresponding dialkylboranes. These dialkylboranes, upon treatment with acetaldehyde and then oxidation, afforded the corresponding trans-2-heteroarylcycloalkanols of 85–86% ee. Similarly, 1-heteroarylcyclohexenes, upon hydroboration with IpcBH2, followed by acetaldehyde treatment and oxidation, gave the corresponding alcohols in ca. 90% ee. The dialkylboranes or alkylboronic acids derived from 1-heteroarylcyclopentenes and IpcBH2 can be recrystallized to furnish materials approaching 100% ee. From such dialkylboranes or alkylboronic acids, the corresponding boronates of ca. 100% ee were prepared and isolated. These are proving to be highly versatile synthetic intermediates for organic synthesis.