A crystalline, internally-coordinated chloroborane for asymmetric hydroboration

Abstract

Asymmetric hydroboration is an important method in the preparation of enantiomerically-enriched compounds that are necessary in many areas of chemistry. Here is reported the preparation of a unique chiral chloroborane-internal ether complex and its applications in asymmetric hydroboration. This chloroborane is easily obtained in crystalline, solvent-free and enantiomerically pure form. The chemical shift of the Lewis-basic methoxy group is sensitive to the boron environment, making this reagent especially amenable to NMR studies in which diastereomer ratios can, in many cases, be determined before oxidation, and even kinetics are relatively easily carried out. It reacts readily with prochiral alkenes to give, after oxidation, alcohols in up to 82% ee. The dialkylchloroborane intermediates can be diastereomerically purified by recrystallization either directly, or as air-stable ethanolamine complexes, to give after oxidation alcohols in up to 99.8% ee. The chloro group is easily replaced by hydride in-situ, allowing for intramolecular asymmetric hydroborations and deuteroborations with a high degree of regiochemical control that intermolecular hydroborations cannot provide.