Ab Initio Computational Examination of Carbonyl Reductions by Borane: The Importance of Lewis Acid−Base Interactions

Abstract

Calculations of the energies and geometries of various complexes of diborane and borane with acetaldehyde, acetone, acetyl chloride, formaldehyde, methyl acetate, THF, dimethyl sulfide (DMS), and ammonia have been performed. Adducts and transition states involving methoxyborane, dimethoxyborane, and trimethyl borate have also been examined. The 6-31+G(d,p) basis set was used to optimize geometries with the Møller−Plesset (second order, MP2) perturbation method. These calculations characterize the probable intermediates and transition states involved in borane, diborane, THF·BH3, and DMS·BH3 reductions of carbonyl compounds as well as THF- and DMS-catalyzed diborane reductions. Four-centered transition states were located for borane reductions of acetaldehyde, acetone, acetyl chloride, formaldehyde, and methyl acetate, the accessibility of which correlates to carbonyl π-orbital energies. Diborane adducts of acetone, acetaldehyde, DMS, THF, and ammonia were also located. The ability to form such adducts depends strongly on Lewis basicity, and subsequent adduct disruption to give free borane and Lewis base−borane adducts is critical in diborane reductions. THF and DMS are predicted to disrupt completely diborane under typical conditions to give THF·BH3 and DMS·BH3. Borane transfer from one Lewis base to another can occur by SN1- or SN2-like pathways of comparable energy. Methoxyborane has a low-energy disproportionation pathway to give borane and dimethoxyborane. Dimethoxyborane is a poor reducing agent that probably does not disproportionate during reductions.