Conformational and Substituent Effects on The Rehybridization of Boron in β-Substituted Ethylboranes

The chemistry of boron-containing molecules pervades the study of biomolecules, new materials, organic synthesis, and beyond. In addition, the stereochemistry and rehybridization capability of boron in molecules influence the reactivity and selectivity of reactions. Therefore, conformational analysis and the study of substituent effects on the modulation of the sp(2)-sp(3) character of boron in beta-substituted ethylboranes may be valuable for the rational design of compounds in which the empty p(B) orbital plays a central role in the reaction intermediate. Accordingly, we have quantum-chemically studied the orbital interactions and bonding properties that govern the conformational behavior and boron hybridization in beta-substituted ethylboranes (substituents, X=H, CH3, F, Cl, NH2, PH2, OH, and SH). Conformations with an anti arrangement along the B-C-C-X path are generally preferred over syn and/or gauche conformations, while a B-H bond tends to eclipse with the C-C bond rather than with a less sterically hindered C-H bond. However, the s character of boron relative to the C-B bond strongly decreases in the syn conformation, where X can participate as an electron donor to the empty p(B) orbital. Indeed, such interaction makes the syn conformation of 2-aminoethylborane the single stable conformer in the gas phase.