Allylboration of azole aldehydes: enantioselective synthesis of homoallylic azole alcohols and reconsideration of the mechanism of enantioselectivity

Allylboration of the 1,2,3-triazole and pyrazole aldehydes with triallylborane or 2,4-pentadienyl(dipropyl)borane under mild reaction conditions provides high yields of the corresponding novel homoallylic and dihomoallylic alcohols that can be used to modify the side chain in the triazole- or diazole-based molecules. Enantioselective allylboration of triazole aldehydes with (+)-B-allyldiisopinocampheylborane gave optically active R(+) homoallylic alcohols in good yields with 80–93% ee. The R-configuration of the reaction products was assigned based on the results of DFT calculations and single-crystal X-ray diffraction study. It was demonstrated that, contrary to early theories, stereo-selectivity is controlled by the stabilizing noncovalent interactions rather than intramolecular repulsive ones.