Umpolung catalysis: assessment of catalyst and substrate reactivities in acyloin type reactions

The umpolung of aldehydes and acylsilanes in acyloin type reactions is computationally studied by a sequence of model reactions (CPCM-THF B3LYP/6-31G(d)//B3LYP/6-31G(d)) with three different types of catalysts. Cyanide, a nucleophilic carbene and a phosphite form adducts, transition structures for [1,2]-H-shifts or [1,2]-SiMe3-Brook rearrangements and generate the umpoled d1-species. Aliphatic and aromatic aldehydes and acylsilanes (i.e., acetaldehyde, benzaldehyde, acylsilane, and benzoylsilane) show that π-conjugation slightly favors the umpolung. For aldehydes, the nucleophilic carbene, N-methylthiazol-2-ylidene, is by far the most reactive catalyst, while cyanide is slightly superior to the glycole based phosphite. For all catalysts, a dramatic decrease of activation barriers is apparent with the acylsilanes due to [1,2]-SiMe3-Brook rearrangements, thermodynamically formations of d1-species with strong Si–O bonds become highly favored.