Computational Study of the Mechanism of Dehydrogenative Borylation of Terminal Alkynes by SiNN Iridium Complexes

This paper presents a comprehensive study of the mechanism of dehydrogenative borylation of terminal alkynes (DHBTA) by Ir complexes of the SiNN pincer ligands. The study uses phenylacetylene as the prototypical alkyne and pinacolborane (HBpin) as the boron source. The original report (J. Am. Chem. Soc. 2013, 135, 3560) on this reactivity proposed, without any substantial evidence, a mechanism similar to that usually ascribed to Ir catalysts for aromatic C-H borylation. However, this work has uncovered a completely different mechanistic picture. Three interlinked mechanistic pathways have been identified. The free energy barriers lie in the range of approximately 16-22 kcal/mol, which is qualitatively compatible with the experimentally observed turnover rate on the order of 0.1 s(-1). The key element in all three pathways is the facile migration of the Bpin group between Ir and the N(amido) of the ancillary SiNN ligand. In particular, migration of Bpin onto N(amido) opens the Ir center electronically and coordinatively for a more facile C-H oxidative addition step. The Si-H moiety of the SiNN ligand is also non-innocent as the nature of its interaction with the Ir center fluctuates between complete oxidative addition of the SiH bond and a a-complex interaction. In addition to the identification of three different catalytic cycles with plausible overall barriers, many transition states within each cycle possess similar energies. Thus, in this system, it is not possible to identify a dominant high-lying transition state with confidence, especially considering that the relative energies of the closely spaced states are likely to vary depending on the nature of the alkyne substrate and the different reagent concentrations throughout the course of the reaction. Nonetheless, it is the presence of the multiple possible low-energy pathways that must be responsible for the effective catalysis of DHBTA by the (SiNN)Ir system.