Abstract
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 co...
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