Mechanistic Study of Silylacetylene Hydroboration Catalyzed by a Cobalt PN5P Pincer Complex: Catalytic Cycles Altering the Reaction Efficiency

The reaction mechanisms of the hydroboration of silylacetylene with pinacolborane catalyzed by a cobalt complex with a PN5P triazine pincer have been studied using density functional-theory (DFT). The calculations reveal multiple reaction routes for catalyst initiation and product formation. The cobalt complex undergoes activation via hydride transfer from pinacolborane, a process confirmed by experimental NMR analysis. Additionally, DFT results indicate that catalyst activation involves dimerization and hydrogen evolution. In contrast to the generally proposed cobalt-boryl intermediates, the active catalyst is identified as a cobalt monohydride species. During the propagation phase, cobalt monohydride preferentially reacts with silylacetylenes rather than pinacolborane, as indicated by kinetic and thermodynamic descriptors, thus impeding the formation of cobalt boryl species. This higher reactivity of alkynes in this reaction is attributed to their Brønsted acidity. The pathways leading to the hydroborated product involve hydrometallation competing with hydrogen evolution, with the latter being followed by borylation with pinacolborane, hydroalkylation, and eventually either reductive elimination or alkyne insertion. Our findings also indicate that transition state energies can be reduced by incorporating electron-withdrawing groups into the silylacetylene substrate. Enhancements in product yield are examined in relation to the silyl substituents, as well as a protocol with broader applicability, formulated under laboratory conditions.