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.