The Atmosphere Matters: The Effect of “Inert” Gas on the Catalytic Outcomes in Cobalt-Mediated Alkyne and Olefin Hydroboration

We present a highly
efficient cobalt-mediated hydroboration
reaction
of alkynes and alkenes enabled by a π-acidic and redox-active
pyrimidinediimine (PPymDI) core. The entry point in the
catalytic cycle is on a cobalt dinitrogen complex rather than cobalt
hydride species, which are commonly postulated in hydroelementation
reactions. Stoichiometric studies have demonstrated that both cobalt
hydrides and dinitrogen complexes can be generated from the same precursors
and under the same reaction conditions, with the sole difference being
the reaction atmosphere (argon vs N2). Nevertheless, while
the PPymDI-based cobalt dinitrogen complex is highly active
(TOF = 1100 h–1 at t1/2, RT), the hydride analogue displays only modest conversions at slow
reaction rates. Under the optimized conditions, a wide range of vinyl
and alkyl organoboron derivatives can be obtained with high catalytic
efficiency. Mechanistic studies suggest that, due to the increased
π-acidity of the PPymDI core, initial formation of
Co-π-complexes is preferred, followed by the oxidative addition
of borane, which is the turnover-limiting step. This sequence of catalytic
steps is supported by a Hammett analysis, which shows that the hydroboration
reaction proceeds more rapidly for electron-rich substrates. The electronic
structure of all relevant species was investigated in detail by computational,
crystallographic, and spectroscopic means, revealing ligand involvement
in the redox processes.