Selective Cobalt(II)-SNS Dithiolate Complex-Catalyzed Bifunctional Hydroboration of Aldehydes: Kinetics and Mechanistic Studies

Anionic ligands containing a nonbondedlone pair (i.e., amido, thiolate) are well-knownto cooperate witha metal centerin bifunctional catalytic reactions. An NHC Co-II(& kappa;(3)-SNS) imine-dithiolate complex (Co-3)effects the selective hydroboration of aldehydes (vs ketones) in 4-30min at a 1 mol % loading in benzene (NHC = 2,5-dimethyl-3,4-dichloroimidazol-2-ylidene).Although the catalyst was unchanged in stoichiometric reactions withthe aldehyde or borane, VTNA kinetic studies showed first-order ratedependence on the concentrations of Co-3, aldehyde, andpinacolborane as well as significant product inhibition. DFT studiesconfirmed a bifunctional transition state with B-H activationat the 5-membered ring thiolate donor being slightly favored overthe 6-membered ring. Stable van der Waals complexes of Co-3 with both benzaldehyde and its hydroboration product were also identified,and catalyst reuse studies further confirmed product inhibition, especiallywith aromatic aldehydes. Reactivation of the unchanged catalyst waseasily accomplished by product removal using hexane.

Automated summary

Anionic ligands with nonbonded lone pairs have been widely utilized in bifunctional catalytic reactions with metal centers. A NHC Co-II (& kappa; (3)-SNS) imine-dithiolate complex (Co-3) selectively catalyzes the hydroboration of aldehydes (vs. ketones) in 4-30 min at a 1 mol% loading in benzene. VTNA kinetic studies reveal first-order dependence on the concentrations of Co-3, aldehyde, and pinacolborane, as well as significant product inhibition. DFT studies confirm a bifunctional transition state with B-H activation favored at the 5-membered ring thiolate donor.