Efficient Cobalt Catalyst for Ambient-Temperature Nitrile Dihydroboration, the Elucidation of a Chelate-Assisted Borylation Mechanism, and a New Synthetic Route to Amides

Abstract

N,N-Diborylamines have emerged as promising reagents in organic synthesis; however, their efficient preparation and full synthetic utility have yet to be realized. To address both shortcomings, an effective catalyst for nitrile dihydroboration was sought. Heating CoCl2 in the presence of (PDI)-P-PyEt afforded the six-coordinate Co(II) salt, [((PDI)-P-PyEt)CoCl][Cl]. Upon adding 2 equiv of NaEt3BH, hydride transfer to one chelate imine functionality was observed, resulting in the formation of (kappa(4)-N,N,N,N-(IPNEtPy)-I-PyEt-N-CHMe)Co. Single-crystal X-ray diffraction and density functional theory calculations revealed that this compound possesses a low-spin Co(II) ground state featuring antiferromagnetic coupling to a singly reduced imino(pyridine) moiety. Importantly, (kappa(4)-N,N,N,N-(IPNEtPy)-I-PyEt-N-CHMe)Co was found to catalyze the dihydroboration of nitriles using HBPin with turnover frequencies of up to 380 h(-1) at ambient temperature. Stoichiometric addition experiments revealed that HBPin adds across the Co-N-amide bond to generate a hydride intermediate that can react with additional HBPin or nitriles. Computational evaluation of the reaction coordinate revealed that the B-H addition and nitrile insertion steps occur on the antiferromagnetically coupled triplet spin manifold. Interestingly, formation of the borylimine intermediate was found to occur following BPin transfer from the borylated chelate arm to regenerate (kappa(4)-N,N,N,N-(IPNEtPy)-I-PyEt-N-CHMe)Co. Borylimine reduction is in turn facile and follows the same ligand-assisted borylation pathway. The independent hydroboration of alkyl and aryl imines was also demonstrated at 25 degrees C. With a series of N,N-diborylamines in hand, their addition to carboxylic acids allowed for the direct synthesis of amides at 120 degrees C, without the need for an exogenous coupling reagent. © 2019 American Chemical Society