Zirconium Complex as an Efficient Catalyst in the Hydroboration of Nitriles, Alkynes, and Carboxylic Esters: A Combined Experimental and Computational Study

We report the synthesis of a zirconium complex {& kappa;(2)-[Ph2P(S)NCH2CH2N(CH2CH2)O]Zr(C5H5)(2)Cl} (2) supported by a flexible amidophosphinosulfideligand [Ph2P(S)N(CH2)(2)N(CH2CH2)(2)O](-) (1). The solid-state structure of zirconium complex 2 was established through single-crystal X-ray diffraction analysis.Complex 2 proved to be a competent catalyst in the hydroborationof organic nitriles, alkynes, and carboxylic esters with pinacolborane(HBpin) in neat reaction conditions. The reactions were accomplishedwith efficient catalytic reactivity demonstrated by complex 2 under mild temperatures and solvent-free conditions, affordinga high yield of the corresponding N-borylamines,vinyl boranes, and benzyl boronate esters. The protocol for the catalyticreaction presented in this paper is simple and efficient, with diversesubstrate scope for nitriles, alkynes, and carboxylic esters, showingexcellent functional group tolerance. The kinetic study reveals thatthe nitrile reduction proceeded with first-order dependence on theconcentrations of nitrile, HBpin, and catalyst 2. DFTcalculations have been performed to gain mechanistic insights intothe catalytic hydroboration of nitriles.