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Zinc Hydride Catalyzed Chemoselective Hydroboration of Isocyanates: Amide Bond Formation and C=O Bond Cleavage.

Herein, a remarkable conjugated bis-guanidinate (CBG) supported zinc hydride, [{LZnH}2; L = {(ArHN)(ArN)-C=N-C=(NAr)(NHAr); Ar = 2,6-Et2-C6H3}] (I) catalyzed partial reduction of heteroallenes via hydroboration is reported. A large number of aryl and alkyl isocyanates, including electron-donating and withdrawing groups, undergo reduction to obtain selectively N-boryl formamide, bis(boryl) hemiaminal and N-boryl methyl amine products. The compound I effectively catalyzes the chemoselective reduction of various isocyanates, in which the construction of the amide bond occurs. Isocyanates undergo a deoxygenation hydroboration reaction, in which the C=O bond cleaves, leading to N-boryl methyl amines. Several functionalities such as nitro, cyano, halide, and alkene groups are well-tolerated. Furthermore, a series of kinetic, control experiments and structurally characterized intermediates suggest that the zinc hydride species are responsible for all reduction steps and breaking the C=O bond.