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

• Published in: Angewandte Chemie International Edition 2021-05, Vol.60 (21), p.11991-12000
• Main Authors: Sahoo, Rajata Kumar, Sarkar, Nabin, Nembenna, Sharanappa
• Format: Article
• Language: English
• Subjects: amides; Amines; Deoxygenation; Hydrides; Hydroboration; hydrodeoxygenation; Intermediates; Isocyanates; Reduction; Zinc
• ISSN: 1433-7851; 1521-3773
• DOI: 10.1002/anie.202100375

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. The zinc‐catalyzed selective reduction of isocyanates via hydroboration is reported for the first time. Amide bond formation by the chemoselective reduction of isocyanates and hydrodeoxygenation of isocyanates to secondary methyl amine have been described.