Intermediates, Isolation and Mechanistic Insights into Zinc Hydride-Catalyzed 1,2-Regioselective Hydrofunctionalization of N-Heteroarenes
Rajata Kumar Sahoo, Nabin Sarkar, Sharanappa Nembenna
Abstract
The conjugated bis-guanidinate-supported zinc hydride [{LZnH}2; L = {(ArHN) (ArN)–C═N–C═(NAr) (NHAr); Ar = 2,6-Et2-C6H3}] (I)-catalyzed highly demanding exclusive 1,2-regioselective hydroboration and hydrosilylation of N-heteroarenes is demonstrated with excellent yields. This protocol is compatible with many pyridines and N-heteroarene derivatives, including electron-donating and -withdrawing substituents. Catalytic intermediates, such as [(LZnH) (4-methylpyridine)] IIA, [(L′ZnH) (4-methylpyridine) IIA′, where L′ = CH{(CMe) (2,6-Et2C6H3N)}2)], LZn(1,2-DhiQ) (isoquinoline) III, [L′Zn(1,2-DhiQ) (isoquinoline)] III′, and LZn(1,2-(3-MeDHQ)) (3-methylquinoline) V, were isolated and thoroughly characterized by NMR, HRMS, and IR analyses. Furthermore, X-ray single-crystal diffraction studies confirmed the molecular structures of compounds IIA′, III, and III′. The NMR data proved that the intermediate III or III′ reacted with HBpin and gave a selective 1,2-addition hydroborated product. Stoichiometric experiments suggest that V and III independently reacted with silane, yielding selective 1,2-addition of mono- and bis-hydrosilylated products, respectively. Based on the isolation of intermediates and a series of stoichiometric experiments, plausible catalytic cycles were established. Furthermore, the intermolecular chemoselective hydroboration reaction over other reducible functionalities was studied.