Intermediates, Isolation and Mechanistic Insights into Zinc Hydride-Catalyzed 1,2-Regioselective Hydrofunctionalization of N-Heteroarenes.

The conjugated bis-guanidinate-supported zinc hydride [{LZnH} 2 ; L = {(ArHN) (ArN)-C═N-C═(NAr) (NHAr); Ar = 2,6-Et 2 -C 6 H 3 }] ( 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-Et 2 C 6 H 3 N)} 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.