Metal-Free Catalytic Hydrogenation of Enamines, Imines, and Conjugated Phosphinoalkenylboranes

摘要

Catalytic heterolytic splitting of dihydrogen had until recently been a domain of metal-containing systems, be it for example in the Fe or Fe/Ni hydrogenase enzymes or the Noyori-type chelate amine ruthenium catalysts and many conceptually related systems. Stephan et al. recently reported the reaction of dihydrogen with frustrated phosphane/borane Lewis pairs, some of which are quite active catalysts for the hydrogenation of bulky imines at elevated temperatures. We reported the formation of the ethylene-bridged P/B system 1 (Mes = 2,4,6-trimethylphenyl), a weakly intramo-lecularly interacting phosphane/borane pair that rapidly activates dihydrogen heterolytically at ambient temperature to form the linked phosphonium/hydridoborate zwitterion 2 [Eq. (1)]. Herein we show that the system 1/2 is able to transfer the H~+/H~- pair under mild conditions and can serve as a catalyst for the hydrogenation of bulky imines and of enamines.