Efficient Z-Selective Semihydrogenation of Internal Alkynes Catalyzed by Cationic Iron(II) Hydride Complexes

摘要

The bench-stable cationic bis(sigma-B-H) aminoborane complex [Fe(PNPNMe-iPr)(H)(eta(2)-H2B = NMe2)](+) (2) efficiently catalyzes the semihydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes. Moreover, selective incorporation of deuterium was achieved in the case of 1,3-diynes and 1,3enynes. The catalytic reaction takes place under mild conditions (25 degrees C, 4-5 bar H-2 or D-2) in 1 h, and alkenes were obtained with high Z-selectivity for a broad scope of substrates. Mechanistic insight into the catalytic reaction, explaining also the stereo- and chemoselectivity, is provided by means of DFT calculations. Intermediates featuring a bisdihydrogen moiety [Fe(PNPNMe-iPr)(eta(2)-H-2)(2)](+) are found to play a key role. Experimental support for such species was unequivocally provided by the fact that [Fe(PNPNMe-iPr)(H)(eta(2)-H-2)(2)](+) (3) exhibited the same catalytic activity as 2. The novel cationic bisdihydrogen complex 3 was obtained by protonolysis of [Fe(PNPNMe-iPr)(H)(eta(2)-AlH4)](2) (1) with an excess of nonafluoro-tert-butyl alcohol.