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

Abstract

The bench-stable cationic bis(σ-B-H) aminoborane complex [Fe(PNP^NMe-<i>i</i>Pr)(H)(η²-H₂B = NMe₂)]⁺ (<b>2</b>) 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 1,3-enynes. The catalytic reaction takes place under mild conditions (25 °C, 4-5 bar H₂ or D₂) in 1 h, and alkenes were obtained with high <i>Z</i>-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(PNP^NMe-<i>i</i>Pr)(η²-H₂)₂]⁺ are found to play a key role. Experimental support for such species was unequivocally provided by the fact that [Fe(PNP^NMe-<i>i</i>Pr)(H)(η²-H₂)₂]⁺ (<b>3</b>) exhibited the same catalytic activity as <b>2</b>. The novel cationic bisdihydrogen complex <b>3</b> was obtained by protonolysis of [Fe(PNP^NMe-<i>i</i>Pr)(H)(η²-AlH₄)]₂ (<b>1</b>) with an excess of nonafluoro-<i>tert</i>-butyl alcohol.