Selective Hydrogenation of Cinnamaldehyde and Other α,β-Unsaturated Substrates Catalyzed by Rhodium and Ruthenium Complexes

Abstract

The complexes [Rh(PhBP3)(cod)] (1) and [{Ru(PhBP3)(μ-Cl)}2] (8), containing the tripodal phosphanoborate ligand [PhB(CH2PPh2)3]−, are efficient catalysts for the selective hydrogenation of cinnamaldehyde to the corresponding allyl alcohol. Complex 8 is one of the most efficient catalysts reported to date for this reaction, in terms of activity (TOF 527 h−1) and selectivity (≥97%) under mild reaction conditions (6.8 atm H2, 75 °C). The rhodium system also displays good catalytic features in the hydrogenation of cinnamaldehyde (TOF 219 h−1), particularly a high selectivity (81%) for this metal in the reduction of the C═O bond. Crotonaldehyde can also be reduced, although the selectivities are not as high as for cinnamaldehyde; 2-cyclohexenone is rapidly and specifically reduced to cyclohexanone by both catalysts. The ruthenium catalyst 8 operates via heterolytic activation of hydrogen, involving monohydride intermediates and possibly ionic hydrogen transfer, while the rhodium complex 1 involves oxidative ad...