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

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 addition of dihydrogen to form dihydride intermediates and follows a substrate route. Indeed, complex 1 reacts with hydrogen in acetonitrile to give the dihydride complex [Rh(PhBP₃)(H)₂(NCMe)] (3), while protonation of one of the phosphane arms of the ligand occurs on treatment of complex 1 with HBF₄ to give the cationic species [Rh{PhB(PH)P₂}(cod)]BF₄. The hydride ligands in 3 are easily removed as molecular hydrogen by reaction with CO under atmospheric pressure to give the rhodium(I) complex [Rh(PhBP₃)(CO)₂]. In this reaction, the replacement of acetonitrile by CO takes place previously to the elimination of hydrogen, which indicates that substrates can coordinate to the metal in 3 by replacement of the labile acetonitrile ligand. Under an atmosphere of argon, complex 3 reacts with chloroform to give an equimolecular mixture of the cis and trans isomers of [{Rh(PhBP₃)(H)(μ-Cl)}₂] and, eventually, complex [Rh(PhBP₃)Cl₂] in one day.