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)(mu-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 H-2, 75 degrees 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)P-2}(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)(mu-Cl)}(2)] and, eventually, complex [Rh(PhBP3)Cl-2] in one day.