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

The complexes [Rh(PhBP<sub>3</sub>)(cod)] (<b>1</b>) and [{Ru(PhBP<sub>3</sub>)(μ-Cl)}<sub>2</sub>] (<b>8</b>), containing the tripodal phosphanoborate ligand [PhB(CH<sub>2</sub>PPh<sub>2</sub>)<sub>3</sub>]<sup>−</sup>, are efficient catalysts for the selective hydrogenation of cinnamaldehyde to the corresponding allyl alcohol. Complex <b>8</b> is one of the most efficient catalysts reported to date for this reaction, in terms of activity (TOF 527 h<sup>−1</sup>) and selectivity (≥97%) under mild reaction conditions (6.8 atm H<sub>2</sub>, 75 °C). The rhodium system also displays good catalytic features in the hydrogenation of cinnamaldehyde (TOF 219 h<sup>−1</sup>), 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 <b>8</b> operates via heterolytic activation of hydrogen, involving monohydride intermediates and possibly ionic hydrogen transfer, while the rhodium complex <b>1</b> involves oxidative addition of dihydrogen to form dihydride intermediates and follows a substrate route. Indeed, complex <b>1</b> reacts with hydrogen in acetonitrile to give the dihydride complex [Rh(PhBP<sub>3</sub>)(H)<sub>2</sub>(NCMe)] (<b>3</b>), while protonation of one of the phosphane arms of the ligand occurs on treatment of complex <b>1</b> with HBF<sub>4</sub> to give the cationic species [Rh{PhB(PH)P<sub>2</sub>}(cod)]BF<sub>4</sub>. The hydride ligands in <b>3</b> are easily removed as molecular hydrogen by reaction with CO under atmospheric pressure to give the rhodium(I) complex [Rh(PhBP<sub>3</sub>)(CO)<sub>2</sub>]. 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 <b>3</b> by replacement of the labile acetonitrile ligand. Under an atmosphere of argon, complex <b>3</b> reacts with chloroform to give an equimolecular mixture of the <i>cis</i> and <i>trans</i> isomers of [{Rh(PhBP<sub>3</sub>)(H)(μ-Cl)}<sub>2</sub>] and, eventually, complex [Rh(PhBP<sub>3</sub>)Cl<sub>2</sub>] in one day.