Selective hydrogenation of alpha, beta-unsaturated ketones in homogeneous and aqueous biphasic catalysis

A series of mononuclear ruthenium complexes Ru(CO)2(CH3COO)2(PnBu3)L (L = P(p-CH3OC6H4)3, PPh3, P(p-ClC6H4)3) were synthesized. All of them contain the basic monophosphine PnBu3 in trans position to a triarylphosphine ligand. Since the chosen triarylphosphines differ only by the pka value and not by the cone angle (145°), they are highly suitable to modulate gradually the electronic effect on the metal centre. Spectroscopic data of the new complexes were correlated with the basicity of the triarylphosphines or with the mesomer effect of the substituent in the para position of the aromatic ring. These Ru(II) complexes were employed in homogeneous and in aqueous biphasic conditions as catalyst precursors for the hydrogenation of alpha,beta-unsatured ketones to yield allyl alcohols in good yields. Indeed, the presence of two different phosphine ligands in the coordination sphere of the ruthenium has a positive effect in the C=O hydrogenation. Applying TPPTS as ligand makes the ruthenium complex water soluble, and on the other hand PnBu3 has a high affinity for organic solvents, which makes an inter-phase transport of the catalyst very easy. The hydrogenation of alpha,beta-unsatured ketones was studied by operando high pressure NMR spectroscopy, in order to observe resting states of the catalytic process. Salvini, A.; Frediani, P.; Giannelli, C.; Rosi, L. J. Organomet. Chem., 2005, 690, 371. (a) Ohkuma, T.; Ooka, H.; Ikariya, T.; Noyori, R. J. Am. Chem. Soc. 1995, 117, 10417. (b) Hannedouche, J.; Kenny, J. A.; Walsgrove, T.; Wills, M. Synlett, 2002, 2, 263. (c) Spogliarich, R.; Vidotto, S.; Farnetti, E.; Graziani, M. Tetrahedron: Asymmetry, 1992, 3, 1001. (d) Hernandez, M.; Kalck, P. Journal of Molecular Catalysis A: Chemical, 1997, 116, 131.