Towards inhibition of yellowing of mechanical pulps .1. Catalytic hydrogenation of lignin model compounds under mild conditions

Di-mu-chloro-bis(eta(4)-1,5-hexadiene) dirhodium(I) effectively catalyzed the hydrogenation of the aromatic rings of the lignin model compounds 4-propylphenol, eugenol, 1,2-dimethoxy-4-propylbenzene and 2,6-dimethoxy-4-propyl-phenol under extremely mild conditions (25 degrees C, I atm H-2) in a two-phase, hexane/aqueous pH 7.5 medium. This catalyst was more active than di-mu-chloro-bis(eta(4)-1,5-cyclooctadiene)-dirhodium(I), rhodium(III) trichloride trihydrate and ruthenium(III) trichloride trihydrate. The reactivity of three predominant functional groups in lignin toward the rhodium(I) catalyzed hydrogenation followed the descending order of carbon-carbon double bond > aromatic ring > carbonyl group. The hydrogenation reactions were sensitive to steric and electronic effects of the substituent(s) on the aromatic rings. Moderate to extremely high diastereoselectivities were observed for the hydrogenated products.