Carbonyl Groups in Lignin. II. Catalytic Hydrogenation of Model Compounds Containing Aryl Carbinol, Aryl Carbinol Ether, Ethylene, and Carbonyl Groups.

Abstract

The catalytic hydrogenation of a variety of lignin model compounds containing aryl carbinol, aryl carbinol ether, ethylene and carbonyl groups was examined. In a reaction mixture containing 93% acetic acid as a solvent , Pd-BaSO4 as a catalyst and HC1 as a promoter, hydrogenolysis of the aryl carbinols I and their methyl ethers proceeded readily, but the rate of reaction was decreased if the [beta]- and gamma-positions of the propane side-chain were substituted by hydroxyl groups. Further inhibition was produced by a [beta]-guaiacoxy substituent and especially by the simultaneous presence of a [beta]-guaiacoxy and a gamma-hydroxy substituent (guaiacylglycerol-[beta]-guaiacyl ether type). Cyclic benzyl ethers, as in pinoresinol or in phenylcoumarans, were readily opened by hydrogenolysis. Ethylenic bonds, either conjugated with the aromatic ring (as in coniferyl alcohol) or isolated (as in eugenol) were rapidly saturated. Keto groups conjugated with the aromatic ring ([alpha]-keto groups) were reduced at satisfactory rates. In coniferyl aldehyde both the carbonyl and the ethylenic group were rapidly hydrogenated. Isolated keto groups ([beta]-keto groups), however, reacted extremely slowly. In a few experiments a nickel boride catalyst and an alkaline solvent were used. In this system the rate of hydrogenation decreased in the following series: C=C [beta]-CO groups > [alpha]-CO groups. No appreciable hydrogenolysis of aryl carbinols and aryl carbinol ethers took place.