Ceric Ammonium Nitrate Oxidative-Cleavage Reaction of Some Lignin Model Compounds: Role of the Benzylic Hydroxyl Group

Article abstract of DOI:10.1021/jo00242a030

A series of β-1 lignin model compounds was synthesized and then oxidized with ceric ammonium nitrate (CAN) in aqueous acetonitrile.The compounds studied were 1,2-diarylethanols with either a p-hydroxy (1a-e) or a p-methoxy (2a-e) on the 1-ring and the following para substituents on the 2-ring: (a) H, (b) Cl, (c) Me, (d) OMe, and (e) NO2.Reactions of p-hydroxyl compounds 1a-e with CAN resulted in the formation of a red cerium complex that did not oxidize under the conditions of the reaction.In contrast all of the methoxy compounds 2a-e were rapidly oxidized by CAN atroom temperature to give p-anisaldehyde in near quatitative yield.The relative rates of these oxidative-cleavage reactions were found to be 0.11, 0.16, 1.00, 1.58, and 2.37 for the 1,2-diarylethanols 2e, 2b, 2a, 2c, and 2d, respectively.A Hammett treatment of this data revealed an excellent correlation with ?(ρ = -1.24).Methylation of the benzylic hydroxy group of 2a gave 1-methoxy-1-(4-methoxyphenyl)-2-phenylethane, which was found to be inert to CAN oxidation.