The Reduction of α-X-Acetophenones (X = PhO, Br, Cl) in Hydrogen-Donating Solvents at Elevated Temperatures

Abstract

The reduction of α-X-acetophenones (X = PhO, Br, Cl), as model compounds for lignin liquefaction studies, has been investigated in the presence of a hydrogen-donating solvent such as 9,10-dihydroanthracene (AnH2) or 2-propanol, between 373 and 573 K. With α-phenoxyacetophenone (PAP) in AnH2, acetophenone and phenol have been obtained with high selectivities. The mechanism involves the reverse radical disproportionation (RRD) with AnH2. Hydrodebromination of α-bromoacetophenone (BrAP) is quantitative at 423 K using AnH2 as a reducing agent. Now, the hydrogen transfer proceeds by an uninhibited radical chain mechanism with anthracenyl radicals as the chain carriers. For the kinetic analysis, the C–X (X = Br, Cl) bond dissociation enthalpies (BDEs) have been determined by means of very low pressure pyrolysis to give BDE(C–Br) = 271 kJ mol–1 and, as a lower limit, BDE(C–Cl) ≈ 309 kJ mol–1, at 298 K. The BDEs are quite at variance with recently published insights derived from an electrochemical study. For comparison, density functional theory calculations (DFT) have been performed.