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

✍ Scribed by Edwin Dorrestijn; Sven Hemmink; Guido Hulstman; Laurent Monnier; Wibo van Scheppingen; Peter Mulder

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 325 KB
• Volume: 1999
• Category: Article
• ISSN: 1434-193X
• DOI: 10.1002/(sici)1099-0690(199903)1999:3<607::aid-ejoc607>3.0.co;2-v

✦ Synopsis

The reduction of α-X-acetophenones (X = PhO, Br, Cl), as the hydrogen transfer proceeds by an uninhibited radical chain mechanism with anthracenyl radicals as the chain model compounds for lignin liquefaction studies, has been investigated in the presence of a hydrogen-donating solvent carriers. For the kinetic analysis, the C-X (X = Br, Cl) bond dissociation enthalpies (BDEs) have been determined by such as 9,10-dihydroanthracene (AnH 2 ) or 2-propanol, between 373 and573 K. With α-phenoxyacetophenone (PAP) means of very low pressure pyrolysis to give BDE(C-Br) = 271 kJ mol -1 and, as a lower limit, BDE(C-Cl) ഠ 309 kJ mol -1 , in AnH 2 , acetophenone and phenol have been obtained with high selectivities. The mechanism involves the reverse at 298 K. The BDEs are quite at variance with recently published insights derived from an electrochemical study. radical disproportionation (RRD) with AnH 2 . Hydrodebromination of α-bromoacetophenone (BrAP) is For comparison, density functional theory calculations (DFT) have been performed. quantitative at 423 K using AnH 2 as a reducing agent. Now, quinone to the phenolic form already occurs at 350 K.