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.

Oxidation of Alcohols and Activated Alkanes with Lewis Acid-Activated TEMPO

“…Complex 23 crystallizes in the orthorhombic space group Pnma, and its solid state molecular structure is shown in Figure 1. 2-phenoxyacetophenone (6), 62 1,2-diphenyl-2-methoxyethanol (8) (mixture of 85:15 u (R,S + S,R): l (R,R + S,S) diastereomers), 21 and anhydrous TEMPOH 63 were prepared according to the previously reported procedures. NMR spectral data for these compounds were consistent with those reported in the literature (Note: the yield.…”

Decomposition of β-O-4 Linked Lignin Model Compound in Anhydrous Ethanol without any Added Catalyst

“…Essentially, the active hydrogen species generated from the solvents (illustrated in Scheme 1) participated in the proceedings and advanced the followup reactions (Guthrie 1975). The active hydrogen species (bonded to the surface of the steel wall, or as Metal-H) first attacked the oxygen atoms in α-carbonyl of the 2-PAP molecule and formed POPE radicals (Huang et al 1995;Dorrestijn et al 1999). The POPE radicals subsequently reacted along three routes, Routes Ia, Ib, or Ic, which led to further cleavage, rearrangement, or over-hydrogenation, respectively.…”

“…Referring to published reports (Guthrie 1975;Netto-Ferreira et al 1990;Andersen et al 1996;Dorrestijn et al 1999;Lanzalunga and Bietti 2000) and based on the main products' distribution listed in Fig. 2, Fig.…”

“…2A and S5A, and Entry 1 in Table 2), the decomposition of 2-PAP mostly proceeded along Route I (Dorrestijn et al 1999). Essentially, the active hydrogen species generated from the solvents (illustrated in Scheme 1) participated in the proceedings and advanced the followup reactions (Guthrie 1975).…”

“…Then, the resultant 1-phenylethenol (an enol) further tautomerized to a more stable ketone form of acetophenone (AP in Scheme 2) (Huang et al 1995), while the phenoxy radicals were terminated with the formation of phenol (P in Scheme 2) by abstracting hydrogen from acetophenone or the solvent (Netto-Ferreira et al 1990). POPE radicals underwent isomerization along Route Ib and further cleaved to benzaldehyde along Route Ib-1 (BAD in Scheme 2) (Dorrestijn et al 1999). The intermediate of POPE radicals also isomerized, and then hydrogenated to generate 2-hydroxy-1,2-diphenylethan-1-one, which then further etherified into 2-ethoxy-1,2-diphenylethan-1-one (EDPE in Scheme 2) along Route Ib-2.…”