C−O Bond Homolysis in a Tungsten Alkoxide:  The Mechanism of Alcohol Deoxygenation by WCl₂(PMe₃)₄ and WH₂Cl₂(PMe₃)₄

Abstract

Reactions of alcohols with WCl2(PMe3)4 (1) or WH2Cl2(PMe3)4 (2) yield W(O)Cl2(PMe3)3 (3), PMe3, and hydrocarbons. Cyclopropanemethanol is deoxygenated to give 1-butene and a trace of trans-2-butene as the organic products; benzyl alcohol yields toluene and bibenzyl. These products indicate the intermediacy of organic radicals. Benzyl radicals in the reaction of 1 with PhCH2OH can be trapped by added 2 or by 9,10-dihydroanthracene (DHA), leading to increased yields of toluene vs bibenzyl. With WD2Cl2(PMe3)4 (2-d 2) or DHA-d 12, PhCH2D is formed. The reaction of methanol with 1 proceeds similarly in the presence of DHA, forming 3 and methane. Kinetic studies on the reaction of 1 with benzyl alcohol indicate that the reaction proceeds via alkoxide intermediates. A mechanism involving homolysis of the C−O bond in an alkoxide intermediate is suggested by these results. The thermodynamics of this unusual transformation are discussed.

Titanium: A Unique Metal for Radical Dehydroxylative Functionalization of Alcohols

“…In this perspective, we highlight that low-valent Ti(III) complexes can be a powerful reagent/catalyst for radical cleavage of the alcohol C-OH bonds. Radical deoxygenative functionalization of alcohols by other transition metals, such as rhenium, 12 vanadium, 13 tungsten, 14 molybdate, 15 and niobium, 16 were not included.…”

A tungsten-mediated closed cycle of reactivity for the reduction of CO2 to CO

“…Considering the perception that metal-oxo multiple bonds in early and middle transition metal complexes are an obstacle against turnover in a cycle for small molecule reduction, it would be useful to have an experimentally determined W O bond dissociation energy. The W O bond strength has been estimated by Mayer to be at least 577 kJ mol -1 , 22c, 59 moderately greater than the 531 kJ mol -1 determined for the C O bond of CO 2 . [60][61] We have computationally assessed the W O bond dissociation energies in [WOCl 2 (PH 3 ) 3 ], [WOCl 2 (PMePh 2 ) 3 ] and [WOCl 2 (PMe 3 ) 3 ] in the gas phase (Table 8) and found them to agree reasonably well with Mayer's estimate.…”

Thermodynamic Influences on C-H Bond Oxidation

“…38 A more detailed thermochemical analysis of C-O bond homolysis has been presented for a tungsten benzyloxide. 7 Thermochemical arguments can also shed light on the mechanism of methane oxidation by soluble methane monooxygenase enzymes. The reactive di-iron center (compound Q) reacts very rapidly with methane 43 suggesting that this step is not substantially endothermic.…”

“…4 -7 The tungsten-oxygen bond is so strong that tungsten alkoxide complexes can simply homolyze to give the WHO and R». 7 Compounds with more than one oxo ligand tend to have weaker metal-oxo bonds. The solution (T| 5 -C 5 Me 5 )(0)2Re=0 bond strength of 117+1 kcal/mol is one of very few for which direct experimental data is available.…”