Applications of Ammonium Formate Catalytic Transfer Hydrogenation. 6. Analysis of Catalyst, Donor Quantity, and Solvent Effects upon the Efficacy of Dechlorination

Article abstract of DOI:10.1021/jo00267a012

Dehalogenation reactions have important synthetic and environmental potential and can be achieved by a variety of chemical methods, including transfer hydrogenation.A catalytic transfer hydrogenation procedure utilizing ammonium formate as an in situ hydrogen donor has previously been used for the dechlorination of polychlorinated biphenyls and for the deuterium labeling of chlorinated peptides.In this study, a systematic evaluation of the effects of catalyst, donor quantity, temperature, and solvent on ammonium formate catalytic transfer hydrogenolyses of aryl chlorides has been undertaken.Results indicate that carbon was the most effective support among those surveyed, and a 3percent palladium loading and 2 equiv of ammonium formate were adequate in terms of overall reaction rate and cost.Catalysts with low Pd content were found to benefit from an increase in temperature, as did reactions involving highly hydrophobic substrates.Alcohols proved to be more effective than polar aprotic (HMPA) and nonpolar (THF, dioxane) solvents.Thus, 1-chloronaphthalene was quantitatively dechlorinated in C2 and C3 alcohols in 2 h and in 4 h in HMPA.The reaction was only 51percent complete in THF and 30percent complete in 1,4-dioxane after 4 h.Improved reaction procedures emerging from these studies were then used for the dechlorination of a variety of mono- and polychlorinated aromatic compounds.