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Efficient aerobic oxidation of arylcarbinols and arylmethylene compounds mediated by Nickel (II)/1,2,4-triazole ligand catalyst system

The oxidation of benzylic alcohols is a ubiquitous transformation in organic chemistry due to the relevance of aryl ketones, present in natural products and pharmaceutically active compounds and also intermediates in the synthesis of agrochemicals, medicines and other functional materials. Numerous oxidizing agents are available to effect this key reaction. In most instances, these reagents are required in stoichiometric amounts and are usually toxic, or hazardous, or both. In this context, considering environmental and safety issues, oxygen would be the reagent of choice for such organic transformation. Although much effort has been devoted to aerobic oxidation of benzyl alcohols, the amounts of metal catalyst are still relatively high ( 0.1-10 mol % ) and, sometimes, oxygen pressures above 5 atms.

In this context, we have discovered that systems based upon 1,2,4-triazole ligands and simple Ni (II) salts show extremely high catalytic activity in the aerobic oxidations of alcohols. In fact, primary and secondary benzylic alcohols have provided the corresponding carboxylic acid derivatives by using molecular oxygen at atmospheric pressure, in the presence of a combination of nickel(II) bromide and 1,2,4-triazole or, alternatively, a 1,2,4-triazole pincer ligand at such a low catalytic loading as 10-5 mol%. This catalytic system has proven to be efficient also for the benzylic C-H oxidation.

Keywords: aerobic oxidation, benzylic alcohols, Nickel, pincer ligand, triazole