Abstract
A new catalytic system that employs water as an environmentally friendly solvent for the dehydrogenative oxidation of alcohols and lactonization of diols has been developed. In this catalytic system, a water-soluble dicationic iridium complex having a functional ligand that comprises α-hydroxypyridine and 4,5-dihydro-1H-imidazol-2-yl moieties exhibits high catalytic performance. For example, the catalytic dehydrogenative oxidation of 1-phenylethanol in the presence of 0.25 mol % of the iridium catalyst and base under reflux in water proceeded to give acetophenone in 92% yield. Additionally, under similar reaction conditions, the iridium-catalyzed dehydrogenative lactonization of 1,2-benzenedimethanol gave phthalide in 98% yield.
Highlights
From the viewpoint of green sustainable chemistry, it is important to accomplish synthetic organic reactions efficiently using water as a solvent
Non-toxic, inexpensive, and available in large quantities, it is important that research aims at using water as a solvent for organic synthesis [1,2,3,4,5,6]; it is generally difficult to use water as a solvent in such reactions, especially in reactions that require homogeneous transition metal catalysts
These limitations have prevented the development of methods for catalytic organic synthesis in aqueous media
Summary
From the viewpoint of green sustainable chemistry, it is important to accomplish synthetic organic reactions efficiently using water as a solvent. We reported some catalytic systems that were active for the dehydrogenative oxidation reaction of alcohols in aqueous media, for the production of aldehydes, ketones, carboxylic acids, and lactones [8,9,10] These achievements were remarkable as uncommon examples of catalytic organic synthesis using water as a solvent [11,12,13,14,15,16,17,18,19,20,21,22]; some issues remained unresolved such as (1) the necessity of using comparatively large amounts of catalyst, (2) the significant effort required to synthesize the functional ligands, and (3) the limited scope of substrates that can be used as a starting material for the dehydrogenative reactions
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