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A tautomeric ligand enables directed C‒H hydroxylation with molecular oxygen

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Science  25 Jun 2021:
Vol. 372, Issue 6549, pp. 1452-1457
DOI: 10.1126/science.abg2362

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Easing oxygen into arenes

Although oxygen is all around us, it is often surprisingly difficult to use it for selective chemical oxidations, necessitating more expensive, wasteful alternatives. Li et al. report that careful ligand optimization produces palladium catalysts that can efficiently activate oxygen to hydroxylate a variety of aryl and heteroaromatic rings adjacent to a carboxylic acid substituent. The ligand binds to palladium through pyridine and pyridone components, and the authors posit that tautomerization between dative and anionic coordination modes plays a role in its effectiveness.

Science, abg2362, this issue p. 1452

Abstract

Hydroxylation of aryl carbon–hydrogen bonds with transition metal catalysts has proven challenging when oxygen is used as the oxidant. Here, we report a palladium complex bearing a bidentate pyridine/pyridone ligand that efficiently catalyzes this reaction at ring positions adjacent to carboxylic acids. Infrared, x-ray, and computational analysis support a possible role of ligand tautomerization from mono-anionic (L,X) to neutral (L,L) coordination in the catalytic cycle of aerobic carbon–hydrogen hydroxylation reaction. The conventional site selectivity dictated by heterocycles is overturned by this catalyst, thus allowing late-stage modification of compounds of pharmaceutical interest at previously inaccessible sites.

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