Scope and Mechanism of the Ruthenium-Catalyzed Dehydrative C–H Coupling of Phenols with α,β-Unsaturated Carbonyl Compounds: Expedient Synthesis of Chromene and Benzoxacyclic Derivatives

Chromene and benzoxacyclic derivatives were efficiently synthesized from the ruthenium-catalyzed dehydrative C–H coupling reaction of phenols with α,β-unsaturated carbonyl compounds. The cationic ruthenium–hydride complex was found to be an effective catalyst for the coupling and annulation of phenols with enals to form chromene products. The coupling of phenols with linear enones afforded 2,4-disubstituted chromene derivatives, whereas the analogous coupling with cyclic enones yielded 9-hydroxybenzoxazole products. The reaction of 3,5-dimethoxyphenol with PhCH═CHCDO resulted in the chromene product with a significant H/D exchange to both benzylic and vinyl positions. The most significant carbon isotope effect from the coupling of 3,5-dimethoxyphenol with 4-methoxycinnamaldehyde was observed on the α-olefinic carbon of the chromene product (C(2) = 1.067). A Hammett plot from the coupling of 3,5-dimethoxyphenol with para-substituted p-X-C6H4CH═CHCHO displayed a linear correlation, with a strong promotional effect by an electron-withdrawing group (ρ = +1.5; X = OCH3, CH3, H, F, Cl). Several biologically active chromenone derivatives were synthesized by using the catalytic coupling method. The catalytic method provides an expedient synthetic protocol for the coupling of phenols with α,β-unsaturated carbonyl compounds without employing reactive reagents or forming any wasteful byproducts.