Quinone monoacetals (QMAs) were found to be convenient substrates for a unique arylation reaction of enol silyl ethers. This arylation process of QMAs typically proceeds through the acetal activation of the QMAs by a hydrogen-bond donor solvent, such as a fluoroalcohol, for the initiating step. The silyl transfer from silyl enol ethers to the carbonyl oxygen of the QMAs appears to be involved in the C–C coupling step, followed by QMA aromatization. By this method, valuable α-aryl carbonyl compounds containing o-phenol moieties can be obtained directly under mild conditions without lactone formation.

AbstractThe reaction can also be performed in the presence of fluorinated terephthalic acid as catalyst albeit products are obtained in somewhat lower yields.

We have developed an efficient Brønsted acid-controlled strategy for the [3 + 2] coupling reaction of quinone monoacetals (QMAs) with nucleophilic alkenes, which is triggered by the particular use of a specific acid promoter, perfluorinated acid, and a solvent, fluoroalcohol. This new coupling reaction smoothly proceeded with high regiospecificity in regard with QMAs for introducing π-nucleophiles to only the carbon α to the carbonyl group, thereby providing diverse dihydrobenzofurans and derivatives with high yields, up to quantitative, under mild conditions in short reaction times. The choice of Brønsted acid enabled us to avoid hydrolysis of the QMAs, which gives quinones, and the formation of discrete cationic species from the QMAs. Notably, further investigations in this study with regard to the acid have led to the findings that the originally stoichiometrically used acid could be reduced to a catalytic amount of 5 mol % loading or less and that the stoichiometry of the alkenes could be significantly improved down to only 1.2 equiv. The facts that only a minimal loading (5 mol %) of perfluoroterephthalic acid is required, readily available substrates can be used, and the regioselectivity can be controlled by the acid used make this coupling reaction very fascinating from a practical viewpoint.