Palladium-catalyzed synthesis of 2,3-dihydro-2-ylidene-1,4-benzodioxins

Palladium-catalyzed condensation of benzene-1,2-diol with various propargylic carbonates afforded regio- and stereoselectively 2,3-dihydro-2-ylidene-1,4-benzodioxins. The reaction is suggested to proceed by the formation of a (sigma-allenyl)palladium complex, followed by the intermolecular attack of the phenoxide ion on this complex to generate a new (sigma-allyl)palladium complex in equilibrium with the corresponding (eta(3)-allyl)palladium complex. Intramolecular attack of the phenoxide ion afforded the corresponding benzodioxan compound. This last attack occurs predominantly at the more electrophilic end of the (eta(3)-allyl)palladium intermediate. The Z- or E-stereochemistry of the products was established by (1)H NMR and proton NOE measurements and also by X-ray analysis on an example. The Z-stereochemistry generally observed is in agreement with the formation of this (eta(3)-allyl)palladium intermediate. However, in the case of tertiary propargylic carbonates, the E-stereochemistry generally observed could be explained by an intramolecular attack of the phenoxide ion on the intermediate (sigma-allyl)palladium complex, in slow equilibrium with the (eta(3)-allyl)palladium complex.