Palladium(0)-Catalyzed Synthesis of 2-Vinyl-2,3-dihydro-benzo[1,4]dioxins

The reaction of 1,4-bis(methoxycarbonyloxy)but-2-ene (2a–3a) or 3,4-bis(methoxycarbonyloxy)but-1-ene (4a) with various substituted benzene-1,2-diols was catalyzed by a palladium(0) complex to give substituted 2-vinyl-2,3-dihydro-benzo[1,4]dioxins in good yields via a tandem allylic substitution reaction. In the case of 4-substituted benzene-1,2-diols, the ratio of regioisomers is determined by the relative acidity of the two phenolic protons. For 3-substituted benzene-1,2-diols, this ratio is determined only by steric effects in the case of alkyl substituents, although it is determined mainly by the relative stabilities of the corresponding phenates for other substituents; however, for 3-nitrobenzene-1,2-diol, this ratio is determined by the relative leaving-group ability of 2-nitro- or 3-nitrophenate. When the cyclisation was performed in the presence of an optically active phosphane, chiral 2-vinyl-2,3-dihydro-benzo[1,4]dioxin (5) was obtained with enantioselectivity of up to 45% using BINAP as the chiral phosphane.