Synthesis of cinnamaldehydes by oxidation of arylpropenes with 2,3-dichloro-5,6-dicyanoquinone

Alkoxylated 1-aryl-1-propenes [l-(4-methoxyphenyl)-1-propene, 1-(3,4-dimethoxyphenyl)-1 -propene, 1-(3,4,5-trimethoxyphenyl)-1-propene] and 3-aryl-1-propenes [3-(4-methoxyphenyl)-1-propene, 3-(3,4-dimethoxyphenyl)-1-propene, 3-(3,4,5-trimethoxyphenyl)-1-propene] gave cinnamaldehydes in 71-84% yield on treatment with 2,3-dichloro-5,6-dicyanoquinone (DDQ) (slight excess) at room temperature for 0.5-2 h in the two-phase system dichloromethane-water (4.1). Arylpropenes lacking electron-donating alkoxy groups (1-phenyl-1-propene, 3-phenyl-1-propene) or carrying an acetoxy group [l-(4-acetoxy-3-methoxyphenyl)-1-propene, 3-(4-acetoxy-3-methoxyphenyl)-1 -propene] were converted into cinnamaldehydes in low to moderate yields on oxidation with a large excess of DDQ in combination with long reaction times (>12 h). All the 1-aryl-1-propenes examined were rapidly converted into a mixture of mono- and bis-(3-aryl-2-propenyl) ethers of 2,3-dichloro-5,6-dicyanohydroquinone (DDHQ) on DDQ oxidation. The rate of formation of DDHQ ethers from alkoxy-substituted 3-aryl-1-propenes was slightly lower. 3-Phenyl-1-propene and also 3-(4-acetoxy-3 -methoxyphenyl)-1-propene were largely unchanged at the initial stage of the oxidation. Significant differences in the compositions of the DDHQ ether mixtures obtained from 1-aryl-1-propenes and 3-aryl-1-propenes were not observed.