Catechol oxidative trimerization

The trimethylsilyloxy (TMSO) group is stable under the coupling conditions in acetonitrile (Table 12, number 6). After oxidative dimerization the TMS-ether can be mildly hydrolyzed (H+ and H2O) to the phenol or converted to a dibenzofuran. 1,2-Dialkoxybenzenes have been trimerized to triphenylenes (Table 5, numbers 7, 8). The reaction product is the triphenylene radical cation, which is reduced to the final product either by zinc powder or in a flow cell consisting of a porous anode and cathode [188]. Anodic trimerization of catechol ketals yields triphenylene ketals, which can function as a platform for receptors, for example, in an artificial caffeine receptor [190]. 

Recently the conditions have been defined under which trimerization of some phenol ethers is viable these require anodic oxidation in a flow cell with porous electrodes, using acetonitrile or trifluoroacetic acid-methylene chloride. Catechol derivative (48) and 1,2-methylenedioxybenzene were thus converted into trimers (49) and (50) respectively, in ca. 30% yield. 

One of them was found as dimeric species (MW 510) of 8-hydroxybentazon, which was stabilized at neutral pH. In acidic condition, the main products were considered as two trimeric isomers (MW 764). Oxidative copolymerization of 8-hydroxybentazon and catechol resulted in the formation of tetrameric oligomer during initial stage of the reaction. The structure of the tetrameric oligomer (MW 728) was thought to be cross-coupled with dimeric species of 8-hydroxybentazon and catechol-derived dimmer. 

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