Issues with the Radical Coupling Mechanism, and a Solution

Although researchers have occasionally attempted to discern pathways by which the oxidized monolignol might react with the ground-state oligomer, none of the hypotheses have withstood testing. Here we add the following to that list. 

Hypothetical two-electron oxidation of monolignols and coupling reactions of potential quinone methides. (See text for details.) 

To date, effecting the subsequent reaction chemistry (as hypothesized in Fig. 12.9) has not been possible in our laboratories. There have been indications that reaction of the epoxide quinone methide QM24 in chloroform with acetic acid may have opened the epoxide before rearomatizing the quinone methide moiety, but this is not well authenticated. However, all attempts to add phenols to this epoxide, particularly aimed at producing 8-0-4-ethers PI, have been unsuccessful. Similar reactions using QM25 have also failed. 

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ISSUES WITH THE RADICAL COUPLING MECHANISM, AND A SOLUTION ... 

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