Preparation of Asymmetric Diarylmethanes

Thirty-five different asym DAMs were synthesized by the condensation reaction of benzyl alcohols and arenes in the presence of an acid catalyst. 

The mechanism of the condensation reaction of pseudocumene and formaldehyde showed that trimethylbenzyl alcohols were formed as intermediates. However, these alcohols were not detected in the reaction products because they are more reactive than formaldehyde and condense immediately with pseudocumene to produce the corresponding HexMeDPM (39). 

Ordinarily, p-toluenesulfonic acid has been used as the catalyst. Although p-toluenesulfonic acid has high activity, its use results in the formation of by-products such as methylbenzyl-p-toluenesulfonates, high molecular weight condensates, and esters. We found that formic acid is a better catalyst for this reaction and gives almost quantitatively the mixture of HexMeDPM in a pure state without any contamination due to undesirable by-products. Formic acid also serves as a solvent. From this study, it was found that benzyl alcohols react easily with arenes to form asym DAMs, and formic acid is the most favorable catalyst for the benzylation reaction. 

Five kinds of methylbenzyl alcohols were prepared through the following routes  

Benzyl alcohols, including commercially available benzyl alcohol and 4-methylbenzyl alcohol, were purified through rectification, followed by recrystallization when the alcohols were solids. 

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