Chemoselectivity in the microwave-assisted solvent-free solid–liquid phase benzylation of phenols: O- versus C-alkylation

The outcome of the solvent-free benzylation of phenol and 4-substituted phenols (such as 4-cresol and 4-chlorophenol) under MW irradiation was found to depend on the absence or presence of the base (K2CO3) and catalyst (triethylbenzylammonium chloride (TEBAC)). Reaction of benzyl halides at 80–120 °C in the presence of K2CO3 and TEBAC resulted in O-alkylation in high (89–96%) chemoselectivities. In the absence of TEBAC, the proportion of C-alkylation was considerable (16–34%). Omitting also K2CO3 from the reaction mixture, C-alkylation became predominant. In the case of 4-tert-butylphenol, the presence of TEBAC promoted C-alkylation. The selectivity of the alkylations can be fine-tuned by choosing the appropriate conditions.