Fries rearrangement of aryl esters catalysed by heteropoly acid Original Research Article

Heteropoly acids (HPAs) are active catalysts for the Fries rearrangement of aryl esters (phenyl acetate, phenyl benzoate, and p-tolyl acetate) to yield the acylated phenols and esters together with phenols in homogeneous or heterogeneous liquid-phase systems at 100–170 °C. Amongst the HPA catalysts studied are bulk, silica-supported, and sol–gel silica-included H3PW12O40 (PW), as well as bulk Cs+ and Ce3+ salts, Cs2.5H0.5PW12O40 and Ce0.87H0.4PW12O40. The reaction with bulk and silica-supported PW occurs homogeneously in aryl esters (without solvent) or in polar solvents, such as nitrobenzene and o-dichlorobenzene. In non-polar solvents such as dodecane, the reaction is heterogeneous. The Cs+ and Ce3+ salts and sol–gel PW catalysts perform heterogeneously in all these media. From heterogeneous systems, the catalysts can be separated and reused, albeit with reduced activity. The homogeneous and silica-supported PW are much more active than H2SO4 and H-Beta zeolite, which is explained by the greater acid strength of HPA compared to the other acids. In contrast to silica-supported PW, the sol–gel PW catalysts show only a negligible activity in the Fries reaction of phenyl acetate, yielding mainly phenol with 92–100% selectivity. This may be explained by a weaker acid strength of the sol–gel catalysts and the presence of relatively high amount of water. 31P MAS NMR shows that the state of the PW anion in the sol–gel and silica-impregnated catalysts is similar, but different from that in the bulk PW.