Fries rearrangement of aryl esters catalysed by heteropoly acid

Abstract

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 H 3PW 12O 40 (PW), as well as bulk Cs + and Ce 3+ salts, Cs 2.5H 0.5PW 12O 40 and Ce 0.87H 0.4PW 12O 40. 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 Ce 3+ 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 H 2SO 4 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. 31 P 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.