Comparative study of phenol alkylation mechanisms using homogeneous and silica-supported boron trifluoride catalysts

The application of silica-supported BF3 as a mild solid acid catalyst to the C- and O-alkylation of various aryl alcohols is described. In the case of O-alkylation, the reactivity of the ether product in both heterogeneous and homogeneous systems is compared, and it is shown that the former can be used to affect selective C-alkylation of ethers, while in the latter system, ether rearrangement to ring poly-alkylated phenols also occurs. Ether rearrangement is thought to require coordination of the ether to an available Lewis acid site, which, in the case of the heterogeneous system, is apparently impossible. The difference between the homogeneous and the supported BF3 may be attributed to steric restrictions and/or relatively weak Lewis acidity of the supported system. Effects of steric hindrance at the reaction centre are examined. Possible routes to ethers and ring-alkylated products are also discussed. (C) 2000 Elsevier Science B.V. All rights reserved.