Facile benzylation of aromatics in ionic liquid solvents promoted by TfOH, Sc(OTf) 3 , and Yb(OTf) 3 ·xH 2 O; New life for a classic transformation

Benzylation of aromatics with PhCH2Cl and PhCH2OH is conveniently performed in [BMIM][OTf] or [BMIM][PF6] ionic liquids (ILs), by using TfOH, Sc(OTf)3 and Yb(OTf)3·xH2O as catalysts. With PhCH2Cl, high conversions were achieved by using 20% Sc(OTf)3 or Yb(OTf)3 hydrate under mild conditions (65–80 °C). Triflic acid is superior to Yb(OTf)3 as promoter for benzylation with PhCH2OH in the IL solvent, since in most cases little or no dibenzyl ether (DBE) was formed as side product. The scope of arene benzylation with benzyl alcohol in the TfOH-catalyzed and Yb(OTf)3-catalyzed reactions was examined in [BMIM][PF6] solvent. Whereas conversions are typically quantitative at 65–70 °C, minor amounts of DBE were produced, along with the corresponding ArCH2Ph (with minor amounts of dibenzylated derivatives being detected in benzylation of mesitylene and biphenyl). Substrate selectivity (KT/KB) and regioselectivity (isomer distribution) measured for benzylation in IL solvents employing TfOH or Yb(OTf)3 as catalyst are similar to those reported previously in molecular solvents employing Nafion-H, AlCl3, TiCl4 or “clayzic”. The observed high yields and chemoselectivities (absence of DBE), coupled to easy isolation of the benzylated products and recycling/reuse of the IL, provide a new life for this classical transformation.