Facile benzylation of aromatics in ionic liquidsolvents promoted by TfOH, Sc(OTf)3, and Yb(OTf)3·xH2O; New life for a classic transformation

Abstract

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 (K_T/K_B) 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.