Flow chemistry enhances catalytic alcohol-to-alkene dehydration†

Hf(OTf)₄ was identified as an excellent catalyst for the low temperature (180 °C) dehydration of 1-hexanol to hexenes and 2-methyl-1-butanol to 2-methylbutenes. Batch conditions limited yields of alkene to 50% despite >90% conversions of 1-hexanol, 16% yield and 55% conversion for 2-methyl-1-butanol, but dramatically better yields were achieved using flow chemistry. For 2-methyl-1-butanol, steady-state conditions were achieved at 180 °C at flow rates of 0.1–0.2 mL min⁻¹ that gave excellent mass balance and allowing selectivities and activities to be meaningfully compared. Hf(OTf)₄ was the most active (51 h⁻¹) with a selectivity of 50% at 50% conversion. Optimising for the production of purer alkene was achieved by raising the pressure producing 2.1 g h⁻¹ of 2-methylbutenes (up to 98% pure by mass).