Kinetic and Thermodynamic Study of the Reduction of 1,1-Diphenylethylene by a Thermally Frustrated Diethyl Ether-BCF Lewis Pair

The reaction enthalpy and rate of reduction of 1,1-diphenylethylene (DPE) by the frustrated Lewis pair formed between tris-pentafluorophenylborane (BCF) and diethyl ether (Et₂O) in dichloromethane have been determined by mixing calorimetry. At 50 °C and 13.6 atm hydrogen, a 0.08 M solution of DPE is reduced to 1,1-diphenylethane, in the presence of 1 equivalent BCF and 0.8 equivalents Et₂O, in 40 minutes. NMR spectroscopy showed>99 % conversion to the reduced product. The rate of conversion of the olefin to the alkane, as monitored by the time-dependent heat flow, showed a linear dependence on the free Et₂O and BCF concentration. Integration of the heat flux provides a measurement of the reaction enthalpy, ΔH, of ca. −116±4 kJ mol⁻¹ for the reaction Ph₂C=CH₂+H₂→Ph₂CHCH₃. The equilibrium constant for dative adduct formation, Et₂O+BCF↔Et₂O−BCF, was determined as a function of temperature by ¹⁹F NMR spectroscopy, and provided an experimental measurement of the enthalpy, ΔH=−54.6±3.3 kJ mol⁻¹, and entropy, ΔS=−154±13 J mol⁻¹ K⁻¹, for dative bond formation in DCM. Extrapolation of the Van’t Hoff plot to 50 °C provides K_eq, which is used to estimate the concentration of free BCF and Et₂O available to activate hydrogen.