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 (Et 2 O) 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 Et 2 O, 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 Et 2 O and BCF concentration. Integration of the heat flux provides a measurement of the reaction enthalpy, Δ H , of ca. −116±4 kJ mol −1 for the reaction Ph 2 C=CH 2 +H 2 →Ph 2 CHCH 3 . The equilibrium constant for dative adduct formation, Et 2 O+BCF↔Et 2 O−BCF, was determined as a function of temperature by 19 F NMR spectroscopy, and provided an experimental measurement of the enthalpy, Δ H =−54.6±3.3 kJ mol −1 , and entropy, Δ S =−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 Et 2 O available to activate hydrogen.