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虠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 Keq, which is used to estimate the concentration of free BCF and Et2O available to activate hydrogen. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.