Thermal stability and kinetics of ylide-borane complexes

Complexes of dimethylsulfoxonium methylide (1) and organoboranes are crystalline for ylide·BH3 (2), ylide·BPh3 (3), ylide·B(C6F5)3 (4), and ylide·BF3 (5). These complexes undergo exothermic rearrangement by 1,2-migration upon heating to produce homologated organoboranes and dimethylsulfoxide. Non-isothermal kinetic analysis of the differential scanning calorimetry (DSC) data for ylide·BPh3 (3) and ylide·B(C6F5)3 (4) complexes was applied using the Flynn–Wall–Ozawa and Kissinger methods. The calculated apparent activation energy for the reaction of ylide·BPh3 (3) yielded consistent results between the A1.5 model (Ea=120 kJ mol−1, A=4.79×1013 min−1) and Kissinger method (Ea=129 kJ mol−1, A=1.73×1017 min−1). The analysis for the reaction of ylide·B(C6F5)3 (4) gave consistent results between R2, R3, and F1 models with the average parameters, Ea=262 kJ mol−1, A=3.33×1033 min−1. The Kissinger analysis for the reaction of ylide·B(C6F5)3 (4) gave Arrhenius activation parameters (Ea=171 kJ mol−1, A=7.80×1019 min−1) that were higher than for the reaction of ylide·BPh3 (3). The kinetic data revealed that the C6F5 electron-deficient group has a higher activation energy for 1,2-migration and a higher entropy of activation for 1,2-migration than the C6H5 group. HF/6-31G(d) ab initio calculations agree with the kinetic data.