Kinetics and isotope effects of the Dehydrochlorination of 1,1-dichloro-2,2-bis-(4-nitrophenyl) ethane with 1,5-diazabicyclo [4.3.0] non-5-ene in aprotic solvents

The kinetics of the reaction of 1,1-dichloro-2,2-bis-(4-nitrophenyl)ethane ( 1 ) with 1,5-diazabicyclo-[4.3.0]non-5-ene(DBN) in acetonitrile (MeCN), dichloromethane (DCM), tetrahydrofuran (THF), isopropylether (IPE) and hexane (Hex) solvents are reported. The reaction is a simple β-elimination forming 1-chloro-2,2-bis-(4-nitrophenyl)ethane ( 2 ) (100% conversion). The rate constants are similar for MeCN, THF, IPE, but lower by a factor of 2 and 7 for DCM and Hex respectively. The deuterium kinetic isotope effects k_H/k_D are between 10 and 6 for MeCN and Hex solvents. The entropies of activation (ΔS≠) are negative and large and show no correlation with the polarity of the solvents. The free enthalpies of activation (ΔG≠) are rather small (about 68 kJ mol⁻¹). The results are interpreted in terms of E2H with a contribution of (E1cB)_I mechanism.