Kinetics, isotope effects, and mechanism of the reaction of 1,1,1-trifluoro-2,2-bis-(4-nitrophenyl)ethane with piperidine and pyrrolidine bases in dipolar aprotic solvents

Abstract

The kinetics of the reaction of 1,1,1-trifluoro-2,2-bis-(4-nitrophenyl)ethane with piperidine and pyrrolidine bases in a series of solvents [acetonitrile (MeCN), benzonitrile (PhCN), and dimethyl sulphoxide (Me₂SO)] are reported. The reaction is complex, leading to a 1-amino-1-fluoro-2,2-bis-(4-nitrophenyl)ethene as the final product via the intermediate 1,1-difluoro-2,2-bis-(4-nitrophenyl)ethene. The reaction is promoted only by primary and secondary amines; tertiary amines appear to be inactive even under reflux conditions. The entropies of activation (ΔS^‡/J mol^–1 K^–1) are negative and large (– 183.7 and –162.3) for the reaction with piperidine in PhCN and MeCN; for reaction in Me₂SO the value is only –77.4. The kinetic isotope effects (k_H/k_D) fall between 1.0 and 1.6 at 30 °C. The results obtained are discussed in terms of a multistep mechanism, (E1cB)_ip, consisting of a pre-equilibrium followed by fast addition–elimination steps.