High-pressure kinetics of electron donor–acceptor complex formation and cycloaddition between tetracyanoethylene and enol ethers

Abstract

The kinetics of the high-pressure cycloaddition reaction between tetracyanoethylene and enol ethers (n-butyl vinyl ether and ethyl 2-methylpropenyl ether) in chloroform have been studied, by following spectrophotometrically the disappearance of the electron donor–acceptor complex (e.d.a. complex) at 25 °C and up to 1 500 bar. It is concluded that the e.d.a. complex is on the pathway to the zwitterionic intermediate and the final cycloaddition product. The reaction volume ΔV1 of e.d.a. complex formation is –11.0 ± 1.4 and –5.8 ± 1.0 cm3 mol–1 for n-butyl vinyl ether and ethyl 2-methylpropenyl ether, respectively. The volume of activation ΔV2‡ of cycloaddition step from e.d.a. complex to the adduct is –30.8 ± 1.5 and –41.8 ± 1.5 cm3 mol–1 for n-butyl vinyl ether and ethyl 2-methylpropenyl ether, respectively. The variation of ΔV1 and ΔV2‡ is discussed from the viewpoint of the ionization potential and the electron density at the β-carbon atom of the enol ether.