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.