The charge-transfer interaction and successive thermal (2+2) cycloaddition of α, β-unsaturated ethers with tetracyanoethylene

The systems of α, β-unsaturated ethers and substituted olefins with tetracyanoethylene (TCNE) give deeply colored 1 : 1 CT complexes immediately after mixing. Subsequently, the complexes react to form colorless 1, 1, 2, 2-tetracyanocyclobutanes only in the case of the ethers. Equilibrium studies on the complex formations and kinetic studies on the successive thermal (2+2) cycloaddition reactions were made in such poor solvents as CH_2Cl_2, CHCl_3 and CCl_4 in the temperature range 10-40℃ by the spectrophotometric method. TCNE-unsaturated ether and TCNE-olefin complexes exhibit gradual increases in stability with the increase of the double-bond substitution, in contrast with Ag^+-unsaturated ether, Ag^+-olefin and I_2-olefin complexes where the complexes become more unstable with increasing substitution at the double bond. This tendency could be interpreted by considering that the electronic and steric effects of the substituents are mutually competing. Furthermore, there is significant difference in the substituent effect on the reactivity between the complex formation and the successive cycloaddition. The difference would be caused by the fact that the steric effect is more important in the activation process of the cycloaddition because the transition state seems much more rigid than the complex state. The cycloaddition reaction proceeds stereoselectively. The experimental results were discussed on the basis of the extended Huckel molecular orbital (EHMO) theory and the molecular orbital correlation diagram

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