Molecular Complexation and Kinetics for Diels−Alder Condensation of Naphthylalkenes with Tetracyanoethylene

The charge-transfer spectra of 1:1 molecular complexes of seven 1-alkyl-1-(2-naphthyl)ethenes (where R = alkyl group: H, Me, Et, n -Pr, i- Pr, t -Bu, Neopent) and 1-vinylnaphthalene with TCNE were determined in ClCH 2 CH 2 Cl as solvent at 27.1 °C. Equilibrium constants, K, for these complexes vary with R, as based on two opposing factors, viz. (a) the polar substituent factor σ* and (b) the angle-of-twist θ between the planes of the naphthyl and vinyl groups. For R = Neopent (σ* dominant) K is 50 times as large as for R = t -Bu (θ dominant). Except for R = t -Bu, kinetics of reaction conform with the equation D + TCNE ⇄ ( K ) D·TCNE ⇄ ( k 1 / k -1 ) P, where D is the donor alkene, P is the Diels−Alder 1,4-cycloaddition product, and k 1 and k - 1 are first-order reaction rate constants. Values of k 1 vary from 0.71 min - 1 (R = H) to 55.5 min - 1 (R = Neopent) and the corresponding relative second-order rate constants k 2 (or k 1 K ) from 1 to 4000. The rate constant k - 1 was measured only for 1b (R = Me, 0.0017 min - 1 ) in the solvent mixture p -xylene/ClCH 2 CH 2 Cl. Formation of 1b ·TCNE complex gives Δ H = 10.0 kcal/mol and Δ S = 38.4 eu, and conversion to P shows an Arrhenius activation energy of E a = 7.24 kcal/mol. It is proposed that the preferred conformation of a naphthylalkene for complexation has the R and vinyl groups projecting outward from opposite sides of the plane of the naphthalene ring. The TCNE molecule then aligns parallel to the naphthalene ring on the vinyl side where (except for R = t -Bu) it can slide into the geometry of the transition state to form P.