Mechanistic investigation of the dipolar [2+2] cycloaddition-cycloreversion reaction between 4-(N,N-dimethylamino)phenylacetylene and arylated 1,1-dicyanovinyl derivatives to form intramolecular charge-transfer chromophores

The kinetics and mechanism of the formal [2 + 2] cycloaddition- cycloreversion reaction between 4-(N,Ndimethylamino)phenylacetylene (1) and para-substituted benzylidenemalononitriles 2b-21 to form 2-donor-substituted 1,1-dicyanobuta-1, 3-dienes 3b31 via the postulated dicyanocyclobutene intermediates 4b-41 have been studied experimentally by the method of initial rates and computationally at the unrestricted B3LYP/6-31G(d) level. The transformations were found to follow bimolecular, second-order kinetics, with ΔH^≠ _exp = 13-18 kcal mol^-1, Δ^≠ _exp≈-30 cal K^-1 mol^-1, and ΔG^≠ _exp = 22-27 kcal mol^-1. These experimental activation parameters for the rate-determining cycloaddition step are close to the computational values. The rate constants show a good linear free energy relationship (ρ = 2.0) with the electronic character of the para-substituents on the benzylidene moiety in dimethylformamide (DMF), which is indicative of a dipolar mechanism. Analysis of the computed structures and their corresponding solvation energies in acetonitrile suggests that the rate-determining attack of the nucleophilic, terminal alkyne carbon onto the dicyanovinyl electrophile generates a transient zwitterion intermediate with the negative charge developing as a stabilized malononitrile carbanion. The computational analysis predicted that the cycloreversion of the postulated dicyanocyclobutene intermediate would become rate-determining for 1,1-dicyanoethene (2 m) as the electrophile. The dicyanocyclobutene 4 m could indeed be isolated as the key intermediate from the reaction between alkyne 1 and 2 m and characterized by X-ray analysis. Facile first-order cycloreversion occurred upon further heating, yielding as the sole product the 1,1-dicyanobuta-1,3-diene 3m.