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,N‐dimethylamino)phenylacetylene (1) and para‐substituted benzylidenemalononitriles 2 b–2 l to form 2‐donor‐substituted 1,1‐dicyanobuta‐1,3‐dienes 3 b–3 l via the postulated dicyanocyclobutene intermediates 4 b–4 l 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 ${{\rm{\Delta }}H_{{\rm{exp}}}^{ {\ne} } }$=13–18 kcal mol−1, ${{\rm{\Delta }}S_{{\rm{exp}}}^{ {\ne} } }$≈−30 cal K−1 mol−1, and ${{\rm{\Delta }}G_{{\rm{exp}}}^{ {\ne} } }$=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 3 m. Intermediate trapped! A detailed investigation of the cycloaddition–cycloreversion reaction between N,N‐dimethylanilinoacetylene and arylated 1,1‐dicyanovinyl derivatives elucidated its bimolecular, dipolar reaction mechanism, in which the relative height of reaction barriers is highly sensitive to the substituents. In one case, the cyclobutene formed in the cycloaddition step could be isolated as a crystalline solid and its structure analyzed (see scheme).