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 DeltaH(exp)(not equal)=13-18 kcal mol-1, DeltaS(exp)(not equal) approximately -30 cal K-1 mol-1, and DeltaG(exp)(not equal)=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 (rho=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.
Wu
Yi-Lin
YL
Laboratorium für Organische Chemie, ETH-Zürich, Hönggerberg, HCI, CH-8093 Zürich, Switzerland.
Jarowski
Peter D
PD
Schweizer
W Bernd
WB
Diederich
François
F