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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

Wu, Y-L, Jarowski, PD, Schweizer, WB and Diederich, F (2010) 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 Chem-Eur J, 16, 1. pp. 202-211.

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Abstract

The kinetics and mechanism of the formal [2+2] cycloaddition-cycloreversion reaction between 4-(N,N-dimethylamino)phenylacetylene (1) and para-substituted benzylidenemalononitriles 2b-2l to form 2-donor-substituted 1,1-dicyanobuta-1,3-dienes 3b-3l via the Postulated dicyanocyclobutene intermediates 4b-4l 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 Delta H-exp(not equal) = 13-18 kcal mol(-1), Delta S-exp(not equal) approximate to -30 cal K-1 mol(-1), and Delta G(exp)(not equal) = 22-27 kcal K-1 mol(-1). These experimental activation parameters for the rate-determining cycloaddition step are close to the computational values. The rate constants show it 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 (2m) as the electrophile. The dicyanocyclobutene 4m could indeed be isolated as the key intermediate from the reaction between alkyne I and 2m 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.

Item Type: Article
Authors :
AuthorsEmailORCID
Wu, Y-LUNSPECIFIEDUNSPECIFIED
Jarowski, PDUNSPECIFIEDUNSPECIFIED
Schweizer, WBUNSPECIFIEDUNSPECIFIED
Diederich, FUNSPECIFIEDUNSPECIFIED
Date : January 2010
Identification Number : https://doi.org/10.1002/chem.200902645
Related URLs :
Depositing User : Symplectic Elements
Date Deposited : 28 Mar 2017 13:11
Last Modified : 28 Mar 2017 13:11
URI: http://epubs.surrey.ac.uk/id/eprint/805654

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