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

Yi Lin Wu, Peter D. Jarowski, W. Bernd Schweizer, François Diederich*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

20 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)202-211
Number of pages10
JournalChemistry - A European Journal
Volume16
Issue number1
DOIs
StatePublished - Jan 4 2010

Keywords

  • Cycloaddition
  • Kinetics
  • Linear free energy relationships
  • Push-pull chromophores
  • Reaction mechanisms

ASJC Scopus subject areas

  • Chemistry(all)

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