Substituent-induced switch of the role of charge-transfer complexes in the Diels-Alder reactions of o-chloranil and styrenes.
Addition of p-substituted styrenes, XSty (X = H, Me, MeO, or Cl) to the solutions of o-chloranil, oCA, in dichloromethane resulted in the transient formation of the charge-transfer complexes, [XSty, oCA], followed by the Diels-Alder reaction. At low temperatures, these reactions led to formation of essentially pure endocycloadducts. As expected for the inverse-electron-demand Diels-Alder reaction, the rate constants of the cycloaddition rose with the increase of the donor strength. However, while facile cycloaddition took place in the neat mixtures of the o-chloranil with p-methyl, p-chloro-, or unsubstituted styrenes at low temperatures, a similar system involving the strongest MeOSty donor was surprisingly persistent. X-ray structural measurements and quantum-mechanical computations indicated that this anomaly is related to the fact that the diene/dienophile orientation in the charge-transfer [MeOSty, oCA] complex is opposite to that in the endocycloadduct and in the lowest-energy transition state leading to this isomer. Thus, the proceeding of the cycloaddition requires dissociation of the (dead-end) complex. For the systems involving the oCA diene and either the HSty, ClSty, or MeSty dienophile, the donor/acceptor arrangements in the charge-transfer complexes apparently are consistent with that in the corresponding products, and the formation of these complexes does not hinder the Diels-Alder reaction.
Department of Biological, Chemical and Physical Sciences, Roosevelt University, Chicago, Illinois 60605, USA. srosokha@roosevelt.edu
Sergiy V
SV
Rosokha
Vasiliy
V
Korotchenko
Charlotte L
CL
Stern
Vladimir
V
Zaitsev
Jeremy T
JT
Ritzert
D002703:Chloranil / Q000031:analogs & derivatives* / Q000737:chemistry; D018360:Crystallography, X-Ray; D003500:Cyclization; D008958:Models, Molecular; D011789:Quantum Theory; D013343:Styrenes / Q000737:chemistry*
D013343:Styrenes; D002703:Chloranil; C039499:2-chloranil