jo300614k_si_001.pdf (1.62 MB)
Substituent-Induced Switch of the Role of Charge-Transfer Complexes in the Diels–Alder Reactions of o-Chloranil and Styrenes
journal contribution
posted on 2012-07-20, 00:00 authored by Sergiy V. Rosokha, Vasiliy Korotchenko, Charlotte L. Stern, Vladimir Zaitsev, Jeremy T. RitzertAddition 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.