CATALYZED SIGMATROPIC REARRANGEMENTS .10. MECHANISM OF THE PALLADIUM DICHLORIDE CATALYZED COPE REARRANGEMENT OF ACYCLIC DIENES - A SUBSTITUENT EFFECT STUDY

The PdCl2-catalyzed Cope rearrangement of eight 2-aryl-1,5-hexadienes was studied. No simple substituent effect relationship was found, since the catalyzed-rearrangement rate was decreased by introducing both electron-withdrawing and electron-donating substituents. The rates of rearrangement of the four most electron-deficient dienes (X = p-CF3, m-CF3, m-Br, m-F) showed good correlation (ρ = -2.0) with σ+. This correlation provides the first direct evidence for the development of significant electron deficiency at C-2 in the transition state and is fully consistent with a cyclization-induced rearrangement mechanism in which the rate-limiting step involves cyclization of PdCl2 alkene complex 5 to form the 4-palladacyclohexyl cation intermediate 6. The fact that electron-donating substituents also decrease the rate of the catalyzed rearrangement is attributed to nonproductive binding of the PdCl2 catalyst with the styrene unit. In support of this suggestion, p-methoxystyrene and 1 -methyl-p-methoxystyrene are effective inhibitors of the PdCl2-catalyzed rearrangement of dienes 3c (X = H), 3b (X = zm-CF3), and 3g (X = p-CF3), whereas l-methyl-p-(trifluoromethyl)styrene is not. © 1990, American Chemical Society. All rights reserved.