Substituent effects Cope rearrangement

Carroll, G.L., Harrison, R., Gerken, J.B., and Little, R.D. (2003) Coping with substituent effects in divinylcydopropyl dizaene rearrangements. Tetrahedron Letters, 44, 2109-2112. 

Like the Cope rearrangement, the Claisen rearrangement is sensitive to substituents on the reacting system. Cyano groups promote the rearrangement by a factor of 10 at positions 2 and 4 and have smaller effects at the other positions, as shown below. Data are also available for methoxy groups at positions 2, 4, 5, and 6. ... 

Even if full potential energy surfaces are not calculated, simple EHT calculations, skilfully coupled with orbital symmetry considerations, can provide insight into complex reactivity problems. This is well exemplified by Hoffmann and Stohrer s analysis of substituent effects on the Cope rearrangement (28). 

The SnCU-catalysed Claisen and Cope rearrangements of A -allylanilines and N-allylenamines, and the effect of meffl-substituents in the aromatic ring on the Claisen aromatic amino rearrangement of a series of fluorinated anilines, have been investigated. 

Figure 12.7. Orbital interactions in the Cope rearrangement (a) symmetry-allowed interactions of the g and orbitals (b) effect of X (or C ) and Z (or C ) substituents in the 3-position (c) effect of Z substituents at the 1-position.

The same is true for Cope rearrangements in general. Most substituents at C-1 and C-3 accelerate the reaction, and with more than one of them, their effects are cooperative. Substituents at C-2, however, shift the balance of the transition structure towards a biradical-like intermediate in which the new a bond is formed ahead of the old one breaking. Substituent effects at this site are not cooperative with substituent effects at C-l and C-3, because they change the nature of the transition structure rather than contribute to it in the same way. 

The Houk and Borden groups °° ° have collaborated on a couple of important studies to address the effect of multiple substituents on the Cope rearrangement. They examined the effects of cyano, phenyl, and vinyl substituents at various positions on 1,5 -hexadiene. All three substituents give similar results, but only the cyano and phenyl cases will be discussed here. The cyano case was also examined by Staroverov and Davidson, " coming to the same conclusions as presented here but with a slightly different analysis. 

These results suggest a competitive interaction between the active and nodal substituents. The geometries of these transition states support this competition their values are quite similar to the distance found in the parent 1,5-hexadiene. Computational examinations of the substituent effects on the Cope rearrangement conclude that the centauric model does not apply. The chameleonic model makes a better accounting of the cooperative and competitive ways the substituents affect the Cope rearrangement. Borden has proposed a simple mathematical model that allows for the prediction of the stabilization of the transition state by substituents solely on the change in... 

Hrovat, D. A. Chen, J. Houk, K. N. Thatcher, B. W. Cooperative and competitive substituent effects on the Cope rearrangements of phenyl-substituted 1,5-hexadienes elucidated by Becke3LYP/6-31G calculations, 7. Am. Chem. Soc. 2000, 722, 7456-7460. 

Delbecq, F., Nguyen Trong, A. A theoretical study of substituent effects. Influence on the rate of the Cope rearrangement. Nouv. J. Chim. 1983,7,505-513. 

Gajewski, J. J., Conrad, N. D., Emrani, J., Gilbert, K. E. Substituent effects on the Cope rearrangement, Neither centaurs nor chameleons can characterize them. ARKIVOC (Gainesville, FL, United States) [online computer file] 2002, 18-29. 

Haeffner, F., Houk, K. N., Reddy, Y. R., Paquette, L. A. Mechanistic Variations and Rate Effects of Alkoxy and Thioalkoxy Substituents on Anionic Oxy-Cope Rearrangements. J. Am. Chem. Soc. 1999, 121, 11880-11884. 

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