Chameleons and Centaurs

Doering examined the Cope rearrangements of l,3,5-triphenylhexa-l,5-diene  

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. 

The calculated activation enthalpies for the Cope rearrangements of various cyano- and phenyl-substituted 1,5-hexadienes were calculated at B3LYP/6-31G and are listed in Table 4.8. 

4-disubstituted cases are degenerate, so no averaging is needed. The chameleonic model suggests that the substituent effects should be additive, or a predicted value of A/ = 31.6 kcal mol for l,4-dicyauo-l,5-hexadiene and AH = 31.4 kcal mol for l,4-diphenyl-l,5-hexadiene. The computed barriers are actually lower thau these values. The barrier for the 1,3,4,6-tetrasubstituted 

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 

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Polo V, Andres J (2005) A joint study based on the electron localization function and catastrophe theory of the chameleonic and centauric models for the cope rearrangement of 1,5-hexadiene and its cyano derivatives. J Comput Chem 26 1427-1437... 

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