Chameleonic transition states

Chapter 30 - Ab initio and DFT calculations on the Cope rearrangement, a reaction with a chameleonic transition state, Pages 859-873, Weston Thatcher Borden... 

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... 

In a related study, Borden and coworkers have examined whether other pericyclic reactions might express chameleonic behavior. Using B3LYP/6-31G calculations, they located transition states for the 1,5-hydrogen shift in phenyl-substituted 1,3-pentadienes (21-23). The activation enthalpy for the... 

Borden concludes that 1,5-hydrogen migration is not chameleonic. Rather than having a shift of the dominant resonance contributor with substitution as occurs in the Cope rearrangement, regardless of substitution, the transition state for the 1,5-H... 

Although, as stated above, olefin epoxidation is commonly referred to as an electrophilic oxidation, recent theoretical calculations suggest that the electronic character of the oxygen transfer step needs to be considered to fully understand the mechanism [451]. The electronic character, that is, whether the oxidant acts as an electrophile or a nucleophile is studied by charge decomposition analysis (CDA) [452,453]. This analysis is a quantitative interpretation of the Dewar-Chatt-Dimcanson model and evaluates the relative importance of the orbital interactions between the olefin (donor) and the oxidant (acceptor) and vice versa [451]. For example, dimethyldioxirane (DMD) is described as a chameleon oxidant because in the oxidations of acrolein and acrylonitrile, it acts as a nucleophile [454]. In most cases though, epoxidation with peroxides occurs predominantly by electron donation from the 7t orbital of the olefin into the a orbital of the 0-0 bond in the transition state [455,456] (Fig. 1.10), so the oxidation is justifiably called an electrophilic process. 

Chameleonic features of carbenes can be further amplified by complexation with transition metals (Figure 5.42). In complexes with low valent/low oxidation state late transition metals (Fischer carbenes), carbenes display electrophilic properties, and often behave similarly to a carbonyl compound. Such carbenes also often have p-donor substituents, such as-OR or-NR, on the carbene carbon and x-acceptor ligands at the metal. In contrast, carbene complexes with high valent/high oxidation state early transition metals (Schrock carbenes) are nucleophilic. The ability of metal in the Schrock carbenes is further enhanced by donor ligands. 

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