Cope rearrangement 1,5-hexadiene, geometry

All three models show broadly similar behavior. Errors associated with replacement of exacf reactant and transition-state geometries by AMI geometries are typically on the order of 2-3 kcal/mol, although there are cases where much larger errors are observed. In addition, AMI calculations failed to locate a reasonable transition state for one of the reactions in the set, the Cope rearrangement of 1,5-hexadiene. 

The Cope and Claisen rearrangements are markedly similar reactions, although they differ in thermodynamic driving force. Whereas the Cope rearrangement of 1,5-hexadiene is thermoneutral (reactant and product are the same), the analogous Claisen rearrangement of allyl vinyl ether is exothermic. Do thermodynamic differences lead to differences in transition state geometries ... 

Table 8. Transition structure geometries, activation energies, and reaction energies for transition structures of Cope rearrangement of 1,5-hexadiene...

The implication of the multiple possible reaction pathways shown in Scheme 4.6 is that any computational approach must allow for the possible contribution of at least these three valence bond structures. " The simplest approach to the nature of the wavefunction for the Cope rearrangement is to just account for the correlation of the active orbitals of the reactants with those of the products. The o-bond between C3 and C4 of the reactant correlates to a(Ci-C ) in the product. Assuming that 1,5-hexadiene has C2 symmetry, both of these orbitals have a synunetry. The in-phase mixing of the two jc-bonds of the reactant (it(Cx-C2)-l-Jc(C5-Cg)) has b synunetry and correlates with (jc(C2-C3)-l-Jt(C4-C5)) of the product. The out-of-phase combination of the reactant Jc-bonds (it(Ci-C2) - it(C5-Cg)) has a synunetry and correlates with (jc(C2-C3) - Jc(C4-C5)) of the product. If the reaction proceeds through a C211 geometry, orbital symmetry demands that these active orbitals of must become Ug aJbJ. So, we may take as the aromatic ... 

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... 

A and at the equilibrium geometry of the diene, minus the difference between the energies of unsubstituted 1,5-hexadiene at the TS for its Cope rearrangement and at its equilibrium geometry. 

Table 30.2 shows that at/f = 1.599 A the C2 phenyl group in 2-phenyl-1,5-hexadiene provides a net stabilization of = —2.9 kcal/mol, and atR = 2.218 A the Cl and C3 phenyl groups in 1,3-diphenyl-1,5-pentadiene provide a net stabilization of = —2.0 kcal/mol. However, the TS for Cope rearrangement of 1,3,5-triphenyl-1,5-pentadiene occurs with interaUyhc bond lengths of about R = 2.110 A (Table 30.1). At this TS geometry, neither the phenyl groups at Cl and C3 nor the phenyl group C5 provides as much stabilization as these phenyl groups furnish in the TSs for the Cope rearrangements of, respectively, 1,3-diphenyl-1,5-pentadiene at / = 2.218 A and...

Analysis of the product ratio from chair and boat TS geometry from a Cope rearrangement of deuterated 1,5-hexadiene indicated that the boat TS is about 6 kcal/mol less stable than the chair TS [35]. It is reflected in the Cope rearrangement of cyclic dienes 59 and 60. Comparison of their reaction rates showed that diene 59 reacted faster by a factor of 18,000. This fact can be rationalized by considering their TS. Compound 59 reacts through a chair-like TS while 60 through a boat-like TS. The chair-like TS has lower activation energy and hence 59 reacts much faster [36]. 

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