Cope rearrangement of 1,5-hexadiene

Step through the sequence of stmctures depicting Cope rearrangement of 1,5-hexadiene. Plot energy (vertical axis) vs. the length of either the carbon-carbon bond being formed or that being broken (horizontal axis). Locate the transition state. Measure all CC bond distances at the transition state, and draw a structural formula for it... 

It was pointed out earlier that a Cope rearrangement of 1,5-hexadiene gives 1,5-hexadiene. This is a degenerate Cope rearrangement (p. 1380). Another molecule that undergoes it is bicyclo[5.1.0]octadiene (105). At room temperature the NMR... 

SCHEME 18. van der Waals volume of activation AV and volume of activation calculated for degenerate Cope rearrangement of 1,5-hexadiene... 

The numbering is written by the order i, j written in a bracket. The letter / and j denote the number of atoms across which the o bond migrates. Let us take the case of cope-rearrangement of 1, 5 hexadiene. 

Cope himself formulated this transformation as what would now be called a synchronous pericyclic reaction . This interpretation was supported by Woodward-Hoffmann s analysis of pericyclic processes. The Cope rearrangement of 1,5-hexadiene derivatives was regarded therefore for a long time as a classical example of an allowed pericyclic reaction... 

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. 

Figure 12.6. a) Cope rearrangement of 1,5-hexadiene (6) oxy-Cope rearrangement (c) divinyl-cyclopropane rearrangement (d) degenerate rearrangements of bullvalene. 

A number of compounds are known to rearrange from one structure to an entirely equivalent structure, sometimes with extraordinary facility. Such compounds are said to be fluxional to distinguish them from tautomers (which usually involve rearrangements between nonequivalent structures). Simple examples are 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 ... 

McGuire, M. J. Piecuch, P. Balancing dynamic and nondynamic correlation for diradical and aromatic transition states a renormalized coupled-cluster study of the Cope rearrangement of 1,5-hexadiene, J. Am. Chem. Soc. 2005,127, 2608-2614. 

The Marcus equation provides a nice conceptual tool for understanding trends in reactivity. Consider for example the degenerate Cope rearrangement of 1,5-hexadiene and the ring-opening of Dewar benzene (bicyclo-[2,2,0]hexa-2,5-diene) to benzene. Figure 15.29. The experimentally observed activation energies are 34 kcahmol and 23 kcal/mol, respectively. The Cope reaction is an example of a Woodward-Hoffmann... 

Again this averaging procedure can only be expected to work when the reactions are sufficiently similar . This is difficult to quantify a priori. The Marcus equation is therefore more a conceptual tool for explaining trends, than for deriving quantitative results. C Figure 15.30 MOJ diagi Bond formation 1 am for the Cope rearrangement of 1,5-hexadiene ... 

Sakai, S. Theoretical analysis of the Cope rearrangement of 1,5-hexadiene and phenyl derivatives. THEOCHEM 2002, 583, 181-188. 

This problem was intensively studied both experimentally and theoretically. The quantum chemical calculations were carried out using various methods at different levels. The earlier calculations for the Cope reanangement based on a CASSCF wave function for six electrons in the bonds rearranged were found to overestimate the diradical character of the wave funclion- --. More recently, MP2 methods for the multireference wave function have been developed whose application to an estimate of the energy of the chair transition state has been described. AMI calculations of altemative transition states for the Cope rearrangement of 1,5-hexadiene derivatives have been discussed by Dewar and colleagues i -217... 

The first is that the chair TS for the Cope rearrangement of 1,5-hexadiene should resemble more closely the lower energy of the two diradical extremes, A and C. The second prediction is that to the extent structures A and C contribute to the Cope TS, radical stabilizing substituents should be capable of lowering the energy of the TS, relative to the reactants, and thus accelerating the Cope rearrangement. Substiments at C2 and C5 of 1,5-hexadiene should stabilize structure A in Fig. 30.1, and substituents placed at Cl, C3, C4, and C6 should stabilize structure C. 

It now seems hard to believe that, 20 years ago, (6/6)CASSCF/3-21G calculations were beyond the capabilities of the computers available to most computational chemists. Nevertheless, in the first ab initio study of the Cope rearrangement, published in 1984, (6/6)CASSCF/3-21G calculations could not actually be performed they could only be simulated [14]. Smaller MCSCF calculations were employed to obtain a partially optimized set of orbitals. Then those orbitals were used to carry out full Cl calculations for the 52 configurations that comprise the complete (6/6) active space at the C h stationary point for the chair Cope rearrangement of 1,5-hexadiene. 

Table 30.2 Dissection of the effects of phenyl substituents on lowering the energy of the TS for the chair Cope rearrangement of 1,5-hexadiene. Energies (kcal/mol) were obtained from B3LYP/6-31G calculations at different interallylic distances (R) in the maimer described in the text...

The coordinated alkene or alkyne ligand can be attacked by other alkene or alkyne molecule to accomplish some metal-catalyzed synthetically useful transformations. Typical examples include dimerization and polymerization of alkenes catalyzed by highly electrophilic cations [PdL2(MeCN)2] + (L = MeCN, PR3) (e.g. Scheme 8.35) [57], and Cope rearrangement of 1,5-hexadiene derivatives catalyzed by PdCl2 (Scheme 8.36) [58], It was proposed that the key step in these reactions was the C-C bond formation via attack of the external alkene at the alkene carbon which was made highly electron-dehcientby coordination to Pd(II) ion. 

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