Rearrangements, Cope states

Fig. 7.29a. The transition state in the Cope rearrangements. Cope rearrangement...

FIGURE 4 Ectocarpene as the product of a [3.3]-sigmatropic rearrangement. The fatty acid accommodates to the active center of the enzyme in a U-shaped fashion. Decarboxylation in conjunction with loss of the C(8) HR hydrogen atom yields, after cyclization between C(4) and C(6) of the precursor, the thermolabile (lS,2R)-cyclopropane. A subsequent spontaneous [3.3]-sigmatropic rearrangement (Cope rearrangement) proceeds via the cis-endo transition state and yields (6S )-ectocarpene. 

Fig. 4.30 The interaction between two allyl SOMOs in the Cope-rearrangement transition state...

This reaction is an example of a 1,3-shift that is suprafacial for both components and involves two 7i-systems, each with 3 electrons. The MOs of 1,5-hexadiene and the Cope-rearrangement transition state show the reacting orbitals. Table 4.3 gives the Woodward-Hoffmann rules for sigmatropic rearrangements between 7C-systems with I and / electrons. 

Fig. 6.7 Cope rearrangement transition state of Cope rearrangement of cw-dipropenyl cyclopropane generated with AMI method...

The Cope rearrangement usually proceeds through a chairlike transition state. The stereochemical features of the reaction can usually be predicted and analyzed on the basis of a chair transition state that minimizes steric interactions between the substituents. Thus, compound 26 reacts primarily ttuough transition state 27a to give 28 as the major product. Minor product 29 is formed flirough the less sterically favorable transition state 27b. 

There is a second possible transition state for the Cope rearrangement in which the transition state adopts a boatlike geometry ... 

The stereochemical features of the Claisen rearrangement are very similar to those described for the Cope rearrangement, and reliable stereochemical predictions can be made on the basis of the preference for a chairlike transition state. The major product has the -configuration at the newly formed double bond because of the preference for placing the larger substituent in the pseudoequatorial position in the transition state. ... 

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... 

Reactions that proceed via cyclic transition states Claisen (18-33) and Cope (18-32) rearrangements. 

As we have indicated with our arrows, the mechanism of the uncatalyzed Cope rearrangement is a simple six-centered pericyclic process. Since the mechanism is so simple, it has been possible to study some rather subtle points, among them the question of whether the six-membered transition state is in the boat or the chair form. ° For the case of 3,4-dimethyl-l,5-hexadiene it was demonstrated conclusively that the transition state is in the chair form. This was shown by the stereospecific nature of the reaction The meso isomer gave the cis-trans product, while the ( ) compound gave the trans-trans diene. If the transition state is in the chair form (e.g., taking the meso isomer), one methyl must be axial and the other equatorial and the product must be the cis-trans alkene ... 

The predictions of the reactivities by the geminal bond participation have been confirmed by the bond model analysis [103-105] of the transition states and the calculations of the enthalpies of activation AH of the Diels-Alder reaction [94], the Cope rearrangement [95], the sigmatropic rearrangement [96], the Alder ene reaction [100], and the aldol reaction [101] as are illustrated by the reactions of the methyl silyl derivatives in Scheme 38 [102], The bond is more electron donating than the bond. A silyl group at the Z-position enhances the reactivity. 

The next homolog, 1,5-hexadiene (1,5-HD), is of special chemical interest because the molecule is capable of undergoing the so-called Cope rearrangement. A GED study of 1,5-HD was also recently reported6. Because of the increased conformational complexity of this molecule compared to that of 1,4-PD, the structural details of the various con-formers could not be resolved and only averaged structure parameters were determined from the gas phase. Molecules in the solid state are frozen, mostly in only one conformation, which may but must not represent the conformational ground state. Therefore, conformational isomerization is usually not discussed with X-ray structures presented in the literature. 

Fig. 16 (a) Comparison of potential energy profile for the formal Cope rearrangement of 3,4-difluorohexa-l,5-diyne-3-ene with that of (Z)-hexa-l,5-diyne-3-ene, (b) Rehybridization in the C(F) bond along the reaction path. EDI = 3,4-difluoro-hex- 3-ene-l,5-diyne ED2 = 1,6-di-fluoro-hex-3-ene-l,5-diyne BZY = difluoro-l,4-didehydrobenzezne TSBC = the transition state for the Bergman cyclization TSRBC = the transition state for the retro Bergman cyclization. 

Originally very few types of such rearrangements were known e.g., Copes rearrangement, Claisen rearrangement and some 1, 5 hydrogen shift in some dienes, but now many others have been discovered. The common feature of such reactions is that they are concerted, uncatalysed and involve a bond migration through a cyclic transition state. 

A common example of Cope rearrangement involving [3, 3] sigmatropic rearrangement in a 1, 5 diene is the pyrolysis of meso 3, 4 dimethyl hexa-1, 5 diene giving exclusively cis, trans isomer of 2, 6 octadiene. The process involves a six electron transition state which has a chair like configuration and substituents occupy equatorial sites. 

Thus a Cope rearrangement proceeding through a boat-like transition state is the rearrangement of cis 1, 2 divinyl cyclo-propane. 

There are few examples of other allowed sigmatropic shifts involving six ten electron transition states, but these are not common reactions. A [3, 4] shift is observed in competition with a [1, 2] shift in cations derived from cyclohexane diols. This is a cationic equivalent of the Cope rearrangement,... 

Given that the boat transition state 8 is unfavourable, it is at first sight surprising that the Cope rearrangements of bullvalene (14), barbaralane (15), and semibullvalene (16) should take place so readily given that the transition states (17) of these reactions are derivatives of 8. We therefore decided 3S-) to calcu-... 

This is perhaps the area where there is the most optimism of attaining the elusive goal of neutral homoaromaticity. It has been suggested that semibullvalene [83], which undergoes degenerate Cope rearrangements through a homoaromatic transition state [83b] (2) with extremely low... 

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