For the Cope rearrangement

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. ... 

The required temperatures for the Cope rearrangement are generally lower, if the starting material contains a substituent at C-3 or C-4 which can form a conjugated system with one of the new double bonds. 

Trifluoromethyl imidates show similar reactivity.262 Imidate rearrangements are catalyzed by palladium salts.263 The mechanism is presumably similar to that for the Cope rearrangement (see p. 555). 

Following the suggestion that donor-acceptor (Dewar-Hoffmann) semi-bullvalenes [83a] would have a lower activation barrier for the Cope rearrangement, or even a homoaromatic ground state (Hoffmann and Stohrer, 1971 Dewar and Lo, 1971), numerous syntheses and studies of appropriately substituted semibullvalenes have been reported. In fact, this aspect of the search for homoaromatic semibullvalenes has been the most extensively investigated (for a partial summary of this work see Quast et al., 1985 Gompper et al., 1988, and references cited therein). 

One of the early examples demonstrating the efficiency of this chemistry was the [3+4] cycloaddition reaction of diazoglutaconate 42 with cyclopentadiene, in which the endo-isomer 43 is exclusively formed in 98% yield (Eq. 5) [73]. The intermediacy of a cis-divinylcyclopropane is consistent with the stereochemical outcome because it would rearrange to the endo-product. Indeed in the case of more highly functionalized vinyl-diazoacetates, the ds-divinylcyclopropane was isolable, in which elevated temperatures were required for the Cope rearrangement [73]. 

The vinylcarbenoid [3-1-4] cycloaddition was applicable to the short stereoselective synthesis of ( )-tremulenolide A 73 and ( )-tremulenediol A 74 (Scheme 14.7) [81]. The key step is the cyclopropanation between the cyclic vinyldiazoacetate 69 and the functionalized diene 70, which occurs selectively at the ds-double bond in 70. Because of the crowded transition state for the Cope rearrangement of the divinylcyclopropane 71, forcing conditions are required to form the fused cycloheptadiene 72. The stereo-... 

Orbital Interaction Analysis. An orbital interaction diagram for the Cope rearrangement is shown in Figure 12.7a. The reaction may be initiated by electron donation from... 

Predict the relative rates for the Cope rearrangement of bullvalene A, barbaralone B, protonated barbaralone B, barbaralane C and octamethylsemibullvalene D. 

Figure 6. Logarithmic plot values of LUm/fc versus pressure for the Cope rearrangement of bullvalene (torr at the experimental temperature of 356 K). Experimental values are signified by solid circles. Pressures are the total sample pressure at 356 K. Errors in frUni/fc are reported to 2o. The solid (upper) line represents the values calculated from RRKM theory using the biradicaloid transition state model. The lower line represents calculated rate constants using the aromatic transition-state model. The collision diameter was 3.6 A in both cases.

The unhomogeneous composition of the products generated by the photochemical reaction is due to another mechanism. While the thermal isomerization of 1,5-dienes proceeds via a cyclic transition state in a synchronous sense, the photochemically induced transformation causes a reorientation of the allyl radicals generated from the educts. Warming up the reaction mixture to 100°C activates a complete transfer from 4c to 5c) of all isomers. This step may be explained by a radical CC bond split of the 1,2-diphenylethylene unit. Since the isomerization of the diastereomeric compound 4c to 5c is activated at much lower temperatures than for the Cope rearrangement (from 3c to 4c), it is clear that the thermal transfer exclusively forms the twofold changed product. 

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 ... 

Anytime a wavefunction may have diradical character, care must be taken to insure that the wavefunction is stable with respect to lifting the requirement of spin restriction. This can be simply tested by examining whether an unrestricted wavefunction is lower in energy. For the Cope rearrangement, the... 

---