Cope rearrangement comparisons

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. 

A kinetic study of the Cope rearrangement of 2-(trifluoromethyl)oeta-l.5-diene [(Z)-7] showed no rate acceleration in comparison with the rearrangement of unsubsliluled hexa-1,5-diene. Equilibrium was reached after 7 days at 207 C (K = 3.46 favoring the is-isomer). at which stage no intermediate 4-ethylhcxa-l,5-diene 8 was observed.6 In contrast, a trifluoromethyl group at position 2 does accelerate the Claisen rearrangement (sec Section 5.1.5.2.). 

Aitken et al. have also oxidized bicyclic sulfone 13 (performic acid 39% yield). Flash vacuum pyrolysis of epoxide 14 (580 °C) gives a 55% yield of 4,5-dihydrooxepine, identified by comparison of its H and NMR data with the literature values. The dihydrooxepine presumably results from initial extrusion of SO2 to give t-2,3-divinyloxirane, which then undergoes a Cope rearrangement (Scheme 7) <1999J(P1)605>. 

From a synthetic viewpoint, a comparison of the thermolyses of the epimers (92) and (94) (Schemes 12 and 13) illustrates the point, previously mentioned, that, in certain cases in which alternative modes of rearrangement are possible, the stereoselective formation of the cts-divinylcyclopropane substrate is important. Thus, while thermal rearrangement of the cis substrate (92) provi s the Cope rearrangement product (98) in reasonable yield, thermolysis of the trans isomer (94) does not. 

Fig. 10.39. Comparison of transition structure geometry for anionic oxy-Cope (right) rearrangement with Cope rearrangement (left) showing atom separation distances and Mulliken charges (B3LYP/6-31IG ). Reproduced from Helv. Chim. Acta, 84, 124 (2001), by permission of Wiley-VCH.

Aza-Cope rearrangement. The rearrangement of Af-alkyl-A-allylanilines is promoted by Lewis acids. In comparison with ZnCE, the catalytic effect of BFi OEti is superior. 

B. Comparison of Transition States for Homo-Cope Rearrangements. 333... 

Cope rearrangements of 2,6-disubstituted bicyclo[5,l,0]octa-2,5-dienes have been studied to determine the extent of bond making and bond breaking at the transition state. The interpretation of the results was complicated by the enforced boat-transition state, but comparison with polycyclic systems supports the hypothesis that, unlike Cope rearrangements in acyclic dienes, bond cleavage is at least synchronous with bond formation. ... 

Similar discussion is possible with respect to the transition state of the Claisen and Cope rearrangements i ). These can be treated similarly. Fig. 7.29a indicates that the symmetry of SO MO suggests ds-cis interaction with the six-membered structure for the transition state, but the chair-boat selectivity is not determined by the SO-SO interaction. The overlapping of LU and HO plays a secondary role. Fig. 7.29 shows that the boat form is unfavourable in comparison with chair form on account of the different signs of LU and HO at the central carbons. Similar consideration is possible with respect to the extended Cope rearrangement (Fig. 7.29.b). The predominance of thechair-form transition state is known both in the Claisen i f and the Cope rearrangements... 

A consequence of these substitution dependent rates can be seen in the comparison of the attempted anionic oxy-Cope rearrangement of 109 and 111. While 109 proceeds efficiently to give the Cope product 110, 111 appears to decompose via an intramolecular backside opening of the neighboring oxetane ring to give 112. 

J. F. M. Oth, K. Mullen, J.-M. Giles, and G. Schroder, Comparison of C- and H-magnetic resonance spectroscopy of technique for the quantitative investigation of dynamic processes. The Cope rearrangement in bullvalene, Helv. Chim. Acta 57 (1974) 1415. 

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]. 

As a means of transferring the site of asymmetry the Claisen-Cope rearrangement is without comparison. Thus in the rearrangement of allyl vinyl ether (123) the original stereogenic centre is destroyed but two new centres are formed with retention of the overall chirality of the molecule to give (124).(53]... 

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