Cope rearrangement chair transition structure

Suprafacial-suprafacial chair transition structure for the Cope rearrangement. 

Products of the Cope rearrangement of racemic (top) and meso (bottom) diastereomers of 3,4-di-methyl-1,5-hexadiene through chair transition structures. 

Because of the dispute over these questions, a large number of different methods have been used for the investigation of the Cope rearrangement the results from some of the most important aie summarized in Table 9. For the C2 -symmetric chair transition structure, most methods predict an aromatic character with C1-C6 bond lengths ranging from 1.93 to 2.19 A. The aromatic character of the transition structures was also confirmed by calculations of the magnetic properties of the transition structure. In contrast, the perturbation... 

The X-ray crystal structure for AZ-28 has a variety of structural features that are consistent with the proposed mechanism operative for the oxy-Cope rearrangement. The antibody binds the transition stage analog in a chair-like conformation, consistent with the preferred chair transition state for this pericyclic reaction (Doering and Roth, 1962). The positions of the C-2 and C-5 atoms are fixed in the antibody-bound hapten molecule in a similar fashion, the C-2 and C-5 positions in the hexadiene substrate should be held in a fixed position by conserved van der Waals interactions locking in the two phenyl substituents in the antibody combining site. This bound conformation of the acyclic (47T + 2er) system of the hexadiene substrate should enforce a molecular conformation close to the transition state for the rearrangement reaction, consistent with the catalysis observed for AZ-28. 

Fig. 30.1. Schematic depiction of the bonding in the transition structure (TS) for the chair Cope rearrangement, showing the diradical resonance contributors (A and C) and the aromatic representation (B). R is the interaUylic distance.

The loss of rotational freedom when the cyclic transition state is formed from the acyclic starting material accounts for the observed negative entropies and volumes of activation. It has been established that the transition state of the closely related Cope rearrangement (see below) resembles the chair conformation of cyclohexane. An analogous transition state structure for the Claisen rearrangement would account for the fact that isomerization of vinyl a-methylallyl ether yields 97% trans 4-hexQm and 3% cw-4-hexenal, since there should be a preference for an equatorial orientation of the methyl group in a cyclohexane chair-like transition state . 

An antarafacial [3,3] sigmatropic rearrangement was proposed for one reaction by Miyashi, T. Nitta, M. Mukai, T. ]. Am. Chem. Soc. 1971, 93, 3441, but an alternative explanation for the experimental results was advanced by Baldwin, J. E. Kaplan, M. S. /. Am. Chem. Soc. 1971,93,3969. Even more complicated pathways can be imagined. Hansen, H.-l Schmid, H. Tetrahedron 1974, 1959, considered seven possible transition structures for the Cope rearrangement, including boat and chair suprafacial-suprafacial, twist, cross, and plane antarafacial-antarafacial transition structures, and one anchor antarafacial-suprafacial transition structure. 

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... 

In cyclic systems, however, conformational constraints can override the inherent preference for chairlike transition states in Cope as well as Claisen rearrangements and lead to a partial involvement if not a dominance of boat-like TS structures. In the Ireland rearrangement of lactones of type (247), for example, chair-like transition state (249) is accessible only when the diaxial bridging methylene chain becomes sufficient in length (n = 7, Scheme 44). The preference of boat-like transition state (250) over (251) is due to a serious A - -type interaction between the endocyclic oxygen atom and pseudoaxial substituent R in (251). 

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