Cope rearrangement boat transition structure

Suprafacial-suprafacial boat transition structure for the Cope rearrangement. 

Products of Cope rearrangement of racemic (top) and meso (bottom) diastereomers of 3,4-dimethyl-1,5-hexadiene through boat transition structures. 

Two minor processes sometimes operate competitively with that illustrated in the scheme. One of these involves 1,4-addition of the second vinyl anion to give a reactive intermediate that differs structurally from 1, but is capable of setting into motion a closely related sequence of chemical events leading to an isomeric diquinane.4 This is the route followed to produce the minor product characterized here. The other option consists of cis-t, 2-addition, an event that is followed by a dianionic oxy-Cope rearrangement via a boat-like transition state.4 When sufficient substitution is present to allow the installation of multiple stereogenic centers, the adoption of this pathway is easily distinguished from the electrocyclic alternative since a cis relationship between relevant substituents is in place, instead of the trans arrangement required by the electrocyclization cascade. 

It is well established that steric effects hinder the Cope rearrangement of divinylcyclopropanes. An interesting example of this steric effect is seen in the reaction of 33 with cis- and trans-l-acetoxy-butadiene (Scheme 13). ° The reaction of 33 with trans-1-acetoxy-l, 3-butadiene leads cleanly to the [3+4] annulation product 34 in 67% yield. In contrast, the product from the reaction of 33 with c/j-l-ace-toxy- 1,3-butadiene is the cw-divinylcyclopropane 35 (80% yield), and high temperatures (220 °C) are required to convert 35 to the [3+4] annulation product 36. The effect of alkene geometry on the stereochemistry and the rate of reaction is readily explained by considering the boat transition state for the Cope rearrangement of divinylcyclo-propanes (structure 37). A trans diene substituent (Y) would generate a trans product (34), whereas a cis substituent (X) would lead to a cis... 

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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