Cope rearrangement boat geometry

Generally, when 1,5 dicncs arc heated, they isomerire in a [3,3]-sigmatropic rearrangement known as a Cope rearrangement (not to be confused with the Cope elimination process). The mechanism is a simple six-membered pericyclic process, the chair form being the usual transition state. However, in thi.s case the cyclopropyi moiety forces the geometry of the transition state into boat form 28. 

In acyclic systems, Claisen rearrangements show a well-established preference for chair-like transition states. With crotyl propenyl ether, the chair selectivity amounts to 97-98% at 142 C, which corresponds to an approx. 3 kcd mol difference between the free energy of activation (AAG ) of chair and boat TS (equation 26). The preference for a chair-like geometry in the TS is even more pronounced in the Cope rearrangement 99.7% of the 3,4-dimethylhexa-1,5-diene rearranges at 225 C via a chair-like TS, corresponding to a AAG ch ir-boat of -5.7 kcal mol". The latter result closely parallels the difference in energy of the chair and boat conformations of cyclohexane (5-6 kcal mol" )- ... 

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... 

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]. 

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