Cope rearrangement conjugating substituents

Substituent effects provide other insights into the nature of the TS for the Cope rearrangement. Conjugated substituents at C(2), C(3), C(4), or C(5) accelerate the reaction. Donor substituents at C(2) and C(3) have an accelerating effect. The net effect on the reaction rate of any substituent is determined by the relative stabilization of the TS and ground state. The effect of substituents on the stabilization of the TS can be analyzed by considering their effect on two interacting allyl systems. We consider the case of phenyl substituents in detail. 

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. 

Cookson has studied substrates that can, in principle, effect a tandem Claisen-Cope-Cope rearrangement. By employing 1,1,3-trimethoxybutane as a surrogate for 1-methoxy-1,3-butadiene (equation 10), allylic alcohol (62) undergoes acid-catalyzed exchange to provide the transient p,y-unsaturated aldehyde (66) which, lacking a substituent at the a-position, suffers a facile conjugation of the double bond that preempts the tandem process. This phenomenon has been also observed by Thomas. ... 

Another important factor, which shifts the equilibrium of the Cope rearrangement to the product side to a large extent, is a conjugation effect with rc-substituents such as carbonyl, cyano, or phenyl groups, e,g, 5701. 6702, and 7703. 

The absence of a substituent a to the carbonyl group in such systems does not necessarily preclude a successful Claisen-Cope reaction sequence. Thus, reaction of allylic alcohol 50 with acetal 51 produces the Claisen product 53. Double-bond isomerization of 53 to give a,/J-unsaturated aldehyde 54 is not observed. Instead, 53 is transformed to Cope product 57, probably via a conjugation-deconjugation-Cope rearrangement sequence1162. 

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