ENERGETIC AND RATE EFFECTS OF THE TRIFLUOROMETHYL GROUP AT C-2 AND C-4 ON THE ALIPHATIC CLAISEN REARRANGEMENT

The rate of the Claisen rearrangement is accelerated by a factor of 73 over the parent system when a trifluoromethyl group is present at C-2 of allyl vinyl ether. Ground-state destabilization by the trifluoromethyl group may be responsible for this rate effect. There is little polar character in the transition state, and the transition-state structure has little carbonyl character and only moderate (ca. 1/3) bonding character between the two terminal carbons. The rate enhancement is not observed in the Cope rearrangement of the all-carbon analogue that has a trifluoromethyl group at C-2. At C-4, the trifluoromethyl group does not bring about a significant rate effect in the Claisen rearrangement relative to the parent system; this result is in contrast to an energetic benefit of 3.5 kcal/mol enjoyed by the system when a cyano group is at C-4, which suggests that radical-stabilizing ability and not electron-withdrawing ability is important in stabilizing the transition state. © 1990, American Chemical Society. All rights reserved.