Organocatalytic claisen rearrangement: theory and experiment.

A combined computational and experimental study on the Claisen rearrangement of a 2-alkoxycarbonyl-substituted allyl vinyl ether in the presence of thioureas as potential noncovalent organocatalysts has been performed. DFT calculations employing different basis sets were utilized to predict a catalytic cycle for the thiourea-catalyzed Claisen rearrangement. The nature of the transition state in the presence and absence of thioureas was studied in detail. Critical geometrical data of the transition state that are indicators for the relative barrier height of the Claisen rearrangement are discussed. Although we did observe a significant transition state stabilization, due to endergonic conformational changes and endergonic complexation the overall effect on the barrier is small, in accordance with experimental results.