Mechanism of N-heterocylic carbene-catalyzed annulation of allenals with chalcones to 3-pyrancarbaldehydes or cyclopentene

The reaction mechanisms of N-heterocylic carbene (NHC)-catalyzed annulation to 3-pyrancarbaldehydes or cyclopentene were theoretically analyzed. Different from what has been reported for normal aldehydes or enals, our calculation results revealed that the intermediate results from the conjugate addition to the central carbon atom of the allene moiety, instead of forming the Breslow intermediate. The next step, 1,4-addition of the chalcone, resulted in the formation of an enolate anion intermediate. Then, intramolecular conjugate addition and elimination of the NHC catalyst afforded the final product. In addition, the computed results showed good agreement with the experimental evidence, suggesting that both the existence of an allene structure and charge distribution play a crucial role in the outcome of chemoselectivity and involvement of operative intermediates in the reaction pathway.