Role of base assisted proton transfer in N-heterocyclic carbene-catalyzed intermolecular Stetter reaction

The mechanism of the NHC-catalyzed intermolecular Stetter reaction between benzaldehyde and cyclopropene has been investigated using the PCM-M062X/6-311++G(3df,2p)//M062X/6-31+G(d,p) level of DFT. Compared to the direct reaction, a substantial reduction in the activation free energy by 10.6-14.4 kcal/mol is observed when the reaction is performed in the presence of water, 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD), and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU). The bases promote the proton transfer step of the reaction to yield the Breslow intermediate. An early concerted transition state has been located for the stereocontrolling C-C bond formation step (Delta G(#) = 26.6 kcal/mol) which is used to explain the diastereomeric ratio observed in the experiment. (C) 2013 Elsevier Ltd. All rights reserved.