Insights into the Morita–Baylis–Hillman reaction of isomeric dibenzofuran carbaldehydes: a theoretical and mass spectral study

Abstract

We herein report the faster Morita–Baylis–Hillman (MBH) reaction of dibenzofuran-4-carbaldehyde (2) than that of its isomer, dibenzofuran-2-carbaldehyde (1), with different activated olefins in the presence of DABCO as the base catalyst. We observed that there is no significant effect of the solvent (methanol) on the reaction rates. In situ mass spectrometry experiments and computational studies were applied to understand the role of the reaction intermediates and their structure implications. MS data revealed that the zwitterionic intermediate obtained from 2 is more stable than that obtained from 1. Computational studies were performed for the gas as well as solvent phase reactions at the mPW1K/6-31 + G(d,p) level. In accordance with the experimental results, aldehyde 2 is found to be more reactive compared to 1. The results are in accordance with McQuade's proposal of the MBH mechanism, wherein the second equivalent of aldehyde plays a key role in the proton migration step during the course of the reaction in the absence of methanol solvent.