Enhancing the reaction rates of morita-baylis-hillman reaction in heterocyclic aldehydes by substitutions

The molecular origin of the experimentally observed pronounced difference in the rates of Morita-Baylis-Hillman (MBH) reaction in heterocyclic aldehydes, depending on the position of the formyl group, is investigated herein by using DFT-based mechanistic studies and free energy computations. These calculations are based on the 1,4-diazobicyclo[2.2.2]octane (DABCO)-catalyzed MBH reaction of methyl acrylate with substituted 4- and 5-isoxazolecarbaldehyde, which are slow- and fast-reacting substrates, respectively. As a result of this study, we propose that by tailoring ring substitutions the reactivity of the formyl group for MBH reactions may be enhanced in slow-reacting heterocyclic aldehydes. This proposition is demonstrated by enhancing the rate of the MBH reaction in 4-isoxazolecarbaldehyde more than 10 ⁴-fold by installing an ester substitution at the C-3 position. Similarly, the reactivity of the formyl group towards the MBH reaction in substituted 3-pyrazolecarbaldehyde and pyridinecarbaldehyde is shown to be increased several-fold by a halo substitution. We also confirm that the reasons for different reactivities of heterocyclic aldehydes and the proposed scheme for improving the reaction rates remains valid for all the three mechanisms proposed for the MBH reaction, namely, Hill-Isaacs, McQuade, and Aggarwal.