A theoretical model study on the cyclic reaction of 4-hydroxybutanal catalyzed by Brønsted acid

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Abstract
For a theoretical model study on the cyclic reaction of 4-hydroxybutanal (4-OH-BL), we have examined five assumed reaction pathways (I–V) by performing the B3LYP calculations in the gas phase and self-consistent isodensity polarized continuum model (SCIPCM)-B3LYP calculations in aqueous solution. Pathways II (4-OH-BL + H+), III (4-OH-BL + H3O+), and IV (4-OH-BL + H3O+ + H2O) represent three models for the cyclic reaction catalyzed by Brønsted acids. The present study leads to the following conclusions concerning the five pathways (mainly on the basis of the calculation results in the solution). The high barrier along pathway I (with no catalyst) implies that the reaction does not occur without a catalyst, and the extremely large stabilization energy of the intermediate implies that pathway II is not a realistic model for the reaction catalyzed by Brønsted acid. Along pathway III, there are two intermediates and a transition state in between, and they are 10–16 kcal/mol lower in energy than the reactants (4-OH-BL + H3O+). Along pathway IV, there is only one intermediate, and it is 20.6 kcal/mol lower in energy than the reactants (4-OH-BL + H3O+ + H2O). Pathways III and IV are predicted to be feasible. Energetically, pathway IV is more favourable than pathway III and it is considered as a rational model for the cyclic reaction of 4-OH-BL catalyzed by Brønsted acid. Our calculations for pathway V (catalyzed by H2O) indicate that the water molecule may also serve as a catalyst for the cyclic reaction. The transition state along pathway V is 20.0 kcal/mol higher in energy than the reactants (4-OH-BL + H2O), and one can clearly see the “proton wire” in its structure. Our calculations show strong solvent effects on energetics of the charged intermediates along pathways II, III, and IV.