A competitive Diels-Alder/1, 3-dipolar cycloaddition reaction of1-H-imidazole 3-oxide toward sulfonyl methane. A DFT study on the energetic and regioselectivity

Document Type: Research Paper

Author

Chemistry Department, Shahrood Branch, Islamic Azad University, Shahrood, Iran

Abstract

The dual diene/1,3-dipolar character of 1-H-imidazole 3-oxide, HIO 1, allows this compound to
participate in a competitive Diels-Alder (DA)/1,3-dipolar cycloaddition (13DC) reaction toward C=S
double bond of the electro-deficient sulfonyl methane SFM 2. The B3LYP/6-311++G(d,p) calculated
relative Gibbs free energies indicate that among the studied 13DC and DA reactions, former is
completely preferred over the latter in the gas phase as well as in the presence of dichloromethane
(DCM). This 13DC reaction takes place through a one-step mechanism to generate corresponding
five-membered cycloadduct via a quite C1-S5 regioselective fashion which is controlled by
theHOMO(HIO 1)- LUMO(SFM 2) interaction.The aromatic character of HIO 1 which is lost along the
studied cycloaddition reaction is responsible for the relatively high activation Gibbs free energies and
the endergonic character of these thermodynamically reversible reactions. The quite C1-S5
regioselectivity, resulted from energetic studies, in the investigated 13DC reactioncan be explained
usingcalculated electrophilic and nucleophilic Parr functions at the reactive sites of reagents.

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