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Cycloaddition of Methyl Acrylate with Substituted Pyridinium-3-olates and Pyrazinium-3-olates: 1,3-Dipolar Cycloaddition versus Diels-Alder
* 1 , 2 , 3 , 1 , 4 , 1
1  Department of Chemistry, University of Mauritius, Réduit, Mauritius
2  The School of Chemistry, The University of Manchester, Manchester M13 9PL, UK
3  Departamento de Química Orgánica, Universidad de Valencia, Dr. Moliner 50, 46100 Burjassot, Valencia, Spain
4  School of Chemical Sciences, Universiti Sains Malaysia, 11800 Penang, Malaysia

Abstract: The 1,3-dipolar cycloaddition (1,3-DC) and Diels-Alder (DA) reactions of substituted pyridinium-3-olates and pyrazinium-3-olates with methyl acrylate are studied using DFT method at the B3LYP/6-31G(d) level of theory. The molecular mechanism of the possible stereo- and regiochemical pathways is characterized both in the condensed and solvent phases. It is found that 1,3-DC is the favourable pathway for the reaction of pyridinium-3-olates with methyl acrylate and the thermodynamically more stable 6-substituted 8-azabicyclo[3.2.1]oct-3-en-2-ones are formed preferentially. This research is further extended to substituted pyrazinium-3-olates. Reactions of unsubstituted pyrazinium-3-olate and 1-methyl pyrazinium-3-olate with both ethene and methyl acrylate show a preference for the DA cycloaddition reaction. However, the addition of methyl acrylate to substituted pyrazinium-3-olates results in competitive mechanistic pathways and the preferred cycloaddition pathway is dependent on the location of the methyl group on the pyrazinium ring. The presence of a methyl substituent on C5 of the pyrazinium-3-olate favours the 1,3-DC over the DA reaction as the 2,6 positions are not sterically crowded. However, when methyl groups are present at the C2 and C6 positions, cycloaddition occurs via a Diels-Alder route which is free from encumbrance and hence bond formation at the C5 position is more enhanced.
Keywords: pyridinium-3-olate, pyrazinium-3-olate, methyl acrylate, 1,3-dipolar cycloaddition, Diels-Alder