Activation of 1,2-diazole as a diene for Diels-Alder reaction

Activation of 1,2-diazole as a diene for Diels-Alder reaction 

A more reliable study of the 1,2-diazole tautomer reactivity as a diene for the Diels-Alder reaction was carried out by the MP2/6-31+G(d) theory model. The predicted order of reactivity was similar to the one generated by the AMI study [2H]-1,2-diazole (0.0 kcal/mol) [4H]-1,2-diazole (-24.3 kcal/mol), and [3LT1-1,2-diazole (-29.1 kcal/mol). Because the reaction is LUMO diene controlled by 

BOx-Y = bond order between atoms X and Y in the heterocycle ring SBO = sum of ring bond orders ABO = average bond order BOD = sum of bond order deviation from the average ring bond order 

Furthermore, protonation of [4//]-1,2-diazole should improve its reactivity. This was demonstrated by a considerably lower electron density on the aromatic ring of protonated (SBO = 5.507, Table 45) in comparison with unprotonated [4H]-1,2-diazole (SBO = 6.729, Table 45). 

The interactions in two isomeric transition state structures must he also responsible for change of the aromaticity of the six-memhered ring of two isomeric transition state structures (Table 46). Both transition state structures have exceptionally low deviation from an ideal distribution of bond order in an six-membered transition state structure. The transition state structure with this heterocycle is more aromatic than the corresponding transition state structure for furan as diene. It is actually comparable in reactivity to cyclopentadiene, although for the endo transition state structure it was predicted to be the most aromatic of all studied transition states here (Table 46). Therefore should have the lowest activation barrier. 

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