Addition of cyclopropene to heterocycles with heteroatoms in the 1, 2, and 5 positions

Addition of cyclopropene to heterocycles with heteroatoms in the 1,2, and 5 positions 

Here we present only activation barriers for heterocycles with heteroatoms in 1,2,5- positions in reaction with the most reactive dienophile, cyclopropene (Table 40). It demonstrates our previous postulate that the activation barriers were too high, even with very reactive dienophiles such as cyclopropene. The most reactive was 1,2,5-oxadiazole but not 1,2,5-tbiadiazole as predicted on the basis of FMO energy gap between reactants. The results indicated that the reaction should not be experimentally achievable. To the best of our knowledge, there is no experimental evidence that heterocycles with heteroatoms in 1,2,5- positions might be acceptable dienes for Diels-Alder reactions. 

HOF = heat of formation computed by AMI E = total energy (a.u.) computed by B3LYP/6-31G(d)/AMl DEj = activation barrier (kcal/mol) computed by AMI DEn = activation barrier (kcal/mol) computed by B3LYP/6-31G(d)/AMl 

I = TS with 1,3,4-triazole II = TS with 1,3,4-oxadiazole III = 1,3,4-thiadiazole a = for addition of acetylene b = for addition of ethylene c = for exo addition of cyclopropene d = for endo addition of cyclopropene A = HOMOts -HOMOdienophile B = HOMOxS HOMOheterocyclei C LUMOdienophile LUMOxS D = LUMOxs LUMOheterocyclei IAI-hIDI Z2= IBI + ICI. 

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