| Abstract
| - An unusual reaction process that produced unexpected heterocyclic systems by a fragmentation−recombination mechanism is described. Thus treatment of the triketone, 3-acetyl-2,6-heptanedione,1, with methanesulfonyl azide gave, in addition to the expected α-diazo ketone 3a, the dihydropyrazole 3c and its oxidation product, the pyrazole 3d. We propose that the initially formed α-diazoketone 3a fragments into the simple α-diazomethyl ketone and methyl vinyl ketone which thenundergo an intermolecular [2,3]-dipolar cycloaddition. Analogous treatment of the trifluoromethyltrione 2 again afforded a pyrazole 4c. Further experiments were carried out to lend evidence toour mechanistic hypothesis. Thus α-diazoacetophenone 5 and MVK underwent a [2,3]-dipolarcycloaddition under mild conditions to give the two regioisomeric dihydropyrazoles 6a and 6b.Interestingly these were formed in a 2:1 ratio, which suggested that 6a was more stable than 6b.The structures of 6a and 6b were optimized by using the B3LYP density functional method andthe 6-31G* basis set and isomer 6a was predicted to be 1.5 kcal/mol more stable than isomer 6b.This energy difference could be rationalized by the greater capacity of the acetyl group than thebenzoyl group to conjugate with the hydrazone. This difference in conjugation is reflected by keybond length differences. Thus we have discovered a novel fragmentation−cycloaddition process.We have also presented evidence for the mechanism of the formation of the dihydropyrazoles andcarried out calculations to support these findings.
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