Transformation of cyclic malonohydrazides into the Diels-Alder reactive 1,3-diazole

Transformation of cyclic malonohydrazides into the DielS Alder reactive 1,3-diazole 

The reaction transformation should start by reacting malonic ester and hydrazine to prepare cyclic malonic hydrazide. It is well known that thionyl chloride can transfer amides to vinyl chloride, but in our case we used the pyrazole tautomer. We believe that this tautomer must be a sufficiently reactive diene for a Diels-Alder cycloaddition reaction because of the findings of one experimental study performed by Adam and coworkers [62] and one computational study presented above. There are at least two key factors that must be addressed before this reaction scheme can be considered seriously the likelihood that 3,5-dichloro-[4//]-pyrazole will participate in a cycloaddition reaction and the stability of the formed cycloadduct. One can also question the stability and the availability of cyclic hydrazides of malonic ester. These compounds are unstable but they can 

As in the case of unsubstituted [4//]-l,2-diazole, 3,5-dichloro-[4H]-l,2-diazole is a LUMO diene controlled cycloaddition reaction. Necessary FMO changes for reactants in the exo and endo addition of cyclopropane to 3,5-dichloro-[4H]-1,2-diazole to adapt the transition state structures are presented in Table 48. The required energy is too small to be compared with the other heterocycles presented in this study (for example see Table 41). Furthermore, the endo addition of cyclopropane should be favored over the exo cycloaddition. 

To confirm these findings, we have computed activation barriers for the acetylene, ethylene and cyclopropane additions to 3,5-dichloro-[4H]-1,2-diazole. The computed activation barriers (Table 49) were even slightly lower than the activation barriers for same reaction with 4,4-dimethyl-[4H]-l,2-diazole as a diene (Table 47). This is a reasonable observation because the cycloaddition reaction is LUMO heterocycle (diene) controlled. The LUMO energy of 3,5-dichloro-[4fir -1,2-diazole was substantially lower than the LUMO energy of 4,4-dimethyl-[4/Tl-1,2-diazole. Subsequently, with a modest activation barrier of 20.1 kcal/mol, even poor dienophiles such as acetylene should be capable to react with 3,5-dichloro-[4i/]-1,2-diazole as a diene in the Diels-Alder reactions (Table 49). 

AEi = activation barrier computed by AMI AEn = activation barrier computed with the B3LYP/6-31G(d)//AMl theory model. 

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