Activation energy substituted cyclopropanes

Table 14 Arrhenius activation energies and free energies of activation (kcal mol ) for isomerization of substituted cyclopropanes in CDClj. ...

Viehe s group has studied cis-trans isomerizations of captodative-substituted cyclopropanes in more detail (Table 14) (Merenyi et al., 1983 De Mesmaeker et al., 1982). The lowest activation energy is observed for X = OCH3 and the highest for X = COjCHj. Thus, a donor instead of a captor... 

The Arrhenius parameters for the gas-phase unimolecular structural isomerizations of 1,1,2-trimethylcyclopropane28 to three isomeric methylpentenes and two dimethyl-butenes, and of 1,1,2,2-tetramethylcyclopropane29 to 2,4-dimethylpent-2-ene have been determined over a wide range of temperatures. Despite previous reports on substantial decreases in activation energies for structural isomerizations of methyl-substituted cyclopropanes, this study has revealed that the trend does not continue beyond dimethylcyclopropane isomerization. 

Similarly, the isomerization of vinylcyclopropane has a lower activation energy than the reactions of saturated cyclopropanes (Table 4) a similar result is obtained with other substituted vinylcyclopropanes . Moreover, these reactions have frequency factors nearly two orders of magnitude lower than the cis-trans isomeriza-tions of saturated cyclopropanes. These results can be explained by the formation of an allylically stabilized biradical as the intermediate, viz. 

Both T-substituted 1-pentene and 3-methyl-1-butene were observed in yields comparable to the other T-pentene isomers. This observation clearly indicates that the primary source of T-pentene in these systems is through the loss of H atoms from the excited radicals formed by hot tritium addition with opening of the cyclopropane ring. The almost lack of isomerization of the T-substituted parent molecule set an upper limit for the energy left in the T-substituted parent molecule due to the reaction with energetic tritium the upper limit is equal to the activation energy for isomerization (65 kcal mol for cyclopropane and methylcyclopropane and about 62 kcal mol for ethylcyclopropane and dimethyl-cyclopropane. ... 

It will be noted that the isomerization to cyclopentene proceeds with a considerably lower energy of activation than the other cyclopropane isomerizations so far discussed. As a result these reactions have been investigated kinetically at temperatures about 100° lower than those not having a vinyl substituent. A number of substituted vinylcyclopropanes have been studied and the Arrhenius parameters for their isomerizations to substituted cyclopentenes determined. The results are shown in Table 4. From the results in Table 4 it can be seen that the isomerizations... 

It is to be expected, especially if a biradical is an intermediate, that the more highly substituted alkylcyclopropanes will isomerize with a lower energy of activation than cyclopropane itself, owing to higher stability of a tertiary or secondary radical than a primary one. Even if a free biradical is not involved, a similar effect... 

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