Cyclopropane activation energy

Examination of cyclopropanation through RT6 and RT7 reveals that a less conventional explanation may be required to rationalize the high reactivity of zinc car-benoids (Fig. 3.29). The structure of RT6 represents a pseudo-dimer as shown in RTS that has been further activated by coordination of zinc chloride to the oxygen of the chloromethylzinc alkoxide. This mode of activation is reminiscent of that observed in RTl. Cyclopropanation proceeding from RT6 through TS3 has an activation energy of 27.8 kcal mol . This represents a negligible decrease in the barrier to methylene transfer when compared to reaction from RTS. 

The activation energy for the favored transition state TS4 (22.8 kcal mol ) is still somewhat high. Still, the qualitative predictions of enhanced reactivity of the zinc alkoxide-zinc chloride complexes are in full agreement with contemporary ideas about this reaction and represent a major advance in the theoretical understanding of the cyclopropanation process. 

Fig. 5. Apparent activation energies of the ethane hydrogenolysis and cyclopropane hydrogenation reactions on the group VIII noble metals. The activation energies were determined at hydrogen and hydrocarbon partial pressures of 0.20 and 0.030 atm, respectively (63).

Bernardi and co-workers investigated the mechanism of the Simmons-Smith reaction theoretically at the DFT (B3LYP) level of theory, using ClCH2ZnCl as a model system.95 Of the two available reaction channels (addition and insertion), the former process was found to have the lower activation energy (107.7 vs. 150.7 kj moP1), and this result correlates well with the exclusive cyclopropanation over the competing insertion reaction. 

While activation energies for these reactions have not yet been obtained, it is clear from the temperatures at which the isomerizations have been found to occur that they must have significantly smaller energies of activations (perhaps by as much as 10 to 15 kcal mole than those for the cis-trans isomerizations of dialkylcyclopropanes. This suggests that the two cyclopropane rings can co-operate, especially if the geometric arrangement is favourable. 

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 methylene triplet adds to ethylene symmetrically through the triplet biradical directly (B). The central methylene group (formed from ethylene) is bent downwards by this process (Fig. 10). At this stage rotation or direct ring closure can occur, with loss of stereochemistry following bond formation to yield cyclopropane. The cyclo-addition of the triplet requires only a small activation energy of about 5 kcal/mole 52). 

Carbenes and carbenoids can add to double bonds to form cyclopropanes or insert into C—H bonds. These reactions have very low activation energies when the intermediate is a free carbene. Intermolecular insertion reactions are inherently nonselective. The course of intramolecular reactions is frequently controlled by the proximity of the reacting groups.53... 

Thietane 1,1-dioxide decomposes cleanly to cyclopropane and sulfur dioxide at 400 °C with an activation energy of 117.5 kJ mol-1 (75MI51401) at higher temperatures some... 

The cyclopropanation reaction using gem-dizinc carbenoids has also been theoretically studied by Phillips and coworkers. The gem-dizinc carbenoids react with ethylene via a synchronous attack with an activation energy of about 15 kcalmoR (Figure 3, D). It was also predicted that the reaction could be accelerated by the addition of zinc iodide. 

Activation energies observed in the hydrogenation of cyclopropane over various metals... 

The cis/trans isomerization of l,l-difluoro-2,3-dimethylcyclopropane (2) has an activation energy about lOkcal mol-1 lower than that of the parent hydrocarbon.5 Unlike the parent hydrocarbon, competing cyclopropane to propene rearrangement is not observed, a generalization that extends to other gem-difluorocyclopropanes studied in the Dolbier laboratories.1... 

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