Activation energy ring closure

The mechanism of titanocene (TiCp2Cl2)-mediated 3-exo cyclization of vinyl epoxides containing carbonyl, phenyl, and vinyl groups has been determined by DFT calculations and experimental studies (Scheme 52).86 The reaction is exothermic and reversible, in contradiction to an earlier mechanistic proposal. The energies of activation for ring closure are among the lowest known for radical reactions. The crucial step for a successful completion of the reaction is rapid trapping of the cyclopropylcarbinyl... 

Finally, examine the transition states for closure of the Ci enolate to the 7-membered ring product, and of the C3 enolate to the 5-membered ring product. Calculate activation energy barriers from their respective enolates. Which ring closure (to the five or the seven-membered ring) occurs more readily ... 

In Case 1 both the ring closure steps (with rate constants ks and kR) are faster than dissociation of the 7r-complexes to reform the alkenol and N-X+ species. Here the [S]/R] product ratio is determined only by the difference in the activation energy leading to the 7is- and rcR-complexes because immediately after these are formed the cyclization occurs. Thus, the chirality is set by the approach of the alkene to the halonium ion. 

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). 

Spiropyran thermal ring closure is faster in nonpolar solvents than in polar solvents. This would be expected due to the polarity of this zwitterionic form, which will be relatively stabilized in a polar solvent, because a transition state would be expected to be less charge separated if it were cis about the central 3-methine bond. The activation energy for ring closure is typically between 75 and 105 kJ mol depending on the solvent polarity [51]. The reaction is generally reported to be first order [25,51]. 

Photolytically generated 1-silabuta-l,3-dienes undergo a thermal reverse reaction to 2-silacyclobutenes. Thus 2-phenylsilacyclobut-2-ene 360 is easily opened to the 2-phenylsilabuta-1,3-diene 361 by irradiation in 3-methylpentane matrix at 77 K or by flash photolysis at ambient temperature (equation 97)183. The rate for the thermal reverse reaction was measured at room temperature and the activation energy for the 1 -siladiene ring closure was estimated to be 9.4 kcalmol-1183. 

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