Theoretical Characterization of Cycloaddition Reactions of the Cyclopropylcarbinyl Cation
Cramer, C. J.; Barrows, S. E.
J. Org. Chem. 1994, 59, 7591.
The gas-phase cycloaddition reactions of ethylene and 1,3-butadiene with the cyclopropylcarbinyl cation (1) to produce the cyclohexyl and cycloocten-5-yl cation, respectively, have been computationally characterized. At levels of theory up to MP2/cc-pVDZ the reactions are quite asynchronous but are concerted (i.e., there is only one transition state prior to formation of the product carbocycles). Intermediate ion-dipole complexes are formed along the path between separated reactants and the transition state. The complex of 1 and ethylene resembles a more typical van der Waals complex, while that between 1 and 1,3-butadiene already incorporates almost a full single bond between the carbinyl methylene and a butadiene methylene. The latter result illustrates that the formation of a cation with allylic character facilitates the overall reaction. The gas-phase vibrationally corrected free energies of activation are 9.0 kcal/mol for the [4+2] reaction and -12.1 kcal/mol for the [4+4] reaction. The effects of polar solvation have been considered using the AM1-SM2 continuum solvation model. Because charge is more diffuse in the transition state structures than in the reactants, solvation decelerates the reaction relative to the gas phase. This deceleration is more pronounced in the case of 1 + 1,3-butadiene since the formation of the allyl-like cation renders charge more diffuse than in the 1 + ethylene case.
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