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Diels–Alder and Retro‐Diels–Alder Cycloadditions of (1,2,3,4,5‐Pentamethyl)cyclopentadiene to La@ C 2 v ‐C 82 : Regioselectivity and Product Stability
Author(s) -
GarciaBorràs Marc,
Luis Josep M.,
Swart Marcel,
Solà Miquel
Publication year - 2013
Publication title -
chemistry – a european journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.687
H-Index - 242
eISSN - 1521-3765
pISSN - 0947-6539
DOI - 10.1002/chem.201203517
Subject(s) - regioselectivity , cyclopentadiene , cycloaddition , chemistry , adduct , diels–alder reaction , computational chemistry , thermal decomposition , fullerene , double bond , medicinal chemistry , stereochemistry , organic chemistry , catalysis
One of the most important reactions in fullerene chemistry is the Diels–Alder (DA) reaction. In two previous experimental studies, the DA cycloaddition reactions of cyclopentadiene (Cp) and 1,2,3,4,5‐pentamethylcyclopentadiene (Cp*) with La@ C 2 v ‐C 82 were investigated. The attack of Cp was proposed to occur on bond 19 , whereas that of Cp* was confirmed by X‐ray analysis to be over bond o . Moreover, the stabilities of the Cp and Cp* adducts were found to be significantly different, that is, the decomposition of La@ C 2 v ‐C 82 Cp was one order of magnitude faster than that of La@ C 2 v ‐C 82 Cp*. Herein, we computationally analyze these DA cycloadditions with two main goals: First, to compute the thermodynamics and kinetics of the cycloadditions of Cp and Cp* to different bonds of La@ C 2 v ‐C 82 to assess and compare the regioselectivity of these two reactions. Second, to understand the origin of the different thermal stabilities of the La@C 82 Cp and La@C 82 Cp* adducts. Our results show that the regioselectivity of the two DA cycloadditions is the same, with preferred attack on bond o . This result corrects the previous assumption of the regioselectivity of the Cp attack that was made based only on the shape of the La@C 82 singly occupied molecular orbital. In addition, we show that the higher stability of the La@C 82 Cp* adduct is not due to the electronic effects of the methyl groups on the Cp ring, as previously suggested, but to higher long‐range dispersion interactions in the Cp* case, which enhance the stabilization of the reactant complex, transition state, and products with respect to the separated reactants. This stabilization for the La@C 82 Cp* case decreases the Gibbs reaction energy, thus allowing competition between the direct and retro reactions and making dissociation more difficult.