Diels–Alder and Retro‐Diels–Alder Cycloadditions of (1,2,3,4,5‐Pentamethyl)cyclopentadiene to La@C_2v‐C₈₂: Regioselectivity and Product Stability

Abstract

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(2v)-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(2v)-C(82)Cp was one order of magnitude faster than that of La@C(2v)-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(2v)-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.

Evaluation of bonding, electron affinity, and optical properties of M@C28 (M = Zr, Hf, Th, and U): Role of d‐ and f‐orbitals in endohedral fullerenes from relativistic DFT calculations

“…Endohedral fullerenes containing interstitial metal atoms have attracted particular interest owing to their novel properties with potential applications in biomedicine and photovoltaics among others issues. The interstitial metal atoms modify the behavior of the external carbon cage, thereby resulting in a different chemical reactivity compared to the corresponding hollow cage . Endohedral metallofullerenes (EMF) can encapsulate actinide metals, including uranium and thorium, leading to molecular materials displaying fullerene stability as well as the chemistry of the interstitial actinide atom.…”

Counter anion in Li+‐encapsulated C60 can further enhance the rate of Diels–Alder reaction: A DFT study

“…One important reaction applied to the chemical functionalization of fullerenes is Diels–Alder cycloaddition reaction . Previous studies have uncovered an interesting finding that fullerenes encapsulated with additional atoms, ions, or clusters, which are known as endohedral metallofullerenes (EMFs), greatly alter the rate of Diels–Alder reaction . The effect of the encapsulation on the reactivity of fullerenes is further clearly shown in a recent study by Kawakami et al who found that [Li + @C 60 ] ( ) experiences a much higher reactivity for Diels–Alder cycloaddition toward cyclopentadiene (CpH) in comparison to that of empty C 60 …”

Reaction Mechanism and Regioselectivity of the Bingel–Hirsch Addition of Dimethyl Bromomalonate to La@C2v‐C82

“…The La@C 2v -C 82 EMF,w hich in the case of Ce@C 2v -C 82 is approximately four times more abundant than the analogous C s -C 82 , [17] has been the subjecto fm any studies. [18][19][20] For example,t he first EMF functionalization was achieved for La@C 2v -C 82 throughaphotochemical reaction with disilirane. [21] The electronic and structural properties of the La@C 2v -C 82 cage have been studied already.A ccordingt othe ionic model, [22][23][24][25] charget ransfer from the La atom to the fullerene cage occurs, and as ar esult, the formal electronic structure of La@C 2v -C 82 is describeda sL a 3 + @C 2v -C 82 3À ,w ithan umber of unpairedelectrons located on the fullerene cage.…”

“…Chemical functionalization of EMFs [5,6] is commonly achieved through cycloaddition reactions, the most important being: Diels-Alder, [18,29] 1,3-dipolar (or Prato), [30] and nucleophilic [2+ +1] Bingel-Hirsch (BH) additions. [31][32][33] The BH addition is acyclopropanation reaction in which af ullerenea nd diethyl bromomalonate react in the presence of as trong base such as 1,8-diazabicycloundec-7-ene (DBU) or sodium hydride to produce am ethanofullerene or af ulleroid.…”