On the regioselectivity of the Diels-Alder cycloaddition to C60 in high spin states

The application of aromaticity and antiaromaticity to reaction mechanisms

Aromaticity, in general, can promote a given reaction by stabilizing a transition state or a product via a mobility of π electrons in a cyclic structure. Similarly, such a promotion could be also achieved by destabilizing an antiaromatic reactant. However, both aromaticity and transition states cannot be directly measured in experiment. Thus, computational chemistry has been becoming a key tool to understand the aromaticity-driven reaction mechanisms. In this review, we will analyze the relationship between aromaticity and reaction mechanism to highlight the importance of density functional theory calculations and present it according to an approach via either aromatizing a transition state/product or destabilizing a reactant by antiaromaticity. Specifically, we will start with a particularly challenging example of dinitrogen activation followed by other small-molecule activation, C-F bond activation, rearrangement, as well as metathesis reactions. In addition, antiaromaticity-promoted dihydrogen activation, CO2 capture, and oxygen reduction reactions will be also briefly discussed. Finally, caution must be cast as the magnitude of the aromaticity in the transition states is not particularly high in most cases. Thus, a proof of an adequate electron delocalization rather than a complete ring current is recommended to support the relatively weak aromaticity in these transition states.

Reactions of C60 with Pyridazine and Phthalazine

Cage-opened bisfulleroids are one of suitable building blocks for making a large hole on fullerenes. This work focuses on the Diels-Alder reaction of C₆₀ with azines, among synthetic methods developed thus far, to provide bisfulleroids. Surprisingly, the computational study predicted that the reaction proceeds with normal electron demand in contrast to hitherto considered inverse-electron-demand pathway. The benzoannulation to the pyridazine ring, i. e., phthalazine, resulted in the remarkably shortened reaction time due to the better interaction between the HOMO of phthalazine and the LUMO of C₆₀ as well as stronger 2,3-diaza-1,3-butadiene character in the phthalazine as confirmed crystallographically. Contrary to expectations, the benzobisfulleroid was converted into corresponding orifice-enlarged derivative via the photooxygenation slightly faster than the fulleroid derived from pyridazine.

Capturing unconventional metallofullerene M@C60 through activation of the unreactive [5,6] bond toward Diels-Alder reaction

Full control of the regioselectivity in the functionalization of fullerenes is important for production of fullerene derivatives with desirable properties. Cycloaddition reactions of C60 usually take place at the hexagon-hexagon ring junction, i.e. the [6,6] bond of the fullerene cage, whereas the [5,6] bond is generally unreactive. The activation of the [5,6] bond toward Diels-Alder reactions is difficult because of its longer bond length than the [6,6] bond. In this study, we computationally demonstrate that the [5,6] bond of C60 can be efficiently activated by encapsulation of a divalent metal atom such as Ca or Sm. Electron transfer from the metal atom to the fullerene cage and the interaction between the metal cation and the cage play critical roles in enhancing the reactivity of the [5,6] bond. The physical origin of the reactivity enchancement of the [5,6] bond is investigated quantitatively by using the activation strain model and the energy decomposition method. The change in the orbital interaction energy along the intrinsic reaction coordinate has a major effect on the thermodynamics and kinetics of the reactions between Ca@C60 and cyclopentadiene. Both mono- and bis-addition reactions of cyclopentadiene with Ca@C60 prefer to take place at the [5,6] bonds of the fullerene cage thermodynamically, which is distinct from the case of pristine C60. The HOMO-LUMO energy gap of Ca@C60 is remarkably enlarged upon mono- and bis-functionalization with cyclopentadienes. Therefore, the covalent derivatization strategy can be used to capture the unconventional, missing metallofullerene M@C60.

Decomposition of the electronic activity in competing [5,6] and [6,6] cycloaddition reactions between C60 and cyclopentadiene

Fullerenes, in particular C60, are important molecular entities in many areas, ranging from material science to medicinal chemistry. However, chemical transformations have to be done in order to transform C60 in added-value compounds with increased applicability. The most common procedure corresponds to the classical Diels-Alder cycloaddition reaction. In this research, a comprehensive study of the electronic activity that takes place in the cycloaddition between C60 and cyclopentadiene toward the [5,6] and [6,6] reaction pathways is presented. These are competitive reaction mechanisms dominated by σ and π fluctuating activity. To better understand the electronic activity at each stage of the mechanism, the reaction force (RF) and the symmetry-adapted reaction electronic flux (SA-REF, JΓi(ξ)) have been used to elucidate whether π or σ bonding changes drive the reaction. Since the studied cycloaddition reaction proceeds through a Cs symmetry reaction path, two SA-REF emerge: JA'(ξ) and JA''(ξ). In particular, JA'(ξ) mainly accounts for bond transformations associated with π bonds, while JA''(ξ) is sensitive toward σ bonding changes. It was found that the [6,6] path is highly favored over the [5,6] with respect to activation energies. This difference is primarily due to the less intensive electronic reordering of the σ electrons in the [6,6] path, as a result of the pyramidalization of carbon atoms in C60 (sp2 → sp3 transition). Interestingly, no substantial differences in the π electronic activity from the reactant complex to the transition state structure were found when comparing the [5,6] and [6,6] paths. Partition of the kinetic energy into its symmetry contributions indicates that when a bond is being weakened/broken (formed/strengthened) non-spontaneous (spontaneous) changes in the electronic activity occur, thus prompting an increase (decrease) of the kinetic energy. Therefore, contraction (expansion) of the electronic density in the vicinity of the bonding change is expected to take place.