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

Abstract

A change in regioselectivity is predicted in the Diels–Alder reaction when going from the singlet to triplet state.

Regioselectivity in Diels–Alder Cycloadditions of #6094C68 Fullerene with a Triplet Ground State

“…For IPR fullerenes, the [6,6] addition is preferred in the singlet closed-shell state, 45 whereas in the triplet state the preferred attack takes place on the [5,6] bond. 18 For non-IPR fullerenes, the [5,5] adduct is the thermodynamic reaction product and the [5,6] F adduct is the one obtained under kinetic control, irrespective of the electronic state being a singlet 19 or a triplet. However, we have found that in the triplet state the regioselectivity in favor of the [5,5] and [5,6] F attacks is enhanced by 5−7 kcal•mol −1 .…”

“…Unfortunately, experimental proof of this change of reactivity was not possible because of the difficulties associated with generating cycloadducts from highly negatively charged fullerenes like C 60 –5 or C 60 –6 . More recently, some of us reported that the regioselectivity of the DA cycloaddition changes from [6,6] to [5,6] addition when the spin of the fullerenic cage is increased . Indeed, for the triplet C 60 , the most favored addition is already the [5,6] addition.…”

“…More recently, some of us reported that the regioselectivity of the DA cycloaddition changes from [6,6] to [5,6] addition when the spin of the fullerenic cage is increased. 18 Indeed, for the triplet C 60 , the most favored addition is already the [5,6] addition. Not only for the triplet, but also for high spin states, the most favored product is the [5,6] adduct, whereas in the ground state it is the [6,6] product.…”

“…Despite computations predicting a change of regioselectivity when moving from S 0 to T 1 , the experimental results showed that the DA cycloaddition between isoindene and photoexcited 3 C 60 also generates the [6,6] adduct instead of the predicted [5,6] one. 18 By analyzing the reaction in more detail, we computationally found that in the T 1 state the reaction goes through an intersystem crossing (ISC) to reach the S 0 state. When this happens, the reaction ends up in the ground state therefore affording the expected [6,6] product.…”

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

“…For chemists working in the reactivity of fullerenes, it would be fascinating to have a full control on the regioselectivity of the DA reactions. Interestingly, the [5,6] attack was computationally found to be favored in strongly reduced C 60 cages [31] and in high spin states [32]. The accumulation of negative charge or spin density takes place in the 5-membered rings of C 60 .…”