Electronic vs Steric Effects on the Stability of Anionic Species: A Case Study on the Ortho and Para Regioisomers of Organofullerenes

Abstract

The stability of the anionic species of the ortho and para regioisomers of (MeO)BnC(2n) (Me = methyl, Bn = benzyl, n = 30 or 35) has been examined. The results show that the ortho adducts (electronically favored regioisomers) are stable upon receiving one or two electrons, while the para ones (sterically favored adducts) decompose by removing the methoxy group under similar conditions. Computational calculations indicate that the stability of the anionic species is significantly affected by the electronic structure, where the [5,6]-double bond is responsible for the instability of the reduced species of the para adducts. Further study with 1,15-(MeO)2-2,4-Bn2C60, an adduct with both the ortho and para positioned methoxy, shows that the reduced species is stable, indicating that the 1,2,4,15-configuration is an electronically preferential structure even though it has a [5,6]-double bond.

Controlled Synthesis of C70 Equatorial Multiadducts with Mixed Addends from an Equatorial Diadduct: Evidence for an Electrophilic Carbanion

“…As for 3a , it is composed of a 50:50 mixture of two mirror-image enantiomers, which are designated as 7,19,23,27,33,37,44,53- and 7,19,23,27,37,44,48,64-adducts. Notably, the methoxy groups are always located at the para positions with respect to the starting benzyls, while the o -BrCH 2 PhCH 2 groups are at the para sites with respect to the methoxys, indicating that the reaction is initiated with the addition of MeO – , followed by a reaction with alkyl bromide, in agreement with previous results …”

“…We have recently reported the reaction of mixed methoxylation and benzylation of C 60 and C 70 with impressive regioselectivity . It would be of interest to examine this reaction with 7,23-Bn 2 C 70 , as it may help to achieve a controlled synthesis of C 70 equatorial multiadducts and reveal more information on the reactivity of the C 70 equatorial carbons.…”

Synthesis of (MeO)2Bn2C70: Regiochemistry of 2-fold Additions to C70 with Addends That Are Preferential for Ortho Addition and Capable of Para Addition

“…We have recently reported the 1-fold addition of the methoxy and benzyl groups to C 60 and C 70 . The methoxy and benzyl groups preferentially undergo the process with the ortho addition pattern, with the ortho adducts ( 1 and 2 , Figure ) being the predominant products.…”

Reductive Activation of C70 Equatorial Carbons and Structurally Characterized C70 δ-Adduct with Closed [5,6]-Ring Fusion

“…The chemistry of fullerenes is governed by the release of the strain energy caused by the deviation from the planarity of sp 2 -hybridized carbon atoms. , The principle is well observed for the reactions of C 70 , where the more strained carbons in the polar region (types a – c in Figure ) are more reactive with preferential formation of derivatives at the most and second most curved bonds (α- and β-bonds, Figure ) for ortho-addition reactions. − In contrast, the less strained carbon atoms in the flat equatorial region of C 70 are inert with very limited reports so far on the addition at the δ-bond. ,, Notably, the δ-adduct is the fifth most stable isomer predicted theoretically using C 70 H 2 as a model, next only to the α- and β-adducts and two para isomers with additions at d , d -carbons across the equator and a , c -carbons, which have all been obtained in significant amounts, − ,,, suggesting that the unavailability of the δ-isomer is likely due to the kinetic rather than thermodynamic factor.…”