Fullerene reactions with radicals

In addition to the radical-induced reduction of the y-CD incorporated [60]fullerene, even the formation of Ceo-radical adducts were demonstrated to occur. Considering the configuration of the y-CD/Ceo/y-CD complex, a reaction with radical species is only possible through the two caps of the cyclodextrin moieties and should thus be restricted to small radicals. In light of this aspect, a reaction with a bulky radical species such as "CH2(CH3)2COH should be made more difficult or even be ruled out. In fact, the lack of any spectral evidence for a (C6o-CH2(CH3)2COH) adduct supports this view. 

Enes et al. (2006) recently presented new fulleropyrrolidines bearing one or two 3,5-di-tert-bvAy 1-4-hydroxyphcny 1 units, the EPR studies of which demonstrated that these derivatives are antioxidants. In this case, the presence of the fullerene unit seems to play a marginal role in the reaction with peroxyl radicals, which is governed by the phenol portion. Despite this, the presence of C60 should contribute to scavenge radicals in hypoxic conditions, where alkyl radicals could be the main oxidative products to be removed. 

Taking into account that ROS produced by irradiated fullerenes C60 may act only in the radius of their short diffusion existence, one may suppose that cytotoxic effect is determined by the interaction of fullerene C60 with the surface of cells and initiation of chain reactions of free radical peroxidation in membranes. That is why the influence of photoexcited fullerene C60 on the course of LPO process was studied and evaluated by the content of generated primary (diene conjugates) and final (Schiff bases) products. The content of diene conjugates in thymocytes was 17.7 4.2, inEAC cells was 21.1 1.3, andinL1210 was 12.8 3.1 nM/mg protein, and Schiff bases -56 7.9,46.5 4.5, and 36.6 4.6 rel. units/mg protein, respectively, and did not alter during 1 h incubation of the cells. 

In principal, electron transfer reactions with fullerenes could occur via both the singlet- and triplet-excited state. However, due to the short singlet lifetime and the efficient intersystem crossing, intermolecular electron transfer reactions usually occur with the much longer lived triplet-excited state. The result of the electron transfer is a radical ion pair of fullerene and electron donor or acceptor. 

In all examples discussed up to now the radical cation of Qo is involved in the reaction mechanism. However, due to the electronic features reduction of the fullerenes leading to radical anions should be much easier performed. For example, a useful method to synthesize 1-substituted l,2-dihydro-[60]fullerenes is the irradiation of Q0 with ketene silyl acetals (KAs) first reported by Nakamura et al. [216], Interestingly, when unstrained KAs are used, this reaction did not yield the expected [2 + 2]-cycloaddition product either by the thermal, as observed by the use of highly strained ketene silyl acetals [217], or by the photochemical pathway. In a typical reaction Q0 was irradiated for 10 h at 5°C with a high pressure mercury lamp (Pyrex filter) in a degassed toluene solution with an excess amount of the KA in the presence of water (Scheme 11). Some examples of the addition of KAs are summarized in Table 11. 

The most likely course of this conversion involves H abstraction by bromine atoms. The resulting radical may undergo homolysis of the fullerene-silicon bond as outlined in Scheme 57. The silyl radical thus formed then undergoes intramolecular cyclization to give 132. While this type of intramolecular reaction readily occurs with radical species, it is not a common one in silicon ring systems. The Si-Si bond of 132 then must react with bromine followed by hydrolysis to give siloxane 131. 

It has been known [6-10] that C6o fullerene reacts easily with proton donors and accepts electrons to form anion radicals. Thus, one might expect the preferred fullerene transformations in electrolysis to be related to the fullerene reactions on the cathode. Anion radical transformations can occur both on the surface and in the solution. It has been known that fullerene forms solvates [16] with many solvents (Sol), in particular with toluene. With electrons (D) in the donor medium, two competitive processes can proceed in the reaction system (donor-C6o-solvent)... 

The reactivity of the molecular fullerene solid resembles the expected pattern for a homogeneous material. Only a small prereactivity at 700 K indicates that a fullcrcne-oxygen complex [12] is formed as an intermediate stoichiometric compound [15, 105], At 723 K the formation of this compound and the complete oxidation are in a steady state [12, 106, 107] with the consequence of a stable rate of oxidation which is nearly independent of the bum-off of the fullerene solid. This solid transforms prior to oxidation into a disordered polymeric material. The process is an example of the alternative reaction scenario sketched above for the graphite oxidation reaction. The simultaneous oxidation of many individual fullerene molecules. leaving behind open cages with radical centers, is the reason for the polymerization. 

Yet another scheme exists besides the principles mentioned above. This is mainly encountered in the addition of radicals and certain nucleophiles like lithium or copper organyls. Upon reaction with an excess of the reagent, the favorable products are a hexakis- or a pentakis-adduct, respectively, that adopts the structure of a 1,4,11,14,15,30-hexahydrofullerene (Figure 2.38). The adduct s characteristic feature is a cyclopentadienyl unit whose double bonds are isolated from the conjugated n-system of the remaining fullerene skeleton. This structure is... 

Apart from reactions with carbanions, Qo may also be included in polymers by radical polymerization in the presence of free fullerene. The method usuaUy employs a radical starter. At first, however, this will not react with the monomer, but with the highly reactive C o. Polymerization thus begins originating from the Coo-center and yields highly substituted derivatives. The chain length and the mass contribution of the fullerene can be controlled by tuning the concentration of the initiator and the amount of Coo present. For instance the copolymerization of styrene and Coo succeeds in the presence of an initiator. 

Prior to the development of tether-directed functionalization methods, regioisomerically pure higher adducts of C50 usually were obtained by additions of transition metal complexes [31-33] or radical halogenations [34, 35]. These reactions either occur under thermodynamic control or lead to the precipitation of the least soluble derivative. Iso-merically pure higher adducts of C o sometimes are also readily isolated out of more complex product mixtures [36]. Tether-directed remote functionalization of CgQ allows the construction of fullerene derivatives with addition patterns that are difficult to obtain by thermodynamically or kinetically controlled reactions with free untethered reagents. Since the description of the first such reaction in 1994 [7], which is the subject of Section 7.3.1, an increasing variety of such regioselective functionalization protocols have... 

Oxidation The radiation-chemically induced ionization of chlorinated hydrocarbons, i.e., dichloroethane (DCE) leads to the initial generation of the corresponding solvent radical cation, [DCE] ". The electron affinity of the latter is sufficient to oxidize the fullerene moiety ([60]fullerene E1/2 = +1.26 versus Fc / Fc ). Pulse radiolytic experiments with [60]fullerene in nitrogen-saturated or aerated DCM solutions yielded a doublet with maxima at 960 and 980 nm (Figure 1) (12-18). This fingerprint is identical to that detected in photolytic oxidation experiments and that computed in CNDO/S calculations. Rate constants for the [60]fullerene oxidation are typically very fast with estimated values IC7 > 2 x 10 ° M s. The 7t-radical cation is short-lived and decays via a concentration-dependent bimolecular dimerization reaction with a ground state molecule (kg = 6 x lO M s ) (13). 

Radiolytic experiments of Ceo/Y CD under conditions that generate carbon-centered radicals, such as CH3, were in line with a radical addition mechanism. It is interesting to note that a reaction of [60]fullerene even with the strongly oxidizing Clj radicals (oxidation potential of +2.3 V versus SCE) give an absorption that suggests a (Ceo-Cl) adduct. This is in contrast to a prediction that is purely based on the... 

Finally, the reaction with the two remaining primary radical species that are generated during the course of the radiolysis of water, e.g., H and "OH radicals, should be discussed. However, due to their rapid reaction with the y-CD host molecule, such reactions with the fullerene core are practically precluded and, accordingly, could not be observed (35). 

C(io(OH)i8 (10) Poly hydroxy lation (Scheme 4) of the hydrophobic [60]fullerene core enhances the water solubility of this carbon allotrope up to 4.0 x 10 M (67). The tt-radical anion, (Ceo )(OH)ig, generated by electron transfer from hydrated electrons and (CH3)2 C0H radicals, absorbs with maxima at 870, 980 and 1050 nm. The bimolecular rate constant for a reaction with hydrated electrons is 4.5 x 10 M s . Based on electron transfer studies with suitable electron donor / acceptor substrates, the reduction potential of the C6o(OH)ig/(C6o )(OH)i8 couple was estimated to be in the range between -0.358 V and -0.465 V versus NHE. 

Two findings are particularly noteworthy. First, the experiments in which the reactivity of water-soluble fullerene derivatives in aqueous media was probed (62-64) Not only, that the intermolecular reactions with hydrated electron and various radicals provided unequivocally evidence for the presence of fullerene clusters. But, furthermore, these investigations helped, in reference to the kinetics of the fullerene monomers, to estimate the agglomeration number for, for example, the mono pyrrolidinium salt in the respective fullerene cluster. Secondly, the intermolecular electron transfer reactions between radiolytically generated arene tt-radical cations and higher fullerenes (25,51) The noted parabolic dependence of the rate constants on the thermodynamic driving force is one of the rare confirmations of the existence of the Marcus-Inverted region in forward electron transfer. 

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