Diels-Alder additions, fullerene

The new absorption band at 435 ran in the C60 spectrum has been attributed to the 1,2 addition to the fullerene cage to the fatty acid chains either across to the double bonds by a Diels-Alder addition or, more simply, by radical addition (Cataldo and Braun, 2007). Thus, fatty acid esters are able to not only dissolve C60, but also react with this molecule causing the addition of the fatty chain to the fullerene cage. In fact, the bands at 435 ran shown in Fig. 13.3 appear only when C60 is stirred at 75°C for a couple of hours in the esters of fatty acids. Only for olive oil the new band appears much weaker than in the other cases and displaced at 450 ran (Fig. 13.3B). Since this oil contains chlorophyll, the displacement may be probably due also to a charge-transfer interaction between C60 and chlorophyll or with other impurities. 

Scheme 1.18. Diels-Alder addition of 2-[(trimethylsilyl)oxy]buta-l,3-diene to Cgo, followed by hydrolysis of the resulting silyl enol ether to the ketone 214 and reduction of the latter, affording the racemic alcohol ( )-215 as a versatile fullerene-containing building block for the attachment of further functionalities.

The cyclohexene substructure resulting from Diels-Alder addition of acyclic dienes to fullerenes can adopt two boat conformations.50,387-389 If a new stereogenic center is generated during the cycloaddition, the boat-to-boat interconversion leads to conformational diastereoisomerism in each configurational stereoisomer.389,390 Such a case was studied by Zhang and Foote who performed VT (variable temperature) XH NMR and NOE measurements on... 

A racemic, bichromophoric C70-pyrene conjugate showing intramolecular photo-induced energy transfer from the excited singlet state of the PAH to the fullerene has been reported by Daub and co-workers.383 Its chirality is based on the stereogenic center resulting from reduction of a fullerene-fused cyclohexanone, itself obtained by Diels-Alder addition of 2-[(trimethylsilyl)oxy]buta-1,3-diene to the C(l)-C(2) bond of C70 and subsequent hydrolysis of the formed silyl enol ether (cf. Section IV.C.l.d and Scheme 1.18). 

Hirsch and co-workers subsequently discovered that the yield of hexakis-adducts such as 8 or 9 can be greatly increased and their separation from side products substantially facilitated, if 9,10-dimethylanthracene (DMA) is used as a template (Scheme 7-2) [14, 15]. Anthracene derivatives such as DMA are well known to undergo reversible Diels-Alder additions with fullerenes at ambient temperamre [16-18]. After addition to the fullerene, the template directs diethyl malonate addends in the Bingel addition tegio-selectively into e positions, ultimately yielding the hexakis-adducts with a pseudo-octahedral, all-e addition pattern. The templated activation of e 6-6 bonds (bonds between two six-membered rings) is also efficient starting from C o mono-adducts, and several examples are shown in Scheme 7-2. Thus, bis(alkynyl)methanofullerene 10 [19] reacted with diethyl 2-bromomalonate/DBU (8 equiv) and DMA (12 equiv) to provide hexakis-... 

As described above, cycloaddition reactions are among the most powerful for functionalization of fullerenes, and microwave irradiation has proven a very useful technique in this kind of chemistry. The reactivity of the walls of SWNTs should be similar to that of fullerenes - albeit significantly slower. It is therefore of interest to apply microwaves for lateral functionalization of SWNTs. The first such example of Diels-Alder addition to the lateral wall of a SWNT has been described [90], and involved use of microwaves as a source of energy (Scheme 21.24). Ester functionalized SWNT 69 [91] was reacted with o-quinodimethane 71, generated in situ from 4,5-benzo-l,2-oxathiin-2-oxide (70), under the action of microwave irradiation for 45 min. The reaction was also performed under classical heating conditions for 72 h in ODCB under reflux but under these conditions the NMR spectrum was indicative of very low conversion. 

The scope of the microwave technique in the preparation of fullerene derivatives was determined in the well known Diels-Alder reaction of C6o with anthracene (1) [71], which has been reported to occur under thermal conditions (13% [71a], reflux, toluene, three days 25% [71b], reflux, benzene, 12 h) (Scheme 9.22). In addition to 76, multiply-substituted adducts that undergo cycloreversion to the starting materials were formed. 

Thus, the addition of fullerenes to the fatty chains cannot be exclusively through a Diels-Alder mechanism, but involves also a radical mechanism. 

A review of Diels-Alder reactions of fullerenes with acyclic and cyclic dienes has been presented. The addition of substituted pyrimidine o-quinodimethanes (75) to [60]fullerenes yields novel organofullerenes (76) bearing a pyrimidine nucleus covalently attached to the Ceo cage (Scheme 26). The Diels-Alder dimerization of cyclopenta[/]phenanthrene (77) with isobenzindene (78) yields the dimer (79) in 85% yield (Scheme 27). Further evidence has been supplied to support an early reorganization of the r-network in the dimerization of 2-methoxycarbonylbuta-1,3-diene. The Lewis acid-catalysed Diels-Alder reactions of acrylate derivatives of new carbohydrate-based chiral auxiliaries with cyclohexadiene show excellent endo. exo... 

The chemistry of all fullerenes is dominated by their ability to react as poorly conjugated and electron-deficient 2ir alkenes they show very few properties typical of dienes or arenes (5). In addition, because of the high cage stability, they never undergo substitutions. C60 shows behavior similar to that of a monosubstituted alkene such as vinyl chloride or acrylate. All fullerenes readily add to electron-rich species such as nucleophiles, bases, radicals, or reducing agents. They are, for example, perfect dienophiles for Dieles-Alder reactions. The types of reactions undergone by fullerenes are illustrated in Scheme 1. 

Further C6o derivatives of menthol include a menthyl malonate adduct119 and a carbamate derived from l,2-epimino[60]fullerene.429 Resin acid derivatives have been prepared by Diels-Alder reaction of C6o with methyl levopi-marate, whose parent acid can be isolated from rosin.430 Finally, a derivative of a-pinene was synthesized in the context of a study on the photochemically induced addition of allyl stannanes to Cr,o, a reaction presumably proceeding via SET from the stannane to the triplet excited fullerene.431... 

Figure 2.59 collects several [2t-2]-cycloadditions that can be carried out with C )-The reaction with dehydrobenzene is instructive as it reveals electronic properties of the fullerene The dehydrobenzene generated in situ from anthranilic acid reacts with Cso exclusively in a [2-i-2]-cycloaddition, although in principle a [4-f2]-reaction would also be possible, and dehydrobenzene usually enters into the latter when adding to electron-rich dienes. The nonoccurrence of this reaction clearly shows the electron-deficient character of C,so. For the same reason it never constitutes the diene part in a Diels-Alder reaction. Furthermore, the [24-2]-cycloaddition may be thermally effected, for example, the addition of long-chain cumulenes, allene amides or quadricyclan. The addition of ketenes as well occurs without irradiating the reaction mixture. Normally a reduced reactivity toward C,so should be expected for the electrophilic ketenes, but in reality the products of a [2-1-2]-cycloaddition are even found to be the major product. 

Altogether the electrophilic character of nanotubes is less pronounced than that of fullerenes. Regarding that in addition their reactivity is diminished by the weaker curvature, one would not expect too strong an affinity to typical dienes. Still further reports on examples of the Diels-Alder reaction on single- and multiwalled nanotubes may be foreseen for the future. 

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