Silylated Fullerenes

SCHEME 6. A possible mechanism for the formation of bis-silylated fullerene... 

Itoh and coworkers [223] have shown that fullerene derivatives as 6/2-113, which to date have been prepared in a stepwise procedure, can be obtained in a three-component domino process by treatment of diynes 6/2-109, dimethylphenylsilane 6/2-110 and fullerene (C60) in the presence of a Rh-catalyst [223]. Interestingly, using maleic anhydride as dienophile failed to give the desired cycloadduct, whereas Cso -in spite of its strong tendency to form complexes with various transition metals [224] - never suppressed the catalytic silylative cyclization step to give the diene 6/2-112 (Scheme 6/2.24). 

Eguchi and Ohno have used silyl nitronate induced 1,3-dipolar cycloaddition for functionalization of fullerene C60 (Eq. 8.76).127a Nitrile oxides also undergo 1,3-dipolar cycloaddition... 

A wide range of olefins can be cyclopropanated with acceptor-substituted carbene complexes. These include acyclic or cyclic alkenes, styrenes [1015], 1,3-dienes [1002], vinyl iodides [1347,1348], arenes [1349], fullerenes [1350], heteroare-nes, enol ethers or esters [1351-1354], ketene acetals, and A-alkoxycarbonyl-[1355,1356] or A-silyl enamines [1357], Electron-rich alkenes are usually cyclopropanated faster than electron-poor alkenes [626,1015],... 

The chemical modification of fullerenes has received considerable attention in the last decade in order to achieve new applications to material sciences [27]. Fuller-ene-bonded polysilane derivatives might be expected to show high conductivity since Ceo-doped polysilane is found to be a good photoconductor [28]. Therefore, a variety of silylated derivatives have been obtained to date, although the available methods are limited to the photoinduced addition of various silanes to Ceo-... 

The photochemical addition of some cyclic oligosilanes to Ceo has also been found interesting. Scheme 8.8 shows some examples of such a transformation. Irradiation (X > 300 nm) of a toluene solution of disilirane 36 with Ceo afforded the fullerene derivative 37 in a 82% yield [37]. The reaction mechanism is still unknown. When toluene is replaced by benzonitrile the bis-silylated product of the solvent was obtained in good yields. In these experiments a photoinduced electron transfer between 36 and Ceo is demonstrated, indicating the role of Ceo as sensitizer [38]. The photoinduced reactions of disilirane 36 with higher fullerenes such as C70, Cv8(C2v)and CuiDi) have also been reported... 

Owing to their stability and low nucleophilicity, metal acetylides are less reactive toward Cjq than other lithium organyls or Grignard reagents [11]. Though the reaction is slower and higher reaction temperatures are necessary, various acetylene derivatives of Cjq could be obtained. The first acetylene Cjq hybrids were (trimethyl-silyl)ethynyl- and phenylethynyl-dihydro[60]fullerene, synthesized simultaneously... 

As well as the Bingel reaction and its modifications some more reactions that involve the addition-elimination mechanism have been discovered. 1,2-Methano-[60]fullerenes are obtainable in good yields by reaction with phosphorus- [44] or sulfur-ylides [45,46] or by fluorine-ion-mediated reaction with silylated nucleophiles [47]. The reaction with ylides requires stabilized sulfur or phosphorus ylides (Scheme 3.9). As well as representing a new route to l,2-methano[60]fullerenes, the synthesis of methanofullerenes with a formyl group at the bridgehead-carbon is possible. This formyl-group can be easily transformed into imines with various aromatic amines. 

Based on bis-silylated dienes another approach to quinoxaline derivatives such as 80 (Scheme 4.10) was found [97]. Fast [4+2] cycloaddition takes place by treatment of Cgo with 2,3-bis(trimethylsilyloxy)butadiene 98, yielding the acyloin-fused fullerene derivative 100 in good yields (Scheme 4.16). The silylated diene is formed in situ by treatment of 98 at 180 °C in o-dichlorobenzene. Controlled bromination of the intermediate 99 leads to the transient diketone 101, which reacts readily in a one-pot reaction with various o-diaminoarenes to yield the quinoxaline-fused fullerenes 102. 

C60 also reacts with silyl ketene acetals when photolyzed with a high-pressure mercury arc. The reaction, illustrated in equation 48, leads to functionalization of the fullerene as an ester, and is said to involve single electron transfer205. 

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. 

Mikami et al. also investigated the addition of ketene silyl acetals. They found that addition of the silyl enol ether of acetone and allylic silanes did not result in the synthesis of substituted l,2-dihydro[60]fullerenes [218a,220], In 1997, Mikami et al. [221] reported the photoaddition of allylic stannanes that leads to monoallylation of C6o (Scheme 13). 

The photochemical addition of cyclic 1,3-diones such as dimedone, 1,3-cylohexandione 62, or their respective silyl enol ethers leads to the formation of two fused furanylfullerenes, (1) achiral 63 and (2) chiral 64 [244], The latter having an unusual bis-[6,5] closed structure. In the initial step of this reaction, [2 + 2] photocycloaddition across a [6,6] bond to form cyclobutanols or the corresponding TMS ethers is involved (Scheme 26). Oxidation with 02 yields in the formation of the radical 65a. Cleavage to 66a followed by cyclization gives furanyl radical 67a. H abstraction by 102 or a peroxy radical finally leads to product 63. In competition, formation of fullerene triplets by absorption of a... 

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. 

A very useful intermediate for the attachment of further functionalities to Cgo is obtained by reaction of the fullerene with 2-[(trimethylsilyl)oxy]buta-1,3-diene, followed by hydrolysis of the resulting silyl enol ether under formation of a fullerene-fused cyclohexanone (214) which is reduced to the racemic alcohol ( )-215 (Scheme 1.18).374 Because of their great synthetic potential, silyloxy-substituted dienes, such as Danishefsky diene type systems,375-377 have been widely used in the preparation of fullerene derivatives, mostly in the form of stereoisomeric mixtures.378-383... 

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). 

The carbon-carbon bond formation via photoinduced electron transfer has recently attracted considerable attention from both synthetic and mechanistic viewpoints [240-243]. In order to achieve efficient C-C bond formation via photoinduced electron transfer, the choice of an appropriate electron donor is essential. Most importantly, the donor should be sufficiently strong to attain efficient photoinduced electron transfer. Furthermore, the bond cleavage in the donor radical cation produced in the photoinduced electron transfer should occur rapidly in competition with the fast back electron transfer. Organosilanes that have been frequently used as key reagents for many synthetically important transformations [244-247] have been reported to act as good electron donors in photoinduced electron-transfer reactions [248, 249]. The one-electron oxidation potentials of ketene silyl acetals (e.g., E°o relative to the SCE = 0.90 V for Me2C=C(OMe)OSiMe3) [248] are sufficiently low to render the efficient photoinduced electron transfer to Ceo [22], which, after the addition of ketene silyl acetals, yields the fullerene with an ester functionality (Eq. 15) [250, 251]. 

The initial monoadduct reacts further to give the corresponding bisadduct at prolonged irradiation times particularly at higher concentration of the ketene silyl acetal (10 equiv.) [22]. When the unsubstituted ketene silyl acetal (H2C=C(OEt)OSiMe3) is employed, only the monoadduct, ethyl l,2-dihydro]60]fullerene-l-acetate, is ob-... 

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