1.3- Dipolar cycloadditions fullerene reactions

In 1995, Boyd and co-workers <95TL7971 > covalently linked a porphyrin to fullerene Cgo through a 1,3-dipolar cycloaddition reaction involving the porphyrinic azomethine ylide 28 (Scheme 8). The ylide was generated in situ from befa-formyl-meso-tetraphenylporphyrin 27 and A -methylglycine, and provided the porphyrin-C6o diad 29 in good yield. 

The demonstration that the 1,3-dipolar cycloaddition process with azomethine ylides works with nanotubes implies that similar reactions developed for use with fullerenes also may be successful with carbon nanotubes. In particular, the cyclopropanation reactions discussed previously for the modification of Cg0, likely will work for derivatization of SWNTs and MWNTs (Zakharian et al., 2005). 

Dipolar cycloadditions of fullerene C6o to nitrones have been studied. Their mechanism, regiochemistry, and nature of addition have been investigated. All of the reactions lead to the formation of fullerene fused heterocycles. Theoretically, these reactions can lead to four types of additions, such as closed [6,6], open [5,6], closed [5,6], and open [6,6] additions (Scheme 2.317). Energetics and thermodynamic analyses of these reactions show that closed [5,6] and open [6,6]... 

In search of a convenient procedure for preparing diazo substrates for the cycloaddition to Cgg, Wudl introduced the base-induced decomposition of tosyl-hydrazones [116]. This procedure allows the in situ generation of the diazo compoimd without the requirement of its purification prior to addition to Cgg. Since they are rapidly trapped by the fullerene, even unstable diazo compounds can be successfully used in the 1,3-dipolar cycloaddition. In a one-pot reaction the tosyUiydrazone is converted into its anion with bases such as sodium methoxide or butylHfhium, which after decomposition readily adds to Cgg (at about 70 °C). This method was first proven to be successful with substrate 142. Some more reactions that indicate the versatility of this procedure are shown in Table 4.4. Reaction of 142 with CgQ under the previously described conditions and subsequent deprotection of the tert-butyl ester leads to [6,6]-phenyl-C5j-butyric acid (PCBA) that can easily be functionalized by esterification or amide-formation [116]. PCBA was used to obtain the already described binaphthyl-dimer (obtained from 149 by twofold addition) in a DCC-coupling reaction [122]. 

A series of pyrazolino[60]fullerenes has been prepared in one-pot reactions by 1,3-dipolar cycloaddition of the corresponding nitrile imines [306-311]. In all cases... 

Semiempirical AMI and DFT (B3LYP/6-31G ) calculations were used to investigate the highly diastereoselective 1,3-dipolar cycloaddition of 1,4-dihydropyridine- (g) containing azomethine ylides to [60]fullerene (Prato s reaction). The activation energy for the four calculated transition state structures favours the formation of SSaS and... 

RSaS stereoisomers.29 The 1,3-dipolar cycloaddition of [60]fullerene with diazomethane, nitrile oxide, and nitrone afforded fullereno-pyrazolines and -isoxazolines. These reactions were modelled at the B3LYP/6-31G(d,p)//AMl level and the reaction mechanisms, regiochemistry, and nature of addition were investigated.30... 

An ab initio and semiempirical (AMI) study on the 1,3-dipolar cycloaddition of a 1,4-dihydropyridine 6, bearing an azomethine ylide at the 3-position, with fullerene to give four diastereomeric products showed the activation energies of the four transition states to be very similar <2005JOC3256>. The formation of the SSaS stereoisomer 7 was determined to be favored and resulted from the kinetically controlled reaction (Scheme 2). The calculations predicted a product ratio for the reaction which was in agreement with experimentally obtained results <2003T9179>. 

Besides the 1,3-dipolar cycloaddition of azomethine ylides to C60, the Bingel cycloprop anation reaction is widely used for regioselective functionalization of fullerenes. In principle, this versatile modification involves the generation of carbon nucleophiles from a-halo esters and their subsequent addition to C60 [19]. The addition takes place exclusively on double bonds between two six-membered rings of the fullerene skeleton, yielding methanofullerenes. As shown in Scheme 2, addition of diethylbromomalonate to C60, in the presence of an auxiliary base... 

Beyond the widely used synthetic methodologies of 1,3-dipolar cycloadditions, Bingel cyclopropanations, and various other cycloadditions on fullerene skeletons, several novel organofullerene materials have been produced utilizing reactions that do not fall into any of the above-mentioned categories. Nevertheless, due to their unique formation, that could not otherwise be reached via those established synthetic methodologies, a few such chemical transformations of fullerenes are presented in this section. 

Several organofullerene donor-acceptor molecular material hybrid systems have been synthesized via 1,3-dipolar cycloaddition reactions of azomethine ylides, via Bingel cyclopropanation and methanofullerene formation intermediates as well as via cycloaddition reactions, that have already been discussed in previous sections. The majority of such hybrid systems possess always as acceptor unit the fullerene core and as donor moieties porphyrins, tetrathiafulvalenes, ferrocenes, quinones, or electron-rich aromatic compounds that absorb visible light [190-193]. The most active research topic in this particularly technological field relies (i) on the arrangement of several redox-active building blocks in... 

Some representative examples of fullerene-porphyrin dyads are shown in Scheme 9. In other examples, porphyrin analogs such as phthalocyanines and subphthalocyanines have been used for the construction of efficient dyads. Again, the most straightforward approach for their synthesis involved 1,3-dipolar cycloaddition of the appropriate azomethine ylides to C60 [203-205]. Also, with the aid of the Bingel reaction, other phthalocyanine-fullerene systems have been prepared [206,207] with the most prominent example being the one that contains a flexible linker possessing an azacrown subunit [208]. The novelty of this dyad can be found in the nature of the linker that could, in principle, induce conformational changes in the multicomponent system when certain ions (e.g., alkaline ions) are present. As a direct consequence this would potentially allow an external control over the electronic interactions between the phthalocyanine and fullerene units. 

The design of covalently linked donor-fullerene systems capable of undergoing photoinduced electron-transfer processes has been widely studied as a result of the remarkable photophysical [35] and electronic [36] properties of fullerenes. Porphyrins, phthalocyanines, tetrathiafulvalenes, carotenes, and ferrocene [37] have been covalently attached to the fullerene sphere, usually as pyrrolidine[ 60] fullerene derivatives by 1,3-dipolar cycloaddition reactions. 

Dipolar cycloaddition reactions of azomethine ylides are probably the most widely used reactions for functionalization of [60]fullerene. This reaction has also been used to obtain SWNT derivatives and, occasionally, microwave irradiation has been used [89]. Pyrrolidino-SWNT 76 was synthesized by reaction of pristine... 

The solvent-free mechanochemical reaction of Ceo with ethyl 2-diazopropionate was investigated by Wang and coworkers [14], For this purpose, ethyl 2-diazopropionate was prepared in situ from alanine ethyl ester hydrochloride and reacted with fullerene. In the course of reaction, three products, methanofuUerene 27 and fulleroids 28 and 29 were obtained in 1.2 1 4.6 ratio (Scheme 7.7). When milling was carried out with preformed ethyl 2-diazopropionate, products 27-29 were obtained in 3.6 1 3.6 ratio and 27% yield. For comparison, reaction carried out in toluene for 16h at room temperature afforded 46% of compounds 27-29 in a ratio of 4 1 10. Although the pyr-azoline intermediate was not observed reaction is presumed to take place by initial 1,3-dipolar cycloaddition and subsequent nitrogen elimination from unstable pyrazo-line adduct. 

Question on stability of the pyrazoline intermediates formed in 1,3-dipolar cycloadditions of alkyl diazoacetates with fullerenes was addressed in subsequent publication [15]. The reaction of glycine ethyl ester hydrochloride (or glycine octyl ester hydrochloride) with sodium nitrite was utilized to prepare ethyl diazoacetate in situ under the HSVM conditions. After 30min of vigorous milling at room temperature, 2-pyrazolines 32a and 32b were formed in 48% and 49% yield, respectively (Scheme 7.8). These pyrazolines were formed via isomerizations of 1-pyrazolines 31a,b which are obtained directly by 1,3-dipolar cycloadditions. 

A variant of the Huisgen 1,3-dipolar cycloaddition reaction provides a new and convenient functionalization of fullerenes... 

Theoretical studies suggest that the cycloaddition of nitrones, to C6o and CNTs, is the least favored reaction of the several 1,3-dipolar cycloaddition reactions studied (03JA10459, 09CEJ13219). Despite these premises, the experimental results show that it is possible to functionalize MWCNTs with cyclic nitrones (09CC252,1 ICMl923). While fullerene and SWCNTs do not apparently react with nitrones, MWCNTs react with cyclic nitrones upon refluxing in DMF. The harsh reaction conditions require the use of stable nitrone 74. This reaction produces functionalized CNTs 75 that are pretty soluble in DMF and dispersible in polymers. The markedly different reactivity of MWCNTs with respect to SWCNTs arises from the higher numbers of defects that are present on the wall of MWCNTs. A Raman measurement and a theoretical study support the hypothesis. 

Perfluorinated BSubPc bearing axial m-formylphenoxy group 196 [99] was used for preparation conjugates with fullerene (Scheme 51) as tunable molecular scaffolds for intramolecular electron and energy transfer processes [110]. The azome-thine ylide formed by treatment of the formyl derivative 196 with A -methylglycine gives the conjugate with fullerene 197 (44 %) in a dipolar 1,3-cycloaddition (Prato reaction). 

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