Fullerene 1, 3-dipolar cycloaddition

Dipolar cycloaddition in the synthesis of fullerene Ceo derivatives containing heterocyclic fragments 98KGS291. 

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... 

Compound 318 used as dipolarophile with ylide 315 (Ar = 2,4,6-Me3C6H2) gives spiro compound 319 (Equation 46) <2001HCA3403>. The 1,3-dipolar cycloaddition of 3-oxo-2-pyrazolidinium ylide 315 (Ar = Ph) with buckminsterfullerene Cgo yields new heterocyclic fullerene derivatives <1995TL2457>. 

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). 

Electroactive 3-(N-phenylpyrazolyl)fullereno[l,2-r/]isoxazolines have been synthesized by using 1,3-dipolar cycloaddition of pyrazole nitrile oxides, generated in situ, to Cgo at elevated temperature or microwave irradiation. The cyclic voltammetry measurements show a strong donor pyrazole ring, and a better acceptor ability of the fullerene moiety than the parent C60 (538). Treating fullerene Cgo with mesitonitrile oxide in toluene gives fullerene-nitrile oxide adduct, which is supposed to be useful for electrical and optical components (539). 

The synthesis of C60-based dyads in which the Ccm core is covalently attached to a strong electron acceptor moiety, has been carried out by 1,3-dipolar cycloaddition of in situ generated nitrile oxides with C(,o- As expected, the obtained adducts show reduction waves of the fullerene core that are anodically shifted in comparison with the parent Cr>o. This indicates that they are remarkably stronger acceptors than Ceo-The electron acceptor organic addend also undergoes an anodic shift due to the electronic interaction with the C(,o moiety (545). 

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]... 

Da Ros T, Prato M, Novello F, Maggini M, Banfi E (1996) Easy access to water soluble fullerene derivatives via 1,3-dipolar cycloadditions of azomethine ylides to C60. J. Org. Chem. 61 9070-9072. 

Another important method for preparation for exohedrally functionalized fullerenes is the 1,3-dipolar cycloaddition of in s/Yw-generated azomethine ylides to C60 yielding fulleropyrrolidines (Maggini et al., 1993). Further functionalization is facilitated either by the use of adequate aldehydes for the azomethine ylide formation or quatemization of the pyrrolidine nitrogen atom. Both bisaddition (Kordatos et al.,... 

Attaching a Ceo cluster to an [Ru(bpy)3] + core has been achieved by 1,3-dipolar cycloaddition of azomethine ylides to the fullerene. The electrochemistry of the complex is complicated a one-electron reversible oxidation of the Ru center, five one-electron reversible reductions associated with the Ceo cage, and five more reversible reductions centered on the bpy ligands. The photophysical properties of the complex have been discussed. ... 

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]. 

Isoxazoline derivatives of Cgo such as 250 (Scheme 4.40) are accessible by 1,3-dipolar cycloadditions of nitrile oxides to [6,6] double bonds of the fullerene [2, 278, 291-305]. The nitrile oxides 249 with R = methyl, ethyl, ethoxycarbonyl and anthryl are generated in situ from the corresponding nitroalkane, phenyl isocyanate and triethylamine. The isoxazoline derivative of Cgo 250 (with R = anthryl) crystallizes in black prisms out of a solvent mixture of CS2 and acetone (3 2) [292]. X-ray crystal structure analysis shows that addition of the nitrile oxide occurs on a [6,6] double bond of the fullerene framework. 

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... 

Photolytic cleavage of 2,3-disubstituted 2H-azirines or monoaryl-2H-azirines provides access to nitrile yiides with the structure 280.1,3-Dipolar cycloaddition of the in situ generated yiides to Cjq affords mono- or disubstituted pyrrolo fullerenes such as 281 (Scheme 4.50) [319, 320]. Aliphatic 2H-azirines are not reactive, as they have shorter excitation wavelengths than the phenylic substituted 2-azirines. 

Bellavia-Lund and Wudl (43) investigated the 1,3-dipolar cycloaddition of 2-(methoxyethoxy)methyl azide with [70]fullerene (199) (Scheme 9.43). Three isomeric triazolines 200-202 were obtained. Thermolysis of these triazolines gave the corresponding azafulleroids and fulleroaziridines, as a mixture, respectively. 

Tandem intramolecular 1,3-dipolar cycloadditions and cycloreversion, phosphinimine alkylidenemalonate cyclization, and retro-malonate additions have been reviewed.52 The origins of the stereoselection in the 1,3-dipolar cycloadditions to chiral alkenes53 and the 3 + 2-cycloadditions of fullerene, Cea, have been reviewed.54 The selectivity of the double 3 + 2-cycloaddition of tethered double vinyl carbene species in die presence of C6o varies witii the nature of the tether.55... 

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. 

Ferrocene is composed of a pair of 6-7r-electron carbon arrays and a 6-d-electron iron(II) atom. Ferrocene-fullerene donor-acceptor dyads carry all the requisites for electron-transfer phenomena. However, data for the formation of ferrocene-fullerene hybrids are not abundant. Some such dyads have already been synthesized following the methodology of 1,3-dipolar cycloaddition of the appropriate azome-thine ylides to C60, with either variable-spacing building blocks or a rigid-bridge all-cj-bonded framework, in order to tune the redox properties of the system [40,234, 248-251]. Another novel dyad that contained two covalently bound ferrocene units was recently synthesized via cyclopropanation of the fullerene core [252]. 

Some more complicated hybrid systems have also been constructed, sharing more than one donor unit. In one of them, a nonlinear triad where all the electroactive units are linked to the same functionality of a pyrrolidine group (e.g., utilized the 1,3-dipolar cycloaddition of the appropriate azomethine ylides as the critical step for connecting the donor units to the fullerene skeleton) was considered [245]. With this arrangement the formation of any bisadducts was minimized. 

The covalent assembly of functional Jt-systems is a general synthetic principle and in some cases they can even be achieved in a multi-component fashion. One of the most impressive examples is the very elegant access to covalently linked donor-fullerene arrangements by 1,3-dipolar cycloadditions with in situ-generated azomethine ylids [59]. However, here only the multi-component de novo synthesis of the chromophore structures will be considered. The major developments have been achieved in condensation-based and cross-coupling strategies. 

Pc with osmium tetroxide and sodium periodate, and subsequent 1,3-dipolar cycloaddition of the azomethine ylide, formed in the presence of an excess of sarco-sine, and Cgo to give the fulleropyrrolidine-Pc conjugate 17. The second protocol relies on the fulleropyrrolidine formation prior to the statistical cyclotetrameriza-tion with 4-tert-bu tv I p It lit a lonitrile. The low yields obtained via the latter strategy are presumably a result of the steric congestion of the benzodinitrile-functionalized fullerene precursor in the statistical crossover condensation. 

A series of isoindazole-C6o dyads based on pyrazolino[60]fullerene were prepared by 1,3-dipolar cycloadditions of the nitrilimines, generated in situ from hydrazones, to C6o <2002T5821, 2004JOC2661>. 

A simple synthesis of pyrazolo[4, 5 ][60]fullerenes from pyrazolyl hydrazones with [60]fullerene was achieved with microwave irradiation evidence of intramolecular charge-transfer interaction was shown <1999TL1587>. Novel C6o-fused isoxazolines have been synthesized from 1,3-dipolar cycloadditions of pyrazole nitrile oxides to Ceo under thermal or microwave irradiation <1999T4889>. A new triad based on pyrazolino[60]fullerene and a conjugated... 

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