1.2- Dihydro fullerenes

In principle, 23 regioisomers of the dihydro fullerene CggH2 are possible. The formal addition of an A-B molecule, for example H2, to the externally Cgg sphere could proceed in three ways [30] (1) Addition to one double bond of the low-energy Kekule structure (Figure 5.3), which would leave all the other bonds unchanged ([6,6] double bonds and [5,6] single bonds) (2) conjugate addition of two atoms, which requires... 

For methanofullerenes, the triplet-triplet absorption appears arround 720 nm, for cyclohexylfused derivatives, pyrrolidino- and dihydro-fullerene, the absorption appears arround 700 nm [62,71,72]. 

The enamine is unique in that it also has a photosensitizer (the dihydro-fullerene chromophore) in the same molecule, and brief exposure to air and room light leads to cleavage of the enamine double bond, producing the ketoamide shown below. The well-known photooxidative cleavage of enamines proceeds via an intermediate 1,2-dioxetane [119,120]. Although many 1,2-dioxetanes are relatively stable, those from enamines are not, and cleave to ketone and amide fragments, below — 40 °C in most cases. The ketoamide was characterized by FAB ms (m/e = 863), IR, and and CNMR (carbonyls at 170 ppm for the amide and 204 ppm for the ketone and overall C, symmetry for the fullerene carbons). 

Since their discovery, fullerenes and their derivatives have been the subject of very extensive research. One of the topics investigated intensively are the linear and nonlinear optical (NLO) properties, owing to a variety of possible applications. Here we review some of the recent work of our group in this area, which is concerned with the ab-initio calculation of molecular NLO properties of two different kinds of fullerene derivatives, a) substituted 1,2-dihydro fullerenes and b) fullerenes endohedrally doped with atoms or small molecules. Apart from the purely electronic response, we also focus on the vibrational contributions to the NLO response, that is, to the response of the nuclei to the external electric fields. 

The H C ratio in hydrocarbons is indicative of the hydrogen deficiency of the system. As mentioned, the highest theoretical H C ratio possible for hydrocarbon is 4 (in CH4), although in electron-deficient carbocationic compounds such as CH5 and even CH/, the ratio is further increased (to 5 and 6, respectively, see Chapter 10). On the other end of the scale in extreme cases, such as the dihydro- or methylene derivatives of recently discovered Cgo and C70 fullerenes, the H C ratio can be as low as 0.03. 

Buchko GW, Wagner JR, Cadet J, Raoul S, Weinfeld M (1995) Methylene blue-mediated photooxidation of 7,8-dihydro-8-oxo-2 -deoxyguanosine. Biochim Biophys Acta 1263 17-24. Burlaka AP, Sidorik YP, Prylutska SV, Matyshevska OP, Golub OA, Prylutskyy YI, ScharIF P (2004) Catalytic system of the reactive oxygen species on the C60 fullerene basis. Exp Oncol 26 326-327. 

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... 

The NMR spectra of C qHR exhibits 37 resonances of the fuUerene carbons, with two of them in the sp region, proving Q-symmetry for the C oHR adducts. This is consistent with an addition to a double bond of a pole corannulene imit (1,2-addition), leading to l,2-dihydro[70]fullerene derivatives. These particular [6,6] bonds of Cyg, located between the carbons of the sets A and B, have almost the same bond length as the [6,6] bonds in Cjq [30], and the pole corannulene unit also exhibits bond alternation with longer [5,6] bonds. NMR data imply that the initial attack of the nucleophile occurred on C-1 and the protonation on C-2. 

Dimers with two methylene bridges and a cyclobutane fusion were synthesized by thermolysis of ethoxycarbonylmethano-l,2-dihydro[60]fullerene. They have a similar structure as 328 [374],... 

Upon hydroboration of C70 followed by hydrolysis of the presumed intermediates C7QHBH2, two isomers of C70H2 are obtained. These are the l,2-dihydro[70]fullerene (4) as the major and the 5,6-dihydro[70]fullerene (5) as the minor reaction product (Figure 5.1) [16, 17]. 

Photoinduced electron transfer [22] from reductants such as l-benzyl-l,4-di-hydronicontinamide [27], the Hantzsch-ester [22] (diethyl-2,6-dimethyl-l,4-di-hydropyridine-3,5-dicarboxylate) or 10-methyl-9,10-dihydroacridine [27, 28] to the fullerene and successive proton transfer leads selectively to l,2-dihydro[60]fullerene. These reductions usually proceed under mild conditions. 

Figure 14.8 Lowest energy VB structures, PM3 calculated lengths of [5,6 - double bonds, HOMO coefficients and energies of different dihydro[60 fullerenes.

It is therefore useful to look at a l,2-dihydro[60]fullerene as a stereoelectronically slightly perturbed Cjq. Introduction of a double bond into a five-membered ring costs about S.Skcalmol (Figure 14.10) [111]. In a 1,4-adduct (l,4-dihydro[60]-fullerene) one, and in a 1,6-adduct (l,16-dihydro[60]fullerene or 1,6- dihydro[60]-fuUerene) two, double bonds in five-membered rings are required for the corresponding lowest-energy Kekule structure. This VB consideration is also confirmed experimentally and by computations (Figure 14.8) [36],... 

Table 14.2 Calculated relative PM3 energies (kcal mol ) of dihydro[50]fullerene derivatives with addends of different steric requirement [36].

Irradiation of 2,3-diphenyl-2//-azitine in the presence of Cgo fullerene leads to the formation of mono- and ohgo adducts (98,99). A monoadduct, l,9-(3,4-dihydro-2,5-diphenyl-2//-pytTolo)fullerene-60 was isolated and characterized. Mechanistic studies showed that under conditions of direct irradiation it was formed by a classic nitrile yhde cycloaddition but in the presence of 1,4-napthalenedicarbonitrile (DCA) it resulted from reaction of the radical cation intermediate 108. Cycloaddition reactions have also been carried out with diaza-phospholes and diazaarsoles (100) to give adducts of the type 189 (A=As,P) and with cyanogen to give 190 and with atyldiazocyanides where addition to both the azo moiety and the cyano group were observed (101). 

Keywords [60]fullerene, zinc, ethyl bromoacetate, 1-ethoxycarbonylmethy 1-1,2-dihydro [60] fullerene... 

As a first example, the photochemical synthesis of substituted 1,2-dihydro-[60]fullerenes will be discussed. These compounds can be synthesized by various photochemical reaction pathways. In the first one the radical cation Qo is involved in the reaction. In 1995, Schuster et al. reported the formation of C6o radical cations by photosensitized electron transfer that were trapped by alcohols and hydrocarbons to yield alkoxy or alkyl substituted fullerene monoadducts as major products [211], Whereas Foote et al. used N-methylacridinium hexafluorophos-phate NMA+ as a sensitizer and biphenyl as a cosensitizer [167], Schuster et al. used 1,4-dicyanoanthracene (DCA) as a sensitizer for the generation of C 6o- The... 

NMA+) and 2,4,6-triphenyl-pyrylium tetrafluoroborate (TPP+) in the presence of biphenyl as cosensitizer were suitable for this reaction [174], The assumed mechanism of formation of do by this cosensitization is shown in Scheme 7. Reaction of do with H-donors such as te/t-butylmethylether, propionaldehyde and alcohols results in the formation of 1 1 adducts, the 1-substituted 1,2-dihydro-[60]fullerenes. Product structure support a H-abstraction process [212,213] rather than nucleophilic addition. In Scheme 8, the general formation of 1-substituted l,2-dihydro-[60]fullerenes is shown. Selected examples of the products obtained by this method are summarized in Table 10. 

Scheme 8 Formation of dihydro[60]fullerene under PET conditions (TPP+ or DCA/BP, 419nm).

Table 10 Selected Examples of 1-Substituted 1,2-Dihydro[60]-Fullerenes...

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). 

Scheme 22 Formation of l,2-dihydro[60]fullerenes 57a-b upon irradiation of C6o and cyclic amino acids 56a-b.

Gan et al. used amino acids such as piperidino acetic acid or morpholino acetic acid in photochemical reactions with C60 [231,232]. Whereas the reaction of the ester derivatives of the amino acids result in the formation of pyrrolidine-fused C6o derivatives, the reaction of the free amino acids give the 1-substituted l,2-dihydro[60]fullerenes 57a-b (Scheme 22). In a typical experiment a methanol solution of the amino acid 56a-b was added to a toluene solution of C6o and irradiated with a 250 W overhead projector lightbulb. Prolonged irradiation led to multiadducts. To avoid hydroxylation of C60 the pH of the amino acid solution should be no more than 9.0. 

Sun et al. further investigated the photoinduced electron transfer reactions of C6o and triethylamine, both in deoxygenated solution and air saturated solution [79], Three types of cycloadducts of fullerenes 33 and 92a-b were obtained, whereas the formation of the monoalkylated l,2-dihydro[60]fullerene 29 as described by Liou et al. [230] in the reaction of trimethylamine and /V,/V-dimcthy-laniline with C6o, was not observed (see Fig. 32). 

Photophysical investigations of C60 and its derivatives have revealed that [60]fullerene and l,2-dihydro[60]fullerene derivatives produce singlet oxygen in high quantum yields. Consequently, the fullerene compounds are used as photo-... 

Figure 1.54. A bichromophoric dyad used for photoinduced electron transfer studies and obtained as a mixture of diastereoisomers. It includes a 1,2-dihydro[60]fullerene acceptor, a [Ru(bpy)3]2+ donor unit, and a rigid spacer derived from 4,5-dihydrotestosterone. (The asterisk marks the stereogenic center giving rise to the diastereoisomerism.)...

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