Azafullerenes

Axle and collar molecular structure, 27 60-61 Axokine, 3 97 Azaboranes, 4 170, 204 Azacarbocyanine dyes, 9 257 Aza[18]crown-6, 24 41 Aza[18]crown-6 appended a-helical barrels / rods, 24 58 Azacrown ethers, 24 44 Azacrowns, 24 41 Azacryptands, 24 42 Azadirachtin, 24 474 Azafullerenes, 22 231, 232, 243 Azahomofullerenes, 22 243 Azaindene compounds, 29 197 Azalactone method (for Emphaze supports), for covalent ligand immobilization, 6 396t Azamethiphos (salmosan)... 

FIGURE 62. 13C NMR data of azafullerenes 112-115. Reprinted with permission from Reference 69. Copyright (1997) American Chemical Society... 

Re-evaluation of pulse delay times used to record fullerene 13C NMR spectra revealed that a 16 s pulse delay, twice the value for a standard detection, allowed the observation of a weak resonance in the sp3 region at 90.4 ppm in the 13C NMR spectrum of the unlabeled heterofullerene 114. Attempts were made to optimize the NMR experimental parameters for a long 7 i, i.e. the variation of delay times and pulse angles. Various conditions were tried on the labeled material without success. This is probably due to the mixture of the labeled and unlabeled 114 which give too low S/N for signal detection. Table 49 summarizes the NMR results obtained and illustrates a distinct pattern of the azafullerenes. 

In Scheme 1.15, path a can be demonstrated with examples of anion-radicals of amino and hydroxy derivatives of 2,1,3-benzothiadiazole (Asfandiarov et al. 1998) and the azafullerene anion-radical, C59HN (Keshavarz et al. 1996). One of the examples, hydrogen atom detachment from C59HN, is as follows ... 

Lull and co-workers showed that the tethered bisazides such as 2,2-dibenzyl-l,3-diazidopropane (124) undergo [3-t2] cycloadditions to adjacent [6 6]-bonds (cis-l addition). Thermal extrusion of N2 afforded a mixture of the corresponding cis-1-1,2,3,4-bisimino[60]fullerenes and twofold cluster-opened 2,3,4,5-bis-aza-homo[60]-fullerenes [36, 37, 111]. The reaction of with optically active bisazides yielded enantiomerically pure bis-azafullerenes with the same addition pattern [111]. 

Since heterofullerene chemistry began, as well as 8, 17 and 18 various further derivatives formed during the formation of azafullerenes or from (C59N)2 (2) itself have been synthesized. Heterofullerene transformation of the ketolactam 6 in the presence of a 15-fold excess of hydroquinone leads to the parent hydroaza[60]-fullerene 25 [14]. The hydroquinone is assumed to reduce the C59N radical intermediate (Scheme 12.6). 

Arylated azafullerenes are also accessible from the heterofullerene precursors 6 and 33 by thermal treatment in the presence of aromatics and air (Scheme 12.12) [3]. Although the yields are lower than those starting from 2, one reaction step can be saved. Conversely, if the reaction is carried out in 1-chloronaphthalene at 220 °C, 34 is obtained in an isolated yield of 46%. 

Heterofullerene chemistry is still a very young discipline within synthetic organic chemistry and even within fullerene chemistry. So far, it is restricted to mono-azafullerenes. However, the potential of structural diversity within heterofullerenes is enormous (Figure 12.2). Preparative challenges for the future are, for example. 

Among other examples, the thermal [3+2] cycloaddition of azides to C60 [72-74] plays a role in the initial step towards the macroscopic synthesis of [60]azafullerenes [75-77]. 1,3-Dipolar cycloaddition of azides to C60 at temperatures below 80 °C occurs preferentially to a 6,6 double bond affording the corresponding thermally labile triazolines. Due to the lower symmetry D5h of C70 as compared to Ih symmetrical C60, the regiochemistry of [3+2] cycloadditions of azides gives isomeric mixtures of [70]triazolines (Scheme 4) that can be eventually separated and serve as precursors in the synthetic route to [70] azafullerenes [79-81]. 

Its application to according derivatives of C70 afforded the achiral azafullerene dimers 97 and 98 for the first time.47,159... 

Fullerenes. A resonance Raman study of C6o in its first allowed electronic excited state shows that the ground state hg(l) mode splits into two components, 265, 281 cm-1. The data are consistent with D5d symmetry for the excited state.175 Raman spectroscopy was used to characterise C60 units in a tantalum oxide lithium fulleride composite.176 A group theoretical analysis has been made of the vibrational normal modes for the azafullerene C4gNi2.177... 

By substituting single atoms, for example, with N, a heterofullerenyl radical is obtained from >. Usually this will form a dimer, which indeed is the species that has first been detected. The bond connecting two azafullerenes is formed between... 

The replacement of carbon atoms in fullerenes by boron and/or nitrogen atoms has been repeatedly discussed in the literature, both theoretically j79-81j and experimentally. Smalley s group at the Rice University in Houston has successfully obtained polyaza-fullerenes from a mixture of graphite and boron nitride under the same conditions in which graphite yields fullerenes [82j by adding azides to buckminsterfullerene, Prato et al. j83j synthesized azafullerenes. 

The state of the art in heterofuUerene chemistry and physics is reviewed with emphasis on azafullerenes. The macroscopic synthetic methods that have been developed for aza[60] fullerene compounds since 1995 have led to a whole new and rich area in the science of fuUe-renes cage modification chemistry. The synthetic routes towards aza[60]fuUerene and its derivatives are reviewed in Sect. 2. The synthetic routes for aza[70]fuUerene and its derivatives are summarized in Sect. 3. Section 4 comprises the theoretical and experimental work on the physicochemical properties of azafuUerene compoimds. Finally, in Sect. 5, the literature regarding heterofullerenes other than monoazafuUerenes is reviewed. 

These gas-phase observations paved the way for the first synthetic route towards azafullerenes, as described in the next section. The cage-opened structures 22 and 26 are asymmetric. Ketolactam 22 was later resolved on a chiral HPLC column, and the circular dichroism of the enantiomers was determined [64]. 

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