Cluster-opening reaction

Scheme 11.5 Reaction of 16 with singlet oxygen, and structure motif of the open ring within the cluster framework of 22 and 25. Curly arrows within the formulas 20 and 23 indicate the cluster opening reaction leading to the valence isomers 22 and 25.

Framework Modifications of [60]Fullerene Cluster Opening Reactions and Synthesis of Heterofullerenes... 

Conversely, cluster degradation reactions lead to more open structures, e.g. ... 

In spite of the apparent difference between conjugate addition and carbocupra-tion reactions (Sect. 10.3.2), the similarities between the key organometallic features of the two reactions are now evident. In both reactions, inner sphere electron-transfer converts the stable C-Cu bond into an unstable C-Cu bond, and the cluster-opening generates a nucleophilic, tetracoordinated alkyl group. The difference is that the product of conjugate addition (PD) remains as a lithium enolate complexed with RCu (Scheme 10.5), while the initial product of carbocupration... 

The accessibility of new techniques such as EXAFS brings researchers a powerful tool for unambiguous determination of the true core metallic framework of such systems. Thus, the relationship between the parent carbonyl precursor, the support and the final metal-supported particles has been studied at the structural atomic level in some cases. This can allow differentiation of the catalytic behavior of supported metal particles with bulk-like properties from that of supported metal clusters, opening the way to understanding the mechanism of metal-catalyzed reactions and extending the concept of sensitive or insensitive structure reactions from metal aggregates to clusters. 

Examples of reductive cluster-opening and oxidative cluster-closing reactions are common in the chemistry of metal-hydrocarbon tt complexes. For example, bases convert nido- (hexa-hapto)arene-manganese tricarbonyl complexes into aracAno(pentahapto)-7T-cyclohexadienyl complexes 129,130, 217) ... 

This exemplifies a quite general reaction in organometallic chemistry, the cluster-opening conversion of an w-hapto ligand into an ( — 1 )-hapto ligand by addition of a nucleophile X . The converse, cluster-closing conversion accompanies removal of X . 

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]. 

Scheme 11.4 Photochemical reaction of 14 with singlet oxygen, leading to the cluster opened fullerene 19. (i) O2 hv, benzene (II) [2+2] cycloreversion, keto-enol tautomerization.

Cluster opened bislactams have been obtained in a one-pot reaction by treating diazidobutadiene 30 with Cjq for four days in ODCB at 55 °C [3, 35]. The first step in the reaction sequence can be assumed to be the formation of the bisazafulleroid 31 (Scheme 11.7). Subsequent addition of 2 leads to the endoperoxide intermediate 32, which isomerizes via a electrocyclic ring opening to 33 [36]. Subsequent oxidation with DDQ or O2 afforded the dehydrogenated bislactam 34. 

The second synthetic approach to heterofullerenes in bulk quantities is based on the fragmentation of 4 in the gas phase (Scheme 12.3) [3, 12]. The reaction of 4 with 20 equiv. p-TsOH in refluxing ODCB in an argon atmosphere leads to 2 in an optimized yield of 26%. Interestingly, together with the dimer 2 the alkoxy substituted monomeric compoimd 8 was formed. This exohedral heterofuUerene adduct, however, is not stable in the long term in solution but decomposes to form a cluster opened system exhibiting carbonyl vibrations in the IR spectra. Nevertheless, 8 was the first heterofuUerene whose NM R spectrum shows the resonance for sp -fuUerene C atoms at 8 = 90.03 [3] similar to those of the interfullerene bond within 2 [15]. 

Treatment of 2-methylbenzotelluroazole 45 (R = Me) with triiron dodecacarbonyl gave instead both a direct coordination compound 29 and a cluster compound 78 as the result of a ring-opening reaction (Equation 4) <19970M3194>. 

The synthesis starts from 2,4,4-trimethyl-4,5-dihydrooxazole, a compound with marked C—H acidity in the exocyclic methyl group. Triple lithiation followed by derivatiza-tion with chlorotrimethylsilane affords a triply silylated, open-chain ketenimine which is reacted with [AuCl(PPh3)] in the presence of caesium fluoride to give a dimeric, octanu-clear gold(I) cluster. The reaction proceeds beyond the stage of a triauriomethyl oxazolinyl derivative which, in fact, can only be observed as a side product. 

Here, a first review is provided which summarizes the more recent development framework modified fullerenes like cluster opened structures and heterofullerenes. The key steps for such framework modifications are always defined activations of the fullerene cluster due to specific covalent addition reactions. Therefore, the principles of covalent fullerene chemistry [3-8] will be considered first ... 

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