Multiple adducts

For the higher fullerenes beyond C70 only a few examples of adducts with unambiguous structural characterizahon have been reported [2], For example, several Diels-Alder and malonate monoadducts of Dj-Cy have been isolated [10, 36], The addends are bound to a-type bonds of the fullerene framework. Nucleophilic cyclopropanahons of a 3 1 mixture of and D3-C78 with a slight excess of diethyl 

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Thilgen C., Cardullo F., Haldimann R., Isaacs L., Seiler P., Diederich F., Boudon C., Gisselbrecht J. P., Gross M. Synthesis of Multiple Adducts of C60 With Specific Addition Patterns by Simple and Reversible (Templated) Tether-Directed Remote Functionalization Proc. - Electrochem. Soc. 1996 96-10 1260-1271 Keywords fullerene C60, regiochemistry... 

The extent of a reaction in these measurements is determined by bare metal cluster ion signal depletion. In most cases products are also observed. Some systems show multiple adducts indicating comparable or higher rates for each successive step up to a saturation level. For other systems the fully saturated product is observed almost as soon as the reaction starts. This later behavior is characteri sti c of an early rate-limiting step. Due to this complexity kinetics have only been reported on the formation of the first adduct, i. e. for the initial chemisorption step. 

Unlike hydrogen these reactions do not appear to be activated. In addition the products distributions observed indicate comparable rates for multiple adduct formation. The mass complexity, relatively high ionization potentials, and the known prevalent dissociative ionization of the fully saturated carbonyls(42) has possibly caused the failure of some initial saturation experi ments(43). The ability to synthesize the stable carbonyl complexes will help this field significantly due to the vast amount of information available, especially their structures. 

L. Isaacs, E Diederich, R. F. Haldimann, Multiple Adducts of C6o by Tether-Directed Remote Functionalization and Synthesis of Soluble Derivatives of New Carbon Allotropes Cn(60+5) , Helv. Chim Acta 1997, 80, 317-342. 

Reuther U, Brandmuller T, Donaubauer W, Hampel F, Hirsch A (2002) A highly regioselective approach to multiple adducts of Cm governed by strain minimization of macrocyclic malonate addends. Chem. Eur. J. 8 2261-2273. 

The products of these reactions are shown in Scheme 3.12. The addition of reactive nucleophiles usually yields a complex mixture of multiple adducts. Thus it is remarkable that in this case the monoaddition products can be obtained in very good yields. The reduction potential of these compoimds is in the range of pristine Cgo or even higher. 

Phthalimidonitrene 348 is derived by reaction of the corresponding amine 347 with lead tetraacetate in chlorobenzene (Scheme 4.72) [404]. Addition to Cgg affords mono and multiple adducts with up to four phfhalimido addends. 

In 1994, Diederich and co-workers reported a very important approach for the regioselective formation of multiple adducts of Cjq by tether-directed remote functionalization [75]. This technique allows for the synthesis offullerene derivatives with addition patterns that are difficult to obtain by thermodynamically or kinetically controlled reactions with free untethered addends. This important subject has been extensively reviewed [26, 76, 77]. 

It is first worth mentioning some general problems of fullerene characterization. Not only can a mixture of various multiple adducts result from a given reaction, but also each of them may exist as a mixture of regioisomers that can often only be separated by HPLC. In addition, there is often a poor signal-to-noise ratio for many spectroscopic techniques owing to the use of only small amounts of relatively high molecular mass and low solubility. 

The only reported examples of a multiple adduct where the addends bind to Causing different atoms are [Ir(CO)(PPh3)2(i72-C60On)Cl] (n = 1,2) (29,36) and [Ir(CO)(AsPh3)2(T 2-C60O)Cl] (39). For both n = 1 and 2, there is disorder with the occurring in as many as seven different sites. The major forms are shown next and involve the Ir and centers coordinating to adjacent double bonds. No doubt the oxophilic Ir center is initially attracted to a double bond adjacent to the epoxide atom. 

In comparison with pristine C6o and C70, the fullerene derivatives (Fig. 2) show partly different photophysical properties due to the pertubation of the fullerene s TT-electron system. The degree of variation is dependent on (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in the case of multiple adducts on the addition pattern at the fullerene core [59-112],... 

Unlike the reversed shift of the emission band compared to the dihydrogen addend type, the singlet lifetime in the bis- and tris(bis-(ethoxycarbonyl)-methylene) derivatives is increased comparable to the former multiple adducts. The values range from 1.7 to 3.1 ns (tris-adduct), depending on the distorted T7-electron system of the fullerene core [67,108], In comparison to C6o, the fluorescence quantum yield for the malonic ester hexaadduct is increased by the factor 10 (30 X 10 4) [67,111], In the case of both pyrrolidino hexa-adducts (Th 14 and D3 15, Fig. 13), the effect is remarkably higher. The fluorescence quantum yields are increased about 100-fold (-0.02) compared to C6o. On the other hand, the singlet lifetime is only partly increased with -3.5 ns [111,112],... 

In this chapter we have described the photophysics and photochemistry of C6o/C70 and of fullerene derivatives. On the one hand, C6o and C70 show quite similar photophysical properties. On the other hand, fullerene derivatives show partly different photophysical properties compared to pristine C6o and C70 caused by pertuba-tion of the fullerene s TT-electron system. These properties are influenced by (1) the electronic structure of the functionalizing group, (2) the number of addends, and (3) in case of multiple adducts by the addition pattern. As shown in the last part of this chapter, photochemical reactions of C60/C70 are very useful to obtain fullerene derivatives. In general, the photoinduced functionalization methods of C60/C70 are based on electron transfer activation leading to radical ions or energy transfer processes either by direct excitation of the fullerenes or the reaction partner. In the latter case, both singlet and triplet species are involved whereas most of the reactions of electronically excited fullerenes proceed via the triplet states due to their efficient intersystem crossing. 

H20, and multiple adduct ions using ESI. 0-Ethyl (V,(V-dimethylphosphoramidic acid, the hydrolysis product of tabun, gave an abundant MH+ plus polymeric ions (28). 

The positive ion ESI spectrum of VX, which has a high proton affinity by virtue of its diiso-propylaminoethyl substituent, gave the protonated molecule as the only significant ion (40), in contrast to the multiple adducts seen with G agents. In fact, LC/API/MS is a useful alternative to GC/CI/MS as... 

Benzyne, generated by the diazotization of anthranilic acid with isoamyl nitrite, is added to Gd Cs2 forming two isolable isomers of mono-adducts. Electrochemical measurements disclosed that the electronic structure of pristine Gd Cs2 has changed dramatically. Because of the high reactivity of benzyne, multiple adducts are not avoidable, even at lower temperatures [146]. 

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