Electron Addends

Cycloaddition is not necessarily restricted to the active positions flanking the heteroatom. 7V-Methylnaphtho[l,8- Ze]triazine and dimethyl acetylenedicarboxylate give an alternative adduct (Fig. 7b) —an addition which is also facilitated by the frontier orbital interactions. 

Simitar reactivity is observed between betaines and electron-deficient alkenes (E CH CHj) and a high degree of regiospecificity is often observed. Pyridinium-3-olates (Section III,A,2) react with acrylonitrile or methyl- 

Cycloadditions of betaines are not restricted to electron-deficient alkenes. Pyridinium-3-olates also react with conjugated olefins (e.g., styrenes) and with electron-rich olefins (e.g., ethyl vinyl ether). In the latter case, the betaine LUMO/alkene HOMO interaction becomes dominant and reaction is only observed with pyridinium-3-olates having a low-energy LUMO 

CHCN Fig. 8. Calculated orbital coefficients and preferred geom- 

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The study of reactions between mesomeric betaines and 1,3-dienes has so far been restricted to pyridinium-3-olates (Section III,A,2) and quinolinium-3-olates (Section III,B, 18). The mode of addition of dienes differs from that of 23T-electron addends in two respects (i) Dienes add across a different... 

With a suitable combination of electron-deficient fluoroalkene and electron-rich addend, cycloaddition can proceed by a dipolar mechanism involving zwitte-rion intermediates Like Us isomer, l,2-bis(trifluoromethyl)-l,2-dicyanoethylene [85], l,l-bis(trifluoromethyl)-2,2-dicyanoethylene forms cyclobutanes by an ionic mechanism [104, 105, 106] (equations 39 and 40)... 

Both the carbon-carbon and carbon-oxygen double bonds of fluoroketenes can take part in [2+2] cycloadditions, but with cyclopentadiene, only cyclo butanones are produced via concerted [2 +2 ] additions [J34] (equation 58) Cycloadditions involving the carbon-oxygen double bonds to form oxetanes are discussed on page 855 Difluoroketene is veiy short lived and difficult to intercept but has been trapped successfully by very electron rich addends to give 2 2 di fluorocyclobutanones m moderate yields [/55] (equation 59)... 

Sensitized cross-dimerizations form a second group with a few examples shown in Eqs. 24—27. Most examples involve electron-deficient olefins as one addend. The reactions are again highly regioselective... 

Qualitatively, the interaction diagram would closely resemble that in Fig. 3, since electron-donating substituents in both addends would raise the molecular levels of both the carbonyl compound and the olefin. Only the energy gap, E(n)-> F(n), would increase, the net result being that the calculated ratio of concerted to biradical reaction, Eqs. 40 and 41, should be even closer to unity than in the formaldehyde-ethylene case. Detailed calculations 38> support this conclusion, so PMO theory predicts that the overall stereochemical results are due to a combination of concerted (singlet) and biradical (triplet) mechanisms. This explanation agrees with the experimental facts, and it bypasses the necessity to postulate differential rates of rotation and closure for different kinds of biradical intermediates. 

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). 

The electrochemistry of derivatized Cjq has also been widely investigated [8, 23-28], As observed by electrochemical reduction, derivatization usually decreases the electron affinity of the CgQ-sphere. Typically, cathodicaUy (more negative) shifted waves have been observed by cyclovoltammetry and other methods. Depending on the addend, the shifts range from 30 to 350 mV per adduct with respect to those of pure Cgfl. Reduction of some derivatives resulted in the loss of the addend. In some cases, like the retro-Bingel-reaction (Section 3.2.2), this can also be advantageous. 

The synthetically most valuable intermediate in heterofullerene chemistry so far has been the aza[60]fulleronium ion C59N (28). It can be generated in situ by the thermally induced homolytic cleavage of 2 and subsequent oxidation, for example, with O2 or chloranil [20-24]. The reaction intermediate 28 can subsequently be trapped with various nucleophiles such as electron-rich aromatics, enolizable carbonyl compounds, alkenes and alcohols to form functionalized heterofullerenes 29 (Scheme 12.8). Treatment of 2 with electron-rich aromatics as nucleophilic reagent NuH in the presence of air and excess of p-TsOH leads to arylated aza[60]fullerene derivatives 30 in yields up to 90% (Scheme 12.9). A large variety of arylated derivatives 30 have been synthesized, including those containing cor-annulene, coronene and pyrene addends [20, 22-25]. 

Systematic investigations of twofold additions of malonates to C70 revealed that the second addition takes place at one of the five a-bonds of the unfunctionalized pole [17, 26], With achiral, C2v-symmerical malonate addends, three constitutionally isomeric bisadducts are formed An achiral one (C2v-symmetrical 1), and two chiral ones (C2-symmetrical 2 and 3), which are obtained as pairs of enantiomers with an inherently chiral addition pattern (Figure 13.5). Twofold addition of chiral malonates leads to the formation of five optically active isomers, two constitutionally isomeric pairs of C2-symmetrical diastereomers and a third constitutional C2-symmetrical isomer (Figure 13.5). Twofold additions of azides to C70 lead to diazabis[70]homo-fullerenes, which served as starting material for the synthesis of bis-(aza[70]-fullerenyl) (Cg9N)2 (Chapter 12) [27]. As further bisadditions, addition reaction to C70 [2+2]cycloaddition of electron-rich bis(diethylamino)ethyne and 1-alkylthio-2-(diethylamino)ethynes [28] and the addition of transition metal fragments have been reported [29-32],... 

The covalent chemistry of fullerenes has developed very rapidly in the past decade in an effort to modify fuUerene properties for a number of applications such as photovoltaic cells, infrared detectors, optical limiting devices, chemical gas sensors, three-dimensional electroactive polymers, and molecular wires [8, 25, 26, 80-82]. Systematic studies of the redox properties of Cgo derivatives have played a crucial role in the characterization of their unique electronic properties, which lie at the center of these potential applications. Furthermore, electrochemical techniques have been used to synthesize and separate new fullerene derivatives and their isomers as well as to prepare fullerene containing thin films and polymers. In this section, to facilitate discussion of their redox properties, Cgo derivatives have been classified in three groups on the basis of the type of attachment of the addend to the fullerene. In group one, the addends are attached via single bonds to the Cgo surface as shown in Fig. 6(a) and are referred to as singly bonded functionalized derivatives. The group includes... 

Finally, cycloadducts (22a-c) exhibit an addend-based first reduction wave, which is very close to the first reduction potential of pristine Ceo- Remarkably, as shown in Table 14, the choice of substituent on the p-benzoquinone moiety affects the redox behavior of these derivatives. By controlling the relative energy of the LUMO of the organic addend with respect to the LUMO of Ceo, the first reduction process may be forced to occur at either the C60 cage or the organic addend, allowing for tunability of these electron-acceptor... 

After addition of two-electron equivalents, the current for the CPE performed earher on (58-62) did not reach background levels. In fact, a substantial amount of current remained. If electrolysis was allowed to continue until the current decreased to background level, a total of six electrons per molecule were added, and analysis of the products after reoxidation indicated that both addends had been completely removed, producing Ceo in about 75% yield. These results are summarized in Sch. 3. 

Clearly, depending on the number of electrons per molecule added during CPE, either removal of the addends [170] or an isomerization reaction was attained. The former reaction was initially called the rctro-Bingd reaction [64], since it was the reverse of the Bingel addition of methano addends to Ceo [171]. The generality of the retro-Bingel and isomerization reactions has since been tested using other multiple-malonate adducts of Ceo [172,173]. In fact, the reaction has made it possible to isolate the Civ isomer of Cyg, a new Cyg bis-adduct, and new isomers of Cg4 [44, 54]. 

One of the most characteristic types of ground-state reaction for alkenes is electrophilic addition, often involving a proton acid as addend or catalyst. In the excited state similar reactions can occur, with water, alcohols or carboxylic acids as commonly encountered addends. However, there is a variety of photochemical mechanisms according to the conditions or substrate used. In a few instances it is proposed that the electronically excited state is attacked directly by a proton from aqueous acid, for example when styrenes are converted to l-arylethanols (2.47 the rate constant for such attack is estimated to be eleven to fourteen orders of magnitude greater than that for attack on the ground state, and the orientation of addition is that expected on the basis of relativecarbonium ion stabilities (Markowni-kov addition). 

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