Radialenes oxidation

The physical properties of the expanded radialenes were greatly enhanced upon donor functionalization, leading to the stable derivatives 76-78 with fully planar conjugated rr-chromophores [110]. These compounds exhibit large third-order nonlinear optical coefficients, can be reversibly reduced or oxidized, and... 

The compound 251 decarbonylates on photolysis to bis(4-hydroxyaryl) acetylene 253, which is easily oxidized to the quinonoid cumulene 254. This is also obtained by thermal decarbonylation of the product of oxidation of cyclopropenone 251, the diquinocyclopropanone 252. Likewise, the blue derivative of 3-radialene 256 (a phenylogue of triketo cyclopropane) is formed from tris-(4-hydroxyaryl) cyclopropenium cation 255 by oxidation34. ... 

Radialenes which are structurally related to 44, i.e. cyclopropanes bearing two quinoid and another acceptor-substituted methylene substituent, were obtained by condensation of bis(4-hydroxyphenyl)cyclopropenones with active methylene compounds, followed by oxidation (Scheme 6)19. Radialenes 45a-f are brilliantly colored solids that are blue or blue-violet in solution but appear metallic gold or red in reflected light. Instead... 

The functionalized [4]radialene 86 offers opportunities for further transformations by hydrolytic cleavage of the O-silylenol moieties and by oxidative desilylation (Scheme 16). Base- and acid-catalyzed hydrolyses lead to different products (130 and 131, respectively)60. By analogy with the formation of 1,4-diketones by oxidative coupling of two siloxyalkene molecules, treatment of 86 with the iodonium salt Phl+—O—+I—Ph BF4 in dichloromethane leads to 132 which is immediately... 

The redox chemistry of [4]radialenes shows similarities as well as differences with respect to [3]radialenes (see elsewhere1 for a more detailed comparison). The simplest [4]radialene for which a redox chemistry in solution is known appears to be octa-methyl[4]radialene (94). It has been converted into the radical anion 94 (with potassium, [2.2.2]cryptand, THF, 200 K) and into the radical cation 94 + (with AICI3/CH2CI2, 180 K)82. Both species are kinetically unstable, but the radical cation is less stable than the radical anion and disappears even at 180 K within 2 hours, probably by polymerization. For the success of the oxidation of 94 with the one-electron transfer system... 

The present volume deals with the properties of dienes, described in chapters on theory, structural chemistry, conformations, thermochemistry and acidity and in chapters dealing with UV and Raman spectra, with electronic effects and the chemistry of radical cations and cations derived from them. The synthesis of dienes and polyenes, and various reactions that they undergo with radicals, with oxidants, under electrochemical conditions, and their use in synthetic photochemistry are among the topics discussed. Systems such as radialenes, or the reactions of dienes under pressure, comprise special topics of these functional groups. 

Deprotonation of 113 generates 114 which is the medium oxidation level of a four-step, reversible redox system enabling the electrochemical transformation of [4]radialene 116 cyclobutadiene (118) via intermediates 115 and 117, if R = CO Et (83LA658) (Scheme 20). 

Scheme 6.1 (A) Synthesis of radialenes 1 by oxidative acetylenic homocoupling [17], and (B) Synthesis ofTTF based [18]annu-lenes 2 and 3 by Pd-mediated homocoupling [25], Reagents and conditions (a) TBAF, THF (b) [Cu(OAc)2], 02, py/PhH (c) Cul, [PdCI2(PPh3)2], Et3N.

As shown in Eq. 9, if we can remove two electrons from the tris (dimethyl-amino) cyclopropenium ion 11), then we will get the hetero (3) radialene system 14) which is iso-ji-electronic with the hexamethyl (3) radialene (75) s3mthesized by Kobrich and Heinemann in 1965. In accordance with this expectation, it has been found that one-electron oxidation of the diamagnetic tris(dimethylamino)cyclopropenium ion with concentrated sulphuric acid is easily effected to give the radical cation (75). 

Many phenoxy-Substituted fulvenes, e.g. 368, are oxidized to the corresponding highly coloured [3]radialenes. Reduction of the latter species can be effected with hydroquinone (equation 107). Oxidative cleavage of some triafulvenes to allenes is... 

Since tetraethynylethenes represent a repeat unit in more than one two-dimensional carbon network including 45 and 46 (Fig. 13-1) [1] the preparation of a specific network cannot be accomplished by simple oxidative polymerization of 20, but rather requires a more characteristic macrocydic precursor as starting material. Macrocyclic precursors to extended carbon sheets are perethynylated dehydroannulenes [56] and expanded radialenes, novel carbon-rich materials with interesting and unusual structures and functions. 

A wide range of substituted [3]radialenes are accessible from perchlorinated cyclopropenylium ion, cyclopropene, and cyclopropane-active methylene compounds through nucleophilic substitution/deprotonation/oxidation sequences. This strategy was pioneered by West and coworkers [15] who used it to obtain an array of [3] radialenes with /)-quinoid substituents. As the example given in Scheme 4.2 shows, tris(4-hydroxyphenyl)cyclopropenylium salts 13 were obtained from trichlorocyclopropenylium tetrachloroaluminate (12) by... 

Similar to the reaction sequence shown in Scheme 4.2, l,2-bis(3,5-di-terl-butyl-4-hydroxyphenyl)cyclopropenone (16) was converted into [3]radialenes 18 by a Knoevenagel-type condensation followed by oxidation of the tria-fulvene intermediates 17 (Scheme 4.3) [16]. Other similar compounds, such as the 2-(dicyanomethylene)-substituted 2,5-thienoquino-, 2,5-selenoquino-,... 

Charge-transfer complexes of variously functionalized, both electron-rich and electron-poor, [3]radialenes have attracted attention because of their magnetic properties [7] and as potential organic metals [17, 18]. Most of the [3]radialenes reported so far have been studied with respect to their oxidation and reduction chemistry (see the reviews [7, 8] and some examples mentioned in this chapter). With appropriate substitution patterns, the range of redox properties extends from the most electron-rich [3]radialene 31 to the very electron-poor radialene 24a, a strong oxidant [21]. The first one-electron reduction step of 18 (X = Y = CN) [16], 19, and 20 [17] occurs at potentials similar to TCNQ, and at potentials similar to chloranil for other derivatives of 18 (Scheme 4.3). 

The redox potentials are summarized in Table 8.7. The CV of 36, 37, and 38a exhibited two two-electron redox waves, whereas the CV of 38b showed one simultaneous four-electron transfer process. The shape of CV chart in 36 and 37 depend remarkably on the scan rate. It is suggested that the oxidation of 36/36 or 3H3l could occur in association with structural or conformational changes. Although the values of 36 and 37 were found to be lower by 0.12-0.15 V than that of 39, they were higher by 0.24-0.32V than that of DT[5]radialene 33a,b and 33e (Table 8.6). This result indicates that a cyclopentadienide structure contributes to 36 or 37 to some extent, in addition to a fulvalene structure (Figure 8.17). Interestingly, the redox potentials of 38a /38a and 38b/38b , ... 

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