Radialenes functionalized

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... 

Fig. 6. Electronic absorption spectra of donor-functionalized expanded radialenes 76-78 in CHCI3 at 298 K...

The parent [3]radialene 1 has been generated from variously functionalized cyclopropane precursors by classical -elimination reactions (Scheme l)2-6. All these reactions have been carried out as gas-phase reactions, and the radialene has been collected at —63 °C or below. At —78 °C, the pure compound is stable for several days, but polymerization occurs when the vapor is exposed to room temperature as well as in carbon tetrachloride at 273 K2, or in contact with oxygen3. 

A variety of functionalized [3]radialenes have been prepared starting from the appropriately substituted cyclopropanes or cyclopropenes. West and Zecher have pioneered the chemistry of [3]radialenes with quinoid substituents. The general strategy of this synthesis is outlined in Scheme 513. A tris(4-hydroxyphenyl)cyclopropenylium... 

The scope of this method is illustrated by the preparation of other peralkylated [4]radialenes, such as 103-10566 and the functionalized radialene 10670,71 (Scheme 12). Removal of the carboxylate groups from the latter provided the very electron-rich radialene 107. 

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 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. 

Molecular scaffoldings with tetraethynylethenes (TEEs, 3,4-diethynylhex-3-ene-l,5-diynes) and trans-1,2-diethynylethenes [DEEs, (E)-hex-3-en-l,5-diynes] are at a particularly advanced stage.114,37 38 441 A collection of dose to one hundred partially protected and functionalized derivatives have been prepared in the meantime, providing starting materials for the perethynylated dehydroannulenes and expanded radialenes shown in Figure 6.136 441 TEEs and DEEs, as well as dimeric derivatives substituted at the terminal alkynes with donor (D, p-(dimethyl-... 

In another metal-mediated coupling reaction, the highly functionalized dienes 136a and 136b were allowed to react with Ni(PPh3)4 in the presence of excess zinc/copper couple at 50 ""C and 1 atm of carbon monoxide to provide the octakis(methoxycarbonyl) 137a and the tetrabenzo derivatives 137b in excellent yields (77 and 84%, respectively), (see Scheme 17 compare also Scheme 12 for the formation of the related [4]radialene 10686-88) 373 s treated with excess LiBr H2O in HMPA at temperatures... 

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. 

Radialenes have been and are still objects of study for a variety of reasons. Their particular structures, the high chemical reactivity of the parent systems, and the quest for derivatives with specific functionalization have spurred the interest of synthetically oriented chemists. Molecular properties such as structure, bonding, thermodynamics, and electronic properties have attracted... 

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). 

All known routes to the parent [4] radialene (3) use elimination reactions of appropriately functionalized cyclobutane derivatives (Scheme 4.12), namely, fourfold Cope elimination or Hofmann degradation of tetra(aminoxide)... 

Similarly, tetrathio-functionalized 2,3-dibromobutadienes 104, which also serve as precursors to [4]- and [5]radialenes (see Schemes 4.15 and 4.22), can be converted into hexakis(l,3-dithiol-2-ylidene)cyclohexanes 116a,b when treated with Ni(PPh3)4 and Zn-Cu couple in DMF (Scheme 4.24) [90]. Interestingly, these highly substituted [6]radialenes are initially formed with a twist-boat conformation of the central six-membered ring, which can be converted thermally into the more stable chair conformation (see below). 

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