Radialenes reduction

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... 

The thermal stability in air of the deeply colored radialenes 37 increases with the efficiency of steric shielding of the carbonyl groups. Thus, 37a was only detected in solution by its UV/Vis spectrum, whereas 37b and 37c are reduced to their precursors, 36b and 36c, when heated in air at 133 and 280 °C, respectively. In solution, this reduction is readily accomplished with hydroquinone13. 

Various other [3]radialenes bearing quinoid substituents have been synthesized analogously, for example 3814,3914,4015, 4116,4215, and the rather unstable 4317. In contrast to most other tris(quino)cyclopropanes, reduction of tris(anthraquino)cyclopropane 38 does not succeed with hydroquinone, but requires more forcing conditions (Sn/HCl or Zn/HCl). Compound 44 represents the only tropoquino-substituted [3]radialene known so far18 the black-blue crystals of this strongly electron-accepting radialene are stable to air and light. 

Hexacyano[3]radialene (50) is a very powerful electron acceptor according to both experiment23,24 35 and MNDO calculations of LUMO energy and adiabatic electron affinity25. The easy reduction to the stable species 50" and 502- by KBr and Nal, respectively, has already been mentioned. Similarly, the hexaester 51 is reduced to 512-by Lil24. Most [3]radialenes with two or three quinoid substituents are reduced in two subsequent, well-separated, reversible one-electron steps. As an exception, an apparent two-electron reduction occurs for 4620. The reduction potentials of some [3]radialenes of this type, as determined by cyclic voltammetry, are collected in Table 1. Due to the occurrence of the first reduction step at relatively high potential, all these radialenes... 

TABLE 1. Reduction potentials of [3]radialenes ( 1/2, V) with quinoid substituents and of some related compounds, as determined by cyclic voltammetry (in CH2CI2 vs SCE)... 

Another interesting property of expanded radialenes is their strong ability to accommodate electrons upon reduction, as revealed by an electrochemical study by Gross, Gisselbrecht, and Boudon [29d]. Thus, in all three series, the first reduction occurred at anodically shifted potentials relative to the reference TEE dimers the following shifts were observed +170-440 mV for 29a-c relative to 19, +210-330 mV for 30a,b relative to 23, and +240-320 mV for 28a-c relative to 21 (Figure 9). Thus, the radical anions of expanded radialenes are very stable, particularly those of the expanded [3]- and [4]radialenes, whatever the nature of the substituents. 

Fig. 9. Comparison of first reduction potentials (ys. Fc/Fc+) for the three differently substituted series of TEE dimers/expanded radialenes [29d], Solvent THF for 19, 29a-c CH2CI2 for 21, 23, 28a-c, and 30a,b.

In the case of tetramethylbutatriene, Ni(0) catalyzes not only the cyclodimerization (formation of [4]radialene 94), but also the cyclotrimerization, leading to [6]radialene 95 and its isomer 96 (see also Section ILD). The product pattern depends to some extent on the nature of the catalyst, but the choice of solvent seems to be more crucial. This is illustrated impressively by the Ni(cod)2-catalyzed reaction of 93, which leads exclusively to the [4]radialene in toluene solution, but to the [6]radialene in DMF. Interestingly, the stoichiometric reaction between 93 and (2,2Tbipyridyl)-(l,5-cyclooctadiene)nickel yields the nickel complex 97, which has been isolated and characterized by X-ray diffraction. On treatment of 97 with two equivalents of maleic anhydride, reductive elimination of nickel takes place and octamethyl[4]radialene (94) is formed in good yield. This reaction sequence sheds light on the mechanism of the Ni-catalyzed reactions mentioned above further ideas on the mechanism of the cyclodimerization and cyclotrimerization reactions have been developed by lyoda and coworkers. ... 

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). 

A reductive 1,8-didehydroxylation of l,2-dialkynyl-3,4-bis(diphenyl-methylene)-cyclobutenes 56, which themselves were prepared from 1,2-dibromocyclobut-l-ene 55 and the appropriate terminal alkyne by a Sonogashira coupling reaction, provided the deeply colored [4]radialenes 57a-d [50, 51]. A remarkable feature of 57 is the fast rotation around the cumulenic bonds at ambient temperature, with AG values of 13.7 (b, -20 °C), 14.9 (c, 0°C), and 17.8 (d, 27 °C) kcal mol" in chlorinated solvents. The bond rotation process was explained as proceeding through a diradical intermediate 58, that is, the geometrical isomerizations at the two cumulenic units occur independently (Scheme 4.13). 

Ito and coworkers have designed an electrochemical route to the elusive hexaanionlc-extended radialene 146 (Scheme 4.32), which was presumably formed in a six-electron reduction of hexakis(6-azulenylethynyl)benzene (145). 

The synthesis of DT[5]radialene is depicted in Scheme 8.6 [44,45]. Ni(0)-catalyzed reductive cyclodimerization from 29c,e in THF under carbon monoxide atmosphere gave the cyclopentanone analogs of 32c,e, respectively. Hydrolysis and subsequent decarboxylation from 32c gave 32d in good yield. The reduction of 32e by LLAIH and treatment with HEF j EtjO, followed by Wittig reaction in the presence of EtjN gave DT[5]radialene 33a,b, and 33e. 

In recent years, several groups have reported on a series of radiaannulenes incorporating TTF or DT rings. Radiaannulenes are hybrid frameworks composed of a cross-conjugated radialene and a hnearly conjugated dehydroannulene [78-81]. These compounds possess a so-called prearomaticity that is, the ability of the compound to form aromatized mesomeric 14jt-electron system as the result of oxidation or reduction (Scheme 8.12). 

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