Radialenes with quinoid substituents

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... 

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... 

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. 

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)... 

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... 

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... 

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See also in sourсe #XX -- [ , ]

See also in sourсe #XX -- [ , ]

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