Radialenes cyclopropanation

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. ... 

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. 

Various alkyl- and aryl-substituted [3]radialenes could be prepared from 1,1-dihaloal-kenes using organometallic pathways. Hexamethyl-[3]radialene (25), the first [3]radialene to be synthesized, was obtained in a very low yield by treatment of l,l-dibromo-2-methyl-1-propene (22) with butyllithium8,9. The lithium carbenoid 23 and the butatriene 24 are likely intermediates of this transformation (Scheme 2), the former being the source of an unsaturated carbene moiety which is transferred onto the latter. However, the outer double bonds of 24 are more readily cyclopropanated than the central one. 

It appears that neither the lithium carbenoid pathway nor the cyclopropanation of buta-trienes are general routes to [3]radialenes. More successful is the cyclotrimerization of 1,1-dihaloalkenes via copper or nickel carbenoids, provided the substituents at the other end of the C=C double bond are not too small. Thus, tris(fluoren-9-ylidene)cyclopropane 27 was formed besides butatriene 28 from the (l-bromo-l-alkenyl)cuprate 26 generated in situ from (9-dibromomethylene)fluorene (Scheme 3)10. The cuprate complexes formed... 

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... 

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. 

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... 

Hexamethyl[3]radialene (25) does not undergo Diels-Alder-reactions with the typical electron-poor dienophiles, probably because of the full substitution at the diene termini. With TCNE, however, a violet-blue charge-transfer complex is formed which disappears within 30 min at room temperature to form a 1 1 adduct (82% yield) to which structure 55 was assigned9. Similar observations were made with tris(2-adamantylidene)cyclopropane (34), but in this case cycloaddition product 56 (81% yield) was identified its allenic moiety is clearly indicated by IR and 13C NMR data12. 

Dichlorocarbene adds to 150 to give the monoadduct 158 as well as the two bis-adducts 159 and 160 (equation 18)101. An ortho -bis-adduct could not be detected among the cyclopropanation products—possibly because of steric hindrance in the immediate vicinity of 158. The anti-arrangement of the two cylopropane rings in 159 and 160 was established by X-ray structural analysis101. Formally, the latter adducts represent hybrids of [6]radialenes and [6]rotanes ( rotaradialenes ). 

K. Takahashi and S. Tarutani, Novel metallic charge-transfer complexes composed of a [3]radialene type acceptor a l,2-bis(p-benzoquino)-3-[2-(dicyanomethylene)-2,5-selenoquino]cyclopropane derivative, Adv. Mater., 1, 639-641 (1995). 

The parent [3]radialene (2) has been obtained by 1,2- and 1,4-elimination reactions from cyclopropane derivatives 8 [2,11], 9 [12], 10 [12], and 11 [13] in very low to moderate yields (Scheme 4.1). Later, a practical large-scale synthesis of triethyl cyclopropane-la,2a,3P-tricarboxylate [14] has facilitated the synthesis of 2 via l,2,3-tris(iodomethyl)cyclopropane (10). All transformations were performed as gas-phase reactions, and the radialene was condensed in a cold trap. Liquid 2 is stable at -78 °C for some days, but it easily polymerizes under other conditions (in the presence of oxygen, in CCl solution at 0 C, by bringingthe vapor to room temperature). 

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... 

Syntheses of the parent [6]radialene (5) were published by three research groups in 1977/1978 4, 5, 6. Among the various gas-phase flash pyrolysis reactions reported, the triple HCl-elimination from l,3,5-tris(chloromethyl)mesitylene (109) is the most practicable one and yields the radialene in 35 -48% yield (Scheme 4.23) [5] other flash pyrolyses were carried out, typically at 900 °C, with cyclododeca-l,5,9-triyne, benzo[l,2 4,5]dicyclobutene derivatives and benzo[l,2-c 3,4-c 5,6-c"]tris(2,5-dihydrothiophene 1,1-dioxide). The sixfold dehydrobromination of hexa(bromomethyl)benzene (110) with methyl lithium at low temperature has been achieved more recently in this case, 5 has been trapped without isolation by cyclopropanation reactions yielding spiro compounds 111 in very low yield [88]. Radialene 5 is a diSicult-to-handle, extremely air-sensitive compound with properties [5] that resemble those described for the parent radialenes 2 and 3 see Sections 4.2.1 and 4.2.2. 

Takahashi and Tarutani have recently prepared radialene-type acceptors 36 and 37 [57], having a hybrid structure of thiophene-TCNQ 1 and tris(p-benzoquino)cyclopropane [58]. Both of them show electron accepting abilities comparable to TCNQ and more decreased on-site coulombic repulsion. Compound 36 forms fairly highly conductive 1 1 CT complexes with TTF and TTT, of which the former exhibits metallic dependence on its pellet conductivity. 

---