Hexa radialene

To demonstrate the quality of this simple approach we show in Figures 6 and 7 the PE spectra of 1,3-butadiene 2, (3 )-hexa-l,3,5-triene 161, (3E, 5 )-ocla-1,3,5,7-tetraene 234, [3]radialene 173, 3,4-dimethylidenecyclobutene 174 and fulvene 175. The observed positions I j1 of the tt-bands are collected in the third column of Table 2, and the eigenvalues xj obtained from standard HMO models in the fourth. A least-squares calculation yields the linear regression... 

Shortly after the hexaaryl[3]radialenes, the analogous syntheses of hexa(2-pyridyl)- and hexa(3-pyridyl) [3]radialene were reported concurrently by the groups of Steel [28] and Oda [29], and hexa(4-pyridyl)[3]radialene followed more recently [30]. These water-soluble radialenes represent a new class of multidentate ligands with applications in metallo-supramolecular coordination chemistry vide infra). 

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. 

Only a few facts about the chemical reactivity of the parent [6]radialene (5) are known - certainly because it is not easyto handle - but they reveal its character as a triple 1,3-diene system (catalytic hydrogenation, triple 1,4-addition of Br2, [4+2] cycloaddition reactions [5, 6]). Much more is known about the alkyl-substituted radialenes 113 and 72, in particular due to the detailed investigations of Hopf and coworkers [88]. Because of the presence of three hexa-2,4-diene subunits in the latter radialenes, it is not surprising that isomerization pathways via sigmat-ropic and electrocyclic reactions exist. Thus, in a gas-phase thermolysis of 113, products 125-129 were formed in relative yields that depended on the reaction temperature (e.g., 127 was the major product at 260 C and 129 at 360 °C). The mechanistic scenario includes the isomerization 113 125 by three consecutive 1,5-H shifts, and the sequence 127 128 129 [88]. The permethylated [6]radi-alene (72) is thermally much more stable than 113 the product mixture obtained from its pyrolysis at 350 "C was dominated by benzocyclobutene 130 (an analog of 127), which, however, could be isolated in only 17% yield [88] (Scheme 4.27). 

---