Radialenes isomerization

A thermal tricyclobutabenzene/[6]radialene isomerization also gave rise to the first alkoxy-substituted [6]radialenes (Scheme 4.25). Both the syn-anti and the... 

Surprisingly little is known about isomerization reactions of [4]radialenes. 5,6,7,8-Tetravinyl[4]radialene 127 is certainly a unique case since it appears to be in equilibrium with its precursors in synthesis, 125 and 126, by electrocyclic reactions (equation 8). As 127 has not been isolated in pure form and is unstable with respect to polymerization, no details on this possible equilibrium are known79. 

With the transition-metal-assisted ring-opening and isomerization of small rings in mind, it is astonishing that only one such study for a [4]radialene is known. The cobalt(I)... 

It should be noted that 175 is the chromium complex of 154, the intermediate postulated in the thermal isomerization of hexamethyl[6]radialene (150). 

The electronic structure of Cgo is best described as a fusion of [5]radialene and cyclohexa-l,3,5-triene substructures (Taylor, 1992 Hirsch, 1994). It is known that the double-bond character in the five-memberend rings in is low. Methano bridging can take place, however, as we have already seen (8.28-8.30), at the [6,5] or the [6,6] ring junctions and, in addition, valence isomerization is possible in both cases. This results in four isomeric methanofullerenes and eight, if the two substituents at the methano C-atom are different. Structure and naming of the four isomers are shown in Figure 8-4. 

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). 

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). 

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