Permethyl radialene

Cycloaddition (Diels-Alder) reactions have been reported for [6]radia-lene (5) and its hexaalkyl derivatives 113 and 115, but not for the permethylated radialene 72, which was inert even to the reactive dienophiles TCNE and Af-phenyltriazolinedione [67]. The sterically least hindered radialene 5 reacted with acetylenic and olefinic dienophiles in a 1 3 ratio to give triphenylene derivatives such as 139 in low yield (Scheme 4.30) [5, 95]. On the other hand, radi-alenes 113 and 115 gave linear,/)-quinodimethane-type 1 2-adducts, when they were exposed to an excess of various common dienophiles inter alia maleic anhydride, tetracyanoethylene, />-benzoquinone, acrolein, ethyl acrylate, acetylenedi-carboxylic acid) [89, 96, 97]. The 1 1 adduct 140, which was isolated so far only from the reaction with an equimolar amount of TCNE (92% yield) [97], presumably prefers the second cycloaddition step in the linear (para) position (141) over that in the angular (meta) position (142) for steric reasons. 

In order to get insight into the preferred orientations of the various radialene systems, we might consider the permethylated derivatives of the parent compounds, since... 

When the organocuprate methodology is applied to l,l-dibromo-2-methylprop-l-ene, octamethyl[4]radialene (94) and decamethyl[5]radialene are the major products73. While this method does not offer any advantage over the Ni(0)-mediated syntheses of 94 (see above), it constitutes the only known synthesis of the permethylated [5]radialene (see Section II.C). For the sake of completeness, we mention that the Ni(0)-mediated dehy-drohalogenation/cyclotetramerization of 1,1-dibromoalkenes is not an efficient route to [4]radialenes66. 

That this difference in (4+2)-cycloaddition behavior most likely has steric origins—the methyl groups in 150 or the derived monoadduct preventing an ortho -addition of two equivalents of the dienophile—is supported by the observation that permethyl[6]radialene 95 is inert even towards the extremely reactive dienophile 4-phenyl-1,2,4-triazolinedione68. 

A typical example of steric control over spin delocalization in cation radicals has been described for permethylated dithia[6]radialene (Gleiter et al. 1996). As shown, the unpaired electron is delocalized only in one half of this cation radical, within the limits of the 2,3-dithiatetramethyl-butadiene unit. Owing to the steric demand of the isopropilidene groups, two of the four-methylene groups are twisted, while the other two are coplanar the authors give Scheme 3-10. 

Among the series of the parent systems 1-4, [5]radialene (3) is still unknown. The simplest derivative described so far is decamethyl[5]radialene (135) which has been obtained from l,l-dibromo-2-methyIpropene (22) by low temperature metalation with n-butyllithium followed by a metal exchange reaction with nickel or (better) copper salts and the thermal decomposition of the carbenoid thus formed (equation 10). The yield of 135 varies it is only 14% with CuBr SMe2, but it more than doubles (32%) when Cul PBu3 is employed . The formation of 135 is accompanied by di-, tri- and tetramer-ization of the dimethylvinylidene unit derived from 22 leading to tetramethylbutatriene and the respective permethylated [3]- and [4]radialenes. It is unlikely, though, that this... 

As far as chemical reactivity is concerned, much more is known about [4]radi-alenes than the other radialene classes, mainly due to the studies of Griffin and Peterson [2], and Hopf et al. [47, 80] on the parent [4]radialene and of Wilke et al. [67] on the permethyl derivative. In spite of its ease of dimerization... 

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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