Decamethyl radialene

The parent [5]radialene (5) has so far evaded preparation. The decamethyl derivative is, however, known, and this molecule is found to have a half-chair conformation, with approximately C2 symmetry179. There are, however, observations indicating that... 

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. 

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... 

A particular situation arises, when the linear C17-pentaquinane 111 is bridged between its two ends to result in the skeleton of the so-called [5]peristylene 117 which essentially is a hexaquinane (Fig. 10). Decamethyl[5]radialene 116 which can be considered as the inner core of 117,has been reported by Iyoda et al. [83], but the fivefold methylene-bridged analog 117 is still unkown. Only the fully saturated skeleton of 117, the so-called peristylane 118 has been prepared by Eaton et al. [84], and its chemistry has also been covered in previous reviews [1,2]. 

The parent [5]radialene is still unknown (see below, however note added in proof). As the simplest substituted derivative, decamethyl[5]radialene (84) has been obtained along with [4]radialene 71 by a formal cyclooligomerization of dimethylvinylidene units via copper [76] or nickel [10, 37] carbenoids (see Scheme 4.17, Section 4.2.2). It is unlikely, however, that other [5]radialenes will become accessible by the same method, because increasing steric demand of the substituents favors the formation of the [4]- and [3]radialene as well as the [3]cumulene structures (see Sections 4.2.1 and 4.2.2). 

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