Octamethyl radialene

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. 

In the case of tetramethylbutatriene, Ni(0) catalyzes not only the cyclodimerization (formation of [4]radialene 94), but also the cyclotrimerization, leading to [6]radialene 95 and its isomer 96 (see also Section ILD). The product pattern depends to some extent on the nature of the catalyst, but the choice of solvent seems to be more crucial. This is illustrated impressively by the Ni(cod)2-catalyzed reaction of 93, which leads exclusively to the [4]radialene in toluene solution, but to the [6]radialene in DMF. Interestingly, the stoichiometric reaction between 93 and (2,2Tbipyridyl)-(l,5-cyclooctadiene)nickel yields the nickel complex 97, which has been isolated and characterized by X-ray diffraction. On treatment of 97 with two equivalents of maleic anhydride, reductive elimination of nickel takes place and octamethyl[4]radialene (94) is formed in good yield. This reaction sequence sheds light on the mechanism of the Ni-catalyzed reactions mentioned above further ideas on the mechanism of the cyclodimerization and cyclotrimerization reactions have been developed by lyoda and coworkers. ... 

Surprisingly little is known about the coordination chemistry of [4]radialenes. The interaction of octamethyl[4]radialene (71) with CpCo(ii -CH2=CH2)2 may begin with the formation of a diene complex CpCo(ii -71), but it continues as a catalytic rearrangement to finally furnish 1,3-diisopropyl-5-methyl-2-(l-methylvinyl)benzene [81]. 

Substituted [4]radialenes can have a planar four-membered ring as in tricyclic radialene 67 [63] or a puckered ring as in octamethyl[4]radialene (71) [67]. Notably, the donor-acceptor-substituted [4]radialene 69, in spite of its four aryl substituents, features a completely planar four-membered ring [64], but steric strain is obviously relieved by pyramidalization of the two dicyanomethylene carbon atoms and their offset from the ring plane. 

Only one account of a [4]radialene reacting with more than one molecule of a dienophUe has been described, and, despite involving a hetero-DA reaction, we believe it important enough to warrant inclusion. When Wilke and coworkers reacted octamethyl[4]radialene (181) with IMequiv. of 4-phenyl-1,2,4-triazole-3,5-dione, the expected cycloadduct 182 was obtained (Scheme 12.39) [60]. When 2 M equiv. were employed, a second DA reaction was not observed instead, the [3-1-2] cycloaddition product 183 was isolated (confirmed by X-ray analysis). 

Scheme 12.39 A double addition of 4-phenyl-1,2,4-triazole-3,5-dione to octamethyl[4]radialene (181).

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