Barbaralane

Another compound for which degenerate Cope rearrangements result in equivalence for all the carbons is hypostrophene W1). In the case of the compound barbaralane (108) (bullvalene in which one CH=CH has been replaced by a CH2) ... 

Given that the boat transition state 8 is unfavourable, it is at first sight surprising that the Cope rearrangements of bullvalene (14), barbaralane (15), and semibullvalene (16) should take place so readily given that the transition states (17) of these reactions are derivatives of 8. We therefore decided 3S-) to calcu-... 

Shortly after this prediction, Schroder (1963) isolated bullvalene. Numerous studies amply demonstrated the facile Cope rearrangement of [84] and its derivatives (see for example Schroder and Oth, 1967 Doering et al., 1967). Theory and experiment agree that, by pinching the methano bridges closer together, the rate of the Cope process increases in the sequence semibullvalene [83] > barbaralane [85] > bullvalene [84] (Dewar and Schoeller, 1971 Anastassiou et al., 1975). 

In principle, the divinylcyclopropane structure discussed here is incorporated into very well known systems such as bullvalene 547, barbaralane 548 and semibullvalene 549, which very easily undergo a Cope rearrangement. 

Barbaralane 548 (tricyclo[3.3.1.04,6]nona-2,7-diene) was first described in 1967 (see Reference 80 and literature cited therein). Barbaralane and its derivatives functionalized in the C(9) position (550a,b) are degenerate Cope systems whose equilibrium can be frozen... 

A number of semibullvalenes and some barbaralanes, which undergo a facile Cope rearrangement via bicyclooctadienyl diradicals, produce various cyclic peroxides on exposure to triplet oxygen ". Thus, cyclopentano semibullvalene 82a is peroxidated with air to give a mixture of 5-membered 83a and 6-membered endoperoxide 84a in a 1 2 ratio, whereas the cyclohexano derivative 82b yields only the 6-membered cyclic peroxide 84b (Scheme 15). The energy in the highly strained mono(Dewar benzene) isomer of... 

Radical cations (21 +) derived from semibullvalene + or barbaralane belong to a different structure type. The ESR spectrum of 21 + (a = 3.62 mT, 2H a = 0.77 mT, and strong CIDNP effects support a structure in which... 

Barbaralane was synthesized by Biethan Klusacek Musso Angew. Chem. Ini. Ed. Engl. 1967, 6. 176 [Angew. Chem. 79, 152) by Tsuruta Kurabayashi Mukai Tetrahedron Lett. 1965, 3775 by Doering Fcrricr Fossel Hartcn-stein Jones Klumpp Rubin Saunders Tetrahedron 1967, 23, 3943 and by Henkel Hane J. Org. Chem. 1983, 48, 3858. 

Bridged 1,5-benzodiazepines (204), prepared by condensation of o-phenyl-enediamine with 4,6-dimethylbicyclo[3.3.1]nona-3,6-diene-2,8-dione, give barbaralanes (205) on electrochemical reduction in acetonitrile in the presence of acetic anhydride338 [Eq. (119)]. The reaction is akin to the reduction of acetylacetone in which cyclopropane derivatives have been formed.339-340... 

Another structural possibility for a hexadiene radical cation arises, when the two allylic moieties are linked in pairwise fashion to two- or three-carbon spacers. This structure type can be approached by oxidation of molecules such as semibullvalene [391-393] or barbaralane [394]. In the resulting radical cations, the two allylic moieties are held in close proximity model considerations suggest a non-bonding C—C distance of 2.2-2.3 A, considerably closer than for the previously discussed structure type. At this distance, a moderately strong interaction of the twin moieties cannot be excluded. Accordingly, we assigned cyclic conjugated structures to radical cations derived from semibullvalene (-> 138 cf. 

Fig. 28) [391] and several barbaralane derivatives (-+ 139) [394]. In these systems, bond-making and bond-breaking processes are frozen at a stage permitting cyclic delocalization of five Jt-electrons. 

We are, of course, aware of the strict Hiickel definition linking aromaticity to the presence of 4n + 2 rc-electrons, On the other hand, we note that the radical cations discussed below have SOMOs, which have one electron less than the Hiickel requirement and which show cyclic homoconjugation, even though the parent molecules assume alternative structures. In view of the precedence established for the radical anion (156) of bicyclo[6.1.0]nonatriene [416,417], in order to emphasize the dramatic difference between the structures of these radical cations and their precursors, and for reasons of convenience, we will refer to these species as homo- or bis-homoaromatic. The paradigm of these remarkable species is found in the barbaralane system. 

In contrast, CIDNP results indicate that the radical cations of barbaralane (157, X = C = 0) and semibullvalene (157, X = —) correspond to the elusive structure type with a single minimum [391, 424]. The spin density resides primarily on the termini (C-2,4,6,8) of the twin allyl moieties, whereas the remaining (internal) carbons of the 5 jr-electron perimeter have negative spin density. This spin density distribution reflects the coefficients of orbital 158, the HOMO of a bis-homoaromatic structure (Fig. 32) [424], More recently, ESR results have confirmed this assignment [392, 393],... 

Fig. 32. High lying molecular orbitals of barbaralane and potential SOMOs of its radical cation...

Predict the relative rates for the Cope rearrangement of bullvalene A, barbaralone B, protonated barbaralone B, barbaralane C and octamethylsemibullvalene D. 

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