Semibullvalenes and barbaralanes

FIGURE 26. Molecular packing of the inclusion crystal of 209 and argon at 103 K. Positions of Ar atoms are marked by circles. Projection down the c axis. Reproduced by permission of VCH VerlagsgesellschaftmbH from Reference 322 

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H. Quast, M. Seefelder, Angew. Chem. Int. Ed. Engl. 38, 1064 (1999). The Equilibrium between Localized and Delocalized States of Thermochromic Semibullvalenes and Barbaralanes—Direct Observation of Transition States of Degenerate Cope Rearrangements. 

Jackman LM, Fernandes E, Heubes M et al (1998) The effects of substituents on the degenerate cope rearrangement of semibullvalenes and barbaralanes. Eur J Org Chem 10 2209-2227... 

Seefelder M, Heubes M, Quast H et al (2005) Expraimental and theoretical study of stabilization of delocalized forms of semibullvalenes and barbaralanes by dipolar and polarizable solvents, observation of a delocalized structure that is lower in free energy than the localized form. J Org Chem 70 3437-3449... 

Goren AC, Hrovat DA, Seefelder M et al (2(K)2) The search for bishomoaromatic semibullvalenes and barbaralanes computational evidence of their identification by UVWis and IR spectroscopy and prediction of the existenee of a blue bishomoaromatic semibullvalene. J Am Chem Soc 124 3469-3472... 

Quast and Seefelder recorded the variable-temperature UV—vis spectra of four of their semibullvalenes and barbaralanes (91—94) in a range of solvents of widely varying dipolarity. ° They concluded from... 

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). 

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... 

MINDO/2 methods have again been used to determine the strain energies of semibullvalene (16), barbaralane (17), and bullvalene (18), and the activation energies for the Cope rearrangement have been calculated. 

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-... 

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. 

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. 8. Correlation between the highest occupied molecular orbitals of dihydrobullvalene, barbaralane, semibullvalene, homotropylidene and cycloheptatriene. The energy values are taken from experiment...

In semibullvalene, Ci and Ce of hexadiene are bonded to one carbon atom, C3 and C4 are bonded to another, and the two additional C-atoms (C7 and Cg respectively) are linked to one another. Barbaralane differs from it only in that C7 and Cs are linked by a third aliphatic carbon atom, C9. Both molecules have C5 symmetry, with the sole mirror plane passing through C7 and Cg, whereas a perpendicular mirror plane and a two-fold rotational axis appear at the TS, that necessarily has the boat (C y) geometry. 

Childs. Cremer, and Elia maintain that there are several examples of neutral homoaromatics. They suggest that 1,2-dihydroborete (70) and some of its derivatives are neutral homoaromatics. This electron-deficient system is isoelectronic with the mono-homocyclopropenium cation (17). It has been investigated by both theory and experiment, and its classification as homoaromatic is justified. In addition to 70, Childs, Cremer, and Elia propose that certain semibullvalenes, barbaralanes, bridged an-nulenes, cycloheptatrienes, and norcaradienes are homoaromatic. They draw particular attention to the semibullvalenes 71-75 and consider that 74 and 75 are the best candidates for experimental work. 

In his efforts to lower the activation energy for the Cope rearrangement, Doerlng designed the first fluxional molecule homotropUldine (79). Barbaralane (80), bullvalene (81), and semibullvalene (68) are each related to 79 as homotropilidines locked in the appropriate Cope conformation (79b). As a conse-... 

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