Semibullvalenes formation

Fig. 16. Structure of the boat-like conical intersection of cyclocta-l,3,5,7-tetraene. This intersection mediates semibullvalene formation and double-bond/single-bond shifting. The values of the relevant structural parameters are given in A and degrees. Data from Refs. 97 and 98.

In mechanism (8.43) the bridgehead hydrogens of barrelene should be found at the a positions of semibullvalene (2a, 0/3,0y). Mechanism (8.44) can give three different hydrogen-label distributions. If the final bond formation is concerted with bond fission, and bond fission and formation take place at the same carbon atom [mechanism (8.44A)], the label distribution should be (la, 0/3, ly). If bond formation is concerted with bond fission but with a preference for bond formation at the carbon allylic to bond fission [mechanism (8.44B)], the label distribution should be (2a, 0/3, Oy). If there is a symmetric allylic biradical which has a finite existence [mechanism (8.44AB)], then the hydrogen-label distribution should be (1.5a, 0/3,0.5y). 

Two possible reaction schemes for the photosensitized formation of semibullvalene are as follows, where the large dot indicates a proton label (all other protons were exchanged for deuterium) ... 

A DFT computation has been used to study the cyclization and dimerization of the phenyl-substituted hexadienyne to a semibullvalene that occurs on heating. A new mechanism for formation of the dimer is proposed (Scheme 118).32... 

Under suitable photochemical conditions, benzene will undergo 1,3-cycloaddition to select olefins with formation of adducts containing the semibullvalene nucleus. 1,2-Dichloroethylene,179 methylenecyclobutane,180 and vinylene carbonate181 are... 

The (non-combustion calorimetrically) experimentally determined enthalpies of formation for benzvalene [31], semibullvalene [32], and triquinacene [33] are 363, 308, and 224 kJ mol1 respectively. Assuming linear dependence of enthalpy of formation on any of these counts (r2 = 0.98), we derive a value for the enthalpy of formation of tetrahedrane of 437 kJ mol1. The equivalent non-statistical derivation is to assume thermoneutrality for the reaction... 

From a purely synthetic viewpoint, triazolinedione adducts have served as substrates for gaining access to numerous target molecules such as prismane, semibullvalene, bridged semibullvalenes, elassovalene, caged compounds, and azoalkanes. Indeed, the title reagent can be used not only as a chiral source, but also as an azo donor. In the synthesis of 4,5-diazatwis-4-ene, for example, (-)- nfirst step involved cycloaddition to cyclooctatetraene dibromide with formation of a separable mixture of diastereoisomers. Eventual removal of the... 

Mehta and Ravikrishna have described the ready formation of monosub-stituted semibullvalenes 11 by photodecarbonylation of the polycyclic ketones 12 in methanol solution, and by a similar procedure, meta and para [2.2] cyclophanes are obtained by bisdecarbonylation of 13 and 14 respectively (Isaji et al). 

Direct irradiation through quartz of the dibenzobarrelene (151) in pentane or benzene results in the formation of the cyclooctatetraene (152) and this process is not affected even when the amino functions are converted into the HCl or HBr salts.In the solid state, however, irradiation of the bis salt yields a semibullvalene (153), as the main product and the authors suggest that this change in reactivity is due to a change in the excited state that is operative. Thus in the solid the chloride or bromide ions act as heavy atoms that enhance the intersystem crossing within the excited state of (151) but apparently this does not occur in solution. 

Naphthobarrelenes " and pyrazinobarrelenes afford mixtures of semibullvalene photoproducts, as both vinyl-vinyl and aryl-vinyl bridging occurs. Semibullvalenes can also be produced from the light-induced isomerization of benzobicyclo[4.2.0]octa-2,4,7-trienes. Their formation can be rationalized by a di-7t-methane rearrangement involving initial bridging between C5 and C7 of the two vinyl portions of the trienes. ... 

When a mixture of benzene and perfluorobutyne was irradiated in a Vycor tube with 253.7 nm light, slow formation of at least eight compounds was detected by gas chromatography. The major compound was l,2-bis(trifluoromethyl)cycloocta-l,3,5,7-tetraene (10, 40%), accompanied by three isomeric bis(trifluoromethyl)tricyclo[3.3.0.0 ]octa-3,6-dienes (semibullvalenes) 11-13 in 25, 12 and 5% yield, respectively. ... 

The valence isomerization of cyclooctatetraene to semibullvalene developed by H. E. Zimmerman may be rationalized in terms of an intramolecular cycloaddition. Direct formation of C-C bonds between C1-C5 and C4-C6 of a hexatriene unit seems to be the most probable reaction pathway. It should be noted that this photoreaction belongs to the rare examples of a gas phase photolysis useful for synthetic purposes. 

Another kind of tetraradicaloid Cl has been invoked and computationally characterized for photoisomerization of cyclooctatetraene. Photoexcitation leads to return to reactant, double-bond shift, and formation of semibullvalene. The tetraradicaloid... 

All the previously reported examples of photoaddition of acetylenes to benzene involve 1,2-cycloaddition and subsequent formation of cyclo-octatetraenes10 (but see ref. 43 for a discussion of the addition of perfluorobut-2-yne to benzene). The report of 1,3-cycloaddition of diphenylacetylene to the trimethyl ester of trimesic acid, is, therefore, of interest as this leads directly to the semibullvalene system.44 The reaction apparently requires 6 days, the yield of the product is low, and the adduct (28) is not isolable in the pure state. The triethyl ester and the acetylene also yield (29) via a stilbene-phenanthrene cyclization of the primary... 

Two cis-structures of homotropilidine are interconvertable 1000 times per second at 180°C. This interconvertabity can be enhanced by eliminating possibility of formation of frans-isomer which can be achieved by connecting 1 and 5-positions either directly or through a methylene group. Ethylene bridge solves the same problem. As a result of these operations we get semibullvalene, barbaralene and bullvalene, respectively. 

---