Homotropylidene

The parent thionine system 1 up to now has not been prepared probably because the C-S bond in valence isomeric forms is too weak giving rise to facile rearrangement or decomposition. The obvious synthetic route, photochemical transformation of cyclooctatetraenccpisulfide 2 (9-thiabicyclo[6.1.0]nona-2,4,6-triene), does not lead to 1, but intriguingly to another valence isomer, the sulfur-bridged homotropylidene system 3.20... 

Access to alkyl substituted derivatives of the homotropylidene complexes is provided via the >j4-tropone iron complex by reaction with diazoalkanes, followed by mild thermolysis of the 3+2 py razoline adduct to give the corresponding homotropone complexes (equation 149)217,218. The 8,8-dimethyl derivative was used as starting material for the preparation of the fluxional ( 5-2,8,8-trimethylbicyclo[5.1.0]octa-2,4-dienylium)Fe(CO)3 cation complex219. More recently (homotropone)Fe(CO)3 was used for the synthesis of unique chiral 1,2-homoheptafulvene iron complexes220 221. 

A few cycloadducts of 7-substituted cycloheptatrienes have been converted to 4(8)-substituted homotropylidenes (bicyclo[5.1.0]octa-2,5-dienes)18,20. 

For the synthesis of oxiranes with more complex structures, the peracid method is combined with other epoxidation procedures examples are the syntheses of ( )-crotoxirane, ( )-epicrotoxirane, and ( )-isocrotoxirane ° or the preparation of cis-trioxatris-(a)homotropylidene. o-Sulfoperbenzoic acid has been used for the stereoselective epoxidation of cholesterol. The selective epoxidation of cholest-5-en-3-one too has been examined. In the synthesis of 25-hydroxycholesterol selective epoxidation occurs on and 26 is formed. The epoxidation of olefin propellanes 27 and 28 can be achieved with MCPBA. As a consequence of the secondary orbital interaction, syn-attack is more marked in the case of 28. ° Epoxypropelladiene can be synthesized in accordance with Eq. 12. ... 

The Cope rearrangement of bicyclo[5.1.0]octa-2,5-diene (3,4-homotropylidene) (5) leads to a product that is identical with the substrate however, six of eight carbon atoms have changed places. The rearrangement is degenerate . As a consequence NMR and C NMR spectra... 

If the vinyl groups of c/. s-l,2-divinylcyclopropane are joined by a methylene bridge. Cope rearrangement of the resulting homotropylidene yields a product identical with the starting material. 

Non-reactions of the sort exemplified by the valence tautomerism of 3,4-homotropylidene obviously cannot be studied by conventional methods which involve measuring the rate of disappearance of a starting material or rate of formation of a product. However, many of these degenerate rearrangements occur so rapidly that they cause averaging of some or all of the nmr frequencies of protons in the substrate at temperatures which are experimentally accessible. The rates of these processes can be estimated by use of temperature-dependent line broadening or spin-echo nmr techniques. 

The Cope rearrangement of 3,4-homotropylidene is facilitated by both pre-orientation and steric strain effects, and should occur quite rapidly. In fact, NMR spectroscopy shows that a molecule of 3,4-homotropylidene rearranges about a thousand times per second at 180°C, and about once per second at — 50°C ... 

Bridged derivatives of homotropylidene also undergo extremely facile degenerate Cope rearrangements. Perhaps the most intriguing of these reactions involves the substance tricyclo[3.3.2.0 ]deca-2,7,9-triene, or bull-valene (XIII). Bullvalene can have its ten carbon atoms arranged in more than... 

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

Treatment of the benzobicyclo[4,l,0]heptene-2,5-diols (86) with diphosphorus tetraiodide gave the corresponding 3,4-benzocycloheptatrienes. ° Similarly, substituted homotropylidenes (87) were obtained from diols (88). " ... 

Complete lineshape analysis of the ABCj DEF2 exchange process observed in the n.m.r. spectra of 2,6-disubstituted homotropylidenes has been employed in the evaluation of the thermodynamic parameters for Cope rearrangement, The phenyl groups at the 2,6-positions of the barbaralane (155) destabilize the transition state relative to the unsubstituted hydrocarbon (154) as evidenced by the AGj g values, respectively 9.3 and 7.6 ( 0.1) kcalmol" Kinetic data for the Diels-Alder addition of acrolein to cydohexa-1,3-diene, and its reverse reaction, in the gas phase have been interpreted in favour of a biradical mechanism. Both the heat and entropy of formation of the endo-adduct have been estimated. Structural modification of 2,3-dimethylenenorbornane (156) or of 2,3-dimethylenenorbornene (158) by the introduction of oxygen in place of C-7, as in (157) and (159) respectively, leads to a decrease in rate of Diels-Alder reaction of the dienes with dimethyl acetylenedi-carboxylate. The double bond in (158) likewise resulted in deactivation relative to (156), and epoxidation of the endocyclic double bond in (158) also afforded an s-cis-diene of decreased reactivity. 

As we have noted several times in this chapter, just because a pericyclic reaction is allowed does not necessarily mean that it is facile. However, one particularly facile reaction is a [3,3] shift involving a (T bond that migrates from and to a cyclopropane ring. A prototypical example is the rearrangement of homotropylidene (shown in the margin). Even more complex examples of this kind of are known, and the most famous involves bullvalene, which is described in the next Going Deeper highlight. 

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