1.3.5- Cyclooctatriene tautomerism

XXIII. Valence Tautomerism of 1.3.5-Cyclooctatriene and Bicyclo(4.2.0)-octa-2.4-diene. J. Amer. chem. Soc. 74, 4867 (1952). 

A bimetallic iron tricarbonyl cation C8H9Fe2(CO)6 + (IV) is derived from cyclooctatriene (157). The Mossbauer resonance of spectrum of (IV) and the related cycloheptatrienyl-Fe2(CO)6 cation (V) (73) suggest that these cations may be considered as containing a bis(7r-allyl) and an allyl-diene system, respectively. In the second case (V), rapid valence tautomerism is invoked to account for the unique proton NMR signal in solution. 

When 18 is treated with acetone, boronic ester 20 with a cyclooctatriene structure is cleanly obtained indicating the valence tautomerism between 18 and 19. In fact, 19 appears to be the kinetic product in the borylation of lithium or potassium cyclononatetraenide with alkylboron dihalides which rearranges quickly to 18 due to the comparatively high thermodynamic stability of the latter.1819 Acetaldehyde,... 

For the related tautomerism between the higher homologues, 1,3,5-cyclooctatriene (120) and bicy-clo[4.2.0]octa-2,4-diene (121), a tabulation of a series of derivatives including thermodynamic data is presented in the treatise by Marvell. The data reveal a large variation in equilibrium constants by introducing seemingly small changes in the structure. While the electronic effects of the substituents at C-7... 

Barton has suggested 35> that in photochemical reactions of cyclic conjugated olefins, ring fission will predominate in rings of n annular atoms containing (n/2)-l double bonds and valence tautomerization will occur in other systems. Consequently, it was anticipated that 1,3,5-cyclo-octatriene upon photolysis should give rise to an acyclic tetraene in the same manner that 1,3-cyclohexadiene opens 35> 36> to 1,3,5-hexatriene. When 1,3,5-cyclooctatriene (55) was irradiated in solution (ether or pentane) 37> 39> two isomerization products were isolated, bicyclo[4.2.0]-octa-2,7-diene (56) and tricyclo[5.1.0.0 4> 8]oct-2-ene (57). The formation of 56 is not exceptional. The formation of 57 has been visualized by... 

A valence-bond tautomerism is also evident in other systems such as the oxepin derivative and cyclooctatriene 78 " as shown below ... 

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