Annulenes—1,3,5,7,9-Cyclodecapentaene Isomers

A number of structures have been prepared that avoid the steric problems associated with the 1,3,5,7,9-cyclodecapentaenes. 1,6-Methano[10]annulene (1) can be prepared from naphthalene in multigram quantities in four steps.  

Quite a number of substituted derivatives have also been prepared. In compound 1 the steric problem is avoided with only a modest loss of planarity in the tt system.  

The NMR spectrum of compound 1 shows a diamagnetic ring current of the type expected in an aromatic system. The NICS calculated for 1 is -17.7, which indicates aromaticity. Both spectroscopic data and MO (MP2/DZV ) and DFT (B3LYP/6-31G) calculations indicate that the ground state of 1 is delocalized. Thus while the t system in 1 is not completely planar, it appears to be sufficiently close to provide a delocalized 10-electron t system. A stabilization energy of 17.2 kcal has been obtained on the basis of an experimental heat of hydrogenation.  

Bianchi, T. Pilati, and M. Simonetta, Acta Crystallogr., Sect. B, B36, 3146 (1980). 

The deviation from planarity that is present in a structure such as 1 raises the question of how severely a conjugated system can be distorted from the ideal coplanar alignment of p orbitals and still retain aromaticity. This problem has been analyzed by determining the degree of rehybridization necessary to maximize /i-orbital overlap in 1. A quantitative VB treatment also finds the periphery of conjugated bonds to be delocalized and the orbitals are quite similar to those in naphthalene. Thus a relatively small amount of rehybridization greatly improves orbital overlap in the twisted system and stabilizes the structure. 

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Larger annulenes permit the incorporation of irons double bonds into the rings hence, isomeric annulenes warrant consideration beginning with the cyclodecapen-taenes. [10]Annulene should, by the Huckel rule, possess aromatic stabilization if it were planar. All the isomeric cyclodecapentaenes suffer serious strain that prevents the planar geometry from being adopted. The trans,cis,trans,cis,cis-isomer, which has minimal angle strain, suffers a severe nonbonded repulsion between the two internal hydrogens ... 

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