1,3,5,7,9-Cyclodecapentaene isomers

As predicted, the NMR spectrum of bullvalene coalesces to a single line at high temperatures due to 3,3-shifts of the cw-divinylcyclopropane moieties, and subsequent work established log k = 14.0 — 13 900/23RT for the conversion. At high temperatures, bullvalene (BV) gives c/.y-9,10-dihydronaphthalene, (C9,10) (Scheme 11.11), an isomer which had been prepared previously in the unfulfilled hope of thermalizing it to 1,3,5,7,9-cyclodecapentaene. ... 

The purely thermal processes that are involved include the formation of cw-9,10-dihydronaphthalene (C9,10) from bullvalene (BV), bicyclo[4.2.2]decatetrene (422) and its intramolecular Diels-Alder adduct, tetracyclo[4.4.0.0 0 ]decadiene (TCD), from lumibullvalene (LBV), isobullvalene (IBV), tricyclo[5.3.0.0 ]deca-3,5,9-triene (TDT), and isolumibullvalene (ILBV), from basketene (BSK), NHC, TRQ, and the syn and anti isomers of tricyclo[4.4.0.0 ]decatriene (S and A, respectively), and from all c/ -cyclodecapentaene (C5), snoutene (SNT), and diademane (DIA). A more recent review includes many heats of formation and enthalpies of activation ... 

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 ... 

Figure 6.30. Isomers of cyclodecapentaene (CioHio). (a) The internally strained all-(Z)-iso-mer with angles greater than 120°. (b) The (Z,Z, ,Z,i )-isomer, strained because of internai hydrogen steric repulsion.

PROBLEM 13.12 The three isomeric cyclodecapentaenes of Rgure 13.35 differ in the stereochemistries of the double bonds. Why isn t there more than one isomer ofbenzene ... 

---