1,3,5,7,9-Cyclodecapentaene

The general term annulene has been coined to apply to completely conjugated mono cyclic hydrocarbons with more than six carbons Cyclobutadiene and benzene retain then-names but higher members of the group are named [jcjannulene where x is the number of carbons m the ring Thus cyclooctatetraene becomes [8]annulene cyclodecapentaene becomes [10] annulene and so on... 

Cyclobutanc, angle strain in. 115-116 conformation of. 115-116 molecular model of, 116 photochemical synthesis of, 1190 strain energy of, 114 torsional strain in, 115-116 Cyclodecane, strain energy of. 114 Cyclodecapentaene, molecular model of, 525, 540... 

The cyclodecapentaene shown here is the cis-trans-cis-cis-trans form. For other stereoisomers, see page 62. 

Many other examples of contrasting behaviour have been discovered. For example all-cis-cyclodecapentaene (VII) photochemically equilibrate at low temperatures with trans 9, 10 dihydronapthalene by a conrotatory six electron electrocyclic reaction but it is converted thermally into cis-9, 10 dihydronaphthalene by disrotatory closure. 

The conjugated 1,3,5,7,9-cyclodecapentaene with the double-bond configuration as in 53 is far less stable than either azulene, 54, or bicyclo[4.4.1 J-1.3.5,7.9-undecapentaene, 55. Explain why this is so on the basis of the VB method (molecular models will be helpful). 

Aromaticity was found to be a general property of many (but not all) cyclic, conjugated jt systems. Moreover, it was found diat aromaticity in molecules can be predicted by Huckel s rule. The structural requirements implicit in Huckel s rule are that there be An + 2 (n is an integer) n electrons in a cyclic, conjugated n system. Obviously benzene, which has six jt electrons (An + 2, n = 1) in a conjugated it system, is aromatic. However, Huckel s rule predicts that molecules such as cyclodecapentaene An + 2 = 10 (n = 2) and [18]-annulene An + 2 = 18 (n = A) should be aromatic, have equal bond lengths, and be planar—and they are. 

It is of interest to examine the aromaticity of the next possible monocyclic aromatic system, Ci0H8. The decagon with f order C-C bonds is shown in Figure 6.6(d) with superimposed o-a-m unit circles. As these circles do not intersect, delocalization of o-a-m is highly unlikely. Experimentally the compound is found to be non-aromatic, cyclodecapentaene. Whereas the effective C-C-C aromatic angle increases with carbon number there is no possibility of monocyclic aromatics beyond benzene. Cyclo-octatetraene (COT) (Figure 6.6(c)) is eliminated by the 4m+2 rule. 

Cyclodecapentaene has An + 2 n electrons (n = 2), but it is not flat. If cyclodecapentaene were flat, the starred hydrogen atoms would crowd each other across the ring. To avoid this interaction, the molecule is distorted from planarity. 

The all-cis cyclodecapentaene shown here is not aromatic because it is not planar. All hydrogens, however, are equivalent and show one absorption in the vinylic region of the molecule s H NMR spectrum. If the molecule were planar and therefore aromatic, the absorption would appear between 6.5-8.0 8. 

Some compounds of types (2, 2) and (0, 0) regroup comparatively easily into (0, 2) pericycles. An example is the spontaneous transformation of unstable 1,6-methane-cyclodecapentaene into double norcaradiene ... 

To be aromatic, a molecule must have 4u + 2 jt electrons and must have cyclic conjugation. 1,3,5.7,9-Cyclodecapentaene lullills one of these criteria but not the other and has resisted all attempts at synthesis. Hxplain. 

The anmdenes from [10]armulene to [16]armulene react quite selectively by 6e disrotatory processes. Thus the low-teirqierature photochemical generation of cyclodecapentaene (143) from rranr-9,10-dihydroiiaphthalene (144) or from (145) was followed by its thermal isomerization to cis-9,10-dihydronaphthalene (146). The E JZJZ -cyclodecapsaUiene (147) has been postulated as inter-... 

Ever since the special stability of benzene was recognized, chemists have been thinking about homologous molecules and wondering whether this stability is also associated with rings that are similar but of different sizes, such as cyclobutadiene (77), cyclooctatetraene (78), cyclodecapentaene (79) , and so on. The general... 

An example is shown in Figure 2.15 where an even perturbation produces the orbitals of azulene and an odd perturbation (/ j) those of naphthalene from the perimeter orbitals of cyclodecapentaene. (Cf. Example 2.5.]) The energies of 0, and 0,. are not affected to the first order by the odd perturbation producing naphthalene, whereas 0s is stabilized and 0,- destabilized. Therefore, 0, becomes the HOMO and 0. the LUMO, and the HOMO-LUMO splitting AEhomo-lumo he same as for cyclodecapentaene. The HOMO- LUMO transition is referred to as L according to Platt. In azulene, on the other hand, 0, is destabilized and becomes the HOMO, whereas 0s- is stabilized and becomes the LUMO. The HOMO-LUMO splitting is markedly smaller than for cyclodecapentaene, and the HOMO-LUMO transition is of the Lb type. 

All Cis cyclodecapentaene is a stable molecule that shows a single absorption in its NMR spectrum at 5.67 S. Tell whether it is aromatic, and explain its NMR spectrum. 

Pyridine has an aromatic sextet of electrons. Cyclodecapentaene is not fiat because of stcric interactions. 

---