Cyclodecapentaene, and

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. 

The central molecules from this saga of the 1960s are the potentially aromatic cyclodecapentaenes and bullvalene, a compound of 10/3 faces. Efforts directed to the synthesis of these compounds involved thermal, photochemical, and metal-catalyzed processes which not only accomplished the major task but also generated... 

A good set of models will show this strain easily. Construct the planar, aU-cis cyclodecapentaene and the strain will be apparent. If you are lucky, the plastic will mimic the molecule and the model will snap into a pretzel shape, which approximates the energy minimum form of the molecule. These molecules are best described as strained polyenes. They fail the test of planarity for aromatic molecules because the planar structures are much more strained than the nonplanar forms. In this case, strain trumps aromaticity. 

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... 

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. 

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. 

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... 

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. 

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 ... 

From Equation 3.33 it follows that cyclic systems, for which the number of electrons can be written as N = 4n + 2, where n is an integer, are especially stable. For n = 1 (N = 6), we have benzene and for n = 2 (N = 10) (cyclodeca— pentaene = quasi-naphthalene ) (Figure 3.9). Naphthalene itself (C,oHg) has a bridge across the middle of the molecule, which makes this molecule even more stable. Also, azulene is closely related to cyclodecapentaene. For larger cyclic systems, the stabilization is less pronounced. This (4n + 2) rule is called HUckeVs rule. 

Metal, Metalloidal, and Non-metal Fluoroacetates.— Fluorine Magnetic Resonance Studies.I.para-Substituted P-DifluorostyTeae [thepP-difluorostyrenesp-XC,H(-CH CFs (X — NMe OMe, Me, H, F, Cl, or CN) were obtained via the CF,Cl-CO,Na-Ph,P-aldehyde route]. Reaction of Difluorocarbene with 2-Benzylidenecyclohexanones to afford Phenylfluorofurans (CF2 generated via thermal decomposition of CF CI-COjNa). Cyclodecapentaene Derivatives (reaction of CF, from CFjCI-COjNa with 1,4,5,8-tetrahydronaphthalenes). Action of Sodimn and Lithium Chlorodifluoroacetates on... 

The resulting molecule, a bridged system containing 10 n electrons, isn t a perfect cyclodecapentaene. The overlap between the 2p orbitals at the bridgehead and adjacent positions isn t optimal, for example (Rg. 13.38). Still, the molecule maintains enough orbital overlap to remain fiiUy conjugated and satisfy the criteria for aromaticity (cyclic, flat, conjugated, and 4m -b 2 7t electrons). Both its chemical and physical properties allow it to be classified as aromatic. 

PROBLEM 13.50 Vogel and Roth s synthesis of the bridged cyclodecapentaene (1) is outlined helow. Supply the missing reagents (a, b, c, and d) and an arrow formalism for the last step. 

In the following two reactions, denoted as (A) and (B), cyclodecapentaene cyclizes into trans- and c/s-9,10-dihydronaphthalene. Apply Woodward-Hofimann rules to determine the reaction condition (thermal or photochemical) and reaction pathway (conrotatory or disrotatory) of each process. 

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