Annulenes,

TABLE 3.26 Relative Energies (kcal mol ) of [lOJAnnulene Isomers 

Unfortunately, the situation became much more complicated. Schaefer and Luthi noted that 44 would give rise to six H NMR signals, not the five noted in the experiment. In addition, single-point computations at CCSD(T) reverse the relative energies of the twist and heart isomers (Table 3.26), suggesting that the heart isomer cannot be described by just a single configuration wavefunction.  

TABLE 3.27 Computed Chemical Shifts for 39,42, and 44 Compared with the Experimental Values for T pe II 

Since the geometries of these isomers were found with what may be inappropriate wavefunctions, reoptimization with the CCSD(T) method is warranted. However, the relative ordering of these three isomers is unchanged upon this reoptimization, and so these best energy computations indicate that the two most favorable conformations of [lOJannulene are 39, which agrees with Masamune s type II, and 42. 

Stanton ° confirmed the assignment of type n as the twist isomer by computing the NMR shifts of 39, 42, and 44, shown in Table 3.27. The heart isomer (44) gives rise to six signals, inconsistent with the five observed in the experiment. The differences between the experimental chemical shifts and the computed values for isomer 39 are less than 1.1 ppm but 13.1 ppm for 42. The relative spacings between the peaks also match much better with 39. This still leaves open for question the exact nature of Masamune s type I isomer. 

The hydrogens on the double bond of the eight-membered ring appear upheld at 4.93 5, indicating that this compound has some antiaromatic character. 

The general name annulene is sometimes given to rings that contain alternating single and double bonds in a single Lewis structure. Thus, benzene can be called [6]annulene, and cyclooctatetraene can be called [8]annulene. A number of larger annulenes have been prepared to determine whether they follow Hiickel s rule and are aromatic when they have 4n + 2 electrons in the cycle. 

The H-NMR spectium of this compound shows absorptions in the region of 9.5 S and other absorptions in the region of —7 S. Explain which hydrogens are responsible for each of these absorptions. 

Build models of [10]annulene and the bridged [10]annulene discussed on the previous page and examine the strain and planarity of each. 

A 14-electron tt system can be generated in circumstances in which the steric problem associated with the internal hydrogens of [14]annulene can be avoided. This can be achieved in 10b,10c-dihydropyrene systems, in which the annulene ring is built around a saturated core. 

Another family of 14 Tr-electron systems is derived from structure 3.  

An isomeric system is related to the benzenoid hydrocarbon phenanthrene. Both the syn and anti stereoisomers have been synthesized.  

The Hiickel rule predicts nonaromaticity for [16]annulene. The compound has been synthesized and characterized. The bond lengths show significant alternation in length (C=C, 1.34 A C—C, 1.46 A), and the molecule is less planar 

The general term annulene refers to completely conjugated monocyclic hydrocarbons with more than six carbons. Cyclobutadiene and benzene retain their names, but higher members of the group are named [jc]annulene, where x is the number of carbons in the ring. Thus, cyclooctatetraene becomes [8]annulene, cyclodecapentaene becomes [10]annulene and so on. 

Use Frost s circle to construct orbital energy diagrams for (a) [lOlannulene and (b) [12]annulene. Is either aromatic according to Huckel s rule  

The size of each angle of a regular polygon is given by the expression (number of sides) - 2 

A second isomer of [10]annulene (the cis, trans, cis, cis, trans stereoisomer) can have bond angles close to 120° but is destabilized by a close contact between two hydrogens directed toward the interior of the ring. To minimize the van der Waals strain between these hydrogens, the ring adopts a nonplanar geometry, which limits its ability to be stabilized by it electron delocalization. It, too, has been prepared and is not very 

Planar geometry required for aromaticity destabilized by van der Waals repulsions between indicated hydrogens 

The term annulene refers to monocyclic compounds that can be represented with alternating single and double bonds. We indicate the size of the 

The original synthesis of COT was a 13-step process Willstatter, R. Heidelberger, M. Chem. Ber. 1913, 46, 517. 

Dewar reported calculations showing that the chair-like conformation of cyclooctatetraene is unstable with respect to the tub conformation shown here Dewar, M. J. S. Merz, K. M., Jr. /. Chem. Soc. Chem. Commun. 1985, 343. 

Theoretical calculations indicate that a Dgj, (planar, antiaromatic) structure is a transition state in the double bond shifting reaction of cyclooctatetraene Hrovat, D. A. Borden, W. T. /. Am. Chem. Soc. 1992, 114, 5879. 

4 APPLICATIONS OF MOLECULAR ORBITAL THEORY AND VALENCE BOND THEORY 

The stabilization imparted by aromaticity has been exploited by nature in the use of the porphyrin ring system. Shown to the right is the basic porphyrin skeleton. 

Franck, B., and Nonn, A. Novel Porphyrinoids for Chemistry and Medicine by Biomimetic Synthesis. Angew. Chem. Int. Ed. Eng., 34,1795 (1995). 

Sample Solution (a) Cycloheptatriene (compound A) is not aromatic because, although it does contain six tt electrons, its conjugated system of three double bonds does not close on itself—it lacks cyclic conjugation. The CH2 group prevents cyclic delocalization of the tt electrons. 

In the next section we ll explore Hiickel s rule for values of n greater than 1 to see how it can be extended beyond cyclobutadiene, benzene, and cyclooctatetraene. 

Most of the synthetic work directed toward the higher annulenes was carried out by Franz Sondheimer and his students, first at Israel s Weizmann Institute and later at the University of London. 

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Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxlrane ting. Cataiytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for trons-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicycKc tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

Large annulenes tend to undergo conformational distortion, cis-trans isomerizations, and sig-matropic rearrangements (p. 40 and p. 100). Methylene-bridged conjugated (4n + 2)-ic cyclopolyenes were synthesized with the expectation that these almost planar annulenes should represent stable HOckel arenes (E, Vogel, 1970, 1975). 

The kekulene macrocycle consists of twelve anellated benzene rings and may be considered as an [iSlaanulene (inside) or a 30]annulene (outside). H. Staab (F. Diederich, 1978) called it a superbenzene , since it has the same symmetry as benzene. 

As noted earlier planar annulenes with 4n tt electrons are antiaromatic A mem ber of this group [16]annulene has been prepared It is nonplanar and shows a pattern of alternating short (average 134 pm) and long (average 146 pm) bonds typical of a nonaromatic cyclic polyene... 

Something interesting happens when we go beyond benzene to apply the aromatic ring current test to annulenes... 

Annulene satisfies the Huckel (4n+2) tt electron rule for aromaticity and many of its proper ties indicate aromaticity (Section 11 20) As shown in Figure 13 10a [18]annulene contains two different kinds of protons 12 he on the ring s periphery ( out side ) and 6 reside near the middle of the molecule ( inside ) The 2 1 ratio of outside/inside protons makes it easy to assign the signals in the NMR spectrum The outside protons have a chemical shift 8 of 9 3 ppm which makes them even less shielded than those of benzene The six inside protons on the... 

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

Which would you predict to be more shielded the inner or outer protons of [24]annulene" 13 41 F IS the only isotope of fluonne that occurs naturally and it has a nuclear spin of j... 

Divide the heats of combustion by the number of carbons The two aromatic hydrocarbons (benzene and [18]annulene) have heats of combustion per carbon that are less than those of the nonaromatic hydrocarbons (cyclooctatetraene and [16]annulene) On a per carbon basis the aromatic hydrocarbons have lower potential energy (are more stable) than the nonaromatic hydrocarbons... 

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