Annulene, Aromaticity

The Hiickel theory [21, 22], which has become the standard description of annulene aromaticity, is one of the cornerstones of physical organic chemistry. Unfortunately, in its qualitative version, from which the famous Hiickel rules are derived, the theory does... 

VIII. ANNULENES AROMATICITY AND ANTIAROMATICITY A. If We Study Cyclooctatetraene, Why Not Benzene ... 

Huckel criterion for aromaticity becomes insufficient where bond alternation is preferred e.v). The critical size which demarks the onset of bond alternation has been calculated and is expected to fall between [22]-and [26]-annulene. Hence among the (4w- -2) tt annulenes, aromatic character is predicted for values of < 5, whereas for > 6 non-aromatic, or polyenic properties are expected. The results reported to date for 4n -1- 2) TT systems seem to be compatible with this prediction, as all of the M < 5 annulenes appear to be aromatic, while a tridehydro-[26]-annulene ( = 6) does not ). The study of the [22]annulene series ( = 5) will serve to demark more clearly the limit at which bond alternation quenches the ring current. Preliminary studies of the n.m.r. of two compounds in the n = 7 series, viz. a pentadehydro-[30]annulene and a tridehydro-[30] annulene also indicate that their ring size is... 

This reference structure gives the annulene aromaticities in Figure 10. These are perhaps slightly worse than the results in Figure 9 since only cyclobutadiene and cyclooctatetraene are computed to be antiaromatic. The order of the 11 compounds in column three of Figure 6 is fairly good. Benzene is now at the top of the list, cyclobutadiene is antiaromatic, and butadiene is nonaromatic. However, three... 

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). 

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

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... 

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... 

The 7/NMR spectrum displays signals of shielded protons = - 2.35, integral level 1) and of deshielded ones = 10.45 and 9.39, integral levels 1 1). This reflects a ring current due to aromaticity as described for annulenes and porphyrins in section 2.5.2. To conclude, the reaction involves an oxidative cyclisatlon of 2,5-bis(2-pyrrolylmethyl)-17/-pyrrole 2 with 47/-trlazole-3,5-dlaldehyde 3 to the corresponding 2,3-diazaporphyrin 4, following the 3-t-l pathway of porphyrin synthesis. Two non-equivalent tautomers may exist these are the diaza[ 18]annulene 4a and the tetraaza[18]annulene 4b. 

Annulene is a very imstable compound that undergoes cyclization to bicyclic isomers and can be kept only at very low temperature. The NMR spectrum has been studied at low temperature." Besides indicating the double-bond geometry shown in the stmcture below, the spectrum reveals a paramagnetic ring current, the opposite to what is observed for aromatic systems. This feature is quite characteristic of the [4 ]annulenes and has been useful in characterizing the aromaticity or lack of it in annulenes." ... 

Annulene, [22]annulene, and [24]annulene have all been reported. The NMR spectrum of [22]annulene is consistent with regarding the molecule as aromatic, whereas those of the [20] and [24] analogs are not. In each case, there is some uncertainty as to the preferred conformation in solution, and the NMR spectra are temperature-dependent. Although the properties of these molecules have not been studied as completely as those of the smaller systems, they are consistent with the predictions of the Huckel rule. 

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