Annulenes spectra

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... 

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 was first prepared in 1960. ° Its NMR spectrum has been investigated and shows that two stereoisomers are in equilibrium ... 

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. 

Badger and coworkers devised a sequential synthesis of [ 18]annulene-l,4 7,10 l 3,16-trioxide which is formally the condensation product of three furan molecules and three ethylenes . The synthesis is illustrated below in Eq. (3.25). The [18]annulene trioxide was obtained as a red solid (mp 215—216 °d) whose proton nmr spectrum showed two peaks of equal area at 8.66 and 8.68 ppm. 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

It is straightforward in the charging of a layered electrophore to increase the interplane distance of the -layers and thus to relax the resulting Coulombic strain this approach is discussed for the doubly layered annulenes [14], In this particular case, a stable tetra-anion is available which can be characterized by a highly resolved H-nmr spectrum (Irmen et al., 1984). 

The electronic spectrum of the fractions containing the pure tridehydro [18]annulene exhibits the strongest absorption maximum (in benzene) at 342 nm. (e 155,000) and the spectroscopic yield, based on the molar extinction coefficient, is 1.17 g. (2.40% from 1,5-hexadiyne). The yield of tridehydro[18]annulene in the mixed fractions, based on the 342 nm. maximum,is 0.27g.(0.55%). The tridehydro [18]annulene is best stored in solution in the refrigerator. 

Annulene was the first macrocyclic annulene containing (4n -j- 2) zr-electrons to be synthesized. The compound is of considerable interest, since it is the type of annulene that was predicted to be aromatic by Hiickel.10 It proved to be aromatic in practice, as evidenced from the proton magnetic resonance spectrum,8-11 the X-ray crystallographic analysis,18 and the fact that electrophilic substitution reactions could be effected.13... 

The platinadehydrobenzo[19]annulene 331 is formed by reaction of the SnMes derivative with rrans-PtCl2(PEt3)2 and Cul (Scheme 72). The molecule is not planar, maximum deviations from the mean plane of the C30H12 macrocycle being between 0.484 A above to 0.283 A below. The UV/vis absorption spectrum suggests that electron delocalization occurs throughout the macrocycle. ... 

On the basis of the calibration by melting points and phase transitions, a number of shift thermometers have been developed for solid-state NMR spectroscopy in various temperature ranges. Wehrle et al. 144), for example, used the line splitting in the N CP/MAS NMR spectrum of the organic dye molecule tetra-methyldibenzotetraaza annulene (TTAA) in the temperature range 123—405 K. A high-temperature shift thermometer for temperatures of up to 790 K was developed by van Moorsel et al. 40) on the basis of Sn MAS NMR spectroscopic... 

The PMR spectrum of 85a agrees well with theoretical considerations. It can be seen from the vicinal coupling constants that the seven-membered ring is planar, containing localized bonds. A paratropic effect of 1.7 ppm in the five- and six-membered rings and one of at least 2.7 ppm in the seven-membered ring follows from comparison with suitable reference compounds. The different paratropism can be explained by the superposition of two ring systems, a diatropic indolizine and a paratropic [ 12]annulene moiety.106,107... 

We have mentioned already (Section 22-3C) the large differences in nmr chemical shifts between the inside and outside hydrogens of [18]annulene —a substance which with 18 tt electrons should be aromatic by the 4n + 2 rule. These differences are observed only at low temperatures. The proton nmr spectrum of [18]annulene at room temperature is a single resonance, which indicates that the inside (Ha) and outside (Hb) hydrogens are equilibrating rapidly. This can take place only if cis-trans interconversion occurs about the double bonds (marked c and t) ... 

At very low temperatures (—155°), the proton nmr spectrum shows the inner hydrogens at 812.9-10.5 and the outer hydrogens at 85.7-6.4, which is in exactly the opposite order to the shifts observed with [18]annulene and the other known [4n + 2]7r-electron annulenes. 

For benzene, [6]annulene, with six it electrons (4n + 2, n = 1), the theory clearly meets both tests. As we have pointed out, there is a substantial stabilization of about 37 kcal mole-1 compared with a hypothetical localized model. The familiar chemical properties also point to a strong tendency for maintenance of the six it electron unsaturated system. The proton magnetic resonance spectrum of benzene and its derivatives shows the proton resonances in the range of 8 = +7 to +8 ppm (downfield from tetramethylsilane), 1-2 ppm lower than protons attached to nonbenzenoid double bonds. Referring to Figure 1.21a, we can see that the prediction is in agreement with this result. The induced field adds... 

Since the ground state and the 1 IT, excited state are made from two sets of Kekule structures that are interchanged by two b2u modes, one will expect to find in the spectrum of the B2u state two b2u modes with exalted frequencies as shown in Fig. 7.8. As can be seen in the figure, one of these modes is benzenic, the other is annulenic. These two exalted modes are indeed observed in the B2u spectrum of anthracene (16,19). The bonding features of the B2u state are expected to involve spin pairing between nonconsecutive carbons... 

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