Aromaticity, aromatic character Hiickel

The most obvious compound in which to look for a closed loop of four electrons is cyclobutadiene (52). Hiickel s rule predicts no aromatic character here, since 4 is not a number of the form 4 + 2. There is a long history of attempts to prepare this... 

The resulting crystal proved amenable to a conclusive XRD analysis. As shown in Eq. (26), the C—C ring bond is lengthened over what it is in the structurally similar cyclopropene (1.304 A) and the B—C bonds shortened relative to the electronically analogous bond in trivinylborane (1.558 A). Thus, one can safely conclude that there is extensive 7r-electron delocalization and Hiickel aromatic character in the borirene ring. 

A rule for helping to predict if a monocyclic planar system of delocalized tt electrons will exhibit aromatic character. The ring system will be aromatic if the number of delocalized tt electrons equals 4n -h 2 where nisa positive integer or zero. For example, the ring system in phenylalanine obeys Hiickel s rule. See Aromaticity... 

The most obvious compound in which to look for a closed loop of four electrons is cyclobutadiene (44).135 Hiickel s rule predicts no aromatic character here, since 4 is not a number of the form 4n + 2. There is a long history of attempts to prepare this compound and its simple derivatives, and, as we shall see, the evidence fully bears out Hiickel s prediction— cyclobutadienes display none of the characteristics that would lead us to call them aromatic. More surprisingly, there is evidence that a closed loop of four electrons is actually ami-aromatic.1 If such compounds simply lacked aromaticity, we would expect them to be about as stable as similar nonaromatic compounds, but both theory and experiment show that they are much less stable.137 An antiaromatic compound may be defined as a compound that is destabilized by a closed loop of electrons. 

Hehre66 69 and independently Jorgensen72,73 pointed out that the Mobius and Hiickel description of homoconjugated molecules (Figure 9) is consistent with the assumed homoaromtic and homoantiaromatic character of these compounds. However, it was also realized that in the general case such a classification might not be sufficient to describe subtle differences in orbital interactions, which determine the homo(anti)aromatic character of a molecule. 

The aromatic character of the dibenzo derivative 2 has also been calculated. Studies on the behavior of the deep red anion 3, as a potential IOji heteroaromatic system, were described previously <1996CHEC-II(9)268>. H NMR spectra (NMR - nuclear magnetic resonance) of l,3-dithiepin-2-carbodithiolate 4, a ligand obtained from the reaction of carbon disulfide with the lithium salt of 1,3-dithiepin, as well as its palladium salt suggest a good deal of Hiickel aromatic character in the seven-membered dithiepin ring <1991ICA(185)169>. 

The porphyrin ring contains 22 electrons in its n orbitals. As explained above, the Hiickel rule cannot be applied to this electron count, because the molecule is not monocyclic. However, the porphyrin ring can be formally derived from neutral [18]annulene, by introduction of appropriate heteroatoms and bridges (Fig. 3). The macrocycle is thus shown to be aromatic in the Hiickel sense, and is denoted [18]porphyrin. As we will show in subsequent sections, this approach is readily generalized to other porphyrinoids, whose aromatic character can be predicted by defining a neutral annulenoid pathway in the macrocycle. These pathways will be... 

One approach to explaining the substituent effect has been suggested by Carpenter.25 It uses simple Hiickel molecular orbitals, and avoids the need to rely on frontier orbitals. The basis of the idea is to return to the picture of a pericyclic transition structure as having aromatic character [see (Section 6.4.1) page 214]. 

It is important to examine aromaticity in its wicommon features in addition to planarity and aromatic stability. MO calculations carried out by Hiickel in the 1930s showed that aromatic character is associated with planar cyclic molecules that contained 2, 6, 10, 14 (and so on) 7i-electrons. This series of numbers is represented by the term An + 2, where n is an integer, and gave rise to Hiickel s An + 2 rule that refers to the number of 7i-electrons in the p-orbital system. In the case of benzene, n= and thus the system contains six 7C-electrons that are distributed in MOs as shown above. 

Now, what evidence is there that other Hiickel numbers—1, 10, 14, etc.— are also magic numbers We cannot expect aromatic character necessarily to appear here in the form of highly stable compounds comparable to benzene and its derivatives. The rings will be too small or too large to accommodate trigonally hybridized atoms very well, so that any stabilization due to aromaticity may be largely offset by angle strain or poor overlap of p orbitals, or both. 

Hiickel (4n + 2) rule Monocyclic planar (or almost planar) systems of trigonally (or sometimes digonally) hybridized atoms that contain [An + 2) Jt-electrons (where n is a nonnegative integer) will exhibit aromatic character. The rule is generally limited ton = 0-5. 

Mobius aromaticity A monocyclic array of orbitals in which a single out-of-phase overlap (or, more generally, an odd number of out-of-phase overlaps) reveals the opposite pattern of aromatic character to Hiickel systems with 4n electrons it is stabilized (aromatic), whereas with 4n + 2 it is destabilized (antiaromatic). In the excited state 4n + 2, Mobius pi-electron systems are stabilized, and 4n systems are destabilized. No examples of ground-state Mobius pi systems are known, but the concept has been applied to transition states of PERI-CYCLIC REACTIONS (see AROMATIC [3]). 

The first theoretical explanation - based on quantum mechanical approach - of the aromatic character of a molecule was given by physical chemist E. Hiickel in 1931. The Hiickel s rule... 

There has been no work published assessing the aromatic character of 1,2,3-thiadiazole. From the Hiickel definition of aromaticity, (4n + 2) 77-electrons in a ring constitute an aromatic compound and by this criterion 1,2,3-thiadiazoles should be considered aromatic compounds. Chemical shifts for 1,2,3-thiadiazoles in both 13C and H NMR, which are related to the electronic environment, are in accord with an aromatic heterocyclic ring. 

We therefore conclude that n.m.r. will differentiate between the [4 ]- and [4n + 2]-annulenes, for it is quite apparent that proton chemical shifts are sensitive to the magnetic properties of the tr-electrons therein. Insofar as this property parallels the Hiickel predictions regarding aromatic and anti-aromatic character (which we now identify with diamagnetic and paramagnetic ring currents) then n.m.r. is a qualitative8) criterion of aromatic character in the annulenes. 

The directly bonded C—coupling constants have been used to give a measure of aromaticity in mono- and polycyclic hydrocarbon ring systems.Cyclo-octatetraene reacts with methylene dichloride and methyl-lithium to afford a mixture of syn and anti-9-chlorobicyclo[6,l,0]nona-2,4,6-triene (40 and 41) which, on treatment with a 30% lithium dispersion in tetrahydrofuran, gives cyclonona-tetraenide, which is isolated as the tetraethylammonium salt. The chemical shift and the coupling constant (7i3c-ih = 137 c./sec.) observed for the lithio-salt (42) are indicative of the aromatic character of the ring, in accord with the predictions of Hiickel s Aromaticity rule. 

---