Aromaticity Hiickel

The presence of a ring-current is characteristic of all Hiickel aromatic molecules and is a good test of aromaticity. For example, benzene, a six-7r-clectron aromatic molecule, absorbs at 7.37 5, but cyclooctatetraene, an eight--iT-electxon nonaromatic molecule, absorbs at 5.78 8. 

In this method, the orbital symmetry rules are related to the Hiickel aromaticity rule discussed in Chapter 2. Huckel s mle, which states that a cyclic system of electrons is aromatic (hence, stable) when it consists of 4n + 2 electrons, applies of course to molecules in their ground states. In applying the orbital symmetry principle, we are not concerned with ground states, but with transition states. In the present method, we do not examine the molecular orbitals themselves but rather the p orbitals before they overlap to form the MO. Such a set of p orbitals is called a basis set (Fig. 15.5). In investigating the possibility of a concerted reaction, we put the basis sets into the position they would occupy in the transition state. Figure 15.6 shows this for both the... 

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. 

Electronically related to such borirenes are the salts of the dianion of 1,2-di-tert-butyl-3-[bis(trimethylsilyl)]methyl]-l, 2-diborirane.85 An XRD analysis of the dipotassium salt (86a,b) uncovered shortened B—B (1.58 A) and B—C (1.50 A) bonds, as suggested by resonance contributions 86a and 86b. Salt 86 can be considered to have largely a 2- r-electron Hiickel aromatic ring. 

Our success in super-stabilization of cation 6 led us to the preparation of a higher homologue, that is, cyclooctatetraene (COT), fully annelated with BCO units 9 (9). As compared with a large number of studies on its radical anion or dianions, the studies on the cationic species of COT have been quite limited. There have been only one study by Olah and Paquette on the substituted COT dication (70), which is a typical 6n Hiickel aromatic system, and few sporadic studies on radical cations, which involve indirect spectral observations, such as electronic spectra in Freon matrix at low temperature (77,72) and constant-flow ESR study (13). 

Multicenter bonding is the key to understanding carboranes. Classical multicenter n bonding gives rise to electron-precise structures characteristic of Hiickel aromatics, which are planar and have 4n + 2 n electrons. Clusters are defined here as ensembles of atoms connected by non-classical multicenter bonding , i.e., all... 

The multicenter bonding interaction in 2A is classic in a sense that it results in a well known Hiickel aromatic, which is planar with cyclic delocalized An + 2 n electrons (no hyper-coordinate atoms). The most favorable site for a proton to bind to 2A is not the carbon atom, but the B-B edge. The non-dassical 1C is 47.6 kcal mol-1 more stable than the classical structure 1A [5],... 

For example, both Hiickel aromatics B and C conform to the Wade-Mingos electron counting rules [2] (see Chapter 1.1.2) and to the structural systematics developed for boranes and heteroboranes [1] the hexagonal bipyramid with the apices removed is in agreement with an arachno electron count of 18 SE for (CH)g [2],... 

Consequently, the reduction of [4n]annu-lenes should be observed at relatively positive potentials with small AE separations for the dianion formation, while the reduction of the [4n + 2]annulenes should occur at more negative potentials with large AE separations for the dianion formation. This is exactly what is observed in (see Table 1). Although benzene [37, 38], the classical Hiickel aromatic with 4 1 + 2 = 6 jr-electrons, is reduced at —3.42 V (vs. Ag/AgCl), the reduction of the [Sjannulene COT occurs at —1.81V. Similarly, the [16]annulene (3) [67, 68] is more easily reduced than the corresponding [ISjannulene (4) [69], although the reduction of the larger jr-systems should be more favorable for electrostatic reasons. 

Effects other than angular distortions can lead to bond fixation. A simple model based on Hiickel aromatic theory accounts for a large number of such cases (e.g., starphenylene and triphenylene). Ideas that electronegative groups in the annelation are responsible for the bond fixation are shown to be inconsistent when tested against a significant sample of data. 

The seven-membered fully unsaturated boron heterocycle is named l//-borepin or, more frequently, borepin. It has six 7r-electrons and is therefore a potential Hiickel aromatic, isoelectronic with the tropylium ion. This similarity has induced several groups to search for a synthesis. Recent theoretical calculations have predicted the -stabilization to be less than that of the tropylium ion. On the other hand, borepins are organoboranes and therefore expected to be sensitive to oxygen. 

For a homoaromatic system, surface delocalization in the cyclopropyl ring is perpendicular to the bridging bond, thus forming a Hiickel aromatic electron ensemble which is delocalized in just one part of the bi(poly)cyclic system. 

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>. 

Day21 has given a careful account of the relationship between the Woodward-Hoffmann rules and Mobius/Hiickel aromaticity, and has defined the terms supra-facial and antarafacial in terms of the nodal structure of the atomic basis functions. His approach makes quite explicit the assumption that the transition state involves a cyclic array of basis functions. Thus the interconversion of prismane (10) and benzene, apparently an allowed (n2s+ 2S+ 2S) process, is in fact forbidden because there are additional unfavourable overlaps across the ring.2... 

It should be noted, however, that planarity had not been excluded for corannulene until the X-ray analysis was performed. The molecule is a non-alternant hydrocarbon because it contains an odd-membered ring. Hence, the it-electron density distribution cannot be uniform in the ground state. One of such polar structures is the double Hiickel aromatic structure 66, c consisting of peripheral 14n and central 6ir systems. The contribution of resonance as shown in 6 c will be at its maximum when the whole molecule is planar like coronene (8). Notwithstanding, the planar corannulene will have huge angle strain. If we assume that all C-C bonds are 1.40 A in length and the... 

This thermal pericyclic reaction involves six electrons and should takes place via a Hiickel aromatic transition state. The following scheme shows that the ring closure must be disrotarory. Therefore, the phenyls are cis to each other. 

The stereospecific cis-addition of diboron tetrachloride to alkynes and alkenes (37) may be interpreted as an interaction of the empty 7r-orbitals of the boron atoms with the 7r-orbital of the organic species. According to this picture, boron-boron bond breaking would lag behind boron-carbon bond formation. The transition state is a 4 + 2 Hiickel aromatic ( .=0), and thermal addition is allowed. If bond making and breaking were synchronous, this four-center reaction would be more like the <7-77 exchange reactions, which we shall discuss later. With regard to (37), there is a discrepant case in which an apparent trans addition of diboron tetrachloride to cyclopentadiene has been found (Saha et al., 1967). 

Silver(diazomethyl)phosphoryl compounds may also be synthesized by treatment of the corresponding diazophosphoryl compound with silver oxide. The thermally stable phosphoryl derivatives undergo electrophilic substitution with alkyl iodides73 but, unlike the carbonyl-substituted derivatives, also undergo electrophilic diazoalkane substitution with a variety of Hiickel aromatic salts (Scheme 1.31).74—79... 

Cyclopentadienide (Cp) 1 is well known as one of the most frequently used ligands in organometallic chemistry. In addition, the cyclopentadienide anion 1 has always been quoted as a classic example of Hiickel aromaticity, to demonstrate along with benzene and the cydoheptatrienyl cation the validity of the (4n + 2) -electron rule. In contrast, a simple and stable cyclopentadienyl cation of the type 1+ remains to be elusive [5]. With the highly unstable neutral cyclobutadiene and the cydoheptatrienyl anion, 1+ shares the character-... 

Phosphinines are Hiickel aromatic systems even though their properties show pronounced differences to those of the homologous pyridines. Thus, for example, phosphininium salts are not stable (in contrast to pyridinium salts), whereas the formation of phosphininyl anions and radicals as reactive intermediates is known (neither species has been described in the case of pyridine). 

There is, however, an important difference between examples 27 and 41. The later compound forms a Hiickel-aromatic orbital system in 41b while the former compound adopts a Mobius orbital system with 4q + 2 electrons, i.e. 27 is Mobius antiaromatic although six electrons participate in cyclic delocalization (see Section III. B). This is in line with a destabilizing resonance energy of 9.9 kcalmoT (Table 2) calculated with the MM2ERWmethod" "l... 

Note the signs of the coefficients. We can conclude from what was said above that the higher or lower stability of a cyclic polyene as compared to an acyclic one depends on the combination of signs of the coefficients at the ends of the demethylized compound. If the signs are identical, the even AS is aromatic due to cyclic stabilization if the signs are different, the system is anti-aromatic due to cyclic destabilization. Hence, the Hiickel aromaticity... 

Anti-Hiickel aromaticity. The aromaticity rules for anti-Hiickel systems are opposite to those for conventional (Hiickel) systems (Table 9). 

Just as with excited state Hiickel aromatics, Mobius molecules in the excited singlet or triplet state are thought to be aromatic if they contain [4n - - 2] rather than [4n] tt-electrons. 

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