Aromatic stabilisation

The requirements necessary for the occurrence of aromatic stabilisation, and character, in cyclic polyenes appear to be (a) that the molecule should be flat (to allow of cyclic overlap of p orbitals) and (b) that all the bonding orbitals should be completely filled. This latter condition is fulfilled in cyclic systems with 4n + 2n electrons (HuckeVs rule), and the arrangement that occurs by far the most commonly in aromatic compounds is when n = 1, i.e. that with 6n electrons. IO71 electrons (n = 2) are present in naphthalene [12, stabilisation energy, 255 kJ (61 kcal)mol-1], and I4n electrons (n = 3) in anthracene (13) and phenanthrene (14)—stabilisation energies, 352 and 380 kJ (84 and 91 kcal) mol- respectively ... 

The operation of (d) is seen in cyclopentadiene (14) which is found to have a pKa value of 16 compared with 37 for a simple alkene. This is due to the resultant carbanion, the cyclopentadienyl anion (15), being a 6n electron delocalised system, i.e. a 4n + 2 Hiickel system where n = 1 (cf. p. 18). The 6 electrons can be accommodated in three stabilised n molecular orbitals, like benzene, and the anion thus shows quasi-aromatic stabilisation it is stabilised by aromatisation ... 

Radical attack on methylbenzene (toluene, 60) results in preferential hydrogen abstraction by Cl leading to overall substitution in the CH3 group, rather than addition to the nucleus. This reflects the greater stability of the first formed (delocalised) benzyl radical, PhCH2 (61), rather than the hexadienyl radical (62), in which the aromatic stabilisation of the starting material has been lost ... 

In a related study on the cyclopentafused pyrenes [94] in which regular ab initio methods were used (RHF/6-31G and B3LYP/6-31G ), we found that the magnetic properties suggested that the aromatic character decreases upon cyclopentafusion. The aromatic stabilisation energies were unaffected, though. 

In a previous study, it was shown that the aromatic stabilisation energies of the compounds 1-7 are all nearly equal [94] i.e. cyclopentafusion has no effect on the resonance energy. This conclusion is confirmed by the VB calculations. The resonance energy (both Eres and E s) of the compounds 1-7 are all of the same magnitude (Fig. 12 and Table 2). 

However, the isolation of N,N -dimethyl substituted NHC [54] and the first saturated NHC, l,3-dimethyl-lH-imidazolin-2-ylidene by Arduengo et al. in 1995 [95] cast serious doubt as to the validity of the need for steric or aromatic stabilisation in these carbenes. Two independent theoretical studies by Boehme and Frenking [10] and Heinemann et al. [11] in 1996 as well as a later one by Tafipolski et al. [94] gave an excellent account of the stabilising factors and the differences between saturated and unsaturated NHC. 

Even though there is still extensive delocalisation of the positive charge that has been introduced by the electrophile, it is limited to that which would have been available to a linear, non-cyclic system. The extra delocalising possibilities that were available to the benzene system, because it was cyclic, are no longer possible. This means that the cyclohexadienyl cation is significantly less stable than the initial benzene ring, because it has lost the aromatic stabilisation of the cyclic sextet of electrons. 

Ab initio calculations carried out on three 1,3,2-diazaphosphole derivatives, eg., 1, at the MP2/6-31 lG(d,p) level, gave rise to structural and energy data that are interpreted in context of its aromaticity. The 1,3,2-diazaphospholenium ion 2 also has a substantial degree of aromatic stabilisation energy (24.0 kcal mol ) in fact it is comparable to that of pyrrole. Cyclic delocalisation is supported by an analysis of computed charge distribution data, natural bond orbital data, bond... 

Substrate analogues in which the 2 -OH is replaced by chlorine or azide are converted by Type I and II enzymes into an avid Michael acceptor, which gains aromatic stabilisation on addition of nucleophiles at The process... 

These two basic processes, with minor variants, cover the majority of the steps involved in classical heteroaromatic ring synthesis. We shall show below how a sequence of such simple steps leads, via a set of equilibria, to the final product, driven to completion by the formation of an aromatic stabilised system. In a few instances, displacements of halide, or other leaving groups, from saturated carbon are also involved. 

BenzoimidazoT2-ylidenes lose less aromatic stabilisation than imidazoT2-ylidenes when they dimerise, and unhindered examples exist as dimers at ambient temperatures. However, two studies have demonstrated that equilibria can be established at higher temperatures. Dissociation of dimer 30 in... 

The equilibrium for unhindered diaminoearbenes that lack any aromatic stabilisation undoubtedly lies on the side of the dimer. Steric hindrance will not only thermodynamically destabilise the dimer, but is also likely to increase the kinetic barrier to dimerisation, so it is currently uncertain how much steric hindrance is required for to prevent dimerisation. All one can say at present is that bis(diisopropylamino)carbene 6, 1,3-diisopropyl-... 

The aromaticity of porphyrins is also indicated by measurements of their heats of combustion (because of aromatic stabilisation, benzene gives out less heat when it is burnt than if it consisted of three alternating double bonds, as cyclohexatriene). Also, X-ray crystallography of many porphyrins... 

The greatest progress in this area has been made in the reduction of bi-cyclic heterocyclic compounds which are presumably easier to hydrogenate than single aromatic rings as aromatic stabilisation of the heteroaryl ring is lowered in bicyclic systems. For example, a variety of catalytic systems have been developed for the synthesis of quinolines as illustrated in Scheme 14.23. Quinoline 63 can be reduced under either metal-catalysed pressure or transfer hydrogenation conditions, or Bronsted acid... 

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