Benzene ring, complexes, electronic

All aluminum complexes were investigated as catalysts for the ROP of l-LA and rac-LA (Table 6.11). These aluminum complexes showed moderate to high activities (81.6-93.0% conversion) with the cocatalysts 2-propanol at 70°C. It is worth noting that the activities of these complexes decreased with the increase in substituent size on the benzene rings, while electron-withdrawing substituents raising the polymerization rate [66]. 

The complex contains 72 atoms with 244 valence electrons distributed in 226 valence atomic orbitals. In order to reduce the computational effort, and to assess the contribution of the ligand 7r-orbitals to the overall spectrum, we examined a "reduced" model, see Figure 2, in which the benzene rings of the ligands are replaced by -HC=CH- groups. This model compound consists of... 

They are thus all electron-withdrawing with respect to the benzene ring, i.e. aromatic species containing them all have a dipole with the positive end located on the benzene nucleus. Taking Y = NR3 as exemplar of the rest, we can write the a complexes for attack by an electrophile, E (e.g. N02), o-, m- and p- to the original NR3 substituent ... 

Sapse et al.41b performed ab initio calculations on benzene-benzene as a model for the interaction between the a phenylalanine residue and the benzoyl portion of a typical folate compound. In addition, that study examined the complex formed hy a benzene ring and a polar serine which acts as a hydrogen donor, between a benzene ring and the amino group of a meta diamino-imidazole group (present in some anti-folates) and between serine as a hydrogen donor and the diamino-imidazole group as the electron donor. 

The competing effects of the electron-releasing ability of the non-complexing benzene ring against the electron-withdrawing character of the substituents (acetyl, nitrile) are illustrated in Fig. 9. 

Fig. 4. Hybridization changes of w and o electrons at different stages of dissociative chemisorption B.R., plane of benzene ring it complex adsorption a bond.

The dehydrodimerization reaction involving aromatic radical-cations is fast only when electron donating substituents are present in the benzene ring. These substituents stabilise the a-intermediate. Benzene, naphthalene and anthracene radical-cations form a a-sandwich complex with the substrate but lack the ability to stabilise the a-intermediate so that radical-cation substrate reactions are not observed. The energy level diagram of Scheme 6.4 illustrates the influence of electron donating substituents in stabilising the Wheland type a-intermediate. 

In aromatic systems, the Lewis acids which activate via coordination are also capable of activating the aromatic system by the formation of a and ir complexes. There are a sufficient number of examples available to indicate that the activation via the latter processes is the more important of these, where all are present. Olivier (52) showed in 1913 that the kinetic behavior of such reactions consists of two portions. When the catalyst, say aluminum chloride, is present in less than the amount required to complex all the functional groups, the reaction is relatively slow and the catalytic activity is due to the small amount of Lewis acid resulting from the dissociation of the complex. As soon as all the functional groups are coordinated, any additional Lewis acid is found to accelerate the rate enormously. In these electrophilic substitutions it seems highly probable that the the activation involves the pi electron system of the benzene ring. Olivier studied the reaction sequence ... 

The reach of cyclic voltammetry is vast. It has been applied to the investigation of simple electron-transfer reactions those with two successive electron transfers (so-called EE reactions) and with multiple electron transfers (EEE) involving electron transfer to and from compounds, say, with several benzene rings. The technique has been applied to complex sequences in which an electron transfer is followed by a chemical reaction step, and then by another electron transfer (ECE reactions), etc. The complexity of some of the reaction sequences investigated by cyclic voltammetry lends itself well to calculations that need computers the classic work of Feldburg in this direction (digital simulation) has been already mentioned (Section 7.5.19.2). 

Annelation on to a benzene ring increases considerably the complexity of the spectra, and indole has absorptions at 216 (4.54), 266 sh (3.76), 270 (3.77), 276 (3.76), 278 (3.76) and 287 (3.68) nm in ethanol solution. Because of the widespread occurrence of the indole ring system in nature and the sensitivity of absorption band position and intensity to substitution type, considerable use has been made of electronic spectroscopy in the past for structure identification. An extensive tabulation of data, primarily for monosubstituted derivatives, is available (71PMH(3)67,p.94). As expected, whereas the effects of alkylation are comparatively slight, introduction of groups capable of mesomeric interaction with the indole it -system may cause profound changes in the appearance of the spectrum representative examples are given in Table 24. 

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