Aromatic systems, dimerization

Pawliszyn J, Szczeskiak MM, Schemer S. Interactions between aromatic systems-dimers of benzene and S-triazine. J Phys Chem 1984 88 1726-1730. 

Pawliszyn, J., Szczgsniak, M.M., and Scheiner, S. (1984) Interaction between aromatic systems dimers of benzene and s-tetrazine. J. Phys. Chem., 88, 1726-1730. 

The chapter opens with a number of general comments (I, II) and then moves to the section on nitroso compounds (II, B). These latter groups can lead to fairly complex spectra because isomerization (enolization) to oximes can occur (II, B4), and in the case of tertiary and aromatic systems dimerization is possible (II, B5). Nitrosoamines also tend to dimerize to give spectra that are consistent with the above assignments (II, D). Finally, there are a detailed discussions of nitro ( NO2 (III, A, B)) and nitrate (O NOa (III, C)) groups. Here the stretching frequencies are enormously intense and are often the most intense bands in the spectrum. 

The existence of a germanium-carbon pjr-pjr double bond in the intermediate complex is likely. The intermediate was not isolated as such, but as its dimer. Compounds containing a carbon-germanium double bond were prepared by Satg6 and coworkers59, like the fluorenylidenedimethylgermanium, where stabilization arises from change transfer in the aromatic system. 

Helical columns of bifunctional ureidotriazines have also been created in water.40 In this solvent the aromatic cores of compound 39 stack and create a hydrophobic environment that favors the formation of intermolecular hydrogen bonds. The chiral side chains can express their chirality within the columnar polymer because of the helicity generated by the backbone. In contrast, for monofunctional 68 water interferes with the hydrogen bonding and 68 does not stack to form a column. As a consequence the chiral side chain does not express its chirality in the aromatic system. For 39, the bifunctional nature allows for a high local concentration of stacking units. A comparison might be made here to the individual DNA bases that also do not dimerize and stack in water, unless they are connected to a polymer backbone. 

Solutions of ytterbium in liquid ammonia are capable of reducing aromatic systems to 1,4-dihydroaromatics or alkynes to trans alkenes [293]. The a,fi-unsaturated cholest-4-en-3-one was reduced to eholestanone in 80% yield by threefold excess of ytterbium in ammonia followed by oxidation with Jones reagent (Scheme 16). In the absence of a proton source, (HOEt) and THF as co-solvent, the pinacol dimer was obtained as the major product. 

Lewis and Petisce [44] have investigated PET reactions between a number of cyano aromatic electron acceptors and electron donating methyl aromatic systems. Botb substitution as well as dimer products have been observed depending on the electron affinity of the acceptor [44,45]. When weak electron acceptors, e.g. m-dicyanobenzene and benzonitrile, were used dibenzyl derivatives were formed predominantly. In contrast, strong electron acceptors produced predominantly substitution products. For example, use of tetracyanobenzene with p-xylene produced predominantly in-cage substitution product while use of m-dicyanoben-... 

Electron transfer from alkali metals to aromatics is very easy in suitable solvents [182]. The radical anions produced in this way do not dimerize. The formation of a covalent C—C bond would be accompanied by the loss of resonance stability in the aromatic system. Paul et. al. [188] have shown that the unpaired electron is placed in the lowest unoccupied orbital of the molecule, and that the stability of the radical anion increases in the order diphenyl < naphthalene < phenanthrene < anthracene. 

Nucleophilic attack was only observed at the carbon in the perhydrothiophenium ring adjacent to the sulfur atom. No sulfur-phenyl bond cleavage occurred consistent with a strong jr-bond between the sulfonium sulfur and the aromatic system. The dimeric zwit-terion formed by the initiation step can react at either end with further monomer or other mers of any DP. In fact, the lability of perhydrothiophenium rings on the ends of macrozwitterions would be expected to be greater because of the loss of the adjacent phenoxide anion. 

Carbon-carbon double bonds conjugated to multiple bonds other than aromatic systems may also be reduced with metals. Again the nature of the reduction product is dependent on the availability of a proton donor in the reaction medium. In the absence of an excess of proton donors, dimerization of the initially formed anion radical is observed. Both the reduction of 1,3-dienes and trapping experi-... 

As seen in Figure 5, addition of a butyl iodide-phenyl butyl nitrone mixture to a coal anion solution of Illinois No. 6 coal does indeed lead to the trapping of butyl radicals. Such radicals, once formed, may alkylate the aromatic substrate, abstract hydrogen from the system, dimerize, or disproportionate (16, 17, 18, 30). In the context of the alkylation step of... 

D. Reductive dimerization of positively charged aromatic systems References... 

Kinetic and mechanistic studies on reductive couplings have accumulated in the last decades, and particularly electrohydrodimerizations and dimerization of aromatic systems have been much studied. The level of the mechanistic discussions in this chapter reflects the somewhat uneven level of knowledge accumulated for the different types of coupling reactions. In some cases where little mechanistic work has been done, the mechanistic rationalizations presented are based on evaluations made by the present authors. No attempts have been made to bring reduction potentials on a common scale since differences in solvent, supporting electrolytes, added acids, electrode material, etc. may lead to considerable differences in the measured potentials. This is particularly so when it comes to values of reduction peak potentials measured under conditions where the electrogenerated intermediate is consumed in a fast follow-up reaction. In some cases, however, the relative values may be of interest in a mechanistic discussion. Unless stated otherwise the cited potentials have been measured versus SCE. 

D. Reductive Dimerization of Positiveiy Charged Aromatic Systems... 

One-electron reduction of positively charged systems leads to neutral radicals, and independently of the structure the most common follow-up reaction is fast dimerization of two neutral radicals. In contrast to neutral radicals formed by cleavage, those formed by reduction of positively charged aromatic systems are not reduced at the potential where they are formed, and formation of 2-F products therefore only competes via H-atom abstraction. For JV-heterocyclic systems in aqueous acidic media, protonation of the radical and further reduction leading to the dihydro-monomeric system may compete with dimerization. 

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