Aromatic ring equivalents

FIGURE 20.25 Acetylenic groups as aromatic ring equivalents. 

The reaction giving A is chloromethylation, a reliable metliod of adding a CH2OH equivalent to an aromatic ring. You may have been surprised at the use of reagent B to make an acid chloride. B is oxalyl chloride and is often used when pure acid chlorides are wanted - the other products are gases (which ). 

Such stmctures are stabilized by having the imine bond conjugated between two aromatic rings and the possibiUty of tautomerism between two equivalent stmctures ... 

In order for a substitution to occur, a n-complex must be formed. The term a-complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the hydrogen that is displaced. As the term implies, a a bond is formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a-complex is a four-7t-electron delocalized system that is electronically equivalent to a pentadienyl cation (Fig. 10.1). There is no longer cyclic conjugation. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive chargex)f the cation is located at the positions ortho and para to the site of substitution. 

Benzotriazole can exist in two tautomeric forms, l-//-benzotriazole (6.46, R = H) and 2-/f-benzotriazole. If the aromatic ring contains a substituent, the 1- and 3-nitrogen atoms of the triazole are not equivalent, and therefore a 3-//-benzotri-azole derivative can also exist. The equilibrium between the 1 -H and 2-H tautomers of benzotriazoles is strongly on the side of the 1 -H tautomer, in contrast to triazole where the 2-H tautomer is dominant. Tomas et al. (1989) compared experimental data (enthalpies of solution, vaporization, sublimation, and solvation in water, methanol, and dimethylsulfoxide) with the results of ab initio theoretical calculations at the 6-31G level. 

In systems of fused six-membered aromatic rings, the principal canonical forms are usually not all equivalent. Structure 36 has a central double bond and is thus... 

Friedel-Crafts disconnection (38a) Is unambiguous because of the synunetry of (39). Further disconnection requires FGA. A carbonyl group next to the aromatic ring gives a 1,4-dicarbonyl compound (40) and allows disconnection of an acyl anion equivalent to give an enone (41). This can be made by Mannich reaction from (42). 

Radicals in which the odd electron is on a nitrogen next to an aromatic ring are stabilized by resonance analogous to that of tri-phenylmethyl. In the case of Wurster s salts, the nitrogen analogs of semiquinones, there are two equivalent resonance structures in the acid form, but in the less stable basic form one of the structures requires separation of charge. Evidence for the unpaired electron has been obtained by measurement of the paramagnetism.144... 

In most paramagnetic sandwich complexes the proton hf structure is not resolved in low temperature powder or single crystal EPR spectra. In vanadium dibenzene, a typical example of this type of compound, the poor resolution is due to the fact that the aromatic rings are rigidly frozen at T < 50 K and thus the proton hfs tensors of the benzene rings are no longer magnetically equivalent. 

The cross-coupling alternative yields halide salts as co-products. During the preparation of aryl halides from the aromatic ring and dihalogen also one equivalent of halide salt is produced and thus there is further room for improvement if we could directly substitute a hydrogen atom by the hetero atoms mentioned ... 

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