RICHTER Aromatic Carboxylation

REIMER TIEMANN Phenol formylation 314 REISSERT - GROSHEINTZ FISCHER Cyanoamme reaction 315 REPPE Acetytane reaction 316 von RICHTER Aromatic carboxylation 317 von RICHTER - WIDMAN - STOERMER Cinnolme synthesis 318 RILEY Selenaim dioxide oxidation 319 Rimmi 4... 

When aromatic nitro compounds are treated with cyanide ion, the nitro group is displaced and a carboxyl group enters with cine substitution (p. 854), always ortho to the displaced group, never meta or para. The scope of this reaction, called the von Richter rearrangement, is variable. As with other nucleophilic aromatic substitutions, the reaction gives best results when electron-withdrawing groups are in ortho and para positions, but yields are low, usually < 20% and never > 50%. 

Vitamin C is another enol-based redox system like the hydroquinones and tyrosine, but it has no aromatic character. The enediol component is stabilized by a conjugated lactone group. The oxidation (Ej = +58 mV) occurs in two one-electron steps, but the ascorbate radical is not as stable as the semiquinone radical (Bielski, and Richter, 1977). The anion radical disproportionates via an initial dimerization in a similar way as semiquinone radicals (Bielski et al., 1981 Sawyer, et al., 1982). The lifetime of the radical is in the order of microseconds. The acidity of ascorbic acid (pk = 4.2) stems from the OH group in the P position to the lactone carbonyl group. It corresponds to the OH group of a vinylogous carboxylic acid (Scheme 7.2.11). Its UV maximum occurs at 260 nm (e = 1 x 1(T). 

The von Richter reaction is the nucleophilic aromatic substitution of a nitroarene (1) with potassium cyanide to give the cme-substituted benzoic acid (2). The reaction is characterized by the observed regiochemistry of the product where the carboxyl group occupies a position ortho to the nitro group that is lost. 

The von Richter reaction warrants special attention, since recent studies have added substantially to our knowledge of its mechanism. This is a reaction between an aromatic nitro compound and alkaline potassium cyanide which results in the loss of the nitro group and the attachment of a carboxyl group in a position ortho to the position previously occupied by the nitro group. Some typical examples are... 

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