Nucleophilic addition and substitution reactions between the cyanide ion and cyclopentadienyliron complexes of chlorobenzenes. Syntheses of benzonitriles and phthalonitriles

Abstract

Reaction of cyclopentadienyliron (CpFe) complexes of chlorobenzenes with NaCN in DMF could give rise to products resulting from the addition of the cyanide ion or from substitution and addition reactions with the cyanide ion. Demetallation–oxidation of such products by treatment with DDQ would lead to the synthesis of benzonitriles and phthalonitriles. With the CpFe complex of chlorobenzene, reaction with NaCN in DMF for 3 min, 30 min, or 3 h gave rise to an approximately 90:10 mixture of (1–5-η5-1-chloro-exo-6-cyanocyclohexadienyl)(η5-cyclopentadienyl)iron (9) and (1–5-η5-1,exo-6-dicyanocyclohexa-dienyl)(η5-cyclopentadienyl)iron (10), a 40:60 mixture of 9 and 10 or pure 10, respectively. When the 3-h reaction was worked up in the presence of an added aqueous solution of NH4PF6, only the substitution product, the (η6-benzonitrile)(η5-cyclopentadienyl)iron cation (11), was obtained as its hexafluorophosphate. Apparently, under the acidic conditions of the added aqueous NH4PF6, 10 could readily revert to 11 which then gave rise to its hexafluorophosphate. DDQ treatment of the 3-min or 3-h reaction product gave, respectively, o-chlorobenzonitrile or phthalonitrile. Similarly reaction of CpFe complexes of o-substituted chlorobenzenes could give rise to CpFe-complexed cyclohexadienyl systems with two cyano groups at the 1- and 6-positions and the original o-substituent at the 2-position. DDQ treatment of such products would lead to the formation of 3-substituted phthalonitriles, and in the present work, 3-methyl, 3-methoxy-, and 3-phenoxyphthalonitriles were prepared in this way. In another set of reactions, depending on the experimental conditions employed, the CpFe complex of 2,6-dichlorotoluene (20) could give products from cyanide addition only, the DDQ treatment of which led to the formation of 2,4-dichloro-3-methylbenzonitrile, or products from substitution and addition and subsequent DDQ treatment of which gave rise to 4-chloro-3-methylphthalonitrile. Keywords: η6-chlorobenzene- η5-cyclopentadienyliron cation, complexed chlorobenzenes, nucleophilic addition of cyanide ion, nucleophilic substitution with cyanide ion, synthesis of benzonitriles and phthalonitriles.