Some aspects of the homolytic Pschorr reaction

Abstract

In furtherance of the study of the mechanism of the Pschorr and related reactions the decompositions of diazotised derivatives of the following 2-aminotriarylmethanols were carried out under three reaction conditions: 2-aminotriphenylmethanol, 2-amino-4'-methyltriphenylmethanol, 2-amino-4'-chlorotriphenylmethanol, 2-ami no-4'-methoxy-triphenylmethanols, 2-amino-4'-chloro-4"-methyltriphenylmethanol, 2-amino-3',5'-dichloro-triphenylmethanol and 2-amino-3',5'-dimethyltriphenylmethanol.

The reactions of 2-aminotriary lmethanols, diazotised using amyl nitrite in benzene solution, resulted in products which resembled those of previously reported homolytic arylations. Accordingly, a homolytic mechanism was postulated for reactions under the present conditions. This was the first known application of these reaction conditions to effect intramolecular arylation, previous reports having been of intermolecular arylations only. The cyclisation reactions were accompanied by yields of phenols which were relatively and unexpectedly high in view of the small amount of water expected to be formed in these reactions.

Decompositions were also carried out in aqueous acidic conditions. Where no catalysts were added, the distribution of attack by the reactive intermediates (formed on dediazoniation) upon the various substrates in the reaction mixtures, was almost statistical. Thus 9-phenylfluoren-9-ols substituted in each aromatic system were obtained in almost equal amounts; fluorene-substituted products resulted from cyclisation onto substituted phenyl rings of the 2-aminotriarylmethanols and 9-arylfluorenols from ring closure onto unsubstituted rings. Phenols were obtained in yields slightly lower than those of cyclisation products, possibly as a result of steric effects which did not allow ready reaction between the formed reactive intermediates and water.

The results of copper-catalysed decompositions in aqueous acidic media were explained by postulating the formation of intermediate diazonium salt-copper complexes. Such complexes are expected to react further by expelling nitrogen with consequent formation of cyclic products and phenols.