Labilization of C—N Linkages by Dibenzocyclohepten-5-yl Ring System

The hydrolysis of several 5-aminodibenzo[a,d]cycloheptenes is discussed. The relative reactivity of the various compounds was closely related to the amino substituents at position 5 of the tricyclic nucleus. Both secondary and tertiary amines were prepared. They showed instability in aqueous solution below the pKa with invariant velocity constants at pH 0.15–7.0. This is indicative of spontaneous decomposition of the molecule via a carbonium ion or through the kinetically equivalent solvent attack on C5 of the dibenzocycloheptene ring system. The fully aromatic ring system leads to a two magnitude enhancement of the velocity constants as compared to those of the corresponding 10,11-dihydrodibenzo[a,d]cycloheptenes. The cycloheptene nucleus is unstable when the adjacent nitrogen is cationic. When positively charged, the compounds undergo decomposition by nucleophilic attack of hydroxylic oxygen either on a carbonium-ion-like substrate or the protonated parent compound. The rate law may be written as kobs = k0 [H+]/([H+] + Ka). These results indicate cleavage of the C—N bond to be SN1 in character. Degradation was primarily to the bis-ether by way of the 5-hydroxy intermediate, with evidence of traces of the ketone. Kinetic analyses were by the acid–dye procedure, utilizing methyl orange as the anionic component. This method is specific for the reactions involved and yields first-order plots showing linearity through several half-lives.