Computational study of reactions of 2'-halo-[1,1'-biphenyl]-2-amines: synthesis of carbazoles by photostimulated intramolecular C?N coupling?

W. D. GUERRA; S. M. BAROLO; A. B. PIERINI; R. A. ROSSI.

Ciudad de Piedra

Congreso; 10th Congress of the World Association of Theoretical and Computational Chemists; 2014

The Unimolecular Radical Nucleophilic Substitution (SRN1) is a chain process that involves radicals and radical anions as intermediates. An interesting feature of the reaction is the possibility to obtain heterocycles by intramolecular ring closure of compounds bearing both the leaving group and the nucleophilic center in adequate position [1].  Following this leading idea, the possibility to obtained carbazoles by the photoinitiated reaction of 2´-halo-2-amines-biphenyls (1) was studied (eq. 1).  Experimentally, the reaction of 2´-chloro-[1,1´-biphenyl]-2-amine (1a), afforded the cyclized product 2a (9H-carbazole, 57% yield) and the reduced product 3a (13% yield). However, 2´-chloro-N-phenyl-[1,1´-biphenyl]-2-amine  (1b) gave only the product 2b (9-phenyl-9H-carbazole) in excellent yield (93%). The experimental findings showed in the previous equation were interpreted by density functional theory (DFT) calculations performed with the B3LYP functional and the 6-311+G* basis set; the solvent was simulated with the PCM continuum model. The conformations of anions 1a- (Fig 1) and 1b- were evaluated. In both cases de only conformer found is responsible to give, after DET (dissociative electron transfer), the distonic radical anion 1a0?- or 1b0?-. These intermediates afforded the conjugated radical anion 2a?- or 2b?- respectively, by C-N cyclization. The activation energy (Eacyclization) calculated for the radical-anion coupling was 4.55 (1a0?-) and 3.35 (1b0?-) kcal/mol. Also, both 1a0?- and 1b0?- could give, after internal rotations (Earotation), the distonic radical anions 1a180?- and 1b180?-.