Mechanistic Approaches of Photostimulated Intramolecular Arylation of 2′- Halo[1,1′-biphenyl]-2-amines

GUERRA, WALTER; ROBERTO A. ROSSI; BAROLO, SILVIA; ADRIANA B. PIERINI; MARIA EUGENIA BUDÉN

Villa Carlos Paz

Congreso; 13th Latin American Conference on Physical Organic Chemistry May 17-21,2015, Carlos Paz, Argentina; 2015

The photosubstitution reactions between an aromatic and a nucleophile could be achieved by several mechanisms. The classification is based on the key intermediates involve [1]. We have recently reported an efficient strategy to afford a family of 9H-carbazoles 2 by a photostimulated intramolecular N-arylation reaction from biphenyl amines 1 (eq 1) [2]. Besides the Radical Nucleophilic Substitution Unimolecular (SRN1), there are other mechanism that could explain the formation of the product like the SN2Ar*, SN(ET)Ar* or radical ? radical collapse [1]. Our proposal is to investigate the mechanism of these photosubstitution reactions supplying information from the study of experimental reaction conditions, photophysical properties (UV-vis and steady state fluorescence) and computational data (B3LYP DFT functional and 6-311+G* basis set. Some initially results reveal that the reaction of 1 (R=H) in DMSO with KOtBu (2 equiv) in 45 min. afforded 2 (R=H) in 26% yield, together with 13% yield of the reduced product 3 (R=H). There was no reaction under dark conditions which excludes a benzyne mechanism. The addition of 25 mol% of mdinitrobenzene (m-DNB), a well-know electron acceptor, caused 46% of inhibition (14% yield of 2). The same effect was observed when 50 mol% of m-DNB was added (85% of inhibition, 4% yield of 2). The reduced product follows the same tendency. Herein, the inhibition was not proportional to the amount of m-DNB used. These evidence shown as that there was a chain process and product 2 could be form from a SRN1 mechanism. Moreover, the reduced product could be formed by hydrogen abstraction from the solvent (supported by computational data). The effect of the base was evaluated by carrying out the photostimulated reaction in the absence of KOtBu. Under these conditions traces of 2 were observed. Also, NaH was used in place of ?OtBu, and not cyclization product 2 was found. This indicates that ?OtBu was responsible to initiate the ET reactions. In general, biphenylamines with an electrondonating group like Me or OMe behave in the same way than R=H giving both, cyclized 2 and reduced 3 products, meanwhile biphenylamines containing electron-withdrawing groups like CN, COOEt, or CF3 gave only the carbazole 2. These differences in regiochemistry could suggest that different mechanism could be involved. A complete mechanistic picture of this cyclization reaction will be discussed.