Development of new visible light assisted photocatalytic methods for the syntheses and functionalization of heterocyclic compounds

Abstract

This thesis comprises of the applications of visible light photoredox catalysis synthesis for the synthesis of 6-arylphenanthridines, synthesis of 2-mercaptobenzothiazole, cyclization of alkyl 1,1 -biaryl-2-ones into 10-alkylphenanthren-9-ols in water and the oxidative functionalization of cyclic/acyclic ethers and thioethers into hemiaminals (thio)ethers. This thesis also provides the plausible mechanisms for these transformations. The study provides a brief insight into the theoretical background, principles and the recent progresses in the field of visible light photocatalysis for the synthesis and functionalization of the heterocyclic compounds. The study investigated the visible light photocatalysed synthesis of 6-arylphenanthridines from aryl diazonium salts of N-(2-aminoaryl)benzoimine generated in situ via reactions of N-(2-aminoaryl)benzoimines and tert-butyl nitrite (Scheme 2) using 2 mol% [Ru(bpy)3]Cl2 as photocatalyst and blue LEDs as source of energy at room temprature. The preparation of 2-mercaptobenzthiazoles from 2-azidoarenediazonium tetrafluoroborates and carbon disulfide in the existence of eosin Y photo catalyst and LEDs, cf. Scheme 4. The visible light-mediated and rose bengal photosensitized oxidative radical C-H cyclization of alkyl-1,1 -biaryl-2-ones into 10-alkylphenanthren-9-ol in water (Scheme 6) was analysed. The protocol offers an atom-economical and naturally affable method for the conversion of promptly accessible alkyl-1,1 -biaryl-2-ones to the corresponding 10-alkylphenanthren-9-ols in good yield. It also deals with and#945;-C(sp3)-H amination of ethers and thioethers to hemiaminal (thio)ethers using DDQ as photoredox catalyst and tert-BuONO as co-catalyst, under a 5W blue LED irradiation (Scheme 8). They key factor in this transformation is, the photoexcited catalyst *DDQ which has a very high reduction potential of Ered = 3.18 V vs SCE in the excited state and is capable of two electron oxidation of the substrate.