The synthesis of N-heterocycles is one of the most active areas due to their structures with extensive applications in the fields of organic, pharmaceutical and materials chemistry. It is an increasingly challenging task to develop greener, milder and cheaper N-heterocycle synthetic strategies through C–N/C–C bond formation owing to the requirement of multistep reactions or harsh reaction conditions in many cases. Herein, this review focuses on the synthesis of N-containing heterocycles such as pyrimidines, imidazoles, 1,2,4-triazoles, 1,3,5-triazines, benzimidazoles, isoquinolines, quinazolines, quinazolinones, 1,2,4-oxadiazoles, thiazoles, and thiadiazoles using amidines as readily available starting materials, with an emphasis on the novel green synthetic methodologies via C–N/C–C bond formation, as well as the mechanisms of these reactions. We hope that this review will help as a reference for researchers who are fascinated with the increasing applications in this prolific area.

Thioyl and sulfonyl radicals are usually produced from various thiols and sulfonyl derivatives in high efficiency by single-electron-transfer (SET) oxidation. The generated sulfur (thioyl/sulfonyl) radicals are also highly reactive intermediates having various applications in the construction of organosulfur compounds in the field of organic synthesis. Recently, photoredox-catalyzed C–S/P–S bond formation via the generation of sulfur centered radicals has been studied extensively. In the photoredox catalytic process, a variety of S–H, S–S, S–C, S–N, and S–X (F, Cl, Br, I) bonds, and even active sulfone-containing skeletons can be easily transformed into the corresponding thioyl/sulfonyl radicals. Some of these transformations are achieved by a combination of photoredox catalysts (i.e., TiO₂, Bi₂O₃, eosin Y, fac-[Ir(ppy)₃], [Ru(bpy)₃]²⁺) and other catalysts such as strong bases, Lewis acids, organocatalysts and transition metal catalysts. Compared with previous methods, photoredox catalysis is inexpensive and features the advantages of high efficiency and easy utilization in addition to being environmentally-benign. In this review, we have focused on the research on photoredox-catalyzed C–S/P–S bond formation via the generation of thioyl/sulfonyl radicals and further functionalization in the past few years. We hope to offer chemists the tools to open the door for further progress in organsulfur chemistry.

An efficient and convenient photocatalytic direct C-3 sulfenylation of indoles with thiophenols has been developed for the construction of 3-sulfenylations.