A Visible‐Light‐Mediated Synthesis of Carbazoles

Abstract

The photosynthetic preparation of N‐aryl‐ and N‐alkyl‐bearing carbazoles utilizes continuous flow, visible light, and an in situ formed Cu‐based sensitizer (see picture). The method is mild and efficient, and allows the straightforward synthesis of a variety of carbazoles with different substituents, heterocycles, and complex carbon architectures.

Cu(Xantphos)(dmp)BF4

“…[5]Helicene 3 is a polycyclic aromatic carbocycle, which is typically synthesized by UV light-mediated dehydrogenative photocyclization from stilbene precursor 2. The approach has many disadvantages, such as the need for high dilution conditions and the use of UV lamps and quartz glassware.…”

“…The approach has many disadvantages, such as the need for high dilution conditions and the use of UV lamps and quartz glassware. The formation of [5]helicene 3 using UV light affords only a 25% isolated yield, and the major by-products result from regioisomeric ring closure and overoxidation of 3 (eq 3). 3 The use of a sensitizer and visible light to synthesize [5]helicene has been investigated.…”

“…The formation of [5]helicene 3 using UV light affords only a 25% isolated yield, and the major by-products result from regioisomeric ring closure and overoxidation of 3 (eq 3). 3 The use of a sensitizer and visible light to synthesize [5]helicene has been investigated. A number of commonly used photosensitizers, such as Ru(bpy) 3 (PF 6 ) 2 (6) and Ir(tbbpy)(phpy) 2 PF 6 (7), were also considered.…”

“…When the reaction mixture was injected through fluorinated ethylene propylene tubing at 1 mL min −1 , a 42% yield of [5]helicene was obtained in a 12th of the reaction time compared with batch reaction.…”

“…4 Three stilbenoid precursors were submitted to previously developed photoredox cyclization conditions employed for the synthesis of [5]helicene using batch conditions, and the pyrene-helicene hybrids 4-6 were obtained in 12-27% yield (eq 5). 3 However, by using continuous flow conditions, yields were improved to 41-46% yield, and the reaction time was reduced by seven times.…”