Triarylamine-Substituted Benzimidazoliums as Electron Donor–Acceptor Dyad-Type Photocatalysts for Reductive Organic Transformations

Abstract

Triarylamine-substituted benzimidazoliums (BI + − PhNAr 2 ), new electron donor−acceptor dyad molecules, were synthesized. Their photocatalytic properties for reductive organic transformations were explored using absorption and fluorescence spectroscopy, redox potential determinations, density functional theory calculations, transient absorption spectroscopy, and reduction reactions of selected substrates. The results show that irradiation of BI + −PhNAr 2 promotes photoinduced intramolecular electron transfer to form a longlived (∼300 μs) charge shifted state (BI • −PhN •+ Ar 2 ). In the pathway for photocatalysis of reduction reactions of substrates, BI • −PhN •+ Ar 2 is subsequently transformed to the neutral benzimidazolyl radical (BI • − PhNAr 2 ) by single-electron transfer from the donor 1,3-dimethyl-2phenylbenzimidazoline (BIH−Ph) serving as a cooperative agent. Among the benzimidazoliums explored, the bromo-substituted analogue BI + −PhN(C 6 H 4 Br-p) 2 in conjunction with BIH−Ph demonstrates the most consistent catalytic performance.

Desulfonylative Radical Truce–Smiles Rearrangement Utilizing the Benzimidazoline and Benzimidazolium Redox Couple

“…In a preliminary effort aimed at this goal, we found that 2-bromo-2-methyl- N -phenyl- N -phenylsulfonylpropaneamide undergoes the rearrangement to form 2-methyl-2-phenyl- N -phenyl-propaneamide when our originally developed organophotocatalysts, such as triarylamine-substituted benzimidazoliums (BI + –PhNAr 2 ), along with N , N -diisopropylethylamine (DIPEA) and acetic acid are used (above in Scheme 2 ). 8 In a recent investigation, we also developed processes in which BIH–R are employed to induce debrominative oxygenation reactions of α-bromoketones and -esters under an air atmosphere, at room temperature, and without light irradiation. 13 This observation suggests that the newly developed redox-based activation protocol would also be applicable to Truce–Smiles rearrangement reaction of 2-bromo- N -phenylsulfonylpropaneamides.…”

“…5 Our past studies of thermal as well as photochemical reactions involving BIH–R and BI + –R redox couples 4d k led to the development of a new series of BIH–R photoreductants 6 and BI + –R photocatalysts. 7 8 9…”

“…In the first stage of this study, we explored desulfonylative Truce–Smiles rearrangement reactions of amides 2a – e photocatalyzed by 1f using the conditions recently developed. 8 Unfortunately, except for the reaction of 2a that generates 3a in good yield (67%), the processes proceed in unexpectedly low yields of other 3 (23–35%, Scheme 3 , see details in Table S1 in the Supporting Information).…”

“…In a preliminary effort aimed at this goal, we found that 2-bromo-2-methyl-N-phenyl-Nphenylsulfonylpropaneamide undergoes the rearrangement to form 2-methyl-2-phenyl-N-phenyl-propaneamide when our originally developed organophotocatalysts, such as triarylamine-substituted benzimidazoliums (BI + -PhNAr 2 ), along with N,N-diisopropylethylamine (DIPEA) and acetic acid are used (above in Scheme 2). 8 In a recent investigation, we also developed processes in which BIH-R…”

Primary and Secondary Amines by Flavin‐Photocatalyzed Consecutive Desulfonylation and Dealkylation of Sulfonamides

“…[29] The same protocol was used for arenesulfonamides, but its application is limited by the use of high temperature and UV light. [30] A few reductive photodesulfonylation methods based on visible light photoredox catalysis have also been described; [31][32][33][34][35][36][37][38][39] recently, Budén, [40] Nicewicz, [41] Kang [42] and Hasegawa [43] published photodetosylation using an excited Hantzsch ester anion, an acridinium salt, diphenyldibenzocarbazole or triarylamine-substituted benzimidazolium, respectively (see example in Scheme 1B). However, all these photochemical methodologies suffer from limited substrate scope from the point of view of sulfonyl group structure and are limited to tosyl derivatives.…”