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Visible-Light-Promoted C−H Arylation by Merging Palladium Catalysis with Organic Photoredox Catalysis Jian Jiang, Wen-Man Zhang, Jian-Jun Dai, Jun Xu, and Hua-Jian Xu* School of Biological and Medical Engineering, Hefei University of Technology, Hefei 230009, China S Supporting Information *

ABSTRACT: The use of a dual palladium/organic photoredox catalytic system enables the directed arylation of arenes with aryldiazonium salts with a broad substrate scope at room temperature under mild reaction conditions. This study thus serves as not only an alternative route for the biaryl motifs but also a new example for the application of an organic photoredox catalyst.

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INTRODUCTION In recent years, visible-light-promoted single-electron transfer (SET) by photocatalysts has been recognized as a key step in organic synthesis.1 In this context, photocatalyzed C−H arylation reactions using aryl diazonium salts have received much attention and have made great progress.2 For instance, Sanford and co-workers in 2011 described the ligand-directed C−H arylation reactions by merging palladium catalysis with the Ru-based visible light photoredox catalysis (Figure 1a).2a Thereafter, König et al. developed direct C−H arylations of heteroarenes using eosin Y as a photoredox catalyst (Figure 1b).2b More recently, Xiao and Rueping have reported C−H arylations of N-heteroarenes using [Ru(bpy)3]Cl2·6H2O and TiO2 as the photoredox catalysts, respectively (Figure 1c).2c,d On the other hand, Fukuzumi et al. have demonstrated that

9,10-dihydroacridine (AcrH2) derivatives could act as an efficient photocatalyst in dehalogenation reactions.3 Inspired by these elegant studies and recently reported dual transition metal (including Pd,4 Au,5 Ni6)/photoredox catalysis, we envisioned that AcrH2 could serve as a photoredox catalyst for further elaboration with a palladium catalyst thus enabling the C−H arylation under mild conditions (Figure 1d). This study not only provides an alternative route for the formation of the biaryl motifs but also further highlights the use of AcrH2 for synthetic applications under photoredox conditions.

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RESULTS AND DISSCUSION We started our investigations with acetanilide 1a as the model substrate. Extensive studies were carried out to optimize the reaction conditions (Table 1). It was found that the C−H arylation product 3a was obtained in 86% (79% isolated) yield with 15 mol % of Pd(OAc)2, 10 mol % of 9,10-dihydro-10ethylacridine (AcrH2-A), and 3 equiv of phenyldiazonium tetrafluoroborate salt 2a in MeOH at room temperature (rt) under visible-light irradiation from blue LEDs (λmax = 425 ± 15 nm) for 12 h (Table 1, entry 1). Although the yields of 3a were inferior, the reactions could also be caused by other AcrH2 derivatives (Table 1, entries 2−6). We also tested some solvents including DCE, DCM, DMF, and CH3CN, but they were not effective (Table 1, entries 7−10). By employing different light sources, blue LED irradiation was found to promote the reaction most efficiently (Table 1, entries 11−13). As expected, the control experiments clearly showed that Pd(OAc)2, the AcrH2 catalyst, and light are essential for this reaction, as no or trace amount of the arylation product was observed in the absence of the palladium catalyst, AcrH2, or blue light irradiation (entries 14−16). Use of 5 mol %, rather than 15 mol %, Pd(OAc)2 resulted in a lower yield of the C−H arylation product (entry 17). Reducing the reaction time to 6 h only led to a 45% yield of 3a (entry 17). As shown in entries 19

Figure 1. C−H arylation reaction using aryl diazonium salts by photoredox catalysis. © 2017 American Chemical Society

Received: January 19, 2017 Published: March 17, 2017 3622

DOI: 10.1021/acs.joc.7b00140 J. Org. Chem. 2017, 82, 3622−3630

Article

The Journal of Organic Chemistry Table 1. Optimization of the Reaction Conditionsa

entry

variation from the standard conditions

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

none AcrH2-B instead of A AcrH2-C instead of A AcrH2-D instead of A AcrH2-E instead of A AcrH2-F instead of A DCE instead of MeOH DCM instead of MeOH DMF instead of MeOH CH3CN instead of MeOH 36 W Green LED 23 W White LED UV no blue light no Pd(OAc)2 no AcrH2-A with 5 mol % Pd(OAc)2 6 h instead of 12 h with 2 equiv of 2a with 1 equiv of 2a

86 (79) 59 71 70 41 68 38 29 trace 0 9 24 23 trace 0