Photoredox-Catalyzed Deoxygenative Intramolecular Acylation of

Photoredox-Catalyzed Deoxygenative Intramolecular Acylation of Biarylcarboxylic Acids: Access to Fluorenones. Rehanguli Ruzi†‡, Muliang Zhang‡, ...
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Photoredox-Catalyzed Deoxygenative Intramolecular Acylation of Biarylcarboxylic Acids: Access to Fluorenones Rehanguli Ruzi,†,‡ Muliang Zhang,‡ Keyume Ablajan,*,† and Chengjian Zhu*,‡ †

Key Laboratory of Oil and Gas Fine Chemicals, Ministry of Education and Xinjiang Uygur Autonomous Region, College of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046, P. R. China ‡ State Key Laboratory of Coordination Chemistry, Jiangsu Key Laboratory of Advanced Organic Materials, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, P. R. China S Supporting Information *

ABSTRACT: An efficient deoxygenative radical cyclization reaction has been reported for the synthesis of fluorenones by employing various biarylcarboxylic acids via photoredox catalysis. Attractive features of this process include generation of acyl radical, which quickly underdone intramolecular radical cyclization. This method marks the first photocatalytic intramolecular acyl radical coupling for constructing carbon−carbon bond, which further synthesizes the valuable fluorenone products with mild conditions, good yields, and good functional-group compatibility.

F

developed using this oxidative mode under mild conditions.10 However, there are only a few reports detailing transformations of carboxylic acids by the single-electron reduction process. Recently, the visible-light-mediated tandem acylarylation of olefins and multicomponent 1,2-acylalkylation of alkenes have been reported by the Wallentin group.11 In the meantime, our group also achieved hydroacylation of olefins by employing carboxylic acids and hydrosilanes through photoredox catalysis.12 Searching to develop more transformation applications for carboxylic acids, we decided to further study acyl radical intermediates produced by a SET reductive process. To the best of our knowledge, current applications of visible-lightinduced acyl radical reactions are only limited to intermolecular radical addition onto carbon−carbon multiple bonds, but have not been seen for intramolecular coupling. We wondered if 2aryl acids would produce acyl radical by a SET reductive process, followed by intramolecular addition to the orthoposition aryl C−H component, which could lead to the formation of fluorenones (Scheme 1). Herein we describe a practical and simple process for synthesis of fluorenones from biarylcarboxylic acids via photoredox catalysis. To ensure effective intramolecular radical cyclization, we embarked upon this investigation using 2-phenylbenzoic acid 1a as the model substrate. As a preliminary experiment, 1a was examined in the presence of dimethyl dicarbonate (DMDC) which made the anhydride intermediates obtained, photocatalyst fac-Ir(ppy)3, and we observed the formation of the desired fluorenones in degassed CH3CN (0.1 M) under irradiation visible light from blue LEDs (5 W, λmax = 455 nm) at room temperature (entry 1). As shown in Table 1, different solvents were tested, and strong polar solvent DMF gave better results for this reaction (entries 2−5). Then we

luorenones are an important class of fundamental constituents of organic compounds, which have been extensively used as optoelectronical materials and biologically active molecules.1 More specifically, with the feature of intense blue photo- and electroluminescence and liquid crystalline properties,2 study of fluorenones has advanced as a vivid field of organic chemistry. Consequently, there are several synthetic strategies available for the preparing of fluorenones. Traditionally, the Friedel−Crafts acylation of biarylcarboxylic acids has often been applied to synthesis of this class of compounds.3 However, the treatment with a large excess amount of Brønsted acids is inevitable in these transformations, which also produces large volumes of waste products.3a In this context, some other synthetic methods have been reported for the construction of fluorenone scaffold such as radical cyclization process,4 the oxidation of fluorenes5 or fluorenols,6 and intramolecular dehydro Diels−Alder reactions.7 In addition, transition-metalcatalyzed cross-couplings involving C−H bond activation has been well developed for the synthesis of fluorenones (Scheme 1).8 However, transition-metal catalysis usually requires high temperature and harsh reaction conditions, which are incompatible with many sensitive functional groups. Due to the aforementioned importance of fluorenones, the availability of simple and efficient strategy for the direct preparation of fluorenones from biarylcarboxylic acids will continue to have a significant impact on chemical synthesis. In recent years, the application of visible-light-induced photoredox catalysis has emerged as a novel and efficient tool for activating carboxylic acids.9 In particular, photocatalyzed radical decarboxylative functionalization has garnered much interest as an ideal pathway to generate radicals from carboxylic acids. Generally, radical decarboxylation of carboxylic acids depends on photoinduced oxidation of carboxylates, giving rise to the formation of reactive radical intermediates after CO2 extrusion, and a series of valuable transformations have been © 2017 American Chemical Society

Received: August 31, 2017 Published: September 26, 2017 12834

DOI: 10.1021/acs.joc.7b02197 J. Org. Chem. 2017, 82, 12834−12839

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The Journal of Organic Chemistry Scheme 1. Metal-Catalyzed Intramolecular C−H Acylation of Biarylcarboxylic Acids

Table 1. Optimization of the Reaction Conditionsa

entry

catalyst

base

solvent

yield (%)b

1 2 3 4 5 6 7 8 9 10 11 12 13 14c

fac-Ir(ppy)3 fac-Ir(ppy)3 fac-Ir(ppy)3 fac-Ir(ppy)3 fac-Ir(ppy)3 fac-Ir(ppy)3 fac-Ir(ppy)3 fac-Ir(ppy)3 fac-Ir(ppy)3 fac-Ir(ppy)3 Ru(bpy)3Cl2 Ir(ppy)2(dtbbpy)PF6 − fac-Ir(ppy)3

2,6-lutidine 2,6-lutidine 2,6-lutidine 2,6-lutidine 2,6-lutidine DABCO TMEDA Cs2CO3 DIPEA Na2CO3 2,6-lutidine 2,6-lutidine 2,6-lutidine 2,6-lutidine

CH3CN DMSO DMF DCM DMA DMF DMF DMF DMF DMF DMF DMF DMF DMF

20 40 78 − 68 10 23 − 12 35 − − − −

the ortho position for chloro (2j). We next investigated metasubstituted aromatics in the form of meta-methyl and metachloro substrates. The reaction proceeded in good yield with little regioselectivity between the two isomers 2k and 2l. This outcome is consistent with typical Pschorr cyclizations of metasubstituted arenes, which rarely demonstrate regioselectivity in the free radical aromatic substitution process.13 A more substituted substrate cyclized in a similarly good yield (2m and 2n) to the para-Me or para-OMe analog 2b and 2d. Substituents on the aromatic moiety of carboxylic acids were also tolerated. Reactions for the substrates with electrondeficient fluoro group or electron-rich methoxyl group processed well and afforded the corresponding products in good yields (2p and 2q). To demonstrate the synthetic utility of this intramolecular acylation of biarylcarboxylic acids, a gram-scale reaction was carried out under otherwise standard conditions, and the desired fluorenone product 2a was obtained in 72% yield on a 5.0 mmol scale (Scheme 2). Scheme 2. Gram-Scale Reaction

a

Reaction conditions: 1a (0.1 mmol), base (0.1 mmol, 1.0 equiv), photocatalyst (0.002 mmol, 2.0 mol %), and solvent (2 mL) under argon atmosphere, irradiation with 5 W blue light LEDs at 25 °C for 20 h. bIsolated yields. cThe reaction was performed without light.

screened other bases including organic bases and inorganic bases in detail, and they show lower efficiency except for 2,6lutidine (entries 6−10). The catalysts Ir(ppy)2(dtbbpy)(PF6)2 and Ru(bpy)3Cl2 only delivered traces of product (entries 11− 12), likely because of their high reduction potential compared to the photocatalyst fac-Ir(ppy)3. Finally, control experiments confirmed the requirement of a photocatalyst and a light source for this intramolecular radical cyclization reaction (entries 13− 14). With the optimized reaction conditions obtained, we examined a variety of functionalized biarylcarboxylic acids and were pleased to find them effective for a wide substrate range (Table 2). A series of para-substituted aromatics bearing electron-rich groups (methyl, tert-butyl, methoxyl, benzyloxy, and thiomethyl) all worked in good yields (2b−2f), whereas electron-deficient groups (fluoro, chloro, and trifloromethyl) afforded moderate yields of fluorenones (2g−2i). There was no obvious difference in yield when the subsituent was placed in

For further insight into the intramolecular acylation cyclization process, 2.0 equiv of TEMPO was added as an additive, and 2,2,6,6-tetramethylpiperidin-1-yl [1,1′-biphenyl]2-carboxylate 8 was observed in 34% yield, which supported that the reaction proceeded through a radical pathway (Scheme 3A). The mixed anhydride intermediate 4 could be isolated and delivered the desired product in the presence of a photocatalyst (Scheme 3B). Next, we also carried out a kinetic isotope effect (KIE) experiment to gain more insights into the mechanism. When a 1:1 mixture of 1a and [D5] 1a was subjected to the photoredox-catalyzed deoxygenative intramolecular acylation reaction conditions, the products 2a and [D4] 2a were obtained in a ratio of 1.31:1 by NMR analysis (Scheme 3C). This KIE value of 1.31 indicated that C−H cleavage was not the first irreversible step in this photoredox cycle. Based on the literature reported and our previous work,11,12 a plausible reaction mechanism for intramolecular radical 12835

DOI: 10.1021/acs.joc.7b02197 J. Org. Chem. 2017, 82, 12834−12839

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The Journal of Organic Chemistry Table 2. Scope of Biarylcarboxylic Acidsa

radical cyclization via photoredox catalysis. The protocol presents a mild and energy-efficient system which offers an alternative method for the formation of fluorenones. Attractive features of this process include generation of acyl radical, which effectively undergoes radical cyclization. Further applications of visible-light-induced photoredox-catalyzed transformation involving carboxylic acids are currently underway in our laboratories.

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EXPERIMENTAL SECTION

General Methods. 1H NMR and 13C NMR were recorded using 400 MHz spectrometer at room temperature, with TMS as internal standard. Chemical shifts (δ) are determined in ppm downfield from tetramethylsilane. HRMS data were recorded by a TOF LC/MS. Melting points (mp) were determined with a digital electrothermal apparatus without further correction. All of other commercially available compounds were used without further purification. Thin layer chromatography (TLC) was used to detect the progress of reactions and to separate products. Biarylcarboxylic acid 1a was purchased from Sigma-Aldrich. Other biarylcarboxylic acids 1 were prepared according to the reported procedure.13 General Procedure for the Intramolecular C−H Acylation of Biarylcarboxylic Acid. An oven-dried Schlenk tube (10 mL) was equipped with a magnetic stir bar, 1 (0.1 mmol), and fac-Ir (ppy) 3 (0.02 equiv, 0.002 mmol, 1.3 mg). The flask was evacuated and backfilled with Ar for three times, and 2.0 mL DMF, 2, 6-lutidine (1.0 equiv, 0.1 mmol, 10.7 mg) and DMDC (3 equiv, 0.3 mmol, 40.2 mg) were added with syringe under Ar. The tube was placed at a distance (∼5 cm) from a 5 W blue LED lamp, and the resulting solution was stirred at ambient temperature under visible-light irradiation and monitored by TLC. After the reaction was finished, the mixture was concentrated under vacuum to remove DMF, and the residue was purified by silica gel chromatography (hexane/EtOAc = 20/1−5/1) to afford the product 2. Fluoren-9-one (2a).8a Yellow solid; (14.0 mg, 78%); mp 82−83 °C; 1H NMR (400 MHz, CDCl3): δ 7.62 (d, J = 7.2 Hz, 2H), 7.42− 7.48 (m, 4H), 7.24−7.28 (m, 2H). 13C NMR (100 MHz, CDCl3) δ 193.9, 144.4, 134.7, 134.1, 129.1, 124.3, 120.3. HRMS (APCI) calcd for C13H9O ([M + H]+): 181.0648, found: 181.0649. 2-Methylfluoren-9-one (2b).8a Yellow solid; (16.1 mg, 83%); mp 90−91 °C; 1H NMR (400 MHz, CDCl3) δ 7.59 (d, J = 7.3 Hz, 1H), 7.41−7.43 (m, 3H), 7.34 (d, J = 7.6 Hz, 1H), 7.20−7.24 (m, 2H), 2.34 (s, 3 H). 13C NMR (100 MHz, CDCl3) δ 194.2, 144.6, 141.8, 139.2, 135.1, 134.6, 134.4, 134.2, 128.6, 125.0, 124.2, 120.1, 120.0, 21.4. HRMS (APCI) calcd for C14H11O ([M + H]+): 195.0804, found: 195.0806. 2-(tert-Butyl)-9H-fluoren-9-one (2c).14 Yellow solid; (20 mg, 85%); mp 94−95 °C; 1H NMR (400 MHz, CDCl3) δ 7.71 (d, J = 1.8 Hz, 1H), 7.62 (d, J = 7.4 Hz, 1H), 7.48−7.50 (m, 1H), 7.40−7.45(m, 3H), 7.22−7.25 (m, 1H), 1.34 (s, 9H). 13C NMR (100 MHz, CDCl3) δ 194.4, 152.8, 144.5, 141.8, 134.6, 134.5, 134.2, 131.5, 128.6, 124.2, 121.6, 120.1, 120.0, 35.1, 31.2. HRMS (APCI) calcd for C17H17O ([M + H]+): 237.1274, found: 237.1275. 2-Methoxy-9H-fluoren-9-one (2d).8a Yellow solid; (16.6 mg, 79%); mp 75−76 °C; 1H NMR (400 MHz, CDCl3) δ 7.57 (d, J = 7.3 Hz, 1H), 7.34−7.42 (m, 3H),7.15−7.19 (m, 2H), 6.93−6.96 (m, 1H), 3.83 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 193.9, 160.8, 144.9, 136.9, 135.9, 134.8, 134.3, 127.9, 124.3, 121.3, 120.2, 119.6, 109.4, 55.7. HRMS (APCI) calcd for C14H11O2 ([M + H]+): 211.0754, found: 211.0754. 2-(Benzyloxy)-9H-fluoren-9-one (2e). Yellow solid; (20.6 mg, 72%); mp 135−136 °C; 1H NMR (400 MHz, CDCl3) δ 7.58 (d, J = 7.3 Hz, 1H), 7.33−7.48 (m, 8H), 7.25 (d, J = 4.2 Hz, 1H), 7.15− 7.19 (m, 1H), 7.01−7.04 (m, 1H), 5.08 (s, 2H). 13C NMR (100 MHz, CDCl3) δ 193.8, 159.9, 144.8, 137.2, 136.3, 135.9, 134.9, 134.3, 128.7, 128.2, 127.9, 127.5, 124.3, 121.4, 121.1, 120.3, 119.6, 118.1, 110.4, 70.4. HRMS (APCI) calcd for C20H15O2 ([M + H]+): 287.1067, found: 287.1068.

a Reaction conditions: 1 (0.1 mmol), DMDC (0.3 mmol, 3 equiv), 2,6lutidine (0.1 mmol, 1.0 equiv), fac-Ir(ppy)3 (0.002 mmol, 2.0 mol %), and DMF (2 mL) under argon atmosphere, irradiation with 5 W blue LEDs at 25 °C for 20−24 h. The regioisomeric ratio (r.r.) was determined by isolated yield.

cyclization reaction is proposed, as shown in Scheme 4. The anhydride intermediate 3 is quickly produced from biarylcarboxylic acid 1a and DMDC in the presence of 2,6-lutidine. Under the visible-light irradiation, the photocatalyst facIr(ppy)3 goes through a metal-to-ligand charge-transfer (MLCT) process to generate the fac-*IrIII(ppy)3. Then a SET step from this strongly reducing excited-state species Ir*III to intermediate 4 would generate the radical anion 5 and IrIV. The radical anion 5, after fragmentation, gives acyl radical 6 along with CO2 and methanoate. Acyl radical 6 undergoes intramolecular addition to the ortho-position of aryl C−H component to deliver the intermediate 7. Subsequently, a second one-electron oxidation and proton loss from intermediate 7 are required to give the final product 2a. In summary, we have successfully developed a redox-neutral approach for the synthesis of fluorenones by intramolecular 12836

DOI: 10.1021/acs.joc.7b02197 J. Org. Chem. 2017, 82, 12834−12839

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The Journal of Organic Chemistry Scheme 3. Mechanistic Experiment

Scheme 4. Possible Mechanism

2-Chloro-9H-fluoren-9-one (2h).8a Yellow solid; (12.2 mg, 57%); mp 122−123 °C; 1H NMR (400 MHz, CDCl3) δ 7.66 (d, J = 7.4 Hz, 1H), 7.61 (m, 1H), 7.51−7.52 (m, 2H), 7.45 (d, J = 1.1 Hz, 2H), 7.29−7.33 (m, 1H). 13C NMR (100 MHz, CDCl3) δ 192.5, 143.7, 142.6, 135.7, 135.0, 134.7, 134.1, 133.9, 129.3, 124.7, 124.6, 121.4, 120.4. HRMS (APCI) calcd for C13H8ClO ([M + H]+): 215.0258, found: 215.0258. 2-Trifluoromethylfluoren-9-one (2i).8a Yellow solid; (13.4 mg, 54%); mp 147−148 °C; 1H NMR (400 MHz, CDCl3) δ 7.86 (s, 1H), 7.68−7.75 (m, 2H), 7.52−7.61 (m, 3H), 7.35−7.39 (m, 1H). 13C NMR (100 MHz, CDCl3) δ 192.1, 147.4, 143.0, 135.1, 134.5, 134.3, 131.6 (q, JCF = 4 Hz), 131.3 (q, JCF = 33 Hz), 130.2, 124.7, 123.7 (q, JCF = 271 Hz), 121.2 (q, JCF = 4 Hz), 121.1, 120.4. HRMS (APCI) calcd for C14H8F3O ([M + H]+): 249.0522, found: 249.0524.

2-(Methylthio)-9H-fluoren-9-one (2f). Orange solid; (16.0 mg, 71%); mp 115−116 °C; 1H NMR (400 MHz, CDCl3) δ 7.60 (d, J = 7.3 Hz, 1H), 7.41−7.49 (m, 3H), 7.37 (d, J = 7.8 Hz, 1H), 7.29−7.31 (m, 1H), 7.22−7.24 (m, 1H), 2.51 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 193.6, 144.4, 141.0, 140.4, 134.9, 134.8, 133.8, 132.0, 128.7, 124.4, 121.6, 120.5, 120.1, 15.7. HRMS (APCI) calcd for C14H11OS ([M + H]+): 227.0525, found: 227.0526. 2-Fluoro-9H-fluoren-9-one (2g).8a Yellow solid; (11.5 mg, 58%); mp 114−115 °C; 1H NMR (400 MHz, CDCl3) δ 7.61 (d, J = 7.4 Hz, 1H), 7.41−7.48 (m, 3H), 7.23−7.31 (m, 2H), 7.10−7.15(m, 1H). 13C NMR (100 MHz, CDCl3) δ 192.4, 163.5 (d, JCF = 249 Hz),143.9, 140.1, 136.3, 135.0, 134.3, 128.7, 124.6, 121.6 (d, JCF = 8 Hz,), 120.8 (d, JCF = 23 Hz,), 120.1, 111.9 (d, JCF = 24 Hz). HRMS (APCI) calcd for C13H8FO ([M + H]+): 199.0554, found: 199.0555. 12837

DOI: 10.1021/acs.joc.7b02197 J. Org. Chem. 2017, 82, 12834−12839

Note

The Journal of Organic Chemistry 4-Chloro-9H-fluoren-9-one (2j).8a Yellow solid; (15.0 mg, 70%); mp 183−184 °C; 1H NMR (400 MHz, CDCl3) δ 8.12 (d, J = 7.6 Hz, 1H), 7.68 (d, J = 7.6 Hz, 1H), 7.49−7.57 (m, 2H), 7.40 (d, J = 8.1 Hz, 1H), 7.31−7.35 (m, 1H), 7.19−7.23(m, 1H). 13C NMR (100 MHz, CDCl3) δ 192.5, 143.1, 140.6, 136.4, 136.1, 135.0, 134.0, 130.0, 129.6, 129.4, 124.4, 124.1, 122.5. HRMS (APCI) calcd for C13H8ClO ([M + H]+): 215.0258, found: 215.0259. 3-Methyl-9H-fluoren-9-one (2k).8a Yellow solid; (14.3 mg, 73%); mp 65−66 °C; 1H NMR (400 MHz, CDCl3) δ 7.60 (d, J = 7.3 Hz, 1H), 7.50 (d, J = 7.5 Hz, 1H), 7.40−7.44 (m, 2H), 7.22−7.27(m, 2H), 7.04 (d, J = 7.5 Hz, 1H), 2.38 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 193.6, 145.8, 144.8, 144.3, 134.7, 134.4, 131.8, 129.6, 128.9, 124.3, 124.1, 121.2, 120.1, 22.2. HRMS (APCI) calcd for C14H11O ([M + H]+): 195.0804, found: 195.0807. 1-Methyl-9H-fluoren-9-one (2k′).15 Yellow solid; (2.1 mg, 11%); mp 98−99 °C; 1H NMR (400 MHz, CDCl3) δ 7.63 (d, J = 7.3 Hz, 1H), 7.45−7.52 (m, 2H), 7.28−7.37 (m, 3H), 7.05 (d, J = 6.6 Hz, 1H), 2.63 (s, 3H). 3-Methoxy-9H-fluoren-9-one (2l).16 Yellow solid; (15.3 mg, 72%); mp 99−100 °C; 1H NMR (400 MHz, CDCl3) δ 7.58−7.61(m, 2H), 7.41−7.46 (m, 2H), 7.25−7.30 (m, 1H), 6.99 (d, J = 2.2 Hz, 1H), 6.70−6.73 (m, 1H), 3.89 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 192.5, 165.4, 147.0, 143.3, 135.3, 134.1, 129.3, 127.1, 126.2, 123.8, 120.1, 113.0, 107.1, 55.8. HRMS (APCI) calcd for C14H11O2 ([M + H]+): 211.0754, found: 211.0755. 1-Methoxy-9H-fluoren-9-one (2l′).16 Yellow solid; (2.7 mg, 13%); mp 138−139 °C; 1H NMR (400 MHz, CDCl3) δ 7.58 (d, J = 7.3 Hz, 1H), 7.34−7.44 (m, 3H), 7.19−7.24 (m, 2H), 7.07 (d, J = 7.3 Hz, 1H), 6.77 (d, J = 8.5 Hz, 1H), 3.92 (s, 3H). 1,3-Dimethyl-9H-fluoren-9-one (2m).8a Yellow solid; (18.1 mg, 87%); mp 111−112 °C; 1H NMR (400 MHz, CDCl3) δ 7.60 (d, J = 7.3 Hz, 1H), 7.41−7.46 (m, 2H), 7.24−7.27 (m, 1H), 7.15 (s, 1H), 6.84 (s, 1H), 2.57 (s, 3 H), 2.36 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 194.7, 145.2, 145.2, 143.8, 139.4, 135.0, 134.1, 132.3, 128.8, 123.7, 119.8, 118.9, 22.0, 17.8. HRMS (APCI) calcd for C15H13O ([M + H]+): 209.0961, found: 209.0963. 1,3-Dimethoxy-9H-fluoren-9-one (2n).17 Yellow solid; (20.0 mg, 83%); mp 142−143 °C; 1H NMR (400 MHz, CDCl3) δ 7.91 (d, J = 7.8 Hz, 1H), 7.52−7.56 (m, 1H), 7.37−7.44 (m, 2H), 6.49 (s, 1H), 6.45 (d, J = 4.5 Hz, 1H), 3.79 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 172.9, 160.4, 143.0, 142.9, 131.9, 130.9, 130.4, 129.6, 127.4, 123.6, 119.9, 106.8, 99.6, 55.4. HRMS (APCI) calcd for C15H13O3 ([M + H]+): 241.0859, found: 241.0861. 4-Methylfluoren-9-one (2o).8a Yellow solid; (15.4 mg, 67%); mp 157−158 °C; 1H NMR (400 MHz, CDCl3) δ 8.41 (d, J = 8.0 Hz, 1H), 7.95 (d, J = 7.6 Hz, 1H), 7.80−7.84 (m, 1H), 7.68−7.74 (m, 2H), 7.63−7.65 (m, 1H), 7.52−7.59 (m, 2H), 7.46−7.50 (m, 1H),7.25− 7.29 (m,1H). 13C NMR (100 MHz, CDCl3) δ 194.4, 145.0, 142.8, 138.0, 134.5, 134.4, 131.8, 129.9, 129.6, 128.8, 128.7, 128.2, 127.7, 124.8, 124.0, 123.3, 119.8. HRMS(APCI) calcd for C17H11O ([M + H]+): 231.0804, found: 231.0805. 2-Fluoro-9H-fluoren-9-one (2p).17 Yellow solid; (12.1 mg, 61%); mp 116−117 °C; 1H NMR (400 MHz, CDCl3) δ 7.62 (d, J = 7.4 Hz, 1H), 7.42−7.48 (m, 3H), 7.23−7.31 (m, 2H), 7.10−7.15 (m, 1H). 13C NMR (100 MHz, CDCl3) δ 192.5, 163.5 (d, JCF = 249.0 Hz), 143.9, 140.1, 136.3, 135.0, 134.3, 128.7, 124.6, 121.6 (d, JCF = 7.0 Hz), 120.9 (d, JCF = 23 Hz), 120.1, 111.9 (JCF = 23 Hz). HRMS (APCI) calcd for C13H8FO ([M + H]+): 199.0554, found: 199.0556. 2-Methyl-9H-fluoren-9-one (2q).18 Yellow solid; (16.7 mg, 86%); mp 125−126 °C; 1H NMR (400 MHz, CDCl3) δ 7.60 (d, J = 7.3 Hz, 1H), 7.42 (d, J = 4.0 Hz, 3H), 7.35 (d, J = 7.6 Hz, 1H), 7.19−7.25 (m, 2H), 2.35 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 194.1, 144.6, 141.8, 139.2, 135.1, 134.6, 134.4, 134.3, 128.6, 125.0, 124.2, 120.1, 120.0, 21.4. HRMS (APCI) calcd for C14H11O ([M + H]+): 195.0804, found: 195.0805. Gram-Scale Reaction Procedure of 2a. An oven-dried Schlenk tube (100 mL) was equipped with a magnetic stir bar, 1a (5.0 mmol), and fac-Ir (ppy) 3 (0.02 equiv, 0.1 mmol, 65 mg). The flask was evacuated and backfilled with Ar for three times, and 60 mL DMF, 2, 6-lutidine (1.0 equiv, 5.0 mmol, and 53.5 mg), and DMDC (3 equiv,

15 mmol, 2.01 g) were added with syringe under Ar. The tube was placed at a distance (∼5 cm) from a 24 W blue LED lamp, and the resulting solution was stirred at ambient temperature under visiblelight irradiation and monitored by TLC. After the reaction was finished, the mixture was concentrated under vacuum to remove DMF, and the residue was purified by silica gel chromatography (hexane/ EtOAc = 20/1−5/1) to afford the product 2a (0.65 g, 72%). General Procedure for Radical Trapping Experiment with TEMPO. An oven-dried Schlenk tube (10 mL) was equipped with a magnetic stir bar, 1a (0.1 mmol), fac-Ir (ppy) 3 (0.02 equiv, 0.002 mmol, 1.3 mg), and TEMPO (0.2 mmol). The flask was evacuated and backfilled with Ar for three times, and 2.0 mL DMF, 2,6-lutidine (1.0 equiv, 0.1 mmol, 10.7 mg) and DMDC (3 equiv, 0.3 mmol, 40.2 mg) were added with a syringe under Ar. The tube was placed at a distance (∼5 cm) from a 5 W blue LED lamp, and the resulting solution was stirred at ambient temperature under visible-light irradiation and monitored by TLC. After the reaction was finished, the mixture was concentrated under vacuum to remove DMF, and the residue was purified by silica gel chromatography (hexane/EtOAc = 20/1−5/1) to afford the product 8. 2,6,6-Tetramethylpiperidin-1-yl [1,1′-biphenyl]-2-carboxylate (8). Colorless liquid (11.5 mg, 34%). 1H NMR (400 MHz, CDCl3) δ 7.83 (d, J = 7.7 Hz, 1H), 7.50−7.54 (m, 1H), 7.39−7.44 (m, 1H), 7.29− 7.37 (m, 6H), 1.55−1.66 (m, 4H), 1.47 (d, J = 12.4 Hz, 2H), 1.01 (s, 6H), 0.94 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 168.6, 142.5, 141.4, 131.1, 130.9, 130.8, 129.6, 128.74, 128.1, 127.3, 127.1, 60.2, 39.2, 31.9, 20.5, 16.9. HRMS (APCI) calcd for C22H28NO2 ([M + H]+): 338.2115, found: 338.2116. Intermolecular Kinetic Isotope Effect (KIE). An oven-dried Schlenk tube (10 mL) was equipped with a magnetic stir bar, 1a (0.1 mmol), [D5]1a (0.1 mmol), and fac-Ir (ppy)3 (0.02 equiv, 0.002 mmol, 1.3 mg). The flask was evacuated and backfilled with Ar for three times, and 2.0 mL DMF, 2,6-lutidine (1.0 equiv, 0.1 mmol, 10.7 mg), and DMDC (3 equiv, 0.3 mmol, 40.2 mg) were added with a syringe under Ar. The tube was placed at a distance (∼5 cm) from a 5 W blue LED lamp, and the resulting solution was stirred at ambient temperature under visible-light irradiation for 24 h. After the reaction was finished, the mixture was concentrated under vacuum to remove DMF, and the residue was purified by silica gel chromatography (hexane/EtOAc = 20/1−5/1) to afford the product. This KIE value was determined by NMR analysis (KH/KD = 1.31/1).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.7b02197. 1 H and 13C NMR spectra of all compounds (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Chengjian Zhu: 0000-0003-4465-9408 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the National Natural Science Foundation of China (21474048, 21372114, 21672099, and 21462041).

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REFERENCES

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