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Photocatalytic Synthesis of 6 Phosphorylated Phenanthridines from 2 Isocyanobiphenyls via Radical C−P and C−C Bond Formation

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15 May 2023

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15 May 2023

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Abstract
A mild, efficient, and photocatalytic synthesis of 6-phosphorylated phenanthridines via tandem radical addition/cyclization/aromatization of 2-isocyanobiphenyls with diarylphosphine oxides is reported. The method features operational simplicity, metal-free conditions, using low-cost rose Bengal as catalyst and sustainable air as terminal oxidant at room temperature, and providing the desired products in moderate to good yields.
Keywords: 
Subject: Chemistry and Materials Science  -   Organic Chemistry

1. Introduction

Phenanthridine is a significant fused N-heterocycle, which is ubiquitous in many bioactive alkaloids, pharmaceuticals, natural products, and functional material molecules.1-3 For example, Nitidine chloride (Figure 1) is an anti-cancer active natural product with phenanthridine motif that has been found to inhibit topoisomerase I and topoisomerase II.4 NK109 (Figure 1) has been found to exhibit anti-tumor effects against a number of human cancer cell lines.5 Fagaronine (Figure 1) is a antileukaemic and antimalarial active alkaloid.6 In addition, trispheridine,7 N-methylcrinasiadine,8 and lycobetaine9 are natural alkaloids with excellent bioactivities (Figure 1).
Extensive study and improvement on the synthesis of highly functionalized phenanthridine derivatives have been done due to their important biological activities. Great progress and developments have been made in the synthetic strategies of phenanthridine derivatives via C-H bond activation, radical addition cyclization, photochemical catalysis, isocyanide chemistry, etc.10-12 The transformation proceeded in the absence or presence of various transition-metal catalysts, such as Pd, Mn, Cu, Ru, Ir, Rh, Fe, Ni and so on. The functionalization mainly focused on the 6-position of phenanthridine, including trifluoromethylation, difluoromethylation, arylation, alkylation, alkynylation, acylation, sulfonylation, phosphorylation, difluoromethylphosphonation, benzylation, trichooromethylation, thiolation, etc.
C-P bond construction has shown dramatic attention due to the potential applications of organophosphorous compounds in pharmaceutical chemistry and materials science.13,14 In 2014, several methods for the synthesis of 6-phosphorylated phenanthridines initiated by Ag(I) or Mn(III) salts were reported via radical cascade cyclization [Scheme 1, eq(a-e)].15-19 In 2016, Lakhdar and co-workers described the 6-phosphorylation of phenanthridines in the presence of diphenyliodonium salt and triethylamine under metal-free conditions [Scheme 1, eq(f)].20 Subsequently, several photocatalytic methods were reported in the presence of photocatalyst [Ir(ppy)2(dtbpy)]PF621/2D-COF-122/4CzIPN-tBu23 using K2S2O8 or TBHP as oxidant [Scheme 1, eq(g-i)].
Recently, we have described the synthesis of 6-aroyl24 and 6-benzylated25 phenanthridines via iron-catalyzed cascade radical addition/cyclization of 2-biphenyl isocyanides. In 2019, we reported a visible-light-induced Mn(acac)3-catalyzed method for the synthesis of 6-β-keto alkyl phenanthridines in the absence of extend oxidant.26 In addition, we also described a Mn(III)-catalyzed radical process of 2-isocyanobiphenyls for the synthesis of 6-phosphorylated phenanthridines.16 Herein, we report another alternative method for the preparation of 6-phosphorylated phenanthridines using low-cost Rose Bengal as catalyst and sustainable air as terminal oxidant under metal-free conditions at room temperature.

2. Results and Discussion

Initially, 2-isocyano-5-methyl-1,1'-biphenyl 1a and diphenylphosphine oxide 2a were chosen as model reaction substrates for optimization conditions. The desired product 3aa was isolated in 15% yield using Rose Bengal (2 mol%) as catalyst and DBU as base in co-solvent MeCN/H2O at room temperature under air atmosphere (Table 1, entry 1). Increasing the loading of catalyst to 5-10 mol% could improve the yields of 3aa (Table 1, entries 2 and 3). An increase in the diphenylphosphine oxide 2a amount from 1.5 equiv to 3.5-4.5 equiv, the results shown that the yields of target product 3aa raised obviously (Table 1, entries 4 and 5). Then, we screened several organic photocatalysts, and the results shown that Eosin Y could promote the reaction smoothly and provide 3aa in 61% yield, while only trace amount of desired product 3aa was observed when Fluorescein and Rhodamine B were used as photocatalysts (Table 1, entries 6-8). In addition, a series of bases, involving Et3N, DABCO, Na2CO3, and K2CO3 were surveyed, but no better yield was obtained (Table 1, entries 9-12). Then we attempted to change the amount of base, and the best result (84%) was obtained in the presence of 3.0 equiv of DBU (Table 1, entries 13-15). The examination of reaction medium shown that the effect of solvent is obvious. The reaction couldn’t offer better yield in pure acetonitrile or other co-solvents, such as MeOH/H2O, EtOH/H2O, and THF/H2O (Table 1, entries 16-19). The control experiment indicated that the irradiation of blue LED lights is important because no reaction was carried out in the dark (Table 1, entry 20).
We next examined the generality and the substrate scope of 2-isocyanobiphenyls under the optimized reaction conditions (Scheme 2). Initially, the electronic effect of substituents R2 was investigated. The reaction could tolerate both electron-donating groups and electron-withdrawing groups, such as methyl, methoxy, halogen (F, Cl), and trifluoromethyl on the ortho-, meta-, or para-position of the phenyl ring, providing the corresponding products in moderate to good yields. For instance, ortho- or meta-methyl substituted substrates 1b and 1c gave the target products 3ba and 3ca in 74% and 59% yields, respectively. Para-trifluoromethyl substituted substrate 1i provided the product 3ia in 53% yield under standard conditions. Unfortunately, when substrate 1n bearing nitro group was used, only trace amount of desired product was observed under the same conditions. Then, 2-(2-isocyanophenyl)naphthalene was applied to the reaction with 2a, the product 3pa was obtained in low yield (38%). The electronic effect of substituent R1 was significant, and the substrates with electron-withdrawing groups halogen (F, Cl) provided the corresponding products in 3ra-3ta moderate yields 37-46%. The reaction didn’t work when substate 1u bears a strong electron-withdrawing group (-CF3).
In addition, the substrate scope of diphenylphosphine oxides 2 was also examined under the optimal conditions (Scheme 3). The results shown that the electronic effect wasn’t obviously, diaryl substituted phosphine oxides 2 with both electron-donating groups and electron-withdrawing groups were proceeded smoothly, providing the corresponding phosphorylation phenanthridines in good yields. Product 3ad was isolated only in moderate yield (46%) due to the steric effect. However, no target product was observed in the case of diisopropyl phosphine oxide 2i.
A control experiment was executed to investigate the possible reaction mechanism (Scheme 4). The reaction was totally inhibited in the presence of radical scavenger of 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 3.5 equiv) under standard reaction conditions. The results indicated that the transformation probably undergo cascade radical addition/cyclization processes.

3. Materials and Methods

3.1. General Information

Nuclear magnetic resonance (NMR) spectra are recorded in parts per million from internal tetramethylsilane on the δ scale. 1H and 13C NMR spectra were recorded on a Bruker AV-400 spectrometer operating at 400 MHz and 100 MHz, respectively. All chemical shift values are quoted in ppm and coupling constants quoted in Hz. High resolution mass spectrometry (HRMS) spectra were obtained on a micrOTOF II instrument.

3.2. General Procedure for the photocatalytic synthesis of 6-phosphorylated phenanthridines from 2-isocyanobiphenyls

To a 25 mL quartz test tube containing a magnetic stir bar, was added 2-biphenyl isocyanides 1 (0.2 mmol), Rose Bengal (0.01 mmol, 5 mol%), DBU (3.0 equiv) under air, added diphenylphosphine oxides 2 (0.6 mmol, 3.0 equiv) and MeCN/H2O (1.0/0.18 mL). The resulting mixture was stirred at room temperature under 30 W blue LEDs irradiation for 5 h. After completion, monitored by TLC, evaporation of the solvent under reduced pressure followed purification by silica gel chromatography using ethyl acetate – petroleum ether (1:3) as eluent to provide the desired products 3.

3.3. Characterization Data for Products 3

(2-Methylphenanthridin-6-yl)diphenylphosphine oxide (3aa): Isolated (Rf = 0.6, EtOAc – petroleum ether = 1:3) as a white solid (66.1 mg, 84% yield), mp: 222 – 223 oC. 1H NMR (400 MHz, CDCl3) δ 9.48 (d, J = 8.4 Hz, 1H), 8.57 (d, J = 8.4 Hz, 1H), 8.31 (s, 1H), 8.05 – 7.84 (m, 5H), 7.76 (t, J = 7.6 Hz, 1H), 7.63 (t, J = 7.6 Hz, 1H), 7.52 – 7.37 (m, 7H), 2.57 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 155.6 (d, JC-P = 128.7 Hz), 141.3 (d, JC-P = 23.3 Hz), 139.1, 133.1 (d, JC-P = 104.0 Hz), 132.4, 132.3 (d, JC-P = 9.1 Hz), 131.6 (d, JC-P = 2.5 Hz), 130.9, 130.8, 130.5, 128.5, 128.2 (d, JC-P = 12.1 Hz), 127.9, 127.7, 124.2 (d, JC-P = 2.4 Hz), 122.1, 121.7, 22.2. 31P NMR (162 MHz, CDCl3) δ 28.2. HRMS (ESI): m/z [M + H]+ calcd for C26H21NOP: 394.1355, found: 394.1358.
(2-Methylphenanthridin-6-yl)di-p-tolylphosphine oxide (3ab): Isolated (Rf = 0.4, EtOAc – petroleum ether = 1:3) as a white solid (64.1 mg, 76% yield), mp: 249 – 251 oC. 1H NMR (400 MHz, CDCl3) δ 9.45 (d, J = 8.0 Hz, 1H), 8.63 (d, J = 8.4 Hz, 1H), 8.37 (s, 1H), 7.95 (d, J = 8.4 Hz, 1H), 7.87 – 7.73 (m, 5H), 7.65 (t, J = 7.6 Hz, 1H), 7.52 (dd, J = 8.4, 0.8 Hz, 1H), 7.26 – 7.21 (m, 4H), 2.63 (s, 3H), 2.37 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 156.0 (d, JC-P = 128.4 Hz), 141.9 (d, JC-P = 2.7 Hz), 141.2 (d, JC-P = 23.4 Hz), 138.9, 132.3 (d, JC-P = 9.5 Hz), 130.9, 130.4, 130.0 (d, JC-P = 106.4 Hz), 128.9 (d, JC-P = 12.5 Hz), 128.6, 127.9 (d, JC-P = 23.0 Hz), 127.7, 124.2, 122.0, 121.6, 22.2, 21.6. 31P NMR (162 MHz, CDCl3) δ 28.8. HRMS (ESI): m/z [M + H]+ calcd for C28H25NOP: 422.1668, found: 422.1670.
(2-Methylphenanthridin-6-yl)di-m-tolylphosphine oxide (3ac): Isolated (Rf = 0.4, EtOAc – petroleum ether = 1:3) as a white solid (56.5 mg, 67% yield), mp: 237 – 239 oC. 1H NMR (400 MHz, CDCl3) δ 9.38 (d, J = 8.4 Hz, 1H), 8.50 (d, J = 8.4 Hz, 1H), 8.24 (s, 1H), 7.85 (d, J = 8.4 Hz, 1H), 7.76 – 7.64 (m, 3H), 7.62 – 7.51 (m, 3H), 7.41 (d, J = 8.3 Hz, 1H), 7.23 – 7.17 (m, 4H), 2.50 (s, 3H), 2.25 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 155.8 (d, JC-P = 128.1 Hz), 141.3 (d, JC-P = 23.2 Hz), 139.0, 138.0 (d, JC-P = 11.9 Hz), 133.0 (d, JC-P = 104.4 Hz), 132.6 (d, JC-P = 8.9 Hz), 132.4 (d, JC-P = 2.7 Hz), 132.3 (d, JC-P = 6.8 Hz), 130.9, 130.7, 130.4, 129.5 (d, JC-P = 9.3 Hz), 128.6, 128.1, 128.0 (d, JC-P = 12.8 Hz), 127.7, 124.2 (d, JC-P = 2.4 Hz), 122.1, 121.7, 22.2, 21.5. 31P NMR (162 MHz, CDCl3) δ 28.6. HRMS (ESI): m/z [M + H]+ calcd for C28H25NOP: 422.1668, found: 422.1671.
(2-Methylphenanthridin-6-yl)di-o-tolylphosphine oxide (3ad): Isolated (Rf = 0.4, EtOAc – petroleum ether = 1:3) as a white solid (38.8 mg, 46% yield), mp: 228 – 230 oC. 1H NMR (400 MHz, CDCl3) δ 9.11 (d, J = 8.4 Hz, 1H), 8.58 (d, J = 8.4 Hz, 1H), 8.31 (s, 1H), 7.73 (d, J = 8.0 Hz, 2H), 7.54 (t, J = 7.6 Hz, 1H), 7.42 (d, J = 8.0 Hz, 1H), 7.36 – 7.26 (m, 4H), 7.24 – 7.18 (m, 2H), 7.07 (t, J = 7.2 Hz, 2H), 2.55 (s, 3H), 2.36 (s, 6H). 13C NMR (100 MHz, CDCl3) δ 156.1 (d, JC-P = 128.3 Hz), 143.4 (d, JC-P = 7.8 Hz), 141.3 (d, JC-P = 23.5 Hz), 139.1, 133.1 (d, JC-P = 11.9 Hz), 132.4 (d, JC-P = 6.5 Hz), 131.8 (d, JC-P = 2.5 Hz), 131.6, 131.5, 131.1, 130.7, 130.5, 130.4, 128.8, 127.7, 127.6 (d, JC-P = 23.0 Hz), 125.3 (d, JC-P = 12.8 Hz), 124.3, 122.2, 121.6, 22.2, 22.0, 22.0. 31P NMR (162 MHz, CDCl3) δ 38.2. HRMS (ESI): m/z [M + H]+ calcd for C28H25NOP: 422.1668, found: 422.1673.
Bis(3,5-dimethylphenyl)(2-methylphenanthridin-6-yl)phosphine oxide (3ae): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:3) as a white solid (54.8 mg, 61% yield). mp: 265 – 268 oC. 1H NMR (400 MHz, CDCl3) δ 9.46 (d, J = 8.4 Hz, 1H), 8.62 (d, J = 8.4 Hz, 1H), 8.36 (s, 1H), 7.97 (d, J = 8.4 Hz, 1H), 7.80 (t, J = 7.4 Hz, 1H), 7.65 (t, J = 7.4 Hz, 1H), 7.55-7.50 (m, 5H), 7.12 (s, 2H), 2.62 (s, 3H), 2.30 (s, 12H). 13C NMR (100 MHz, CDCl3) δ 156.0 (d, JC-P = 127.6 Hz), 141.2 (d, JC-P = 23.2 Hz), 138.9, 137.7 (d, JC-P = 12.6 Hz), 133.4 (d, JC-P = 2.8 Hz), 132.9 (d, JC-P = 104.5 Hz), 132.3 (d, JC-P = 6.8 Hz), 131.0, 130.5 (d, JC-P = 30.6 Hz), 129.9 (d, JC-P = 9.2 Hz), 128.2 (d, JC-P = 100.6 Hz), 128.0 (d, JC-P = 23.0 Hz), 124.2 (d, JC-P = 2.4 Hz), 122.0, 121.6, 22.2, 21.4. 31P NMR (162 MHz, CDCl3) δ 29.0. HRMS (ESI): m/z [M + H]+ calcd for C30H29NOP: 450.1981, found: 450.1986.
Bis(3-methoxyphenyl)(2-methylphenanthridin-6-yl)phosphine oxide (3af): Isolated (Rf = 0.6, EtOAc – petroleum ether = 1:3) as a yellow solid (60.7 mg, 67% yield), mp: 241 – 243 oC. 1H NMR (400 MHz, CDCl3) δ 9.33 (d, J = 8.0 Hz, 1H), 8.51 (d, J = 8.2 Hz, 1H), 8.25 (s, 1H), 7.85 (d, J = 8.4 Hz, 1H), 7.69 (t, J = 7.6 Hz, 1H), 7.55 (t, J = 7.6 Hz, 1H), 7.49 – 7.40 (m, 3H), 7.36 (d, J = 7.6 Hz, 1H), 7.33 (d, J = 8.0 Hz, 1H), 7.27 – 7.18 (m, 2H), 6.93 (d, J = 7.6 Hz, 2H), 3.68 (s, 6H), 2.51 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 159.3 (d, JC-P = 15.0 Hz), 155.4 (d, JC-P = 129.3 Hz), 141.2 (d, JC-P = 23.5 Hz), 139.1, 134.2 (d, JC-P = 103.4 Hz), 132.3 (d, JC-P = 6.8 Hz), 130.9, 130.8, 130.5, 129.3 (d, JC-P = 14.3 Hz), 128.4, 127.9 (d, JC-P = 23.5 Hz), 127.7, 124.7 (d, JC-P = 9.2 Hz), 124.3, 122.1, 121.7, 118.0 (d, JC-P = 2.5 Hz), 117.0 (d, JC-P = 10.1 Hz), 55.4, 22.2. 31P NMR (162 MHz, CDCl3) δ 28.2. HRMS (ESI): m/z [M + H]+ calcd for C28H25NO3P: 454.1567, found: 454.1568.
Bis(3-fluorophenyl)(2-methylphenanthridin-6-yl)phosphine oxide (3ag): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:3) as a yellow solid (58.4 mg, 68% yield), mp: 257 – 259 oC. 1H NMR (400 MHz, CDCl3) δ 9.44 (d, J = 8.4 Hz, 1H), 8.67 (d, J = 8.4 Hz, 1H), 8.40 (s, 1H), 7.98 (d, J = 8.4 Hz, 1H), 7.86 (t, J = 7.6 Hz, 1H), 7.77 – 7.62 (m, 5H), 7.58 (d, J = 8.0 Hz, 1H), 7.48 – 7.40 (m, 2H), 7.22 (td, J = 8.4, 2.0 Hz, 2H), 2.65 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 161.4 (dd, JC-F = 247.9, JC-P = 17.0 Hz Hz), 160.0, 153.4 (d, JC-P = 131.6 Hz), 140.1 (d, JC-F = 23.9 Hz), 138.5, 134.4 (dd, JC-P = 103.5, JC-F = 5.6 Hz Hz), 131.4 (d, JC-P = 7.1 Hz), 130.0, 129.8, 129.7, 129.2 (d, JC-F = 7.3 Hz), 129.1 (d, JC-F = 7.3 Hz), 128.3, 128.1, 127.1, 127.0 (d, JC-P = 3.0 Hz), 126.9, 126.8, 123.3, 121.2, 120.7, 118.3 (d, JC-F = 9.9 Hz), 118.1, 118.0 (d, JC-F = 9.1 Hz), 117. 9 (d, JC-P = 2.6 Hz), 21.2. 31P NMR (162 MHz, CDCl3) δ 24.9. HRMS (ESI): m/z [M + H]+ calcd for C26H19F2NOP: 430.1167, found: 430.1169.
(2-Methylphenanthridin-6-yl)di(naphthalen-1-yl)phosphine oxide (3ah): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:3) as a yellow solid (66.1 mg, 67% yield), mp: 293 – 296°C. 1H NMR (400 MHz, CDCl3) δ 9.41 (dd, J = 8.2, 2.0 Hz, 1H), 8.49-8.44 (m, 3H), 8.20 (s, 1H), 7.88 – 7.79 (m, 3H), 7.75-7.72 (m, 4H), 7.69 (d, J = 8.0 Hz, 2H), 7.64 (t, J = 7.6 Hz, 1H), 7.51 (t, J = 7.6 Hz, 1H), 7.41 – 7.31 (m, 5H), 2.45 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 155.7 (d, JC-P = 129.5 Hz), 141.3 (d, JC-P = 23.4 Hz), 139.2, 134.7 (d, JC-P = 2.3 Hz), 134.0 (d, JC-P = 8.9 Hz), 132.5 (d, JC-P = 7.3 Hz), 132.4 (d, JC-P = 6.9 Hz), 130.9, 130.8, 130.5, 130.4 (d, JC-P = 103.8 Hz), 129.1, 128.5, 128.1, 127.9, 127.8 (2C), 127.7, 127.6, 126.7, 124.3, 122.2, 121.7, 22.2. 31P NMR (162 MHz, CDCl3) δ 28.9. HRMS (ESI): m/z [M + H]+ calcd for C34H25NOP: 494.1668, found: 494.1672.
(2,10-Dimethylphenanthridin-6-yl)diphenylphosphine oxide (3ba): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:3) as a white solid (60.3 mg, 74% yield), mp: 222 – 225 oC. 1H NMR (400 MHz, CDCl3) δ 9.32 (d, J = 8.0 Hz, 1H), 8.52 (s, 1H), 7.89 – 7.73 (m, 5H), 7.55 (d, J = 7.2 Hz, 1H), 7.46 (t, J = 8.0 Hz, 1H), 7.43 – 7.37 (m, 3H), 7.37-7.30 (m, 4H), 3.01 (s, 3H), 2.52 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 156. 1 (d, JC-P = 129.0 Hz), 142.3 (d, JC-P = 23.8 Hz), 138.0, 135.3, 135.0, 133.2 (d, JC-P = 104.3 Hz), 132.3 (d, JC-P = 9.1 Hz), 131.9 (d, JC-P = 6.7 Hz), 131.6 (d, JC-P = 2.5 Hz), 131.2, 129.5, 129.3 (d, JC-P = 23.4 Hz), 128.1 (d, JC-P = 12.1 Hz), 127.2, 127.0, 126.4, 125.7 (d, JC-P= 2.4 Hz), 27.1, 22.5. 31P NMR (162 MHz, CDCl3) δ 29.4. HRMS (ESI): m/z [M + H]+ calcd for C27H23NOP: 408.1512, found: 408.1515.
(2,8-Dimethylphenanthridin-6-yl)diphenylphosphine oxide (3da): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:3) as a white solid (48.1 mg, 64% yield), mp: 231 – 233 oC. 1H NMR (400 MHz, CDCl3) δ 9.28 (s, 1H), 8.48 (dd, J = 8.4, 1.2 Hz, 1H), 8.29 (s, 1H), 7.97 – 7.87 (m, 5H), 7.61 (dd, J = 8.4, 1.2 Hz, 1H), 7.53 – 7.36 (m, 7H), 2.58 (s, 3H), 2.53 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 155.0 (d, JC-P = 129.0 Hz), 140.9 (d, JC-P = 23.4 Hz), 139.0, 137.9, 133.6 (d, JC-P = 104.0 Hz), 132.7, 132.4 (d, JC-P = 9.1 Hz), 131.5 (d, JC-P = 2.6 Hz), 130.8, 130.2 (d, JC-P = 6.9 Hz), 130.0, 128.4, 128.1 (d, JC-P = 12.0 Hz), 127.6, 124.3 (d, JC-P = 2.5 Hz), 122.0, 121.5, 22.2, 21.9. 31P NMR (162 MHz, CDCl3) δ 28.0. HRMS (ESI): m/z [M + H]+ calcd for C27H23NOP: 408.1512, found: 408.1516.
Diphenyl(2,7,9-trimethylphenanthridin-6-yl)phosphine oxide (3ea): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:3) as a white solid (62.3 mg, 74% yield), mp: 255 – 257 oC. 1H NMR (400 MHz, CDCl3) δ 8.21 (s, 2H), 7.72 – 7.65 (m, 4H), 7.51 (d, J = 8.0 Hz, 1H), 7.40-7.35 (m, 2H), 7.34 – 7.27 (m, 5H), 7.17 (s, 1H), 2.84 (s, 3H), 2.48 (s, 3H), 2.43 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 155.4 (d, JC-P = 129.9 Hz), 140.5, 139.8 (d, JC-P = 24.1 Hz), 138.9, 137.4, 135.1 (d, JC-P = 106.9 Hz), 134.2 (d, JC-P = 6.7 Hz), 133.3, 132.0 (d, JC-P = 8.9 Hz), 131.1 (d, JC-P = 2.5 Hz), 130.3, 130.2, 128.0 (d, JC-P = 12.1 Hz), 125.7 (d, JC-P = 23.7 Hz), 124.1 (d, JC-P = 2.6 Hz), 121.8, 120.0, 25.0, 22.2, 21.9. 31P NMR (162 MHz, CDCl3) δ 36.2. HRMS (ESI): m/z [M + H]+ calcd for C28H25NOP: 422.1668, found: 422.1672.
(2-Methyl-[1,3]dioxolo[4,5-j]phenanthridin-6-yl)diphenylphosphine oxide (3fa): Isolated (Rf = 0.4, EtOAc – petroleum ether = 1:1) as a white solid (53.3 mg, 63% yield), mp: 262 – 265 oC. 1H NMR (400 MHz, CDCl3) δ 8.98 (s, 1H), 8.11 (s, 1H), 7.98 – 7.84 (m, 6H), 7.54 – 7.38 (m, 7H), 6.08 (s, 2H), 2.58 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 153.4 (d, JC-P = 131.3 Hz), 151.1, 148.2, 141.0 (d, JC-P = 23.2 Hz), 138.5, 133.8, 132.2 (d, JC-P = 103.9 Hz), 132.3 (d, JC-P = 9.1 Hz), 131.6, 130.7 130.5 (d, JC-P = 7.4 Hz), 129.9, 128.1 (d, JC-P = 12.0 Hz), 125.2 (d, JC-P = 23.4 Hz), 124.4, 121.4, 105.6, 102.0, 99.8, 22.1. 31P NMR (162 MHz, CDCl3) δ 28.1. HRMS (ESI): m/z [M + H]+ calcd for C27H21NO3P: 4038.1254, found: 438.1257.
(9-Chloro-2-methylphenanthridin-6-yl)diphenylphosphine oxide (3ga): Isolated (Rf = 0.4, EtOAc – petroleum ether = 1:3) as a white solid (48.8 mg, 57% yield), mp: 211 – 214 oC. 1H NMR (400 MHz, CDCl3) δ 9.50 (d, J = 8.8 Hz, 1H), 8.57 (s, 1H), 8.27 (s, 1H), 7.97 – 7.86 (m, 5H), 7.60 (dd, J = 8.8, 1.6 Hz, 1H), 7.58 – 7.48 (m, 3H), 7.47-7.41 (m, 4H), 2.62 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 155.3 (d, JC-P = 130.1 Hz), 141.5 (d, JC-P = 22.9 Hz), 139.5, 137.4, 133.8 (d, JC-P = 6.7 Hz), 132.8 (d, JC-P = 104.4 Hz), 132.3 (d, JC-P = 9.2 Hz), 131.8, 131.2, 130.9, 130.2, 128.4, 128.2 (d, JC-P = 12.0 Hz), 126.3 (d, JC-P = 23.4 Hz), 123.2, 121.7 (d, JC-P = 12.1 Hz), 21.2. 31P NMR (162 MHz, CDCl3) δ 27.9. HRMS (ESI): m/z [M + H]+ calcd for C26H20ClNOP: 428.0966, found: 428.0972.
(8-Chloro-2-methylphenanthridin-6-yl)diphenylphosphine oxide (3ha): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:3) as a white solid (56.5 mg, 66% yield), mp: 229 – 231 oC. 1H NMR (400 MHz, CDCl3) δ 9.62 (d, J = 2.0 Hz, 1H), 8.46 (dd, J = 8.8, 1.2 Hz, 1H), 8.23 (s, 1H), 8.06 – 7.90 (m, 5H), 7.69 (dd, J = 8.8, 2.0 Hz, 1H), 7.57 – 7.47 (m, 3H), 7.46 – 7.39 (m, 4H), 2.58 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 154. 5 (d, JC-P = 128.6 Hz), 141.1 (d, JC-P = 22.7 Hz), 139.7, 133.8, 132.9 (d, JC-P = 104.6 Hz), 132. 3 (d, JC-P = 9.1 Hz), 131.8 (d, JC-P = 2.6 Hz), 131.5, 130.9, 130.8, 130.7 (d, JC-P = 6.8 Hz), 128.8 (d, JC-P = 23.1Hz), 128.2 (d, JC-P = 12.1 Hz), 127.5, 123.7, 123.6 (d, JC-P = 2.4 Hz), 121.5, 22.2. 31P NMR (162 MHz, CDCl3) δ 27.2. HRMS (ESI): m/z [M + H]+ calcd for C26H20ClNOP: 428.0966, found: 428.0969.
(2-Methyl-8-(trifluoromethyl)phenanthridin-6-yl)diphenylphosphine oxide (3ia): Isolated (Rf = 0.6, EtOAc – petroleum ether = 1:3) as a yellow solid (48.9 mg, 53% yield), mp: 199 – 201 oC.1H NMR (400 MHz, CDCl3) δ 10.00 (s, 1H), 8.70 (d, J = 8.8 Hz, 1H), 8.35 (s, 1H), 8.05 – 7.95 (m, 6H), 7.60 (d, J = 8.4 Hz, 1H), 7.55 – 7.48 (m, 2H), 7.47 – 7.41 (m, 4H), 2.63 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 155.8 (d, JC-P = 127.7 Hz), 141.8 (d, JC-P = 12.5 Hz), 139.9, 134.4 (d, JC-P = 6.2 Hz), 132.8 (d, JC-P = 105.4 Hz), 132.3 (d, JC-P = 9.1 Hz), 131.8 (d, JC-P = 2.6 Hz), 131.7, 131.0, 129.4 (q, JC-F = 32.6 Hz), 128.3 (d, JC-P = 12.1 Hz), 127.3 (d, JC-P = 22.9 Hz), 126.6 (q, 1JC-F= 3.1 Hz), 126.2 (d, JC-P = 4.3 Hz), 123.9 (q, JC-F = 270.8 Hz), 123.2 (d, JC-P = 2.2 Hz), 123.1, 122.0, 22.2. 31P NMR (162 MHz, CDCl3) δ 26.8. HRMS (ESI): m/z [M + H]+ calcd for C27H20F3NOP: 462.1229, found: 462.1232.
(10-Fluoro-2-methylphenanthridin-6-yl)diphenylphosphine oxide (3ja): Isolated (Rf = 0.3, EtOAc – petroleum ether = 1:3) as a white solid (55.1 mg, 67% yield), mp: 192 – 194 oC. 1H NMR (400 MHz, CDCl3) δ 9.35 (d, J = 8.2 Hz, 1H), 8.78 (s, 1H), 7.96 – 7.86 (m, 5H), 7.65 – 7.58 (m, 1H), 7.57 – 7.48 (m, 4H), 7.47 – 7.42 (m, 4H), 2.61 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 160.5 (d, 1JC-F = 253.5 Hz), 155.0 (d, JC-P = 129.1 Hz), 141.5 (d, JC-P = 23.0 Hz), 139.8, 132.8 (d, JC-P = 104.8 Hz), 132.3 (d, JC-P = 9.1 Hz), 131.7 (d, JC-P = 2.5 Hz), 130.7 (d, JC-F = 17.7 Hz), 129. 9 (d, JC-F = 24.3), 128.2 (d, JC-P = 12.1 Hz), 128.1 (d, JC-P = 8.9 Hz), 126.6 (d, 2JC-F = 23.0 Hz), 124.6 (d, JC-P = 4.0 Hz), 121.8 (m), 117.3 (d, 2JC-F = 23.2 Hz), 22.4. 31P NMR (162 MHz, CDCl3) δ 28.6. HRMS (ESI): m/z [M + H]+ calcd for C26H20FNOP: 412.1261, found: 412.1267.
(9-Fluoro-2-methylphenanthridin-6-yl)diphenylphosphine oxide (3ka): Isolated (Rf = 0.4, EtOAc – petroleum ether = 1:3) as a white solid (32.1 mg, 39% yield), mp: 205 – 208 oC. 1H NMR (400 MHz, CDCl3) δ 9.60 (dd, J = 9.2, 6.0 Hz, 1H), 8.23 (s, 1H), 8.21 (d, J = 9.2 Hz, 1H), 7.98 – 7.89 (m, 5H), 7.57 – 7.49 (m, 3H), 7.47 – 7.36 (m, 5H), 2.62 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 163.8 (d, 1JC-F = 251.6 Hz), 155.1 (d, 2JC-P = 128.9 Hz), 141.3 (d, 2JC-F= 23.2 Hz), 139.2, 135.0 (d, JC-P = 9.3 Hz), 134.9 (d, JC-P = 9.3 Hz), 132.9 (d, JC-P = 104.4 Hz), 132.4, 132.3, 131.7 (d, JC-P = 2.7 Hz), 131.6, 131.2, 130.9, 128.2 (d, JC-P = 12.1 Hz), 125.1 (d, 2JC-P = 24.3 Hz), 123.8, 121.8, 116.9 (d, JC-F = 23.3 Hz), 107.2 (d, JC-F = 22.2 Hz), 22.1.31P NMR (162 MHz, CDCl3) δ 27.9. HRMS (ESI): m/z [M + H]+ calcd for C26H20FNOP: 412.1261, found: 412.1265.
(8-Fluoro-2-methylphenanthridin-6-yl)diphenylphosphine oxide (3la): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:3) as a yellow solid (50.2 mg, 61% yield), mp: 216 – 219 oC. 1H NMR (400 MHz, CDCl3) δ 9.29 (dd, J = 10.2, 2.6 Hz, 1H), 8.61 – 8.54 (m, 1H), 8.27 (s, 1H), 7.98 – 7.90 (m, 5H), 7.56 – 7.47 (m, 4H), 7.46 – 7.40 (m, 4H), 2.60 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 161.2 (d, 1JC-F = 247.4 Hz), 154.6 (d, JC-P = 129.2 Hz), 140.9 (d, 2JC-F = 23.5 Hz), 132.8 (d, JC-P = 104.3 Hz), 132.3 (d, JC-P = 9.1 Hz), 131.7 (d, JC-P = 2.5 Hz), 130.9, 130.4, 129.3 (d, 3JC-F = 9.3 Hz), 129.0 (d, 3JC-F = 8.8 Hz), 128.2 (d, JC-P = 12.1 Hz), 124.6 (d, JC-P = 8.5 Hz), 123.9, 121.4 120.3 (d, JC-F = 24.2 Hz), 113.1 (d, 2JC-F= 23.1 Hz), 22.2. 31P NMR (162 MHz, CDCl3) δ 27.4. HRMS (ESI): m/z [M + H]+ calcd for C26H20FNOP: 412.1261, found: 412.1263.
(7,9-Difluoro-2-methylphenanthridin-6-yl)diphenylphosphine oxide (3ma): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:1) as a white solid (42.9 mg, 50% yield), mp: 245 – 247 oC. 1H NMR (400 MHz, CDCl3) δ 8.20 (s, 1H), 8.08 (d, J = 9.6 Hz, 1H), 7.80 – 7.70 (m, 5H), 7.56-7.51 (m, 3H), 7.48 – 7.43 (m, 4H), 7.04 (t, J = 9.6 Hz, 1H), 2.62 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 163.6 (d, 1JC-F= 252.5 Hz), 160.2 (d, 1JC-F = 246.6 Hz), 141.2, 140.1, 136.1, 133.1 (d, JC-P = 109.2 Hz), 131.9, 131.8, 131.4, 131.3, 130.9, 128.8, 128.0 (d, JC-P = 12.2 Hz), 122.5, 121.9, 104.3 (d, JC-F = 27.2 Hz), 103.9, 22.2. 31P NMR (162 MHz, CDCl3) δ 34.8 (d, J = 4.1 Hz). HRMS (ESI): m/z [M + H]+ calcd for C26H19F2NOP: 430.1167, found: 430.1171.
(3-Methylphenanthridin-6-yl)diphenylphosphine oxide (3oa): Isolated (Rf = 0.4, EtOAc – petroleum ether = 1:3) as a white solid (45.6 mg, 58% yield), mp: 197 – 200 oC. 1H NMR (400 MHz, CDCl3) δ 9.39 (d, J = 8.0 Hz, 1H), 8.46 (d, J = 8.4 Hz, 1H), 8.32 (dd, J = 8.2, 2.4 Hz, 1H), 7.88-7.80 (m, 4H), 7.72 (s, 1H), 7.68 (t, J = 7.2 Hz, 1H), 7.53 (t, J = 7.6 Hz, 1H), 7.42 – 7.37 (m, 3H), 7.36 – 7.30 (m, 4H), 2.42 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 156.7 (d, JC-P = 128.2 Hz), 142.9 (d, JC-P = 23.1 Hz), 139.0, 133.1 (d, JC-P = 104.1 Hz), 132.7 (d, JC-P = 6.9 Hz), 132.3 (d, JC-P = 9.1 Hz), 131.6 (d, JC-P = 2.6 Hz), 130.9, 130.6, 130.5, 128.5, 128.2 (d, JC-P = 12.1 Hz), 127.5 (d, JC-P = 12.6 Hz), 127.4, 122.0 (d, JC-P = 2.5 Hz), 121.9 (d, JC-P = 5.0 Hz), 21.4. 31P NMR (162 MHz, CDCl3) δ 28.1. HRMS (ESI): m/z [M + H]+ calcd for C26H21NOP: 394.1355, found: 394.1358.
Benzo[i]phenanthridin-5-yldiphenylphosphine oxide (3pa): Isolated (Rf = 0.5, EtOAc – petroleum ether = 1:1) as a white solid (32.6 mg, 38% yield), mp: 221 – 223 oC. 1H NMR (400 MHz, CDCl3) δ 9.40 (d, J = 8.8 Hz, 1H), 9.09-9.06 (m, 1H), 9.05 – 9.00 (m, 1H), 8.16 – 8.12 (m, 1H), 8.04-8.00 (m, 1H), 7.98 – 7.90 (m, 5H), 7.77– 7.66 (m, 4H), 7.54 – 7.40 (m, 6H). 13C NMR (100 MHz, CDCl3) δ 155.3 (d, JC-P = 128.2 Hz), 144.8 (d, JC-P = 23.3 Hz), 133.1 (d, JC-P = 104.6 Hz), 132.4 (d, JC-P = 9.1 Hz), 131.9 (d, JC-P = 6.8 Hz), 131.7, 131.6, 130.9, 128.9, 128.8, 128.7, 128.5, 128.4 (d, JC-P = 2.7 Hz), 128.2 (d, JC-P = 12.0 Hz), 128.1, 127.4 (d, JC-P = 22.6 Hz), 127.2, 126.7, 124.6, 124.0. 31P NMR (162 MHz, CDCl3) δ 29.1. HRMS (ESI): m/z [M + H]+ calcd for C29H21NOP: 430.1355, found: 430.1359.
(10-Methoxyphenanthridin-6-yl)diphenylphosphine oxide (3qa): Isolated (Rf = 0.3, EtOAc – petroleum ether = 1:1) as a white solid (45.9 mg, 56% yield), mp: 244 – 246 oC. 1H NMR (400 MHz, CDCl3) δ 9.58 – 9.45 (m, 1H), 9.14 (d, J = 8.2 Hz, 1H), 8.03 – 7.98 (m, 1H), 7.93-7.86 (m, 4H), 7.72 – 7.64 (m, 2H), 7.62 (t, J = 8.0 Hz, 1H), 7.53 – 7.47 (m, 2H), 7.46-7.40 (m, 4H), 7.30 (d, J = 8.0 Hz, 1H), 4.10 (s, 3H). 13C NMR (100 MHz, CDCl3) δ 158.1 (d, JC-P = 2.7 Hz), 156.4 (d, JC-P = 129.1 Hz), 143.3 (d, JC-P = 23.2 Hz), 133.1 (d, JC-P = 104.5 Hz), 132.3 (d, JC-P = 104.5 Hz), 132.3 (d, JC-P = 9.1 Hz), 131.6 (d, JC-P = 2.4 Hz), 130.9, 129.7 (d, JC-P = 23.8 Hz), 128.7, 128.2, 128.1 (d, JC-P = 12.1 Hz), 128.0 (d, JC-P = 5.6 Hz), 124.2, 123.1 (d, JC-P = 7.0 Hz), 120.8, 112.1, 55.8. 31P NMR (162 MHz, CDCl3) δ 29.2. HRMS (ESI): m/z [M + H]+ calcd for C26H21NO2P: 410.1304, found: 410.1309.
(2-Chlorophenanthridin-6-yl)diphenylphosphine oxide (3ra): Isolated (Rf = 0.4, EtOAc – petroleum ether = 1:3) as a white solid (38.1 mg, 46% yield), mp: 241 – 243 oC. 1H NMR (400 MHz, CDCl3) δ 9.41 (d, J = 8.0 Hz, 1H), 8.48 (d, J = 8.0 Hz, 1H), 8.47 (s), 7.89 (d, J = 8.8 Hz, 1H), 7.86 – 7.80 (m, 4H), 7.77 (d, J = 8.0 Hz, 1H), 7.64 (t, J = 7.6 Hz, 1H), 7.56 (dd, J = 8.8, 2.0 Hz, 1H), 7.47-7.42 (m, 2H), 7.40-7.34 (m, 4H). 13C NMR (100 MHz, CDCl3) δ 156.3 (d, JC-P = 128.2 Hz), 140.1 (d, JC-P = 23.4 Hz), 138.2, 133.9, 131.6 (d, JC-P = 104.7 Hz), 131.5, 131.3, 131.2, 130.8 (d, JC-P = 2.6 Hz), 130.3, 128.3, 127.7, 127.5, 127.2 (d, JC-P = 12.2 Hz), 124.4, 121.1, 120.7, 113.0. 31P NMR (162 MHz, CDCl3) δ 28.5. HRMS (ESI): m/z [M + H]+ calcd for C25H18ClNOP: 414.0809, found: 414.0813.
(2-Fluorophenanthridin-6-yl)diphenylphosphine oxide (3sa): Isolated (Rf = 0.3, EtOAc – petroleum ether = 1:3) as a yellow solid (31.8 mg, 40% yield), mp: 235 – 237 oC. 1H NMR (400 MHz, CDCl3) δ 9.41 (d, J = 8.4 Hz, 1H), 8.45 (d, J = 8.0 Hz, 1H), 8.13 – 8.09 (m, 1H), 7.99 – 7.93 (m, 1H), 7.86 – 7.76 (m, 5H), 7.64 (t, J = 7.6 Hz, 1H), 7.48 – 7.33 (m, 7H). 13C NMR (100 MHz, CDCl3) δ 161.4 (d, JC-F = 248.9 Hz), 155.1 (d, JC-P = 131.7 Hz), 138.6 (d, JC-P = 23.1 Hz), 132.5 (d, JC-P = 9.3 Hz), 131.7 (d, JC-P = 104.3 Hz), 131.2 (d, JC-P = 9.1 Hz), 130.7, 130.1, 127.6 (d, JC-F = 16.3 Hz), 127.2 (d, JC-P = 12.1 Hz), 126.6, 124.9, 121.3, 116.8 (d, JC-F = 24.5 Hz), 106.1 (d, JC-F = 23.2 Hz). 31P NMR (162 MHz, CDCl3) δ 28.5. HRMS (ESI): m/z [M + H]+ calcd for C25H18FNOP: 398.1105, found: 398.1112.
(3-Chlorophenanthridin-6-yl)diphenylphosphine oxide (3ta): Isolated (Rf = 0.4, EtOAc – petroleum ether = 1:3) as a white solid (30.6 mg, 37% yield), mp: 194 – 197 oC. 1H NMR (400 MHz, CDCl3) δ 9.44 (d, J = 8.4 Hz, 1H), 8.48 (d, J = 8.0 Hz, 1H), 8.40 (d, J = 8.8 Hz, 1H), 7.95 (d, J = 1.6 Hz, 1H), 7.87-7.80 (m, 4H), 7.76 (t, J = 7.6 Hz, 1H), 7.64 – 7.54 (m, 2H), 7.47 – 7.41 (m, 2H), 7.41 – 7.32 (m, 4H). 13C NMR (100 MHz, CDCl3) δ 158.5 (d, JC-P = 125.7 Hz), 143.2 (d, JC-P = 23.4 Hz), 134.5, 132.6 (d, JC-P = 104.5 Hz), 132.2 (d, JC-P = 9.1 Hz), 131.9, 131.5, 130.1, 129.3, 128.8, 128.3 (d, JC-P = 12.3 Hz), 127.8 (d, JC-P = 22.7 Hz), 123.6, 122.9 (d, JC-P = 2.4 Hz), 122.0. 31P NMR (162 MHz, CDCl3) δ 28.3. HRMS (ESI): m/z [M + H]+ calcd for C25H18ClNOP: 414.0809, found: 414.0816.

4. Conclusions

Rose Bengal was found to be an available photocatalyst for the cascade phosphorylation cyclization of 2-isocyanobiphenyls. A wide range of 6-phosphorylated phenanthridines have been synthesized efficiently via visible-light-induced radical addition cyclization under metal-free conditions. Biological screening of these P-containing compounds is in progress in our laboratory.

Supplementary Materials

The following supporting information can be downloaded at the website of this paper posted on Preprints.org. The experimental procedures and characterization (1H- and 13C-NMR and 31P-NMR) for all of the products are provided in the supporting information.

Author Contributions

Conceptualization, Q.D. and L.W.; methodology, Q.D. and X.S.; validation, L.W. and Q.Z.; writing—original draft preparation, L.W.; writing—review and editing, X.S. and Q.D.; supervision, Q.D. All authors have read and agreed to the published version of the manuscript.

Funding

We gratefully acknowledge the National Natural Science Foundation of China (21961016), the Foundation for Academic and Technical Leaders of Major Disciplines of Jiangxi Province (20225BCJ22007) and the Natural Science Foundation of Jiangxi Province (20224ACB203009) for financial support.

Data Availability statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Representative bioactive phenanthridine derivatives.
Figure 1. Representative bioactive phenanthridine derivatives.
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Scheme 1. Synthesis of 6-phosphorylated phenanthridines from 2-isocyanobiphenyls.
Scheme 1. Synthesis of 6-phosphorylated phenanthridines from 2-isocyanobiphenyls.
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Scheme 2. Substrate scope of 2-isocyanobiphenyls.
Scheme 2. Substrate scope of 2-isocyanobiphenyls.
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Scheme 3. Substrate scope of diphenylphosphine oxides.
Scheme 3. Substrate scope of diphenylphosphine oxides.
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Scheme 4. Control experiment in the presence of TEMPO.
Scheme 4. Control experiment in the presence of TEMPO.
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Table 1. Optimization of the reaction conditions a.
Table 1. Optimization of the reaction conditions a.
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Entry 1a/2a Catalyst Base Solvent Yield (%)b
1 1/1.5 Rose Bengal (2 mol%) DBU MeCN/H2O (1.0/0.18 mL) 15
2 1/1.5 Rose Bengal (5 mol%) DBU MeCN/H2O (1.0/0.18 mL) 36
3 1/1.5 Rose Bengal (10 mol%) DBU MeCN/H2O (1.0/0.18 mL) 32
4 1/3.5 Rose Bengal (5 mol%) DBU MeCN/H2O (1.0/0.18 mL) 78
5 1/4.5 Rose Bengal (5 mol%) DBU MeCN/H2O (1.0/0.18 mL) 77
6 1/3.5 Eosin Y (5 mol%) DBU MeCN/H2O (1.0/0.18 mL) 61
7 1/3.5 Fluorescein (5 mol%) DBU MeCN/H2O (1.0/0.18 mL) trace
8 1/3.5 Rhodamine B (5 mol%) DBU MeCN/H2O (1.0/0.18 mL) trace
9 1/3.5 Rose Bengal (5 mol%) NEt3 MeCN/H2O (1.0/0.18 mL) 69
10 1/3.5 Rose Bengal (5 mol%) DABCO MeCN/H2O (1.0/0.18 mL) trace
11 1/3.5 Rose Bengal (5 mol%) Na2CO3 MeCN/H2O (1.0/0.18 mL) trace
12 1/3.5 Rose Bengal (5 mol%) K2CO3 MeCN/H2O (1.0/0.18 mL) trace
13 1/3.5 Rose Bengal (5 mol%) DBUc MeCN/H2O (1.0/0.18 mL) 38
14 1/3.5 Rose Bengal(5 mol%) DBUd MeCN/H2O (1.0/0.18 mL) 84
15 1/3.5 Rose Bengal (5 mol%) DBUe MeCN/H2O (1.0/0.18 mL) 80
16 1/3.5 Rose Bengal (5 mol%) DBUd MeCN 50
17 1/3.5 Rose Bengal (5 mol%) DBUd EtOH/H2O (1.0/0.18 mL) 43
18 1/3.5 Rose Bengal (5 mol%) DBUd MeOH/H2O (1.0/0.18 mL) 56
19 1/3.5 Rose Bengal (5 mol%) DBUd THF/H2O (1.0/0.18 mL) 61
20f 1/3.5 Rose Bengal (5 mol%) DBU THF/H2O (1.0/0.18 mL) NR
a Reaction conditions: 1a (0.2 mmol), base (2.0 equiv) and reaction were irradiated by 30 W blue LEDs at room temperature, stirring for 20 h under air atmosphere. b Isolated yields of 3aa. c DBU (1.0 equiv). d DBU (3.0 equiv). e DBU (4.0 equiv). f The reaction was carried out in the dark.
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