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Visible Light-Driven C-3 Functionalization of Indoles over Conjugat-ed Microporous Polymers Weijie Zhang, Juntao Tang, Wenguang Yu, Qiao Huang, Yu Fu, Gui-Chao Kuang, Chunyue Pan, and Guipeng Yu ACS Catal., Just Accepted Manuscript • Publication Date (Web): 17 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018
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ACS Catalysis
Visible Light-Driven C-3 Functionalization of Indoles over Conjugated Microporous Polymers Weijie Zhanga, Juntao Tanga, Wenguang Yua, Qiao Huanga, Yu Fua, Guichao Kuanga, Chunyue Pana*and Guipeng Yua,b* a
College of Chemistry and Chemical Engineering, Hunan Provincial Key Laboratory of Efficient and Clean Utilization of Manganese Resources, Central South University, Changsha 410083, China. b State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 110762, China E-mail:
[email protected] (C. Pan);
[email protected] (G. Yu). ABSTRACT: Metal-free and heterogeneous organic photocatalysts provide an environmentally-friendly alternative to traditional metalbased catalysts. This paper reports a series of carbazole-based conjugated microporous polymers (CMPs) with tunable redox potentials and explores their photocatalytic performance with regard to C-3 formylation and thiocyanation of indoles. Conjugated polymers were synthesized through FeCl3 mediated Friedel-Crafts reactions, and their redox potentials were well regulated by simply altering the nature of the core (i.e. 1,4-dibenzyl, 1,3,5-tribenzyl or 1,3,5-triazin-2,4,6-triyl). The resulting CMPs exhibited high surface areas, visible light absorptions and tunable semiconductor-range band gaps. With the highest oxidative capability, CMP-CSU6 derived from 1,3,5-tri(9Hcarbazol-9-yl)benzene, showed the highest efficiency for C-3 formylation and thiocyanation of indoles at room temperature. Notably, the as-made catalysts can be easily recovered with good retention of photocatalytic activity and reused for at least 5 times, suggesting good recyclability. These results are significant for constructing high-performance porous polymer catalysts with tunable photoredox potentials targeting an efficient material design for catalysis.
KEYWORDS: conjugated microporous polymers, visible light, photoredox catalysis, heterogeneous catalysis, C-3 functionalization, indoles
INTRODUCTION 3-Substituted indoles, as important intermediates, have been widely utilized in agrochemicals, alkaloids and pharmaceutical drug candidates.1 Formylation and thiocyanation of indoles are two important pathways for direct C-3 functionalization of indoles. Generally, unrecyclable toxic transition metal (Cu or Ru) catalysts under constant heating conditions (e.g. 400 K) are required, leading to environment issues and excessive consumption of high-cost noble catalysts.2 Photocatalytic transformation driven by sunlight shows promise in producing 3-substituted indoles, which are advantageous over transition metal catalysts. Current photocatalysts like small dye molecules, however, suffer from insufficient stability and difficulty in tuning optical and electronic properties.2 Therefore, it is highly desirable to develop metal-free, recyclable and heterogeneous catalyst systems for effective C-3 formylation and thiocyanation of indoles under visible light irradiations. To the best of the authors’ knowledge, the use of organic frameworks as photocatalysts for C-3 functionalization of indoles has not yet been reported. As an emerging class of organic porous materials, conjugated microporous polymers (CMPs) combine extensive π-conjugated skeletons with nanometer-scale pores to offer some advantages in addressing energy and environmental issues such as gas capture and adsorption,3 heterogeneous catalysis,4 light emitting,5 energy storage.6 Concerning the tunable optoelectronic properties of CMPs at the molecular level, systematic studies have been carried out to demonstrate their utility in various heterogeneous photocatalytic reactions, including selective oxidation of thioether,7 oxidative coupling of primary amines,4a,8 radical polymerization,9 hydrogen evolution10 and others.11 CMPs as catalysts possess many noteworthy advantages. First, the robust porosity-rich nature and large surface area offer a shelter for substrates, rendering substrate diffusion and product mass transfer more effective. Also, the good chemical and thermal stability of CMPs, especially
compared with other catalysts such as small dye-molecules, ensure high photocatalytic performance and good recyclability. In addition, the flexible tuning of band gaps makes CMPs ideal candidates for specific photocatalytic process. For example, strategies like copolymerizations have been reported by doping appropriate comonomer, and the obtained polymers have shown high photocatalytic activity for hydrogen evolution,10 oxidation of benzylic β-O-4 alcohols,12 and cyclization of N, N-dimethylanilines with maleimides.13 This study designed a series of carbazole-containing CMPs as efficient heterogeneous photocatalysts for C-3 functionalization of indoles. The paper investigates the structural effect of CMPs on their photocatalytic performance by incorporating different electron donating or withdrawing building blocks (Figure 1a). The as-made CMPs show high porosity and their redox potentials can be well-tuned to meet the requirements of photo-induced transformation. The effects of redox potential and pore structure on catalytic performance for the formylation and thiocyanation of indoles are examined. The CMPs exhibit high efficiency in C-3 transformation for a broad range of indole substrates and demonstrate high functional group tolerance. Compared to the reported catalysts for C-3 formylation and thiocyanation of indoles,2 the designed CMP photocatalytic system works efficiently at room temperature without the addition of strong acids or bases while avoiding the use of strong oxidants. The system therefore possesses several advantages including cost effectiveness (metal-free and recyclability), simplicity (easy operation and separation) and environmental friendliness.
RESULTS AND DISCUSSION The non-planar rigid conformation, high light-absorption coefficient, and electron donating ability of carbazole make it an ideal scaffold in optoelectronic materials such as dye-sensitized solar cells,14 perovskite solar cells15 and organic field-effect transistor.16
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cleophilic substitution was adopted to prepare the precursors.17 Carbazolic CMPs (hereafter denoted as CMP-CSU5, CMP-CSU6 and CMP-CSU7, synthesized from DCB, TCB and TCT, respectively) were obtained as insoluble solids via FeCl3-promoted oxidative polymerizations in anhydrous CHCl3 under N2 protection (Scheme S1). in high yields (90-95).17
Owing to these unique properties, incorporation of photosensitizing carbazole moiety into a polymer backbone may be conducive to facilitating the migration of generated charges and suppressing charge recombination. For this purpose, three carbazolic precursors called 1,4-di(9H-carbazol-9-yl)-benzene (DCB), 1,3,5-tri(9Hcarbazol-9-yl)benzene (TCB) and 2,4,6-tri(9H-carbazol-9-yl)1,3,5-triazine (TCT) were synthesized from 1-bromo-4iodobenzene, 1,3,5-tribromobenzene and 2,4,6-trichloro-1,3,5triazine, respectively. Either copper-promoted N-arylation or nu-
hv
1
O2 andO2
Donor
a)
LUMO
e O2
N
Photoactive center O N
S
N N
N
HOMO
Acceptor
h
N
N N
N
N
The porosity features of CMPs were characterized using N2 adsorption-desorption measurement. CMP-CSU5, CMP-CSU6 and CMP-CSU7 revealed high Brunauer-Emmett-Teller (BET) surface areas of 978, 1281 and 811 m2 g−1, total pore volumes of 0.87, 0.96 and 0.58 cm3 g−1, respectively (Table 1). All CMPs featured a hierarchical micro-mesoporous structure; more importantly, over 43 % of the pore volume is contributed by mesopores, which appears to be promising shelters for the transformation of large-sized substrates (Figure S1 and Table 1).7b,18Scanning electron microscopy (SEM) images of CMPCSU5, CMP-CSU6 and CMP-CSU7 displayed different morphological structures (graphic flake-like, fiber-like and plate-like) at the micrometer scale (Figure 2). Additionally, the residual Fe3+ contents of CMP-CSU5, CMP-CSU6 and CMP-CSU7, which were detected using inductively coupled plasma-atomic emission spectrometer (ICP-AES) measurements after fully degrading the samples in nitrohydrochloric acid, were low at 1.1, 1.3 and 1.1‱, respectively. These results indicate that the residual metal ions were nearly completely removed. Table 1. Pore Parameters of CMPs
NH4SCN
Vtotalb
Vmesoc
(m2g-1)
(cm3g−1)
(cm3g−1)
CMP-CSU5
978
0.87
0.49
55
CMP-CSU6
1281
0.96
0.63
44
CMP-CSU7
811
0.58
0.25
43
CMP-CSU6@70
1717
1.36
0.69
51
Sample N N
N
N
N
CMP-CSU7
CMP-CSU6
c)
b)
0 day
Vmeso/Vtotal (%)
N
N
CMP-CSU5
SABETa
N
N N
N
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CMP-CSU6@140 1451 1.32 0.74 56 a Calculated using adsorption branches over the pressure range 0.01-0.1 bar of N2 isotherm at 77 K. b Calculated from the N2 isotherm at P/P0 = 0.99 cm3 g−1. c The volume of mesopores with pore width >2 nm.
2 days
Figure 1. a) The structures of the photoredox catalysts CMPCSU5, CMP-CSU6 and CMP-CSU7 and their applications in C-3 functionalization of indoles; b), c) C-3 formylation of 1-methyl1H-indole tests were performed under solar radiation Changsha, China (08/04/2018-09/04/2018, 25-28 °C)
Figure 2. Scanning electron microscopy (SEM) images of CMPCSU5 (a), CMP-CSU6 (b) and CMP-CSU7 (c) The chemical structures of the as-prepared CMPs were confirmed using Fourier transform infrared spectroscopy (FT-IR), solid-state 13C nuclear magnetic resonance (NMR) and the ultraviolet-visible spectroscopy (UV/Vis) diffuse reflectance spectrum. Generally, the elimination of the absorption peak at around 720 cm−1 (assigned to the disubstituted phenyl ring in the carbazole monomer) and newly generated peaks at around 800 cm-1 (attributed to the vibration of C-H bonds on the di-or tri-substituted
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ACS Catalysis carbazole ring) implied successful polymerizations (Figures S2, S3, S4 and Figure 3a)19. The 13C CP/MAS NMR spectrum for CMP-CSU6 (Figure S5) also offered valuable evidence for structure identification. From the UV/Vis diffuse reflectance spectra, the designed networks appeared to exhibit a broad absorption band centered around 400 nm (Figure S6 and Figure 3b), and their absorption band edges extended to the visible range (λ > 620 nm); the corresponding precursors exhibited minimal visible light absorption. Such red-shift behavior could be attributed to the formation of extended π-conjugation chains, which indeed enhances the visible range absorption. A detailed comparison between the designed CMPs and a carbazole-free polymer such as PPCMP@mmm indicated that the introduction of carbazolic donors benefited the broad absorption band.20
a)
Cyclic voltammetry measurements were conducted to investigate the energy band structure of the polymers. The lowest unoccupied molecular orbital (LUMO) position of CMP-CSU5, CMP-CSU6 and CMP-CSU7 was nearly the same and found to be lower than0.86 V (Figure S7), sufficiently negative to reduce oxygen into the active species O2•−.6 Based on a free energy change (E0–0) between the vibrational related excited state and the ground state of 2.14, 2.29 and 2.08 eV (E0–0 = 1240/λ),7a the highest occupied molecular orbital (HOMO) positions of CMP-CSU5, CMP-CSU6 and CMP-CSU7 were calculated to be 1.26, 1.42 and 1.19 V, respectively (Figures S8 and 3d). Among the three samples, CMP-CSU6 exhibited the highest oxidation potential; therefore, it is expected to perform better in photocatalytic oxidation reaction than the widely reported transition metal photocatalysts such as [Ru(bpy)3]2+ (+1.29 V vs. SCE).21 Furthermore, the LUMO (0.87 V) and HOMO (1.42 V) levels of CMP-CSU6 were sufficient for the redox process; the photoactive macromolecular systems could effectively mediate the electron transfer from N, N, N’N’-tetramethylbenzene-1,4-diamine (+ 0.12 V vs SCE)8 to molecular oxygen, as verified by the occurrence of the colored cationic radical (Figure 3c insert).8 Among the three polymers, CMP-CSU6 demonstrated the highest intensity in UV/Vis absorption spectra, and CMP-CSU7 ranked the lowest, which is consistent with the color depth of obtained solutions catalyzed by the designed CMPs. These findings suggested the superior photooxidative activity of CMP-CSU6 over CMP-CSU5 and CMP-CSU7, as evidenced by the highest oxidation potential (Figure 3c and 3d). The electron paramagnetic resonance (EPR) spectra showed an enhanced signal upon light irradiation, relative to that taken in darkness, indicating photogenerated radicals, i.e., electron−hole pairs in CMP-CSUs (Figure S9). Given the promising photoredox properties of the designed CMPs, their photocatalytic activities towards C-3 formylation of indoles were investigated. 1-methyl-1H-indole was chosen as a model substrate, tetramethyl ethylenediamine (TMEDA) served as the carbon source and molecular oxygen was a clean oxidant. The reaction conditions were screened and optimized (Table 2 and Table S1). C-3 formylation of indoles catalyzed by CSU-CMP6 afforded a satisfactory yield of 86%. Replacing CMP-CSU6 with CMP-CSU5 or CMP-CSU7 was deleterious for the product yield (Table 2. entries 1, 2 and 5). The higher photocatalytic activity of CMP-CSU6 could presumably be ascribed to its higher HOMO level (compared to others) considering the comparatively lower LUMO levels (-0.88 V, -0.87 V and -0.89 V) of the three CMPs. A notably high HOMO level (1.42 V) could oxidize TMEDA (TMEDA+•/TMEDA=0.65 V vs SCE) (Figure S10) more easily to form radical cations. Most importantly, the designed CMP-CSU6 outperformed at a yield increase of 10% when compared to that
c)
b)
N + O2
N
(I)
(II)
hv CMP-CSUs
O2
N
N
(III) (IV)
d)
Figure 3. a) Partial FT-IR spectra of CMP-CSU5 (black), CMPCSU6 (red) and CMP-CSU7 (blue); b) normalized solid state UVVis absorption spectra of CMP-CSU5 (black), CMP-CSU6 (red) and CMP-CSU7 (blue); c) UV-vis absorption spectra and photograph of the cationic radical of TMPD generated by CMP-CSUs in the presence of light and oxygen. Inserted images: (I): CMPCSU5, (II): CMP-CSU6, (III):CMP-CSU7, (IV): No catalyst and d) the LUMO and HOMO of CMP-CSU5, CMP-CSU6, and CMP-CSU7 catalyzed by Rose Bengal with constant heating up to 60 oC (Table 2. entries 2 and 6). Also, a comparably high yield without the
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aid of high temperatures was obtained for the designed catalytic system compared to Rose Bengal, CuCl2, and I2 catalytic systems (Table 2. entries 6, 7 and 8).2c,2f,22 The g-C3N4, a state-of art metal-free photocatalyst was also investigated under optimized condition. However, only 31% yield of the product was obtained. (Table 2. entry 9). Compared to metal complex Ru(bpy)3Cl2 and the dyes, the designed CMP catalysts exhibited certain advantages. First, the robust porosity-rich nature and large surface area offered a shelter for substrates, leading to more effective substrates diffusion and the product mass transfer. Second, the chemical and thermal stability of CMPs remained good, even after being irradiated for a long time. Comparatively, for metal complex Ru(bpy)3Cl2 and the dye were usually unstable after being irradiated under visible light for a long time, the dye molecules are usually unstable. The dye could also be subjected to “bleaching” during the reaction. These patterns indicated that a high performance catalytic system which operates under moderate reaction conditions has been developed by simply altering the core nature of CMPs. When referring to the methods of copolymerization to tune the redox potential of CMPs,8,12,13 altering the core nature of the precursors is an efficient strategy to fine tune of the redox potential of task-specific CMPs for certain photoredox transformations. Except for the redox potentials of CMPs, the mass transfer of the substrate (related to pore features) was also thought to exert certain effects on photocatalytic performance. To verify this assumption, CMP-CSU6 with different porosity features was prepared by polymerization at different temperatures (denoted as CMP-CSU6@T, Table 1 and 2, Figure S11) and the catalytic performance of each was compared. Again, insoluble powders were obtained with surface areas, ranging from 1451 to 1717 m2 g−1 with total pore volumes of 1.36 and 1.32 cm3g-1 were obtained (Table 1). In general, CMPs with a higher surface area and larger porosity demonstrate favorable mass transfer in chemical transformations; results of this study are similar (Table 2. entries 2, 3 and 4). However, as the catalyst increases in surface area and pore volume, the reaction efficiency in terms of yield did not increase significantly. This trend suggests that the energy band structure is more critical than porosity nature in developing CMPs for C-3 formylation of indoles when they deliver satisfactory porosity. To demonstrate the product regioselectivity, 1,3-dimethylindole was subjected to the reaction system under the optimized conditions. No desired product was observed during this process, indicating that the formylation of indoles occurred at the C-3 position rather than others. The same conclusion can be drawn from the 1H NMR spectrum analysis (Supporting information). To further investigate the mechanism for C-3 formylation of indoles, several control experiments were carried out. Only trace of or no target product were detected by getting rid of either photocatalysts or light irradiations (Table S1. entries 1 and 2), which verified the necessity of a light source and photocatalyst in the reaction process. Additional experiments were performed to explore the specific role of photogenerated electron-hole pairs in the photocatalytic C-3 formylation (Table S1. entries 3 and 4). By adding TEMPO as a radical scavenger,2c a substantially reduced yield of 11% was obtained, while adding benzoquinone as a superoxide radical anion O2•− scavenger to the reaction mixture, led to an even lower yield (9%). These results suggest that the superoxide radical participated in the photocatalytic reactions. When O2 was replaced by N2, the yield was quite poor, highlighting the important role of oxygen in the reaction process (Table S1. entry 5). O2 thus appeared to have been converted into superoxide radical anion O2•− via single electron transfer from the excited state of
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CMPs (Figure 3c), consistent with known results.10 Research has reported that the formation of singlet oxygen (1O2) is possible when CMPs are irradiated.7a To probe the feasibility of this phenomenon, sodium azide (NaN3) which acts as a 1O2 scavenger was added during photocatalytic transformation. A yield of 3a was declined to 45% in the presence of NaN3. It is clear that 1O2 also played an important role in the process of photocatalytic C-3 formylation of N-methylindole (Table S1. entry 6). The effect of the water amount on yield for the formylation of N-methylindole was investigated. At first, high yield retained with an increase in water. (Table S1. entries 7 and 8). When the water reached 2 mL or 3 mL, the yield decreased dramatically (Table S1. entries 9 and 10). A yield lower than 10% was observed when the water was used as the solvent (Table S1, entry 11). These results demonstrate that the proportion of H2O played a central role in the formylation of N-methylindole. Table 2. Selected Catalysts and Their Catalytic Efficiency for C-3 Formylation of N-Methylindolea
entry
catalyst
light
tempb
yieldc
ref
1
CMP-CSU5
+
RT
67
This work
2
CMP-CSU6
+
RT
86
This work
3
CMP-CSU6@70
+
RT
90
This work
4
CMP-CSU6@140
+
RT
88
This work
5
CMP-CSU7
+
RT
43
This work
6
Rose Bengal
+
60oC
75
[2c]
7
CuCl2
-
80oC
93
[2f]
8
I2
-
120 oC
86
[22]
9
g-C3N4
+
RT
31
This work
a
Reaction conditions: N-methylindole (0.5 mmol), TMEDA (2 equiv), KI (4 eq), photocatalyst (20 mg), CH3CN (5 mL) and 50 µL of H2O, RT= 25 ± 2oC, O2 balloon (~0.1 MPa), using a 14 W LED lamp (0.20 W/cm2) as the light source, 48 h. btemp=reaction temperature. cIsolated yield (%). A potential reaction mechanism for C-3 formylation of indoles was also proposed (Scheme 1). Upon irradiation, the CMP-CSU6 catalyst was excited to CMP-CSU6*, and then reductively quenched by oxygen to produce superoxide radical anion O2•− along with CMP-CSU6+ via singlet electron transfer (SET). Similar to the mechanism of oxidation of amines catalyzed by Ru with tert-Butyl hydroperoxide (TBHP)9b,23, TMEDA (2) (+ 0.65 V vs SCE) (Figure S10) was oxidized by CMP-CSU6+ to form radical cations 2A and to regenerate CMP in a photoredox cycle. Subsequently, a hydrogen atom of a radical cation transferred to the superoxide radical anion to form hydrogen peroxide anion and iminium ion (2A→2B). An indole iminium ion (2B→3A) was obtained by electrophilic addition of iminium ion to N-
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ACS Catalysis methylindole, which then released a hydrogen ion to provide an intermediate 3B (detected by LC-MS, Figure S12) and presumably H2O2. After a second light photoredox cycle (3B→3C), an iminium ion was formed and subjected to hydrolysis to generate 3-formyl-N-methylindole (3C→3a).
Scheme 1. Proposed Mechanism of C-3 Formylation of Indoles. Table 3. Substrate Scope for C-3 Formylation of Indolesa,b
O
O
O R
R1 N
N
N
R2 3a, R2=CH3 (86%) 3d, R2=Boc (NR) 3b, R2=Et (72%) 3e, R2=Ts (NR) 3c, R2=Bn (64%) 3f, R2=H (67%)
O
R 3n, R=F (64%) 3o, R=Cl (44%)
N 3p, R=Br (67%) 3q, R= OMe (64%)
3g, R1=CH3 (72%) 3h, R1=Ph (70%)
R
3i, R=CH3 (70%) 3l, R=Br (41%) 3j, R=F (51%) 3m, R= OMe (55%) 3k, R=Cl (38%)
O
O
N 3r, R=OBn (54%) 3s, R=CH3 (66%)
obtained, suggesting that the reaction was not affected by steric hindrance at C-2 position. For methyl substitution at different benzene ring positions (i.e., 4-, 6- and 7-position), the methyl group at different positions was found to exert no obvious impact on yields (3i: 70%; 3s: 66%; 3t: 61%). The reactions between 2 and indoles functionalized by halogen (e.g., F, Cl and Br) on their 5- or 6-positions resulted in the desired products in moderate yields of 41-67 %. Also, the halogen groups retained well after being subjected to optimized reaction conditions, providing a potential site for further functionalization. The yields from indoles with an electron-donating group (OMe) and an electronwithdrawing group (F, Cl, Br) at the C-6 position were better than those at the C-5 position (3j, 3k, 3l, 3m, 3n, 3o, 3p, 3q). Moreover, 1,7-dimethyl-1H-indole and 4-(benzyloxy)-1-methyl-1Hindole could also be subjected to the formylation reactions, and satisfactory yields (61–70%) (3i, 3r, 3t) were obtained. To further verify the attracting photocatalytic properties of the designed CMP-CSU6, efficiency of the C-3 thiocyanation of indoles was examined. 1H-indole was applied as a model substrate and the conditions for light-mediated thiocyanation were screened and optimized (Table 4 and Table S2). A high yield of 95% could be obtained using 2 equiv of NH4SCN as an SCN source and tetrahydrofuran as the reaction solvent. Notably, the reaction conditions for CMP-CSU6 catalytic system were much milder and free of strong oxidants or metal-based catalysts (Table 4) compared to the catalysts disclosed 2b, 2d, 2e, 24. Further control experiments revealed that light, catalyst and O2 played critical roles in chemical transformations (Table S2. entries 1, 2 and 3), of which the reaction mechanism was highly similar to the aforementioned formylations (see Scheme 1 and Scheme 2). Upon light irradiations, the CMP-CSU6 was excited to CMP-CSU6* and reductively quenched by oxygen to produce hydrogen peroxide and CMPCSU6+ via SET. The photoredox cycle is completed by a single electron transfer between −SCN and CMP-CSU6 + affording •SCN and CMP-CSU6 (Figure S10; Table S2. entry 9). Then, an electrophilic addition of radical attacks 1H-indole afforded the intermediate 4A. 4A was then attacked and oxidized to produce 4B.2b,25 Rearomatization of intermediate 4B by losing a proton generated the final 3-thiocyanoindole product 5f.
N R 3t, R=CH3 (61%)
a
Reaction conditions: indole (0.5 mmol), TMEDA (2 equiv), KI (4 eq), photocatalyst (20 mg), 5 mL of CH3CN and 50 µL of H2O, RT=25 ± 2oC, O2 (~0.1 MPa), using a 14 W LED lamp (0.20 W/cm2) as the light source, 48h.b NR=No Reaction Next, the general feasibility of CSU-CMP6 as a photocatalyst for the C-3 formylation of indoles was elevated. A broad range of substrates containing either electron-donating or electronwithdrawing groups that connected to different indole positions were investigated. The substrate scope of C-3 formylation of 1methyl-1H indoles was explored in Table 3. First, indoles containing electron-donating groups (e.g., methyl, ethyl and benzyl) on the nitrogen atom were all efficiently converted to their corresponding products at respective in the yields of 86%, 72% and 64%. However, this investigation failed to achieve formylation of indoles with an electron-withdrawing group such as t-Butyloxy carbonyl (Boc) and 4-Toluene sulfonyl (Ts) on the nitrogen atom. This finding may result from the low electron density of indole derivatives. The formylation of free (N-H) indoles afforded 3f in a 67% yield. For N-methyl indoles with a methyl or phenyl group blocked at C-2 position (3g and 3h), yields of 72% and 70% were
Scheme 2. Proposed Mechanism of C-3 Thiocyanation of Indoles Having established conditions for the standard C-3 thiocyanation of indoles, the study next examined the generality and selectivity of thiocyanation on various indole substrates. For N-substituted indoles, such as N-methyl-, N-ethyl-, and N-benzyl indoles, the corresponding 3-thiocyano products were afforded in high yields of 98%, 96%, and 94%, respectively (Table 5. 5a, 5b and 5c). However, the reaction failed when introducing a strong electronwithdrawing nitro group such as Boc and Ts group on N atom of indole (Table 5. 5d and 5e). For an indole with a phenyl or me-
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thyl group at the C-2 position, 5f and 5g were obtained in high yields, which may suggest that the steric hindrance at C-2 position exerts no effect on the photocatalytic performance (Table 5. 5g and 5h). Results also revealed that high yields (92-99%) (5i, 5j, 5k, 5l, 5m, 5n, 5o, 5p, 5q, 5r, 5s, 5t) could be obtained by incorporating either an electron-donating or electron-withdrawing group on the benzene ring of indoles. Table 4. Selected Catalysts or Oxidants and Their Catalytic Effi-
carried out using CMP-CSU6 as the photocatalyst and 1-methyl1H-indole as the substrate. CMP-CSU6 could be used for another five repeating cycles without a substantial decline in its catalytic efficiency (Figure 4 Black), consistent with the observed trend of almost no change in the FT-IR spectra for samples before and after the photocatalytic reaction (Figure S13). High recyclability of the catalyst was also found for C-3 thiocyanation (Figure 4 Red). These results demonstrate the apparent reusability of the designed CMP-CSU networks as heterogeneous photocatalysts.
ciency for C-3 Thiocyanation of 1H-Indolea
entry
oxidant
catalyst
tempb
yieldc
ref
1
O2
Cu(OTf)2
80 oC
83
[2d]
2
Oxone
-
RT
98
[24]
3
Mn(OAc)3
-
120 oC
83
[2e]
4
O2
Ru(bpy)3Cl2
RT
87
[2b]
5
O2
Ir(ppy)3
RT
47
[2b]
6
O2
CMP-CSU6
RT
95
This work
a
Reaction conditions:1H- indoles (0.5 mmol), NH4SCN (1.0 mmol, 2 equiv), catalyst (20 mg), THF (5 mL), 10 h, RT=25 ± 2oC, O2 (~0.1 MPa), using a 14 W LED lamp (0.20 W/cm2) as the light source. btemp=reaction temperature. cIsolated yield (%) Table 5. Substrate Scope for 3-Thiocyanation of Indole a,b
Figure 4. Recyclability of CMP-CSU6 in C-3 formylation and thiocyanation of indoles To illustrate the scalability of CMP-CSU6 as the photocatalyst, the study scaled up the optimized conditions by 25 times using 1H-indole as the substrate for C-3 thiocyanation. The starting material was consumed as indicated by thin layer chromatography (TLC) after 20 h. The residual mixtures were purified by column chromatography to afford the desired product in 94% yield, demonstrating the practical nature and scalability of the proposed protocol. After purifications, CMP-CSU6 could be recovered using simple filtration and before rinsing and then being re-used in the transformation. The starting material was converted to the desired product after 12 h, again reflecting the inherent robustness of the desighed CMP-CSUs polymers as the photocatalysts.
CONCLUSION
a
Reaction conditions: Indoles (0.5 mmol), NH4SCN (1.0 mmol, 2 equiv), catalyst (20 mg), THF (5 mL), RT=25 ± 2oC,10 h, O2 (~0.1 MPa), using a 14 W LED lamp (0.20 W/cm2) as the light source. b NR=No Reaction
In summary, this paper has developed a series of CMP catalysts to effectively catalyze C-3 formylation and thiocyanation of indoles. Altering the nature of the core (phenyl or triazine units) was found to be an effective method for regulating the photoredox potentials of the CMPs. Furthermore, the photocatalytic process appeared not to introduce toxic metals or organic oxidizing agents and the photocatalysts showed good recyclability without a substantial decrease in photocatalytic efficiency. This study also highlights that this design strategy for optimizing photocatalytic efficiency could be extended to prepare a wider range of conjugated microporous polymeric photocatalysts for other organic transformations.
ASSOCIATED CONTENT Supporting Information
Aside from photocatalytic activity and substrate tolerance, the recyclability of the catalyst remained one of the most important parameters. For C-3 formylation, recycling experiments were
Additional details such as materials and instruments, synthesis procedures, FT-IR spectroscopy, NMR spectra, UV/Vis diffuse reflectance spectrum, cyclic voltammetry, LC-MS data. The Sup-
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ACS Catalysis porting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Corresponding Authors *C. Pan (
[email protected]) *G. Yu (
[email protected]).
Notes The authors declare no competing financial interests.
ACKNOWLEDGMENTS We acknowledge the financially support from the National Science Foundation of China (Nos. 21674129 and 21636010), the Hunan Provincial Science and Technology Plan Project, China (No.2016TP1007), State Key Laboratory of Fine Chemicals (KF1604) and Innovation Mover Program of Central South University (2018CX046). Q. Huang acknowledge the financially support from Fundamental Research Funds for the Central Universities of Central South University (2016zzts253). W. J. Zhang gratefully also acknowledge the NMR measurements by The Modern Analysis and Testing Center of CSU. REFERENCES (1) Kochanowska-Karamyan, A. J.; Hamann, M. T. Marine Indole Alkaloids: Potential New Drug Leads for the Control of Depression and Anxiety. Chem. Rev. 2010, 110 (8), 44894497. (2) (a) Wu, W.; Su, W. Mild and Selective Ru-Catalyzed Formylation and Fe-Catalyzed Acylation of Free (N-H) Indoles Using Anilines as the Carbonyl Source. J. Am. Chem. Soc. 2011, 133, 11924-11927. (b) Fan, W.; Yang, Q.; Xu, F.; Li, P. A visible-light-Promoted aerobic metal-free C-3 thiocyanation of indoles. J. Org. Chem. 2014, 79, 10588-10592. (c) Li, X.; Gu, X.; Li, Y.; Li, P. Aerobic Transition-MetalFree Visible-Light Photoredox Indole C-3 Formylation Reaction. ACS Catal. 2014, 4, 1897-1900. (d) Jiang, H.; Yu, W.; Tang, X.; Li, J.; Wu, W. Copper-Catalyzed Aerobic Oxidative Regioselective Thiocyanation of Aromatics and Heteroaromatics. J. Org. Chem. 2017, 82, 9312-9320. (e) Pan, X. Q.; Lei, M. Y.; Zou, J. P.; Zhang, W. Mn(OAc)3promoted regioselective free radical thiocyanation of indoles and anilines. Tetrahedron Lett. 2009, 50, 347-349. (f) Zhao, D.; Wang, Y.; Fu, H. J.; Shen, Q.; Li, J. X. Cu (II)-catalyzed C-H (SP3) oxidation and C-N cleavage: base-switched methylenation and formylation using tetramethylethylenediamine as a carbon source. Chem. Commun. 2012, 48, 5928-5930. (3) (a) Dawson, R.; Stöckel, E.; Holst, J. R.; Adams, D. J.; Cooper, A. I. Microporous organic polymers for carbon dioxide capture. Energy Environ. Sci. 2011, 4, 4239-4245. (b) Xie, Y.; Wang, T.T.; Liu, X.H.; Zou, K.; Deng, W.Q. Capture and conversion of CO2 at ambient conditions by a conjugated microporous polymer. Nat. Commun. 2013, 4, 19601967. (c) Liao, Y.; Cheng, Z.; Zuo, W.; Thomas, A.; Faul, C. F. J. Nitrogen-Rich Conjugated Microporous Polymers: Facile Synthesis, Efficient Gas Storage, and Heterogeneous Catalysis. ACS Appl. Mater. Interfaces. 2017, 9, 38390-38400. (d) Gu, S.; Guo, J.; Huang, Q.; He, J.; Fu, Y.; Kuang, G.; Pan, C.; Yu, G. 1,3,5-Triazine-Based Microporous Polymers with Tunable Porosities for CO2 Capture and Fluorescent Sensing. Macromolecules. 2017, 50, 8512-8520. (4) (a) Zhang, X.; Kormos, A.; Zhang, J. Self-Supported
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ACS Catalysis
Table of Contents A series of carbazole-based conjugated microporous polymers (CMPs) with tunable redox potentials were constructed by regulating the core structure, demonstrating high photocatalytic efficiency towards C-3 formylation (up to 90%) and thiocyanation (exceeding 93% yield) of indoles under mild conditions
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