Yuiki
Kawada‡
a,
Shunsuke
Ohmura‡
a,
Misaki
Kobayashi
a,
Wataru
Nojo
b,
Masaki
Kondo
cd,
Yuka
Matsuda
a,
Junpei
Matsuoka
a,
Shinsuke
Inuki
a,
Shinya
Oishi
a,
Chao
Wang
cd,
Tatsuo
Saito
c,
Masanobu
Uchiyama
*cd,
Takanori
Suzuki
*b and
Hiroaki
Ohno
*a
aGraduate School of Pharmaceutical Sciences, Kyoto University, Sakyo-ku, Kyoto 606-8501, Japan. E-mail: hohno@pharm.kyoto-u.ac.jp
bDepartment of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060-0810, Japan
cGraduate School of Pharmaceutical Sciences, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
dCluster of Pioneering Research (CPR), Advanced Elements Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
First published on 10th September 2018
The gold-catalysed annulation of conjugated alkynes bearing an azido group with arenes gave annulated [c]carbazoles. Using benzene, pyrrole, and indole derivatives as the nucleophiles, benzo[c]-, pyrrolo[2,3-c]-, and indolo[2,3-c]carbazoles were produced, respectively. The reaction proceeded through pyrrole and benzene ring construction accompanied by the formation of two carbon–carbon and one carbon–nitrogen bond and the cleavage of two aromatic C–H bonds. The mechanism of the reaction with pyrrole was investigated by density functional theory calculations. N,N′-dimethylated indolo[2,3-c]carbazole showed dual ultraviolet-visible-near-infrared and fluorescence spectral changes upon electrolysis.
The general synthetic approaches to carbazoles including aryl-annulated [c]carbazoles are shown in Scheme 1A and B.1a,3 Pyrrole ring formation based on a combination of carbon–nitrogen and carbon–carbon bond formation provides an efficient route to carbazole synthesis (Scheme 1A).4,5 Reliable coupling reactions such as the Suzuki–Miyaura, Buchwald, and oxidative coupling reactions can be employed for this purpose. Benzene ring formation using vinyl- or aryl-substituted indoles including Diels–Alder-type reactions,6 hydroarylation,7 and related reactions8 leads to various carbazoles including aryl-annulated carbazoles (Scheme 1B). However, the double cyclisation approach for synthesising aryl-annulated [c]carbazoles has not been investigated until recently.9–11 We expected that the gold carbenoid-based cascade cyclisation of conjugated diynes would directly provide aryl-annulated [c]carbazoles in a single operation via the sequential cleavage of two aromatic C–H bonds (Scheme 1C).
Scheme 1 General synthetic approaches to carbazoles including aryl-annulated carbazoles and this work. |
Homogenous gold catalysis has emerged as a powerful tool for atom-economical transformations.12 The π-acidity of gold catalysts enables the activation of C–C multiple bonds to promote various transformations. Recent investigations using diynes in gold-catalysed reactions have revealed that both conjugated and unconjugated diynes are useful precursors of complex molecules.13 For example, we recently reported a gold-catalysed formal [4 + 2] reaction between 1,3-diynes and pyrroles for synthesising 4,7-disubstituted indoles (Scheme 2A, n = 0).14a This reaction proceeded through a double hydroarylation cascade involving the initial intermolecular hydroarylation of 1,3-diyne at the 2-position of pyrrole, followed by intramolecular hydroarylation. When using skipped diynes as substrates, the formal [5 + 2] reaction efficiently proceeded to produce 1,6-dihydrocyclohepta[b]pyrrole derivatives,14b which can be considered as homologs of 4,7-disubstituted indoles (Scheme 2A, n = 1).
We then turned our attention to synthesising aryl-annulated [c]carbazoles based on gold-carbenoid formation15 using conjugated diynes. In 2011, Gagosz16a and Zhang16b independently reported the gold(I)-catalysed synthesis of indoles bearing an electron-donating group at the 3-position (Scheme 2B).17 The reaction can be rationalised by the formation of a gold carbenoid intermediate followed by a nucleophilic reaction at the carbenoid moiety. As the coupling partners, alcohols and arenes can be used for the reaction to produce 3-substituted indoles. We envisaged that incorporating gold carbenoid chemistry into diyne cyclisation would provide direct access to aryl-annulated [c]carbazoles (Scheme 3). Thus, the gold(I)-mediated nucleophilic attack of the azido group of diyne 1 on the proximal alkyne followed by the elimination of nitrogen would produce gold carbenoid species A. The electrophilic aromatic substitution of benzene-type arenes with A would produce intermediate 2. Finally, intramolecular hydroarylation toward alkynes18 would occur to produce benzo[c]carbazole 3. The challenge of this strategy is controlling the regioselectivity when using pyrrole-type heteroarenes as the coupling partner: whereas the first nucleophilic attack at the pyrrole 3-position would produce pyrrolo[2,3-c]carbazole 6, the first nucleophilic attack at the pyrrole 2-position would give pyrrolo[3,2-c]carbazole 7, through 3-pyrrolylindole intermediates 4 and 5, respectively. Attention should also be given to the regioselectivity in the second arylation in the reaction with the benzene-type nucleophile (2 to 3).
Herein, we report a full account of our study on the direct synthesis of aryl-annulated [c]carbazoles by the regioselective gold-catalysed annulation of conjugated diynes and arenes such as benzene, pyrrole, and indole derivatives.19 Computational investigations for elucidating the mechanism as well as redox and fluorescence properties of the pyrrolo[2,3-c]carbazoles are also presented.
We initially screened a variety of different gold catalysts (5 mol%) for the synthesis of benzo[c]carbazole using diyne 1a and anisole 8A (10 equiv.) (Table 1, entries 1–5). Ph3PAuSbF6 in 1,2-dichloroethane (DCE) did not promote even the first arylation of the desired transformation (entry 1). IPr, XPhos, and BrettPhos (Fig. 2) were ineffective ligands for bis-cyclisation; however, 3-phenylindole intermediate 2aA was formed in 36–65% yields (entries 2–4). Fortunately, the annulation reaction was promoted by JohnPhosAu(MeCN)SbF6 (entry 5) to provide the desired fused carbazole 3aA in 44% yield. Next, we examined the choice of reaction solvent using JohnPhosAu(MeCN)SbF6 as the catalyst. The reaction using benzene, propan-2-ol, and 1,4-dioxane gave the monocyclisation product 2aA (63–87% yields) without forming carbazole 3aA (entries 6–8). Carrying out the reaction in 1,1,2,2-tetrachloroethane (TCE) at 140 °C increased the yield of 3aA to 55% (entry 9). In this case, the decomposition of JohnPhosAu(MeCN)SbF6 at high reaction temperature was anticipated. Thus, the first arylation was conducted at 80 °C and, after the disappearance of the starting material and formation of 2aA (monitored by TLC), the reaction temperature was raised to 140 °C for the second arylation, giving rise to a higher yield of fused carbazole 3aA (75% yield, entry 10). Finally, an examination of the stoichiometry revealed that the reaction using excess anisole (as solvent) and 5 mol% BrettPhosAu(MeCN)SbF6 at 140 °C improved the yield to 86% (entry 12), whereas the reaction at 80 °C did not reach completion (entry 11). Thus, we used the conditions shown in entry 10 (10 equiv. of arene, condition A) and entry 12 (arene as the solvent, condition B) for further investigations of benzo[c]carbazole synthesis.
Entry | Catalystb | Solventc | Temperature (time) | Yieldd (%) | |
---|---|---|---|---|---|
3aA | 2aA | ||||
a Reactions were carried out using 1a (1 equiv.), 8A (10 equiv.), and the gold catalyst (5 mol%). b The ligand structures are shown in Fig. 2. BrettPhosAu(MeCN)SbF6, JohnPhosAu(MeCN)SbF6, and IPrAuNTf2 were prepared in advance. The other catalysts were prepared in situ by mixing the AuCl ligand with AgNTf2 or AgSbF6. c DCE = 1,2-dichloroethane, TCE = 1,1,2,2-tetrachloroethane. d Isolated yields. | |||||
1 | Ph3PAuCl/AgSbF6 | DCE | 80 °C (44 h) | 0 | 0 |
2 | IPrAuNTf2 | DCE | 80 °C (24 h) | 0 | 36 |
3 | XPhosAuCl/AgNTf2 | DCE | 80 °C (21 h) | 0 | 57 |
4 | BrettPhosAu(MeCN)SbF6 | DCE | 80 °C (30 h) | 0 | 65 |
5 | JohnPhosAu(MeCN)SbF6 | DCE | 80 °C (26 h) | 44 | 26 |
6 | JohnPhosAu(MeCN)SbF6 | Benzene | 80 °C (10 h) | 0 | 78 |
7 | JohnPhosAu(MeCN)SbF6 | Propan-2-ol | 80 °C (10 h) | 0 | 63 |
8 | JohnPhosAu(MeCN)SbF6 | 1,4-Dioxane | 80 °C (10 h) | 0 | 87 |
9 | JohnPhosAu(MeCN)SbF6 | TCE | 140 °C (13 h) | 55 | 0 |
10 | JohnPhosAu(MeCN)SbF 6 | TCE (condition A) | 80 °C (1 h), 140 °C (16 h) | 75 | 0 |
11 | BrettPhosAu(MeCN)SbF6 | Anisole | 80 °C (15 h) | 13 | 28 |
12 | BrettPhosAu(MeCN)SbF 6 | Anisole (condition B) | 140 °C (19.5 h) | 86 | 0 |
Using these optimised reaction conditions, we then explored the scope of the reaction. Variation of the substitution on the aryl moiety of nucleophile 8 was initially investigated (Table 2). 1,2-Dimethoxybenzene (8B) and 1,3-dimethoxybenzene (8C) served as suitable nucleophiles for gold-catalysed annulation when used as the solvent (condition B) to give benzo[c]carbazoles 3aB (quant) and 3aC (95%) in excellent yields. In these cases, the reaction using 10 equiv. of nucleophile (condition A) also permitted 70% and 40% yields of 3aB and 3aC, respectively. The reaction with benzodioxole (8D) afforded pentacyclic benzo[c]carbazole (3aD) in 76% yield. Less nucleophilic o-xylene (8E) also gave the corresponding benzo[c]carbazole (3aE) in moderate yield (42%), although an increased loading of the gold catalyst (20 mol%) was required. Benzene and toluene did not provide fused carbazoles owing to their lower reactivities. In all cases using arenes 8A–E, the cascade reaction proceeded in a regioselective manner: the first arylation occurred at the para-position of the electron-donating substituent of 8, and the second hydroarylation occurred at the less-sterically-hindered carbon of the introduced aryl group. We next investigated the reaction using various diynes 1b–g under condition B.21 A methyl substituent at the ortho-, meta-, or para-position of the terminal phenyl group was tolerated, producing the corresponding benzo[c]carbazoles (3bA–3dA) in good to excellent yields (70–94%). Similarly, the reaction of 1e–g bearing an electron-donating or -withdrawing group (Cl, NO2, or OMe) at the para-position gave the desired products 3eA–3gA (43–74% yield). The lower yield of the nitro derivative 3fA can be attributed to the less efficient coordination ability of the electron-deficient alkyne(s) to the gold catalyst, which would decrease the probability of the catalyst being activated.
Entry | Pyrrole | R | Time (h) | Yieldb (%) | Ratioc (6:7) |
---|---|---|---|---|---|
a Reaction conditions: 9 (5 equiv.), BrettPhosAu(MeCN)SbF6 (5 mol%), DCE, and 80 °C. b Combined isolated yields. c Determined by 1H NMR spectroscopy. d Contained small amounts of impurities. e Reaction carried out in TCE at 140 °C using 10 mol% of the catalyst. f Separation of the minor isomer from other by-products was difficult. | |||||
1 | 9A | H | 8 | <62%d | 25:75 |
2 | 9B | Bn | 10 | 62% | 18:82 |
3e | 9C | Ts | 0.5 | 34% | 58:42 |
4 | 9D | CO2Me | 1.5 | 62% | 81:19 |
5 | 9E | Piv | 1.5 | 60% | 82:18f |
6 | 9F | Boc | 1.5 | 60% | 92:8 |
The structural elucidation of 6aF and 7aF was unambiguously made by X-ray crystallographic analyses of the methylation products 6aF-Me2 and 7aF-Me2 (Fig. 3). The pyrrolocarbazole moiety adopted a planar geometry as expected, and the twist angle of the phenyl group was 71.1° (for 6aF-Me2) and 26.2–44.0° (for 7aF-Me2).23 The larger twist angle of the phenyl group in 6aF-Me2 was attributed to the presence of an N-methyl group in close proximity to the phenyl group.
Fig. 3 Synthesis and X-ray structures of dimethylated pyrrolocarbazoles. The phenyl group in the latter adopted two orientations in the crystal structure. |
We then optimised the reaction conditions for pyrrolocarbazole formation using diyne 1a, N-Boc-pyrrole 9F (5 equiv.), and various gold catalysts (5 mol%) (Table 4). Whereas Ph3PAuCl/AgNTf2 showed low reactivity (<5% yield, entry 1), other gold complexes bearing IPr, JohnPhos, XPhos, or BrettPhos as the ligand resulted in the formation of pyrrolo[2,3-c]carbazole 6aF in sufficient regioselectivities (>91:9) and moderate yields (55–62%, entries 2–5). Using the most efficient ligand BrettPhos in terms of regioselectivity (6:7 = 94:6, entry 5), two other silver salts were tested (AgSbF6 and AgOTf, entries 6 and 7, respectively); however, the regioselectivity was not improved. The use of a gold complex prepared in advance slightly improved the reactivity (reaction completed within 0.5 h) and regioselectivity (6:7 = 95:5, entries 8 and 9). Solvent screening and investigations of reaction temperature did not improve the yields and product ratios (see ESI†), whereas the reaction at 80 °C was found to be acceptable (entry 10). From these results, we used the conditions shown in entry 8 (condition C) and entry 10 (condition D) for further studies.
Entry | Catalyst | Time (h) | Yieldb (%) | Ratioc (6:7) |
---|---|---|---|---|
a Reaction conditions: 9F (5 equiv.), gold catalyst (5 mol%), TCE, and 110 °C. b Combined isolated yields. c Determined by 1H NMR spectroscopy. d Contained small amounts of impurities. e The reaction was conducted in DCE at 80 °C. | ||||
1 | Ph3PAuCl/AgNTf2 | 24 | <5d | 87:13 |
2 | IPrAuCl/AgNTf2 | 1 | 60 | 91:9 |
3 | JohnPhosAuCl/AgNTf2 | 1 | 56 | 92:8 |
4 | XPhosAuCl/AgNTf2 | 1 | 62 | 93:7 |
5 | BrettPhosAuCl/AgNTf2 | 1 | 55 | 94:6 |
6 | BrettPhosAuCl/AgSbF6 | 3 | 51 | 89:11 |
7 | BrettPhosAuCl/AgOTf | 20 | <12d | 75:25 |
8 | BrettPhosAu(MeCN)SbF6, (TCE, 110 °C: condition C) | 0.5 | 58 | 95:5 |
9 | BrettPhosAuNTf2 | 0.5 | 58 | 95:5 |
10 | BrettPhosAu(MeCN)SbF6 (DCE, 80 °C: condition D)e | 1.5 | 60 | 92:8 |
We subsequently investigated the scope of pyrrolo[2,3-c]carbazole formation (Table 5). The conjugated diynes 1b–i bearing electron-donating or -withdrawing substituents on both the aryl groups reacted smoothly with pyrrole 9F to afford the corresponding carbazoles 6bF–6iF under condition C. The position of the methyl group or introduction of chloro or methoxy substituents at the terminal aryl group did not significantly affect the reaction, and the desired annulation products were efficiently produced (6:7 = 95:5). The regioselectivity was slightly decreased when using electron-deficient diyne 1f substituted by a nitro group. Diynes 1h and 1i substituted by a cyano or methoxy group at the para-position to the azido group also showed relatively low selectivities (6:7 = 81:19–91:9).
a Reaction conditions: 9F (5 equiv.), BrettPhosAu(MeCN)SbF6 (5 mol%), TCE, and 110 °C (condition C). |
---|
We then applied indole derivatives as the nucleophile for the annulation reaction (Table 6). The reactions of azide-diyne 1a with N-protected indoles 10A–C (R1 = Boc, Piv, or CO2Et) under condition C regioselectively gave the indolo[2,3-c]carbazoles 11A–C as well as several unidentified by-products. The structure of 11A was confirmed by X-ray analysis after cleavage of the N-Boc group and dimethylation,24 similar to the cases of 6aF and 7aF (Fig. 3). Indoles possess reactive sites other than the desired 2- and 3-positions, which may cause undesired side reactions.16b Thus, the introduction of an electron-withdrawing group at the 5-position of indole was examined. As expected, indoles 10D–F bearing a bromo, chloro, or ethoxycarbonyl group at the 5-position reacted more efficiently to afford indolo[2,3-c]carbazoles 11D–F in better yields (50–67%) under condition D.
Competition experiments using two different arenes were then carried out (Scheme 6). The gold-catalysed reaction of 1a with anisole 8A (10 equiv.) and toluene 8F (10 equiv.) gave the anisole-derived products 2aA (30%) and 3aA (29%) along with a small amount of toluene derivative 2aF (2%) (eqn (1) in Scheme 6). Thus, the first arylation was highly dependent on the nucleophilicity of the arene.25 The competition between anisole 8A (5 equiv.) and N-Boc-pyrrole 9F (5 equiv.) led to the formation of the anisole-derived monocyclised product 2aA (27%) and pyrrole-derived biscyclised product 6aF (41%), the latter being the preferred product (eqn (2) in Scheme 6). This result suggested that N-Boc-pyrrole 9F was a slightly more efficient partner in the first arylation than anisole 8A, and that anisole-derived intermediate 2aA was significantly less reactive for the second arylation than the pyrrole-derived intermediate. The competition reaction using dimethoxybenzene 8B and N-Boc-pyrrole 9F gave the biscyclisation products 3aB (37%) and 6aF (34%) in comparable yields (eqn (3) in Scheme 6). This result suggested that the second arylation was accelerated by the additional methoxy group located at the para-position to the reacting carbon.26 We then examined the kinetic isotope effect (eqn (4) in Scheme 6). The competition reaction using N-Boc-pyrroles 9F (2.5 equiv.) and 9F-d4 (2.5 equiv.) under condition C gave the corresponding pyrrolocarbazoles 6aF and 7aF, where the D/H ratios were 1:1 in both products. Thus, deprotonation was not the rate-determining step for the formation of these products. This result suggested that electrophilic aromatic substitution was more likely for the first arylation than C–H insertion.27
To further elucidate the reaction mechanism, we undertook density functional theory (DFT) calculations. The calculations were conducted at the M06L/6-31G** (for H, C, N, and P) and SDD (for Au) levels using the formation of pyrrolo[2,3-c]carbazole from 1a and N-methylpyrrole as the model reaction (Fig. 4A). As previously proposed,16 the reaction is initiated by the intramolecular nucleophilic attack of the azide group on the activated alkyne through TS1/2 to form an indolyl-gold intermediate INT2 with a small barrier of 10.9 kcal mol−1 and a rather large endothermicity (10.5 kcal mol−1 higher than INT1). This unfavourable energy loss is compensated for by successive reaction(s). INT2 ejects nitrogen to form a gold carbenoid intermediate INT3-1 with a large stabilisation energy (45.6 kcal mol−1). Next, the key arylation step occurs by the intermolecular nucleophilic attack of N-methylpyrrole on the gold carbenoid INT3-2 through TS3/4, with a small barrier of 1.4 kcal mol−1, to produce INT4. The gold rearrangement from C to N, with a reasonable barrier of 16.6 kcal mol−1, gives an N-aurated indole intermediate INT5-1. This occurs with the simultaneous re-aromatisation, protodeauration, and re-complexation of the gold catalyst with the internal acetylene, and exothermically provides the pyrrole-substituted indole intermediate INT5-2. Finally, 6-endo-dig cyclisation of INT5-2 is promoted by the gold catalyst to produce pyrrolo[2,3-c]carbazole (PD), which regenerates the active gold catalyst. The entire reaction profile is illustrated in Fig. 4B. All the transition states have reasonable energy barriers (1.4–13.1 kcal mol−1). The overall exothermicity is very large because of the formation of one C–N bond, two C–C bonds, and two aromatic rings. This provides the driving force for the overall reaction.
Fig. 4 DFT calculations for cyclisation of 1a with N-methylpyrrole [M06L/6-31G** (H, C, N, P) & SDD (Au)]. |
Entry | Compound | R | Oxidation potentiala |
---|---|---|---|
a E/V vs. SCE, CH2Cl2 containing 0.1 M Bu4NPF6, Pt electrode, and 100 mV s−1. Eox = Epa − 0.03 V (for entries 1–5). E(Fc/Fc+) = +0.53 V under similar conditions. b Reversible redox reaction was observed. | |||
1 | 6aF-H | H | +0.85 |
2 | 6aF-Me2 | Me | +0.79 |
3 | 7aF-H | H | +0.73 |
4 | 7aF-Me 2 | Me | +0.75 |
5 | 11A-Me 2 | — | +0.80b |
Upon the electrochemical oxidation of 11A-Me2 in CH2Cl2, the colourless solution turned green, which demonstrated its electrochromic nature. A continuous change in ultraviolet-visible-near-infrared (UV-Vis-NIR) absorption was accompanied by several isosbestic points, indicating that 11A-Me2 was cleanly oxidised into the corresponding cation radical species (Fig. 6). Wurster's blue exhibits absorption only in the visible region (λ < 700 nm). Thus, the observed red shift was induced through π-extension by the fusion of the indole rings. The carbazole skeleton gives rise to fluorescence properties,30 so the electrolysis of 11A-Me2 also caused a change in the fluorescence (FL) spectrum [λem 424, 442 (sh) nm in CH2Cl2 (λex 354 nm)]. The steady decrease in fluorescence with increasing electrochemical oxidation time could be rationalised by the non-fluorescent nature of its cation radical. Such dual electrochromism in which changes occur in both UV-Vis-NIR and FL spectra is rare,30 but was also realised in our previous study on benzo[g]indolo[2,3-c]carbazole derivatives,9c–e which were synthesised through a different mode of the gold(I)-catalysed cascade reaction.9a Thus, the gold-catalysed synthesis of annulated carbazoles is a powerful tool for exploring the little developed category of advanced electrochromic systems.
Footnotes |
† Electronic supplementary information (ESI) available: Experimental details and spectral data are provided. CCDC 1860899 and 1860900 (6aF-Me2), 1860901 and 1860902 (7aF-Me2), and 1860904 and 1860905 (11A-Me2). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c8sc03525c |
‡ Y. K. and S.O. contributed equally. |
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