Fe-catalyzed cycloaddition of indoles and o-phthalaldehyde for the synthesis of benzo[b]carbazoles with TMSCl- or acid-responsive properties

Abstract

The one-pot iron-catalyzed cycloaddition of indole and o-phthalaldehyde afforded indolyl benzo[b]carbazoles via sequential carbon–carbon bond-forming addition, cyclization involving intramolecular alkylation and aromatization forming a benzene ring. In addition, the fluorescence properties of such indolyl benzo[b]carbazoles were investigated, in which significant changes in fluorescent intensity were observed upon the addition of trimethylchlorosilane (TMSCl) or trifluoroacetic acid (TFA).

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Carbazoles are important alkaloids exhibiting significant biological activities and physicochemical properties.¹ In the past years, the construction of carbazoles and its derivatives has attracted considerable attention because of their applications in pharmacological chemistry and material science.² For example, carbazole derivatives are widely used as building blocks for potential organic light-emitting diodes (OLED)³ and electroluminescent materials.⁴ Recently, they have been observed as potential anticancer agents,⁵ and especially in 2011, benzocarbazole analogues were discovered to be highly selective and potent anaplastic lymphoma kinase (ALK) inhibitors.⁶

Consequently, the syntheses of carbazoles and benzocarbazoles have been extensively studied in the past decades.⁷ Notably, in this context, Ma and co-workers recently reported a route to construct carbazole alkaloids via the PtCl₂-catalyzed cyclization of 1-(indol-2-yl)-2,3-allenols.^7h,i In particular, a number of useful synthetic approaches are available now for the catalytic synthesis of benzo[b]carbazoles.⁸ For instance, Tsuchimoto and Shirakawa^2a reported a concise synthesis of benzocarbazoles through indium-catalyzed annulation of 2-arylindoles with propargyl ethers. Saá and coworkers^8a described an approach to carbazoles by intramolecular dehydro Diels–Alder reactions of ynamides, which allowed the synthesis of carbazoles and benz-annulated carbazoles in moderate to good yields. Snieckus et al.^8b found that a general anionic N–C carbamoyl migration of 2-arylindoles offered an alternative to benzocarbazoles in low to moderate yields. Recently, Turner and Procter^8e developed a new synthetic strategy for benzo[b]carbazoles in which the induction and removal of a phase tag in combination with tag-assisted purification has been used to trigger cyclization events in the synthesis of benzo[b]carbazole end-capped oligothiophenes. On the other hand, intramolecular cyclodehydration has also attracted considerable attention for their application in the construction of benzo[b]carbazole-based building blocks. As an example, Jana and coworkers^8j demonstrated an iron(III)-catalyzed novel and efficient strategy for the synthesis of structurally diverse benzo[b]carbazoles in which the intramolecular domino isomerization/cyclization/aromatic reaction substituted 2-[(indoline-3-ylidene)(methyl)]benzaldehyde derivatives could be easily obtained in the presence of FeCl₃. However, many of these previous protocols have some limitations such as difficulty in the preparation of the starting substrates, lengthy synthetic sequences, harsh reaction conditions, and unsatisfactory chemical yields calculated from commercially available materials. Therefore, the development of new approaches for the construction of benzo[b]carbazoles derivatives is highly desirable. Here, we report a facile process for the synthesis of indolyl benzo[b]carbazoles using easily available compounds via the intermolecular cycloaddition of indoles and o-phthalaldehyde. In addition, the novel indolyl benzo[b]carbazoles, which are readily prepared by the current one-pot approach, are revealed to exhibit very strong fluorescence intensity because of the presence of π-conjugated system in benzo[b]carbazoles.

Our initial investigation began with the coupling reaction of indole 1a with o-phthalaldehyde 2 in the presence of TsOH (4-methylbenzenesulfonic acid). Interestingly, when the reaction was carried out in various solvents at room temperature with 10 mol% of TsOH as catalyst, two different indolyl benzo[b]carbazoles 3a and 4a were obtained unexpectedly in varied ratios.⁹ Under the Brønsted acid catalysis, the reaction of indole 1a and o-phthalaldehyde led to the formation of a mixture of 3a and 4a (entries 1–4, Table 1). Although the total yield was excellent in acetonitrile, there was no selectivity in the domino addition/cyclodehydration reaction. Dichloromethane and tetrahydrofuran resulted in the 4a-selective product in the ratio of 45/55 to 18/82 (3a/4a), while methanol facilitated the formation of 3a in good selectivity (3a/4a = 87/13, entry 1 of Table 1). These preliminary results of this reaction prompted us to investigate the effect of various Brønsted acid or Lewis acid catalysts, which were expected to determine a highly efficient catalyst system for the synthesis of atropisomeric benzo[b]carbazoles 3a or 4a. To test the abovementioned idea, we attempted to carry out the screening experiment. As shown in Table 1, representative and commercially available Brønsted acid or Lewis acid catalysts provided different results in terms of yield and chemoselectivity. Except THF, almost all the catalysts investigated in this work resulted in the formation of major product 4a and minor product 3a in non-protic solvents (such as CH₃CN and DCM). For example, catalytic amount of I₂ gave the mixture of 3a/4a in 67% yield (entry 5, 3a/4a = 37/63), and other Lewis acid or Brønsted acid catalysts, including FeCl₂, Al(OTf)₃, Bi(OTf)₃, TfOH, and TMSOTf, gave similar selectivities (up to 27/73 of 3a/4a, entries 7, 13, 15, 18 and 20, respectively). Fortunately, it was found that FeCl₂ was a good catalyst for the coupling reaction of indole 1a and o-phthalaldehyde 2 in methanol. High yield and good selectivity were achieved under mild reaction conditions (80% yield and 91/9 of 3a/4a).

Table 1 Optimization of Lewis acid-catalyzed cycloaddition of indole (1a) and o-phthalaldehyde (2)^a

Run	Cat.	Solvent	Temperature (°C)	Time (h)	3a/4a (S3/4)^b	Yield^c (%)
a Reaction conditions: 1a (0.5 mmol), 2a (3.5 mmol), Brønsted acid or Lewis acid catalyst (10 mol%).b The ratio of 3 to 4 (also simplified as S3/4) was determined by ^1H-NMR.c Isolated yields.d Not determined.
1	TsOH	MeOH	RT	24	87/13	80
2	TsOH	CH3CN	RT	24	50/50	88
3	TsOH	THF	RT	24	45/55	54
4	TsOH	DCM	RT	24	18/82	67
5	I2	CH3CN	RT	24	37/63	78
6	FeCl2	MeOH	RT	48	91/9	80
7	FeCl2	DCM	RT	48	27/73	59
8	FeCl2	CH3CN	RT	48	34/66	88
9	FeCl2	THF	RT	48	52/48	43
10	FeCl2	H2O	RT	48	ND^d	17
11	InCl3	THF	RT	48	ND^d	<10
12	Al(OTf)3	THF	RT	24	52/48	46
13	Al(OTf)3	CH3CN	RT	24	32/68	70
14	Bi(OTf)3	THF	RT	24	52/48	49
15	Bi(OTf)3	CH3CN	RT	24	34/66	91
16	Bi(OTf)3	CH3OH	RT	24	83/17	83
17	Bi(OTf)3	EtOH	RT	24	83/17	83
18	TfOH	CH3CN	RT	24	32/68	93
19	TfOH	THF	RT	24	50/50	50
20	TMSOTf	CH3CN	RT	24	33/67	86
21	Mg(OTf)2	THF	RT	24	ND	<10
22	FeCl3	MeOH	RT	24	86/14	90
23	FeCl3	THF	RT	24	54/46	60

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Subsequent studies (entries 7–23) further supported the important role of FeCl₂ in this reaction because a significant amount of investigation using various catalysts revealed the difficulty in the improvement of chemoselectivity for the product 3a. Our other goal was the highly selective synthesis of isomer 4a of the interesting benzo[b]carbazole, but all attempts to prepare benzo[b]carbazole 4a with higher chemoselectivity (>90/10 of 4a/3a) failed. Notably, pure benzo[b]carbazole 3 could be obtained by recrystallization, thus providing a novel backbone for the development of fluorescent molecules. Therefore, FeCl₂ (10 mol%) in MeOH at room temperature has been defined as the optimized reaction for the possible study of the synthesis of benzo[b]carbazole 3.

Under optimal reaction conditions, we then investigated various indoles in these reactions. As shown in Scheme 1, the substituted indoles were also proven to be effective substrates under optimal reaction conditions, and moderate to good yields were observed for the gram-scale synthesis of the corresponding indolyl benzo[b]carbazoles. However, no reaction occurred for 3-substituted indoles, indicating the key nucleophilic effect on the reactivity of 3-position on indoles. When 5-methylindole was employed, the corresponding product 3b was obtained in 79% yield. The reaction yield decreased dramatically when the methyl group was located at the C-7 position in the indole ring (3c, 48% yield). Slightly higher yields could be achieved when 5-methoxylindole was employed in this reaction (3d, 85% yield). Interestingly, when the C-5 position of the indole ring was occupied by a halo group, the desired products were obtained in good yields. Therefore, this method provides a simple and facile approach for the straightforward synthesis of indolyl benzo[b]carbazoles with good selectivities (Scheme 1).

Scheme 1 Synthesis of indolyl benzo[b]carbazoles by Fe-catalyzed coupling of indoles and o-phthalaldehyde.

Based on these results and previous reports on iron catalysis,¹⁰ a plausible mechanism for this intermolecular cycloaddition of indoles and o-phthalaldehyde towards divergent indolyl benzo[b]carbazoles is proposed in Scheme 2. Initially, Lewis acidic FeCl₂ would activate o-phthalaldehyde to give possible intermediate I, followed by the addition of indole to intermediate I to give an adduct II. Subsequently, another molecule of indole attacked the activated intermediate II containing aldehyde moiety, which led to the formation of diol III. Then, Lewis acid-catalyzed intramolecular alkylation of III would furnish cyclization to give intermediate IV. Because the catalytic dehydration of compound IV in the presence of Lewis acid easily occurred, the oxidative aromatization of IV may occur smoothly with the aid of FeCl₂ to give the product 3 (path A of Scheme 2). As a different pathway, the formation of the minor product 4 may arise from the key alkylation of intermediate V with indole in the presence of Lewis acidic FeCl₂. Thus, the formation of key intermediate VI would lead to the intramolecular and desymmetric addition of indole motif to aldehyde group, followed by dehydration/aromatization to generate the desired compound 4 (path B of Scheme 2).

Scheme 2 Proposed mechanism (path A or B) for iron-catalyzed synthesis of benzo[b]carbazole 3a or 4a from indole and o-phthalaldehyde.

With the successful synthesis of indolyl benzo[b]carbazoles 3, we speculated whether such conjugated heterocycles could exhibit special photophysical properties and be used as a fluorescent probe for Lewis acid or Brønsted acid because of the NH-group on the indole ring. In the past years, the fluorescence-based detection technique has been the most powerful method in the field of sensors, information displays, chirality chemistry, and catalysis.¹¹ Especially, various fluorescent sensors provide a simple and efficient method for the detection of a wide range of chemical species in many diverse areas, including host–guest chemistry, pharmaceutical and biological science, industry, and environmental science.¹² However, to the best of our knowledge, no such fluorescent probe for silicon-based Lewis acids has been reported yet. In addition, there are no reports on the application of indolyl benzo[b]carbazoles as a fluorescent probe in analytical chemistry.

With regard to fluorescence properties, it was found that their fluorescent spectra excited at three different wavelengths (307 nm, 425–445 nm, and 604 nm). Moreover, all the indolyl benzo[b]carbazoles show a strongly intense emission with a maximum wavelength of 307 nm that may be attributed to the possibility of photoinduced electron transfer (PET) involving nitrogen lone pairs of the indole moiety of indolyl benzo[b]carbazoles (Fig. 1). Similarly, the benzo[b]carbazole moiety of compounds 3a–e was crucial to the 425–445 nm excitation. Notably, indolyl benzo[b]carbazole 3e containing bromide group exhibited strong fluorescence in emission intensity at 307 nm but showed quite weak intensity between 425–445 nm. These results suggest that these indolyl benzo[b]carbazoles are effective building backbones for the π-electron systems with well-extended π-conjugation.

Fig. 1 Fluorescence emission spectra of indolyl benzo[b]carbazoles 3a–f (5 × 10⁻⁶ M) in CH₃CN.

To elucidate the electronic structure of indolyl benzo[b]carbazoles 3, density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) level were performed for representative compounds 3a, 3b, 3d, and 3e. As shown in Table 2, the HOMO of compounds 3a–e is delocalized in the benzo[b]carbazole core with the contribution of π-conjugated skeleton. Despite the substitution of electron-donor groups on this skeleton, the HOMO level of bromo-substituted benzo[b]carbazole 3e is 0.27–0.49 eV lower than that of 3a, 3b, or 3d, which is indicative of the electron-withdrawing effect or halide p–π conjugation of the substitution. Similarly, the characteristics of the LUMOs for 3a–e are almost the same and are mainly localized on the benzo[b]carbazole core. As a result, 3e could be used as a representative indolyl benzo[b]carbazole in the subsequent FL analysis.

Table 2 Frontier orbital energy (in eV) calculated at B3LYP/6-31G(d,p) level of theory¹³

Compounds	LUMO (eV)	HOMO (eV)
3a (R = H)
3b (R = Me)
3d (R = OMe)
3e (R = Br)

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Considering the FL emission peak at 307 nm, we expected that the interaction of metal-free Lewis acid or Brønsted acid (guests) with the indolyl benzo[b]carbazoles would result in significant changes to their excited state properties because of the Lewis basic NH-group on the backbone of indole (Fig. 2). To evaluate the effects of organic Lewis acid or Brønsted acid on the photophysical properties of indolyl benzo[b]carbazole 3e, their fluorescent spectra were measured using trimethylchlorosilane (TMSCl) or trifluoroacetic acid (TFA) titration. In this process, the addition of TFA resulted in the decreasing emission intensity of the bands at 307 and 430 nm. Upon the addition of TFA to indolyl benzo[b]carbazole 3e, the lowest emission peak was observed at 307 nm or 430 nm when 40 eq. of TFA was used. Such changes suggest that the acid–base interaction involving the nitrogen lone pairs of indolyl benzo[b]carbazoles is responsible for the FL changes observed with excess TFA. Interestingly, the addition of a larger amount of TFA in this solution of 3e (>40 eq.) turns on fluorescence slightly (Fig. S2 of ESI†), which could be attributed to the anion–π or cation–π interaction between TFA and the aromatic rings of protonated indolyl benzo[b]carbazole 3e.

Fig. 2 Working hypothesis for the turning off of fluorescence emission of indolyl benzo[b]carbazole 3e with trimethylchlorosilane (TMSCl) or trifluoroacetic acid (TFA).

The benzo[b]carbazole 3e also exhibited an FL change with the addition of TMSCl. Similarly, the fluorescence emission intensity of 3e decreased as the concentration of TMSCl increased, as shown in Fig. 3. Thus, indolyl benzo[b]carbazole exhibited a high sensitivity toward organic Lewis acid or Brønsted acid, quenching 94% of its fluorescence intensity with 40 eq. of TMSCl or TFA. The FL system was further extended to estimate TMSCl in the presence of triethylamine (see Fig. S4 and S5 of ESI†). The fluorescence titration of indolyl benzo[b]carbazole 3e with TMSCl/Et₃N (Fig. S5†) or the mixture of indolyl benzo[b]carbazole 3e and TMSCl (40 eq.) with Et₃N (Fig. S4†) was carried out. Interestingly, upon the addition of TMSCl and Et₃N (1 : 1), the fluorescence emission intensity was almost identical to that obtained in the presence of TMSCl. Notably, the fluorescence response of the mixture of indolyl benzo[b]carbazole 3e with TMSCl (1/40) toward Et₃N was concentration-dependent, and remarkable fluorescence turn-on was observed in the presence of low or high concentration of Et₃N, whereas 40 eq. Et₃N did not cause any interference in the estimation of the quenching of 3e with TMSCl (see Fig. S4 of ESI†). This also indicates that the strange FL properties or the 40-fold-effect in this work can be used to support the idea that highly complex and multiple parameters can impact the outcomes of intermolecular interaction between indolyl benzo[b]carbazole and metal-free Lewis acid or Brønsted acid.

Fig. 3 Changes observed in the fluorescence emission spectrum of 3e (5 × 10⁻⁶ M) upon the addition of TMSCl (1–100 eq.) in CH₃CN.

In summary, we have developed an iron-catalyzed one-pot cycloaddition for the synthesis of indolyl benzo[b]carbazoles from indole and o-phthalaldehyde. The coupling/cyclization approach is environmentally benign and proceeds via sequential carbon–carbon bond-forming addition, cyclization involving intramolecular alkylation and aromatization forming a benzene ring. The possible mechanism of divergent processes was also proposed on the basis of experimental results. In addition, preliminary results regarding the fluorescence properties of such indolyl benzo[b]carbazoles were also investigated. It showed an extremely high selectivity for TFA or TMSCl with quenching 94% of its fluorescence intensity. Although the exact reason for abnormal intramolecular interaction between indolyl benzo[b]carbazole and 40 eq. TMSCl (or TFA) is unclear at present, the sensing protocol can be applicable for the quantification of TMSCl or TFA in consumer products. Nevertheless, this type of benzo[b]carbazoles forms a rare example of a fluorescent sensor containing indole moiety, which would contribute to the development of fluorescent sensors or dyes. Furthermore, this chemistry would be of considerable interest to chemists in diverse areas for further applications in this area. Further investigations on the chemistry of indolyl benzo[b]carbazole are currently undergoing in our laboratory.

Acknowledgements

The authors gratefully thank the financial support from the National Natural Science Foundation of China (NSFC, no. 51203037 and 21173064), the Zhejiang Provincial Natural Science Foundation of China (LR14B030001 and LQ12B02004), and the Hangzhou Science and Technology Program (20130432B06).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra08012b

‡ Mr H. Wang and Miss J. F. Zou contributed equally to this work.

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