Rh-Catalyzed C–C/C–N bond formation via C–H activation: synthesis of 2H-indazol-2-yl-benzo[a]carbazoles

Sundaravel Vivek Kumar, Sonbidya Banerjee and Tharmalingam Punniyamurthy*
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati 781039, India. E-mail: tpunni@iitg.ac.in

Received 10th September 2019 , Accepted 13th October 2019

First published on 14th October 2019


Rh-Catalyzed tandem site-selective C–H activation and ring opening/cyclization of 7-azabenzonorbornadienes with aryl-2H-indazoles is developed to furnish indazolyl-benzocarbazoles. The mechanistic studies suggest that the second C–H activation is the product-determining step. The use of an air stable cationic Rh-catalyst and site-selectivity, and substrate scope are the important practical features.


Introduction

Transition-metal-catalyzed C–H activation occupies a prominent position in modern organic synthesis and has been broadly used for the construction of site-selective carbon–carbon and carbon–heteroatom bonds in an atom economical way.1,2 Specifically, the blending of C–H activation with strained small ring compounds has recently attracted attention and emerged as a powerful synthetic tool for the generation of structurally diverse scaffolds.3,4 The Li group reported the oxidative annulation of arenes with azabenzonorbornadienes using an Rh/Ag catalytic system (Scheme 1a).4b Recently, they have shown the asymmetric coupling of benzamide with azabenzonorbornadiene (Scheme 1b).4k Indazoles and their derivatives obtained are important structural scaffolds due to their potential biological5 and material applications.6 In addition, they are widely used7 as organic light-emitting diodes7a and photoredox catalysts.7b However, investigation on their functionalization is scarce. While, benzocarbazoles are vital structural frameworks, which have exhibited significant biological,8 photophysical and electronic properties.9 Recently, we reported a roll-over cyclometallation of indazoles with alkynes using Rh-catalysis (Scheme 1c).10 During the preparation of this manuscript, the Zhang group showed a roll-over cyclometallation using Rh-catalysis for fused polyheterocycles (Scheme 1d).11 In this report, we exemplify the successful realization of non-rollover cyclometallation using the air stable cationic Rh(III)-complex for the site-selective (C-2′ and C-3′) and C–C/C–N bond formation by the reaction of aryl-2H-indazoles with azabenzonorbornadienes to give indazolyl-dihydro-11H-benzo[a]-carbazole scaffolds (Scheme 1e). Under these conditions, the other possible roll-over (C-2′ and C-3) C–H activation was not observed. The utilization of an air stable Rh-catalyst, selectivity and late stage functionalization are important features.
image file: c9qo01120j-s1.tif
Scheme 1 Summary of [3 + 2] annulation with aza-bicyclic olefins and functionalization of indazoles using C–H activation.

Results and discussion

We began the investigation on optimizing the reaction parameters employing 7-azabenzonorbornadiene 1a and 2-phenyl-2H-indazole 2a as the model substrates (Table 1). To our delight, the reaction occurred to furnish 3a in 23% yield when the substrates were reacted utilizing 4 mol% [Cp*RhCl2]2 as a catalyst, 2.5 equiv. Cu(OAc)2·H2O as an oxidant and 1 equiv. CsOAc as an additive in (CH2Cl)2 at 110 °C for 24 h under N2 (entry 1). Interestingly, the yield increased to 50% when the reaction was performed using AgSbF6, which may be due to the involvement of the cationic RhIII catalysis (entry 2). Subsequent screening revealed that [Cp*Rh(CH3CN)3](SbF6)2 (5 mol%) catalyzed the reaction more efficiently to furnish 61% yield (entry 3). In a set of oxidants studied, Cu(OAC)2·H2O was superior to AgOAc, Ag2CO3 and Ag2O (entries 4–7). 1,2-Dichloroethane was the solvent of choice, whereas CH3CN, THF, 1,4-dioxane, toluene and chlorobenzene gave inferior results (entries 8–12). Among the additives investigated, CsOAc, KOAc, Cs2CO3 and CsOPiv, the former afforded the best results (entries 13–15). Control experiments confirmed that the optimum temperature is 110 °C and 2.5 equiv. Cu(OAc)2·H2O afforded the best results (entries 16–19). The combination of a Rh-catalyst and Cu(OAc)2·H2O was essential and, in their absence, the formation of 3 was not observed (entries 20 and 21).
Table 1 Optimization of the reaction conditionsa,b

image file: c9qo01120j-u1.tif

Entry Oxidant Additive Solvent 3a[thin space (1/6-em)]b, %
a Reaction conditions: 1a (0.25 mmol), 2a (0.3 mmol), [Cp*Rh(CH3CN)3](SbF6)2 (5 mol%), oxidant (0.62 mmol), additive (0.25 mmol), solvent (2 mL), 110 °C, 24 h, N2.b Isolated yield.c 4 mol% [Cp*RhCl2]2 used.d 4 mol% [Cp*RhCl2]2 and 20 mol% AgSbF6 used.e 120 °C.f 100 °C.g Cu(OAc)2·H2O (0.75 mmol) used.h Cu(OAc)2·H2O (0.37 mmol) used.i No Rh catalyst. n.d. = not detected.
1c Cu(OAc)2·H2O CsOAc (CH2Cl)2 23
2d Cu(OAc)2·H2O CsOAc (CH2Cl)2 50
3 Cu(OAc)2·H2O CsOAc (CH2Cl)2 61
4 Cu(OPiv)2 CsOAc (CH2Cl)2 46
5 AgOAc CsOAc (CH2Cl)2 29
6 Ag2CO3 CsOAc (CH2Cl)2 n.d.
7 Ag2O CsOAc (CH2Cl)2 n.d.
8 Cu(OAc)2·H2O CsOAc CH3CN 32
9 Cu(OAc)2·H2O CsOAc THF 45
10 Cu(OAc)2·H2O CsOAc 1,4-Dioxane n.d.
11 Cu(OAc)2·H2O CsOAc Toluene n.d.
12 Cu(OAc)2·H2O CsOAc Cl-Benzene Trace
13 Cu(OAc)2·H2O KOAc (CH2Cl)2 26
14 Cu(OAc)2·H2O Cs2CO3 (CH2Cl)2 12
15 Cu(OAc)2·H2O CsOPiv (CH2Cl)2 43
16e Cu(OAc)2·H2O CsOAc (CH2Cl)2 60
17f Cu(OAc)2·H2O CsOAc (CH2Cl)2 52
18g Cu(OAc)2·H2O CsOAc (CH2Cl)2 61
19h Cu(OAc)2·H2O CsOAc (CH2Cl)2 44
20i Cu(OAc)2·H2O CsOAc (CH2Cl)2 n.d.
21 CsOAc (CH2Cl)2 Trace


With the optimized conditions, the scope of the procedure was examined for the reaction of a series of substituted azabenzonorbornadienes 1b–r with 2-phenyl-2H-indazole 2a as a standard substrate (Table 2). The reaction of the substrates bearing electronically varied substituents in the aryl ring of the N-arylsulfonyl motif was well tolerated. The substrates bearing 3-methyl 1b and 3-trifluoromethyl 1c groups underwent reaction to provide 3b and 3c in 64 and 66% yields, respectively. Similar results were observed with the substrates substituted at the 4-position with bromo 1d, chloro 1e, iodo 1f, methyl 1g, methoxy 1h, nitro 1i and tert-butyl 1j functionalities, furnishing 3d–j in 47–63% yields. The stereochemistry of the products was determined using the single-crystal X-ray analysis of 3g. The substrates with 2-naphthyl 2k and 2-thiophene 1l substituents in the N-sulfonyl group worked well to afford 3k and 3l in 65 and 70% yields, respectively, while 1m bearing a pyridine-2-sulfonyl substituent yielded a trace amount of 3m, which may be due to the chelation of the pyridine nitrogen with the Rh-catalyst. However, the reaction of the substrates bearing methylsulfonyl 1n, N-Boc 1o and N-Cbz 1p protecting groups produced 3n–p in 37–62% yields, whereas the substrates having difluoro 1q and dimethoxy 1r groups in aryl ring A underwent reaction to produce the desired products 3q and 3r in 72 and 66% yields, respectively. However, the reaction using 7-azanorbornadiene 1s was not successful to yield 3s, which suggests that the presence of an aromatic ring enhances the reactivity.

Table 2 Reaction of 7-azabenzonorbornadienes 1 with indazole 2a[thin space (1/6-em)]a,b
a Reaction conditions: 1b–l (0.25 mmol), 2a (0.3 mmol), [Cp*Rh(CH3CN)3](SbF6)2 (5 mol%), Cu(OAc)2·H2O (0.62 mmol), CsOAc (0.25 mmol), (CH2Cl)2 (2 mL), 110 °C, 24 h, N2.b Isolated yield. Ind = 2H-indazole.
image file: c9qo01120j-u2.tif


Next, the scope of the procedure was investigated for the reaction of substituted aryl-2H-indazoles 2b–r with 1a as a standard substrate (Table 3). The substrates bearing 2-methoxy 2b and 2-difluoromethoxy 2c groups in ring B underwent reaction to deliver the target products 3t and 3u in 71 and 76% yields, respectively, whereas 2d having a 3-acetyl group failed to afford 3v, which may be due to the coordination of the acetyl group to the catalyst. However, the substrates having 3-chloro 2e, 3-fluoro 2f, 3-methyl 2g and 3-methoxy 2h groups proved to be excellent to provide the target products 3w–z in 72–80% yields. The substrates with strong electron-demanding groups such as 3-trifluoromethyl 2i and 3-nitro 2j substituents exhibited moderate reactivity, affording 3aa and 3ab in 43 and 51% yields, respectively. This is because the acidity of the H-6 proton in the C–H activation product is increased due to the electron withdrawing effect and consequently acetate may mediate the elimination reaction to afford sulfonamides 3aa and 3ab. In addition, the reaction of 2,3-cyclopentane fused substrate 2k provided the target product 3ac in 61% yield. Under these conditions, the substrates having 4-fluoro 2l, 4-chloro 2m and 4-methyl 2n groups failed to produce 3ad–af due to the electronic and steric factors. Similarly, the reaction of a less reactive pyridine 2o backbone gave a trace amount of 3ag, which may be due to the coordination of Rh with pyridine nitrogen. Attention was next turned towards the reactivity of the substrates bearing substituents in the C ring. The reaction of 5-fluoro 2p and 5-methoxy 2q bearing substrates efficiently delivered the desired products 3ah and 3ai in 68 and 49% yields, respectively. Under these conditions, 2-(2,5-dimethylphenyl)-2H-indazole 2u showed no reaction, which may be due to the steric hindrance of the methyl groups. However, 2-benzyl-2H-indazoles 2r–t reacted to produce 3aj–al in 58–65% yields, whereas the substrates with pyrazole 2v, imidazopyridine 2w and tetrazole 2x directing groups were unsuccessful, which suggest that the electronic nature of the chelating group is crucial.

Table 3 Reaction of 2H-indazoles 2 with 7-azabenzonorbornadiene 1a[thin space (1/6-em)]a,b
a Reaction conditions: 1a (0.3 mmol), 2b–r (0.25 mmol), [Cp*Rh(CH3CN)3](SbF6)2 (5 mol%), Cu(OAc)2·H2O (0.62 mmol), CsOAc (0.25 mmol), (CH2Cl)2 (2 mL), 110 °C, 24 h, N2.b Isolated yield. Ind = 2H-indazole. n.d. = not detected.
image file: c9qo01120j-u3.tif


To illustrate their synthetic utilities, some interesting transformations were performed (Scheme 2). For instance, 3a can be converted into 4a in 95% yield employing Pd-catalyzed hydrogenation. It is pertinent to note that 1,3-diene12 of indazole and dihydronaphthalene unit were selectively reduced. In addition, 3a can be converted into the heterocycle 6a in 73% yield via Rh-catalyzed roll-over double C–H activation, while the base mediated elimination reaction of 3a afforded 7a in 75% yield that can be converted into 8a in 78% yield using the Cu-catalyzed oxidative C–N bond formation,13 which on treatment with the base delivered 9a in 82% yield.


image file: c9qo01120j-s2.tif
Scheme 2 Synthetic transformation of 3a: Reaction conditions: (i) 3a (0.15 mmol), 10% Pd/C, H2 (balloon), EtOAc, rt, 24 h; (ii) 3a (0.15 mmol), 5 (0.18 mmol), 4 mol% [Cp*RhCl2]2, Cu(OAc)2·H2O (0.18 mmol), K2CO3 (0.15 mmol), (CH2Cl)2 (1.5 ml), 110° C, 10 h, N2; (iii) 3a (0.20 mmol), NaOH (0.30 mmol), MeOH[thin space (1/6-em)]:[thin space (1/6-em)]CH2Cl2 (4[thin space (1/6-em)]:[thin space (1/6-em)]1 v/v), reflux, 8 h; (iv) 7a (0.15 mmol), Cu(OTf)2 (5 mol%), PhI(OAc)2 (2 equiv.), CF3CO2H (0.45 mmol), (CH2Cl)2 (3 mL), 70 °C, 20 h.

To gain insight into the catalytic pathway, a series of control experiments were performed. The H–D exchange studies with 2a using D2O as a co-solvent produced C-2′ with 58% D and C-3 with 51% D incorporation (Scheme 3a), whereas the reaction of 1a and 2a using D2O furnished 3a with C-2′ in 40% D and C-3 in 28% D incorporation (Scheme 3b). These results indicated that the initial C–H activation step is reversible. Next, the one-pot kinetic isotope experiment14 yielded [PH/PD] = 1.56, while the parallel and intramolecular studies provided kH/kD = 1.03 and [PH/PD] = 1.36, respectively (Scheme 4a, b, c–i). These studies indicated that the first C–H activation may not be the rate-determining step. Furthermore, the intramolecular kinetic isotope experiment with meta-deuterated indazole m-[D]-2a showed [PH/PD] = 3.03 (Scheme 4c-ii), which suggests that the second C–H activation might be the product-determining step. In addition, the reaction of [Cp*Rh(CH3CN)3](SbF6)2 with indazole 2a (1[thin space (1/6-em)]:[thin space (1/6-em)]1) produced the rhodacycle 10 in 61% yield, which readily catalyzed the reaction of 1a and 2a to furnish 3a in 54% yield (Scheme 4d). These results suggest that the Rh-complex first reacts with indazole 2 to produce the rhodacycle 10, which leads to the subsequent reaction with azabenzonorbornadienes 1 to give the target scaffolds 3. Thus, the indazole directed reversible C–H activation could afford the five-membered rhodacycle 10, which can coordinate with 1a to generate a (Scheme 5). The subsequent migratory insertion of an alkene into the Rh–C bond can lead to the formation of b. The second C–H activation at C-3′ can give c, which may undergo β-N-elimination4b to yield d. The reductive elimination of d can furnish the target heterocycles 3 and Rh(I) species that can oxidize to an active Rh(III) complex using Cu(OAc)2·H2O to complete the catalytic cycle.


image file: c9qo01120j-s3.tif
Scheme 3 H/D exchange studies.

image file: c9qo01120j-s4.tif
Scheme 4 Kinetic isotope experiments and catalytic activities of rhodacycle.

image file: c9qo01120j-s5.tif
Scheme 5 Proposed reaction pathway.

Conclusion

In summary, we have presented the Rh-catalyzed site-selective C–H activation and coupling of 7-azabenzonorbornadienes with aryl-2H-indazoles to furnish 2H-indazolylbenzocarbazoles. The kinetic isotope experiments indicate that the second C–H activation is the product-determining step. The isolation of rhodacycle and selectivity make this procedure attractive.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the Science and Engineering Research Board (SERB) (CRG/2018/000406) for the financial support. S. V. K. thanks SERB for the National Postdoctoral Fellowship (PDF/2016/001460). We thank Mr R. Saravanan, IIT Bombay, for solving the crystal structure. We also thank the Central Instrumentation Facility and Department of Chemistry, IIT Guwahati for NMR (COE, FAST) and mass analyses (FIST, DST).

Notes and references

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Footnote

Electronic supplementary information (ESI) available: CIF file, procedure and NMR spectra. CCDC 1917915. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9qo01120j

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