Manganese(II)-catalyzed modular synthesis of isoquinolines from vinyl isocyanides and hydrazines

Abstract

An efficient manganese(II)-catalyzed oxidative radical cascade reaction was developed for the modular synthesis of multi-substituted isoquinolines from easily accessible vinyl isocyanides and hydrazines. Pyrrolo[1,2-a]quinoxalines and phenanthridines could also be afforded efficiently by this method and a wide range of alkyl, (hetero)aryl and alkoxycarbonyl substitutions could be easily introduced.

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Isoquinolines represent one of the most important molecules in pharmaceutical and agrochemical industries due to their biological and pharmacological activities as therapeutic agents in medicinal chemistry^1,2 and natural products,³ and for this reason the practical synthesis of the core skeleton is of considerable interest in organic synthesis.^4,5 Following the earlier established traditional reactions such as Bischler–Napieralski, Pomeranz–Fritsch and Pictet–Spengler,⁴ many synthetic methods have been developed recently to construct isoquinoline frameworks.⁵ Among them, one of the most common synthetic strategies was attributed to C–H bond activation through transition metal catalyzed cyclization of alkynes with amines,^5c aryl hydrazones,^5b aryl imines,^5a,d,h–j azides,^5f benzamidines,^5g or oximes.^5e However, the reaction products are usually less substituted or lack diversity and the use of noble metals (Pd, Rh, Ru) would limit their applications in synthetic chemistry although these reactions have overcome many drawbacks of the previously reported reactions such as the use of strong acidic conditions and elevated temperatures.

Manganese salts, which exist widely in nature, are ideal catalysts or promotors in organic synthesis due to their inexpensiveness, chemical stabilities and special reactivity properties.⁶ However, as a first-row transition metal, manganese has been less studied except for some well-known oxidants like high-valent species KMnO₄ and MnO₂. Recently, promising and wide applications of manganese reagents have been explored based on their existence in various valences of manganese. For example, Wang and co-workers reported a MnBr(CO)₅ catalyzed dehydrogenative annulation of imines and alkynes,⁷ which suggested that Mn(I) could be an efficient catalyst to replace noble metals such as palladium or rhodium in the field of C–H bond activation. More typically, Mn(III) salts are recognized as efficient promotors in radical reactions due to their oxidation tendency.⁸ In this context, the use of stoichiometric amounts of manganese reagents in some reactions will limit their application, and result in the unwanted production of quantities of waste metals in the meantime. Although Mn(II)-catalyzed reactions have been developed in cross coupling reactions^9a–c and asymmetric oxidations,^9d,e it is still rare for manganese compounds to act as radical initiators catalytically in comparison with the use of a stoichiometric amount of manganese in radical reactions.^9f,g Therefore, from the viewpoint of sustainable development, green chemistry and the fact that obtaining an efficient catalytic turnover remains challenging, the development of manganese-catalyzed reactions, especially using the cheaper Mn(II) salts, will be highly desirable and of great value.

Isocyanides are uniquely versatile C1 building blocks and have been widely applied in the synthesis of various complicated compounds.¹⁰ They are not only capable of reacting with nucleophiles and electrophiles, but also can serve as effective radical acceptors. Although 2-isocyanobiphenyls have been fully reported for the synthesis of phenanthridines with various radical precursors through the somophilic isocyanide insertion reaction,^8a,11 the chemistry of vinyl isocyanides remains largely unexplored and few examples have been demonstrated to construct useful isoquinolines from them.¹² An elegant example was observed in the visible light-promoted annulation in the catalysis of noble iridium.^12a Recently, we have demonstrated the feasible synthesis of isoquinolines through Mn(II)/O₂-promoted vinyl isocyanide insertion with organoboronic acids (Scheme 1),¹³ however, two equivalents of manganese were necessary to accelerate the reaction and limited substrates could be applied as no desired 2-alkyl substituted products could be obtained from the corresponding alkyl boronic acids. In continuation of our recent research interest in isocyanide chemistry,^13–15 herein, we disclose an oxidative radical cascade reaction which, to the best of our knowledge, represents the first example of an Mn(II)-catalyzed protocol for the synthesis of multi-substituted isoquinolines from easily accessible vinyl isocyanides and various hydrazines,¹⁶ whereby the formation of two C–C bonds was involved in one step (Scheme 1). For the given approach, various functional groups including aryl, alkyl, alkoxycarbonyl and acyl could be introduced efficiently from hydrazines onto the C1-position of isoquinolines in good to excellent yields.

Scheme 1 Mn(II)-catalyzed radical cascade reaction of vinyl isocyanides.

At the outset of this investigation, we commenced our study by exploring the catalytic reaction of vinyl isocyanide 1a with phenylhydrazine 2a in the presence of manganese(II) acetate tetrahydrate in DMF using TBPB as an oxidant under a nitrogen atmosphere at 70 °C. Intriguingly, the desired product 3a was isolated in 31% yield after reaction for 23 h (Table 1, entry 1). Various solvents were examined next, which demonstrated that CH₃CN was the most suitable solvent in this catalytic system (entries 2–6). Other oxidants such as TBHP and DTBP also worked in the reaction and afforded 3a in a relatively low yield compared with TBPB (entries 7 and 8). Not much better results were observed when the reaction temperature was increased or decreased (entries 9 and 10), and a nitrogen atmosphere was preferred after the screening of atmospheres (entries 11 and 12). Extensive optimization studies of the amount of Mn(OAc)₂·4H₂O (entries 13 and 14), phenyl hydrazine (entries 15 and 16) and TBPB (entry 17) as well as other manganese catalysts (entries 18–20) suggested that the use of 20 mol% of Mn(OAc)₂·4H₂O and TBPB in CH₃CN solvent at 70 °C under a nitrogen atmosphere turned out to be the best choice and afforded the target product 3a in 82% yield (entry 6). A low yield of 3a (30%) was afforded in the absence of Mn(II) catalyst, which indicated that the manganese catalyst was crucial for this cyclization reaction (entry 21).

Table 1 Optimization of the reaction conditions^a

Entry	Catalyst (mol%)	Solvent	Oxidant	Atmos.	Temp. (°C)	Yield^b (%)
a Reaction conditions: 1a (0.4 mmol), 2a (1.6 mmol), catalyst (20 mol%), oxidant (2.4 mmol) in solvent (3.0 mL), performed in a nitrogen-purged tube at 70 °C, 23 h. TBHP = tert-butyl hydroperoxide. TBPB = tert-butyl peroxybenzoate. DTBP = di-tert-butyl peroxide. b Isolated yield. c 2a (0.8 mmol) was used. d 2a (1.2 mmol) was used. e TBPB (1.6 mmol) was used.
1	Mn(OAc)2·4H2O (20)	DMF	TBPB	N2	70	31
2	Mn(OAc)2·4H2O (20)	THF	TBPB	N2	70	38
3	Mn(OAc)2·4H2O (20)	EtOH	TBPB	N2	70	24
4	Mn(OAc)2·4H2O (20)	Toluene	TBPB	N2	70	42
5	Mn(OAc)2·4H2O (20)	PhCN	TBPB	N2	70	40
6	Mn(OAc) 2 ·4H2O (20)	CH 3 CN	TBPB	N 2	70	82
7	Mn(OAc)2·4H2O (20)	CH3CN	TBHP	N2	70	52
8	Mn(OAc)2·4H2O (20)	CH3CN	DTBP	N2	70	18
9	Mn(OAc)2·4H2O (20)	CH3CN	TBPB	N2	80	74
10	Mn(OAc)2·4H2O (20)	CH3CN	TBPB	N2	60	77
11	Mn(OAc)2·4H2O (20)	CH3CN	TBPB	Air	70	56
12	Mn(OAc)2·4H2O (20)	CH3CN	TBPB	O2	70	63
13	Mn(OAc)2·4H2O (15)	CH3CN	TBPB	N2	70	65
14	Mn(OAc)2·4H2O (10)	CH3CN	TBPB	N2	70	63
15	Mn(OAc)2·4H2O (20)	CH3CN	TBPB	N2	70	53^c
16	Mn(OAc)2·4H2O (20)	CH3CN	TBPB	N2	70	57^d
17	Mn(OAc)2·4H2O (20)	CH3CN	TBPB	N2	70	77^e
18	Mn(OAc)3·2H2O (20)	CH3CN	TBPB	N2	70	66
19	Mn(acac)3 (20)	CH3CN	TBPB	N2	70	72
20	Mn(acac)2 (20)	CH3CN	TBPB	N2	70	65
21	—	CH3CN	TBPB	N2	70	30

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With the optimized reaction conditions established, the scope of hydrazine substrates was next examined (Table 2). Gratifyingly, the catalytic reaction proceeded smoothly with products afforded in moderate to good yields no matter whether the electron-donating (3b, 3d, 3g, 3k) or electron-withdrawing groups (3c, 3e–3f, 3h–3j) were on the aryl ring of the C1-position of products. Substrates bearing ortho- (3b–3c), meta- (3d–3e), para- (3f–3k) groups or with multi-substitutions (3l) all proceeded well. Furthermore, halo-substituted phenylhydrazines (3c, 3e–3f, 3h–3i, 3l) also accomplished the reactions smoothly with the halo-atoms still remaining in the isoquinolines, which appeared to be the key intermediates for further transformation of these products in the classic cross coupling reactions. To our delight, hydrazines with a heterocyclic substituent (3m) or an ester group (3n) were also proved to be suitable components to react with isocyanide 1a under the optimized conditions. It should be noted that 1-alkyl substituted isoquinolines (3o–3s) could be obtained in moderate to good yields, which would significantly expand the application scope of the reaction and provide a good complement and improvement to our previous work, where only aryl and limited alkenyl boronic acids could be used.¹³ Additionally, alkyl hydrazines worked smoothly, not only for those compounds with a linear (3o, 3s) or tertiary carbon center (3p), but also with cycloalkyl groups (3q–3r). 1-Phenethyl quinoline (3s) was achieved in 58% yield, however, no desired benzyl substituted isoquinoline product could be isolated from benzylhydrazine, while the corresponding benzoyl substituted product (3t) was isolated in 20% yield instead. The reason may be the rapid oxidation of the generated 1-benzyl isoquinoline to the unusual 1-benzoyl product under the reaction conditions,^17a while currently we cannot rule out the possibility of the oxidative transformation of the instable benzyl radical to the corresponding benzoyl radical during the reaction.^17b–e

Table 2 Scope of hydrazines^a,b

a Reaction conditions: the reaction was carried out with 1a (0.4 mmol), 2a–t (1.6 mmol), Mn(OAc)₂·4H₂O (0.08 mmol) and TBPB (2.4 mmol) in CH₃CN (3.0 mL) at 70 °C under a nitrogen atmosphere. b Isolated yield. c tert-Butylhydrazine hydrochloride or cyclopentylhydrazine hydrochloride was used in the presence of NaHCO₃ (5.0 equiv.). d Benzylhydrazine was used as the substrate.

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To further study the generality and scope of this reaction, a variety of substituted vinyl isocyanides were investigated with phenyl hydrazine. As illustrated in Table 3, substrates with ester (1a–1e, 1i–1m) and amide (1f–1h) substituents would proceed efficiently to afford the corresponding isoquinolines in moderate to good yields. Notably, vinyl isocyanides derived from diaryl ketones (4a–4h), alkyl aryl ketones (4l–4m) and aryl aldehydes (4i–4k) all proceeded smoothly under the optimized conditions to obtain the corresponding products, regardless of their different electronic properties and substitution positions. Furthermore, the product type was not limited to isoquinolines, fused rings such as benzo[h]isoquinoline derivatives could also be produced successfully (4k and 4l).

Table 3 Scope of vinyl isocyanides^a,b

a Reaction conditions: the reaction was carried out with 1a–m (0.4 mmol), 2a (1.6 mmol), Mn(OAc)₂·4H₂O (0.08 mmol) and TBPB (2.4 mmol), in CH₃CN (3.0 mL) at 70 °C under a nitrogen atmosphere. b Isolated yield.

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This newly established protocol was not limited to vinyl isocyanides, the substrate scope could also be extended further to include aryl isocyanides providing direct access to the corresponding pyrrolo[1,2-a]quinoxalines (5a–5b) and phenanthridine products (5c–5d) in moderate to good yields (Scheme 2), which will further illustrate the broad substrate scope and product diversity of this reaction.

Scheme 2 Construction of other heterocyclic skeletons.

To understand the mechanism of this Mn(II)-catalyzed cyclization reaction, preliminary mechanistic investigation was carried out, as shown in Scheme 3. The reaction afforded 3a in only 8% yield with the addition of 2,2,6,6-tetramethyl-piperidine-1-oxy (TEMPO) under the standard conditions, and the addition of p-nitrophenol caused the reaction to become sluggish and decreased the product yield to 22%, which implied that the reaction may experience a single electron transfer pathway.

Scheme 3 Prevention reaction by radical inhibitors.

On the basis of the above-mentioned experimental observations, a plausible mechanism was proposed for this radical cascade reaction (Scheme 4). Initially, the tert-butoxy radical was generated from Mn(II) and TBPB by a single electron transfer process, together with the formation of Mn(III) and benzoate anions.¹⁸ The given Mn(III) was then reacted with phenylhydrazine 2a to afford the phenylhydrazine radical A through sequential single electron transfer and proton transfer steps, and the Mn(II) catalyst was regenerated to complete the catalytic cycle. With the assistance of the tert-butoxy radical, the phenyl radical was formed through stepwise hydrogen abstractions with the concomitant release of nitrogen gas.¹⁹ The produced phenyl radical then underwent intermolecular addition to isocyanide 1a to form the imidoyl radical B, which subsequently gave the cyclohexadienyl type radical C by an intramolecular attack of the imidoyl radical on the aromatic ring. Finally, the isoquinoline 3a could be afforded by another hydrogen abstraction from intermediate C in the presence of the tert-butoxy radical with the release of tBuOH.

Scheme 4 Plausible mechanism for the synthesis of 3a from 1a.

In conclusion, we have developed a manganese(II)-catalyzed cascade radical reaction from easily accessible vinyl isocyanides and hydrazines. This approach offers a unique strategy and alternative route for the convenient preparation of pharmacologically interesting isoquinolines in moderate to good yields with a wide substrate scope, good functionality tolerance and efficient synthesis modularity. The present manganese catalyst system was also successfully applied to the synthesis of valuable pyrrolo[1,2-a]quinoxalines and phenanthridines and a wide range of alkyl, (hetero)aryl and alkoxycarbonyl substitutions could be easily introduced by this method. Further research into applications of this catalytic system is now underway in our laboratory.

We thank the National Natural Science Foundation of China (no. 21272149), and Innovation Program of Shanghai Municipal Education Commission (no. 14ZZ094) for financial support. Prof. Hongmei Deng's (Laboratory for Microstructures, SHU) help with NMR spectroscopy measurements is highly appreciated. This work was partly supported by Shanghai Key Laboratory of High Temperature Superconductors (no. 14DZ2260700).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: General experimental procedures, characterization data and copies of the ¹H, ¹³C and ¹⁹F NMR spectra for all compounds. See DOI: 10.1039/c6qo00048g

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