Electrochemical Acceptorless Dehydrogenation of N-Heterocycles

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Electrochemical Acceptorless Dehydrogenation of NHeterocycles Utilizing TEMPO as Organo-Electrocatalyst Yong Wu, Hong Yi, and Aiwen Lei ACS Catal., Just Accepted Manuscript • Publication Date (Web): 08 Jan 2018 Downloaded from http://pubs.acs.org on January 8, 2018

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Electrochemical Acceptorless Dehydrogenation of NHeterocycles Utilizing TEMPO as Organo-Electrocatalyst Yong Wu,1‡ Hong Yi,1‡ and Aiwen Lei1,2* 1. Institute for Advanced Studies, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, Hubei 430072, P. R. China. 2. National Research Center for Carbohydrate Synthesis, Jiangxi Normal University, Nanchang 330022, Jiangxi, P. R. China. ABSTRACT: Catalytic acceptorless dehydrogenation (CAD) has been a basically important organic transformation to ubiquitous unsaturated compounds without the usage of a sacrificial hydrogen acceptor. In this work, we successfully develop the first electrochemical acceptorless dehydrogenation (ECAD) of N-heterocycles using TEMPO as the organo-electrocatalyst. We have achieved the catalytic dehydrogenation of N-heterocycles in anode, and the release of H2 in cathode under undivided-cell system. A variety of six-member and five-member nitrogen-heteroarenes can be synthesized in good yields in this system. In addition, this protocol can also be used in the application of important molecular synthesis. Our electrochemical strategy provides a mild and metal-free route for (hetero)aromatic compounds synthesis via CAD strategy. KEYWORDS: Electrochemical acceptorless dehydrogenation, mediators, organo-electrocatalyst, N-Heterocycles, applica-

tion,

Catalytic acceptorless dehydrogenation (CAD) from saturated organic molecules, such as N-heterocycles or hydrocarbons, to generate unsaturated molecules with the release of hydrogen gas has attracted great attention and been widely applied in organic synthesis to access ubiquitous (hetero)aromatic compounds.1 This chemical process not only avoids using stoichiometric oxidants in the organic transformations, but also displays a potential application for “hydrogen storage”. Although hydrogen generation from saturated hydrocarbons is entropically favored, it is thermodynamically unfavored as it is a highly endothermic process.2 Pioneering contributions from groups of Yamaguchi,3 Fujita4 and Crabtree5, iridium, ruthenium, iron, cobalt complexes, and organic borane compounds have been used as efficient catalysts for the dehydrogenation of N-heterocycles.1a, 1d, 6 While these advances are impressive, these methods often require long reaction times or high temperature. Recently, the groups of Li and Kanai independently developed catalytic acceptorless dehydrogenation of N-heterocycles using visible light catalysis, which made a breakthrough in achieving this transformation under room temperature.2b, 6g Although the photocatalysis provided an alternative mild route to CAD reaction, expensive photocatalyst and transition-metals were still used in these two systems. Due to the importance of (hetero)aromatic compounds, the development of mild method via CAD strategy is in highly demand. Electroorganic methods can offer sustainable alternatives to traditional redox (oxidation and reduction) reactions in organic synthesis.7 The overall electrochemical transformation avoids the use of any sacrificial reagents, hydrogen gas is the only byproduct and only consumes electrical energy during the reaction.8 In recent years, great advances have been achieved in electrochemical synthesis, which provides new opportunities for the construction of carbon-carbon and carbon-heteroatom bonds in an environmentally friendly way.9 While in most of this direct electrochemical transformation, the electron-

transfer between substrate and electrode is in a heterogeneous manner, which is relative inefficient.10 The indirect electrolysis using redox mediators to achieve indirect electro-processes has attracted increased attention,11 since it offers many advantages compared to a direct electrolysis. In the indirect electro-synthesis, the heterogeneous electron transfer between the electrode and substrate is replaced by a homogeneous redox reaction in solution, which occurs between an electrochemically activated species and the substrate. In addition, electron transfer mediators are useful in avoiding over oxidation/reduction of the substrates and high selectivity can be achieved. In this work, we achieve an ECAD of Nheterocycles using TEMPO as the organo-molecular electrocatalyst. The N-heterocycle I can be initially oxidized to form the imine IV via the radical cation intermediate III. The further oxidation of imine IV under the electrochemical conditions occurs to generate the (hetero)aromatic compound II with the further loss of two electrons and two protons.

Scheme 1. ECAD of N-Heterocycles using TEMPO as organo-electrocatalyst Based on above scenario, we first optimized the electrolysis conditions for the ECAD of 1,2,3,4-tetrahydroquinoldine (1a), which failed in the absence of TEMPO. Extensive

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experimental investigations indicated that electrolyzing 1a at room temperature in the mixed solvent of CH3CN/H2O (6:0.1) using 10 mol% of TEMPO as the organo electrocatalyst, resulted in the highest yield (82%) (Table 1, entry 1). Previous work showed that related nitroxyl derivatives were important catalysts for electrochemical alcohol oxidation,12 which led to inferior results in the reactions (entries 3,4). DDQ was also competent mediators albeit in slightly lower yield (entry 5). Using other redox mediators such as KI, ferrocene or Ph3N all resulted in no reaction (entries 6-8). H2O was also indispensable to this process and trace product was achieved without water (entry 9). Other electrode materials such as platinum anode (entry 10) or Ni plate cathode (entry 11) were proved to be less efficient. Notably, a slightly decreased reaction yield was obtained when the reaction directly carried out under air (entry 12). Table 1. Optimization of the Reaction Conditions a

entry

variation from the standard conditions

yield(%)b

1 2 3 4 5 6 7 8 9 10 11 12

none without M1 M2 M3 M4 KI FeCp2 Ph3N no H2O Pt plate anode Ni plate cathode under air

86(82)c trace 46 trace 78 n.d. n.d. n.d. trace 25 61 72

a

Standard conditions: C cloth anode, Pt cathode, constant current= 7 mA, tempo (10 mol%), 1a (0.25 mmol), Bu4NBF4 (40 mol%), MeCN (6.0 mL), H2O (0.1 mL), RT, 4 h. bThe yield of 2a was determined by GC analysis with biphenyl as the internal standard; n.d.=not detected. c Isolated yield.

The scope of ECAD reaction to a catalytic version using 10 mol% of TEMPO and a series of six-membered Nheterocycles were next investigated (Scheme 2). The parent quinoline 2a was obtained in 82% isolated yield. Aryl and alkyl substitutions at the position 2-8 have all been proved to be feasible substrates, thus affording the desired products 2b2j in good yields. The N-hetercycles substituted with 2-furyl group and 6-thienyl group also led to favorable yields (2d, 2k), indicating no inhibition by the heterocyclic substituent. The strong electron-withdrawing group (2l) were tolerated in this transformation, albeit afford relative low product with lot of residual starting materials. Halide-substituent was able to furnish the corresponding products in this process (2m), thereby facilitating additional modifications. In addition, the present protocol could be applicable to dehydrogenation of octahydro compound 1n in satisfied yield (2n), which theoretically release four molecules of H2. The excellent tolerance of the present protocol was exhibited by the oxidative aromatiza-

tion of Hantzsch dihydropyridines, which were transformed into the desired aromatic products with high yields (2o-2p). This method also provided an appealing route to 2q and 2r, which the corresponding starting materials may be accessed readily via simple condensation and Pictet−Spengler reactions. Scheme 2. ECAD of Six-Membered N-Heterocycles a

a

Standard conditions: C cloth anode, Pt cathode, constant current= 7 mA, 1 (0.25 mmol), tempo (10 mol%), Bu4NBF4 (40 mol%), MeCN (6.0 mL), H2O (0.1 mL), RT, 4 h. b1 (0.125 mmol), tempo (20 mol%), Bu4NBF4 (80 mol%), c1 (0.5 mmol), tempo (5 mol%), Bu4NBF4 (20 mol%), d5.5 h.

Then, we applied our ECAD strategy to other six-member N-heterocycles. The quinazolines are important and useful moieties in pharmaceutical molecules.13 To our delight, the target quinazolines could be obtained by ECAD of tetrahydroquinazolines in moderate to high yields (4a-4g). (Scheme 3). Especially, the halide-substituents and electronwithdraw groups could be tolerated in this system (4e-4g). Scheme 3. ECAD of Tetrahydroquinazolines a

a Standard conditions: C cloth anode, Pt cathode, 3 (0.5 mmol), constant current=7 mA, tempo (5 mol%), Bu4NBF4 (20 mol%), MeCN (6.0 mL), H2O (0.1 mL), RT, 4 h.

This reaction also showed a broad scope with respect to the saturated five-membered N-heterocycles, which corresponding substrates could be prepared in simple and diverse ways. Either simple indole or N-methyl indole afforded the desired products in good yields (6a-6b). Introduction of methyl and

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phenyl groups on the on the indole rings did not affect the reactivities (6c-6e). The presence of halogen groups such as Cl−, and Br− also led to good yields (6f-6g). Moreover, 2-aryl substituted 2,3-dihydrobenzothiazoles were smoothly converted into benzothiazoles in high yields under our standard conditions (6h-6k). Thus, direct ECAD reaction provides an advantageous strategy to synthesis of indoles and benzothiazoles. Scheme 4. ECAD of Five-Membered N-Heterocycles a

Scheme 6. Investigation for Mechanistic Insights (A) Radical inhibiting experiments

(B) The intermediate experiments MeO MeO

C Cloth (+) Pt (-), 7 mA Bu4NBF4 40 mol% MeO tempo 10 mol% NH MeCN/H2O, r.t., 4 h MeO Ph

The synthetic utility of our ECAD strategy was further demonstrated with rapid synthesis of the well-known useful molecules (Scheme 5A). The 2-phenylquinazolin-4(3H)-one was improved a useful β-Glucuronidase inhibitor.14 Our strategy started with readily available 3a, finally β-Glucuronidase inhibitor was formed in high yield in two steps. In addition, we also conducted the electrolysis reaction on a gram scale, which showed the potential in industrial process (Scheme 5B). The corresponding N-heteroarenes such as 2a, 6a, 2p and 6h were smoothly synthesized using a constant current of 75 mA in satisfied yields (see the Supporting Information for details), thus attesting to the high reaction rate and practicality. Scheme 5. Further Application of this ECAD reaction (A) The synthesis of β-Glucuronidase inhibitor

(B) Gram-scale experiments n

a

N

MeO

Ph

Ph

7

2q yield of 2q

4h

37%

52%

5.5 h

trace

82%

Preliminary mechanistic studies were conducted to gain insights into this transformation. When 2 equivalent of (2,2,6,6butylated hydroxytoluene (BHT) was added in the reaction under the standard conditions (Scheme 6A), the significant decrease of the yield indicated that radical process was possibly involved under the electrochemical conditions. As reported in previous references, the imine can be the key intermediate during the dehydrogenation process.15 We then tried to probe the intermediate formed in the ECAD reaction of the Nheterocycles (Scheme 6B). When using 6,7dimethoxytetrahydroisoquinoline (1q) as the selected substrate under the electrochemical conditions, both dihydrobackebergine (7) and 6,7-dimethoxyisoquinolines could be observed in 4 hours. The single 2q product could be achieved when the reaction prolonged to 5.5 hours. These experiments results indicated that partially hydrogenated imine intermediate 7 might be an important intermediate in the whole transformation.

Pt cathode

a Standard conditions: C cloth anode, Pt cathode, 5 (0.5 mmol), constant current=7 mA, tempo (5 mol%), Bu4NBF4 (20 mol%), MeCN (6.0 mL), H2O (0.1 mL), RT, 4.5 h. b4 h

MeO N +

yield of 7

reaction time

C cloth cathode

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n

Standard conditions: C cloth anode, Pt cathode, N-Heterocycles (10 mmol), constant current=75 mA, tempo (5 mol%), Bu4NBF4 (20 mol%), MeCN/H2O (60/2 mL), RT, 8 h. bconstant current=100 mA, tempo (10 mol%), Bu4NBF4 (40 mol%), MeCN/H2O (120/5 mL), RT, 13 h.

Scheme 7. Proposed mechanism According to the aforementioned studies and previous studies,2b, 16 a possible mechanism for the ECAD reaction of Nheterocycles was outlined in Scheme 7. Initially, TEMPO was oxidized at the anode via ingle-electron-transfer (SET) process to afford TEMPO cation intermediate, which next reacted with 1,2,3,4-tetrahydroquinoldine 1a to generate a radical-cation intermediate 8. Then, dihydroquinoline 9 was formed after losing an electron and two protons. Subsequently, isomeriza-

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tion of 9 afforded 10. Finally, 11 was obtained from 10 by the similar electrochemical oxidation process. The H2 was released in cathode via the electrochemical reduction of protons. Overall, an ECAD of N-heterocycles is developed using TEMPO as the organo-electrocatalyst. The dehydrogenation of N-heterocycles and release of H2 are achieved in one system under oxidant-free conditions. The mild condition, high activity and broad substrate scope of the strategy enabled the process an appealing route for the synthesis of various Nheteroarenes with great synthetic values. Mechanistic studies indicated the imine intermediate served as the intermediate in this transformation. The synthetic applications and gram-scale experiments further demonstrated the potential usage of our reaction.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org, which included NMR data; extended data about scale-up reaction and characterization (PDF).

AUTHOR INFORMATION Corresponding Author * Corresponding author, E-mail: [email protected].

Author Contributions ‡ These authors contributed equally to this work

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the 973 Program (2011CB808600, 2012CB725302, 2013CB834804), the National Natural Science Foundation of China (21390400, 21272180, 21302148, 2109343 and 21402217), and the Research Fund for the Doctoral Program of Higher Education of China (20120141130002) and the Ministry of Science and Technology of China (2012YQ120060) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT1030). The Program of Introducing Talents of Discipline to Universities of China (111 Program) is also appreciated.

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