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Letter
Aerobic Copper-Catalyzed Synthesis of Benzimidazoles from Diaryl- and Alkylamines via Tandem Triple C-H Aminations Taoyuan Liang, Zhenda Tan, He Zhao, xiuwen chen, Huanfeng Jiang, and Min Zhang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00082 • Publication Date (Web): 06 Feb 2018 Downloaded from http://pubs.acs.org on February 7, 2018
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ACS Catalysis
Aerobic Copper-Catalyzed Synthesis of Benzimidazoles from Diaryl- and Alkylamines via Tandem Triple C-H Aminations Taoyuan Liang, Zhenda Tan, He Zhao, Xiuwen Chen, Huanfeng Jiang, and Min Zhang* Key Lab of Functional Molecular Engineering of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China. ABSTRACT: Through radical-induced tandem triple C-H aminations with free amines as the aminating agents, we herein present a precedent on aerobic copper-catalyzed synthesis of 5-diarylamino benzimidazoles, a class of optoelectronic device analogues by combining two molecules of diarylamines and one molecule of alkylamine in one single operation. The developed chemistry proceeds with the merits of a natural abundant copper/O2 catalyst system, readily available feedstocks, broad substrate scope, good functional group tolerance, exclusive regio- and chemoselectivity, high step and atom efficiency, which offers an important basis for further construction of functional products that are inaccessible or difficult to prepare with the existing methods by employing catalytic tandem C-H amination strategy.
KEYWORDS: tandem triple C-H aminations, areobic copper catalysis, radical, functional benzimidazoles, free amines
Carbon-nitrogen (C-N) bond formation constitutes a significantly important subject in the scientific community, as it enables key steps in production of fine and bulk chemicals, pharmaceuticals, agrochemicals, materials and other valuable products. Pioneered by the metal-catalyzed cross-coupling of halogenated substrates with amines (i. e. Ullmann-Goldberg1 and Hartwig-Buchwald aminations2 ), the high step and atom efficiency of C-H amination coupled with the ubiquity and low-cost of hydrocarbons makes it a particularly attractive manner in creation of N-containing products.3 Since the seminal work via nitrene insertion into the C-H bonds reported by Breslow and the co-workers,4 significant advances have been achieved in this area over the past decades.5 In recent years, the strategies via oxidative C-H bond functionalizations,6 directing group-assisted cross-couplings,7 and the radical-induced C-N bond formations8 have also been nicely demonstrated. Despite these important achievements, most of the transformations rely on the utilization of specific aminating agents (i.e. nitrenes, amino group with a leaving substituent) and the directing groups to position the reaction sites. Hence, the development of new C-H amination strategies with free amines as the aminating agents, enabling straightforward access to functional products that are inaccessible or difficult to prepare with the existing methods, would be of great significance.
As our continuous research interest in construction of functional N-heterocycles,9 we have recently reported a cross-coupling reaction of tetrahydroquinolines with 1,8-naphthyridines10. This work prompted us to test the dehydrogenative synthesis of N-bi-heteroarenes s12 from tetrahydroquinoline (s1) and 1,8-naphthyridine (s2) by employing a natural abundant CuCl2/O2 catalyst system. Interestingly, we observed that, instead of compound s12, a para-aryl C-H amination product s1’ arising from dehydrogenative homo-coupling of s1 was observed in 3% yield (eq 1). This observation led us to envisage a direct aryl
C-H amination protocol as depicted in Scheme 1, the aryl radical cations (B and C), arising from single electron oxidation (SEO) of the N-atom of arylamine 1 followed by internal electron transfer (from A to B and C), would trap nucleophilic amine 2, and release the major para- and minor ortho-C-H amination products (3-1 and 3-2) directed by the conjugate effect-induced electron distribution of the aryl ring. Scheme 1. Envisaged aryl C-H amination strategy
However, it is important to note that the presence of copper/O2 system to anilines can easily form the azo compounds.11 Hence, we anticipated that the diarylamines could be desirable precursors to form radical cations regarding the SOMOs stabilized by two aryl rings. To test the feasibility of this idea, we performed the reaction of diphenylamine 1a with p-tolylmethanamine 2a in the presence of CuCl2/O2. Surprisingly, a 1,2-diaryl-5-diarylamino benzimidazole (3aa), the analogue of well-defined optoelectronic devices that need multistep synthesis,12 was obtained in 33% yield in a single operation via two aryl C-H amination and one benzylic C(sp3)-H amination (eq 2). To date, although there are many methods reported for the synthesis of benzimidazoles, including the representative condensation of 1,2-diaminoarenes with aldehydes or the coupling of the surrogates of both coupling partners,13 the protocols via oxidative annulation or C-H functionalization,14 and the conventional cross-coupling reactions,15 the construction of benzimidazoles incorporated with an additional functionality by selective C-H aminations with free amines as the aminating agents has not been reported, yet. Based on our new observation, we wish herein to report, for the first time by radical-induced tandem triple C-H aminations, a general synthesis of functional benzimidazoles from readily available diaryl- and alkylamines with exclusive regio- and chemoselectivity.
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Our initial studies focused on screening an efficient catalyst system for the coupling of 1a and 2a as a model system. The effect of eight copper pre-catalysts, representative solvents and temperatures were evaluated (see Table S2 in the supporting information (SI)). An optimal yield (84%) of 3aa was obtained by treating the mixture of 1a (0.5 mmol), 2a (0.25 mmol), CuCl (15 mol%), i-butanol (1.5 mL) at 100 oC in the presence of O2 (standard conditions).
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benzimidazoles (3aj, 3ak), which have the potential to be applied as hemilabile bidentate ligands in organometallic chemistry and catalysis. Gratifyingly, alkylamines including both linear (2l and 2m) and branched (2n) ones served as effective coupling partners to generate the 2-alkyl benzimidazoles in moderate yields (3al-3an). Noteworthy, under aerobic copper-catalyzed conditions, alkylamines can easily undergo dehydrogenative hydrolysis and dimerization to form the aldehydes and imines.16 However, such by-products were not observed in all the tested examples, revealing that the reaction proceeds in a chemoselective manner. Scheme 3. Variation of both coupling partners
Scheme 2. Variation of alkylamines
H N Ar
1
R1 N
R' R
R1
CuCl (15 mo l%)
NH2
i-butanol, O2 100 oC, 12 h
2
R N N 3, isolated yield% R1
Ar
N
N O
N N 3bo,46% Ph Br
N 3bd, 78%
N Ph
F
F
Br
N N Ph
N O
N
N Ph
3ca, 75%
N 3dd, 73% CF3
CN
F3C
NC
N
N N Ph
N 3ea, 51%
N Ph
O
N 3fa, 40%
COOEt
N EtOOC
N
N
N N 3ga, 40% Ph
N 3ha, 85%
N N Ph
With the availability of the optimal conditions, we then examined the generality of the synthetic protocol. Diphenylamine (1a) was further employed to couple with various benzylic amines (2a-2i, for their structures see Scheme S1 in SI). As shown in Scheme 2, all the reactions proceeded smoothly and delivered the desired products in moderate to excellent yields upon isolation (Scheme 2, 3ab-3ai), and the structure of 3ab was confirmed by single-crystal X-ray diffraction (see SI, Figure S2). In all cases, the diarylamino group was selectively introduced to the C5-site of the benzimidazole skeleton. Moreover, the α-heteroaryl methylamines (2j and 2k) were also amenable to the transformation to result in the 2-heteroaryl substituted
N 3ia, 26%
Subsequently, we turned our attention to the variation of both coupling partners. Hence, the combinations of various unsymmetrical diarylamines (1b-1h) and alkylamines (2) were evaluated. Pleasingly, all the reactions selectively furnished the benzimidazoles 3 in moderate to excellent isolated yields (Scheme 3). It was found that the substituents on the aryl ring of diarylamines (1) affected the product formation to some extent. Especially, the electron-rich ones gave much higher yields (3bd, 3ha) than those with strong electron-withdrawing groups (3ea-3ga), presumably because the electron-rich diarylamines (1) result in more stable radical intermediates (Scheme 1, B and C,). In particular, 10,11-Dihydro-5H-dibenzo[b,f]azepine 1h could efficiently couple with amine 2a to produce the polycyclic benzimidazole
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1a-1 with dipheylamine 1a failed to yield the benzimidazoles 3ia (eq 7) and 3aa (eq 8), respectively, it indicates that the first aryl para-C-H amination of 1a to 1aa plays a decisive role in the product formation, which occurs prior to the aryl ortho-C-H amination of 1aa with 2a. Finally, the treatment of 2a under the standard conditions did not give 4-methylbenzonitrile 2a’ (SI, eq S8), and the reaction of diarylamine 1a with nitrile 2a’ under the identical conditions was unable to generate product 3aa (SI, eq S9). These two experiments prove that both nitrile 2a’ and amidine arising from addition of 1a to 2a’ are not the reaction intermediates.14a Scheme 5. The control experiments
Scheme 4. The synthetic utility of the developed chemistry Reported procedure NH2
step-1
NO2 reduction Br
Br
1) CH2Cl2, 1 h, 0 oC; 4-bromobenzaldehyde
toleuene, 100 C Ar2NH step-2
Ar N
N N H Ar2N
Ar
RHN N Br N 3ah Ph
R=
Ph 1j step-5
Ar
N
Ar
C-N coupling Br
2) Bromosuccinimide, overnight, rt 3) NaOH, H2O step-3 Ar N
NH2
o
NH2
Ar
NH2
t-BuONa, t-Bu3P Pd2(dba) 3
NH2
step-4
R
N N N Ph 4ahj (Ar = Ph) 12% overall yield
Ph
Our streamline synthesis 2 Ph
H N 1a
Br Ph + 2h
1j our protocol 4ahj, 92% 3ah, 78% step-2 step-1 C-N coupling 72% overall yield
NH2
To gain the mechanistic insights the transformation, the model reaction under standard conditions was interrupted after 1 h to analyze the product system. 3aa was detected in 51% yield along with a para-aryl C-H amination compound 1aa in 1% yield (Scheme 5, eq 3). Then, the prepared 1aa was able to couple with amine 2a to give product 3aa in 90% yields (eq 4), showling that 1aa serves as a key reaction intermediate. Moreover, addition of excess TEMPO into the reaction completely suppressed the product formation, and 1a trapping a TEMPO motif was observed (SI, eq S3), reaveling that the reaction involves a radical pathway. Further, the 2-benzylamino diphenylamine 1a-1 underwent dehydrogenative cyclization to form benzimidazole 1a-2 in 96% yield (eq 5), whereas 2-amido diphenylamine 1a-3 was unable to give the same product (eq 6). Thus, product 3aa is formed via an aryl ortho-C-H amination of 1aa with benzylic amine 2a followed by an oxidative intramolecular benzylic C(sp3)-H amination, instead of benzylic oxidation followed by an intramolecular condensation process. Further, both reactions of diarylamine 1i blocked two para-sites with 2a and
Based on the above findings, the possible reaction pathways are depicted in Scheme 6. The [Cun]/O2 induced single electron oxidation (SEO)17 of 1a, or SEO of amine 2a followed by single electron transfer from 1a to 2a’ forms the more stable radical cation 1a’ and species A. Then, 1a’ chemoselectively interacts with the aryl ring of 1a at the sterically less-hindered para-site to form a transition state 1aa-1. Meanwhile, via internal electron transfer of 1a’, the delocalized phenyl radical cation interacts with the N-atom of diarylamine 1a to give transition state 1aa-2. Then, the first aryl para-C-H amination (1aa) is furnished by species A-promoted deprotonations and SEO with regeneration of the catalyst [Cun]. Similar to the first C-H amination, radical cation 1aa-3 is formed via a SEO process, and the sterically less-hindered alkylamine 2a interacts with the less-crowded ortho-site of 1aa-3, thus affording the secondary aryl o-C-H amination adduct 3aa-1 via further SEO and deprotonations. Finally, product 3aa is produced by oxidation of 3aa-1 to imine 3aa-3 followed by intramolecular nucleophilic addition and dehydroaromatization processes (path 1). Alternatively, the successive formation of cabocation (3aa-2 to 3aa-4), cyclization18 and dehydroaromatization (path 2) also rationalizes the result. Noteworthy, except for such proposed cation-nucleophile coupling mode, the C-H aminations via radical-radical cross-coupling may also be involved.
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ACKNOWLEDGMENT
Scheme 6. Plausible reaction pathways
The authors are grateful to the financial support of the National Key Research and Development Program of China (2016YFA0602900), Science and Technology Program of GuangZhou (201607010306), the Fundamental Research Funds for the Central Universities (2017ZD060), the “1000 Youth Talents Plan”, Science Foundation for Distinguished Young Scholars of Guangdong Province (2014A030306018).
REFERENCES
In summary, via radical-induced tandem triple C-H aminations with free amines as the aminating agents, we have developed an unprecedented aerobic copper-catalyzed synthesis of 5-diarylamino benzimidazoles, a class of optoelectronic device analogues by combining two molecules of diarylamines and one molecule of alkylamine in one single operation. The synthesis proceeds with the merits of a natural abundant copper/O2 catalyst system, readily available feedstocks, broad substrate scope, good functional group tolerance, exclusive regio- and chemoselectivity, high step and atom efficiency, which offers an important basis for further construction of functional products that are inaccessible or difficult to prepare with the existing methods by employing catalytic tandem C-H amination strategy.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Detailed experimental procedures, characterization data, crystallographic data for 3ab (CIF) and structures, copies of 1 H NMR and 13 C NMR spectra for all isolated compounds (PDF)
Accession Codes CCDC 1508571 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB21EZ, UK; fax: + 44 1223 336033.
AUTHOR INFORMATION Corresponding Author *
[email protected] Notes The authors declare no competing financial interests.
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