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Photocatalytic Cross-Dehydrogenative Amination Reactions between Phenols and Diarylamines Yating Zhao, Binbin Huang, Chao Yang, Bing Li, Baoquan Gou, and Wujiong Xia ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00192 • Publication Date (Web): 24 Feb 2017 Downloaded from http://pubs.acs.org on February 24, 2017
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ACS Catalysis
Photocatalytic Cross-Dehydrogenative Amination Reactions between Phenols and Diarylamines Yating Zhao, Binbin Huang, Chao Yang, Bing Li, Baoquan Gou and Wujiong Xia* State Key Lab of Urban Water Resource and Environment, School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin, 150080, China
ABSTRACT: The direct intermolecular aryl C-N coupling reaction from precursors without preactivated C-H and N-H bonds has been challenging all the time. Herein, an oxidative system, combining catalytic amount of organic-photocatalyst with stoichiometric amount of persulfate, was developed to enable the successful cross-dehydrogenative-coupling amination between phenols and acyclic-diarylamines in non-metallic method. This protocol precludes both coupling partners from pre-functionalization and achieved single regioselectivitiy of amination products under genuinely simple and benign conditions. Broad scopes of substrates were evaluated with moderate to high efficacy, and the reaction efficiency of electron-deficient phenothiazine and phenol was highly improved. A radical/radical cross-coupling pathway was proposed based on mechanistic studies, wherein a radical chain propagation process was involved.
KEYWORDS: dehydrogenation • oxidative amination • visible light • radical cross-coupling • radical chain propagation
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INTRODUCTION Particular interest in developing new strategies that allow the direct and selective formation of aryl C-N bonds is driven by the ubiquity of aryl C-N bonds in pharmaceutical, agrochemical and material science.1 Recently, the cross-dehydrogenative coupling (CDC)2 has becoming a fascinating technique for bonds construction, which averts the requirement for pre-activation of both coupling precursors and leads to atom and step economy in chemical synthesis. In contrast to the prosperous achievements in intermolecular C-C bonds formation via the CDC route3, research on forging the crosscoupled aryl C-N bonds remains elusive in its early groping stage, despite some inspiring reports have been recorded on the intramolecular C-H amination.4 Among the recent advances in CDC-amination, transition metals were frequently utilized to activate the aryl C-H bonds to achieve direct amination of arenes that attached to coordinating directing groups (DGs).5 These metal-chelation-assisted approaches enabled absolutely selective aryl C-H functionalization corresponding to the DG appendage. Beyond the chelation-assisted methods, metal-free amination by employing electronic techniques or stoichiometric strong oxidants were also proposed for directly building intermolecular C-N bond, giving modest selectivity of aminated product.6 Despite some advances on CDC-amination reaction via radical addition,7 radicals crosscoupling,8 and nucleophilic attack to cation radical,9 the radical engaged process for direct C-N bond construction would remain challenging for a while, because 1) The N-radical, which would add to benzene ring in the process of radical addition (Scheme 1 A, path A), was typically produced through reductive scission of the weak N-X (X= O, S, N and halogen) bond.10 However, it was extremely difficult to be generated by cleavage of the
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strong N-H bond in mild conditions. Moreover, this radical addition method was widely considered to be inherently unselective. 2) It is not readily available to provide an appropriate situation in the radical cross-coupling manner between C-radical and Nradical simultaneously. Even if the two radical species were formed equally, they were even required to follow the principle of persistent radical effect (PRE)11 to selectively afford the anticipated cross-coupling product rather than the homo-coupling one. 3) Alternatively if amination reaction proceeds through the nucleophilic attack pathway, the aminating agent that attacks to the oxidized cation radial of arene demands definite nucleophilic ability (Scheme 1 A, path B), whereas a number of amines could not satisfy the demands.9,12 Among these radical-based amination reactions, the Ritter group lately introduced an interesting concept for almost absolute para-amination by charge-transfer-directed radical substitution (Scheme 1 B).13 The specialized aminating reagent employed in their work was the doubly cationic radical TEDA2+• that derived from a single-electron reduction of Selectfluor by bimetallic catalyst. It held very high electron affinity of 12.4 eV (calculated by DFT), which was proposed to contribute to charge transfer in the transition state of radical addition and resulted in high positional selectivity. Apparently this amination avenue suffers from certain limitations, as radicals with comparable high electron affinity are uncommon, and strategies to access them are unexplored for now. In our previous work on CDC-amination reaction,8 acyclic diarylamine was incapable of coupling with phenol to furnish the intermolecular aryl C-N bond only in the presence of oxidant and light irradiation (Scheme 1 C), which could attribute to its higher bond dissociation energy (BDE) and oxidation potential value than phenothiazine. Besides, another possibility for this
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A) Dehydrogenative C-H amination of aromatics R1
Ar R1
path A
N
R2
R1
N X
R2 H
N-center ed radical addition
X = O, S, N halogen
N Ar
R1
R2
-
+
N
R2
- e , -H
Ar
Ar
R H R2 N H 1
path B
R1
X =H
N H
R2
regioselective isomers
N-centered nucleophilic attack
B) Charge-transfer-directed radical substitution N N
N N
Cl 2+
(TEDA
Cl
N N
Cl
- e-, -H+
)
par a-selective pr oduct
high electron af f inity
C) Non-metallic cross-dehydrogenative amination between phenol and diarylamine Me Me
Me
OH Conditions
NH
via:
N OH
Ar
Ar N
OMe MeO Me
O
F. W . Patureau Conditions7: O2, cumene, 130~170 oC n.d.
W . J. Xia Conditions8: K 2S2O8, visible light, rt n.d.
This work Conditions: OPC, S2O82-, visible light, rt 38% (62% br sm)
R radicals cross-coupling
Scheme 1. Radical-based intermolecular amination reactions.
situation, we hypothesized, is the intrinsic photo-physical properties of phenothiazine derivatives to absorb visible light,14 which might enable a possible energy transfer process to make the transformation happened. Unfortunately, acyclic diarylamines were generally not documented to possess similar optical absorption abilities. Moreover, the cross-dehydrogenative amination between phenol and diarylamine also failed to carry out under oxidation conditions at high temperature (Scheme 1 C)7a or modified lower temperature7b, or even in Ru-catalyzed heating conditions7c. Considering the inert reactivity of N-H bond, we wondered if it could be activated by any catalysts to promote the N-H bond dissociation and furnish the N-radical. Herein, to address the presented challenge, we disclose a non-metallic oxidative system through merging organic-photocatalyst (OPC) with persulfate to enable the direct cross-coupling reaction between
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phenols and acyclic diarylamines to produce triarylamines in genuinely benign and facile conditions (Scheme 1 C). The resulted triarylamine units have been extensively investigated as opto- and electro-active materials in the field of solar cells and as hole transport materials in organic light-emitting diode (OLED) fields as well.15 RESULTS AND DISCUSSION We commenced our studies on this challenging target by adding photocatalysts (PCs) with different oxidation potentials into the starting mixtures of di-p-tolylamine 1a and 4methoxyphenol 2a in the same reaction conditions8 as shown in Scheme 1 C. After irradiation for 48 h, the expected cross-coupling amination product 3a was obtained in 7% isolated yield in the case of 2,4,6-triphenylpyrylium salt A1 as a catalyst (Table 1, entries 1-2). Scanning on solvents proved DCM to be the optimal reaction medium (entries 3-5), and further screening on diverse oxidants revealed that (NH4)2S2O8 could provide a significant improvement in reaction efficiency (entries 6-8). Considering the oxidation potential of PC plays a crucial rule in reaction, alternative PCs, such as different 2,4,6-substituted pyrylium salts (A2, A3)16 and 9-mesityl-10methylacridinium salt (B)17, which had comparable high oxidation abilities as catalyst A1 were therefore evaluated and turned out to affect subtly on the yield of desired product (entries 9-11). Finally, catalyst A1 was preferable when compared to catalyst B that could provide similar reaction efficiency, owing to its preparation was much easier (only one step without metal application) than the latter. Moreover, control experiment highlighted the important role of the light in this transformation (entry 12).
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Table 1. Optimization of the Reaction Conditions for Cross-Aminationa
0.015
0.01
Current (A)
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0.005
0
-0.005 0
0.5
1
1.5
2
2.5
3
Potential (V)
entry
PC (photocatalyst)
oxidant
solvent
yield (%)b
1c
Ir[dF(CF3)ppy]2(dtbbpy) PF6
K2S2O8
MeCN
trace
2c
Catalyst A1
K2S2O8
MeCN
7
3
Catalyst A1
K2S2O8
DCM
18
4
Catalyst A1
K2S2O8
DMF
0
5
Catalyst A1
K2S2O8
DMSO
0
6
Catalyst A1
(NH4)2S2O8
DCM
38 (62)d
7
Catalyst A1
O2
DCM
0
8
Catalyst A1
t-BuOOH
DCM
trace
9
Catalyst A2
(NH4)2S2O8
DCM
25
10
Catalyst A3
(NH4)2S2O8
DCM
27
11
Catalyst B
(NH4)2S2O8
DCM
37
12e
Catalyst A1
(NH4)2S2O8
DCM
trace
a
Reaction conditions: using 1a (0.3 mmol), 2a (0.6 mmol), oxidant (0.6 mmol), and PC (5 mol%) in solvent (0.1 M) under irradiation of 8 W blue LED strips for 5 h. bIsolated yields. cReaction for 48 h. dIsolated yields in brackets was based on recovered starting material. eReaction was performed in the dark.
Having the optimal synergistic oxidation conditions in hand, we turned our attention to examine the scope of the amine component in this new aryl C-N bond formation protocol. As shown in Scheme 2, a broad range of structually diverse amines undergo efficient CDCamination reactions with 2a to afford the desired cross-coupling products smoothly. However, it was found that the reaction efficiency was closely related to electronic effects, wherein amine
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bearing electron-donating groups, e.g. methyl and tert-butyl, was more readily to deliver product (3b and 3c). Unsymmetrical diarylamines consisting of substituents on one or two of aromatic rings were also viable participants for corresponding C-N bond-forming compounds (3d-3k and 3n-3o). Especially, amine bearing a steric bulky moiety proceeded C-N coupling reaction smoothly in moderate efficiency (3i).To be noted, the yield of product 3j that based on recovered OH
OH
PC, (NH4)2S2O8
Ph
HN
N
DCM, blue LED, rt
Ar1
Ar2
OMe
PC: Catalyst A1
Ar2
Ar1
Ph
2a
BF4
3
1
Ph
O
OMe
Substrates within scope OMe
R
OH
OH
N
N
OMe
OH
2
R
R
4
OMe
OMe
OMe
3d, R = 2-Me, 65% (81% brsm) 3e, R = 3-Me, 68% (89% brsm) 3f, R = 3-iPr, 53% (79% brsm) 3g, R = 4-Me, 37% 3i, R = 2,3-(CH2)4, 64% (72% brsm)
3b, R = t-Bu, 32% (50% brsm) 3c, R = H,
