N-Arylamines Coupled with Aldehydes, Ketones, and Imines by Means of Photocatalytic Proton-Coupled Electron Transfer

Article abstract of DOI:10.1002/chem.201801886

A photoredox-catalyzed umpolung strategy for coupling reactions between aldehydes, ketones, imines, and N-arylamines is reported. These reactions proceed by a Br?nsted acid-activated proton-coupled electron transfer pathway, and the protocol was used to synthesize a broad scope of 1,2-amino alcohols and vicinal diamines, both of which are common motifs in biologically active natural products, pharmaceutically active molecules, and ligands.

Full text of DOI:10.1002/chem.201801886

A Journal of
Accepted Article
Title: N Arylaminomethanes Coupled with Aldehydes, Ketones, and
Imines via Photocatalytic Proton-Coupled Electron Transfer
Authors: Qingmin Wang, qing xia, Hao Tian, Jianyang Dong, Yi Qu, Lili
Li, Hongjian Song, and Yuxiu Liu
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.201801886
Link to VoR: http://dx.doi.org/10.1002/chem.201801886
Supported by

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COMMUNICATION
N-Arylamines Coupled with Aldehydes, Ketones, and Imines via
Photocatalytic Proton-Coupled Electron Transfer
[
a]
[a]
[a]
[a]
[a]
[a]
[a]
Qing Xia , Hao Tian , Jianyang Dong , Yi Qu , Lili Li , Hongjian Song , Yuxiu Liu , Qingmin
Wang[a][b]*
Dedication ((optional))
Abstract: Herein, we report
a
photoredox-catalyzed umpolung
enantioselective reductive cross-coupling reaction of nitrones
with aromatic ketyl radicals (Scheme 1c).[8]
strategy for coupling reactions between aldehydes, ketones, imines
and N-Arylamines. These reactions proceed by a Brønsted acid-
activated proton-coupled electron transfer pathway, and the protocol
was used to synthesize broad scope of 1,2-amino alcohols and
vicinal diamines, both of which are common motifs in biologically
active natural products, pharmaceutically active molecules and
ligands.
Carbonyl (C=O) and iminyl (C=N) groups, which are strongly
polarized, are valuable synthons for the construction of β-
functionalized alcohols and amines by means of reactions with
nucleophiles.[1] When reductive umpolung strategies are
employed, reactions of carbonyl and iminyl groups can produce
ketyl/α-amino radicals, which are versatile synthetic
intermediates in numerous important bond-forming and bond-
breaking processes.[2] However, the strongly negative reduction
potentials of aldehydes (E1/2red
=
=
red
−1.93
−2.11
V
vs. SCE for
benzaldehyde),[3] ketones (E1/2
red
V vs. SCE for
acetophenone),[ and imines (E1/2 = −1.91 V vs. SCE for N-
benzylideneaniline),[3] as well as the need to use toxic and air-
and moisture-sensitive reducing agents greatly restricts the
practical utility of umpolung strategies.[4] Recently, photoredox-
catalyzed proton-coupled electron transfers (PCETs), in which a
proton transfer occurs in concert with an electron transfer, were
successfully used to facilitate the formation of ketyl/α-amino
radicals by lowering the energy barrier to single-electron
transfer.[ For example, Knowles et al. adopted a concerted
PCET strategy for ketyl formation, successfully using dual
Brønsted acid/photoredox catalysis for a highly enantioselective
intramolecular aza-pinacol coupling reaction (Scheme 1a).[6] In
3]
Scheme 1. Ketyl Radical Generation and Follow-up Chemistry
1,2-Amino alcohols and vicinal diamines represent one of the
most significant synthetic building blocks and key subunits of
bioactive natural products, pharmaceutically active molecules,
chiral auxiliaries and ligands.[ In seeking to develop more
efficient and straightforward protocols for the construction of
diverse 1,2-amino alcohols and vicinal diamines under mild
conditions using readily available starting materials, we
hypothesized that the introduction of proton shuttle,[ might
mediate proton transfer from N-arylamines to aldehydes,
ketones, or imines. Acceleration of one-electron reduction by the
proton shuttle might make aldehydes, ketones, and imines
attractive candidates for PCET activation, thus presenting an
opportunity to develop catalytic ketyl and α-amino radical
chemistries that employ comparatively mild outer-sphere
5]
9]
2017, Ngai et al. harnessed an elegant dual-catalyst system
(
comprising a Lewis acid and a transition-metal photosensitizer)
10]
under visiblelight irradiation to accomplish β-selective additions
of ketyl or α-amino radicals to Lewis acid-activated
alkenylpyridines (Scheme 1b).[7] Recently, Huang et al.
employed a synergistic catalysis of Ru-photocatalyst and chiral
N,N′-dioxide ligand-coordinated rare earth ion to realize
[
6a]
reductants in combination with a proton donor catalyst. Herein,
we report the realization of this hypothesis in the form of
intermolecular radical coupling reactions of ketyl and α-amino
radicals under Brønsted acid-activated condition. This mild,
broad-scope method can be expected to be valuable for the
synthesis of 1,2-amino alcohols and vicinal diamines.
[
[
a]
b]
Q. Xia, H. Tian, J.-Y. Dong, Y. Qu, L.-L. Li, H.-J. Song, Y.-X. Liu,
Prof. Q.-M. Wang
State Key Laboratory of Elemento-Organic Chemistry, Research
Institute of Elemento-Organic Chemistry, College of Chemistry,
Nankai University, Tianjin 300071, People’s Republic of China
E-mail: wangqm@nankai.edu.cn
Prof. Q.-M. Wang
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin), Tianjin 300071, People’s Republic of China
We began by focusing on the reaction between N-phenyl-
tetrahydroisoquinoline (1a) and p-(trifluoromethyl)benzaldehyde
(
2) (Table 1). The successful implementation of oxidized base
Supporting information for this article is given via a link at the end of
the document.
radical cation activated the aldehyde or imine by the PCET
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COMMUNICATION
mechanism urged us firstly investigate the base additive in the
With the optimal conditions for this radical-radical coupling
protocol in hand (Table 1, entry 5), we sought to determine the
substrate scope with respect to the N-arylamines (Table 2). N-
reaction condition.[5h,
11]
In the presence of base 1,4-
diaza[2.2.2]bicyclooctane (DABCO) and photoredox catalyst
Ru(bpy) Cl ·6H O, irradiation of a CH CN solution of the two
3
2
2
3
Aryl
and
N-heteroaryl
tetrahydroisoquinolines
bearing
substrates under Ar with 14 W white LEDs as a light source
gave 1,2-amino alcohol 3a in 43% yield as a 1.4:1 mixture of
diastereomers (entry 1). Extensive screening of alternative
bases did not improve the yield (entries 2 and 3; see also the
Supporting Information). Previous reports of visible-light induced
ketyl radical formation reactions are enhanced by the addition of
acids,[11b, 12] which were consistent with this reaction when
catalytic amount of oxalic acid was added (entry 4, 63% yield).
Futhermore, to suppress the possible formation of undesired
diol,[12b] and coordinate α-amino radical and ketyl radical cross-
substituents with varying electronic properties smoothly
furnished the desired products in 52–72% yields (3a–3i). N,N-
Dimethylanilines (3j and 3k) were amenable to the reaction,
whereas
a
low yield of 3l was observed with N-
methyldiphenylamine as a substrate. N-Phenyl-substituted five-
membered-ring substrates readily afforded the corresponding
1,2-amino alcohols (3p and 3q) in moderate yields. We also
evaluated some secondary amines. Reactions of arylglycine
ester substrates, as well as a substrate in which the ethyl ester
was replaced with a cyano group, gave moderate yields of the
corresponding products (3m–3o, 49–62%). N-(Naphthalen-1-
ylmethyl)aniline also tolerated the reaction conditions, affording
1,2-amino alcohol 3r in 34% yield. Finally, we used this reaction
system for late-stage functionalization of a derivative of the
antidepressant agent fluoxetine,[15] obtaining 3s in good yield.
coupling by Li+,[13] we added a lithium salt (LiBF
reaction mixture and found that the yield improved to 72% (entry
, 70% isolated yield). In the absence of DABCO and acid, a
, 1 equiv) to the
4
5
sharply decrease in efficiency was found (entry 6), indicated that
the lithium salt serves as a Lewis acid to induce Lewis acid-
activated single electron transfer (SET) process to afford
Table 2. Substrate Scope with Respect to the N-Arylamines [a]
[
14]
corresponding ketyl radical is impossible. Remarkably, we did
not observe any of the product 3a under the acid conditions
without DABCO and lithium salt (entry 7). Replaced DABCO with
DABCO conjugate acid (DABCO-oxalic acid) also lowered the
reaction efficiency (entry 8). Control experiments demonstrated
the critical roles of the photocatalyst, light, and an Ar
atmosphere; none of the desired product was detected in the
absence of any of these components (entries 9–11).
Table 1. Optimization Studies[a]
entry
base
acid
lithium salt
yield(%)[b]
1
2
3
4
5
6
7
8
DABCO
quinuclidine
Hantzsch ester
DABCO
DABCO
-
-
-
-
-
-
-
-
43
40
33
63
72 (70)
trace
0
25
0
oxalic acid
oxalic acid
-
oxalic acid
-
LiBF
LiBF
LiBF
LiBF
LiBF
-
LiBF
LiBF
LiBF
LiBF
4
4
-
DABCO-oxalic acid
DABCO
DABCO
DABCO
4
4
4
4
[
c]
9
1
1
4
4
4
0[d]
1[e]
0
0
[a]Reaction conditions, unless otherwise stated: 1a (0.3 mmol), 2 (0.2
[a]Reaction conditions: Table 1, entry 5. Isolated yields are provided.
Diastereomeric ratios (dr) were detected by 1H NMR. [b]Yield of gram-scale
reaction. [c]Dimethylacetamide was used as the solvent.
mmol), photocatalyst (1 mol%), base (1 equiv), lithium salt (1 equiv), and
3
acid (10 mol%) in CH CN (1 mL) were irradiated with 14 W white LEDs at
rt for 12 h under Ar. See Supporting Information for the details of the
optimization studies. [b]Yields were determined by 19F NMR analysis of the
crude reaction mixture with trifluoroanisole as an internal standard. The
value in parentheses is an isolated yield. [c]Without photocatalyst.
[d]Without light. [e]Under air atmosphere. Abbreviations: DABCO, 1,4-
diaza[2.2.2]bicyclooctane.
Next, we focused on evaluating the variety of aldehydes,
ketones, and imines that could be used in this protocol (Table 3).
Benzaldehydes bearing a wide variety of electron-withdrawing
groups smoothly yielded the desired products in 30–74% yields
red
(
4b–4i). Benzaldehyde, which is electroneutral (E1/2 = −1.93 V
vs. SCE),[ provided desired product 4a in moderate yield.
3]
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COMMUNICATION
Pyridinecarboxaldehyde and pyrimidinecarbaldehyde were both
amenable to the reaction conditions, efficiently affording 4j and
and off for the reaction of 1a and 2 was performed, and the
results clearly excluded the possibility of a radical chain process
in this reaction system (Scheme 2d). Electrochemical analysis
4k, respectively. To explore the generality of the reaction further,
3
+
III/II
we tested several more challenging aromatic ketones, which
afforded moderate yields of the corresponding products (4l–4o).
We wondered whether an aldimine could be used as a substrate. (E1/2 = +0.87 V vs. SCE, Supporting Information). A Stern-
demonstrated that Ru(bpy)
3
(E1/2
= +1.29 V vs. SCE), not
2
+
(E1/2*II/I = +0.77 V vs. SCE), is able to oxidize 1a
*Ru(bpy)
3
red
We were pleased to find that α-amino radicals generated from
aldimines could indeed undergo coupling reactions with 1a,
affording 4p–4s. To demonstrate further the synthetic utility of
the protocol, we tested several more-complex substrates,
including a sugar (diacetone-D-glucose, 4t) and an amino acid
derivative (4u). To our delight, they all afforded the desired
products in synthetically useful yields.
Volmer experiment (Scheme 2e) showed no measurable
2
+
red
luminescence quenching of *Ru(bpy)
3
by 2 (E1/2 = −1.93 V vs.
SCE) or by 2/DABCO in MeCN. However, inclusion of DABCO-
oxalic acid with 2 resulted in a decrease in the measured
fluorescence, thus suggested an oxidative quenching cycle
[
7]
[
6, 16]
through the PCET mechanism.
DABCO radical cation
+
•
red
(DABCO , E1/2 = +0.69 V vs. SCE for DABCO) also cannot be
able to oxidize 1a, therefore excluded the possibility that
DABCO act as an electron transfer agent.[12a]
Table 3. Substrate Scope with Respect to Aldehydes, Ketones, and Imines[a]
[a]Reaction conditions: Table 1, entry 7. Isolated yields are provided.
1
Diastereomeric ratios (dr) were detected by H NMR.
We carried out several reactions designed to provide insight
into the reaction mechanism. When the radical scavenger
TEMPO (2,2,6,6-tetramethylpiperidinooxy) was present under
standard conditions, the reaction was completely inhibited
(
Supporting Information). A reaction carried out under the
standard conditions but with an added allyl sulfone produced
radical addition product 5 in 29% yield. This result indicates the
involvement of an α-N-phenyl-tetrahydroisoquinoline radical (1a•,
Scheme 2a). When di(pyridin-2-yl)methanone 2v was inject into
the standard condition, reductive product 2v' was only produced
in 53% yield, suggested the existence of ketyl in this reaction
(
Scheme 2b). A competition experiment involving a 1/1 mixture
of 1a and deuterated substrate 1a-d revealed a mixed products
of 3a and 3a-d in 59% yield with a 1.6:1 ratio, indicating that
proton transfer may not be the rate-determining step (Scheme
c). An experiment in which the light source was switched on
2
Scheme 2. Mechanistic Experiments
2
2
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COMMUNICATION
A proposed mechanism for the transformation reported Acknowledgements
herein is depicted in Scheme 3. We envisioned a catalytic cycle
initiated by in situ generated DABCO conjugate acid
We are grateful to the National Natural Science Foundation of
China (grant nos. 21732002, 21562023, 21672117) and the
Tianjin Natural Science Foundation (grant no. 16JCZDJC32400)
for generous financial support for our programs.
+
−
(
HDABCO X ), which to be the Brønsted acid to activated the
aldehydes, ketones or imines through hydrogen bond.
Concerted PCET would follow, with electron transfer from the
photoexcited state *Ru(bpy) 2+ (E1/2*II/III = −0.81 V vs. SCE)
2
3
occurring concomitantly with proton transfer to the 2 along the
hydrogen-bond coordinate to generate a neutral ketyl or α-amino
Keywords: PCET • photoredox catalysis • ketyl radical • α-
amino radicals • radical reactions
•
3+
(E1/2III/II = +1.29
radical intermediate 2 . The resulting Ru(bpy)
V vs. SCE) is able to be reduced by N-arylamines 1 by means of
3
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•
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+
or vicinal diamines (3 and 4). Lastly, irradiation of Ru(bpy)
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3
3
and close the catalytic
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protocol for the coupling between a wide variety of aldehydes,
ketones, imines, and functionalized amines by photoredox
catalysis. Mechanistic studies offer valuable insight into the key
step that involves Brønsted acid-activated PCET process to
generate ketyl or α-aminoalkyl radicals. It can be expected to
facilitate syntheses of 1,2-amino alcohols and vicinal diamines,
both of which are common motifs in biologically active natural
products, pharmaceutically active molecules and ligands.
Studies directed toward the development of an asymmetric
version of the reaction are currently underway in our laboratory.
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Experimental Section
[
[
[
[
[
9]
8
General Procedure: To a solution of an N-arylamine (0.3 mmol, 1.5
Chem. Soc. Rev. 2010, 39, 89-102.
equiv) in fresh distilled CH
and 1h) was added Ru(bpy)
DABCO (22 mg, 0.2 mmol, 1.0 equiv), LiBF
3
CN (1.0 mL, 0.2 M) (dimethylacetamide for 1e
Cl ·6H O (1.5 mg, 0.002 mmol, 1 mol%),
(19 mg, 0.2 mmol, 1.0
10]
11]
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equiv), oxalic acid (2.0 mg, 0.02 mmol, 10 mol%), and aldehyde, ketone
or imine 2,2a–2u (0.2 mmol, 1.0 equiv) in a 8 mL vial. The mixture was
bubbled with Ar for 1 min, and the reaction executed under argon
atmosphere. The reaction was then stirred and photolyzed with two white
LEDs (10 W + 4 W) from opposite sites at distances to the vial of
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Afterwards, the mixture was diluted with CH
chromatography on silica gel to afford target products 3a–3s and 4a–4u.
2 2
Cl and purified by flash
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Entry for the Table of Contents (Please choose one layout)
COMMUNICATION
Qing Xia, Hao Tian, Jianyang Dong, Yi
Qu, Lili Li, Hongjian Song, Yuxiu Liu,
Qingmin Wang*
Page No. – Page No.
N-Arylamines Coupled with
Aldehydes, Ketones, and Imines via
Photocatalytic Proton-Coupled
Electron Transfer
Text for Table of Contents
Photoredox-catalyzed umpolung strategy for coupling reactions between aldehydes, ketones, imines and N-Arylamines was
demonstrated. Mechanistic studies offer valuable insight into the key step that involves Brønsted acid-activated PCET process to
generate ketyl or α-aminoalkyl radicals. The protocol was used to synthesize broad scope of 1,2-amino alcohols and vicinal diamines,
both of which are common motifs in biologically active natural products, pharmaceutically active molecules and ligands..
This article is protected by copyright. All rights reserved.