Copper-Catalyzed Asymmetric Ring-Opening of Cyclic Diaryliodonium with Benzylic and Aliphatic Amines

Article abstract of DOI:10.1002/adsc.201800637

A Cu-catalyzed asymmetric ring-opening reaction between cyclic diaryliodonium and benzylic or aliphatic amines has been developed. At low concentration of the amines, realized by either mixing the amines with Lewis acid or slow addition of the amines, the reaction afforded the products in high enantioselectivity. (Figure presented.).

Full text of DOI:10.1002/adsc.201800637

Title: Copper-catalyzed Asymmetric Ring-opening of Cyclic
Diaryliodonium with Benzyl and Aliphatic Amines
Authors: Shibo Xu, Kun Zhao, and Zhenhua Gu
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10.1002/adsc.201800637
Advanced Synthesis & Catalysis
VERY IMPORTANT
PUBLICATION
COMMUNICATION
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Copper-catalyzed Asymmetric Ring-opening of Cyclic
Diaryliodonium with Benzylic and Aliphatic Amines
Shibo Xu†, Kun Zhao† and Zhenhua Gu*
Department of Chemistry, Center for Excellence in Molecular Synthesis, and Hefei National Laboratory for Physical
Sciences at the Microscale, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P.
R. China, E-mail: zhgu@ustc.edu.cn
†These authors contributed equally to this work.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. A Cu-catalyzed asymmetric ring-opening
reaction between cyclic diaryliodonium and benzylic or
aliphatic amines has been developed. At low
concentration of the amines, realized by either mixing the
amines with Lewis acid or slow addition of the amines,
the reaction afforded the products in high
enantioselectivity.
Keywords: atropisomer; axial chiral; copper;
amination; asymmetric catalysis
Figure 1. Biaryl Atropisomeric Natural Products and
Ligand
Owing to the difficulty for their preparation, cyclic
Axial chirality is one of the most important ways for
molecules to manifest their three-dimensional
structures. They not only are the key motifs of natural
products or biologically active molecules,[1] but
become the privileged skeletons of some chiral ligands
and catalysts (Figure 1). The atropisomers require a
rotation restricted single bond, which are usually hard
to be constructed with respect to both reactivity and
selectivity. Despite the growing attention paid to
axially chiral compounds, the (catalytic) asymmetric
synthesis of atropisomers with high enantiopurity is
compelling yet still challenging.[2] Besides the aryl-
aryl cross-coupling and oxidative coupling,[3] several
new strategies, namely, asymmetric cycloaddition,[4]
(dynamic) kinetic resolution,[5] desymmetrization[6]
diaryliodonium salts have received much less attention
compared to their acyclic counterparts. Though there
is no aromaticity in the iodine heterocycles, the cyclic
diaryliodoniums showed poor reactivity toward
nucleophilic substitutions.[10] It was not until recently
that transition-metal-catalyzed substitution reaction of
cyclic diaryliodonium compounds has been
significantly explored.[11]
Table 1. Temperature Effect with Benzylamine as
Nucleophilea)
and
organocatalyzed
rearrangements,[7]
were
developed to access atropisomers with different ortho
functional groups. Bringmann and Hayashi groups
creatively
developed
catalytic
ring-opening
approaches for the efficient synthesis of biaryl
atropisomers.[8] Recently, our group reported a
copper/bis(oxazolinyl)-pyridine-catalyzed
ring-
opening reaction of cyclic diaryliodonium salts with
anilines or benzylamines for the synthesis of
atropisomeric
2'-iodo-[1,1'-biphenyl]-2-amine
derivatives with very high enantiopurity.[9]
1
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10.1002/adsc.201800637
Advanced Synthesis & Catalysis
Entry T. (oC) Time (h) Yield
ee (%)
coordination, would significantly influence the
enantioselectivity of this reaction. To decrease
obnoxious coordinative effect of the amines, some
Lewis acids were added to the reaction mixture (Table
3). To more clearly show the effect of Lewis acid,
benzylamine (2a) has been chosen as the nucleophile
since benzylamine (2a) displayed more coordinative
ability and gave worse enantioselectivity than 4-
(trifluoromethyl)benzylamine (2b). To our delight, the
ee value was improved from 66% to 78% by the
addition of tri(iso-propyl) borate (entries 1-2).
BF3·OEt2 was harmful for the yield though the
enantioselectivity jumped to 93% (entry 3).
Subsequently aryl boronic acids were tested and the
electron-withdrawing groups at the phenyl rings were
beneficial for the enantioselectivity (entries 4-6). The
reaction with (perfluorophenyl)boronic acid gave best
results, the ee value of 3b reached 95% (entry 6).
(%)b)
99
1
2
3
4
5
6
25
0
-5
-10
-15
-20
5
66
70
76
90
64
42
24
24
24
24
72
99
99
95
48
63
a) The reactions were performed with 0.10 mmol of 1a
,
2a (0.12 mmol, 1.2 equiv), 4a (5.0 mol%), and Na2CO3
(0.30 mmol, 3.0 equiv) in CH2Cl2 (2.0 mL). b) Isolated
yields.
During our studies on the Cu-catalyzed asymmetric
ring-opening reaction of cyclic diaryliodoniums, it
was found that under the standard conditions the
reaction with aniline gave 98% ee while the one with
benzylamine afforded the product in 70% ee at 0 ºC.
In our previous report, to achieve high
enantioselectivity, it was necessary for benzylamine
(2a) to be used as the limiting reagent (1a/2a =
1.2:1).[9] However, since the cyclic diaryliodonium
salts are more precious than benzylamine, compound
1a was used as the limiting reagent (1a/2a = 1:1.2) to
optimize the reaction conditions in this study (Table 1).
The enantiomeric excess of the product 3a increased
by lowering the reaction temperature gradually before
Table 3. Lewis Acids Effectsa)
Entry Lewis Acid
Yield (%)b) ee (%)
o
1
2
3
4
5
6
-
99
88
60
99
66
78
93
80
81
95
reaching the highest ee at -10 C (entries 1-4). The
B(OiPr)3
BF3·OEt2
PhB(OH)2
2,4-(CF3)2C6H3B(OH)2 99
C6F5B(OH)2 99
reactions performed at the temperature lower than -10
oC gave decreased enantioselectivity and yields
(entries 5-6). Furthermore, the challenge is that the
optimal reaction temperature significantly depends on
the electronic properties of the benzylamine
derivatives. For example, the reaction of 1a with (4-
(trifluoromethyl)benzylamine (2b) gave 3b in 83%
a)
The reactions were performed with 1a (0.10 mmol),
2a (0.12 mmol, 1.2 equiv), 4a (5.0 mol %), Na2CO3
o
(0.30 mmol, 3.0 equiv), and Lewis acid (0.14 mmol, 1.4
ee at -10 C (Table 2, entry 1). Surprisingly, the
b)
equiv) in CH2Cl2 (2.0 mL) at room temperature.
enantioselectivity was improved to over 95% when the
reaction were performed at to 0-25 oC (entries 2-5).
Isolated yields.
Table 2. Temperature Effect with 4-(Trifluoromethyl)
benzylamine as Nucleophilea)
The requirement of careful optimization of reaction
temperature for different substituted benzylamines or
addition of C6F5B(OH)2 renders the reaction either
inconvenient or costly. In our previous studies it was
found that excessive bis(oxazolinyl)pyridine (PyBox)
relative to copper was necessary to achieve high
enantioselectivity.[9] We reasoned that the excessive
PyBox maintains a proper concentration of 5 by
suppressing the releasing of ligand to form 6 or other
related copper species, which might serve as catalyst
to deliver racemic products. To verify that
Cu(PyBox)PF6 (5) is the active catalyst rather than
Cu(PyBox)2PF6 (4), we performed the following
studies and successfully developed a practical protocol
for this ring-opening reaction by the use of benzyl or
aliphatic amines.
Entry T. (oC) Time (h) Yield (%)b ee (%)
1
2
3
4
5
-10
-5
0
10
25
36
24
24
12
12
85
99
99
99
99
83
90
96
95
95
a)The reactions were performed with 0.10 mmol of 1a, 2b
(0.12 mmol, 1.2 equiv), 4a (5.0 mol %), and Na2CO3 (0.30
mmol, 3.0 equiv) in CH2Cl2 (2.0 mL). b)Isolated yields.
These observations indicate that the basicity of the
amines, which might destabilize the Cu(I)/bis-
(oxazolinyl)pyridine complex by competitive
2
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10.1002/adsc.201800637
Advanced Synthesis & Catalysis
(3b-d, 3f) or electron-withdrawing groups (3e) on the
phenyl rings. The use of pyridin-4-ylmethanamine
diminished the enantioselectivity to 88% (3g). The
center chirality of 1-phenylethanamine has marginal
effect on the enantioinduction, as reflected by their
high diastereoselectivity ratios (3h and 3i). To our
delight, the reaction with furan-2-ylmethanamine also
afforded the product in 99% ee with a slightly declined
yield (3j).
Scheme 1. Plausible Equilibrium of Copper Complexes
Table 5. Substrate Scopea)
The observed ee improvement by Lewis acid clearly
indicates that low concentration of benzylamine is
beneficial for enantioselectivity. We initially used
complex 4b (PyBox/Cu = 1:1) as the catalyst (Table
4),[12] which usually gave poor enantioselectivity even
for aniline analogues. Unsurprisingly, the reaction of
1a with benzylamine and 5 mol% of 4b resulted in a
low ee value of the product 3b (entry 1). Addition of
2a via a syringe pump within 5 h did not improve the
enantio- selectivity (entry 2), while prolonging the
addition time to 25 h can increase ee to 76% (entry 3).
The ee value was further improved to 92% and 97%
when the addition time reached 40 h or longer (entries
4-5). These results clearly demonstrate that PyBox/Cu
(1:1), such as 5, can give excellent stereoinduction. It
is worth noting that the overall reaction rate by slow
addition of benzylamine was faster than that of one-
pot reaction.
Table 4. Addition time effect on the enantioselectivitya)
Entry Addition
Time (h)
Temp. Yield
Ee (%)
(oC)
-10
25
(%)b)
68
1
2
3
4
5
one pot c)
58
53
76
92
97
5
99
25
40
67
25
99
25
99
25
99
a)A solution of
2 (0.12 mmol, 1.2 equiv) in CH2Cl2 (2.0
a)The reactions were performed with 1a (0.10 mmol), 2a
(0.12 mmol, 1.2 equiv), 4b (5.0 mol%), and Na2CO3 (0.30
mL) was added to a mixture of 1a (0.10 mmol), 4a (5
mol%), and Na2CO3 (0.30 mmol, 3.0 equiv) in (CH2Cl)2
(2.0 mL) by syringe pump within 15-24 h.
mmol, 3.0 equiv) in CH2Cl2 (2.0 mL). b) Isolated yields.
c)
The reaction time: 24 h.
The reaction also proceeded very successfully with
With this optimized conditions in hand, we tested
the substrate scope of this reaction with respect to
amines (Table 5). Generally, catalyst 4a showed more
efficient in stereoinduction than complex 4b. The
catalyst can either be prepared as a stable complex 4a
or by mixing Cu(CH3CN)4PF6 with Inda-PyBox in situ
(1:2) before addition of the substrates. Generally, the
reaction with benzylamine derivatives gave very high
enantioselectivity regardless of the electron-donating
aliphatic
amines.
The
reactions
of
3-
aminopropanenitrile, allylamine and propynylamine
gave 98-99% ee (3k-m). In comparison, the
corresponding one-pot reaction with allylamine gave
84% yield with 60% ee (3k). The amines tethered with
cyclic aliphatic substituents were also compatible
substrates, which gave 93-99% ee (3n-p). The reaction
of the amines bearing other coordinative atoms or
active functional groups, such ether, thioether or free
3
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10.1002/adsc.201800637
Advanced Synthesis & Catalysis
hydroxyl groups worked equally efficiently (3q-s).
The amine with a long aliphatic chain, such as
octadecan-1-amine smoothly coupled with cyclic
iodonium to give the product in excellent selectivity
(3t).
We also studied the reaction of various substituted
cyclic diaryliodonium salts to further probe the
generality of this reaction (Table 6). Introducing
methyl groups into the diaryliodoniums did not affect
the
selectivity
(3u-v).
The
Cu
catalyst
regiospecifically cleaved the less sterically hindered
C-I bond when groups were introduced to the adjacent
position to the IIII atom. This regioselectivity is
consistent with the observation of Gaunt, Sanford and
Wen.[11b,13]
The
atropisomerism
of
cyclic
Figure 2. Anomalies of This Reaction: The reactions were
performed with 1a (0.10 mmol), for details see ESI.
iodonium(III) salts is not stable, whose two
conformers undergo fast equilibrium.[9] Thus, good to
excellent enantioselectivities (80-93%) were still
achieved for those non-symmetric substrates(3w, 3y-
3cc). The OTs and CF3 groups at the ortho positions of
chiral axis were also tolerated (3x, 3z and 3cc).
Pleasingly, the reaction with substrate containing a
naphthalene skeleton also gave high yield and
enantioselectivity (3bb).
In some cases, the reaction with extra C6F5B(OH)2
as Lewis acid is superior over the slow addition of
amines (Figure 2). In the presence of C6F5B(OH)2, the
reaction gave excellent yields of 3z, while the
corresponding reaction by slow addition of
benzylamine afforded moderate yields. It is interesting
to see that the reactions by slow addition of 2,2,3,3,3-
pentafluoropropan-1-amine or morpholin-4-amine
gave poor yields, while the one-pot reactions afforded
74-89% yields with excellent enantioselectivity (3dd
and 3ee). Unfortunately, currently under either
condition of one-pot or slow addition of the amines,
the ring-opening reaction of 1a with secondary amines,
such as morpholine, is unsuccessful (3ff).
Table 6. Substrate Scopea)
Based on the observation during this study, we
propose a plausible catalytic cycle for this ring-
opening reaction (Scheme 2). It was reasoned that the
dissociation of complex 4a or 4b would generate
monomer 5, which was supposed to be the active
catalyst for this ring-opening reaction. Oxidative
addition of 1 with 5 stereoselectively delivered CuIII
intermediate 8. However, the coordination of 5 with
benzylamine would form complex 7 or multi-
coordinated complexes. This unexpected coordination
would decrease the overall reaction rate and
stereoinduction. The coordination of CuIII complex 8
with benzylamine gave 9, which, after deprotonation
and reductive elimination, would afford the product
and regenerate catalyst 5.
a)A solution of
(2.0 mL) was added to a mixture of
2
(0.12 mmol, 1.2 equiv) in (CH2Cl)2
(0.10 mmol), 4a
1
(5 mol%), and Na2CO3 (0.30 mmol, 3.0 equiv) in
(CH2Cl)2 (2.0 mL) by syringe pump.
4
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10.1002/adsc.201800637
Advanced Synthesis & Catalysis
General Procedures for One Pot Reactions
An oven-dried Schlenk tube containing a magnetic
stir bar was charged with 1 (0.10 mmol, 1.0 equiv), 2
(0.12 mmol, 1.2 equiv), Cu((+)-indaPyBOX)2PF6
(0.006 mmol, 5 mmol %), and Na2CO3 (0.30 mmol,
3.0 equiv). The Schlenk tube was capped with a rubber
septum and then evacuated and backfilled with
nitrogen (this sequence was repeated three times). Dry
dichloromethane (2.0 mL) was added through the
septum via syringe. The reaction mixture was stirred
at room temperature for the indicated hours, then was
filtered through a thin pad of silica gel (eluting with 30
mL of ethyl acetate/dichloromethane/triethylamine =
15:15:1) and concentrated under reduced pressure. The
crude residue was purified by flash chromatography on
silica gel to deliver the corresponding product.
General Procedures for One Pot Reactions with
Lewis Acids
Scheme 2. Plausible Catalytic Cycle
An oven-dried Schlenk tube containing a magnetic
stir bar was charged with 2 (0.12 mmol, 1.2 equiv) and
(perfluorophenyl)boronic acid (0.14 mmol, 1.4 equiv).
The Schlenk tube was capped with a rubber septum
and then evacuated and backfilled with nitrogen (this
sequence was repeated three times). Dry dichloro-
methane (2.0 mL) was added through the septum via
syringe and then the Schlenk tube was sealed. After
stirred for 1 h, 1 (0.10 mmol, 1.0 equiv), Cu((+)-
indaPyBOX)2PF6 (5.0 mg, 0.005 mmol, 5 mmol %)
and Na2CO3 (32.0 mg, 0.30 mmol, 3.0 equiv) were
added to the reaction mixture. The reaction mixture
was stirred at room temperature for 20 h and then
filtered through a thin pad of silica gel (eluting with 30
mL of ethyl acetate/dichloromethane/ triethylamine =
15:15:1) and concentrated under reduced pressure. The
crude residue was purified by flash chromatography on
silica gel to deliver the corresponding product.
In conclusion, we have realized a Cu/(PyBox)-
catalyzed ring-opening reaction of cyclic diaryl-
iodonium and benzyl or aliphatic amines. Mechanistic
insight provided opportunity for enhancing the
catalytic efficiency and substrate scope. In the
presence of Lewis acid or by slow addition of amines,
the substrate scope regarding the amines was
significantly improved and generally high
enantioselectivity was achieved for this ring-opening
reaction.
Experimental Section
The synthesis of the starting materials cyclic
diaryliodonium salts was described in our previous
report,[9] which followed by the known procedures
with slight modifications.[11a,11c,11g,14]
General Procedures for The Preparation of
Substituted 2-Aminobiphenyls from 2-Nitro Aryl
Iodides
General Procedures for Reactions with Slow
Addition of Amines via Syringe Pump
An oven-dried Schlenk tube containing a magnetic
stir bar was charged with 1 (0.10 mmol, 1.0 equiv),
Cu((+)-indaPyBOX)2PF6 (0.006 mmol, 5 mmol %),
and Na2CO3 (0.30 mmol, 3.0 equiv). The Schlenk tube
was capped with a rubber septum and then evacuated
and backfilled with nitrogen (this sequence was
repeated three times). Dry dichloromethane (2.0 mL)
was added through the septum via syringe. Amine 2
(0.12 mmol, 1.2 equiv) in dichloromethane (2.0 mL)
was added dropwise to the reaction mixture by a
syringe via a syringe pump over the indicated time at
the indicated temperature. After the addition
completed, the reaction mixture was filtered through a
thin pad of silica gel (eluting with 30 mL of ethyl
acetate/dichloromethane/triethylamine = 15:15:1) and
concentrated under reduced pressure. The crude
residue was purified by flash chromatography on silica
gel to deliver the corresponding product.
The preparation of substituted 2-nitrobiphenyls: An
oven-dried Schlenk flask containing a magnetic stir
bar was charged with the appropriate α-nitro aryl
iodide (1.0 equiv), Pd2dba3 (2.0 mol%), S-Phos (8.0
mol%), the aryl boronic acid (1.5 equiv) and
anhydrous powdered K3PO4 (3.0 equiv). The Schlenk
flask was capped with a rubber septum and then
evacuated and backfilled with nitrogen (this sequence
was repeated three times). Dry toluene (0.15 M) was
added through the septum via syringe and the resulting
mixture was stirred at room temperature for ~2 min.
The Schlenk flask was sealed. The reaction mixture
was heated at 90 °C with vigorous stirring for 24H.
The reaction mixture was cooled to room temperature,
diluted with ethyl acetate, filtered through a thin pad
of silica gel (eluting with ethyl acetate) and
concentrated under reduced pressure. The crude
5
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10.1002/adsc.201800637
Advanced Synthesis & Catalysis
material was purified by flash chromatography on
silica gel.
washed with Et2O three times, dried under vacuum to
afford the appropriate cyclic diaryliodonium.
The reduction of 2-nitrobiphenyls: In a round
bottom flask, a mixture of substituted 2-nitrobiphenyl
(1.0 equiv), and zinc powder (6.0 equiv) in acetic acid
(0.2 M) was stirred at room temperature overnight.
The reaction mixture was filtered through a pas of
celite (ethyl acetate). The most of acetic acid was
removed by evaporation under reduced pressure and
the residue was diluted with ethyl acetate and
neutralized with saturated NaHCO3 solution. The
organic layer was separated and filtered through a pad
of MgSO4 and concentrated. The residue was purified
by column chromatography on silica gel to provide the
desired product.
Acknowledgements
We are grateful for financial supported from NSFC (21622206,
21472179), the '973' project from the MOST of China
(2015CB856600), the Fundamental Research Funds for the
Central Universities (WK2060190086).
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Gulder, M. Breuning, Chem. Rev. 2011, 111, 563-639.
General Procedures for The Preparation of 2-
Aminobiphenyls from 2-Bromo Anilines
[2] a) G. Bringmann, A. J. P. Mortimer, P. A. Keller, M. J.
Gresser, J. Garner, M. Breuning, Angew. Chem. Int. Ed.
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C. Sparr, ACS Catal. 2018, 8, 2981-2988.
An oven-dried Schlenk flask containing a magnetic
stir bar was charged with the appropriate boronic acid
(1.5 equiv), Pd2dba3 (2.0 mol%), S-Phos (8.0 mol%)
and anhydrous K2CO3 (4.0 equiv). The Schlenk flask
was capped with a rubber septum and then evacuated
and backfilled with nitrogen (this sequence was
repeated three times). 2-Bromo anilines (1.0 equiv) in
toluene/EtOH/H2O (8: 2: 2, 0.25 M) was added
through the septum via syringe and the Schlenk flask
was sealed. The reaction was stirred at 90 °C for 12H
under nitrogen atmosphere before being allowed to
cool to room temperature. The mixture was extracted
with ethyl acetate, and the combined organic layers
were washed with H2O and brine, dried over
anhydrous MgSO4, concentrated by rotary evaporation.
The crude material was purified by flash
chromatography on silica gel to give the product.
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General Procedure for the Preparation of Cyclic
Diaryliodoniums from 2-Aminobiphenyls
To a stirred mixture of 2-aminobiphenyl (1.0 equiv)
and 4 M HCl (10.0 equiv) in THF (1.0 M) was added
an aqueous solution of NaNO2 (1 M, 1.2 equiv)
dropwise at 0 °C and stirred for 20 min. Aqueous KI
(2 M, 2.5 equiv) was added dropwise at 0 °C and
stirred for 10 min at 0 °C and 1H at room temperature.
The mixture was extracted with ethyl acetate, and the
combined organic layers were washed with H2O, brine
and saturated NaHSO3 solution. The organic phase
was dried over anhydrous MgSO4, filtrated and
concentrated by rotary evaporation. The crude product
was filtered through a pad of silica gel (haxanes) and
used in the next step without further purification.
To a stirred solution of above crude product in
dichloromethane (0.25 M) was added m-CPBA (75%,
2.0 equiv) in one portion. After m-CPBA being
completely dissolved, trifluoromethanesulfonic acid
(3.0 equiv) was added dropwise at 0 °C, and was
stirred at room temperature for 1H. Dichloromethane
was removed by rotary evaporation before the addition
of Et2O. The mixture was stirred for 20 min, and the
solid was collected by filtration. The crude solid was
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COMMUNICATION
Copper-catalyzed Asymmetric Ring-opening of
Cyclic Diaryliodonium with Benzyl and Aliphatic
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Adv. Synth. Catal. Year, Volume, Page – Page
Shibo Xu, Kun Zhao, Zhenhua Gu*
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