Palladium-Catalyzed Asymmetric Arylation of C(sp3)-H Bonds of Aliphatic Amides: Controlling Enantioselectivity Using Chiral Phosphoric Amides/Acids

Article abstract of DOI:10.1021/acs.orglett.5b00968

Enantioselective arylation of secondary β-C(sp³)-H bonds of 8-aminoquinoline amides was realized with a palladium catalyst. Chiral phosphoric amides and acids were used for the first time to control the stereoselectivity at the C-H bond cleavag

Full text of DOI:10.1021/acs.orglett.5b00968

Letter
pubs.acs.org/OrgLett
Palladium-Catalyzed Asymmetric Arylation of C(sp3)−H Bonds of
Aliphatic Amides: Controlling Enantioselectivity Using Chiral
Phosphoric Amides/Acids
Shao-Bai Yan,† Song Zhang,‡ and Wei-Liang Duan*,†
†State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, China
‡School of Pharmacy, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China
S
* Supporting Information
ABSTRACT: Enantioselective arylation of secondary β-C(sp3)−H
bonds of 8-aminoquinoline amides was realized with a palladium
catalyst. Chiral phosphoric amides and acids were used for the first
time to control the stereoselectivity at the C−H bond cleavage step
in the C−H activation reactions.
ransition-metal-catalyzed direct functionalization of un-
reactive C−H bonds has been extensively investigated in
the C−H activation reaction, consequently generating a chiral
center. To realize our hypothesis, we attempted to apply CPAs
for the enantioselective β-arylation reactions of aliphatic
carboxylic amides bearing an 8-aminoquinoline12,13 moiety
(equation in Table 1). The direct enantioselective β-function-
alization of such monosubstituted carboxylic amides with flexible
carbon chains has not been realized yet.14−16
T
recent years. Numerous synthetic methods have been developed
to build diverse complex compounds from simple and non-
functionalized molecules.1 By contrast, enantioselective C−H
activation has not received much attention probably because of
the absence of suitable ligands to control the stereoselectivity in
the C−H activation reaction. Chiral phosphine or nitrogen
ligands, which are widely used in asymmetric catalysis, have also
been applied to C−H activation reactions but with limitations.
Successful examples have only appeared recently.2 The most
frequently used strategies to generate enantioenriched products
are monoprotected amino acids in combination with a Pd
catalyst,3 chiral phosphine/NHC ligand-promoted Pd(0)-
catalyzed cyclization,4 and a chiral Cp* Rh catalyst.5 Despite
great progress, developing new protocols is still highly desirable
for enantioselective C−H activation reactions.
Carboxylic amide 1a was reacted with 4-iodoanisole 2a in the
presence of Pd(OAc)2 catalyst to examine an array of CPAs17 in
p-xylene at 140 °C. The use of 3,3′-bis(2,4,6-triisopropylphenyl)-
1,1′-binaphthyl-2,2′-diyl hydrogen phosphate L1a only gener-
ated a racemic product (Table 1, entry 1). The use of CPAs (L1b
and L1c) bearing 1- or 2-naphthyl groups at the 3,3′-position of
the binaphthyl skeleton generated products with 61.5:38.5 and
54:46 er, respectively (entries 2 and 3), which proved the
feasibility of our initial hypothesis. Furthermore, various CPAs
(L1d−L1l) were examined (entries 4−12), and up to 71:29 er of
product was obtained when the catalyst with a 3,3′-bis(SiPh3)
moiety was tested (entry 10). The simplest CPA L1m without
any substituents unexpectedly generated products with 83:17 er
(entry 13). To further examine the substituent effects of CPAs on
the er of products, several CPAs (L2a−L2f) that only bear
monosubstituents at the 3-position of the binaphthyl skeleton
were prepared and examined. The mono-3-substituted CPAs L2
generally induced an er (entries 14−19) higher than that of the
corresponding 3,3′-disubstituted CPAs, but still inferior to L1m.
To further improve the er, CPAs with a spiro skeleton, L3a and
L3b, were also examined, but a racemic product was generated
(entries 20 and 21). However, examining several bases and
solvents with CPAs L1m did not further improve the er (see
Supporting Information for details). Given that the different
binding energies and bond distances between nitrogen and
Carboxylic acids, such as pivalic acid, are frequently involved in
C−H activation reactions6 because these acids can facilitate the
cleavage of the unreactive C−H bond. This process is known as
the concerted metalation deprotonation mechanism.7 Con-
sequently, chiral carboxylic acids have been utilized to control
enantioselectivity in Pd(0)-catalyzed intramolecular cyclization
reactions with moderate enantiomeric excess.4d,f Chiral phos-
phoric acids (CPAs), as the prominent catalysts, have been
extensively applied in catalytic reactions with excellent
enantioselectivities.8 Although phosphoric acids have recently
been reported as additives in C−H activation reactions,9 the
detailed roles of these acids remain ambiguous.10 The use of
CPAs in C−H activation reactions to control stereoselectivity at
the C−H bond cleavage step has not been reported.11
Considering the high basicity of the PO function in the
phosphoric acid8d,g and good chiral environment induced by the
binaphthyl skeleton of BINOL-based CPAs, we envisioned
whether CPAs can serve as ligands to control stereoselectivity in
Received: April 3, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b00968
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
With the established conditions, the substrate scope of the
reaction was examined, and the results are shown in Scheme 1.19
Table 1. Palladium-Catalyzed Enantioselective Arylation of
Carboxylic Amide 1a with 4-Iodoanisole
a b
,
Scheme 1. Substrate Scope
temp (°C)/time
yield
a
b
entry ligand
Pd salt
(h)
(%)
er
1
2
L1a
L1b
L1c
L1d
L1e
L1f
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
PdCl2(MeCN)2
PdCl2(MeCN)2
140/7
40
55
54
69
71
52
53
38
47
70
48
24
racemic
61.5:38.5
54:46
140/10
140/12
140/10
140/12
140/10
140/10
140/10
140/5
3
4
54:46
5
67:33
6
71:29
c
7
L1g
L1h
L1i
59:41
8
53.5:46.5
67.5:32.5
71:29
9
10
11
12
13
14
15
16
17
18
19
20
21
L1j
140/10
140/5
L1k
L1l
60:40
140/8.5
140/12
140/10
140/5
55.5:44.5
83:17
d
L1m
L2a
L2b
L2c
L2d
L2e
L2f
64
23
86
33
91
50
58
38
31
80
53.5:46.5
70:30
140/5
63:37
140/11
140/12
140/10
140/8
75:25
75:25
77:23
L3a
L3b
L4a
L1m
L4a
L4a
racemic
racemic
86.5:13.5
88.5:11.5
90:10
a
Isolated yields; er was determined with chiral HPLC using hexane/
140/6
b
isopropyl alcohol. The absolute configuration of 3 was deduced from
22
140/8.5
140/12
140/7.5
120/7.5
the configuration of 3a, which was determined by comparison of the
e
d
23
83
specific optical rotation of a 3a derivative with the value reported in
e
e
d
c
24
25
97
literature (see Supporting Information). Reactions were run at 120 °C
d
PdCl2(MeCN)2
80
90.5:9.5
for 12 h.
a
1
Yields were determined by H NMR analysis of the crude product.
The er was determined with chiral HPLC using hexane/isopropyl
b
c
d
e
Substrates bearing various substituents on the iodoarene rings,
such as methyl, alkoxy, fluoro, chloro, and ester groups, were
well-tolerated, and the corresponding products were generated
in good yields and moderate to good enantiomeric ratio (Scheme
1, 3a−3h). In terms of the substituent effects on the aliphatic
carboxylic amides, the substrates also contained different
moieties, such as methyl, methoxy, trifluoromethyl, and halogen
under the standard conditions, and moderate to good er of
products was formed. A sterically hindered ortho-methoxy group
on the phenyl ring of substrate 1 or iodoarene 2 was tolerated,
and the desired products 3i and 3j were generated with decreased
yield and er (76.5:23.5 and 73.5:26.5). Coupling of substituted
amides with 4-iodoanisole yielded various β,β-diaryl-function-
alized carboxylic amides (3k−3o) in moderate to good er.
Notably, the heterocycle 2-iodothiophene was also used in the
reaction, and the β-functionalized product was isolated in 86:14
er. Finally, challenging substrates with alkyl substituents, such as
n-propyl and isopropyl, were examined, and the desired products
alcohol. (S)-L1g was used. Isolated yields. 10 mol % of Pd catalyst
and 20 mol % of ligand were used.
oxygen with Pd metal often exhibit varying results in the
reactions, whether chiral phosphoric amides L4 could enhance
stereoselectivity was also determined. The use of the chiral
phosphoric amide L4a enhanced the er of products to 86.5:13.5
(entry 22), which is better than the results derived from L4b−
L4f bearing different substituents on the nitrogen atom (see
Supporting Information for details). Carboxylic acids have
certain accelerated effects on the C−H activation reaction.6
Thus, different Pd sources were investigated, and the use of
PdCl2(MeCN)2 with ligand L1m or L4a increased the er to
88.5:11.5 and 90:10 (entries 23 and 24), respectively. Others
(Pd(OTFA)2, PdCl2, and Pd(acac)2) afforded inferior er (see
Supporting Information for details). Decreasing the temperature
to 120 °C slightly enhanced the er to 90.5:9.5 but with a
decreased 80% yield (entry 25).18
B
DOI: 10.1021/acs.orglett.5b00968
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
3q and 3r were produced with relatively low enantioselectivities
and yields.
Considering the observed important effects of phosphoric
acid/amide in the reaction, we conducted a series of experiments
to compare their effects on the reaction rate (Figure 1). The
This process effectively generated an array of β,β-diaryl
carboxylic derivatives in moderate to good enantiomeric ratios.
Chiral phosphoric amides and acids have been found to have
prominent effects on the control of enantioselectivity and the
acceleration of the reaction rate. Further improvement on the
stereoselectivity of the current reaction and extending the utility
of chiral CPAs in C−H bond activation are underway in our
laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures and characterization of the products.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b00968.
AUTHOR INFORMATION
Corresponding Author
■
Figure 1. Additive effects on the reaction rate.
*E-mail: wlduan@mail.sioc.ac.cn.
Notes
reaction rate with L4a was increased by 2.6 times compared with
that in the blank reaction. More acidic CPA L1m had a stronger
acceleration effect (3.7 times) than L4a. Meanwhile, widely used
pivalic acid exhibited an acceleration effect (1.3 times) inferior to
that of L1m and L4a. Then, the deuterium-labeled substrate was
used to examine the kinetic isotope effects, and kH/kD = 3.9 was
observed (see Supporting Information for details). This value
indicates that the C−H bond cleavage is the rate-limiting step in
this reaction,20 which is consistent with the reported data for a
similar reaction by Shi et al.13e
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by NSFC (20902099,
21172238, and 21472218). We thank Prof. Qi-Lin Zhou and
Prof. Shou-Fei Zhu at Nankai University for kindly donating
ligand L3a, and Prof. Tamio Hayashi at IMRE, Singapore, for
helpful suggestions.
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D
DOI: 10.1021/acs.orglett.5b00968
Org. Lett. XXXX, XXX, XXX−XXX