Simple branched sulfur-olefins as chiral ligands for Rh-catalyzed asymmetric arylation of cyclic ketimines: Highly enantioselective construction of tetrasubstituted carbon stereocenters

Article abstract of DOI:10.1021/ja3110818

New, simple, sulfinamide-based branched olefin ligands have been developed and successfully used in Rh-catalyzed asymmetric arylations of cyclic ketimines, providing efficient and highly enantioselective access to valuable benzosultams and benzosulfamidat

Full text of DOI:10.1021/ja3110818

Communication
pubs.acs.org/JACS
Simple Branched Sulfur−Olefins as Chiral Ligands for Rh-Catalyzed
Asymmetric Arylation of Cyclic Ketimines: Highly Enantioselective
Construction of Tetrasubstituted Carbon Stereocenters
Hui Wang, Tao Jiang, and Ming-Hua Xu*
Shanghai Institute of Materia Medica, Chinese Academy of Sciences, 555 Zuchongzhi Road, Shanghai 201203, China
S
* Supporting Information
their application in the more challenging but much less studied
ABSTRACT: New, simple, sulfinamide-based branched
olefin ligands have been developed and successfully used in
Rh-catalyzed asymmetric arylations of cyclic ketimines,
providing efficient and highly enantioselective access to
valuable benzosultams and benzosulfamidates containing a
stereogenic quaternary carbon center. This is the first
example of applying a sulfur−olefin ligand in catalytic
asymmetric addition of imines.
enantioselective 1,2-addition to cyclic N-sulfonyl ketimines to
construct functionalized sultams and sulfamidates bearing
quaternary stereogenic centers. To date, the use of chiral SOLs
in catalytic asymmetric additions of CN bonds remains
unprecedented. Herein we describe the first example of Rh/SOL-
catalyzed highly enantioselective arylation of ketimines with
arylboronic acids. With a structurally simple chiral sulfonamide-
based branched olefin ligand, the reaction provides efficient
access to a wide range of highly enantiomerically enriched α-
quaternary cyclic amines, benzosultams, and benzosulfamidates.
In conjuction with our interest in optically active non-
proteinogenic amino acids,15 we initially wished to access 3-
carboxy-substituted benzosultams containing a unique α-amino
acid framework. Such molecules would be attractive to organic
and medicinal chemists, but there are no reports of their
asymmetric synthesis via enantioselective catalysis.16 We started
with the reaction between cyclic N-sulfonyl α-iminoester 1a and
p-anisylboronic acid (2a) using the previously developed SOLs
L1−L3 as chiral ligands (Table 1). We found that the reaction
proceeds very well in the presence of 1.5 mol % [Rh(COE)2Cl]2
(COE = cyclooctene) and 3.3 mol % L1 in aqueous KF (1.5 M)/
toluene at room temperature (rt), giving a 96% isolated yield of
the expected 3-aryl-3-carboxybenzosultam 3a with 50% ee (entry
1). A solvent screen showed that dioxane, MeOH, DME, and
THF led to only trace amounts of product (entries 4−7).
Though slightly higher ee (56%) was obtained in CH2Cl2, the
yield decreased significantly (58%) (entry 8). Changing the
inorganic base failed to improve the enantioselectivity (entries
9−12).
After showing that simple N-(sulfinyl)cinnamylamines can
promote this arylation reaction, we systematically modified the
chiral SOL framework in an attempt to improve the catalyst
performance (Scheme 1). Like L2, N-sulfinyl homoallylic amine
L4 gave a low yield and enantioselectivity. It was also
disappointing that N-(sulfinyl)cinnamylamine analogues L5−
L7 containing substituents with different steric and electronic
natures were either less effective or almost inactive and gave no
higher ee. In sharp contrast, the equally simple but branched
olefin ligand L8 exhibited both high catalytic activity and
enantioselectivity, giving 3a in 89% yield with a promising 70%
ee. This led us to design and synthesize the new sulfonamide-
based branched olefin ligands L9−L15 with different R
yclic amines bearing sulfonamide functionality in the ring,
C
such as sultams and sulfamidates, have emerged as an
intriguing class of synthetic targets because they often exhibit a
broad spectrum of biological activities1 and are useful as chiral
auxiliaries2 and important synthetic reagents.3 However, despite
the development of many synthetic strategies,4−6 catalytic
asymmetric procedures for the generation of optically active
sultams and sulfamidates are very limited.5,6
Transition-metal-catalyzed asymmetric reduction of cyclic N-
sulfonyl ketimines is an efficient approach for the enantiose-
lective synthesis of benzosultams or benzosulfamidates contain-
ing an α-stereogenic center in the amine moiety,6 but it cannot be
used for the construction of quaternary carbon stereocenters.
Katsuki recently disclosed an Ir-catalyzed enantioselective
intramolecular C−H amination approach,7 but it also is not
applicable to α-quaternary benzosultams. Catalytic asymmetric
addition of organometallic reagents to cyclic N-sulfonyl
ketimines is an attractive and direct route to this target class,
but it is far less developed because of the significant difficulty of
reaction stereochemical control.8 Only very recently has Rh-
catalyzed highly enantioselective addition of arylboroxines using
chiral diene ligands been successfully realized,9 giving α-triaryl-
substituted benzosultams with excellent enantioselectivities; also,
a related report described the construction of a quaternary
benzosultam structure using potassium (E)-1-hexenyltrifluor-
oborate.10 Therefore, new approaches for catalytic enantiose-
lective addition are still in high demand.
We recently developed a new and promising class of chiral
sulfur-based olefin ligands (SOLs)11,12 and successfully
employed them in a series of Rh-catalyzed asymmetric
reactions.13,14 A particularly fascinating finding is that simple
chiral N-(sulfinyl)cinnamylamine ligands can promote highly
enantioselective 1,2-additions of arylboronic acids to both α-keto
esters and α-diketones to give tertiary α-hydroxy carbonyl
compounds with excellent enantioselectivities (up to 99% ee) at
ambient temperature.13d These results encouraged us to explore
Received: November 11, 2012
Published: January 8, 2013
© 2013 American Chemical Society
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Journal of the American Chemical Society
Communication
a
substituent is essential. Gratifyingly, an improved ee (89%) was
observed when benzyl-substituted L16 was used. The incorpo-
ration of a large naphthyl group was even more beneficial, as L17
and L18 gave the best enantioselectivity (90% ee) and yield
(>90%). Remarkably, these optimal ligands can be easily
prepared in high yields (Scheme 2).17 It should be noted that
Table 1. Screen of Ligands, Solvents, and Bases
Scheme 2. Synthesis of Optimal Ligands L16−18
b
c
d
entry
ligand
solvent
base
yield (%)
ee (%)
1
2
L1
L2
L3
L1
L1
L1
L1
L1
L1
L1
L1
L1
toluene
toluene
toluene
dioxane
CH3OH
DME
KF (1.5 M)
KF (1.5 M)
KF (1.5 M)
KF (1.5 M)
KF (1.5 M)
KF (1.5 M)
KF (1.5 M)
KF (1.5 M)
K3PO4 (1.5 M)
K2CO3 (1.5 M)
KH2F (1.0 M)
96
50
33
−
−
−
−
−
56
36
48
50
49
11
the widely used diphosphine ligand BINAP was ineffective in this
arylation reaction, indicating the unique catalytic performance of
the class of chiral SOLs.
3
trace
trace
trace
trace
trace
58
4
5
With effective ligands in hand, we examined the substrate
scope under the optimized conditions (Table 2).18 A wide range
6
7
THF
8
CH2Cl2
toluene
toluene
toluene
toluene
a
Table 2. Rh/L18-Catalyzed Asymmetric Arylation of 1
9
19
10
11
12
15
67
e
KOH (0.1 M)
73
a
Conditions: 1a (0.25 mmol), 2a (2.0 equiv), [Rh] (3 mol %), ligand
b
b
c
(3.3 mol %), and base in 2.0 mL of solvent. Unless noted, 1.0 equiv of
base was used. Isolated yields. Determined by chiral HPLC. 0.1
entry
R
Ar
3
yield (%)
ee (%)
c
d
e
1
2
H
H
H
H
H
H
H
H
H
H
4-MeOC6H4
4-MeC6H4
4-ClC6H4
Ph
3a
3b
3c
3d
3e
3f
94
94
68
89
90
90
78
84
62
35
96
84
91
80
59
86
76
81
91
85
94
90
90
93
93
99
94
91
92
97
95
90
92
99
90
99
99
90
94
88
97
85
equiv of KOH (0.1 M) was used.
d
3
Scheme 1. Further Ligand Screen
d
4
5
6
1-naphthyl
2-naphthyl
3-MeOC6H4
3-MeC6H4
3-ClC6H4
2-MeOC6H4
4-MeOC6H4
2-naphthyl
1-naphthyl
3-MeC6H4
3-ClC6H4
1-naphthyl
3-MeC6H4
2-naphthyl
3-MeC6H4
1-naphthyl
4-MeOC6H4
d
7
3g
3h
3i
d
8
d
9
d
10
3j
11
12
13
5-OMe
5-OMe
5-Me
5-Me
5-Me
5-F
3k
3l
3m
3n
3o
3p
3q
3r
3s
3t
d
14
d
15
16
d
17
5-F
18
5-Cl
d
19
5-t-Bu
7-Cl
20
21
6,7-(CH)4
3u
a
Conditions: 1 (0.25 mmol), ArB(OH)2 (2.0 equiv), [Rh] (3 mol %),
L18 (3.3 mol %), and KF (1.5 M, 1.0 equiv) in 2.0 mL of toluene at rt
b
c
d
for 3−5 h. Isolated yields. Determined by chiral HPLC. The
reaction was performed at 60 °C.
substituents for evaluation. Changing the phenyl group in L8 to
the more bulky 1-naphthyl group in L10 increased the
enantiocontrol (86% ee). L11 and L12 with ortho substitution
(CH3, Cl) on the phenyl ring gave similar results as L10, but L13
with the larger i-Pr substituent afforded decreased ee, suggesting
that having the appropriate steric demand of the R group is
important for the stereocontrol. Surprisingly, when the aryl
group was replaced with an alkyl group (CH3, L14), the ee was
maintained at 80%. L15 with an i-Pr group directly attached to
the double bond gave 3a with higher enantioselectivity (84% ee),
although the yield was much lower. These results showed that
fine-tuning of the ligand rigidity and the steric demand of the R
of arylboronic acids with varying electronic and steric demands
were successfully reacted with cyclic N-sulfonyl α-iminoesters 1,
giving the corresponding addition products mostly in high yields
with excellent enantioselectivities. In some cases with electron-
deficient or sterically bulky arylboronic acids, the reaction was
performed at 60 °C to improve the yield. Higher enantiose-
lectivities were observed with less reactive arylboronic acids
bearing an electron-withdrawing group on the benzene ring
(entries 1 and 2 vs 3; 7 and 8 vs 9; and 14 vs 15). Notably, the
reactions with more sterically hindered arylboronic acids such as
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Journal of the American Chemical Society
Communication
a
1-naphthylboronic acid gave the products with extremely high
enantioselectivities (99% ee; entries 5, 13, and 16). The absolute
configuration at the newly created stereocenter was determined
to be R by X-ray crystallographic analysis of 3e.19
Table 4. Rh/L18-Catalyzed Asymmetric Arylation of 6
After demonstrating the first catalytic enantioselective syn-
thesis of 3-carboxysubstituted benzosultams 3, we applied our
catalytic system to the construction of benzosultams bearing an
α-triaryl-substituted stereogenic center.9 The reaction of various
cyclic N-sulfonyl ketimines 4 with sodium tetraarylborates was
successfully promoted by the Rh/L18 complex (3 mol %) at 80
°C in dioxane/MeOH (Table 3). In all cases, excellent
b
c
entry
Ar
7
time (h)
yield (%)
ee (%)
1
2
3
4
5
6
7
4-MeC6H4
4-FC6H4
7a
7b
7c
7d
7e
7f
3
3
3
5
8
5
3
75
81
78
72
26
75
84
99
99
98
99
99
99
99
4-ClC6H4
3-MeOC6H4
2-MeC6H4
1-naphthyl
2-naphthyl
a
Table 3. Rh/L18-Catalyzed Asymmetric Arylation of 4
7g
a
Conditions: 6 (0.25 mmol), ArB(OH)2 (2.0 equiv), [Rh] (3 mol %),
L18 (3.3 mol %), and KF (1.5 M, 1.0 equiv) in 2.0 mL of toluene at 80
b
c
°C. Isolated yields. Determined by chiral HPLC.
b
c
Ar1
Ar2
5
yield (%)
ee (%)
a
entry
Table 5. Rh/L18-Catalyzed Asymmetric Arylation of 8
1
2
Ph
Ph
Ph
4-MeOC6H4
4-ClC6H4
3-MeC6H4
Ph
5a
90
90
81
84
85
91
74
85
82
83
88
72
97 (98)
97 (94)
96 (97)
96
5b
5c
3
4
4-MeOC6H4
4-MeOC6H4
4-MeC6H4
4-MeC6H4
4-ClC6H4
5a′
5d
5e
5
3-MeC6H4
4-MeOC6H4
4-ClC6H4
4-MeC6H4
3-MeC6H4
Ph
95
b
c
6
98
entry
R
Ar
9
yield (%)
ee (%)
7
5f
96
1
2
3
4
5
6
7
8
H
H
H
4-MeC6H4
3-ClC6H4
2-MeC6H4
Ph
9a
9b
9c
9d
9e
9f
96
98
99
96
99
98
99
98
99
99
99
99
99
99
99
99
8
5f′
5g
5c′
5d′
5h
97 (97)
96
9
4-ClC6H4
10
11
12
3-MeC6H4
3-MeC6H4
3-ClC6H4
97
5,6-(CH)4
6-Me
4-MeOC6H4
3-MeC6H4
98
4-MeC6H4
3-ClC6H4
Ph
96
6-Cl
a
Conditions: 4 (0.25 mmol), NaBAr2 (1.5 equiv), [Rh] (3 mol %),
4
7-OMe
8-Me
9g
9h
L18 (3.3 mol %), and CH3OH (3.0 equiv) in 1.0 mL of dioxane at 80
°C for 8−12 h. Isolated yields. Determined by chiral HPLC. Values
in parentheses were reported in ref 9.
4−Br-C6H4
b
c
a
Conditions: 8 (0.25 mmol), ArB(OH)2 (2.0 equiv), [Rh] (3 mol %),
L18 (3.3 mol %), and KF (1.5 M, 1.0 equiv) in 1.0 mL of toluene at rt
for 5−6 h. Isolated yields. Determined by chiral HPLC.
b
c
enantioselectivities (95−98% ee) comparable to those obtained
using a Rh/chiral diene complex (5 mol %)9 were observed. In
the case of 5b, a slightly better ee was obtained (97 vs 94%). In
entries 1 and 4, 3 and 10, 5 and 11, and 7 and 8, both enantiomers
of the product were obtained using the same catalyst simply by
switching the corresponding aryl acceptor and donor.
We next investigated the asymmetric arylation of benzo-fused
six-membered cyclic imine 6 to yield cyclic benzosulfamidates 7
bearing a CF3 group on the quaternary carbon stereogenic center
(Table 4). F-containing heterocyclic chiral amines are partic-
ularly interesting because of their wide range of biological
activities.20 The reaction proceeded smoothly with a sterically
and electronically diverse range of arylboronic acids in the
presence of [Rh(COE)2Cl]2 (1.5 mol %) and L18 (3.3 mol %) in
aqueous KF (1.5 M)/toluene at 80 °C, providing 7a−g in good
yields with extremely high enantioselectivities (98−99% ee).
Finally, we found that cyclic aldimines 8 were also viable
substrates (Table 5). Under the same catalytic conditions,
aldimines with either an electron-donating or electron-with-
drawing group (R) on any aromatic carbon readily underwent
the asymmetric arylation with arylboronic acids at rt, affording
chiral cyclic sulfamidates 9 in extremely high yields and
enantioselectivities (99% ee). To our knowledge, enantioselec-
tive aryl addition of cyclic imines 8 has not been reported to date;
it represents the most efficient and convenient route to 9.
On the basis of the reaction stereochemical outcome, an
empirical transition state model21,22 is proposed (Figure 1). We
Figure 1. Proposed transition state model.
assume that the arylrhodium species has a preferred
conformation with a specific geometry in which the aryl group
is positioned trans to the olefin ligand and the t-Bu moiety is
staggered. To minimize unfavorable steric interactions with the R
group attached to the double bond, the sulfonyl moiety of the
imine substrate coordinates to the Rh in such a way that the ring
oriented away from R; thus, carborhodation takes place from the
si face of the imine to give the R product.
In summary, we have discovered new simple sulfonamide-
based branched olefin ligands for asymmetric catalysis. With
these readily available ligands, highly efficient Rh-catalyzed
asymmetric additions of arylboronic acids to cyclic N-sulfonyl
ketimines have been successfully developed. The reaction
provides the first access to valuable benzosultams and
benzosulfamidates having a carboxylic or CF3 function at the
quaternary carbon in a highly enantioselective manner. More-
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Journal of the American Chemical Society
Communication
(7) Ichinose, M.; Suematsu, H.; Yasutomi, Y.; Nishioka, Y.; Uchida, T.;
Katsuki, T. Angew. Chem., Int. Ed. 2011, 50, 9884.
over, this is also the first example of the use of a sulfur−olefin
ligand for asymmetric addition of imines. This study further
demonstrates the usefulness of this recently developed class of
ligands13,14 and sets the stage for further exploration of their use
in other asymmetric transformations and the development of
other kinds of unique olefin ligands.
(8) Selected examples of catalytic enantioselective addition of carbon
nucleophiles to ketimines: (a) Berger, R.; Duff, K.; Leighton, J. L. J. Am.
Chem. Soc. 2004, 126, 5686. (b) Lauzon, C.; Charette, A. Org. Lett. 2006,
8, 2743. (c) Wada, R.; Shibuguchi, T.; Makino, S.; Oisaki, K.; Kanai, M.;
Shibasaki, M. J. Am. Chem. Soc. 2006, 128, 7687. (d) Fu, P.; Snapper, M.
L.; Hoveyda, A. H. J. Am. Chem. Soc. 2008, 130, 5530. (e) Shintani, R.;
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Recent related reviews: (f) Shibasaki, M.; Kanai, M. Chem. Rev. 2008,
108, 2853. (g) Kobayashi, S.; Mori, Y.; Fossey, J. S.; Salter, M. M. Chem.
Rev. 2011, 111, 2626.
(9) Nishimura, T.; Noishiki, A.; Tsui, G. C.; Hayashi, T. J. Am. Chem.
Soc. 2012, 134, 5056.
(10) Luo, Y.-F.; Carnell, A. J.; Lam, H. W. Angew. Chem., Int. Ed. 2012,
51, 6762.
(11) Reviews of chiral olefin ligands in asymmetric catalysis:
(a) Glorius, F. Angew. Chem., Int. Ed. 2004, 43, 3364. (b) Johnson, J.
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ASSOCIATED CONTENT
* Supporting Information
Experimental methods and characterization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
xumh@mail.shcnc.ac.cn
■
Notes
The authors declare no competing financial interest.
Grutzmacher, H.; Carreira, E. M. Angew. Chem., Int. Ed. 2008, 47, 4482.
̈
(d) Shintani, R.; Hayashi, T. Aldrichimica Acta 2009, 42, 31. (e) Feng,
C.-G.; Xu, M.-H.; Lin, G.-Q. Synlett 2011, 1345. Two pioneering
references: (f) Hayashi, T.; Ueyama, K.; Tokunaga, N.; Yoshida, K. J.
Am. Chem. Soc. 2003, 125, 11508. (g) Fischer, C.; Defieber, C.; Suzuki,
T.; Carreira, E. M. J. Am. Chem. Soc. 2004, 126, 1628.
ACKNOWLEDGMENTS
■
This paper is dedicated to Professor Guo-Qiang Lin on the
occasion of his 70th birthday. We thank the National Natural
Science Foundation of China (20972172 and 21021063) and the
Chinese Academy of Sciences for support.
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(a) Maire, P.; Deblon, S.; Breher, F.; Geier, J.; Bohler, C.; Ruegger, H.;
̈
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Schonberg, H.; Grutzmacher, H. Chem.Eur. J. 2004, 10, 4198.
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