Rhodium-catalyzed, highly enantioselective 1,2-addition of aryl boronic acids to α-ketoesters and α-diketones using simple, chiral sulfur-olefin ligands

Article abstract of DOI:10.1002/anie.201106972

Simply the best: The title reaction has been achieved by asymmetric rhodium catalysis employing an extremely simple, chiral N-(sulfinyl)cinnamylamine ligand. A variety of highly enantioenriched, tertiary α-hydroxy carbonyl derivatives were easily accessed

Full text of DOI:10.1002/anie.201106972

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Angewandte
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DOI: 10.1002/anie.201106972
Asymmetric Catalysis
Rhodium-Catalyzed, Highly Enantioselective 1,2-Addition of Aryl
Boronic Acids to a-Ketoesters and a-Diketones Using Simple, Chiral
Sulfur–Olefin Ligands**
Ting-Shun Zhu, Shen-Shuang Jin, and Ming-Hua Xu*
Optically active a-hydroxy carbonyl compounds are not only
very important structural motifs in numerous biologically
interesting compounds, but also serve as fundamental build-
ing blocks for numerous applications in organic synthesis.[1]
Over the past decades, various methods have been developed
for the synthesis of these valuable chiral compounds.[2–5]
Among these, the transition-metal-catalyzed asymmetric
nucleophilic addition of organometallic reagents to a-keto
carbonyl compounds is a particularly powerful and straight-
forward approach.[4] In recent years, the use of stable,
commercially available aryl boronic acids in place of
common organometallic reagents in transition-metal-cata-
lyzed carbon–carbon bond-forming reactions has attracted
considerable attention.[5,6] However, despite these successes
with asymmetric reactions involving a,b-unsaturated carbon-
yl compounds[7] and aldimines,[8] the transition-metal-cata-
lyzed asymmetric addition of aryl boronic acids to carbon-
oxygen double bonds in ketones (producing enantioenriched
tertiary alcohols) remains under-studied and elusive. To date,
only very limited progress has been achieved in the rhodium-
catalyzed asymmetric addition of these species to isatins (up
to 91% ee),[5a,b] a-ketoesters (up to 95% ee),[5c,d,9] and a-
trifluoromethyl ketones (up to 83% ee);[10] all methods that
employ chiral phosphine, phosphite, or phosphoramidite
ligands.[11] With respect to a-diketones, to the best of our
knowledge, there are no examples of their catalytic, enantio-
selective coupling with aryl boronic acids to provide optically
active, tertiary a-hydroxyketones.[12] Thus, the development
of new catalytic systems that are capable of the efficient
synthesis of enantiomerically pure a-hydroxy carbonyl com-
pounds with a quaternary stereogenic center is highly
desirable.
readily available chiral sulfur–olefin hybrid ligands can also
display great catalytic activities and enantioselectivities in
asymmetric catalysis. We have recently become intrigued by
the possibility of their application in the more challenging
enantioselective 1,2-addition of aryl boronic acids to more
activated ketones, such as a-ketoesters and a-diketones, to
provide optically active, functionalized tertiary alcohols.
Herein we describe our efforts to address this issue. By
employing an extremely simple, chiral N-(sulfinyl)cinnamyl-
amine as a ligand, aryl boronic acids were effectively
activated under rhodium catalysis to react stereoselectively
with both a-ketoesters and a-diketones, giving products with
excellent enantioselectivities under exceptionally mild con-
ditions.
In our initial studies, we attempted to test whether our
recently designed chiral sulfinamide/sulfoxide–olefins 1–3 can
act as chiral ligands in the rhodium-catalyzed 1,2-addition of
aryl boronic acids to a-ketoesters (Scheme 1). The addition of
methyl phenylglyoxylate (9a) with p-tolylboronic acid was
chosen as a model reaction. To our delight, the reactions all
proceeded well in the presence of [{Rh(coe)2Cl}2] (3 mol%)
in aqueous K3PO4 (1.5m)/THF at 408C, giving the expected
We have recently designed a series of chiral sulfinamide/
sulfoxide-based olefin ligands and successfully employed
them in the rhodium-catalyzed asymmetric 1,4-addition of
aryl boronic acids to a,b-unsaturated carbonyl com-
pounds.[13,14] These studies demonstrated that simple and
[*] T.-S. Zhu, S.-S. Jin, Prof. Dr. M.-H. Xu
Shanghai Institute of Materia Medica, Chinese Academy of Sciences
555 Zuchongzhi Road, Shanghai 201203 (China)
E-mail: xumh@mail.shcnc.ac.cn
[**] Financial support from the National Natural Science Foundation of
China (20972172, 21021063), the Chinese Academy of Sciences,
SIMM, and the National Science & Technology Major Project is
greatfully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201106972.
Scheme 1. Ligand screening. coe=cyclooctene, Bz=benzyl.
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tertiary a-hydroxyester 11a in moderate to good yields with
very promising enantioselectivities (69–85% ee). Interest-
ingly, the structurally extremely simple chiral N-(sulfinyl)cin-
namylamine 3[15] showed the greatest potential, both in terms
of catalytic activity and enantiocontrol (90% yield and 85%
ee).[16] With this result in mind, we further elaborated the
ligand structure and designed several new sulfonamide–
olefins 4–6 for screening. However, these efforts were
unsuccessful. In all cases, the new ligands were either less
effective or altogether inactive. It should be noted that,
although this 1,2-addition reaction could be catalyzed by
chiral diene ligand 7 and diphosphine ligand 8 (BINAP), only
very low enantioselectivities (9% and 13% ee, respectively)
were attained. These results highlight the unique catalytic
performance of chiral, sulfur-based, olefin-class ligands.
Following up on the encouraging results obtained with
chiral N-(sulfinyl)cinnamylamine 3, the reaction conditions,
particularly the effect of the ester group in phenylglyoxylate,
were thoroughly explored (Table 1). Under conditions similar
to those noted above, a survey of solvents indicated that the
hydrolysis of the aromatic ester substrates (entries 7–9). After
extensive studies, we found that the use of even milder
conditions employing less base could solve the problem.
Finally, 2-naphthyl phenylglyoxylate underwent reaction
smoothly, producing tertiary a-hydroxyester 11i in good
yield (86%), with the highest ee (94%) obtained in aqueous
KOH (0.1m)/THF at room temperature (entry 14).
Having observed that the rhodium/sulfur–olefin-catalyzed
addition to 2-naphthyl ester under the above optimized
conditions can be highly selective, we then decided to
investigate the scope of this process. As summarized in
Table 2, a variety of aryl boronic acids with diverse steric and
electronic properties were tested with 2-naphthyl a-ketoest-
ers 9i–k. We were pleased to find that the addition reactions
consistently gave the expected products in moderate to good
yields and excellent ee values (90–95%). Unlike the previ-
ously reported catalytic system,[5c,d] the addition reaction
seems insensitive to the steric effects of the substrates and
reagents, as both sterically encumbered 2-naphthyl 2-naph-
thylglyoxylate and ortho-substituted aryl boronic acids could
be successfully employed (Table 2, entries 5,6 and 19–21).
Notably, the system also allows for the efficient synthesis of
products where both Ar1 and Ar2 are substituted phenyl or
Table 1: Optimization of reaction conditions for different esters.[a]
Table 2: Asymmetric 1,2-addition of arylboric acids to a-ketoesters
catalyzed by [{Rh(coe)2Cl}2]/3.[a]
Entry
R
Base[b]
T [8C]
11
Yield[c]
[%]
ee[d]
[%]
1
2
3
4
5
6
7
8
Me
Et
iPr
tBu
Bn
Ph
4-ClC6H4
1-Np
2-Np
Me
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
LiF[e]
40
40
40
40
40
40
40
40
40
RT
RT
RT
RT
RT
11a
11b
11c
11d
11e
11 f
11g
11h
11i
11a
11a
11b
11h
11i
90
96
92
68
81
65
33
41
42
82
95
95
67
86
85
87
85
86
85
92
94
95
94
87
87
88
93
94
Entry
Ar1
Ar2
11
Yield[b]
[%]
ee[c,d]
[%]
1
2
3
4
5
6
7
8
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
Ph (9i)
4-ClC6H4 (9j)
4-ClC6H4 (9j)
4-ClC6H4 (9j)
4-ClC6H4 (9j)
2-naphthyl (9k)
2-naphthyl (9k)
2-naphthyl (9k)
4-MeC6H4
4-ClC6H4
4-FC6H4
4-MeOC6H4
2-MeC6H4
2-MeOC6H4
3-MeC6H4
3-ClC6H4
3-FC6H4
3-MeOC6H4
3,5-Me2C6H3
1-naphthyl
2-naphthyl
n-PhC6H4
Ph
11i
11j
11k
11l
11m
11n
11o
11p
11q
11r
86
84
86
94
72
73
82
67
72
72
74
87
76
82
66
66
70
75
89
87
54
94 (S)
94 (S)
94 (S)
92 (S)
92 (S)
90 (S)
93 (S)
95 (S)
95 (S)
93 (S)
92 (S)
91 (S)
92 (S)
93 (S)
95 (R)
92 (R)
95 (R)
93 (S)
91 (R)
94 (R)
93 (S)
9
10
11
12
13
14
Me
Et
1-Np
2-Np
KOH[f]
KOH[f]
KOH[f]
9
10
11
12
13
14
15
16
17
18
19
20
21
11s
11t
[a] Reactions were carried out with a-ketoester (0.25 mmol) and
1.5 equiv of p-tolylboronic acid, in the presence of 3 mol% of [{Rh-
(coe)2Cl}2], 3.3 mol% of ligand 3, and base in 1.0 mL of THF. [b] Unless
noted, base (1.5m, 0.5 equiv) was used. [c] Yield of isolated product.
[d] Determined by chiral HPLC analysis. [e] 1.2 equiv of LiF (1.5m) was
used. [f] 0.08 equiv of KOH (0.1m) was used.
11u
11v
11j’
11w
11x
11y
11u’
11y’
11z
4-MeC6H4
4-FC6H4
2-naphthyl
Ph
4-ClC6H4
2-MeC6H4
use of toluene, CH2Cl2, MeOH, or 1,2-dimethoxyethane leads
to a decrease in enantioselectivity (60% ee, 39% ee, 70% ee,
and 81% ee, respectively; not shown in Table 1). Varying the
ester substituent from methyl to ethyl, isopropyl, tert-butyl, or
benzyl did not improve the enantioselectivity of the reaction
(Table 1, entries 2–5). Gratifyingly, when phenyl phenylglyox-
ylate was employed, product 11 f was obtained with improved
enantioselectivity (92% ee, entry 6), as expected. Further,
varying the R group to p-chlorophenyl, 1-naphthyl, or 2-
naphthyl gave slightly better enantioselectivities (94–95%),
but led to a large decrease in yields owing to significant
[a] Reactions were carried out with a-ketoester (0.25 mmol) and
1.5 equiv of aryl boronic acid, in the presence of 3 mol% of [Rh],
3.3 mol% of ligand 3, and KOH (0.1m, 0.08 equiv) in THF(1.0 mL) for
3 h. [b] Yield of isolated product. [c] Determined by chiral HPLC analysis.
[d] The absolute configurations of 11i and 11t were determined by
comparing their optical rotation values with known data[18] after removal
of the 2-naphthyl group by ester hydrolysis under basic conditions (2.5m
KOH, see Supporting Information for details); the other configurations
were assigned by analogy.
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naphthyls groups (entries 16–18 and 20,21). The preparation
of such tertiary a-hydroxycarboxylate derivatives by the
direct addition of aryl boronic acids to aryl glyoxylates has not
been extensively explored before.[17] It is particularly remark-
able that a great level of enantiofacial discrimination can be
achieved despite only small differences between the aryl
groups (entries 16,17). Moreover, by simply switching the
corresponding Ar1 and Ar2 groups of the a-ketoesters and
boronic acids, both enantiomers of the product were obtained
with the same high level of enantioselectivity (entries 2 and
15, entries 13 and 19, and entries 18 and 20).
(entries 8–10). To further evaluate the substrate scope, the
reaction of other commonly used a-diketones with p-tolyl-
boronic acid was investigated. In the case of the less sterically
hindered aliphatic biacetyl 12e, a decreased yield and
moderate enantioselectivity was obtained (entry 11). Inter-
estingly, when unsymmetrical a-diketone 12 f was employed,
our system exhibited both regio- and enantio-selectivity.
Products 13l and 13m were both observed, with addition to
the less hindered and less electron deficient carbonyl group
predominating (entry 12). In all of the reaction examples, it is
worth noting that no diarylation occurs.
After our success with the 1,2-addition to a-ketoesters, we
turned our attention to the more challenging a-diketone
substrates in an attempt to generate optically active tertiary
a-hydroxyketone derivatives (Table 3).The catalytic enantio-
To demonstrate the synthetic utility of the current
method, the addition products 11m and 13g were subjected
to bromination by NBS/CCl4 to give the corresponding
bromides (14a and 14b). Upon treatment with NaHCO3,
the quaternary-carbon-containing, chiral 1,3-dihydroisoben-
zofuran (phthalan) products 15a and 15b were generated in
79% and 69% yield, respectively, without loss of optical
purity (Scheme 2). This result is notable because chiral 1,3-
dihydroisobenzofuran compounds are valuable pharmacolog-
ical compounds, as exemplified by the antidepressant drug
citalopram.[19] However, they are usually difficult to access
through asymmetric catalysis.[20]
Table 3: Asymmetric 1,2-addition of arylboric acids to a-diketones
catalyzed by [{Rh(coe)2Cl}2]/3.[a]
Entry Substrate
Ar
13 Yield[b] ee[c,d
[%]
[%]
1
2
3
4
5
6
7
8
R1 =R2 =Ph (12a)
4-MeC6H4
4-MeOC6H4 13b
4-ClC6H4
4-FC6H4
4-PhC6H4
13a
97
99
97
98
98
95
54
93
99
98
45
85
97 (S)
97 (S)
99 (S)
98 (S)
97 (S)
97 (S)
95 (S)
98 (R)
96 (R)
97 (R)
63 (S)
80 (S)
R1 =R2 =Ph (12a)
R1 =R2 =Ph (12a)
R1 =R2 =Ph (12a)
R1 =R2 =Ph (12a)
R1 =R2 =Ph (12a)
R1 =R2 =Ph (12a)
R1 =R2 =4-BrC6H4 (12b)
R1 =R2 =4-FC6H4 (12c)
13c
13d
13e
3-MeOC6H4 13 f
2-MeC6H4
4-MeC6H4
4-MeC6H4
13g
13h
13i
13j
13k
13l
Scheme 2. Synthesis of chiral 1,3-dihydroisobenzofurans. NBS=N-bro-
mosuccinimide, Np=naphthyl.
9
10
11
R1 =R2 =4-MeOC6H4 (12d) 4-MeC6H4
R1 =R2 =Me (12e)
4-MeC6H4
4-MeC6H4
12[e] R1 =Me, R2 =Ph (12 f)
In summary, we have developed a highly efficient
rhodium-catalyzed, asymmetric 1,2-addition of aryl boronic
acids to a-ketoesters and a-diketones through the use of
extremely simple, chiral N-(sulfinyl)cinnamylamine ligand.
The method offers easy, general, and practical access to a
variety of highly enantioenriched tertiary a-hydroxy carbonyl
compounds under exceptionally mild conditions. The catalytic
asymmetric addition to a-diketones represents an unprece-
dented synthesis of optically active, tertiary a-hydroxyketone
derivatives. Moreover, aside from their use in 1,4-addition to
a,b-unsaturated carbonyl compounds,[13,14] this is the first
example of the successful application of the recently devel-
oped, chiral sulfur–olefin ligands in asymmetric catalysis,
wherein the unique sulfonamide–olefin 3 has been shown to
be highly effective. It opens new opportunities for the use of
this novel class of readily available ligands in related
asymmetric processes. Efforts to realize such a goal are
currently underway in our laboratory.
[a] The reaction was carried out with a-diketone (0.25 mmol) and
1.5 equiv of aryl boronic acid, in the presence of 3 mol% of [Rh],
3.3 mol% of ligand, and 0.1m aq KOH (0.2 mL, 8% equiv) in 1.0 mL
THF. [b] Yield of isolated product. [c] Determined by chiral HPLC
analysis. [d] The absolute configuration of 13a was determined by
comparing the optical rotation value and HPLC chromatogram with
those obtained by Grignard addition of phenylmagnesium chloride to the
above-defined tertiary a-hydroxyester (S)-11i. See Supporting Informa-
tion for details; the other configurations were assigned by analogy. [e] A
regioisomer (13m, R1 =Ph, R2 =Me) was obtained in 10% yield with
76% ee.
selective addition of aryl boronic acids to a-diketones to
provide tertiary a-hydroxyketones has not yet been reported.
Initially, benzil (12a) was examined under the optimized
conditions. The reaction led to a 97% yield of the desired
product 13a with an excellent ee value of 97% (Table 3,
entry 1). Reactions involving other aryl boronic acids with
different electronic and steric demands were all found to be
successful, giving addition products with similarly high levels
of enantioselectivity (up to 99% ee; entries 2–7). Further-
more, the substituted benzyl species (such as 12b, 12c, and
12d) also underwent 1,2-addition to afford the corresponding
products in very high yields and enantioselectivities
Experimental Section
General procedure for the rhodium-catalyzed asymmetric 1,2-addi-
tion: Under an argon atmosphere,
a solution of a-dicarbonyl
compounds (0.25 mmol), [{Rh(coe)2Cl}2] (3 mol%, 2.7 mg,
0.0075 mmol of Rh), ligand 3 (2.0 mg, 0.00825 mmol), and aryl
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boronic acid (0.375 mmol) in 1.0 mL of THF was stirred at room
temperature for 30 minutes. Aqueous KOH (0.2 mL, 0.1m,
0.02 mmol) was then added to this mixture. After being stirred at
room temperature for 3 hours, the mixture was concentrated under
reduced pressure. The residue was purified by silica gel column
chromatography to afford the corresponding addition product (11 or
13).
[7] For reviews, see: a) K. Fagnou, M. Lautens, Chem. Rev. 2003,
103, 169; b) T. Hayashi, K. Yamasaki, Chem. Rev. 2003, 103,
2829; c) J. Christoffers, G. Koripelly, A. Rosiak, M. Rossle,
Synthesis 2007, 1279.
[8] a) Z.-Q. Wang, C.-G. Feng, M.-H. Xu, G.-Q. Lin, J. Am. Chem.
Soc. 2007, 129, 5336, and references therein; b) Z. Cui, H.-J. Yu,
R.-F. Yang, W.-Y. Gao, C.-G. Feng, G.-Q. Lin, J. Am. Chem. Soc.
2011, 133, 12394.
[9] For non-asymmetric addition of aryl boronic acids to a-
ketoesters, see: a) P. He, Y. Lu, C.-G. Dong, Q.-S. Hu, Org.
Lett. 2007, 9, 343; b) P. He, Y. Lu, Q.-S. Hu, Tetrahedron Lett.
2007, 48, 5283; c) G. R. Ganci, J. D. Chisholm, Tetrahedron Lett.
2007, 48, 8266; d) S. Miyamura, T. Satoh, M. Miura, J. Org.
Chem. 2007, 72, 2255; For non-asymmetric addition of arylstan-
nanes to a-ketoesters, see: e) S. Oi, M. Moro, H. Fukuhara, T.
Kawanishi, Y. Inoue, Tetrahedron 2003, 59, 4351.
[10] a) S. L. X. Martina, R. B. C. Jagt, J. G. de Vries, B. L. Feringa,
A. J. Minnaard, Chem. Commun. 2006, 4093; b) V. R. Jumde, S.
Facchetti, A. Iuliano, Tetrahedron: Asymmetry 2010, 21, 2775.
[11] For an example of palladium-catalyzed, asymmetric intramolec-
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J. Am. Chem. Soc. 2006, 128, 16504.
[12] For related non-asymmetric addition examples, see Refs [9c–e].
[13] a) S.-S. Jin, H. Wang, M.-H. Xu, Chem. Commun. 2011, 47, 7230;
b) W.-Y. Qi, T.-S. Zhu, M.-H. Xu, Org. Lett. 2011, 13, 3410; c) S.-
S. Jin, Ph.D. Thesis, Shanghai Institute of Materia Medica,
Chinese Academy of Sciences, 2011.
[14] For other recent reports on using chiral sulfur–olefin ligands for
asymmetric 1,4-additions, see: a) T. Thaler, L.-N. Guo, A. K.
Steib, M. Raducan, K. Karaghiosoff, P. Mayer, P. Knochel, Org.
Lett. 2011, 13, 3182; b) X. Feng, Y. Wang, B. Wei, J. Yang, H. Du,
Org. Lett. 2011, 13, 3300; c) G. Chen, J. Gui, L. Li, J. Liao,
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7256; f) Y. Wang, X. Feng, H. Du, Org. Lett. 2011, 13, 4954.
[15] Ligand 3 can be prepared on a multi-gram scale in one pot, see
the Supporting Information for details.
Received: October 2, 2011
Published online: December 1, 2011
Keywords: 1,2-addition · a-hydroxy carbonyl compounds ·
.
asymmetric catalysis · rhodium · sulfur–olefin ligands
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Angew. Chem. Int. Ed. 2012, 51, 780 –783
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