C1-symmetric Rh/Phebox-catalyzed asymmetric alkynylation of α-ketoesters

Article abstract of DOI:10.1002/anie.201100252

Thinking outside the box: A newly developed C₁-symmetric Rh/Phebox complex efficiently catalyzed the asymmetric alkynylation of α-ketoester 1 with various aryl and alkyl substituted terminal alkynes to provide the corresponding chiral tertiary propargylic alcohols with up to 99 % ee (see scheme; TMS=trimethylsilyl). Copyright

Full text of DOI:10.1002/anie.201100252

Communications
DOI: 10.1002/anie.201100252
Asymmetric Catalysis
C1-Symmetric Rh/Phebox-Catalyzed Asymmetric Alkynylation of
a-Ketoesters**
Takashi Ohshima,* Takahito Kawabata, Yosuke Takeuchi, Takahiro Kakinuma,
Takanori Iwasaki, Takayuki Yonezawa, Hajime Murakami, Hisao Nishiyama, and
Kazushi Mashima*
Catalytic asymmetric alkynylation of carbonyl compounds is
one of the most efficient routes for the synthesis of optically
active propargylic alcohols, which are useful and versatile
building blocks for a variety of functionalized molecules, such
as biologically active natural products.[1] In the initial stages of
development of this transformation, stoichiometric amounts
of metal reagents such as organolithium, organomagnesium,
and diorganozinc compounds were used to increase the
nucleophilicity of the alkyne and to prevent an undesired
retro reaction.[1,2] In terms of atom economy,[3] however, the
direct in situ generation of a metal alkynylide species from
terminal alkynes using a catalytic amount of the metal reagent
is highly desirable. Since the pioneering work by Carreira and
co-workers, who utilized catalytic amounts of Zn(OTf)2, N-
methylephedrine, and Et3N,[4] several efficient methods for
the catalytic asymmetric alkynylation of aldehydes have been
developed using chiral Zn,[5] In,[6] Cu,[7] and Ru[8] catalysts.[9]
In contrast to the substantial progress made with alde-
hydes, the development of a catalytic asymmetric alkynyla-
tion of ketones for the construction of a tetrasubstituted
carbon center in an enantioselective manner has had limited
success due to low reactivity, difficulty in obtaining enantio-
facial differentiation, and the ease of the retro reaction as
compared with aldehydes.[10] Jiang et al. succeeded in pro-
moting the asymmetric alkynylation of a-ketoesters with
broad substrate scope and high enantioselectivity (up to
94% ee)[11a] by modifying Carreiraꢀs Zn system.[4] Later,
Shibasaki and co-workers reported Cu catalysis of trifluoro-
methyl ketone with up to 52% ee,[11b] and the Rh catalysis of
an a-diketone reported by Chisholm and co-workers gave the
product in 5% yield with 20% ee.[11c] Although the method of
Jiang et al. is useful for accessing chiral propargylic alcohols,
there remains much room for improvement because this
system requires 20 mol% catalyst loading, 30 mol% of
external amine base, and a rather high reaction temperature
(708C).[11a] Herein, we report the catalytic asymmetric
alkynylation of a-ketoester 1 using various aryl- and alkyl-
substituted terminal alkynes 2 catalyzed by as little as 3 mol%
of C1-symmetric Rh/Phebox complexes 3i and 3j (Figure 1) to
[*] Prof. Dr. T. Ohshima
Figure 1. Structure of C2- and C1-symmetric Rh/Phebox complexes 3.
Bn=benzyl.
Graduate School of Pharmaceutical Sciences
Kyushu University, CREST
Maidashi Higashi-ku, Fukuoka 812-8582 (Japan)
Fax: (+81)92-642-6650
E-mail: ohshima@phar.kyushu-u.ac.jp
afford the corresponding propargylic alcohols with greater
than 99% ee. Because the acetate ligand on the Rh complex
acted as an internal base, the reactions proceeded at 258C
without any additives. An indanyl substituent on the oxazo-
line ligand was effective for obtaining high enantioselectivity
and, in most cases, the C1-symmetric complex gave better
results than the C2-symmetric complex. The electronic tuning
of the Rh complex was achieved by introducing a nitro group
at the para position to Rh and greatly improved both the
reactivity and selectivity of the reaction. Moreover, the Rh
complex had unique chemoselectivity; it selectively reacted
with a a-ketoester over an aldehyde, thus allowing for the
direct use of 4-ethynylbenzaldehyde as the nucleophile.
We began to develop an efficient catalytic asymmetric
alkynylation of CF3-substituted a-ketoester 1 because of the
T. Kawabata, Y. Takeuchi, T. Kakinuma, Dr. T. Iwasaki, T. Yonezawa,
H. Murakami, Prof. Dr. K. Mashima
Department of Chemistry, Graduate School of Engineering Science
Osaka University, CREST, Toyonaka, Osaka 560-8531 (Japan)
Fax: (+81)6-6850-6649
E-mail: mashima@chem.es.osaka-u.ac.jp
Prof. Dr. H. Nishiyama
Department of Applied Chemistry, Graduate School of Engineering,
Nagoya University, Chikusa, Nagoya 464–8603 (Japan)
[**] This work was supported by a Grant-in-Aid for Science Research on
Innovative Areas “Chemistry of Concerto Catalysis” (No. 10313123)
and a Grant-in-Aid for Scientific Research (B) (21390003) from
MEXT, CREST from JST, and the Takeda Science Foundation.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100252.
6296
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Angew. Chem. Int. Ed. 2011, 50, 6296 –6300

increasing demand for CF3-containing chiral com-
pounds,[10p,12] such as the anti-HIV drug Efavirenz,[13] in
pharmaceutical science. First, we screened various metal
complexes such as Zn, Cu, Ag, and In complexes. Among the
complexes initially tested, only the Rh/Phebox complex 3a[14]
promoted the reaction of 1 with phenylacetylene (2a; Table 1,
symmetric Rh complex 3i at room temperature to afford the
products in high yield (up to 99%) and high enantioselectivity
(up to 94% ee), while C2-symmetric Rh complex 3 f was less
effective (Table 2, entries 1–4). The Rh complex 3i also gave
slightly better results than 3 f in the alkylation of phenyl-
acetylenes 2 f–h, which contain electron-withdrawing groups
(Table 2, entries 5–7).
Alkynylation using alkyl-substituted alkynes, however,
resulted in unsatisfactory results, even when the optimized
catalyst 3i was used.[15] For example, the reaction using the
phenethyl-substituted acetylene 2i gave only a 29% yield of
the product, thus requiring the development of a more
powerful catalyst. An attractive feature of the metal/Phebox
complex is the ease of its electronic tuning by the introduction
of an electron-donating or electron-withdrawing group at the
para position to the metal.[14,21,22] To investigate the electronic
effects of the Rh/Phebox catalyst, we synthesized dimethyl-
amino-, bromo-, and nitro-substituted complexes 3c–e and
applied them to the alkynylation of 1 with 2i (Scheme 1).
Table 1: Alkynylation catalyzed by C2- and C1-symmetric 3.
Entry 3 (C2) Yield [%][a] ee [%][b] Entry 3 (C1) Yield [%][a] ee [%][b]
1
2
3
3a
3b
3 f
35
87
94
88
83
91
4
5
6
3g
3h
3i
99
41
95
86
92
91
[a] Yield of the isolated product. [b] Determined by HPLC analysis.
entry 1).[15] Although the yield of product 4a was modest
(35%), the reaction proceeded at 258C and the enantiose-
lectivity was 88% ee.[16] Encouraged by this result, we
examined the ligand effects of the catalyst. We recently
reported that the trifluoroacetate-bridged tetranuclear zinc
cluster Zn4(OCOCF3)6O efficiently catalyzed the direct
conversion of esters, carboxylic acids, and nitriles into oxazo-
line.[17] Under these zinc-cluster-catalyzed conditions, carbox-
ylic acids reacted much faster than nitriles or esters, thus
allowing for easy access to a variety of C1-symmetric
bis(oxazoline) ligands, which contain different oxazoline
moieties.[18,19] Therefore, we tested both C2-symmetric Rh/
Phebox complexes (3a, 3b, and 3 f) and C1-symmetric
complexes (3g–i) for the alkynylation of 1 with 2a. Although
Rh complex 3h gave the best enantioselectivity (92% ee;
Table 1, entry 5), the Rh complexes that have indanyl
substituents (3 f and 3i)[20] were the best in terms of both
yield (up to 95%) and enantioselectivity (91% ee; Table 1,
entries 3 and 6).
Scheme 1. Electronic tuning of Rh/Phebox complex 3.
Although the introduction of the electron-donating dimethyl-
amino group did not induce a reaction, the bromo- and nitro-
substituted complexes 3d[20] and 3e remarkably improved
both yield and enantioselectivity of the product 4i compared
with 3b (Scheme 1), thus suggesting that the Lewis acidity of
the Rh complex 3 is another important factor for catalytic
activity and selectivity.
These results led us to examine the nitro-substituted C1-
symmetric Rh complex 3j (X = NO2) as the catalyst for the
alkynylation of less-reactive substrates. As expected, the Rh-
catalyst 3j efficiently catalyzed the alkynylation of various
alkyl-substituted acetylenes 2i–p (Table 3). With all the tested
substrates, 3j (X = NO2) gave much higher yield and/or
enantiomeric excess than 3i (X = H). For example, the
enantioselectivity of the reaction with cyclopropylacetylene
(2l) was improved from 29% ee to 74% ee (Table 3, entry 4).
In addition, the use of catalyst 3j successfully accelerated the
reactions with 2m–o (Table 3, entries 5–7). The obtained
functionalized propargylic alcohols 4n–p are of great interest
because they can be easily converted into the corresponding
terminal alkynes and propiolaldehyde derivatives. Catalyst 3j
was also effective for the reaction of aryl-substituted alkyne
2e, thus affording product 4e in 85% yield with > 99% ee
(Table 3, entry 9).
With catalysts 3 f and 3i in hand, we investigated the scope
and limitations of aryl-substituted alkynes 2b–h (Table 2).
The alkynylation using phenylacetylenes that contain elec-
tron-donating groups was smoothly catalyzed by the C1-
Table 2: Alkynylation with aryl-substituted alkynes catalyzed by 3 f (C2)
and 3i (C1).
Entry
2
Ar
Using catalyst 3 f
Using catalyst 3i
yield [%][a] ee [%][b] yield [%][a] ee [%][b]
1
2
3
4
5
6
7
2b 4-MeC6H4
2c 3-MeC6H4
2d 2-MeC6H4
82
83
84
95
89
93
80
92
94
86
90
99
99
88
90
89
90
94
90
92
90
90
95
92
During the investigation of substrate scope,[23] we found
that the present Rh catalysis has unique chemoselectivity.
Aldehydes are generally much more reactive electrophiles
than ketones. However, the alkynylation of benzaldehyde 5
was not promoted by Rh catalyst 3i. We therefore performed
2e 4-MeOC6H4 78
2 f 4-FC6H4
2g 4-BrC6H4
2h 4-CF3C6H4
83
86
85
[a] Yield of the isolated product. [b] Determined by HPLC analysis.
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Communications
Table 3: Alkynylation with alkyl-substituted alkynes catalyzed by 3i and
3j.
tate ligand of 3 with a more electron-withdrawing ligand, such
as trifluoroacetate, triflate, or chloride, severely retarded the
reaction.[15] These data, taken together with the fact that the
addition of an external base (Et3N) did not affect either the
yield or enantioselectivity, suggested that the acetate ligand
on 3 acted as an internal base to efficiently deprotonate the
coordinated terminal alkyne to form an Rh/alkynylide
intermediate. Indeed, the formation of this Rh/alkynylide
complex was confirmed by X-ray crystallographic analysis.[24]
The linear relationship between the enantioselectivity of the
chiral ligand and the extent of the asymmetric induction
indicated the involvement of a monomeric active species.[15] In
addition, crossover experiments suggested that a retro
reaction of the alkynylation was not involved in the catalytic
cycle.[15] Although the detailed reaction mechanism remains
unclear, based on these results, we propose the bifunctional[25]
catalytic cycle shown in Scheme 2, where the Rh center acts as
a p acid[26] and/or Lewis acid to activate the alkyne (A!B)
and the ketone (D!E), and the acetate moiety on the Rh acts
as a Brønsted base to deprotonate the terminal alkyne in an
intramolecular fashion (B!C).
Entry
2
R
Using catalyst 3i
Using catalyst 3j
yield [%][a] ee [%][b] yield [%][a] ee [%][b]
1
2
2i
2j
CH2CH2Ph
nPr
82
66
81
85
91
91
92
93
3
2k
85
87
93
96
4
2l
94
29
97
74
5
6
2m tBu
2n TMS
42
52
79
82
81
72
92
84
7
2o
44
94
79
94
8
9
2p CH(OEt)2
68
83
61
85
86
>99
2e
4-MeOC6H4 88
90[c]
[a] Yield of the isolated product. [b] Determined by HPLC analysis.
[c] Reaction time was 24 h. TMS=trimethylsilyl.
competition experiments using an equimolar mixture of
ketoester 1 and aldehyde 5, thus resulting in the exclusive
formation of tertiary alcohol 4a [Eq. (1)]. In contrast, other
Scheme 2. Proposed catalytic cycle of 3-catalyzed alkynylation.
representative alkynylation conditions (lithium alkynylide,
Zn catalyst, and In catalyst) led to only a nonselective or
sluggish reaction.[15] Even though 1 is a rather reactive ketone,
to the best of our knowledge, this is the first example of a
chemoselective alkynylation of a ketone over a aldehyde. To
demonstrate the usefulness of this chemoselectivity that is
induced by the Rh/Phebox catalyst 3, we attempted the direct
use of 4-ethynylbenzaldehyde (2q) as the nucleophile
[Eq. (2)]. Under the optimized reaction conditions, the Rh/
Phebox complex 3i smoothly catalyzed the alkynylation of 1
with 2q to give the corresponding propargylic alcohol 4q
(83%, 89% ee), the synthesis of which generally requires a
protection/deprotection process.[15]
In summary, we have developed a catalytic asymmetric
alkynylation of a-ketoesters with aryl- and alkyl-substituted
alkynes that is promoted by 3 mol% of the C1-symmetric Rh/
Phebox complexes at room temperature. Introduction of an
electron-withdrawing nitro group at the para position to Rh
greatly improved both the reactivity and selectivity of the
reaction. The present catalytic system has a unique chemo-
selectivity, thus resulting in the a-ketoester being alkynylated
in preference to an aldehyde. Investigations into the applica-
tion of this method to the synthesis of bioactive compounds
are ongoing.
To gain insight into the mechanism of this Rh catalysis, we
performed the following experiments. Replacement of ace-
Experimental Section
Rh/Phebox complex (0.0060 mmol, 3.0 mol%), ethyl trifluoropyru-
vate (1, 34.0 mg, 0.20 mmol), and 2 (0.24 mmol, 1.2 equiv) in Et2O
(2.0 mL) were stirred at 258C for 24 h under an argon atmosphere.
The resulting mixture was concentrated in vacuo and purified by flash
column chromatography on silica gel (eluent: ethyl acetate/hexanes
(1:8)) to give the desired product 4.
Received: January 12, 2011
Published online: May 30, 2011
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Angew. Chem. Int. Ed. 2011, 50, 6296 –6300

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Keywords: alkynylation · asymmetric catalysis ·
chemoselectivity · ketones · rhodium
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