Enantioselective copper-catalyzed decarboxylative propargylic alkylation of propargyl B-ketoesters with a chiral ketimine P,N,N-ligand

Article abstract of DOI:10.1002/anie.201309182

The first enantioselective copper-catalyzed decarboxylative propargylic alkylation has been developed. Treatment of propargyl b-ketoesters with a catalyst, prepared in situ from [Cu(CH3CN)4BF4] and a newly developed chiral tridentate ketimine P,N,N-ligand

Full text of DOI:10.1002/anie.201309182

.
Angewandte
Communications
DOI: 10.1002/anie.201309182
Synthetic Methods
Enantioselective Copper-Catalyzed Decarboxylative Propargylic
Alkylation of Propargyl b-Ketoesters with a Chiral Ketimine
P,N,N-Ligand**
Fu-Lin Zhu, Yuan Zou, De-Yang Zhang, Ya-Hui Wang, Xin-Hu Hu, Song Chen, Jie Xu,* and
Xiang-Ping Hu*
Abstract: The first enantioselective copper-catalyzed decar-
boxylative propargylic alkylation has been developed. Treat-
ment of propargyl b-ketoesters with a catalyst, prepared in situ
from [Cu(CH3CN)4BF4] and a newly developed chiral triden-
tate ketimine P,N,N-ligand under mild reaction conditions,
generates b-ethynyl ketones in good yields and with high
enantioselectivities without requiring the pregeneration of
ketone enolates. This new process provides facile access to
a range of chiral b-ethynyl ketones in a highly enantioenriched
form.
alkylation of simple ketones. However, few examples have
been realized for the catalytic decarboxylative propargylic
transformation to date,[5] and to the best of our knowledge,
there is only one report, by Stoltz and co-workers, on the
successful palladium-catalyzed asymmetric propargylation of
enol carbonates with 12–44% ee .[6] Catalytic asymmetric
decarboxylative propargylation is still a far less explored field.
Recent progress in the copper-catalyzed asymmetric
propargylic substitution has indicated that the reaction
proceeded via copper allenylidene complexes as key inter-
mediates, which were formed by the elimination of an ester
group from propargylic substrates.[7] We therefore envision
that an asymmetric decarboxylatve propargylic alkylation
may have occurred when a substrate, tethering the propargyl
moiety and nucleophile together with an ester functional
group, is subjected to a chiral copper catalyst (Scheme 1). As
T
ransition-metal-catalyzed propargylic substitutions repre-
sent an important class of reactions because of their abilities
to introduce an electron-rich triple bond which is a versatile
entity for further chemical transformations.[1] Recent achieve-
ments have led to a catalytic asymmetric version of this
reaction.[2] Although a variety of nitrogen and carbon
nucleophiles are suitable reagents for these catalytic asym-
metric propargylic substitutions, the use of simple ketone
enolates as nucleophiles is still very limited.[3] The develop-
ment of catalytic asymmetric propargylic alkylation with
simple ketone enolates as nucleophiles remains a great
challenge.
Scheme 1. General reaction scheme for enantioselective copper-cata-
In the past decades, an excellent method for the gener-
ation of nonstabilized ketone enolates as nucleophiles in situ
by decarboxylation under mild reaction conditions has been
elegantly developed and known as transition-metal-catalyzed
decarboxylative allylic alkylations.[4] In this method, the loss
of CO2 replaces the need to selectively prepare preformed
enolate equivalents, and both the nucleophile and the
electrophile are formed in situ in a catalytic concentration.
It therefore can be envisaged that this strategy should also
provide an ideal solution for the catalytic propargylic
lyzed decarboxylative propargylic alkylation.
a result, herein we report the first copper-catalyzed enantio-
selective decarboxylative propargylic alkylation of propargyl
b-ketoesters, which provides a new and facile approach for
the synthesis of chiral b-ethynyl ketones in a highly enan-
tioenriched form.[8]
An initial attempt to carry out a reaction using 1-phenyl-2-
propynyl 3-oxo-3-phenylpropanoate (1aa)[9] was conducted in
MeOH at room temperature in the presence of 1.2 equiv-
alents of iPr2NEt with 5 mol% of a chiral copper catalyst
[*] F.-L. Zhu, Y. Zou, D.-Y. Zhang, Y.-H. Wang, X.-H. Hu, S. Chen,
Prof. Dr. J. Xu, Prof. Dr. X.-P. Hu
.
prepared in situ from Cu(OAc)2 H2O and (S)-binap. The
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
457 Zhongshan Road, Dalian 116023 (China)
E-mail: xujie@dicp.ac.cn
xiangping@dicp.ac.cn
Homepage: http://www.asym.dicp.ac.cn
reaction delivered the desired b-ethynyl ketone product 2aa
albeit in low yield with nearly no enantioselection (Table 1,
entry 1). After the evaluation of several ligands which have
proven to be efficient in copper-catalyzed asymmetric prop-
argylic amination,[7] the chiral tridentate P,N,N-ligand (R)-
L4[7j,10] developed in our group was identified as a promising
ligand structure (Table 1, entry 4). We therefore evaluated the
effects of modifications to the ligand structure of (R)-L4 on
the reaction outcome. Two new P,N,N-ligands (S)-L5 and (S)-
L6, bearing a ketimine moiety were then prepared as shown in
Scheme 2. Pleasingly, (S)-L6, derived from 2-benzopyridine,
F.-L. Zhu, D.-Y. Zhang
University of the Chinese Academy of Sciences
Beijing 100049 (China)
[**] Support for this research from the Dalian Institute of Chemical
Physics (CAS) is gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201309182.
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Table 1: Screening of chiral ligands and reaction conditions.[a]
Entry [Cu]
L* Solvent Base
T [8C] Yield [%][b] ee [%][c]
1
2
3
4
5
6
7
8
Cu(OAc)2·H2O L1 MeOH iPr2NEt 25
Cu(OAc)2·H2O L2 MeOH iPr2NEt 25
Cu(OAc)2·H2O L3 MeOH iPr2NEt 25
Cu(OAc)2·H2O L4 MeOH iPr2NEt 25
Cu(OAc)2·H2O L5 MeOH iPr2NEt 25
Cu(OAc)2·H2O L6 MeOH iPr2NEt 25
Cu(CH3CN)4BF4 L6 MeOH iPr2NEt 25
38
92
85
90
91
90
94
85
84
93
–
85
92
95
92
96
<10
12
55
65
68
84
84
60
55
92
Figure 1. Crystal structure of CuCl/(S)-L6 complex. Hydrogen atoms
and solvent are omitted for clarity.
CuI
CuCl
L6 MeOH iPr2NEt 25
L6 MeOH iPr2NEt 25
best result was obtained with Et3N, in which 2aa was obtained
in 92% yield and 94% ee (Table 1, entry 13). The nature of
the solvent showed little influence on both the yield and the
enantioselectivity, and all of the solvents tested showed
excellent performance, with toluene as the optimal in terms of
the enantioselectivity (Table 1, entry 16). This result is differ-
ent compared to those observed with the copper-catalyzed
asymmetric propargylic substitution, in which only a polar
protic solvent was found to be suitable.[7]
Having developed the optimized reaction conditions, we
subsequently explored the scope of this process with respect
to the ketone moieties of propargyl b-ketoesters. As shown in
Table 2, a broad range of 1-phenyl-2-propynyl 3-oxo-3-
arylpropanoates worked efficiently to provide the desired b-
ethynyl ketones in good to excellent yields and with very high
enantioselectivities (Table 2, entries 1–12). Electron-donating
and electron-withdrawing substituents in the para (Table 2,
9
10
11
12
13
14
15
16
Cu(CH3CN)4BF4 L6 MeOH iPr2NEt
Cu(CH3CN)4BF4 L6 MeOH none
Cu(CH3CN)4BF4 L6 MeOH DBU
Cu(CH3CN)4BF4 L6 MeOH Et3N
Cu(CH3CN)4BF4 L6 CH2Cl2 Et3N
0
0
0
0
0
0
0
[d]
–
71
94
94
92
95
Cu(CH3CN)4BF4 L6 THF
Et3N
Cu(CH3CN)4BF4 L6 toluene Et3N
[a] Reaction conditions: 1aa (0.3 mmol), [Cu] (0.015 mmol, 5 mol%), L*
(0.0165 mmol, 5.5 mol%), base (0.36 mmol, 1.2 equiv), 1.2 mL of solvent,
indicated temperature, 12 h. [b] Yield of isolated product. [c] The ee values
were determined by HPLC analysis using a chiral stationary phase. [d] Not
determined because of low conversion. THF=tetrahydrofuran.
Table 2: Copper-catalyzed decarboxylative propargylic alkylation of b-
ketoesters: scope of ketone moieties.[a]
Entry Substrate
Product Yield [%][b] ee [%][c]
Scheme 2. Synthesis of the new chiral tridentate P,N,N-ligands (S)-L5
and (S)-L6 bearing a ketimine moiety.
1
2
3
4
5
6
7
8
1aa: R1 =Ph
2aa
2ba
2ca
2da
2ea
2 fa
2ga
2ha
2ia
96
65
92
96
96
95
93
95
96
95
96
91
88
85
90
95
93
95
94
95
95
95
93
94
96
98
95
85
86
83
1ba: R1 =2-ClC6H4
1ca: R1 =3-ClC6H4
1da: R1 =4-ClC6H4
1ea: R1 =4-FC6H4
1 fa: R1 =4-BrC6H4
1ga: R1 =4-MeC6H4
1ha: R1 =4-MeOC6H4
1ia: R1 =4-NO2C6H4
1ja: R1 =2-naphthyl
was found to be optimal and afforded 2aa in 90% yield and
with 84% ee (Table 1, entry 6). Other copper sources,
including [Cu(CH3CN)4BF4], CuI, and CuCl, were also
tested (Table 1, entries 7–9), and the results demonstrated
that [Cu(CH3CN)4BF4] was the best choice (Table 1, entry 7).
The tridentate coordination mode of (S)-L6 with CuI was
unambiguously confirmed by X-ray analysis of the CuCl/(S)-
L6 complex (Figure 1).[11]
Lowering the reaction temperature to 08C significantly
improved the enantioselectivity to 92% ee without loss of the
yield (Table 1, entry 10). The addition of a base proved to be
crucial for this reaction, and none of the desired b-ethynyl
ketone product 2aa was observed in its absence (Table 1,
entry 11). However, a stronger base, such as 1,8-diazabicyclo-
[5.4.0]undec-7-ene (DBU), had a detrimental effect on both
the yield and the enantioselectivity (Table 1, entry 12). The
9
10
11
12
13[d]
14[d]
15[d]
2ja
1ka: R1 =6-MeO-2-naphthyl 2ka
1la: R1 =thienyl
1ma: R1 =Me
1na: R1 =nPr
1oa: R1 =Bn
2la
2ma
2na
2oa
[a] Reaction conditions: 1 (0.5 mmol), [Cu(CH3CN)4BF4] (0.025 mmol,
5 mol%), (S)-L6 (0.0275 mmol, 5.5 mol%), Et3N (0.6 mmol, 1.2 equiv),
2 mL of toluene, 08C, 12 h. [b] Yield of isolated product. [c] The ee values
were determined by HPLC analysis using a chiral stationary phase.
[d] The reaction was performed in MeOH at a catalyst loading of
10 mol%.
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Table 3: Copper-catalyzed decarboxylative propargylic alkylation of b-
entries 4–9) or meta (Table 2, entry 3) position of the phenyl
ring were compatible with this transformation. However, the
reaction was sensitive to the substituent at the ortho position
of the phenyl ring, and the use of the substrate 1ba, bearing
a 2-Cl substituent, resulted in a decreased yield (Table 2,
entry 2). Although 1ba was less reactive than its 3-Cl (1ca)
and 4-Cl (1da) analogues, the observed selectivity was
nonetheless excellent (Table 2, entry 2). Other aromatics
and heteroaromatics, such as 1-naphthyl, 6-methoxy-1-naph-
thyl, and 2-thiophenyl groups proved to be good substrates for
the reaction (Table 2, entries 10–12). Notably, the reaction
could also be realized for aliphatic b-ketoesters although
a catalyst loading of 10 mol% was required to reach complete
conversion (Table 2, entries 13–15). The reaction also toler-
ated substituents at the a-position. Thus, the cyclic b-
ketoester 1pa also performed well under the optimized
reaction conditions (Scheme 3).
ketoesters: scope of propargyl moieties.[a]
Entry
Substrate
Product
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
1aa: R2 =Ph
2aa
2ab
2ac
2ad
2ae
2af
2ag
2ah
2ai
96
92
95
96
96
93
91
94
96
94
92
–
95
93
96
95
94
95
89
85
95
97
88
1ab: R2 =2-ClC6H4
1ac: R2 =3-ClC6H4
1ad: R2 =4-ClC6H4
1ae: R2 =4-FC6H4
1af: R2 =4-BrC6H4
1ag: R2 =4-MeC6H4
1ah: R2 =4-MeOC6H4
1ai: R2 =4-CF3C6H4
1aj: R2 =2-naphthyl
1ak: R2 =thienyl
1al: R2 =Me
9
10
11
12
2aj
2ak
2al
[d]
–
[a] Reaction conditions: 1 (0.5 mmol), Cu(CH3CN)4BF4 (0.025 mmol,
5 mol%), (S)-L6 (0.0275 mmol, 5.5 mol%), Et3N (0.6 mmol, 1.2 equiv),
2 mL of toluene, 08C, 12 h. [b] Yield of isolated product. [c] The ee values
were determined by HPLC analysis using a chiral stationary phase.
[d] Not determined because of low conversion.
Scheme 3. Copper-catalyzed asymmetric decarboxylative propargylic
alkylation of cyclic b-ketoester.
The scope of the propargyl moieties was next investigated
(Table 3). We were pleased to find that the reaction pro-
ceeded smoothly for all 1-aryl-2-propynyl 3-oxo-3-phenyl-
propanoates tested, with the desired products obtained in
good yields and with high levels of enantioinduction. The
position of the substituent on the phenyl ring had less
influence on this process. Thus, all three substrates having a Cl
at the different positions on the phenyl ring gave similar
results (Table 3, entries 2–4). It appears that the electronic
properties of the substituent at the para position had some
effect on the enantioselectivity, and the substrate with an
electron-donating group tended to give lower enantioselec-
tivity (Table 3, entries 7 and 8). 1-Naphthyl and 2-thiophenyl
substrates were also suitable reaction partners, thus giving the
corresponding b-ethynyl ketones in good results. However,
the present catalytic system did not tolerate an aliphatic
substrate (1al), thus giving very low conversions (Table 3,
entry 12). This result is consistent with the observation in the
catalytic asymmetric propargylic substitution.[7f–g] The abso-
lute configuration of chiral b-ethynyl ketones was unambig-
uously determined by X-ray structure analysis of 2af, and is
assigned as having an R configuration.[11]
ion pair (C),[7f] which isomerizes to give a copper allenylidene
complex enolate ion pair (D). The crossover experiment in
Scheme 4b suggested the dissociation of the copper alleny-
lidene complex and the enolate ion during the reaction. The
enolate attacks at the Cg atom of the copper allenylidene
By considering all the experimental evidence (Scheme 4),
a reaction pathway is proposed in Scheme 5. In the first step,
a copper complex forms the p-complex A with the propargyl
b-ketoester 1aa. Deprotonation of A with Et3N would then
give the copper acetylide complex B, which would explain
why a propargyl b-ketoester bearing an internal alkyne
moiety did not react at all (Scheme 4a). The dissociation of
B likely produces a copper allenylidene complex carboxylate
Scheme 4. Mechanistic investigations into the copper-catalyzed decar-
boxylative propargylic alkylation.
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Scheme 5. Proposed mechanism.
complex with subsequent loss of CO2 to give the copper p-
alkyne complex F. The a,a-disubstituted ester 6 did not
undergo the reaction (Scheme 4c) and may suggest that
decarboxylation occurs after propargylic alkylation. How-
ever, the possibility that decarboxylation occurrs before
propargylation alkylation is still not ruled out. Further
investigations to elucidate the reaction mechanism are
underway.
In conclusion, we have developed the first copper-
catalyzed asymmetric decarboxylative propargylic alkylation
of propargyl b-ketoesters with a new chiral ketimine P,N,N-
ligand, and provides a new and facile access to a variety of b-
ethynyl ketones in good yields and with high enantioselectiv-
ities. Since both the nucleophile and the electrophile are
formed in situ in catalytic concentration by the loss of CO2,
instead of the need to prepare preformed enolate equivalents,
we believe that this new reaction should provide a comple-
mentary strategy for catalytic asymmetric propargylic sub-
stitution. The further development and application of this
reaction, as well as study of the mechanism, is underway in
our laboratory.
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Received: October 21, 2013
Published online: December 18, 2013
Keywords: alkylation · asymmetric catalysis · copper ·
.
ligand design · synthetic methods
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