Intermolecular asymmetric carboesterification of alkenes by using chiral amine auxiliaries under o2: Synthesis of enantioenriched α-methylene-γ-lactones through chloropalladation of alkynes

Article abstract of DOI:10.1002/chem.201500397

Herein, the first example of chloropalladation-initiated asymmetric intermolecular carboesterification of alkenes with alkynes by using chiral amine auxiliaries is reported. The use of (1S,2S)-N¹,N¹-dimethylcyclohexane-1,2-diamine auxiliaries is essential for providing α-methylene-γ-lactones products in moderate to high yields and excellent enantioselectivities at room temperature. Moreover, the chiral amine auxiliaries can be readily removed by hydrolysis during the reaction process to keep the absolute configuration. This oxygen- and water-promoted asymmetric reaction opens a new window to study asymmetric processes in halopalladation reactions.

Full text of DOI:10.1002/chem.201500397

DOI: 10.1002/chem.201500397
Communication
&
Asymmetric Synthesis
Intermolecular Asymmetric Carboesterification of Alkenes by
Using Chiral Amine Auxiliaries under O2: Synthesis of
Enantioenriched a-Methylene-g-Lactones through
Chloropalladation of Alkynes
Zhenming Zhang,[a] Wanqing Wu,*[a] Jianhua Liao,[a] Jianxiao Li,[a] and Huanfeng Jiang*[a, b]
its the coordination of the palladium species with other chiral
Abstract: Herein, the first example of chloropalladation-in-
itiated asymmetric intermolecular carboesterification of al-
ligands.[2d,9e]
To achieve an asymmetric version of the halopalladation-ini-
kenes with alkynes by using chiral amine auxiliaries is re-
ported. The use of (1S,2S)-N1,N1-dimethylcyclohexane-1,2-
tiated reaction, a suitable strategy must be found. In our previ-
ous research, many different types of chiral ligands were
diamine auxiliaries is essential for providing a-methylene-
tested in the palladium-catalyzed intermolecular asymmetric
carboesterification of styrene with an alkylamide base;[10,11]
g-lactones products in moderate to high yields and excel-
lent enantioselectivities at room temperature. Moreover,
however, no satisfactory results were obtained (see the Sup-
the chiral amine auxiliaries can be readily removed by hy-
porting Information for details). Using a chiral auxiliary in
drolysis during the reaction process to keep the absolute
asymmetric synthesis is an effective tool for constructing opti-
configuration. This oxygen- and water-promoted asym-
cally active compounds in organic reactions,[12] such as in the
metric reaction opens a new window to study asymmetric
Diels–Alder reaction,[13] in radical reactions,[14] and so on.[15]
processes in halopalladation reactions.
Thus, we reasoned that employment of a chiral auxiliary might
prove suitable for halopalladation-initiated asymmetric reac-
tions of alkenes with alkynes. Therefore, a palladium-catalyzed
Over the past decade, nucleopalladation-initiated cross-cou-
pling reactions of alkenes and alkynes have emerged as a pow-
erful approach to construct C(sp2)ÀC(sp3) and carbonÀhetero-
atom bonds in one step.[1,2] Since the initial studies in this area
reported by Lu and co-workers,[3] different types of nucleopal-
ladation-initiated cyclization reactions have been developed.[4,5]
In particular, intermolecular and intramolecular carbocycliza-
tion of alkenes and alkynes have been explored and estab-
lished as an effective method for constructing multimembered
heterocyclic compounds.[6–8] Despite the numerous synthetic
approaches for racemic products that have been developed,
nucleopalladation-initiated asymmetric reactions remain highly
desirable for the synthesis of optically active compounds. How-
ever, compared with the impressive development of oxypalla-
dation-initiated asymmetric cyclization reactions,[9] halopallada-
tion-initiated enantioselective transformations through chiral li-
gands and palladium complexes still remain a challenge, as the
halide ions act as both a nucleophile and a ligand, which inhib-
asymmetric intermolecular carboesterification of styrene with
(S)-3-phenyl-N-(1-phenylethyl)propiolamide (1a, 97% ee) was
designed, which used 5 mol% PdCl2 and 2 equivalents of
CuCl2·2H2O in MeCN at room temperature under an O2 balloon
(Scheme 1). To our delight, the cyclic product 3a could be ach-
ieved in a good yield (81%) and moderate enantioselectivity
(42% ee) with Z/E>98:2 as determined by gas chromatogra-
phy (GC). Moreover, the auxiliary (R)-1-phenylethanamine could
be auto removed and kept the absolute configuration during
the reaction process.
Inspired by these initial attempts, different chiral amine aux-
iliaries on the alkyne amide in the Pd-catalyzed intermolecular
asymmetric carboesterification of styrene were investigated
(Table 1). Intriguingly, the study demonstrated that the car-
boesterification occurred smoothly within a few hours. The de-
sired cyclic product 3a was obtained in good yields, but with
moderate enantioselectivities when chiral a-arylamines (1b
and 1c) were used in the reactions (Table 1, entries 1 and 2).
Examination of chiral amino-alcohols (1d–1h) gave lower
yields of 3a, but better enantioselectivity was obtained by in-
creasing steric hindrance at the C3-position of the amino-alco-
hol (Table 1, entries 3–7). A good yield (77%) but poor enantio-
selectivity (12% ee) were achieved when (S)-methyl 2-amino-2-
phenylacetate (1i) was employed in the reaction (Table 1,
entry 8).
[a] Z. Zhang, Dr. W. Wu, J. Liao, J. Li, Prof. Dr. H. Jiang
School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou 510640 (P. R. China)
Fax: (+86)20-8711-2906
E-mail: cewuwq@scut.edu.cn
jianghf@scut.edu.cn
[b] Prof. Dr. H. Jiang
Next, various chiral cyclohexane amines were examined.
Only moderate yield (45%) but high enantioselectivity (78%
ee) was observed when (1S,2S)-cyclohexane-1,2-diamine (1k)
was employed (Table 1, entry 10 in parentheses). To improve
State Key Laboratory of Applied Organic Chemistry
Lanzhou University, Lanzhou 730000 (P. R. China)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201500397.
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thoxy groups on the styrene,
were well-tolerated, and the re-
action was also applicable to
aryl moieties with different sub-
stitution patterns (Table 2, 3a–
m). Pleasingly, 99.2% ee was ob-
tained after recrystallization of
the product 3h from isopropa-
Scheme 1. Pd-catalyzed intermolecular asymmetric carboesterification of styrene with (S)-3-phenyl-N-(1-phenyl-
ethyl)propiolamide (1a).
Table 1. Pd-catalyzed intermolecular asymmetric carboesterification of
styrene with a chiral alkyne amide.[a]
Table 2. Substrate scope of alkenes.[a]
Entry
Substrates
Time
3a
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
1b
1c
1d
1e
1 f
1g
1h
1i
3 h
3 h
6 h
6 h
6 h
6 h
3 h
3 h
6 h
3 h
3 h
73
74
33
46
46
39
49
77
58
24
68
40
12
66
28
12
9
1j
1k
1l
51
72 (45)
77
30
82 (78)
88
10[d]
11[d]
[a] Reactions were carried out at 208C under an O2 balloon with
(0.20 mmol), 2a (0.24 mmol), PdCl2 (5 mol%), and CuCl2·2H2O
1
(0.4 mmol) in acetonitrile (0.5 mL) for 3–6 h. [b] Isolated yield. [c] Deter-
mined by HPLC. [d] 5 equiv of water was used.
the reactivity, optimization of the reaction conditions was un-
dertaken (see the Supporting Information for details). Fortu-
nately, a good yield (72%) and high enantioselectivity (82% ee)
was achieved when an additional 5 equivalents of H2O were
used (Table 1, entry 10), a result that might be due to the abili-
ty of water to promote the amide hydrolysis. Further explora-
tion of auxiliary in the carboesterification reaction indicated
that the (1S,2S)-N1,N1-dimethylcyclohexane-1,2-diamine (1l)
auxiliary gave the cyclic product in good yield (78%) with ex-
cellent enantioselectivity (88% ee; Table 1, entry 11).
[a] Reaction conditions: 1l (0.20 mmol), alkenes (0.24 mmol), PdCl2
(5 mol%), CuCl2·2H2O (0.4 mmol), and H2O (1 mmol) in acetonitrile
(0.5 mL) for 3–6 h at208C under an O2 balloon. The ee values were deter-
mined by HPLC. [b] The ee value determined after recrystallization is
shown in parentheses, yield: 68%.
With the optimized reaction conditions in hand, we next es- nol (IPA). The absolute configurations of the products were
tablished the substrate scope of this intermolecular asymmet- confirmed based on a single-crystal X-ray analysis of 3h (see
ric carboesterification of alkenes with N-((1S,2S)-2-(dimethyla- the Supporting Information for details). Slightly lower yield
mino)-cyclohexyl)-3-phenylpropiolamide (1l). It was found that and enantioselectivity (71% yield, 77% ee) were observed
moderate to good yields and high enantioselectivities were when 2-vinylnaphthalene was employed in this reaction
achieved with aryl alkenes and alkyl alkenes as the substrates (Table 2, 3n), but the thiophene heteroaryl substrate afforded
(Table 2). Both electron-deficient substituents, such as fluoro the corresponding product 3o with good results (65% yield,
and CN groups, and electron-rich substituents, such as me- 90% ee). Linear olefins can be used in the reaction to give the
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desired products, 3p and 3q, in high yields with good enan-
tioselectivities (40% ee and 78% ee, respectively) (Table 2). Un-
fortunately, for activated alkenes, such as methyl acrylate, the
asymmetric process did not occur (Table 2, 3r).
Next, we screened a series of alkyne amides with (1S,2S)-
N1,N1-dimethylcyclohexane-1,2-diamine as the auxiliary and
found that both electron-rich and electron-poor aryl groups
were also well tolerated (Table 3, 5a–11 a). A wide range of
To gain insight into the mechanism of the asymmetric car-
boesterification process, several control experiments were con-
ducted. Under the standard conditions, a clear decrease in
yields and enantioselectivities was observed when substitu-
tions such as acetyl (Ac) and tert-butyloxycarbonyl (Boc)
groups were employed on the nitrogen atom of the substrates,
an observation that indicated coordination between the palla-
dium species and N atom of substrates occurred [Eq. (3)]. In
addition, 3-phenylpropiolamide was transferred to the racemic
product 3a when 1 equivalent of (1S,2S)-N1,N1-dimethylcyclo-
hexane-1,2-diamine was used, suggesting that the enantiose-
lectivity was determined by the chiral substrate, rather than
the chiral amine removed from the substrate [Eq. (4)].
Table 3. Substrate scope of alkyne amide.[a]
Base on the above observations and previous reports,[7,8,10]
[a] Reaction conditions: alkyne amides (0.20 mmol), 2a (0.24 mmol), PdCl2
(5 mol%), CuCl2·2H2O (0.4 mmol), and H2O (1 mmol) in acetonitrile
(0.5 mL) for 3–6 h at 208C under an O2 balloon. The ee values were deter-
mined by HPLC. [b] With an ice-water bath.
a tentative mechanism for this intermolecular asymmetric
alkene carboesterification is proposed (Scheme 2). Firstly, PdII
coordinates with nitrogen and the carbonÀcarbon triple bond
of the substrate to generate intermediate I. Subsequently, the
substrates with different substituents on the aryl group provid-
ed the desired products with high enantioselectivities of 78–
87% ee (Table 3, 5a–7a). To our delight, this reaction was suc-
cessfully applied to 3-alkyl substituted alkyne amides, and
these substrates were smoothly converted to the asymmetric
carboesterification products in good yields with good enantio-
selectivities (Table 3, 12a and 13a).
Under the standard conditions, a satisfactory result (62%
yield, 84% ee) was obtained when the reaction was performed
on a 5-mmol scale [Eq. (1)]. The newly formed g-lactones were
easily transformed to more useful frameworks of biologically
active natural products. For example, a simple aminolysis of 3a
was achieved with n-butylamine to give chiral g-hydroxy
amide 3aa in 94% yield and kept the absolute configurations
under mild reaction conditions [Eq. (2)].
Scheme 2. Plausible reaction mechanism.
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repulsion between the alkene substituent and the chiral auxili-
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absolute configuration observed in the carboesterification
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In summary, we have developed a chloropalladation-initiated
intermolecular asymmetric carboesterification of alkenes under
O2 for the first time with a chiral diamine auxiliary, providing
optically enriched a-methylene-g-lactones in good yields and
enantioselectivities. The chiral diamine auxiliary not only gave
the cyclization products in moderate to excellent enantioselec-
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through hydrolysis during the reaction process and could be
recovered in good yield with the original enantioselectivity.
This work could provide a guide for further asymmetric synthe-
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clarify the mechanism of this novel asymmetric reaction and
develop applications for the enantioenriched products are cur-
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Acknowledgements
We thank the National Natural Science Foundation of China
(21202046 and 21472051), National Basic Research Program of
China (973 Program) (2011CB808600), and the Fundamental
Research Funds for the Central Universities (2014ZP0004 and
2014ZZ0046) for financial support.
Keywords: asymmetric synthesis · carboesterification · chiral
auxiliary · chloropalladation · g-lactones
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Received: January 30, 2015
Published online on && &&, 0000
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COMMUNICATION
&
Asymmetric Synthesis
Z. Zhang, W. Wu,* J. Liao, J. Li, H. Jiang*
&& – &&
Intermolecular Asymmetric
Carboesterification of Alkenes by
Using Chiral Amine Auxiliaries under
O2: Synthesis of Enantioenriched a-
Methylene-g-Lactones through
Chloropalladation of Alkynes
Chiral auxiliary: The first example of
asymmetric intermolecular carboesterifi-
cation of alkenes with alkyne amides ini-
tiated by chloropalladation is achieved.
The use of (1S,2S)-N1,N1-dimethylcyclo-
hexane-1,2-diamine auxiliary is essential
for providing the cyclization products in
moderate to high yields with excellent
enantioselectivities, and can be readily
removed by hydrolysis to maintain the
absolute configuration. This oxygen-
and water-promoted reaction also gives
a novel route to enantioenriched a-
methylene-g-lactones under mild condi-
tions.
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Chem. Eur. J. 2015, 21, 1 – 6
www.chemeurj.org
6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!