A d -Camphor-Based Schiff Base as a Highly Efficient N,P Ligand for Enantioselective Palladium-Catalyzed Allylic Substitutions

Article abstract of DOI:10.1002/cctc.201600084

New Schiff bases derived from chiral d-camphor were determined to be effective phosphine ligands for the asymmetric palladium-catalyzed allylic alkylation of activated methylene compounds, the allylic etherification of alcohols, and the allylic amination of primary amines or secondary amines, in which the corresponding products with various functional groups were achieved in good yields with excellent enantioselectivities (up to >99 % ee). Remarkably, the palladium catalyst derived from Schiff base L2 afforded the highest level of enantioselectivity reported to date for allylic substitution reactions, including allylic etherification and allylic amination, which revealed the privileged role of d-camphor-derived Schiff bases in palladium-catalyzed allylic substitution reactions.

Full text of DOI:10.1002/cctc.201600084

DOI: 10.1002/cctc.201600084
Communications
A d-Camphor-Based Schiff Base as a Highly Efficient N,P
Ligand for Enantioselective Palladium-Catalyzed Allylic
Substitutions
Qiao-Ling Liu,[a] Weifeng Chen,*[a] Qun-Ying Jiang,[a] Xing-Feng Bai,[a, b] Zhifang Li,[a]
Zheng Xu,[a] and Li-Wen Xu*[a, b]
New Schiff bases derived from chiral d-camphor were deter-
mined to be effective phosphine ligands for the asymmetric
palladium-catalyzed allylic alkylation of activated methylene
compounds, the allylic etherification of alcohols, and the allylic
amination of primary amines or secondary amines, in which
the corresponding products with various functional groups
were achieved in good yields with excellent enantioselectivities
(up to >99%ee). Remarkably, the palladium catalyst derived
from Schiff base L2 afforded the highest level of enantioselec-
tivity reported to date for allylic substitution reactions, includ-
ing allylic etherification and allylic amination, which revealed
the privileged role of d-camphor-derived Schiff bases in palla-
dium-catalyzed allylic substitution reactions.
gands bearing phosphorus and nitrogen atoms are widely
used in transition-metal-catalyzed asymmetric syntheses.[4] No-
tably, despite impressive and extensive advances in this field,
the development of highly efficient, readily accessible, and
structurally novel chiral N,P ligands to meet the demand of
perfect enantioselective transformations with potential indus-
trial value remains a formidable challenge.[5] This has motivated
researchers to develop new and practical approaches to estab-
lish structurally simple backbones for the modulation of chiral
N,P ligands. As a consequence, in consideration of the exciting
importance of d-camphor in chiral synthesis,[6] the develop-
ment of d-camphor-based, structurally simple N,P ligands for
asymmetric palladium catalysis is very interesting and may fill
an important gap in modern ligand design for catalytic trans-
formations.
The development of practical asymmetric catalysis routes
mediated expediently by biocatalysts, transition-metal cata-
lysts, and organocatalysts to access a wide range of optically
pure compounds has been attracting extensive attention
owing to the importance of such routes in chiral synthesis.[1] In
the past four decades, tremendous effort has been devoted to
transition-metal-complex-promoted asymmetric catalysis be-
cause it is a practical strategy toward the stereoselective syn-
thesis of high-value molecules owing to the facts that it gives
products with high enantioselectivity and good atom econom-
ics with a low catalyst loading.[2] In general, transition-metal-
complex-based approaches usually rely on the rational design
and facile synthesis of chiral ligands from cheap chiral sources
with at least one inherent stereogenic center. In this regard,
commercially available d-camphor is extremely attractive for
the development of effective phosphine ligands because of its
low price, multiple stereogenic centers on its backbone, rigid
structure, and reactive ketone moiety.[3] Moreover, chiral li-
Herein, we report a modularly synthetic strategy to d-cam-
phor-based chiral N,P ligands on the basis of previous findings
in Schiff base and chiral ligand chemistry.[7] Our approach pro-
vides a convergent and highly modular method for the synthe-
sis of rigid Schiff bases containing phosphorus and nitrogen
atoms that can be used to tailor the steric and electronic prop-
erties of the ligand even further (Scheme 1). Importantly, the
stereogenic centers present in this N,P ligand are derived from
a chiral primary amine and d-camphor. The phosphorus atom
or -PPh2 moiety can be easily introduced by using chlorodi-
phenylphosphine (Ph2PCl) with the aid of n-butyllithium. The
linked reaction is a simple condensation of a ketone and a pri-
mary amine, which leads to the formation of the Schiff base
moiety on the ligand. Finally, the effectiveness of this new
Schiff base with chirality matching that of a bidentate chelat-
ing N,P ligand is demonstrated in palladium-catalyzed allylic
substitutions with various nucleophiles.
Both d-camphor and chiral 1-phenylethanamine are cheap,
readily available sources of chirality on which the chiral Schiff
base containing a phosphorus atom can be built to afford the
[a] Q.-L. Liu, Prof. Dr. W. Chen, Q.-Y. Jiang, X.-F. Bai, Prof. Dr. Z. Li, Dr. Z. Xu,
Prof. Dr. L.-W. Xu
Key Laboratory of Organosilicon Chemistry and
Material Technology of Ministry of Education
Hangzhou Normal University,
No 1378, Wenyi West Road, Science Park of HZNU, Hangzhou (P.R. China)
E-mail: chenwf@hznu.edu.cn
liwenxu@hznu.edu.cn
[b] X.-F. Bai, Prof. Dr. L.-W. Xu
State Key Laboratory for Oxo Synthesis and Selective Oxidation
Lanzhou Institute of Chemical Physics (CAS)
and University of the Chinese Academy of Sciences (P.R. China)
Supporting Information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600084.
Scheme 1. Modular strategy for the preparation of chiral Schiff base based
N,P ligands. R represents adjustable substituents.
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Table 1. Screening of ligands for the enantioselective allylic alkylation re-
action.[a]
Entry
Ligand
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
L1
L2
L3
L4
L5
L6
99
99
99
98
99
99
97 (S)
99 (R)
94 (S)
90 (R)
87 (S)
35 (R)
[a] Unless otherwise noted, reactions were performed with 1a
(0.396 mmol), 2a (1.189 mmol), [Pd(h3-CH2CH=CH2)Cl]2 (2 mol%), ligand
(5 mol%), CsOAc (2 mol%), and BSA (1.189 mmol) in toluene (2.5 mL) at
room temperature. [b] Yield of isolated product. [c] Determined by HPLC
on a chiral stationary phase.
Scheme 2. Synthesis of d-camphor-based Schiff base ligands.
N,P ligand. The chiral 1-phenylethanamine-derived phosphine
is a known molecule that was prepared in two steps under re-
ported reaction conditions (Scheme 2).[7d,8] Subsequent treat-
ment of d-camphor with primary amines A2 in the presence of
trifluoroacetic acid for 12 h at reflux formed desired Schiff base
ligands L1–L6 in yields of 72–83%.
Table 2. Substrate scope of the asymmetric allylic alkylation reaction.[a]
With chiral Schiff base ligands L1–L6 with N,P centers in
hand, the palladium-catalyzed allylic alkylation reaction, a reli-
able and powerful tool for the stereoselective construction of
carbon–carbon and carbon–heteroatom bonds,[9] was attempt-
ed as a model transformation. Despite the fact that numerous
chiral ligands have been reported for regioselective and enan-
tioselective allylic alkylation reactions, highly efficient palladi-
um-catalyzed asymmetric allylic alkylation with perfect enantio-
selectivity (ꢀ99%ee) has not been widely reported in the past
decades. Specifically, it is still an important challenge in terms
of chemoselectivity, regioselectivity, and enantioselectivity; fur-
thermore, efficiency of the chiral ligands as well as the adapta-
bility of the reaction for application in industry remain difficult
to achieve. Initially, we evaluated the catalytic activity of N,P
ligand L2 in the palladium-catalyzed allylic alkylation of di-
methyl malonate with (E)-1,3-diphenylallyl acetate. Upon using
L2 as the chiral ligand, the effects of palladium salts or sources,
the base, solvent, and temperature on the enantioselectivity
and conversion of this catalytic asymmetric allylic alkylation re-
vealed the optimal reaction conditions to involve
[Pd(h3-CH2CH=CH2)Cl]2, N,O-bis(trimethylsilyl)acetamide (BSA,
3 equiv.), CsOAc (2 mol%), and toluene at room temperature
(see Tables S1–S4 in the Supporting Information). Perfect enan-
tioselectivity and yield were achieved under the optimal reac-
tion conditions (99% yield and 99%ee). Notably, other ligands
with varied groups on the amine moiety were also tested, and
the enantioselectivity was inferior to that obtained with N,P
ligand L2 (Table 1).
Entry
1
2
Product
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1 f
2a
2b
2c
2d
2e
2 f
2g
2h
2i
2j
2k
2l
2a
2a
2a
2a
2a
3a
3b
3c
3d
3e
3 f
3g
3h
3i
3j
3k
3l
3m
3n
3o
2p
3q
99
98
98
98
98
95
98
97
98
98
99
98
93
93
92
98
97
99 (R)
98 (R)
97 (R)
96 (R)
99 (R)
96 (S)
96 (S)
99 (S)
96 (S)
98/98[d]
95/96[e]
98/94[f]
90 (R)
88 (R)
92 (R)
90 (R)
97 (R)
9
10
11
12
13
14
15
16
17
[a] Unless otherwise noted, reactions were performed with
(0.396 mmol),
(1.189 mmol), [Pd(h3-CH2CH=CH2)Cl]2 (2 mol%), L2
1
2
(5 mol%), CsOAc (2 mol%), and BSA (1.189 mmol) in toluene (2.5 mL) at
room temperature for 12 h. [b] Yield of isolated product. [c] Determined
by HPLC on a chiral stationary phase; the absolute configuration was es-
tablished by comparison with literature data. [d] The diastereoselectivity
(dr) was 1:1. [e] The dr was 1:1.7. [f] The dr was 1:1.4.
Then, the substrate scope of the reaction was examined by
using the palladium–L2 catalyst system in the presence of
CsOAc and BSA as base in toluene (Table 2). As shown in
Table 2, a wide range of allylic acetates 1 and activated methyl-
ene carbonyl compounds 2 were suitable for the palladium-
catalyzed allylic alkylation, and they afford branched products
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in high yields with high enantioselectivities (96–99%ee for
most cases). The ligand was quite effective for the palladium-
catalyzed allylic alkylation of symmetric dialkyl malonates 2a–i,
and the desired products were obtained with 96–99%ee and
almost quantitative conversions (Table 2, entries 1–9). Notably,
the use of methyl, propyl, and phenyl-substituted dialkyl malo-
nates 2h–i as substrates in this reaction also resulted in the
same level of enantioselectivity, and corresponding products
3h–i having quaternary-carbon-substituted carbonyl groups
were obtained with excellent ee values (96–99%). However,
the diastereoselectivity was not good for unsymmetrical dicar-
bonyl compounds 2j–l (Table 2, entries 10–12). The allylic alky-
lation of unsymmetrical dicarbonyl compounds was a challeng-
ing task. In addition, despite the fact that the catalytic per-
formance was also good for allylic acetates 1b–f, the enantio-
selectivity was sensitive to the substituent on the aromatic al-
lylic acetates (Table 2, entries 13–17).
Table 3. Substrate scope of the asymmetric allylic amination reaction.[a]
Entry
1
4
Product
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1c
1d
1e
1 f
4a
4b
4c
4d
4e
4 f
4g
4h
4i
5a
5b
5c
5d
5e
5 f
5g
5h
5i
5j
5k
5l
5m
5n
5o
5p
5q
5r
98
94
98
97
91
98
97
98
97
98
99
98
98
98
92
89
98
93
97
96
91
99 (S)
97 (S)
86 (S)
86 (S)
98 (S)
86 (S)
98 (S)
87 (S)
99 (S)
97 (S)
71 (S)
97 (S)
99 (S)
98 (S)
98 (S)
94 (S)
98 (S)
97 (S)
99 (S)
97 (S)
96 (S)
Encouraged by the excellent enantioselectivities in the
asymmetric allylic alkylation of malonates and motivated by
the importance of allylic amination in organic synthesis,[10] we
continued to investigate the palladium-catalyzed allylic amina-
tion of (E)-1,3-diphenylallyl acetate with various amines. Initial-
ly, we selected the allylic amination of (E)-1,3-diphenylallyl ace-
tate with phenylmethanamine as the model reaction to test
the feasibility of chiral N,P ligand L2. On the basis of the results
of the experimental screening with various additives, ligands,
solvents, and temperatures (Tables S5–S8), we were pleased to
find that the palladium-catalyzed allylic amination reaction
with chiral N,P-ligand L2 was clean and gave desired produc-
t 5a with almost perfect enantioselectivity and yield (99%ee
and 98% yield) under the optimized reaction conditions
(2 mol% of [Pd(h3-CH2CH=CH2)Cl]2, 5 mol% of ligand L2,
3 equiv. BSA, toluene, room temperature). Notably, other d-
camphor-derived Schiff bases L1 and L3–L6 gave poor or infe-
rior enantioselectivity (23–97%ee) than ligand L2 in this reac-
tion (Table S7).
9
10
11
12
13
14
15
16
17
18
19
20
21
4j
4k
4l
4m
4n
4o
4p
4a
4a
4a
4a
4a
5s
5t
5u
[a] Unless otherwise noted, reactions were carried out with
(0.396 mmol),
(1.189 mmol), [Pd(h3-CH2CH=CH2)Cl]2 (2 mol%), L2
1
4
(5 mol%), BSA (1.189 mmol) in toluene (2.5 mL) at room temperature for
12 h. [b] Isolated yield. [c] Determined by chiral HPLC, the absolute con-
figuration was established by comparison with literature data.
Under the optimized reaction conditions, the asymmetric al-
lylic amination of various amines and allylic acetates was sub-
sequently examined. As shown in Table 3, a variety of amines
4a–p were directly allylated to give desired products 5a–p in
high yields (Table 3, entries 1–16). Except for 4-methoxyben-
zenamine (4k), most amines, including primary and secondary
amines, gave the desired products in good to excellent enan-
tioselectivities (up to 99%ee). The major findings in this allylic
amination reaction are summarized as follows:
with excellent enantioselectivity (97%ee and 98% yield;
Table 3, entry 12).
3) Modification of the aromatic ring on the allylic acetates by
introducing various substituents resulted in the same level
of enantioselectivity in this reaction (Table 3, Entries 17–21),
which showed that the palladium-catalyzed allylic amina-
tion was not sensitive to the aromatic allylic acetates.
1) Substituents on alkylamines can influence the enantioselec-
tivity. Upon using alkyl primary amines as the substrates,
excellent enantioselectivities were obtained (97–99%ee).
Unfortunately, some cyclic secondary amines, such as
pyrrolidine, morpholine, and indoline, only gave the corre-
sponding allylated amines with 86–87%ee in this reaction.
2) Although, in general, aromatic amines were not suitable
substrates in this reaction in terms of enantioselectivity
(Table 3, entries 6 and 11), the use of 4-nitrobenzenamine
containing a strong electron-withdrawing group as the sub-
strate led to the formation of the corresponding product
Therefore, this process provided a robust and practical
method for the construction of aryl-, alkyl-, and alkenyl-substi-
tuted allylic amines with a stereogenic carbon center, which
are important functional amines for synthetic applications.
Although numerous phosphine-containing ligands have
been applied for enantioselective allylic alkylation reactions
with improved performance,[4] the palladium-catalyzed asym-
metric allylic etherification of alcohols with 1,3-diphenyl-2-pro-
penyl acetate, especially with a high level of enantioselectivity
in this reaction, has not been widely reported in the past
decade.[11] Notably, in this regard, since Chan and co-workers[12]
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reported
a method for palladium–(SP,R)-FerroNPS-catalyzed
Table 4. Substrate scope of the asymmetric allylic etherification
(FerroNPS=ferrocenyl-based N,P ligands) asymmetric allylic
etherification with good to excellent enantioselectivities (83–
96%ee), several groups,[13] including our group,[14] have further
developed the ligand to show that bidentate phosphine and
phosphite ligands are effective ligands in palladium-catalyzed
allylic etherification with good enantioselectivities. However,
the previously reported methods still need substantial im-
provement, because the enantioselectivities and chemical
yields are not perfect from an industrial application viewpoint.
Inspired by the above studies and encouraged by our findings
on the palladium-catalyzed allylic alkylation of malonates and
on allylic amination, we continued on the application of d-
camphor-derived Schiff base L2 in the palladium-catalyzed
asymmetric allylic etherification of alcohols with 1,3-diaryl-2-
propenyl acetate.
reaction.[a]
Entry
1
6
Product
Yield[b] [%]
ee[c] [%]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
6a
6b
6c
6d
6e
6 f
6g
6h
6i
6j
6k
6l
6m
6n
7a
7b
7c
7d
7e
7 f
7g
7h
7i
7j
7k
7l
7m
7n
98
99
99
98
98
92
91
98
99
99
94
95
96
95
>99 (S)
99 (S)
97 (S)
>99 (S)
>99 (S)
99 (S)
98 (S)
72 (S)
99 (S)
98 (S)
99 (S)
96 (S)
98 (S)
97 (S)
With the d-camphor-derived Schiff base in hand, the effi-
ciency and stereoselectivity of ligand L2 were then examined
in the model palladium-catalyzed asymmetric allylic etherifica-
tion of benzyl alcohol. Similarly to the above work, after evalu-
ating palladium sources, solvents, bases, and temperatures, we
are pleased to find that Schiff base L2 was still the best choice
in this palladium-catalyzed allylic etherification. Furthermore,
the inorganic base K2CO3 was found to be a suitable promoter
in THF (Tables S9–S11). As shown in Table S11, the palladium-
catalyzed allylic etherification of 6a with 1a at room tempera-
ture afforded perfect enantioselectivity (>99%ee) and almost
quantitative yield (98%). To the best our knowledge, for this
type of allylic etherification, the Pd–L2 system affords the high-
est level of performance reported to date in terms of stereo-
selectivity.
9
10
11
12
13
14
[a] Unless otherwise noted, reactions were performed with
(0.396 mmol),
(1.189 mmol), [Pd(h3-CH2CH=CH2)Cl]2 (2 mol%), L2
1
6
(5 mol%), and K2CO3 (1.189 mmol) in THF (2.5 mL) at 408C for 12 h.
[b] Yield of isolated product. [c] Determined by HPLC on a chiral station-
ary phase; the absolute configuration was established by comparison
with literature data.
On the basis of the above findings, the use of L2 as a chiral
ligand in the palladium-catalyzed asymmetric allylic etherifica-
tion of various alcohols 6 with aromatic allylic acetates 1 was
further evaluated under the optimized conditions. As shown in
Table 4, except for trifluoroethanol, a possible Brønsted acid
[pKa (H2O)=12.5], the allylic etherification reaction proceeded
smoothly to afford the desired ethers with a stereogenic
carbon center in excellent yields (96 to >99%ee). Notably,
Schiff base ligand L2 also worked efficiently in the asymmetric
allylic etherification of alkyl alcohols (e.g., 6i and 6j), which led
to the formation of the desired ethers in excellent yields (99%
yield) and excellent enantioselectivities (98 and 99%ee, respec-
tively). More intriguingly, the palladium-catalyzed asymmetric
allylic etherification of furan-2-ylmethanol (6k) and pyridin-2-
ylmethanol (6m) also gave the corresponding heterocyclic
ethers in excellent yields and enantioselectivities (94–96%
yield and 98–99%ee), which is very important in terms of or-
ganic transformations because of the importance of hetero-
cyclic derivatives.
a chiral N,P ligand in palladium-catalyzed asymmetric allylic al-
kylation reactions with structurally diverse nucleophiles, includ-
ing alkyl malonates, dicarbonyl compounds, primary amines,
secondary amines, heterocycles, and simple alcohols. Notably,
most of the catalytic asymmetric allylic substitution reactions
resulted in excellent yields and enantioselectivities (up to
>99%ee). To the best our knowledge, the catalyst system de-
rived from Schiff base L2 and palladium afforded the highest
level of enantioselectivity reported to date for allylic substitu-
tions, including the allylic alkylation of activated methylene
compounds, the allylic etherification of alcohols, and the allylic
amination of primary alkylamines, which revealed the privi-
leged role of d-camphor-derived Schiff bases in palladium-cat-
alyzed allylic substitutions. Further investigations involving the
application of the Pd/L2 complex in synthetic chemistry will be
performed and reported in the future.
In summary, we developed a family of new Schiff bases for
asymmetric catalysis that are derived from chiral d-camphor
and have nitrogen and phosphorus atoms. This scaffold has
several beneficial features, including ease of accessibility, air
stability, chirality matching of multiple stereogenic centers, and
the incorporation of an imine onto the rigid backbone (d-cam-
phor). We demonstrated successfully its effectiveness as
Acknowledgements
This project was supported by the National Natural Science
Founder of China (No. 21173064, 21202031, and 21472031) and
Zhejiang Provincial Natural Science Foundation of China
(LR14B030001).
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Keywords: allylic compounds · asymmetric catalysis · N,P
ligands · palladium · Schiff bases
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Received: January 24, 2016
Published online on March 22, 2016
ChemCatChem 2016, 8, 1495 – 1499
www.chemcatchem.org
1499
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim