Fei-Phos ligand-controlled asymmetric palladium-catalyzed allylic substitutions with structurally diverse nucleophiles: Scope and limitations

Article abstract of DOI:10.1039/c6ra09657c

To shed light on the scope and limitations of palladium-catalyzed allylic alkylation in the presence of chiral trans-1,2-diaminocyclohexane-derived Fei-Phos as an effective phosphine ligand, the asymmetric palladium-catalysed alkylation of structurally diverse hard/soft nucleophiles, including allylic etherification of alcohols and the allylic alkylation of activated methylene compounds, indoles, and aromatic amines were investigated in this study; the corresponding products with various functional groups were achieved in good yield and with high enantioselectivity (up to 99% ee).

Full text of DOI:10.1039/c6ra09657c

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Fei-Phos ligand-controlled asymmetric palladium-
catalyzed allylic substitutions with structurally
diverse nucleophiles: scope and limitations†
Cite this: RSC Adv., 2016, 6, 45495
Jian-Xing Xu,a Fei Ye,a Xing-Feng Bai,ab Jin Zhang,a Zheng Xu,a Zhan-Jiang Zhenga
ab
*
and Li-Wen Xu
To shed light on the scope and limitations of palladium-catalyzed allylic alkylation in the presence of chiral
trans-1,2-diaminocyclohexane-derived Fei-Phos as an effective phosphine ligand, the asymmetric
palladium-catalysed alkylation of structurally diverse hard/soft nucleophiles, including allylic
etherification of alcohols and the allylic alkylation of activated methylene compounds, indoles, and
aromatic amines were investigated in this study; the corresponding products with various functional
groups were achieved in good yield and with high enantioselectivity (up to 99% ee).
Received 14th April 2016
Accepted 25th April 2016
DOI: 10.1039/c6ra09657c
www.rsc.org/advances
catalytic asymmetric allylic alkylation reactions. Although
numerous chiral ligands have been reported in the catalytic
asymmetric allylic alkylation reaction in recent years,1–5 it is still
highly desirable to add a highly efficient ligand to this list that
can readily accommodate substrates for a wide of catalytic
asymmetric allylic alkylation reactions. Also, the catalysis
chemistry of asymmetric allylic alkylation reactions with
improved catalytic activity and performance as well as stereo-
selectivity continues to receive much attention from synthetic
chemists.
Introduction
Allyl-substituted compounds are an extremely useful class of
molecules that have been used as widespread building blocks in
organic synthesis for more than 40 years.1 Among the synthetic
methods for the preparation of allyl-substituted compounds by
alkylation reactions, transition-metal-catalyzed allylic alkyl-
ations offer one of the simplest and most practical approaches
for the construction of allyl-substituted molecular frameworks.2
Accordingly, the allylic alkylation reaction has become
Despite the recent success of the general allylic alkylation of
carbon nucleophiles, the development of new palladium
complexes with extremely high enantioselectivity and activity is
a hot topic and is also highly sought aer. In addition, the
a
fundamental carbon–carbon bond-forming reaction in
synthetic chemistry. Of especial importance, the catalytic
asymmetric allylic alkylation is now a classic, powerful and
useful carbon–carbon and carbon–heteroatom bond forming
reaction which has satised the growing demand for enantio-
selective synthesis of a range of structurally and stereochemi-
cally rich functional alkenes in recent decades.3 In particular,
the resulting substituted alkenes are prevalent intermediates in
numerous natural products and biologically important building
blocks in organic synthesis. In this respect, recent research has
focused on transition-metal-catalyzed asymmetric allylic alkyl-
ation with varied allylic acetates. In this approach, the catalytic
asymmetric allylic alkylation usually relies on the rational
design of chiral ligands.4 In this regard, numerous phosphine
ligands have been found to play extremely effective roles in
palladium-catalyzed
asymmetric
allylic
alkylation
of
heteroatom-based nucleophiles, such as alcohols, cyanoace-
tates, and heterocycles, with high levels of enantioselectivity has
not been widely reported, probably due to unfavorable deacti-
vation of the metal–ligand complexes.6 Among catalytic asym-
metric alkylation reactions, the catalytic asymmetric allylic
alkylation of alcohols is one of the most important C–O bond
cleavage and new C–O bond forming transformations for the
synthesis of chiral ethers. In this context, various palladium
catalyst systems associated with bidentate phosphine or phos-
phite ligands have afforded the chiral ethers with moderate to
good stereoselectivity.7
In 2013, we developed a new multidentate and multifunc-
tional phosphine with multiple stereogenic centers (CycloN2P2-
Phos), also called Fei-Phos in this work, derived from chiral
trans-1,2-diaminocyclohexane, for the catalytic asymmetric
alkylation of allylic acetates with benzyl alcohols and silanols
(Scheme 1).8 Also in this work, it was found that catalytic allylic
alkylation of alcohols and silanols resulted in good to excellent
yields and high enantioselectivities (up to 99% ee). However, the
aKey Laboratory of Organosilicon Chemistry and Material Technology of Ministry of
Education, Hangzhou Normal University, Hangzhou 311121, P. R. China. E-mail:
liwenxu@hznu.edu.cn; Fax: +86 2886 5135; Tel: +86 2886 5135
bState Key Laboratory for Oxo Synthesis and Selective Oxidation Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China. E-mail:
licpxulw@yahoo.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra09657c
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Scheme 1 Pd/Fei-Phos catalyzed asymmetric allylic etherification/
silylation of allylic acetate with alcohols or silanols.
detailed catalytic activity and possible substrate scope as well as
the limitations of Fei-Phos in the allylic substitution reaction
remain unclear. Meanwhile, in order to expand the synthetic
application of Fei-Phos in asymmetric palladium catalysis,
especially the stereoselectivity of Fei-Phos in the catalytic
asymmetric allylic alkylation and the determination of the
privileged role of the Pd/Fei-Phos catalyst system in such reac-
tions, the clarication of the corresponding asymmetric catalysis
chemistry based on related reaction results is an interesting task
in this eld. In addition, we became intrigued by the possibility
of generating a coordinated palladium/Fei-Phos complex by
a nitrogen or phosphine center, a true catalyst that heretofore
has not been claried in previous studies; this prompted us to
carry out a comparative investigation of asymmetric palladium-
catalyzed allylic alkylation with other ligands in this work.
Results and discussion
Pd/Fei-Phos-catalyzed allylic alkylation with activated
methylene compounds
Scheme 2 Synthesis of diamine-derived multifunctional biphosphines
L1 and L2.
Although Fei-Phos has been introduced as a chiral ligand in
palladium-catalyzed allylic substitutions, our previous work
only focused on the allylic etherication/silylation reaction. In
addition, it is unclear how the stereoselective allylic substitu- diaminocyclohexane with meta-bromobenzaldehyde or para-
tions are induced by the multifunctional Fei-Phos because of bromobenzaldehyde. Then, similarly to a previous report,8 the
the two nitrogen centers and two phosphine centers on this resulting Schiff base S3 was converted into the substituted
ligand. Therefore, it is necessary to investigate the catalytic diphenylpiperazine S4 by a reductive coupling reaction.9 The
functionality and stereoselective induction of Fei-Phos in methylation of secondary amine S4 to tertiary amine S5 was
various allylic substitutions and its potential applications in subsequently performed with MeI in the presence of strong
organic synthesis. To exclude the possibility of two molecular base. Then, in the last step, bromide–lithium exchange followed
ligands with one palladium catalyst (PdL2), we wished to by reaction with ClPPh2 gave the desired biphosphine ligands
prepare the analogous ligands from the same chiral source, L1a and L1b. Similarly, we also prepared the ligand L2 (Scheme
trans-1,2-diaminocyclohexane, for the corresponding catalytic 2), an analogue of Fei-Phos ligand, with
a dicyclohex-
allylic alkylation of various nucleophiles; we felt it would also be ylphosphine moiety. Therefore, these ligands could be applied
useful to indirectly compare studies of the intermolecular to the comparative study of catalytic asymmetric allylic alkyl-
coordination (Pd/Fei-Phos) and intramolecular interaction ations with or without Fei-Phos ligand.
(PdL2). In addition, to determine the importance of the phos-
The catalytic asymmetric allylic alkylation of activated
phorous atom on the Fei-Phos, we carried out the synthesis of methylene compounds is one of the most attractive and
diamine-derived multifunctional biphosphines L1a and L1b. As important approaches for the synthesis of chiral allyl-
shown in Scheme 2, the synthesis of the two novel multifunc- substituted carbonyl derivatives. To better understand the
tional biphosphine ligands started with the preparation of observed trends corresponding to the powerful potential of Fei-
Schiff base S3 via the simple condensation of trans-1,2- Phos related to the catalytic activity of Pd/Fei-Phos complex in
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various asymmetric allylic alkylation reactions, further evalua- However, except for methyl or ethyl-2-cyanoacetate, substituted
tion of the substrate scope was performed to probe the general alkyl-2-cyanoacetates with other groups on their ester moieties,
structure of nucleophiles; we anticipated good enantiose- such as i-Pr, n-Bu, t-Bu, or Bn, gave a mixture of diastereoisomers
lectivity in catalytic asymmetric allylic alkylation with activated with low diastereoselectivity (52 : 48).
methylene compounds. With the optimized reaction conditions
The asymmetric allylic alkylation of activated methylene
in hand (see Tables S1–S4 of the ESI†), a variety of activated compounds with 1,3-diphenyl-2-propenyl acetate (1) using L1–
methylene compounds were used in the catalytic asymmetric L2 (Scheme 4) and chiral Schiff base-based P,N-ligands as chiral
allylic alkylation reaction (Scheme 3). Gratifyingly, various ligands was also investigated. Although the catalytic reactivities
methylene nucleophiles (C1–C4) reacted smoothly with 1,3- of these ligands were comparable to that of Fei-Phos in the
diphenyl-2-propenyl acetate (1) to deliver highly optically pure catalytic asymmetric allylic alkylation of ethyl-2-cyanoacetate
allylic methylene compounds (90% to 94% ee), with good yields (Scheme 3), the expected products 3b, 3d, or 3e were still ob-
of the isolated products (72% to 92% yield). As shown in tained with low enantioselectivity. For example, only 28% ee of
Scheme 3, this approach is potentially useful for the enantio- 3b was achieved in the presence of ligand L1a or L1b. As a ESI†
selective construction of important compounds containing experiment, we also investigated the palladium-catalyzed allylic
aromatic alkenes and unsaturated carbonyl groups. Notably, all etherication of benzylic alcohol in the presence of the new
the activated methylene compounds with various relative acid- ligands L1 and L2 (Scheme 2). These observations in Scheme 5
ities of their a-hydrogen atoms (pKa ¼ 11.1 to 16.4)10 gave the are consistent with the allylic etherication, where biphos-
desired products at almost the same high level of enantiose- phines L1 and L2, derived from trans-1,2-diaminocyclohexane,
lectivity (90% to 94% ee) with undifferentiated induction. The exhibited inferior stereocontrol in this reaction; this indirectly
electronic properties of the activated methylene compounds provides useful information that Fei-Phos is a structurally
show no signicant impact on the selectivity and reactivity of reasonable phosphine ligand in palladium catalysis.
the allylic alkylation, which possibly reveals the high level of
stereoselectivity of the multifunctional Fei-Phos ligand in the chiral Schiff base-based P,N-ligands that were obtained from in
allylic alkylation reaction of activated methylene compounds. situ condensation of (S)-1-(2-(diphenylphosphino)phenyl)
Furthermore, we have also investigated the reactivity of
In addition, to the best of our knowledge, there are only a few ethanamine12 with simple aromatic aldehydes (Scheme 6) in the
examples of the asymmetric palladium-catalyzed allylic alkylation palladium-catalyzed allylic alkylation of ethyl-2-cyanoacetate.
of 2-cyanoaetates with allylic acetates;11 unfortunately, the yields However, in contrast to the multifunctional P,N-type Fei-Phos
and enantioselectivities in previous reports were not good. In fact, ligand, these chiral P,N-ligands resulted in poor yields (10%
chiral ligands with good catalytic activity were still insufficient for to 65%) and inferior enantioselectivities (0% to 90% ee) in this
the allylic alkylation of unsubstituted 2-cyanoacetates.11b,c Very reaction. The above results provided indirect evidence that the
recently, we reported an efficient palladium-catalyzed asymmetric effects of nitrogen and phosphorous atoms on the Fei-Phos
allylic alkylation of alkyl-2-cyanoacetates using a BINOL-derived backbone in the palladium-catalyzed allylic alkylation are not
multifunctional N,O,P-ligand (HZNU-Phos), in which the Pd/ as simple as imagined, which also supports the privileged role
HZNU-Phos complex exhibited high diastereo- and enantiose- of multifunctional Fei-Phos in this reaction.
lectivity (up to >99 : 1 dr and up to 96% ee) in this reaction.12
Therefore, under the optimized reaction conditions, we
further investigated the substrate scope of the Pd/Fei-Phos-
Scheme 4 Comparison of the results of the palladium-catalyzed
Scheme 3 Pd/Fei-Phos catalyzed asymmetric allylic alkylation of allylic alkylation of activated methylene compounds in the presence of
activated methylene compounds (carbon-based nucleophiles).
Fei-Phos and its analogues.
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Scheme
5 Catalytic activity of diamine-derived multifunctional
biphosphines L1a and L1b in palladium-catalyzed allylic etherification.
Scheme 7 Pd/Fei-Phos catalyzed asymmetric allylic alkylation of
alkyl-2-cyanoacetates.
synthesis of indole derivatives is of signicant importance
because they are commonly found in bioactive natural products
and ne chemicals.13,14 In this regard, although numerous
chiral palladium–phosphine complexes have been reported to
catalyze enantioselective intra- or intermolecular allylic alkyl-
ations of indoles that afforded important indole-based chiral
molecules/heterocycles,15 there are only a few examples related
to the intermolecular palladium-catalyzed allylic alkylation of
indoles with high enantioselectivities (>95% ee).16 Herein, with
commercial available indoles 4 in hand, palladium-catalyzed
asymmetric allylic alkylation was attempted under the opti-
mized reaction conditions (see Table S5 of the ESI† and Scheme
7; 2.5 mol% of palladium catalyst, [Pd(h3-C3H3)Cl]2, 5.5 mol% of
Fei-Phos, and 3 equiv. K2CO3 in toluene were determined to be
the optimal reaction conditions).
To our delight, Fei-Phos was also highly effective in the
asymmetric allylic alkylation of indoles with allylic acetate. The
results presented in Scheme 8 clearly demonstrate the efficiency
of Fei-Phos in this catalytic asymmetric transformation. The
corresponding products 5 or 6 were obtained with good yields
and excellent enantioselectivities (91% to 99% ee).17 Therefore,
we have succeeded in the development of a highly efficient
strategy for the palladium-catalyzed asymmetric allylic alkylation
of indoles with the highest level of enantioselective induction to
date in certain cases (for example, 99% ee for 5b/6b). Notably,
when acetic acid 3-phenyl-allyl ester was used in the palladium/
Fei-Phos-catalyzed allylic alkylation of indole (4a), only the
linear product 6h was obtained in good yield (Scheme 9).
Scheme 6 Chiral Schiff base-based P,N-ligand-controlled palladium-
catalyzed allylic alkylation of ethyl-2-cyanoacetate: a comparison with
multifunctional Fei-Phos.
promoted asymmetric allylic alkylation reaction of various alkyl-
2-cyanoacetates with 1,3-diphenyl-2-propenyl acetate (1). As
shown in Scheme 7, good yields and high ee values were ob-
tained for various substituted 2-cyanoacetates (up to 94% ee).
Similarly to previous reports,11,12 other alkyl groups on 2-cya-
noacetates, such as i-Pr, n-Bu, t-Bu, or Bn, gave mixtures of
diastereoisomers with low diastereoselectivity (about 55 : 45).
Notably, the ee values of these isomers were good to excellent
(Scheme 7, 90% to 94% ee).
Pd/Fei-Phos-catalyzed allylic alkylation with indole
compounds
Pd/Fei-Phos-catalyzed allylic amination with aromatic amines
Encouraged by these experimental results, we then turned our Chiral allylic amines are very important compounds in organic
attention to the asymmetric allylic alkylation of indoles. The synthesis because they are useful synthetic intermediates for
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Pd(PPh3)2Cl2, and [Pd(h3-C3H5)Cl]2, could promote the desired
transformation in excellent yields (Table 1). Among the palla-
dium catalysts evaluated in this work, [Pd(h3-C3H5)Cl]2 and
Pd2(dba)3$CHCl3 gave the best enantioselectivity in the pres-
ence of Fei-Phos (entry 6 of Table 1). Then, a broad screen of
commercially available P-based ligands, including our HZNU-
Phos ligand, which exhibited good performance in the allylic
alkylation of alkyl-2-cyanoacetate,12 was carried out for
a comparative study to identify Fei-Phos as the privileged ligand
in this reaction (Scheme 10).
Table 1 Screening of palladium salts in the catalytic asymmetric
amination of 1,3-diphenyl-2-propenyl acetate with aniline
Entry
Cat. Pd
Yield (%)
ee (%)
1
2
3
4
5
6
Pd(OAc)2
PdCl2
Pd2(dba)3$CHCl3
Pd(PPh3)4
99
99
99
94
99
99
81
80
83
52
81
83
Scheme
8 Pd/Fei-Phos-catalyzed asymmetric allylic alkylation of
indoles (a) [Pd(h3-C3H3)Cl]2 (2.5 mol%), Fei-Phos (5.5 mol%), toluene,
K2CO3 (3 eq.), RT, 12 h. (b) Boc2O, DMAP (1 mol%), DCM.
Pd(PPh3)2Cl2
[Pd(h3-C3H5)Cl]2
Scheme 9 Palladium/Fei-Phos-catalyzed allylic alkylation of indole
(4a) with acetic acid 3-phenyl-allyl ester.
the preparation of natural products and drug candidates.18
Accordingly, nding a reliable and efficient entry into chiral allylic
amines is a highly important task. In this regard, the palladium-
catalyzed allylic amination of allylic acetates with amines has
been developed to address this task with the aid of chiral
ligands.19 Multifunctional Fei-Phos-controlled catalytic asym-
metric allylic amination reactions for accessing chiral allylic
amines should be useful for the development of new synthetic
methods as well as for further determination of the scope, limi-
tation, and mechanistic understanding of the palladium/Fei-Phos-
catalyzed alkylation of various nucleophilies in the near future.
Based on the experimental studies described above that
aimed at accessing allyl-substituted compounds with the
synthetic strategy of palladium-catalyzed allylic alkylation, we
envisioned that catalytic asymmetric allylic amination using the
Pd/Fei-Phos catalyst system could also afford the corresponding
products in good yields and with promising stereoselectivity.
Initial attempts of catalytic allylic amination reactions to
generate allylic amines revealed that various Pd-based catalysts,
Scheme 10 Comparison of the results of the palladium-catalyzed
including Pd(OAc)2, PdCl2, Pd2(dba)3$CHCl3, Pd(PPh3)4, asymmetric allylic amination with aniline.
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Under the optimized reaction conditions that were estab- alkyl amines, secondary aromatic amine, amide, and carbamate
lished in the screening of solvents, temperatures, and other are not suitable substrates in this allylic amination. In other
factors (see Tables S6–S9 of the ESI†), the substrate scope of this words, these negative results are useful for understanding how
palladium-catalyzed allylic amination with aromatic amines the Pd/Fei-Phos catalyst system-involved mechanistic procedure
was examined. As summarized in Scheme 11, for the aromatic is affected by the electronic and steric factors of hard/so
amines, substituents on different positions of the phenyl substrates in this reaction.
moiety were well tolerated in this reaction, and all the corre-
sponding products 8 were obtained in excellent yield (86% to
97% isolated yields) with moderate to good enantioselectivity
Conclusions
(75% to 86% ee).
In summary, we have determined the benecial features of Fei-
Although the asymmetric palladium-catalysed alkylation of Phos in the palladium-catalyzed asymmetric allylic ether-
structurally diverse nucleophiles, including the allylic ether- ication of alcohols and the asymmetric allylic alkylation of
ication of alcohols and the allylic alkylation of activated activated methylene compounds, indoles, and aromatic
methylene compounds, indoles, and aromatic amines, has amines. All these catalytic transformations resulted in good to
supported the powerful potential of the enantioselective excellent yields and high enantioselectivities (up to 99% ee),
induction of Fei-Phos ligand in palladium catalysis, this ligand which also demonstrates that this catalytic system, Pd/Fei-Phos
still has unavoidable limitations in the palladium-catalyzed complex, affords a high level of enantioselectivity. In view of the
allylic alkylation of nitrogen-based nucleophiles. As shown in established utility of the Pd/Fei-Phos catalyst system in various
Scheme 12, we have found that heterocycles (such as imidazole), allylic alkylation reactions, we anticipate that it will be neces-
sary to carry out further mechanistic studies on the Pd catalysis
in this reaction in the near future. Finally, we believe that Fei-
Phos is an effective ligand in synthetically useful palladium-
catalyzed asymmetric transformations because of the incorpo-
ration of a tertiary diamine onto the diphosphine-type ligand.
Acknowledgements
The authors are grateful for the nancial support of the
National Natural Science Foundation of China (No. 21173064,
51303043, 21472031, and 21503060), and the support of the
Zhejiang Provincial Natural Science Foundation of China
(LR14B030001) is appreciated. The authors also thank Prof. Z.
R. Qu, Dr K. Z. Jiang, Dr C. Q. Sheng, and Dr X. Q. Xiao (all at
HZNU) for their technical and analytical support.
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Scheme 11 Palladium-catalyzed asymmetric allylic amination of 1,3-
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