Asymmetric Strecker Reactions Catalyzed by Thiourea Phosphonium and Ammonium Salts

Article abstract of DOI:10.1002/adsc.201700029

The application of asymmetric phase-transfer catalysis to the Strecker reaction of ketimines was realized utilizing bifunctional thiourea-phosphonium salts. The asymmetric Strecker reaction of aldimines was also realized utilizing quaternary ammonium salt

Full text of DOI:10.1002/adsc.201700029

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DOI: 10.1002/adsc.201700029
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Asymmetric Strecker reactions catalyzed by thiourea
phosphonium and ammonium salts
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Hongyu Wang,a, b Kaiye Wang,a Yanfei Ren,a Na Li,a Bo Tang,a,* and
Gang Zhaob,*
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a
College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Functionalized
Probes for Chemical Imaging in Universities of Shandong, Key Laboratory of Molecular and Nano Probes, Ministry of
Education, Institute of Molecular and Nano Science, Shandong Normal University, Jinan 250014, People’s Republic of
China
E-mail: tangb@sdnu.edu.cn
b
Key Laboratory of Synthetic Chemistry of Natural Substances, Shanghai Institute of Organic Chemistry, Chinese Academy
of Sciences, 345 Lingling Lu, Shanghai 200032, Peopleꢀs Republic of China
E-mail: zhaog@sioc.ac.cn
Received: January 10, 2017; Revised: February 23, 2017; Published online: && &&, 0000
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201700029
efficiency.[6] To the best of our knowledge, there has
been no report on the application of asymmetric
Abstract: The application of asymmetric phase-
transfer catalysis to the Strecker reaction of
phase-transfer catalysis to the reaction of ketimines(-
ketimines was realized utilizing bifunctional thiour-
Scheme1A).
ea-phosphonium salts. The asymmetric Strecker
reaction of aldimines was also realized utilizing
quaternary ammonium salts derived from amino
acids.
Keywords: Thiourea-phosphonium salt; Thiourea-
ammonium salt; Strecker reaction; Asymmetric
catalysis; Phase-transfer catalysis
38 Since its discovery in 1850, the Strecker reaction has
39 become an important method for building new CÀC
40 bonds, which is especially useful for synthesizing
41 natural and unnatural amino acids.[1] In the realm of
42 the asymmetric Strecker reaction, compared to aldi-
43 mines,[2] ketimines are less electrophilic than aldi-
44 mines. Therefore, it is more challenging to achieve
45 excellent enantiocontrol in catalyzing the addition of
46 cyanide reagents to ketimines, which involves the
47 construction of chiral quaternary stereocenters.
Scheme 1. Asymmetric cyanation of imines.
48
Recently, some impressive advances have been
Chiral betaine catalysts have evolved as effective
49 accomplished in the asymmetric Strecker reaction organocatalysts,[7] which have been demonstrated to
50 with ketimines catalyzed by both metal catalysts[3] and be efficient in a series of asymmetric organic reactions.
51 organocatalysts.[4] However, asymmetric phase-trans- In addition, our group has successfully applied chiral
52 fer catalysis (APTC), which is an important catalytic phosphonium betaine catalysts to asymmetric Man-
53 strategy that has been largely applied to various fields nich, aza-Henry and Strecker reactions with excellent
54 of organic reactions because of its mild reaction enantioselectivities and yields, in particular, this family
55 conditions and environmentally benign experimental of catalysts was shown toefficiently catalyze the
56 procedures,[5] has only been successfully applied to asymmetric cyanation of the ketimines derived from
57 catalyze the Strecker reaction of aldimines with great isatins (Scheme1B).[8] Inspired by these works, we
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wondered if the highly modular structures of the both the thiourea and phosphonium center, furnished
bifunctional phosphonium salts as phase-transfer cata- a superior catalyst e (67% ee, Table1, entry 5).
lysts derived from amino acids could catalyze asym- Subsequent investigation of the effects of solvent and
metric Strecker reactions with ketimines. Although base with this catalyst revealed that a higher enantio-
there are some structural similarities between the control could be obtained in CHCl3 using NaOAc as
chiral phosphonium betaine and the phosphonium the base (Table1, entry8). Pleasingly, lowering the
salt, they have displayed completely different catalytic reaction temperature toÀ408C improved the enantio-
effects in our previous work,[8a] which gives us a selectivity significantly to give the highest ee of 91%
9
challenging topic for this ongoing project.
with almost quantitative yield, albeit with a prolonged
10
Notably, since the first work with thiourea contain- reaction time of 1h (Table1, entry 17).
11 ing ammonium salts was reported by Fernandez and
12 Lassaletta,[9] a new series of (thio)urea(or squara-
13 mide)-phosphonium (ammonium) salts as highly effi-
14 cient catalysts for various asymmetric organic reac-
15 tions have been realized.[10] Herein, we describe the
16 first application of chiral phase-transfer catalysts
17 derived from amino acids to the Strecker reaction of
18 ketimines.
Table 1. Optimization of the asymmetric Strecker reaction
of ketimine 1a under phase-transfer conditions.[a]
Entry cat. Solvent Base
T(oC) Yield[b] ee[c]
19
Oxindole is an important skeleton occurring in
20 numerous natural and biologically active compounds.-
(%)
(%)
21 Therefore, asymmetric catalysis centered on this
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a
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e
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
DCE
NaOAc 20
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22 structure has received much attention, with many
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
–
–
–
–
–
–
–
–
–
–
–
–
–
23 impressive accomplishments.[11] Moreover, the asym-
24 metric addition reactions of ketimines derived from
25 isatins conveniently allow for the preparation of
26 various synthetically useful chiral quaternary stereo-
27 centers on the oxindole.[12] Therefore, we report that
28 chiral amino acid-based thiourea-phosphonium salts
29 (Figure1) are highly efficient catalysts for the asym-
30 metric Strecker reaction of isatin-derived ketimines.
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15[d]
16[e]
17[f]
18[g]
Toluene NaOAc
TBME
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
NaOAc
KOAc
Na2CO3
K2CO3
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NaOAc À10
NaOAc À20
NaOAc À40
NaOAc À40
[a] Unless otherwise noted, all the reactions were carried out
with 1a (0.1mmol), TMSCN (0.2mmol) and base (0.2mmol)
in solvent (1mL) in the presence of a catalyst (2mol%) for
1min.
[b] Isolated yield.
[c] Determined by HPLC analysis.
[d] The reaction was carried out for 5min.
[e] The reaction was carried out for 10min.
[f] The reaction was carried out for 1h.
[g] The reaction was carried out with 1a(0.1mmol), TMSCN
(0.2mmol) and NaOAc (0.2mmol) in CHCl3(5mL) in the
presence of e(2 mol%) for 1h.
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Figure 1. Chiral thiourea-phosphonium salts used in this
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study.
Initially, the reaction between N-Boc ketimine 1a
47 and TMSCN was selected as the model reaction for
48 optimizing reaction conditions (Table1). First, cata-
49 lysts a–d based on different amino acid skeletons were
50 examined in CH2Cl2 at 208C, and it was found that
51 catalyst d derived from tert-butyl L-leucine was the
52 most promising, giving 54% ee (Table1, entry 4). Of
The effect of dilution on this reaction was also
53 note is the exceptionally short reaction time (1min) explored, which exhibited a strong effect on the
54 required for this PTC system, which is in sharp enantioselectivity in previous work reported by Waser
55 contrast to the cinchona-thiourea system that usually and co-workers,[10k] however, no palpable difference
56 requires 2–4 days.[13] Further modification of the was observed in the diluted solution (Table1, entry
57 structure of d, including varying the substituents on 18). We also meticulously explored the influences of
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the conditions of the reaction, such as TMSCN achieve comparable results irrespective of their elec-
concentration, NaOAc concentration, catalyst loading tronic nature or positions, except for substrate 1k with
and temperature (see Supporting Information for a strongly electron-withdrawing NO2 group that likely
more details). From these tables, we found that resulted in better reactivity of this substrate enabling
lowering the temperature improved the enantioselec- more non-catalytic background cyanation. Notably,
tivity, while increasing the NaOAc concentration this reaction was also performed on a gram scale in
decreased the enantiocontrol. When we surveyed the the presence of 2mol% of the bifunctional catalyst e
concentration of the reagent TMSCN, no influence to provide almost quantitative yield, however, a
was detected. To our delight, we were able to lower slightly decreased enantioselectivity was obtained
10 the catalyst loading to 2mol% to achieve the same (89% ee) together with prolonged reaction time
11 enantioselectivity, but a modest enantioselectivity (12h).
12 (75%ee) was found when 0.5mol% e was loaded.
To obtain some insight into the mechanism, we
13
Under the optimized conditions, the substrate explored the influences of the bifunctional catalyst on
14 scope of the reaction was investigated with different control experiments (Table3). When we performed
15 ketimines (Table2). Changing the N-protecting group the asymmetric reaction catalyzed by h, which lacks
16 benzyl in 1a to substituted-benzyl groups bearing the quaternary phosphonium center, we obtained a
17 electron-withdrawing or electron-donating groups or a low yield, long reaction time and racemic product.
18 methyl group did not show any apparentchanges in Similarly, the use of catalyst j, with one blocked H-
19 either the yield or ee value. To our delight, various bonding site, also gave inferior results with low yield
20 substituents on the benzene ring of the isatin skeleton and long reaction time, but with a 50% ee, which
21 of the ketimines were well tolerated in the reaction to suggested that the H-bond is not crucial for the
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enantioselectivity. However, the bifunctional catalyst i
gave 55% ee and 99% yield in 1h at the same
conditions. We also performed some NMR experi-
ments as a mechanistic study (see SI, FigureS1).The
acidic hydrogen could be extracted by the basic
NaOAc. If KCN as cyanide source was added to the
reaction system under the optimized conditions, al-
most no product was obtained, even when stirring for
a long time, which all illustrated the importance of
TMSCN as the cyanide source. Therefore, according
to the experimental results and previous work,[14] we
propose a plausible reaction pathway as shown in
Scheme2. Firstly the catalyst reacts with the basic
NaOAc, then the newly generated betain i’ will
activate TMSCN to form the actual catalyst i“, which
then induces the asymmetric Strecker reaction. We
also propose a possible transition state according to
the results. The hydrogen-bonding interaction between
the NÀH and the ketimine strengthens the electro-
philic capacity of the ketimine.The CNÀanion and the
quaternary phosphonium center constitutes a nucleo-
phile-phosphonium ion pair.The backbone of the
catalyst blocks the Si face and makes the approach of
the nucleophile from the Re face more favorable.
Next, in order to expand the scope of the
application of the phase-transfer catalysts derived
from amino acids, we continued to explore the
asymmetric Strecker reaction of aldimines.[6] Based on
our previous and above work, we speculated that the
novel phase-transfer catalysts derived from amino
acids may be applicable for N-Boc protected aldi-
mines. However, when the thiourea-phosphonium salt
was used, poor results were obtained. To our delight,
we found that the N-Boc protected aldimines could be
efficiently catalyzed by the quaternary ammonium salt
k (for more details, see SI).
Table 2. Scope of the reaction of ketimines.[a]
[a] The reactions were carried out with 1(0.1mmol), TMSCN
(0.2mmol) and NaOAc(0.2mmol) in CHCl3(1mL) catalyzed
by e (2mol%) atÀ408C for 1 h, the absolute configurations
of 2 were determined by comparison of the optical rotation
values with literature data[13a]
.
[b] The reaction was performed with 1a(30mmol), TMSCN
(60mmol) and NaOAc (60mmol) in CHCl3(100mL) cata-
lyzed by e (2mol%) atÀ408C for 12h.PMB:p-MeOC6H4,
PNB:p-NO2C6H4.
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Table 3. Control experiments for the mechanistic study.[a]
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According to previous work and our work,[10j,15] we
supposed that the narrower space around the
ammonium center than around the phosphonium
center may be beneficial to control the stereoselectiv-
ity of the asymmetric cyanation of aldimine. In
addition, the scope of the reaction is shown in Table4.
N-Boc imines with electron-donating and sterically
different aromatic groups, except for the aliphatic
group, were all well tolerated to provide the products
10 in excellent yields and enantioselectivities. The imines
11 with electron-withdrawing groups also gave good
12 enantioselectivities and excellent yields.
Entry
cat.
Time
Yield(%)[b]
ee(%)[c]
13
In summary, we have realized the first application
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h
i
j
24h
1h
24h
40
99
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14 of asymmetric phase-transfer catalysis to the enantio-
15 selective addition of TMSCN to ketimines derived
16 from isatins.The asymmetric Strecker reaction of the
17 aldimines was also realized for the first time utilizing a
18 quaternary ammonium salt derived from an amino
19 acid. The novel thiourea-phosphonium (ammonium)
20 salts demonstrated high efficiency in catalyzing the
21 asymmetric reactions in excellent yields and high
22 enantioselectivities within short reaction times. Fur-
23 ther investigation of the mechanism of the asymmetric
24 reaction andextension of the application scope of
[a] The reactions were carried out with 1a (0.1mmol),
TMSCN (0.2mmol), NaOAc (0.2mmol) and the catalyst
(2mol%) in CHCl3 atÀ408C. [b] Isolated yield. [c]
Determined by HPLC.
General procedure for the asymmetric reactions of TMSCN
to aldimines:To a vial containing catalyst k (3mol%), a
solution of aldimine (0.1mmol) and NaOAc (5equiv) in CH2
25 these thiourea-phosphonium(ammonium) salts are Cl2 (1mL) was added, and TMSCN (0.2mmol) was added
atÀ458C. The mixture was stirred until the reaction was
26 underway in our laboratory.
completed (monitored by TLC), and then, the crude product
was purified by column chromatography on silica gel to
afford product 4.
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Experimental Section
General procedure for the asymmetric reactions of TMSCN
to ketimines: To a vial containing catalyst e (2mol%), a
solution of ketimine (0.1 mmol) and NaOAc (5equiv) in CH2
Cl2 (1mL) was added, and TMSCN (0.2mmol) was added
atÀ408C. The mixture was stirred until the reaction was
completed (monitored by TLC), and then, the crude product
was purified by column chromatography on silica gel to
afford product 2.
Acknowledgments
This work was supported by the 973 Program
(2013CB933800), Chinese Academy of Science (XDB
20020100), National Natural Science Foundation of China
(21272247, 21572247, 21290184, 21390411, 21535004,
Scheme 2. Plausible reaction pathway.
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Table 4. Asymmetric Strecker reactions of aldimines cata-
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Entry
4
Ar
Yield (%)[b]
ee (%)[c]
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1
2
3
4
5
6
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8
9
4a
4b
4c
4d
4e
4f
4g
4h
4i
C6H5
p-MeOC6H4
p-FC6H4
m-MeC6H4
m-MeOC6H4
o-MeOC6H4
2-thiophyl
a-naphthyl
cyclohexyl
90
90
93
90
86
89
92
90
92
85
85
75
87
87
82
87
92
53[d]
[a] Unless other noted, the reactions were carried out with 3
(0.1mmol), NaOAc (0.2mmol) and TMSCN (0.2mmol)
catalyzed by k (3mol%) in toluene (1mL) atÀ458C for
24h, the absolute configurations of 4 were determined by
comparison of the optical rotation values with literature
data.[6d]
[b] The isolated yield.
[c] Determined by HPLC analysis.
[d] Determined by the derivatization of 4i, see SI.
[8] a) H.-Y. Wang, K. Zhang, C.-W. Zheng, Z. Chai, D.-D.
Cao, J.-X. Zhang, G. Zhao, Angew. Chem.2015, 127,
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Province(ZR2016BQ38).
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COMMUNICATIONS
Asymmetric Strecker reactions catalyzed by thiourea
phosphonium and ammonium salts
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