Cobalt-Catalyzed Asymmetric Allylation of Cyclic Ketimines

Article abstract of DOI:10.1002/chem.201704760

A Co^II/Box-catalyzed [Box=bis(oxazoline)] enantioselective addition of potassium allyltrifluoroborate to cyclic ketimines was developed, providing the corresponding chiral α-tertiary amines in high yields and with good enantioselectivity values. Alkoxycarbonyl- and alkyl-substituted saccharin-derived ketimines are suitable substrates for this allylation reaction. The product can be converted to complex molecules over several simple steps, including a precursor of MK-0371, which is a kinesin spindle protein inhibitor. In addition, this catalytic system showed a strong positive nonlinear effect.

Full text of DOI:10.1002/chem.201704760

A Journal of
Accepted Article
Title: Cobalt-Catalyzed Asymmetric Allylation of Cyclic Ketimines
Authors: Liang Wu, Qihang Shao, Guoqiang Yang, and Wanbin Zhang
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Cobalt-Catalyzed Asymmetric Allylation of Cyclic Ketimines
Liang Wu, Qihang Shao, Guoqiang Yang,* and Wanbin Zhang*
Abstract:
A
CoII/Box-catalyzed enantioselective addition of
have been made in the allylation of aldimines for direct access to
chiral homoallylic amines using allylboron reagents,[7] the
enantioselective allylation of ketimines is not a trivial task due to
their low reactivity. Pioneering work on the catalytic
enantioselective allylation of ketimines has been reported by
Shibasaki and co-workers in which the reaction of acyclic
ketimines with allylboronate using a Duphos-CuF catalyst gave
the corresponding products in high yields with high
enantioselectivities.[8] In 2012, Lam and co-workers reported a
chiral diene-ligated rhodium complex-catalyzed enantioselective
addition of potassium allyltrifluoroborates to cyclic ketimines.[9]
In recent decades, a strong tendency has emerged to utilize
the more economical and abundant late first-row transition-
metal-catalysts for sustainable chemical synthesis.[10] Cobalt, as
a relatively harmless first-row transition metal, has attracted
much attention. A number of successful examples have been
reported for the application of this metal in catalytic
organometallic reactions, such as for use in hydrogenations,[11]
potassium allyltrifluoroborate to cyclic ketimines was developed,
providing the corresponding chiral α-tertiary amines in high yields
and with good enantioselectivities. Alkoxycarbonyl and alkyl
substituted saccharin-derived ketimines are suitable substrates for
this allylation reaction. The product can be converted to complex
molecules over several simple steps, including a precursor of MK-
0371, which is a kinesin spindle protein inhibitor. In addition, this
catalytic system showed a strong positive non-linear effect.
O
ptically active homoallylic amines, especially α-tertiary
homoallylamines, are important chiral building blocks for a wide
variety of medicinally important agents and natural products
(Figure 1).[1] The asymmetric addition of organometallic reagents
hydroborations,[12]
hydroacylations[13]
and
C-H
functionalizations.[14] Rh-catalyzed asymmetric addition of
organoboron reagents to imines has been well studied by
several groups, initiated by Hayashi.[15] However, the using of
other metals, especially earth abundant late first row transition
metals, have not been widely studied in this reaction. The use of
cobalt in such reactions is attractive to synthetic chemists and
the pharmaceutical industry. Indeed, several reports concerning
the CoII-catalyzed addition of arylboronic acids to unsaturated
double bonds have recently been divulged by the Cheng and
Zhao groups.[16] In a continuation of our studies on the late first-
Figure 1.α-Trisubstituted Homoallylamines-containing Natural Products and
Drugs.
row
transition-metal-catalyzed
asymmetric
addition
of
organoboron reagents to cyclic imines,[17] herein we report an
allylation reaction of potassium allyltrifluoroborates with cyclic N-
sulfonyl ketimines that yields α-tertiary cyclic homoallylamines
with excellent enantioselectivities using a cobalt-bisoxazoline
complex as a chiral catalyst. More interestingly, a significant
positive nonlinear effect was observed and a dimetal-catalytic
mechanism has been proposed for this catalysis.[18]
Initially, we chose potassium allyltrifluoroborate 2a and
cyclic N-sulfonyl α-ketiminoester 1a as the model substrate to
study the allylation reaction. As shown in Table 1, the reaction
proceeded smoothly to afford the desired adduct 3a in 66% yield
to ketimines is the most attractive and effective methodology for
the preparation of α-trisubstituted homoallylamines due to the
simultaneous construction of the carbon-skeleton and
stereocenter. Although the allylation of carbonyls and imines has
been developed into one of the most powerful C-C bond-forming
reactions,[2] the asymmetric allylation of ketimines is not often
reported because of the steric and electronic factors of this type
of substrate.[3-5] Recently, a significant advance has been made
in catalytic allylation using allylboronates.[6] The reactions
possess several advantages: they only require relatively mild
experimental conditions, and have high functional group
tolerance; allylboronates are readily available, stable, and
possess relatively low toxicity. Although significant achievements
and
with
a
low
ee
when
the
complex
of
Co(OTf)2·2MeCN/bisoxazoline L1 was used as a catalyst (entry
1). Ligand examination revealed that substituents on the
oxazoline rings were critical for the enantioselectivity. Good ee
was obtained with L4 (entry 4), while L2 and L3 provided very
poor enantioselectivities (entries 2 and 3). Seemingly, the phenyl
groups on the oxazoline ring are required for obtaining high
enantioselectivity. One of the possible explanations for this
observation is the presence of π-x (x = H, π…) interaction in the
catalytic cycle. Other ligands were also examined, including L5-
L9 which possess different linkers between the two oxazoline
rings. However, they all gave 3a in lower ees compared to L4
(entries 5-9). Thus, (S)-Ph-Box L4 provided the best results
[]
L. Wu, Q. Shao, Dr. G. Yang, Prof. Dr. W. Zhang
School of Chemistry and Chemical Engineering
Shanghai Jiao Tong University,
800 Dongchuan Road, Shanghai 200240, China
E-mail: gqyang@sjtu.edu.cn
wanbin@sjtu.edu.cn
Homepage: http://wanbin.sjtu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
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giving product 3a in 92% isolated yield and 90% ee (entry 4).
Solvent screening showed that MTBE (methyl tert-butyl ether)
was the best solvent (for more detail information of condition
screening, see SI). Subsequently, a variety of cobalt salts were
evaluated. The product 3a was obtained in good yields and with
higher enantioselectivities in the presence of Co(BF4)2·6H2O or
Co(ClO4)2·6H2O (entries 10 and 11), with Co(ClO4)2·6H2O
proving to be the best CoII source giving the desired product with
excellent ee (97%) (entry 11). Moreover, the reaction could be
conducted at 40 °C to furnish 3a in a slightly lower yield but with
98% ee (entry 12). Notably, a low yield of product was obtained
when the temperature was reduced to 30 °C (entry 13). The
catalyst loading can be reduced to 3 mol% by extending the
reaction time to 48 h, giving the desired product in good yield
and ee (entries 14 and 15).
With the optimal reaction conditions in hand, we
investigated the scope of this reaction as shown in Scheme 1.
All the reactions of cyclic N-sulfonyl α-ketiminoesters bearing
either electron-donating or electron-withdrawing substituents on
the phenyl ring occurred smoothly to afford the desired products
with excellent enantioselectivities and in modest to excellent
yields (3a-3l). Halides, CF3, OCF3, OMe, and alkyl groups
attached at the C5-C7 position of the N-sulfonyl α-ketimines
were found to be tolerant to the reaction conditions (3b-3m).
Naphthofused sultam 3o was also isolated in good yield and
excellent enantioselectivity. An exception to these excellent
results was substrate 1n bearing 4,6-diMe groups, which gave a
significantly lower ee, showing that the enantioselectivities were
also influenced by the sterics of the substrate. Moreover, good
yields and good to excellent enantioselectivities were obtained
i
n
for substrates bearing a variety of ester groups (Me, Pr, or Bu)
(3p-3r). The allylation of cyclic N-sulfonyl-ketiminoesters with
substituted potassium allyltrifluoroborates was also carried out.
In these reactions, a clean allylic transposition took place to form
a new carbon-carbon bond at the more substituted carbon atom
of the allyltrifluoroborate; high enantioselectivities were obtained
but the diastereomeric ratios were only modest (3s and 3t). To
confirm the scalability of the present protocol, a larger scale
reaction of N-sulfonyl α-ketiminoester 1a (4 mmol) with
potassium allyltrifluoroborate 2a was carried out, and product 3a
(1.07 g) was readily isolated in 95% yield and 99% ee.
Table 1. Reaction optimization.[a]
Entry
Solvent
Ligand[b]
L1
Yield [%][b]
66
ee [%][c]
5
Co(OTf)2·2MeCN
Co(OTf)2·2MeCN
Co(OTf)2·2MeCN
Co(OTf)2·2MeCN
Co(OTf)2·2MeCN
Co(OTf)2·2MeCN
Co(OTf)2·2MeCN
Co(OTf)2·2MeCN
Co(OTf)2·2MeCN
Co(BF4)2·6H2O
Co(ClO4)2·6H2O
Co(ClO4)2·6H2O
Co(ClO4)2·6H2O
Co(ClO4)2·6H2O
Co(ClO4)2·6H2O
1
85
9
2
L2
53
5
3
L3
92
90
40
43
45
85
80
94
97
98
98
95
93
4
L4
86
5
L5
78
6
L6
95
7
L7
87
8
L8
96
9
L9
94
10
L4
95
11
L4
12[d]
13[e]
14[d,f,g]
15[d,f,h]
L4
94
80
L4
84
L4
78
L4
[a] Reaction conditions: 1a (0.10 mmol, 1.0 equiv), 2a (0.2 mmol, 2.0 equiv),
MTBE (1.5 ml), Co salts (10 mol%), L1−L9 (12 mol%), 50 oC, 24 h. [b] Isolated
yields. [c] Enantioselectivity was determined by HPLC using a chiral column.
[d] 40 oC. [e] 30 oC. [f] 48 h. [g] CoII (5 mol%) and L4 (6 mol%). [h] CoII (3
mol%) and L4 (3.6 mol%).
Scheme 1. Substrate scope of N-sulfonyl α-ketiminoesters. [a] 4 mmol scale,
1.07 g of product was obtained.
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A
range of N-sulfonyl ketimines possessing alkyl
the tricyclic compound 12 bearing a tetrasubstituted carbon
stereogenic center on the ring system.
substituents were also tolerable to the enantioselective cobalt-
catalyzed allylation reaction (Scheme 2). Excellent yields and
high ees were obtained when the reaction was carried out in
MTBE at 50 °C using Co(OTf)2·2MeCN/bisoxazoline L5 as a
catalyst. All of the substrates gave good yields and good
enantioselectivities of the desired products (5a-5d). Acyclic
aldimine and ketimine are unreactive for our CoII-catalytic
system.
Considering the mechanism of the reaction, on one hand
CoII is a good Lewis acid so that it may activate the ketimine
(Figure 2, A).[20] On the other hand, the transmetalation of
allylboron with CoII to form an allyl-CoII species has also been
reported (corresponding to models B or C).[21] To shed some
light on the mechanism of this reaction, several other Lewis
acids were also tested by replacement of the CoII salt with other
metal salts using the same reaction conditions. Cu(ClO4)2·6H2O,
Cu(OTf)2, Mg(OTf)2, Sc(OTf)3 and Fe(OTf)3 could also catalyze
this reaction to give the allylation product 3a in 54-92% yield, but
the enantiomeric excess of the product was very poor (0-7% ee,
see SI). We thus hypothesize that the allylation reaction of
potassium allyltrifluoroborate with cyclic N-sulfonyl α-
ketiminoesters can be catalyzed by metal Lewis acids (A), but
CoII may also play other roles in the asymmetric catalysis to give
the product in high ee (B, D). Interestingly, significant positive
nonlinear effects of the catalyst’s enantiomeric composition on
reaction enantioselectivity are observed in this reaction (Figure
2).[22] Although we cannot exclude the formation of unreactive
Scheme 2. Substrate scope of N-sulfonyl ketimines possessing alkyl
substituents.
catalytic
oligomers
of
heterochiral
complexes
([ConLS LR ](ClO4)2n) which may be one possible explanation for
x
y
non-linear effect, we propose a dimetal catalysis model (D) for
this reaction based on the aforementioned results.[23,24] The two
Co catalytic center may or may not ligates with each other.
Scheme 3. Transformations of product 3a.
To highlight the synthetic utility, we have applied this
methodology to the synthesis of
a series of biologically
interesting compounds (Scheme 3). For example, treatment of
amino ester 3a (99% ee) with LiAlH4 in refluxing THF afforded
the corresponding cyclic N-sulfonamido alcohol 6 in 70% yield
and without loss of enantiopurity. 6 was readily converted into 7
in equivalent yield. This product could be further transformed in
moderate yield to the oxazolidinone structure 8 bearing a chiral
α-tertiary amine motif (99% ee) via removal of the SO2 group.
Oxazolidinone 8 is an important intermediate for the total
synthesis of MK-0731.[19] In another example, reaction of 8 with
allyl bromide and sodium hydride in THF and under reflux
conditions furnished the N-allylated derivative 9 in 76% yield.
Subsequent ring-closing metathesis using the Grubbs’ first-
generation RCM catalyst in DCM afforded the bicycle 10 in
excellent yield after flash-filtration with no loss in optical purity.
Figure 2.Nonlinear Effect Experiment Using 1a as Substrate and L4 as
Ligand (10 mol% Co, 12 mol% L4), and Proposal of Catalysis Models.
In summary, we have developed an efficient and
enantioselective cobalt-catalyzed addition of potassium
allyltrifluoroborates to N-sulfonyl ketimines. A wide scope of
optically active α-trisubstituted homoallylamines can be obtained,
which are anticipated to be potential bioactive compounds or
drug precursors for medical applications. Further studies of the
synthetic utilities of this protocol reveal it to be a promising
To this end, treatment of oxazolidinone
8
with ethyl
bromoacetate followed by Friedel-Crafts cyclization furnished
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This work was partly supported by the NSFC (Nos. 21772119,
21620102003, 21302127), STCSM (No. 15JC1402200) SHMEC
(No. 201701070002E00030) and the Fundamental Research
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Keywords: cobalt • allylation • asymmetric catalysis • ketimine •
α-tertiary amine
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Liang Wu, Qihang Shao, Guoqiang
Yang,* and Wanbin Zhang*
Page No. – Page No.
Cobalt-Catalyzed Asymmetric
Allylation of Cyclic Ketimines
Co2+operation: A CoII/Box-catalyzed asymmetric allylation of cyclic ketimines was
developed using potassium allyltrifluoroborate. The desired homoallylic amines
were obtained in excellent yields and enantioselectivities.
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