Enantioselective allylic amination of Morita-Baylis-Hillman acetates catalyzed by chiral thiourea-phosphine

Article abstract of DOI:10.1002/cjoc.201500697

The enantioselective allylic amination of Morita-Baylis-Hillman acetates catalyzed by chiral cyclohexane-based thiourea-phosphine catalysts was investigated. In the presence of 20 mol% rosin-derived thiourea-phosphine 3j, the chiral amines were obtained i

Full text of DOI:10.1002/cjoc.201500697

COMMUNICATION
DOI: 10.1002/cjoc.201500697
Enantioselective Allylic Amination of Morita-Baylis-Hillman
Acetates Catalyzed by Chiral Thiourea-Phosphine
Xuan Zhao, Tianchen Kang, Jie Shen, Feng Sha,* and Xinyan Wu*
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, Shanghai 200237, China
The enantioselective allylic amination of Morita-Baylis-Hillman acetates catalyzed by chiral cyclohexane-based
thiourea-phosphine catalysts was investigated. In the presence of 20 mol% rosin-derived thiourea-phosphine 3j, the
chiral amines were obtained in up to 88% yield and up to 85% ee.
Keywords allylic amination, chiral phosphine, enantioselective organocatalysis, Morita-Baylis-Hillman acetate,
phthalimide
Introduction
applied in the enantioselective allylic amination of
MBH acetates by Shi’s group[6d] in 2011, and the de-
sired products were provided in up to 99% yield and up
to 90% ee by using 25 mol% of chiral organocatalyst at
10 ℃. Considering the variety of chiral phosphine
catalysts in other asymmetric reactions,[7] the explora-
tion of efficient chiral phosphine-based organocatalysts
for allylic amination remains necessary.
In our previous works, we have found the chiral
cyclohexane-based phosphine compounds are highly
efficient organocatalysts in several enantioselective re-
actions, such as allylic alkylation,[8a] [4＋2] cycloaddi-
tion,[8b] and MBH-type reactions.[8c-8k] As a part of our
continuing efforts to expand the application of these
phosphine organocatalysts, herein we report the enanti-
oselective allylic amination of MBH adducts catalyzed
by the chiral cyclohexane-based phosphines.
The asymmetric allylic substitution reaction, which
is a powerful tool in organic synthesis for the construc-
tion of carbon-carbon and carbon-heteroatom bonds
with a carbon-stereogenic center, allows easy access to
diverse allylic compounds.[1,2] The substitution with
nitrogen-containing nucleophiles, as known as allylic
amination, is one of the straightforward method of syn-
thesizing α-methylene-β-amino compounds, which is
widely applied in the synthesis of natural products and
biologically active pharmaceuticals.[3] Over the past
decade, great progress has been made in the chiral
amine-catalyzed allylic amination,[4,5] and the N-
nucleophiles involve 4-methyl benzene-sulfonamide,[5a]
phthalimide,[5b,5c] indole,[5d] cyanopyrrole,[5d,5i] ena-
mide,[5e] benzophenone imine,[5f] carbamate and tosyl-
carbamate,[5g,5k] allylic amine,[5h] isatin[5j] and hy-
drazine[5l] in the allylic amination of allylic alcohols or
their derivatives. However, there are only a few exam-
ples of enantioselective allylic amination using nucleo-
philic chiral tertiary phosphine as catalyst.[6] In 2004,
Krische[6a] demonstrated the first chiral phosphine-
catalyzed enantioselective allylic amination of phthali-
mide with Morita-Baylis-Hillman (MBH) acetate in
80% yield and in 56% ee, using (R)-Cl-MeO-BIPHEP
as a chiral catalyst. Hou and co-workers[6b] reported the
allylic amination of MBH acetates catalyzed by planar-
chiral [2,2] paracyclophane monophosphines in 2007.
Later on, Shi and co-workers[6c] developed a chiral BI-
NOL-derived amide-phosphine for this reaction, giving
the chiral amines in good yields (70%－95%) and up to
78% ee. The corresponding thiourea-phosphine was also
Experimental
General procedure for the enantioselective allylic
amination
To a solution of chiral catalyst 3j (0.04 mmol) in 1
mL CHCl3 was added the MBH adduct 5 (0.2 mmol),
phthalimide (0.4 mmol) and triethylamine (0.04 mmol)
at 25 ℃. The corresponding mixture was stirred at this
temperature until the completion of the reaction (moni-
tored by TLC). The solvent was removed under reduced
pressure and the residue was purified by a flash column
chromatography on silica gel to afford the desired
products. The ee values were determined by HPLC
analysis with a chiral column.
*
E-mail: shaf@ecust.edu.cn, xinyanwu@ecust.edu.cn; Tel.: 0086-021-64252011
Received September 29, 2015; accepted November 8, 2015; published online December 15, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500697 or from the author.
Chin. J. Chem. 2015, 33, 1333—1337
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zhao et al.
COMMUNICATION
Results and Discussion
Table 1 Screening of chiral organocatalysts for the allylic ami-
nation between phthalimide and MBH acetate 5aa
Initially, we chose allylic amination of phthalimide
with the MBH acetate 5a derived from methyl vinyl
ketone as a model reaction to evaluate the chiral cyclo-
hexane-based phosphine catalysts (Figure 1). The reac-
tion could not take place in the absence of a basic addi-
tive. Considering that the pKa value of pronucleophile
phthalimide (pKa 8.3) is larger than that of acetic acid
(pKa 4.8), which should not allow the acetate to depro-
tonate the phthalimide.[6d] To achieve the deprotonation
of phthalimide, triethylamine was added to the reaction
system. With the same loading of triethylamine and
chiral catalyst, allylic amination was performed at 25
℃ in CHCl3, and the results are summarized in Table 1.
Firstly chiral bifunctional organocatalysts bearing dif-
ferent H-bonding donors were investigated. The results
indicated that the thiourea-phosphine 3f provided higher
yield than the corresponding amide-phosphine 1 and
squaramide-phosphine 2 (Entry 8 vs. 1 and 2). There-
fore, chiral cyclohexane-based thiourea-phosphines
containing different scaffolds were evaluated. All the
thiourea-phosphines screened achieved good-to-excel-
lent yields in the allylic amination (Entries 3－9).
Among them, organocatalyst 3f provided the best enan-
tioselectivity. The additional chiral group in the alkyl
thiourea moiety would improve the enantioselectivity,
especially the thiourea-phosphines 3j and 4 containing a
dehydroabietic amine unit (Entries 10－13).[9] The
rosin-derived thiourea-phosphine 3j was proved to be
the best catalyst for the allylic amination achieving 88%
yield with 85% ee (Entry 12), probably due to good
chiral match between the chiral cyclohexyl backbone
OAc
20 mol% Catalyst
20 mol% Et3N
COMe
+
CHCl3, 25 oC
O
O
O2N
N
H
5a
O
O
N
COMe
O2N
6a
Entry Catalyst
Time/d
Yieldb/%
70
eec/%
56
1
1
4
2
2
4
72
55
3
3a
3b
3c
3d
3e
3f
3g
3h
3i
3j
4
1
86
55
4
2
80
48
5
1.5
4.5
1.2
1.2
3.5
3
95
50
6
96
55
7
95
34
8
89
64
9
95
55
10
11
12
13
87
68
3
83
77
3
88
85
3
87
−80
a The reactions were carried out with substrate 5a (0.2 mmol),
phthalimide (2 equiv.), 20 mol% Et3N and 20 mol% of chiral
catalyst in CHCl3 (1.0 mL) at 25 ℃. b Isolated yield. c The ee
values were determined by chiral HPLC.
S
O
R
N
N
N
Ph
H
H
and dehydroabietic amine unit. By comparing the opti-
cal rotation value with that reported in the literature,[6d]
the absolute configuration of the allylic amination
products was assigned as “R”.
H
PPh2
PPh2
3a: R = C6H5
1
3b: R = 3,5-(CF3)2C6H3
3c: R = 4-ClC6H4
3d: R = 4-MeOC6H4
3e: R = n-C12H25
O
O
Next, the effect of reaction solvent was investigated
using 20 mol% 3j as a catalyst (Table 2). The allylic
amination is sluggish in non-polar solvent such as tolu-
ene, and only 72% yield and 70% ee were obtained even
after 7 d (Entry 1). Halogen solvents such as CHCl3 and
CH2Cl2 resulted in good yields and enantioselectivities
(Entries 2 and 3), and so did 1,4-dioxane (Entry 4). In
the case of ether, good yield was achieved while a
longer time was required due to the slow reaction rate
(Entry 5). Complex by-products were generated in THF,
EtOAc and CH3CN (especially in CH3CN), so the
chemical yields of allylic amination decreased obvi-
ously (Entries 6－8). In non-protonic polar solvent
DMSO, catalyst 3j was inefficient for the allylic amina-
tion (Entriy 9). Then CHCl3 was selected as a solvent
for the subsequent optimization.
Bn
N
3f: R = Bn
N
H
H
3g: R = 4-MeOC6H4CH2
3h: R = (R)-1-phenylethyl
3i: R = (S)-1-phenylethyl
PPh2
2
H
N
S
H
HN
3j
Ph2P
H
N
S
H
HN
4
Ph2P
Figure 1 Structures of the chiral phosphine organocatalysts
screened.
Among the bases screened, triethylamine was the
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Chin. J. Chem. 2015, 33, 1333—1337

Enantioselective Allylic Amination of Morita-Baylis-Hillman Acetates
Table 2 Optimization of the reaction conditionsa
(0.1 mol/L) led to an obvious decrement of the enantio-
selectivity, while the higher concentration (0.3 mol/L)
caused a lower yield (Entries 17 and 18 vs. 2). More-
over, the higher temperature sped up the reaction, but
resulted in lower enantioselectivity (Entries 2, 19 and
20). When the catalyst loading of 3j was reduced from
20 mol% to 10 mol%, the yield and enantioselectivity
were slightly decreased (Entry 21 vs. 2).
OAc
20 mol% 3j
COMe
20 mol% base
+
solvent
O
O
N
O2N
H
5a
Under the established optimal reaction condition (20
mol% catalyst 3j, 2 equiv. of phthalimide in CHCl3 at
25 ℃), the substrate scope of allylic amination was
surveyed. The results are summarized in Table 3. The
MBH acetates with electron-withdrawing group at the
para- or meta-position of the phenyl group could afford
better yields than those with electron-donating group or
without substituent on the aromatic ring (Entries 1, 2
and 4－10 vs. 11 and 12). Moreover, the MBH acetates
containing strong electron-withdrawing group provided
higher enantioselectivities (Entries 1, 2, 4 and 6). How-
ever, the MBH acetate bearing a nitro group at the or-
tho-position of the phenyl ring could not generate the
product (Entry 3), probably due to the ortho-effect. The
MBH acetates with two electron-withdrawing groups at
both 3- and 4-positions of the phenyl group also gave
O
O
N
COMe
O2N
6a
Entry Solvent Base
Time/d
Yieldb/%
72
eec/%
1
Toluene Et3N
7
70
85
84
82
75
67
69
68
ndd
55
57
8
2
CHCl3
Et3N
Et3N
3
88
85
87
84
59
73
23
trace
49
81
90
65
86
86
84
86
75
55
89
85
3
CH2Cl2
3
4
Dioxane Et3N
3
5
Ether
THF
Et3N
Et3N
Et3N
6
6
3.5
3.5
3
7
EtOAc
8
CH3CN Et3N
9
DMSO
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
CHCl3
Et3N
5
10
11
12
13
14e
15f
16g
17h
18i
19j
20k
21l
C6H5N
DIPEA
3
Table 3 Substrate scope of the enantioselective allylic amina-
tiona
3
DMAP 1.5
20 mol% 3j
OAc
DBU
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
Et3N
3
34
35
81
83
75
82
85
77
84
20 mol% Et3N
COR1
1.5
3
+
Ar
O
CHCl3, 25 oC
O
N
O
O
COR1
N
Ar
5
H
3
6
3
Entry R1 Ar
Time/d Product Yieldb/% eec/%
4.5
7
1
Me 4-NO2C6H4
Me 3-NO2C6H4
Me 2-NO2C6H4
3
4
5
6a
6b
－
6c
6d
6e
6f
88
78
trace
80
73
81
71
76
74
67
58
57
75
66
24
85
84
ndd
81
61
80
77
79
76
65
74
68
79
74
67
2
1.5
6
3
a Unless stated otherwise the reactions were carried out with sub-
strate 5a (0.2 mmol), phthalimide (2 equiv.), 20 mol% base and
20 mol% chiral catalyst 3j in solvent (1.0 mL) at 25 ℃. b Isolated
yield. c The ee values were determined by chiral HPLC. d Not
4
Me 4-CNC6H4 1.5
Me 3-CNC6H4 1.5
5
6
Me 4-CF3C6H4
Me 4-BrC6H4
Me 4-ClC6H4
Me 3-ClC6H4
Me 4-FC6H4
Me C6H5
2
7
3.5
3.5
4
determined. e The amount of triethylamine was 100 mol%. The
f
8
6g
6h
6i
reaction was conducted with 1 equiv. of phthalimide. g The reac-
tion was conducted with 3 equiv. of phthalimide. h The reaction
was carried out in 2.0 mL CHCl3. i The reaction was carried out in
9
10
11
12
13
14
3.5
4
j
k
0.67 mL CHCl3. The reaction temperature was 0 ℃. The reac-
tion temperature was 40 ℃. l The catalyst loading was 10 mol%.
6j
Me 4-MeC6H4
5
6k
6l
Me 3,4-Cl2C6H3 3.5
Me 3,4-F2C6H3 3.5
optimal one for both yield and enantioselectivity (Entry
2 vs. 10－13). Using 1 equiv. of triethylamine could
accelerate the reaction but with a negative effect on the
enantioselectivity (Entry 14 vs. 2). The ratio of
phthalimide to MBH acetate had a slight effect on the
chemical yield and enantioselectivity (Entries 2, 15 and
16). With the concentration of MBH acetate ranging
from 0.1 to 0.3 mol/L in CHCl3, the lower concentration
6m
6n
15
MeO 4-NO2C6H4
5
a
The reactions were carried out with substrate 5 (0.2 mmol),
phthalimide (2 equiv.), 20 mol% Et3N and 20 mol% chiral cata-
lyst 3j (0.04 mmol) in CHCl3 (1.0 mL) at 25 ℃. b Isolated yield.
c The ee values were determined by chiral HPLC. d Not deter-
mined.
Chin. J. Chem. 2015, 33, 1333—1337
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Zhao et al.
COMMUNICATION
the corresponding amines in moderate yields with enan-
tioselectivities (Entries 13 and 14).
moiety and the carbonyl group. Triethylamine deproto-
nates the acidic NH group of phthalimide to give a nu-
cleophile. The steric repulsion between the chiral cata-
lyst and deprotonated phthalimide forces the nucleo-
phile to attack the cationic intermediate from the Re-
face to generate the product with an (R)-configuration.
This asymmetric catalytic system is not suitable for
the MBH acetate derived from methyl acrylate, which
exhibited a lower yield and enantioselectivity than the
MBH acetate derived from methyl vinyl ketone under
the identical conditions (Entry 15 vs. 1). Except
phthalimide, other N-nucleophiles such as aniline, ben-
zylamine and p-toluene-sulfonamide could not react
with the MBH acetates under this catalytic system.
We also examined MBH carbonates as electrophile
in the allylic amination (Scheme 1). To our surprise,
MBH carbonate 7a derived from methyl vinyl ketone
afforded racemic product in good yield. The MBH car-
bonate 7b derived from methyl acrylate provided the
desired product in better yield than the corresponding
MBH acetate, and the addition of triethylamine led to a
decrease of enantioselectivity.
Conclusions
In summary, the chiral cyclohexane-based thiourea-
phosphine 3j was an efficient organocatalyst for the
enantioselective allylic amination of phthalimide with
MBH adducts. With 20 mol% organocatalyst 3j and 20
mol% triethylamine in CHCl3, the enantioselective ally-
lic amination could provide the chiral amines in up to
85% ee and moderate-to-good yields.
Acknowledgement
We are grateful for the financial support from the
National Natural Science Foundation of China (Nos.
21242007, 21102043), Science and Technology Com-
mission of Shanghai Municipality (No. 15ZR1409200),
and the Fundamental Research Funds for the Central
Universities.
Scheme 1 The allylic amination of MBH carbonates
20 mol% 3j
1 20 mol% Et3N
OBoc
COR
CHCl3, 25 oC
+
O2N
O
O
N
H
7
7a: R1 = Me,
7b: R1= OMe,
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© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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