Organocatalytic enantioselective amination of Morita-Baylis-Hillman carbonates with masked ammonia: A facile method for the synthesis of unprotected α-methylene-β-amino esters

Article abstract of DOI:10.1002/chem.201102522

Amination under the mask: The asymmetric allylic amination of MBH carbonates was achieved with masked ammonia (benzophenone imine) under the catalysis of modified cinchona alkaloids to give good yields with excellent enantioselectivities (up to 99% ee) for a wide range of MBH carbonates (see scheme).

Full text of DOI:10.1002/chem.201102522

DOI: 10.1002/chem.201102522
Organocatalytic Enantioselective Amination of Morita–Baylis–Hillman
Carbonates with Masked Ammonia: A Facile Method for the Synthesis of
Unprotected a-Methylene-b-Amino Esters
Aijun Lin,[a] Haibin Mao,[a] Xi Zhu,[a] Huiming Ge,[b] Renxiang Tan,[b]
Chengjian Zhu,*[a] and Yixiang Cheng*[a]
Optically active a-alkylidene-b-amino carbonyl com-
pounds and their derivatives are highly valuable building
blocks, which are widely applied in the synthesis of medici-
nally relevant compounds as well as complex natural prod-
ucts.[1] Therefore, the development of efficient methods for
the synthesis of these multifunctional compounds is current-
ly of particular significance. Giving access to valuable com-
pounds and structural complexity, the aza-Morita–Baylis–
Hillman (aza-MBH) reaction provides one of the most
direct and versatile methods to these compounds.[2] Howev-
er, the aza-MBH reaction remains to be restricted in enan-
tiocontrol and substrate scope; the nucleophilic catalysts
typically utilized in aza-MBH reactions may induce product
racemization.[3] Furthermore, the tosyl groups commonly
used as activating groups for the poorly reactive azomethine
in the aza-MBH adducts were hard to remove,[4] which limit-
ed the application of amino adducts in organic synthesis.
Compared with the aza-MBH reaction, the substitution of
MBH carbonates with nitrogen nucleophiles is another
straightforward strategy to the synthesis of a-methylene-b-
amino esters through a simple operation is still a highly de-
sirable goal.
The smallest nitrogen nucleophile, ammonia, is theoreti-
cally the optimal nitrogen nucleophile, concerning atom
economy,[10] but its volatile nature, high reactivity and toxici-
ty makes it difficult to work with in catalytic systems. Ben-
zophenone imine, which is industrially produced by conden-
sation of benzophenone and ammonia,[11] has previously
been shown to be an appropriate ammonia equivalent in the
asymmetric aza-Michael addition reaction of nitroalkenes
reported by Jørgensen.[12] The unprotected b-amino nitro
compounds were obtained in good yields and enantioselec-
tivities, and the ammonia carrier benzophenone could be re-
covered and reused in the preparation of the starting benzo-
phenone imine. Inspired by this elegant progress, and con-
À
tinuing our interest in chiral C X (X=N, O, S, Se) bond for-
mation reactions,[13] herein we report the organocatalytic
asymmetric allylic amination reaction of MBH carbonates
with benzophenone imine. High enantioselectivities (up to
99% enantiomeric excess (ee)) were achieved for a wide
range of MBH carbonates; the optically pure unprotected
a-methylene-b-amino esters were easily obtained by a one-
step acidic hydrolysis protocol (Scheme 1).
À
amino carbonyl compounds through a direct C N bond for-
mation reaction.[5] Recently, the asymmetric substitution of
MBH carbonates has been established as an effective proto-
[6]
[7]
[8]
À
À
À
col in C C bond, C O bond, C P bond formation re-
actions and cycloaddition reactions[9] with high enantioselec-
tivities for a broad range of substrates; however, the asym-
metric amination progress was not fruitful. Cyclic imide
such as phthalimide was usually employed as the nitrogen
nucleophile in this procedure; however, the enantioselectivi-
ties of most reported reactions were low to moderate.
Searching more suitable nitrogen nucleophiles to achieve
higher enantioselectivities and unprotected a-methylene-b-
Scheme 1. Organocatalytic asymmetric allylic amination of MBH carbo-
nates with masked ammonia (benzophenone imine). Boc=tert-butoxycar-
bonyl.
[a] A. Lin, H. Mao, X. Zhu, Prof. Dr. C. Zhu, Prof. Dr. Y. Cheng
School of Chemistry and Chemical Engineering
Nanjing University, Nanjing, 210093 (China)
Fax : (+86) 25-83594886
E-mail: cjzhu@nju.edu.cn
Initial examination was carried out by using the MBH car-
bonate 2a (1.5 equiv) and benzophenone imine 3 (1.0 equiv)
as the substrates in the presence of 1,4-diazabicyclo-
yxcheng@nju.edu.cn
[b] H. Ge, Prof. Dr. R. Tan
School of Life Sciences
Nanjing University, Nanjing, 210093 (China)
AHCTUNGRTEGNUN[N 2.2.2]octane (DABCO) with MgSO4 (30 mg) in 1,2-dichlor-
oethene (DCE, 0.5 mL) at room temperature.The corre-
sponding N-allylic amination product 4a was isolated in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201102522.
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Chem. Eur. J. 2011, 17, 13676 – 13679

COMMUNICATION
product 4a with 98% ee (Table 1, entry 6). The examina-
tion of solvent effects with catalyst 1e revealed that 1,4-
dioxane was the best reaction media for this reaction,
which can significantly facilitate the reaction to produce
the corresponding product in 84% yield with 99% ee
(Table 1, entry 14). The reaction was found to be sluggish
in the absence of MgSO4 (Table 1, entry 15). Decreasing
the catalyst loading to 10 mol% and 5 mol% had a nega-
tive impact on both the yields and the enantioselectivities
(Table 1, entries 16 and 17).
Having established the optimal reaction conditions, we
then examined a spectrum of MBH carbonates 2 to ex-
plore the generality of this asymmetric transformation.
The reactions were conducted in 1,4-dioxane (0.5 mL)
with catalyst 1e (20 mol%) at room temperature. The re-
sults are summarized in Table 2. The position and elec-
tronic properties of the substituent on the aromatic ring
had little or no effect on the enantioselectivity and all
these MBH carbonates participated in this process in
high efficiency (Table 2, entries 1–10). When 2-thienyl-
Table 1. Optimization of the reaction conditions.[a]
Table 2. Catalytic enantioselective allylic amination of MBH carbonates
2 with benzophenone imine.[a]
Entry
Cat.
Solvent
t
Yield
[%][b]
ee
[h]
[%][c]
Entry
Ar
R
t
Product
Yield
[%][b]
ee
[h]
[%][c]
1
2
3
4
5
6
7
8
DABCO
1a
1b
1c
1d
1e
1 f
1e
1e
1e
1e
1e
1e
1e
1e
1e
1e
DCE
DCE
DCE
DCE
DCE
DCE
DCE
hexane
CH2Cl2
toluene
xylene
THF
PhCF3
1,4-dioxane
1,4-dioxane
1,4-dioxane
1,4-dioxane
48
60
60
60
60
60
60
60
84
84
84
60
72
60
60
84
96
78
62
76
42
58
70
74
13
44
58
50
75
52
84
65
68
57
–
69
1
2
3
4
5
6
7
8
4-ClPh
Ph
4-FPh
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OMe
OEt
60
72
84
84
72
72
96
96
84
84
96
84
84
84
144
4a
4b
4c
4d
4e
4 f
4g
4h
4i
4j
4k
4l
4m
4n
4o
84
80
78
75
71
68
86
81
63
70
65
81
61
74
53
99
95
98
99
99
97
98
99
99
96
90
93
90
95
91
À47[d]
À35[d]
84
4-BrPh
4-NO2Ph
4-CF3Ph
4-OMePh
4-MePh
2-ClPh
2-C10H7
2-thienyl
4-ClPh
4-ClPh
4-ClPh
Ph
98
81
98
98
98
98
98
99
9
10
11
12
13
14
15
16
17
9
10
11
12
13
14
15
99
OPh
OBn
Me
97[e]
96[f]
94[g]
[a] The reaction was carried out with 2 (0.15 mmol), 3 (0.10 mmol), cata-
lyst 1e (20 mol%, 0.02 mmol) and MgSO4 (30 mg) in 1,4-dioxane
(0.5 mL) at room temperature. [b] Yield of isolated product. [c] ee values
were determined by chiral HPLC analysis.
[a] The reaction was carried out with 2a (0.15 mmol), 3 (0.10 mmol), cat-
alyst 1 (20 mol%, 0.02 mmol) and MgSO4 (30 mg) in solvent (0.5 mL) at
room temperature. [b] Yield of isolated product. [c] ee values were deter-
mined by chiral HPLC analysis. [d] The opposite configuration. [e] The
absence of MgSO4. [f] 10 mol% of catalyst 1e was used. [g] 5 mol% of
catalyst 1e was used.
substituted MBH carbonate was used as the substrate, the
asymmetric amination reaction could be completed with
90% ee (Table 2, entry 11). Further exploration of the sub-
strate scope focused on the ester group. Different esters
yielded the desired products in good yields with high enan-
tioselectivities (Table 2, entries 12–14). MBH carbonates de-
rived from methyl vinyl ketone (MVK) could also be trans-
formed with a good ee value under the same reaction condi-
tion (Table 2, entry 15). Unfortunately, when b-alkyl-substi-
78% yield (Table 1, entry 1). Encouraged by this prelimina-
ry result, we then examined the enantioselective variant in
the presence of chiral tertiary amine catalysts. When catalyst
1a was tested, the product 4a was obtained in 62% yield
with 69% ee (Table 1, entry 2). Then we tested other tertiary
amine catalysts derived from cinchona alkaloids[14] and
found that 1e served as the best catalyst, which gave the
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13677

Y. Cheng, C. Zhu et al.
tuted MBH carbonates were tested under the optimal proto-
col the results were not as positive as presented above and
they failed to afford the desired allylic amination prod-
ucts.[15]
As shown in Scheme 2, the unprotected a-methylene-b-
amino esters could be easily obtained by one-step acidic hy-
drolysis protocol. Upon completion of the substitution step,
products 4 were quickly isolated by flash column chromatog-
face, affording the corresponding b-amination product 4b in
the R configuration.
In summary, we have provided a highly efficient asymmet-
ric allylic amination of MBH carbonates with masked am-
monia (benzophenone imine) under the catalysis of modi-
fied cinchona alkaloids. The reaction proceeded in good
yields with excellent enantioselectivities (up to 99% ee) for
a wide range of MBH carbonates. Furthermore, the unpro-
tected a-methylene-b-amino esters were easily obtained
with retentive enantioselectivities through a one-step acidic
hydrolysis protocol, and the recovered benzophenone could
be reused in the preparation of the starting benzophenone
imine. We believe that this methodology presented herein
may potentially expand the synthetic utility of a-methylene-
b-amino esters in organic chemistry and medicinal chemis-
try. Further synthetic application of this transformation and
mechanistic studies are underway in our laboratory.
Scheme 2. One-step acidic hydrolysis protocol for the synthesis of unpro-
tected a-methylene-b-amino esters.
Experimental Section
raphy, then 2m HCl (aq) and Et2O were added. Isolating the
aqueous phase, and subsequent removal of water as azeo-
trope with toluene, the hydrochloride salt of the primary b-
amino esters 5 were obtained as a white solid with retained
enantiopurity; the benzophenone could be recovered in
90% yield from the organic phase. The absolute configura-
tions of product 4a and 4b were assigned to be R by chemi-
cal correlation with the corresponding described N-Boc-pro-
tected amines.[16] The remaining configurations of adducts 4
and salts 5a and 5b were assumed by analogy.
General procedure for the organocatalytic asymmetric allylic amination
of Morita–Baylis–Hillman carbonates with benzophenone imine: A mix-
ture of catalyst 1e (17.2 mg, 0.02 mmol), MBH carbonate 2a (48.9 mg,
0.15 mmol) and MgSO4 (30 mg) in 1,4-dioxane (0.5 mL) was stirred for
30 min at room temperature before the benzophenone imine 3 (18.1 mg,
0.10 mmol) was added, then the resulting mixture was stirred for another
60 h. The crude reaction mixture was diluted with ethyl acetate and then
directly purified by flash chromatography on silica gel (petroleum ether/
ethyl acetate PE/EA) to afford the corresponding product 4a.
On the basis of the experimental results, a plausible acti-
vation model for the R-selective formation of 4b catalyzed
by 1e is outlined in Scheme 3. The MBH carbonate presum-
Acknowledgements
We gratefully acknowledge the National Natural Science Foundation of
China (20832001, 20972065, 21074054) and the National Basic Research
Program of China (2007CB925103, 2010CB923303) for their financial
support.
Keywords: allylic compounds · amination · ammonia ·
benzophenone imine · organocatalysis · Morita–Baylis–
Hillman carbonates
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Organocatalytic Enantioselective Amination
COMMUNICATION
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Received: August 15, 2011
Published online: November 3, 2011
Chem. Eur. J. 2011, 17, 13676 – 13679
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