A highly enantioselective Darzens reaction between diazoacetamides and aldehydes catalyzed by a (+)-pinanediol-Ti(OiPr)4 system

Article abstract of DOI:10.1039/c2ob27179f

A highly efficient enantioselective Darzens reaction of aldehydes with diazoacetamides catalyzed by a (+)-pinanediol-Ti(OⁱPr)₄ system has been developed. The cis-glycidic amides were obtained in high yields and with moderate to excellent enantioselectivity (up to 99%).

Full text of DOI:10.1039/c2ob27179f

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Highly Enantioselective Darzens Reaction between Diazo-Acetamides
and Aldehydes Catalyzed by A (+)-Pinanediol-Ti(OiPr)4 System
Gang Liu,[a] Daming Zhang,[a] Jian Li,[a] Guangyang Xu[a] and Jiangtao Sun[a,*]
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A highly efficient enantioselective Darzens reaction of
aldehydes with diazoacetamides catalyzed by a (+)-
Pinanediol and Ti(OiPr)4 has been developed. The cis-
glycidic amides were obtained in high yields and with
10 moderate to excellent enantioselectivity (up to 99%).
Optically pure epoxides have been recognized as one of the
most important chiral synthons in organic synthesis and
substructures in natural products and pharmaceuticals.[1,2] Chiral
glycidic esters and amides, as important types of epoxides, have
15 broad applications because they can easily be converted to
various target molecules just by simple transformation.[3] To
access these compouds, many elegant approaches have been
developed, including the asymmetric catalytic epoxidation of α,
β-unsaturated carbonyl compounds. However, asymmetric
20 epoxidation of alkene catalyzed by organocatalysts or Lewis
acids,[4] to some extent, have some limits because the alkene
precursors have to be prepared in advance. So in order to broaden
the substrate generality as well as to explore other efficient
methods, many efforts have been made to achieve this goal.
Scheme 1. The reaction of diazo carbonyl compounds with aldehydes
Diazo compounds have been widely used in organic
50 transformations for decades.[12] Generally, the reaction of diazo
carbonyl compounds and aldehydes can proceed in three different
ways, namely the Aldol reaction, Roskamp reaction and Darzens
reaction, which generate the corresponding diazo alcohols,
dicarbonyl compounds and epoxides respectively (Scheme 1).
55 However, the competition of the three rearrangement pathways
often leads to poor chemoselectivity. Moreover, the major
product of the reaction depends on the types of substrates and
Lewis acids used as well as other reaction conditions. Wang et
al[13] and Trost et al[14] reported the asymmetric Aldol-type
60 reaction between α-H-diazoacetates and aldehydes using chiral
Zr(OtBu)4 and Bu2Mg based complexes. Feng et al disclosed an
asymmetric Roskamp-type reaction between α-alkyl-diazoesters
with aromatic aldehydes using chiral N,N′-dioxide-scandium(III)
complexes.[15] Just recently, Hwang and Ryu developed a highly
65 enantioselective Roskamp reaction of α-alkyl diazoesters with
aldehydes using an oxazaborolidinium ion.[16] In addition, as
reported by Wulff et al[17] and Maruoka et al[18], when N-Boc
imines instead of aldehydes reacted with diazo compounds, the
chiral Mannich-type products (chiral aziridines) were obtained.
70 Inspired by the above outcomes, we considered whether it is
possible to initiate the Darzens-type reaction using an appropriate
chiral catalytic system. Herein, we wish to report a highly
enantioselective Darzens reaction between aldehydes and
diazoacetamides catalyzed by a readily accessible (1S, 2S, 3R,
75 5S)-(+)-Pinanediol /titanium (IV) complex, which yields the cis-
glycidic amides in good yields and with moderate to excellent
enantioselectivity.
25
Among the methods developed, the Darzens reaction offers an
elegant way to achieve this goal by using readily available
aldehydes as the substrates.[5] Using chiral phase transfer
catalysts, Arai et al[6] and Bakó et al[7] realized the asymmetric
Darzens reaction between phenacyl chloride and aldehydes with
30 moderate enantioselectivity. In 2011, Deng and co-workers found
6’-OH cinchonium salts can catalyze the Darzens reaction more
efficiently with high enantioselectivity.[8] For the Lewis acids
catalyzed asymmetric Darzens reaction, North and co-workers
utilized a chiral cobalt complex as catalyst to give the epoxy
35 esters with moderate diastereo-selectivity and enantioselectivity.[9]
By using stoichiometric amounts of camphor-derived sulfonium
salts, Aggarwal and co-workers reported the synthesis of trans-
glycidic amides with high enantioselectivity.[10] In 2009, a
breakthrough was made by Gong and co-workers, which reported
40 the highly enantioselective Darzens reaction between
diazoacetamides and aldehydes with a simple and efficient
catalytic system.[11a] Later, they also reported that a 3,3’-di-
iodobinaphthol based zirconium complex could catalyze this
reaction with excellent results.[11b]
45
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Organic & Biomolecular Chemistry
Page 2 of 4
positive effect on the yield and the enantioselectivity. Without
30 any additive, moderate yield and moderate enantioselectivity was
obtained (Table 2, entry 1). Next, a solvent screen showed
dichloromethane was the best one according to the yield and
enantioselectivity. Acetonitrile gave similar enantiosVelieecwtiAvirttyiclbeuOt nline
a little lower yield (Table 2, entry 8). Lowering the catalyst
35 loading resulted in
a slower reaction with diminished
enantioselectivity (Table 2, entry 11). In addition, as
a
competitive reaction, the Darzens-type product and Roskamp-
1
type product were measured by H-NMR analysis of the crude
products. Obviously, different solvents gave different amounts of
40 the Roskamp-type compound. Under the same reaction condition,
higher temperature resulted in lower enantioselectivity and a
small amount of Roskamp-type product 3-carbonyl-N-
phenylpentanamide was detected (Table 2, entry 10). So lower
temperature is essential for high chemoselectivity. Notably, when
45 the reaction was carried out at -10oC in CH2Cl2, good yield and
excellent enantioselectivity were observed, and no Roskamp-type
product detected (98.2% ee, Table 2, entry 9), which means the
reaction can be converted completely in 12 hours at -10oC with
excellent chemoselectivity. Moreover, this reaction exhibited
50 excellent diastereoselectivity and no trans-diastereomer was
detected by 1H NMR analysis of the crude product. So we choose
-10oC as the standard temperature for further investigations.
Fig 1. Chiral diols utilized in the asymmetric Darzens reaction
Initially, we examined the reaction of N-phenyl-
diazoacetamide with propionaldehyde in the presence of
molecular sieves 4Å at 0oC in dichloromethane using different
chiral titanium complexes, which formed in situ from various
chiral diols and Ti(OiPr)4 (Figure 1). Unexpectedly, chiral
TADDOL ligands (L1-L4) showed lower reactivity and very low
10 enantioselectivity (Table 1, entries 1 to 4). The chiral 1,2-vicinal
diols (L6-L8) gave low enantioselectivity too (Table 1, entries 6
to 8). The mannitol derivative ligand L9 gave moderate yield and
very low enantioselectivity (Table 1, entry 9). To our delight, the
reaction went smoothly and furnished the cis-glycidic amide in
15 high yield (92%) with high enantioselectivity (93% ee) when
chiral pinanediol (L5) was used (Table 1, entry 5). Significantly,
these reactions exhibited excellent chemoselectivity and no
formation of Aldol-type products and only small amounts of
Roskamp-type products were detected in some cases.
5
Table
2 Asymmetric Darzens reaction of propionaldehyde with
55 diazoacetamide using L5/Ti complexesa
20
Yield
(%)b
Ee
Entry
Additive
Solvent
(%)c
Table
1 Asymmetric Darzens reaction of propionaldehyde with
diazoacetamide using various diol/Ti complexesa
1
2
3
CH2Cl2
CH2Cl2
CH2Cl2
CHCl3
Toluene
THF
77(6)
92(-)
91(2)
82(-)
56(8)
52(7)
66(9)
88(2)
92(-)
93(4)
80(-)
66.4
93.2
91.1
52.5
61.2
47.8
80.5
92.3
98.2
82.4
93.1
-
M.S.4Å
MgSO4
M.S.4Å
M.S.4Å
M.S.4Å
M.S.4Å
M.S.4Å
M.S.4Å
M.S.4Å
M.S.4Å
4
5d
6d
7
Yield
Ee
Entry
Ligand
(%)b
(%)c
DCE
8
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
1
2
3
4
5
6
7
8
9
69(5)
76(5)
72(2)
76(3)
92(-)
86(-)
84(-)
71(7)
52(14)
<5
<5
29
18
93
38
52
17
<5
L1
L2
L3
L4
L5
L6
L7
L8
L9
9e
10f
11g
a Unless otherwise noted, all reactions were carried out with CH3CH2CHO
(1.2 mmol), diazoacetamide (1 mmol), Ti(OiPr)4 (0.1 mmol), L5 (0.12
mmol) in 5 mL solvent at 0oC for 12 hours. b Isolated yield. The number
in the parenthesis is the percentage of b in crude products. c Determined
by chiral HPLC analysis on a chiral stationary phase. The configuration
was determined by comparison of the specific optical rotation with
literature.[11] d Reaction time is 24 hours. e The reaction was carried out at -
10oC. f The reaction was carried out at 25oC for 4 hours. g Ti(OiPr)4 (5
mmol%) and L5 (6 mmol%) were used and the reaction time was 24
hours.
a Unless otherwise noted, all reactions were carried out with CH3CH2CHO
(1.2 mmol), diazoacetamide (1 mmol), Ti(OiPr)4 (0.1 mmol), chiral ligand
(0.12 mmol) and M.S. 4Å (150 mg) in CH2Cl2 (5 mL) at 0oC for 12 hours.
bIsolated yield of a based on diazoacetamide. The number in the
parenthesis is the percentage of b in crude products. cDetermined by
chiral HPLC analysis on a chiral stationary phase; The configuration was
determined by comparison of the specific optical rotation with
literature.[11]
The influence of different substituted diazoacetamides was
next investigated. The results were summarized in table 3. When
60 N-(p-methoxylphenyl) and N-(p-chlorophenyl)-diazoacetamide
were used as the diazo substrate, they all gave poor yields and
low enantioselectivity (Table 3, entries 1 and 2). Moreover, the
N-benzyl-diazoacetamide reacted sluggishly with aldehyde and
only gave a trace amount of Darzens reaction product (Table 3,
25
Based upon the above results, we then used (+)-Pinanediol
(L5)/Ti(OiPr)4 for further investigations. Firstly, we found that
additives play an important role in the reaction. Both 4Å
molecular sieves and MgSO4 (Table 2, entries 2 and 3) gave
2
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Page 3 of 4
Organic & Biomolecular Chemistry
entry 3). So we choose N-phenyl-diazoacetamide as the standard
substrate for further investigations.
8
9
83
85
98.1
Table
3 Asymmetric Darzens reaction of propionaldehyde with
96.2
View Article Online
5 diazoacetamide using L5/Ti complexesa
a
All reactions were carried out with RCHO (1.2 mmol), N-phenyl-
diazoacetamide (1 mmol), Ti(OiPr)4 (0.1 mmol), L5 (0.12 mmol), M.S.
4Å (150 mg) in CH2Cl2 (5 mL) at -10oC for 12 hours. Isolated yield
b
c
based on diazoacetamide. Determined by chiral HPLC analysis on a
Yield
(%)b
Ee
chiral stationary phase. The configurations were determined by
comparison of the specific optical rotation with literature and the others
were assigned by analogy.[11]
Entry
R
(%)c
1
2
3
p-ClPh
p-CH3OPh
Bn
49
68
58.6
79.2
-
trace
We next utilized different aromatic aldehydes as substrates
25 under the optimized reaction conditions. As shown in Table 5, the
Darzen-type products were obtained in good yields and with
moderate to high ee values. Generally, benzaldehyde derivatives
bearing electron-withdrawing substituents at the para or meta
position in the phenyl ring or neutral aromatic aldehydes
30 provided cis-epoxides in high yields and with high
enantioselectivity (Table 5, entries 2 to 6), the highest ee value
was obtained with p-Nitro-benzaldehyde (95.2% ee, entry 3).
Electron-rich aromatic aldehydes also participated in clean
Darzens reactions gave the cis-epoxides with moderate to high
35 enantioselectivity and in moderate yields (Table 5, entries 8 to
10). Also, no Roskamp-type products formed.
a Unless otherwise noted, all reactions were carried out with CH3CH2CHO
(1.2 mmol), diazoacetamide (1.0 mmol), Ti(OiPr)4 (0.1 mmol), L5 (0.12
b
mmol) and M.S. 4Å (150 mg) in CH2Cl2 (5 mL) at -10oC for 12 hours.
Isolated yield.
c
Determined by chiral HPLC analysis on a chiral
stationary phase. The configuration was determined by comparison of the
specific optical rotation with literature.[11]
Under the optimized reaction conditions, a series of aliphatic
aldehydes were investigated. As shown in Table 4, with 10 mol%
10 catalyst, all of the aldehydes reacted smoothly with N-phenyl-
diazoacetamide, delivering the corresponding epoxy products in
good yields and with excellent enantioselectivity. Moreover, we
did not find any Roskamp-type products under these reaction
conditions. The highest ee value was obtained when n-
15 butyraldehyde used (99.1% ee; Table 4, entry 2). Besides the
linear aliphatic aldehydes, even for the more sterically hindered
branched aldehydes, excellent enantioselectivity was observed
too (Table 4, entries 8 and 9).
Table 5 Asymmetric Darzens reaction of N-phenyl-diazoacetamide with
different aromatic aldehydesa
40
Yield
(%)b
Ee
Entry
Products
20 Table 4 Asymmetric Darzens reaction of N-phenyl-diazoacetamide with
(%)c
different aliphatic aldehydesa
O
O
1
2
81
90
85.1
88.5
NHPh
Yield
(%)b
Ee
Entry
Products
(%)c
3
4
88
83
95.2
92.6
1
2
3
92
90
91
98.2
99.1
97.1
O
MeO
O
NHPh
O
5
6
7
84
78
81
92.5
89.2
73.6
O
NHPh
4
5
6
7
88
83
84
81
96.7
95.4
97.7
97.7
8
9
67
76
70.4
84.3
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50 5
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View Article Online
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diazoacetamide (1 mmol), Ti(OiPr)4 (0.1 mmol), L5 (0.12 mmol), M.S.
55
4Å (150 mg) in CH2Cl2 (5 mL) at -10oC for 12 hours. Isolated yields
b
c
based on diazoacetamide. Determined by chiral HPLC analysis on a
chiral stationary phase; The configurations were determined by
comparison of the specific optical rotation with literature and the others
were assigned by analogy.[11]
60 7
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In conclusion, we have developed an excellent enantioselective
and chemoselective Darzens reaction between aldehydes and
diazoacetamides catalyzed by a chiral titanium complex formed
in situ from commercially available (+)-Pinanediol and Ti(OiPr)4.
When aliphatic aldehyes were used, the cis-glycidic amides were
obtained in good yields (up to 92%) and with excellent
enantioselectivity (up to 99% ee). For the aromatic aldehydes, the
Darzens reaction products were obtained in good yields and with
5
65
8
Y. Liu, B. A. Provencher, K. J. Bartelson, L. Deng, Chem. Sci. 2011,
2, 1301-1304.
10 moderate to high enantiomeric purities. Compared with Gong’s
BINOL/Ti system,[11a] our system showed better yields and
slightly better enantioselectivities for some aliphatic aldehydes,
but lower enantioselectivities for most of the aromatic aldehydes.
Under the optimized reaction condition, no Roskamp-type
15 products formed for all of the cases. Also, no trans-diastereomer
was detected by 1H NMR analysis of the crude products.
Currently, we are investigating other catalytic systems in the
asymmetric Darzens reaction.
We gratefully acknowledge the National Natural Science
20 Foundation of China (21172023), a Project Funded by the
Priority Academic Program Development of Jiangsu Higher
Education Institutions (PAPD), Changzhou's Key Laboratory
of Pharmaceutical Manufacture and Quality Control
Engineering for their financial support.
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a School of Pharmaceutical Engineering & Life Science, Changzhou
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Fax: +86 519 86334598; Tel: +86 519 86334597
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