A Triazole Organocatalyst with Spiropyrrolidine Framework and its Application to the Catalytic Asymmetric Addition of Nitromethane to α,β-Unsaturated Aldehydes

Article abstract of DOI:10.1002/adsc.201500582

A series of new water-compatible "spiropyrrolidine triazole" catalysts was designed and synthesized. The asymmetric Michael addition of nitromethane and α,β-unsaturated aldehydes in an aqueous system was investigated to evaluate these new catalysts, and the resulting adducts were obtained with excellent enantioselectivity (up to 95.5% ee) and moderate to good yield (63-88%).

Full text of DOI:10.1002/adsc.201500582

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DOI: 10.1002/adsc.201500582
A Triazole Organocatalyst with Spiropyrrolidine Framework
and its Application to the Catalytic Asymmetric Addition of
Nitromethane to a,b-Unsaturated Aldehydes
Wen-Xing Liu,a,b Si-Kai Chen,b Jin-Miao Tian,b Yong-Qiang Tu,a,b,*
Shao-Hua Wang,b and Fu-Min Zhangb,*
a
School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, Peopleꢀs Republic of
China
Fax: (+86)-931-8912582; e-mail: tuyq@sjtu.edu.cn
State Key Laboratory of Applied Organic Chemistry & College of Chemistry and Chemical Engineering, Lanzhou
b
University, Lanzhou 730000, Peopleꢀs Republic of China
E-mail: tuyq@lzu.edu.cn or zhangfm@lzu.edu.cn
Received: June 16, 2015; Revised: August 20, 2015; Published online: December 4, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500582.
Abstract: A series of new water-compatible “spiro-
pyrrolidine triazole” catalysts was designed and syn-
thesized. The asymmetric Michael addition of nitro-
methane and a,b-unsaturated aldehydes in an aque-
ous system was investigated to evaluate these new
catalysts, and the resulting adducts were obtained
with excellent enantioselectivity (up to 95.5% ee)
and moderate to good yield (63–88%).
Here we report the design and synthesis of a series of
new spiropyrrolidine organocatalysts bearing a triazole
unit and their application to the asymmetric Michael
addition of nitromethane to a,b-unsaturated alde-
hydes under water-compatible conditions.[6]
Catalysts 2a–2g were synthesized as shown in
Scheme 1. Starting from the known chiral 1-azaspi-
ro[4.4]nonane-6-one (3),[5,7] the ketone group in 3 was
oximated and stereoselectively reduced with NaBH4/
NiCl2 in MeOH to give the primary amine intermedi-
ate, which was stereospecifically converted to azide 4
via a diazo transfer reaction.[8] The azide 4 then un-
derwent a “click reaction”[9] with alkynes substituted
with various functional groups, affording the precur-
sors 5a–5f. Next, removal of the protecting group of
5a–5f gave the desired “spiropyrrolidine triazole” cat-
Keywords: azaspiro skeleton; Michael addition; ni-
tromethane; organic catalysis; 1,2,3-triazoles; a,b-
unsaturated aldehydes
Asymmetric organocatalysis has become an important alysts 2a–2f (Scheme 1). For synthesis of catalyst 2g,
method for constructing valuable chiral building the last two steps of the reaction sequence were re-
blocks in both industrial and fundamental research in versed. The absolute configuration of these catalysts
organic synthesis.[1] In recent decades, a broad array
of organocatalytic asymmetric reactions has been ex-
tensively investigated,[2] one class of which is particu-
larly important because they are water-compatible
transformations[3] that can benefit humans and the en-
vironment. These transformations require effective
water-compatible organocatalysts that contain appro-
priate hydrophobic and hydrophilic moieties in their
structures.[3h] Considering the unique characteristics of
triazole as a privileged framework,[4] and encouraged
by the excellent results of our newly developed “spi-
ropyrrolidine silyl ether” organocatalyst 1 (Figure 1)
for the asymmetric construction of quaternary carbon
centers,[5] we imagined that 1-azaspiro[4.4]nonane
could be further derived to other organocatalysts,
such as a water-compatible triazole-type (2, Figure 1). lysts.
Figure 1. The design of new “spiropyrrolidine triazole” cata-
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Wen-Xing Liu et al.
Scheme 1. Synthesis of “spiropyrrolidine triazole” catalysts.
was further confirmed based on the X-ray analysis of quired longer reaction times and still gave a low yield
catalyst 2a.[10] It is noteworthy that all catalysts could (entry 11). Conducting the reaction in water gave
be smoothly prepared from 3 in approximately 65% complex mixtures, and the use of brine also gave un-
overall yields (for details, see the Supporting Informa- satisfactory results (entries 12 and 13). Use of the
tion).
mixed solvent of MeOH/H2O (v/v=4/1) led to a sig-
With the desired organocatalysts in hand, we then nificantly higher yield (94%) and ee (90%) (entry 14).
evaluated the ability of these organocatalysts to effi- Encouraged by this result, the ratio of the solvent
ciently promote the Michael addition of nitromethane mixture was fine-tuned, and it was found that modify-
(1 mmol) to cinnamaldehyde (6a, 0.1 mmol). To our ing the proportion of H2O strongly influenced both
delight, all catalysts could promote the reaction in the reactivity and stereoselectivity (entries 14--16).[11] In
presence of benzoic acid (BA) in methanol as solvent. contrast, various additives (entries 17–20) and lower
Substituents on the triazole ring obviously affected temperature (entry 21) failed to improve the reaction
the enantioselectivity of this reaction (Table 1, en- performance. Pleasingly, replacing MeOH in the
tries 1–8). Aryl-substituted catalysts 2a–2c gave mixed solvent by TFE gave the product 7a in 88%
adduct 7a with 40–49% ee and 77–84% yield (en- yield and 95% ee (entry 22), albeit with a longer reac-
tries 1–3), indicating that the electronic property of tion time. Finally, further increasing the loading of
substituent in the phenyl ring slightly affected the re- catalyst 2f and PhCO2H to 20 mol% and 35 mol%,
action results. Alkyl-substituted catalysts 2d–2f gave respectively, could both shorten the reaction time to
higher enantioselectivity with similar yield (entries 4– 36 h and slightly increase the ee value to 95.5%
6). The TIPS-substituted catalyst 2g led to slightly (Table 1, entry 23).
lower selectivity than catalyst 2f (entry 7). Using the
Under the optimal reaction conditions (Table 1,
enantiomer of catalyst 2f completely reversed the entry 23), we next studied the generality of this cata-
enantioselectivity of the current reaction (entry 8). lytic asymmetric conjugate addition reaction of nitro-
Therefore, catalyst 2f was selected for further investi- methane with a variety of a,b-unsaturated aldehydes,
gation of the reaction conditions.
which showed that the reaction could be achieved
Extensive solvent screening[11] showed polar alcohol with excellent enantioselectivity (83–95.5% ee) and
solvents to be more suitable than others, although 63–88% yield (Table 2). Notably, cinnamaldehyde de-
yield and enantioselectivity significantly decreased in rivatives with electron-withdrawing or electron-donat-
the sequence MeOH<EtOH<i-PrOH (entries 7–10). ing groups at the 2- or 4-position of the phenyl ring
Replacing methanol with trifluoroethanol (TFE) produced the corresponding products in 63–88% yield
slightly improved the enantioselectivity, but it re- and with 93–95.5% ee (entries 1–7). However, sub-
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A Triazole Organocatalyst with Spiropyrrolidine Framework
Table 1. Screening of the reaction conditions[a]
Entry
Catalyst
Solvent
Additive
Time [h]
Yield[b]
ee[c]
1
2
3
4
5
6
2a
2b
2c
2d
2e
2f
2g
ent-2f
2f
2f
2f
2f
2f
2f
2f
2f
2f
2f
2f
2f
2f
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
MeOH
EtOH
i-PrOH
TFE
H2O
BA[d]
BA
BA
BA
BA
BA
BA
BA
BA
BA
BA
BA
BA
BA
BA
BA
HOAc
Et3N
TsOH
–
24
34
34
29
24
24
24
24
36
60
120
7
84%
79%
77%
87%
82%
87%
88%
85%
75%
19%
38%
N.D.[f]
38%
94%
47%
95%
95%
trace
trace
45%
85%
88%
88%
44%
49%
40%
71%
73%
83%
80%
À83%
72%
68%
84%
N.D.
82%
90%
84%
93%
93%
N.D.
N.D.
77%
94%
95%
95.5%
7
8[e]
9
10
11
12
13
14
15
16
17
18
19
20
21[g]
22
23[h]
brine
6
MeOH/H2O 4:1
MeOH/H2O 1:4
MeOH/H2O 1:1
MeOH/H2O 1:1
MeOH/H2O 1:1
MeOH/H2O 1:1
MeOH/H2O 1:1
MeOH/H2O 1:1
TFE/H2O 1:1
TFE/H2O 1:1
40
36
36
36
8
120
21
48
72
36
BA
BA
BA
2f
2f
[a]
Unless otherwise stated, the reaction was performed employing cinnamaldehyde (0.1 mmol), nitromethane (1 mmol), cat-
alyst 2 (0.01 mmol), additive (0.02 mmol) and solvent (1.0 mL) at room temperature.
Isolated yield after reduction to the corresponding alcohol.
Determined by chiral HPLC analysis on a Chiralpak OD-H column.
BA =PhCO2H.
The enantiomer of catalyst 2f was used, for the synthetic details of ent-2 f see the Supporting Information.
N.D. =not determined.
The reaction was performed at 48C with catalyst 2f (20 mol%) and BA (20 mol%).
Catalyst 2f (20 mol%) and BA (35 mol%) were used.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
stituents at the 3-position slightly decreased the ee their high catalytic activity and stereoselectivity for
value of the products (entries 8 and 9). In addition, iminium-type activation of a,b-unsaturated aldehydes.
acrolein substituted at the 1-position with naphthyl or The present results provide the basis for generating
a heteroaromatic group such as a furyl moiety led to novel organocatalysts based on the 1-azaspiro[4.4]no-
the expected products 7j and 7k in excellent ee (en- nane backbone and applying them to synthetic chal-
tries 10 and 11). Aliphatic enals also worked well, lenges. Such work is already underway in our labora-
giving the desired adducts in moderate yield and good tory.
enantioselectivity (Table 2, entries 12–14). An optical
rotation analysis of the 7a showed it to have the R ab-
solute configuration as reported in the literature.[12]
Experimental Section
Furthermore, we have tried to use nitroethane as sub-
strate to check the corresponding diastereoselectivity
of the current process under the optimal reaction con-
ditions. However, it was frustrating that only moder-
ate diastereoselectivity and enantioselectivity (dr=
1:1.2, 76% ee for syn-8) were obtained (Scheme 2).
In conclusion, we have developed a new series of
General Procedure for Catalyst 2f-Promoted Michael
Addition of Nitromethane to a,b-Unsaturated
Aldehydes
To a solution of a,b-unsaturated aldehyde (0.1 mmol) and
catalyst 2f (0.2 equiv.) in TFE/H2O (1 mL, v/v 1:1) was
“spiropyrrolidine triazole” catalysts and demonstrated added BA (0.35 equiv.) and CH3NO2 (10 equiv.) at room
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Table 2. Catalyst 2f-promoted Michael addition of nitrome-
thane to a,b-unsaturated aldehydes.[a]
silica gel (petroleum ether:EtOAc=3:1 to 1.5:1) to yield the
desired product 7.
Acknowledgements
This work was supported by the NSFC (No.: 21202073,
21290180, 21272097, 21372104 and 21472077), the “973” Pro-
gram of MOST (2010CB833203), the “111” Program of
MOE, the Project of MOST (2012ZX 09201101-003).
Entry R’
Product Time [h] Yield[b] ee[c]
1
Ph
7a
36
58
24
24
24
30
16
28
36
20
20
24
18
18
88%
75%
75%
82%
66%
80%
63%
80%
79%
65%
66%
73%
73%
74%
95.5%
93%
95%
95%
94%
95%
95%
87%
87%
94%
90%
83%
88%
85%
2
4-MeOC6H4 7b
3
4
5
4-NO2C6H4
4-ClC6H4
4-BrC6H4
7c
7d
7e
6
7
8
2-MeOC6H4 7f
2-NO2C6H4 7g
3-MeOC6H4 7h
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A Triazole Organocatalyst with Spiropyrrolidine Framework
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[12] The absolute configuration of products was determined
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