W. Zheng et al. / Catalysis Communications 62 (2015) 34–38
35
Fig. 1. Structure of POSS supported (S)-α,α-diphenylprolinol trimethylsilyl ether catalyst.
2.2. Synthesis of POSS supported (S)-α,α-diphenylprolinol trimethylsilyl
ether catalyst
3-azidopropylheptaphenyl POSS 1 (3.0 g, 2.9 mmol), 2 (1.6 g,
4.3 mmol) and dry tetrahydrofuran (40 mL) were charged to a 100 mL
three-neck round-bottomed flask equipped with a magnetic stirrer.
The system was degassed and purged with nitrogen, then CuBr
(41.5 mg, 0.29 mmol) and N,N,N′,N′,N′′-pentamethyldiethylenetriamine
(PMDETA) (60.3 μL, 0.29 mmol) were added. The reaction mixture was
stirred at room temperature for 30 h and dropped into dilute hydrochlo-
ric acid (1 wt.%, 30 mL). The resultant mixture was extracted with CH2Cl2
(20 mL × 3) and dried over Na2SO4. After the removal of the solvent, the
residue was washed with methyl tert-butyl ether (MTBE) (50 mL) and
dried in vacuo at 50 °C for 24 h to give the desired product as a white
solid which was pure enough to be used for the catalytic reactions
(3.62 g, 88%). IR: 3358, 3198, 2933, 1659, 1594, 1459, 1134, 1106,
Scheme 1. Synthesis of the POSS supported (S)-α,α-diphenylprolinol trimethylsilyl ether
catalyst.
3.2. Optimizing of the asymmetric Michael addition conditions
To evaluate the catalytic capacity of our POSS supported catalyst, the
Michael addition of aldehydes with arylnitroalkenes, which is a power-
ful tool for carbon–carbon bond formation and affords synthetically use-
ful γ-nitro carbonyl compounds, was carried out. The reaction between
propionaldehyde and nitrostyrene was selected as the model reaction
for optimizing the reaction conditions with 3 as the catalyst (Table 1).
Initially, a screening of the solvents at 25 °C for 24 h with 10 mol% of
3 was investigated (Table 1, entries 1–5), the results showed that di-
chloromethane was the choice of solvent to give the highest yield and
ee value (Table 1, entry 1). The reaction proceeded with the best
diastereoselectivity when performed in trichloromethane, however,
1028 cm−1 1H NMR (400 MHz, DMSO-d6): δ −0.09 (s, 9H), 0.85 (t,
;
J = 8.0 Hz, 2H), 1.85–1.90 (m, 2H), 1.98 (br, 1H), 2.02–2.15 (m, 2H),
2.76 (dd, J = 12.0, 4.0 Hz, 1H), 3.21 (dd, J = 16.0, 4.0 Hz, 1H), 3.95 (dd,
J = 16.0, 8 Hz, 1H), 4.27 (t, J = 8.0 Hz, 2H), 4.48 (dd, J = 12.0, 4.0 Hz,
1H), 4.53 (s, 2H), 7.31–7.46 (m, 31H), 7.61 (s, 1H), 7.74–7.76 (m, 14H);
13C NMR (100 MHz, DMSO-d6): δ 143.0, 139.0, 133.6, 131.4, 129.1,
129.0, 128.3, 128.1, 127.9, 127.5, 81.3, 76.3, 60.9, 50.5, 33.0, 28.3, 23.1,
13.4, 7.5, 2.3; 29Si NMR (79.5 MHz, DMSO-d6): δ −56.43, −69.63,
−69.95; HRMS (ESI) calcd for [C68H70N4O14Si9]+: 1419.2885, found
1419.2960.
2.3. General procedure for the asymmetric Michael addition reactions
Table 1
To a solution of aldehyde (0.3 mmol) in CH2Cl2 (1 mL) was added
arylnitroalkene (0.2 mmol) and POSS supported organocatalyst
(57 mg, 0.04 mmol). The homogeneous reaction mixture was stirred
at 10 °C until complete conversion of the starting materials (monitored
by TLC). MTBE (10 mL) was added to the mixture and a precipitate
formed. After filtration, the solid was separated and washed with
MTBE to afford the recycled catalyst with quantitative yield. The filtrate
was concentrated and purified by column chromatography on silica gel
to give the Michael adduct as an oil. The syn/anti ratio was determined
by 1H NMR spectroscopy of the crude mixture and the enantiomeric ex-
cess (ee) of the adduct was determined by HPLC on a chiral phase.
Optimization of Michael addition reaction conditions between propionaldehyde and
nitrostyrene.a
Entry
Loading
(mol%)
Solvent
Temp.
(°C)
Time
(h)
Yieldb
(%)
drc
(syn/anti)
eed
1
2
3
4
5
6
7
8
10
10
10
10
10
5
15
20
30
20
20
20
20
20
CH2Cl2
CHCl3
THF
25
25
25
25
25
25
25
25
25
0
24
24
24
24
24
24
24
24
24
24
24
48
72
96
69
26
67
30
Trace
Trace
75
78
77
58
65
80
82
82:18
97:3
68:32
76:24
n.d.
96
68
83
70
n.d.
n.d.
92
97
96
98
99
99
99
99
CCl4
Toluene
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
n.d.
3. Results and discussion
78:22
86:14
84:16
89:11
90:10
92:8
3.1. Preparation of POSS supported catalyst
9
10
11
12
13
14
The synthetic route of POSS supported catalyst 3 was depicted in
Scheme 1. The intermediate 3-azidopropylheptaphenyl POSS 1 [33]
and pyrrolidine moiety 2 [13] were prepared according to the
established procedure, respectively. Then a CuAAC reaction between 1
and 2 was performed to give 3 in good yield, which is soluble in typical
organic solvents, such as toluene, CHCl3, CH2Cl2, CH3CN, EtOAc, THF, and
insoluble in Et2O, MTBE or H2O. This solubility allowed a simple recov-
ery of POSS support catalyst 3 when precipitated with a poor solvent.
10
10
10
10
92:8
91:9
82
a
Reaction conditions: nitrostyrene (0.2 mmol), propionaldehyde (0.3 mmol), 3 (0.001–
0.06 mmol), solvent (1 mL).
b
Yield of isolated product.
c
Determined by 1H NMR spectroscopy.
d
Determined by HPLC analysis on a chiral phase (chiralcel OD-H).