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C. Lv et al. / Catalysis Communications 12 (2011) 1242–1245
Table 1
matic and aliphatic derivatives catalyzed by 3 was investigated under
the optimized conditions, and the data were listed in Table 2. The results
indicated that introduction of an electron-withdrawing group on the
benzene ring disfavored to the stereocontrol of the reaction (Table 2,
entries 9–12). On the other hand, the electron-rich aromatic aldehydes
were transformed into the corresponding chiral products of cyanohy-
drins in similar or a slightly lower enantioselectivities than that of
benzaldehyde. The steric properties of the aromatic aldehydes have
great effect on the ee values of the products in this reaction. The ortho-
substituted aromatic aldehydes gave lower ee value and yield than the
meta-substituted and para-substituted aromatic aldehydes. Furfural
also gave rise to a lower ee value than that of benzaldehyde (Table 2,
entry 13). Aliphatic aldehydes, such as heptaldehyde, also reacted
smoothly in a 64% ee. Unfortunately, trimethylacetaldehyde did not
furnish a satisfactory result due to the hindrance of corresponding tert-
Butyl group (Table 2, entry 15). These results also provide evidence that
steric congestion is determinant for the reactivity and enantioselec-
tivity. In terms of reactivity, reaction of all aromatic and aliphatic
aldehydes could give good chemical yield with relatively low catalyst
loading except trimethylacetaldehyde and cinnamaldehyde.
The screening of reaction conditions for the addition of TMSCN to benzaldehyde using
catalyst 3a.
OTMS
CHO
Ti-Catalyst 3
CN
+ TMSCN
Entry Catalyst (mol%) TMSCN Temp. (°C) Solvent
Conv. (%)b Ee (%)c
1
2
3
4
5
6
7
8
9
0.2
0.2
0.2
0.2
0.2
0.1
0.1
0.05
0.1
0.1
1.5
1.5
1.5
1.5
1.5
1.2
1.05
1.05
1.05
1.05
0
0
CH2Cl2
Et2O
99
60
99
97
82
35
69
61
22
81
83
74
49
55
0
0
toluene
CHCl3
0
ClCH2CH2Cl 99
0
0
0
−10
20
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
99
99
98
86
99
10
a
All reactions were carried out at listed temperature in table for 24 h.
Determined by GC with n-nonane as internal standard.
Enantiomeric excess of silyl ether is determined by GC with a CP-Chirasil-Dex CB
b
c
column.
4. Conclusion
indicated that oxo-bridges were formed in the Ti catalyst 3 [16,27].
Based on the earlier reports [15], the data of the elemental analyses
and mass spectrum also supported the existence of the intramolecular
Ti\O\Ti bonds in catalyst 3 (Fig. 2).
In summary, we synthesized a dimeric salen ligand and the
corresponding Ti complex from chiral pyrrolidine diamine. The Ti-
salen complex was successfully utilized as catalyst in the asymmetric
silylcyanation of aldehydes, showing moderate to good catalytic activity
and stereoselectivity under mild conditions. Further studies will focus
on expanding the scope of these ligands derived from chiral pyrrolidine
diamine in asymmetric reactions.
The asymmetric addition of trimethylsilyl cyanide (TMSCN) to
benzaldehyde was investigated with Ti catalyst 3. Indeed, the addition
reaction proceeded smoothly even in a relatively low catalyst loading
(as low as 0.1 mol%) [28,29]. Further reducing the catalyst loading
resulted in a lower enantioselectivity (Table 1, entry 8). Among
the tested solvents, CH2Cl2 was the best choice for the asymmetric
silylcyanation of aldehydes. The effect of the temperature was also
surveyed, and the reaction at 0 °C for 24 h was a viable option.
Based on the results achieved with benzaldehyde, the asymmetric
addition of trimethylsilylcyanide to various aldehydes including aro-
Acknowledgments
We are grateful for financial support from the Chinese Academy
of Sciences and National Natural Science Foundation of China
(21073210, 20873166).
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1
2
3
4
5
6
7
8
Benzaldehyde
98
97
90
90
73
92
94
98
95
96
96
95
93
66
70
90
82 (S)
75 (S)
86 (S)
66 (S)
75 (S)
76 (S)
85 (S)
81 (S)
57 (S)
37
4-methylbenzaldehyde
3-methylbenzaldehyde
2-methylbenzaldehyde
4-methoxybenzaldehyde
3-methoxybenzaldehyde
2-methoxybenzaldehyde
4-chlorobenzaldehyde
2-chlorobenzaldehyde
2,6-dichlorobenzaldehyde
2-bromobenzaldehyde
4-trifluoromethylbenzaldehyde
Furfural
9
10
11
12
13
14
15
16
41
62 (S)
70 (S)
12 (S)
13 (S)
64 (S)
Cinnamaldehyde
Trimethylacetaldehyde
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24 h.
b
c
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