The cyanosilylation of prochiral aldehydes catalyzed by lanthanide complexes

Article abstract of DOI:10.1016/j.molcata.2004.10.028

A new series of silylene-bridged rare-earth complexes involving fluorenyl are shown to be the very efficient Lewis acidic catalysts, giving cyanotrimethylsilyl ethers of aldehydes >99% conversion.

Full text of DOI:10.1016/j.molcata.2004.10.028

Journal of Molecular Catalysis A: Chemical 227 (2005) 183–186
The cyanosilylation of prochiral aldehydes catalyzed by
lanthanide complexes
Luo Mei∗
Key Laboratory of Biorganic Chemistry and Molecular Engineering, College of Chemistry and Molecular Engineering,
Ministry of Education, Peking University, Beijing 100871, China
Received 16 October 2004; accepted 16 October 2004
Available online 10 December 2004
Abstract
A new series of silylene-bridged rare-earth complexes involving fluorenyl are shown to be the very efficient Lewis acidic catalysts, giving
cyanotrimethylsilyl ethers of aldehydes >99% conversion.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Silylene-bridged rare-earth complexes involving fluorenyl; Lewis acidic catalysts; Cyanotrimethylsilyl ethers
1. Introduction
Recently, Corey [16] develops a series of rare-earth
trichloride as Lewis acidic catalysts in cyanosilylation of
aldehydes. Inspired by the pioneering works, I used five rare-
earth organometallic complexes as catalysts, of many prod-
ucts in 3 h, they could get the nearly 100% conversions.
Rare-earth organometallic complexes are the most impor-
tant catalysts in polymerization of alkenes [1–3] and organic
reactions [4,5]. I have recently succeeded in developing the
new family Lewis acid catalysts 1–5, and could demonstrate
that complex 5 is the most general catalyst for the cyanosily-
lation of aldehydes.
2. Experimental
Chiral cyanohydrins from aldehydes and ketones are
highly versatile synthetic intermediates which can be easily
converted into a wide variety of important synthetic interme-
diates including chiral ␣-hydroxy acids, ␣-amino acids, and
␤-amino alcohols. As we all know that they are very impor-
tant in medical synthesis and natural products synthesis such
as l-biotin [6], 20S-amptothecin [7] synthesis, etc. For these
reasons, there has been intense research activity in this area in
recent years. Shibasaki first put forward bifunctional catalyst
concept [8–10], he and co-workers developed many heterod-
ifunctional metallic complexes, later, Jacobsen [11], Garcia
[12–14], and Nakai [15] have developed many catalysts in
cyanosilylation of aldehydes.
2.1. General procedures [17]
All cyanosilylation reactions were performed using chlo-
roform as solvent, ligands and lanthanum complexes were
synthesized, reactions were monitored by thin-layer chro-
matography using 0.25 mm E. Merck silica gel-coated glass
plates (60F-254) using UV light to visualize the course of re-
action. Flash column chromatography was performed using
E. Merck silica gel (60, particle size 0.02–0.03 mm). Chem-
ical conversion was obtained by H NMR, 13C NMR. H
and 13C NMR spectra were obtained using either a Bruker
AM-300 or a AM-500 spectrometer. The following abbre-
viations were used to designate chemical shift multiplici-
ties: s = singlet, d = doublet, t = triplet, m = multiplet. Infrared
spectra were recorded on a Mattson Galaxy Series FTIR 3000
1
1
∗
Tel.: +86 10 62763771; fax: +86 10 62763771.
E-mail address: luomeihuahua@sohu.com.
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.10.028

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L. Mei / Journal of Molecular Catalysis A: Chemical 227 (2005) 183–186
Spectrometer. High-resolution mass spectra were obtained on
a MASPEC.
2.5. Preparation of (methyl)(phenyl)silylene-bis(9-
fluorenyl)dysprosium chloride
2.2. Preparation of bis(9-fluorenyl)(methyl)-
(phenyl)silane [18]
Following the procedure described in Section 2.4. Com-
plex 2 as a white crystals (80%yield). HRMS (EI): 646.6505,
Dy% 25.98 (25.13%); IR (KBr, cm−1) ν 3069.1, 2961.1,
2924.1, 2851.2, 1634.6, 1476.3, 1428.2, 1260.9, 1098.0,
1028.2, 872.9, 798.6, 1098.0, 1028.2, 872.9, 798.6, 437.4.
Anal. Calc. for CH3PhSi (C13H9)2 DyCl: C, 60.97%; H,
3.67%. Found: C, 61.30%, H:3.71%.
Ten grams (60.24 mmol) of dry fluorene and 2 g
(288.33 mmol) of lithium were added under free-water and
free-oxygen condition in a dry 100 ml Schlenk flask. They
were dissolved in 100 ml of dry THF. The yellow mixture
was stirred for 48 h, and it is to be used.
4.9 ml (30.12 mmol) of dichloro(methyl)(phenyl)silane
was added dropwise at room temperature to the flask con-
taining a THF solution of fluorenyllithium. After the con-
dition was complete, the reaction mixture was stirred for
an additional 24 h. The solvent was removed under vac-
uum and the residue was treated with 30 ml of n-hexane, the
crude product was washed by n-hexane to afford white solid.
m.p.: 208–212 ◦C; C33H26Si, found (calc., %):C, 88.02%
2.6. Preparation of (methyl)(phenyl)silylene-bis(9-
fluorenyl)praseodymium chloride
Following the procedure described in Section 2.4. Com-
plex 3 as yellow crystals (75% yield). HRMS (EI): 625.0642,
Pr% 21.98 (22.54%); IR (KBr, cm−1) ν 3066.0, 3047.7,
2956.6, 2923.2, 2852.1, 1646.2, 1615.9, 1516.7, 1448.7,
1427.8, 1114.8, 1020.2, 983.7, 738.5, 484.2, 442.7. Anal.
Calc. for CH3PhSi (C13H9)2 PrCl:C, 63.23%; H, 3.77%.
Found: C, 63.42%; H, 3.87%.
1
(88.00%); H, 5.79% (5.78%). H NMR (500 MHz, CDCl3,
27 ◦C), δ (ppm) = 7.71–7.77 (d, 6H, aromH), 7.30–7.33
(t, 8H, aromH), 7.15–7.18 (t, 2H, aromH), 7.09–7.10 (t,
1H, aromH), 6.85–6.88 (t, 2H, aromH), 6.49–6.51 (t, 2H,
aromH), 4.64 (s, 2H, H-9, Flu), −0.32 (s, 3H, CH3);
13CNMR: −10.001, 39.623, 120.109, 120.186, 124.603,
125.700, 125.750, 126.262, 126. 295, 126.773, 129.266,
134.076, 141.044, 141.159, 144.397, 144.440. MS: m/z 450
[M+, 4.66%].
2.7. Preparation of (methyl)(phenyl)silylene-bis(9-
fluorenyl)samarium chloride
Following the procedure described in Section 2.4. Com-
plex 4 as yellow crystals (90% yield). HRMS (EI):
634.0565, Sm% 23.59 (23.70%); IR (KBr, cm−1) ν 3068.1,
2962.7, 1635.3, 1475.6, 1446.8, 1260.9, 1098.8, 1027.7,
790.6, 484.3, 443.5. Anal. Calc. for CH3PhSi (C13H9)2
SmCl: C, 62.51%, H, 3.62%. Found: C, 62.47%; H,
3.81%.
2.3. Preparation of (methyl)(phenyl)silylene-bis(9-
fluorenyl)lithium
A certain amount of 2 g (288.18 mmol) of lithium and 1 g
(2.22 mmol) of bis(9-fluorenyl)(methyl)(phenyl)silane were
diluted with 30 ml dry THF under argon in a 100 ml Schlenk
flask. The stirred solution was cool down to 0 to −5 ◦C. The
resulting green solution was stirred for 48 h, and it is to be
used.
2.8. Preparation of (methyl)(phenyl)silylene-bis(9-
fluorenyl)neodymium chloride
Following the procedure described in Section 2.4. Com-
plex 5 as yellow crystals (86% yield). HRMS (EI): 627.9565,
Nd% 22.31 (22.96%); IR (KBr, cm−1) ν 3067.9, 2961.2,
2923.5, 1634.7, 1476.7, 1260.6, 1049.1, 1028.1, 789.1, 789.8,
737.8, 481.8, 430.5. Anal. Calc. for CH3PhSi (C13H9)2
NdCl: C, 62.98%; H, 3.79%. Found: C, 63.08%; H,
3.84%.
2.4. Preparation of (methyl)(phenyl)silylene-bis(9-
fluorenyl)yttrium chloride
A solutionof rare-earthtrichlorine wascooled down to 0to
−5 ◦C and bis(9-fluorenyl)(methyl)(phenyl)silylene lithium
was added dropwise under argon. The resulting yellow mix-
ture was stirred for 48 h and then was allowed to warm up to
room temperature. The resulting solution was concentrated
in vacuum to about 20 ml. By addition of 30 ml of n-hexane,
solid precipitated out which was recrystallized in THF/n-
hexane. It was washed twice with THF/n-hexane and dried
in vacuum gave complex 1 as a white crystals (72% yield).
2.9. Preparation of ␣-(trimethylsilyoxyl)-
phenylacetonitrile
CH3PhSi (C13H9)2 LnCl 0.015 mmol was dissolved in
CHCl3 phenyl aldehyde (1 mmol) and TMSCN (297 ␮l, 2.2
mmol) was successively added at room temperature. Af-
ter 0.5 h, the reaction was quenched. Further purification
HRMS(EI):657.1306, Yb%25.98(26.33%), IR(KBr, cm−1
)
1
ν3065.4, 2962.6, 2925.9, 2835.2, 1632.1, 1457.3, 1428.7,
1261.3, 790.0, 737.7, 403.3, 390.7. Anal. Calc. for CH3PhSi
(C13H9)2 YbCl: C, 59.97%; H, 3.64%. Found: C, 60.32%,
H3.68%.
was performed by silica gel. H NMR (300 MHz, CDCl3)
7.39–7.39 (t, 0.9 Hz, 2H), 7.31–7.34 (m, 3H), 5.43 (s, 1H),
0.16(s, 9H). 13CNMR(75 MHz, CDCl3)148.0, 129.4, 128.9,
124.3, 115.8, 62.9, −0.26.

L. Mei / Journal of Molecular Catalysis A: Chemical 227 (2005) 183–186
185
2.10. Preparation of ␣-(trimethylsilyoxyl)-o-
CDCl3) 134.9, 131.8, 128.7, 128.4, 127.4, 126.6, 118.7, 62.5,
methyloxylphenylacetonitrile
−2.67.
Following the procedure described in Section 2.9. 1H
NMR (300 MHz, CDCl3) 7.57–7.61 (d, 11.4 Hz, 1H),
7.28–7.36 (t, 5.8 Hz, 1H), 6.99–7.07 (t, 0.9 Hz, 1H),
6.89–6.92 (d, 11.7 Hz, 1H), 5.80 (s, 1H), 3.75 (s, 3H), 0.158
(s, 9H). 13C NMR (75 MHz, CDCl3), 13C NMR (75 MHz,
CDCl3) 162.5, 130.2, 126.9, 126.7, 124.6, 120.5, 110.3, 57.9,
55.2, −2.18.
2.16. Preparation of ␣-(trimethylsilyoxyl)-p-
nitrophenylacetonitrile
Following the procedure described in Section 2.9. 1H
NMR (300 MHz, CDCl3) 8.28–8.38 (d, 2H), 7.67–7.77 (d,
2H), 5.62 (s, 1H), 0.38 (s, 9H). 13C NMR (75 MHz, CDCl3)
143.8, 130.8, 127.3, 126.9, 123.8, 123.3, 117.9, 62.3, −2.16.
2.11. Preparation of ␣-(trimethylsilyoxyl)-o-
fluorophenylacetonitrile
2.17. Preparation of ␣-(trimethylsilyoxyl)-1-
naphthylacetonitrile
Following the procedure described in Section 2.9. 1H
NMR (300 MHz, CDCl3) 7.17–7.20 (d, 9 Hz, 2H) 7.04–7.08
(d, 12 Hz, 2H), 5.70 (s, 1H), 0.098 (s, 9H). 13C NMR
(75 MHz, CDCl3) 162.6, 131.3, 128.3, 127.1, 124.7, 115.7,
115.2, 57.5, −1.95.
Following the procedure described for 2.9. 1H NMR
(300 MHz, CDCl3) 7.84–7.88 (d, 12 Hz, 2H), 7.18–7.54 (m,
5H), 6.09 (s, 1H), 0.32 (s, 9H).13C NMR (75 MHz, CDCl3)
133.7, 130.4, 129.6, 128.8, 126.9, 126.2, 125.5, 124.9, 122.9,
122.1, 118.9, 62.8, −0.36.
2.12. Preparation of ␣-(trimethylsilyoxyl)-o-
nitrophenylacetonitrile
3. Results and discussion
Following the procedure described in Section 2.9. 1H
NMR (300 MHz, CDCl3) 8.03–8.16 (d, 10.5 Hz, 1H),
7.94–7.98 (d, 12 Hz, 1H), 7.62–7.71 (t, 2.7 Hz, 1H),
7.54–7.58 (t, 5.4 Hz, 1H), 6.15 (s, 1H), 0.31 (s, 9H). 13C
NMR (75 MHz, CDCl3): 134.9, 132.4, 130.6, 128.8, 127.6,
125.7, 118.2, 60.5, −1.54.
Rare-earth organometallic complexes are prepared from
bis(9-fluorenyl)(methyl)(phenyl)silylene lithium with LnCl3
in THF under argon, they were characterized by MS, IR and
elemental analysis, the concrete procedure and data can be
seen in Asian J. Chem. [17,18]. The synthesis routes as fol-
lows:
2.13. Preparation of ␣-(trimethylsilyoxyl)-p-
methyloxylphenylacetonitrile
Following the procedure described in Section 2.9.
1H NMR (300 MHz, CDCl3) 7.38–7.42 (d, 12 Hz, 2H),
6.94–6.96 (d, 8 Hz, 2H), 5.45 (s, 1H), 3.83 (s, 3H), 0.23 (s,
9H). 13C NMR (75 MHz, CDCl3) 161.3, 129.8, 128.4, 127.8,
119.2, 114.4, 114.1, 113.7, 63.2, 55.2, −0.40.
2.14. Preparation of ␣-(trimethylsilyoxyl)-p-
fluorophenylacetonitrile
Following the procedure described in Section 2.9. 1H
NMR (300 MHz, CDCl3) 8.09–8.12 (d, 10.5 Hz, 1H),
7.94–7.98 (d, 11.7 Hz, 1H), 7.71–7.73 (t, 3.3 Hz, 1H),
7.54–7.58 (t, 9 Hz, 1H), 6.16 (s, 1H), 0.31 (s, 9H). 13C NMR
(75 MHz, CDCl3) 161.4, 130.8, 128.4, 128.3, 118.9, 116.2,
115.9, 63.3, −0.33.
2.15. Preparation of ␣-(trimethylsilyoxyl)-p-
chlorophenylacetonitrile
Following the procedure described in Section 2.9. 1H
NMR (300 MHz, CDCl3) 7.38–7.40 (d, 6 Hz, 2H), 7.21–7.27
(d, 3 Hz, 2H), 5.48 (s, 1H), 0.245 (s, 9H). 13C NMR (75 MHz,

186
L. Mei / Journal of Molecular Catalysis A: Chemical 227 (2005) 183–186
Table 1
A range of aldehydes underwent efficient reaction under
these conditions, including electron-deficient and electron-
rich substrates, and they were all displayed high reactivity.
The results can be seen in Table 2.
The effect of different Ln3+ to cyanosilylation of phenyl aldehydea
From Table 2, no matter the electron-deficient and
electron-rich substrates, the conversions are very high after
3 h, but as to the entry 9, 5 h later, the conversion has not
gotten to 100%, so the steric factor is the main reason for the
reactivity.
Entry
Catalyst
Time (h)
Conversion (%)b
1
2
3
4
5
6
7
8
a
CH3PhSi(C13H9)2YbCl
CH3PhSi(C13H9)2DyCl
CH3PhSi(C13H9)2PrCl
CH3PhSi(C13H9)2SmCl
CH3PhSi(C13H9)2NdCl
NdCl3
0.5
0.5
0.5
0.5
0.5
1.5
1.5
1.5
44
89
91
32
>99
74
0
4. Conclusion
No
CH3PhSi(C13H9)2
0
In summary, the first example of the highly Lewis acidic
catalysts in cyanosilylation of aldehydes to give stable cyan-
otrimethylsilyl ethers has been reported in excellent yields at
room temperature and with broad substrate generality. Fur-
ther efforts are being directed toward the cyanosilylation of
ketones, separation the planar chiral complexes and appli-
cation to cyanosilylation of prochiral ketones and aldehydes.
This study clearly demonstrates that the structure of the com-
plexes and different rare-earth ions can greatly change the
yield. Silylene-bridged ligands and no catalyst will lead the
slow reaction or no reaction. All in all, no matter the electron-
deficient and electron-rich substrates, the steric factor is the
main reason for the reactivity.
The temperature is at 25 ◦C,CHCl3 solvent.
The conversion (%) was given by 1H NMR (300 MHz, CDCl3).
b
Table 2
Cyanosilylation of aldehydes catalyzed by catalyst 5a
Entry
R1
Time (h)
Conversion (%)b
1
2
3
4
5
6
7
8
9
9
a
C6H5
2-OCH3C6H4
2-FC6H4
2-NO2C6H4
4-OCH3C6H4
4-FC6H4
4-ClC6H4
4-NO2C6H4
1-Naphthyl
1-Naphthyl
0.5
3
1
1
3
3
3
3
1
>99
>99
81
>99
>99
>99
>99
>99
61
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90
The temperature is at 25 ◦C,CHCl3 solvent.
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b
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In this study, I first use rare-earth trichloride as catalyst in
freshly purified chloroform (0.015 M), TMSCN and pheny-
laldehyde were added in room temperature, but it was not
nearly dissolved in CHCl3, maybe this is the poor perfor-
mance in reactivity, 1.5 h later, no reaction, this prompted
me to use the complexes 1–5 that I have synthesized, they
all dissolved in solvent, and the reaction rate is greatly im-
proved, 1 h later, of one product (entry 3), the yield is getting
nearly 100%. The effects of different Ln3+ are as follows:
Nd3+ > Pr3+ > Dy3+ > Yb3+ > Sm3+, the results are listed in
Table 1.
At the same time, I compared silylene-bridged catalyst
CH3PhSi (C13H9)2 and no catalyst, the results were found
that after 1.5 h, no reactivity.
The mechanism of cyanosilylation can be proposed that
one pair isolated electron in oxygen atom of C O bond, could
complex with the Ln3+, so the catalysts greatly actived sub-
strates, the reactivity became very high, and leading to the
smooth reaction.
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