Cyanosilylation of prochiral ketones catalyzed by lanthanide complexes

Article abstract of DOI:10.1080/00397910500212957

A new series of silylene-bridged rare-earth complexes involving fluorenyl are shown to be the very efficient Lewis acid catalysts, giving some cyano trimethylsilyl ethers of ketones in >99% yields. Copyright Taylor & Francis, Inc.

Full text of DOI:10.1080/00397910500212957

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Cyanosilylation of Prochiral
Ketones Catalyzed by
Lanthanide Complexes
Professor Luo Mei a & Ma Huai Zhu b
a Hefei University of Technology, Department of
Chemical Engineering , Hefei, China
b Anhui Normal University , Wu Hu, Anhui, China
Published online: 22 Aug 2006.
To cite this article: Professor Luo Mei & Ma Huai Zhu (2005) Cyanosilylation of
Prochiral Ketones Catalyzed by Lanthanide Complexes, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 35:20,
2615-2623
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Synthetic Communicationsw, 35: 2615–2623, 2005
Copyright # Taylor & Francis, Inc.
ISSN 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910500212957
Cyanosilylation of Prochiral Ketones
Catalyzed by Lanthanide Complexes
Luo Mei
Hefei University of Technology, Department of Chemical Engineering,
Hefei, China
Ma Huai Zhu
Anhui Normal University, Wu Hu, Anhui, China
Abstract: A new series of silylene-bridged rare-earth complexes involving fluorenyl
are shown to be the very efficient Lewis acid catalysts, giving some cyano trimethyl-
silyl ethers of ketones in .99% yields.
Keywords: Cyano trimethylsilyl ethers of ketones, Lewis acid catalysts, silylene-
bridged rare-earth complexes involving fluorenyl
Rare-earth organometallic complexes are the most important catalysts in
polymerization of alkenes[1–3] and organic reactions.[4–6] Chiral cyano-
hydrins from aldehydes and ketones are highly versatile synthetic intermedi-
ates that can be easily converted into a wide variety of important synthetic
intermediates including chiral a-hydroxyacids, a-amino acids, and b-amino
alcohols. They are very important in medical synthesis and natural product
synthesis, such as (S)-oxybutynin[7] and (20S)-camptothecin[8] synthesis. In
recent years there has been intense research activity in this area.
Shibasaki,[9–11] Deng,[12] Snapper and Hoveyda,[13] and Feng[14–16] have
developed many catalysts in cyanosilylation of ketones.
Received in Japan March 18, 2005
Address correspondence to Professor Luo Mei, Hefei University of Technology,
Department of Chemical Engineering, Hefei 230009, China. Tel.: 86-551-2907386;
E-mail: luomeihuahua@sohu.com
2615

2616
L. Mei and M. H. Zhu
Recently, I was inspired by Jacobsen,[17] who developed a series of rare-
earth trichlorides as Lewis acidic catalysts in cyanosilylation of imines. Using
six rare-earth organometallic complexes as catalysts for similar cyanosilyla-
tion reactions of many cyano trimethylsilyl ethers of ketones, I obtained
.99% yields.
Rare-earth organometallic complexes were 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 another article.[18,19] The synthesis routes are shown in
Scheme 1.
Scheme 1.

Cyanosilylation of Prochiral Ketones
2617
By applying these complexes to the cyanosilylation of ketones, it was
found that the complex 3 works the best in freshly purified chloroform
(0.015 M). After 0.5 h it gave excellent yields. When rare-earth trichloride
was employed, no reaction occurred because of its poor solubility. Silylene-
bridged ligands and no catalysts in this reaction will lead to a slow reaction
or even no reaction after 1 h. Results are listed in Table 1.
The following mechanism of cyanosilylation of ketones can be proposed:
one pair of isolated electrons in the oxygen atom of the C55O bond complexes
with Ln3þ, so the catalysts greatly activate the substrates. The reactivity
become very high and leads to a smooth reaction.
A range of ketones underwent efficient reactions under these conditions,
including electron-deficient and electron-rich substrates. They all displayed
different reactivity. The results are shown in Table 2 (entries 1–10).
The four-position aromatic ketones and the electron-deficient substrates
have poor reactivity after 15 h, except entry 10; its electron-rich counter-
parts have better reactivity. The conversion of two-position substitution sub-
strates are better than that of four-position substitution substrates. Other
substrates display less reactivity (entries 2 and 3).
All in all, no matter the electron and deficient and electron-rich substrates,
the steric factor is the main reason for reactivity.
In summary, this is the first example of Lewis acid catalysts in cyano-
silylation of ketones to give stable cyano trimethylsilyl ethers in excellent
yields at room temperature and with broad substrate generality. Additionally,
these lanthanide complexes have been applied to the cyanosilation of
aldehydes. Further efforts are being directed toward separating planar chiral
Table 1. Effect of different Ln3þ to cyanosilylation of acetophonea
Time
(h)
Conversion
(%)b
Entry
Complex
1
2
3
4
5
6
7
8
9
CH3PhSi(C13H9)2YbCl
CH3PhSi(C13H9)2DyCl
CH3PhSi(C13H9)2PrCl
CH3PhSi(C13H9)2LaCl
CH3PhSi(C13H9)2SmCl
CH3PhSi(C13H9)2NdCl
PrCl3
0.5
0.5
0.5
0.5
0.5
0.5
1
10
64
.99
.99
79
16
0
CH3PhSi(C13H9)2
No
1
0
5
0
aThe temperature is at 258C, CHCl3 solvent.
bThe conversion (%) was given by 1H NMR (300 MHz, CDCl3).

2618
L. Mei and M. H. Zhu
Table 2. Cyanosilylation of ketones catalyzed by rare-earth complex 3a
Time
(h)
Conversion
(%)b
Entry
R1
R2
CH3
1
2
C6H5
C6H5
C6H5
0.5
15
15
15
15
15
15
15
15
15
.99
32
CH2CH3
CH(CH3)2
CH3
3
28
4
2-OCH3C6H4
2-CH3C6H4
4-CH3C6H4
4-OCH3C6H4
4-BrC6H4
.99
.99
52
5
CH3
CH3
6
7
CH3
CH3
48
20
8
9
10
4-ClC6H4
4-NO2C6H4
CH3
CH3
56
79
aThe temperature is at 258C, CHCl3 solvent.
bThe conversion (%) was given by 1H NMR (300 MHz, CDCl3).
complexes, and application to cyanosilylation of prochiral ketones, aldehydes,
and some other asymmetric catalytic reactions.
EXPERIMENTAL
General Procedures[18]
All cyanosilylation reactions were performed using chloroform as solvent.
Ligands and lanthanum complexes were synthesized, and reactions were
monitored by thin-layer chromatography using 0.25-mm E. Merck silica
gel–coated glass plates (60F-254) with UV light to visualize the course of
the reaction. Flash column chromatography was performed using E. Merck
silica gel (60, particle size 0.02–0.03 mm). Chemical conversions were
obtained by 1H NMR and 13C NMR. 1H and 13C NMR spectra were
obtained using either a Bruker AM-200 or AM-300 spectrometer.
The following abbreviations were used to designate chemical shift mutipli-
cities: s ¼ singlet, d ¼ doublet, t ¼ triplet, and m ¼ multiplet. Infrared
spectra were recorded on a Mattson Galaxy Series FTIR 3000 spectrometer.
High-resolution mass spectra were obtained on a MASPEC.
Preparation of Bis(9-fluorenyl)(methyl)(phenyl) Silane[19]
10 g (60.24 mmol) of dry fluorene and 2 g (288.18 mmol) of lithium were added
under free-water and free-oxygen conditions in a dry 100-ml Schlenk flask.

Cyanosilylation of Prochiral Ketones
2619
They were dissolved in 100 ml of dry THF. The yellow mixture was stirred for
48 h. 4.9 mL (30.12 mmol) of dichloro(methyl)(phenyl) silane was added
dropwise at room temperature to the flask containing a THF solution of fluor-
enyllithium. After the condition was complete, the reaction mixture was stirred
for an additional 24 h. The solvent was removed under vacuum and the residue
was treated with 30 ml of n-hexane. The crude product was washed with
n-hexane to afford a white solid. Mp (208–2128C), C33H26Si found
1
(calc, %): C, 88.02% (88.00%); H, 5.79% (5.78%). H NMR (500 MHz,
CDCl3, 278C), d (ppm) ¼ 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), 20.32
(s, 3H, CH3). 13CNMR, 210.001, 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 (Mþ, 4.66%).
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 of dry THF
under argon in a 100-ml Schlenk flask. The stirred solution was cooled to
between 0 and 58C. The resulting green solution was stirred for 48 h.
Preparation of (Methyl)(phenyl) Silylene-bis(9-fluorenyl)
Yatterium Chloride
Bis (9-fluorenyl)(methyl)(phenyl) silylene lithium was added dropwise to a
solution of rare-earth trichlorine, and the mixture was cooled down to
between 0 and 58C and under argon. The resulting yellow mixture 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. After adding
30 ml of n-hexane, the solid precipitated out and was recrystallized in THF/
n-hexane. It was washed twice with THF/n-hexane and dried in vacuum to
give complex 1 as white crystals (72% yield). HRMS (EI): 657.1306, Yb%
25.98 (26.33%); IR (KBr, cm21): y 3065.4, 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 CH3-
PhSi(C13H9)2YbCl: C, 59.97%; H, 3.64%. Found: C, 60.32%; H, 3.68%.
(Methyl)(phenyl) Silylene-bis(9-fluorenyl) Dysprosium Chloride
Following the procedure described earlier, complex 2 as a white crystals was
obtained (80% yield); HRMS (EI): 646.6505, Dy% 25.98 (25.13%); IR (KBr,
cm21): y 3069.1, 2961.1, 2924.1, 2851.2, 1634.6, 1476.3, 1428.2, 1260.9,

2620
L. Mei and M. H. Zhu
1098.0, 1028.2, 872.9, 798.6, 1098.0, 1028.2, 872.9, 798.6, 437.4. Anal. calc. for
CH3PhSi(C13H9)2DyCl: C, 60.97%; H, 3.67%. Found: C, 61.30%; H, 3.71%.
(Methyl)(phenyl) Silylene-bis(9-fluorenyl) Praseodymium Chloride
Following the procedure described earlier, complex 3 was obtained yellow as
crystals (75% yield); HRMS(EI): 625.0642, Pr% 21.98 (22.54%), IR (KBr,
cm21): y 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)2PrCl: C, 63.23%; H, 3.77. Found: C, 63.42%; H, 3.87%.
(Methyl)(phenyl) Silylene-bis(9-fluorenyl) Lanthanum Chloride
Following the procedure described earlier, complex 3 was obtained as
yellow crystals (83% yield); HRMS(EI):623.0036, Pr% 21.86 (22.35%); IR
(KBr, cm21): y 3065.3, 3046.2, 2952.4, 2923.2, 2851.8, 1611.3, 1601.1,
1505.8, 1476.8, 1115.2, 984.8, 781.8, 738.2, 484.2, 456.3. Anal. calc. for
CH3PhSi(C13H9)2PrCl: C, 63.62%: H, 3.89. Found: C, 63.52%; H, 3.87%.
(Methyl)(phenyl) Silylene-bis(9-fluorenyl) Samarium Chloride
Following the procedure described earlier, complex 4 was obtained as yellow
crystals (90% yield); HRMS (EI): 634.0565, Sm% 23.59 (23.70%); IR (KBr,
cm21): y 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)2SmCl: C, 62.51%; H,
3.62%. Found: C, 62.47%; H, 3.81%.
(Methyl)(phenyl) Silylene-bis(9-fluorenyl) Neodymium Chloride
Following the procedure described earlier, complex 5 was obtained as yellow
crystals (86% yield); HRMS(EI): 627.9565, Nd% 22.31 (22.96%); IR (KBr,
cm21): y 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-(Trimethylsilyloxy)-2-phenyl-propanenitrile
CH3PhSi(C13H9)2 LnCl (0.015 mmol) was dissolved in 1 M CHCl3 aceto-
phone (1 mmol) and TMSCN (297 ml, 2.23 mmol) was successively added
at room temperature. After 0.5 h, the reaction was quenched. Further

Cyanosilylation of Prochiral Ketones
2621
purification was performed by silica gel (petroleum/dichloromethane: 4/1).
The title compound was obtained as a colorless oil (0.22 g, conversion
.99%). The physical and spectral data were identical to those previously
reported for this compound.[20] 1H NMR (300 MHz, CDCl3): 7.44–7.47
(m, 3H), 7.24–7.32 (m, 2H), 1.76 (s, 1H), 0.079 (s, 9H). 13C NMR
(75 MHz, CDCl3): 0.98 (ꢀ3), 22.56, 33.51, 71.53, 121.54, 124.53 (ꢀ2),
128.57 (ꢀ2), 141.92.
2-(Trimethylsilyloxy)-2-phenylbutanenitrile
The title compound was obtained as a colorless oil (74.56 mg, conversion 32%).
The physical and spectral data were identical to those previously reported for
this compound.[20] 1H NMR (300 MHz, CDCl3): 7.36–7.39 (m, 2H), 7.21–
7.29 (m, 3H), 1.80–1.92 (q, J ¼ 12.6 Hz, 2H), 0.79–0.88 (t, J ¼ 9.9 Hz, 3H),
20.052 (s, 9H). 13C NMR (75 MHz, CDCl3): 1.35 (ꢀ3), 22.58, 30.87, 69.96,
120.67, 126.88, 128.58, 128.75, 129.51, 130.95, 135.11.
2-(Trimethylsilyloxy)-2-phenyl-i-pentanenitrile
The title compound was obtained as a colorless oil (69.65 mg, conversion
1
28%). H NMR (300 MHz, CDCl3): 7.46–7.51 (m, 2H), 7.34–7.42 (m, 3H),
2.02–2.08 (m, 1H), 1.06–1.08 (d, J ¼ 6.6 Hz, 3H), 0.79–0.82 (d,
J ¼ 6.9 Hz, 3H), 0.08 (s, 9H). 13C NMR (75 MHz, CDCl3): 0.80 (ꢀ3),
17.15, 17.26, 41.48, 80.09, 120.19, 125.74 (ꢀ2), 128.23 (ꢀ2), 128.52,
140.19. HRMS: calcd. for C13H19SiNO: 247.13924, found: 247.13812.
2-(Trimethylsilyoxy)-(20-methyloxylphenyl) Propanenitrile
The title compound was obtained as a colorless oil (0.25 g, conversion
.99%), the physical and spectral data were identical to those previously
reported for this compound. 1H NMR (300 MHz, CDCl3): 7.44–7.48
(m, 1H), 7.23–7.29 (m, 1H), 6.85–6.95 (m, 2H), 3.83 (s, 3H), 1.83 (s, 3H),
0.229 (s, 9H). 13C NMR (75 MHz, CDCl3): 1.28 (ꢀ3), 30.09, 55.53,
68.53, 111.63, 120.56, 125.72, 129.80 (ꢀ2), 157.9. HRMS: calcd. for
C13H19SiNO2: 249.11851, found: 249.11720.
2-(Trimethylsilyoxy)-2-(20-methylphenyl) Propanenitrile
The title compound was obtained as a colorless oil (0.23 g, conversion
.99%). 1H NMR (300 MHz, CDCl3): 7.53–7.58 (m, 1H), 7.18–7.27
(m, 3H), 2.55 (s, 3H), 1.94 (s, 3H), 0.077 (s, 9H). 13C NMR (75 MHz,

2622
L. Mei and M. H. Zhu
CDCl3): 1.09 (ꢀ3), 20.68, 30.51, 71.68, 121.62, 125.29, 125.97, 128.66,
132.64, 135.50, 138.41. HRMS: calcd. for C13H19SiNO: 233.12359, found:
233.12213.
2-(Trimethylsilyoxy)-2-(40-methylphenyl) Propanenitrile
The title compound was obtained as a colorless oil (0.12 g, conversion 52%).
The physical and spectral data were identical to those previously reported for
this compound.[21] 1H NMR (300 MHz, CDCl3): 7.33–7.37 (m, 2H), d 7.09–
7.17 (m, 2H), 2.28 (s, 3H), 1.18 (s, 3H), 0.068 (s, 9H). 13C NMR (75 MHz,
CDCl3): 1.00 (ꢀ3), 20.98, 33.45, 71.44, 121.67, 124.51 (ꢀ2), 129.19 (ꢀ2),
138.43, 139.03.
2-(Trimethyloxy)-2-(40-methyloxylphenyl) Propanenitrile
The title compound was obtained (0.12 g, conversion 48%). The physical and
spectral data were identical to those previously reported for this compound.[21]
1H NMR (300 MHz, CDCl3), 7.33–7.36 (d, J ¼ 9 Hz, 2H), d 6.78–6.81
(d, J ¼ 9 Hz, 2H), d 3.71 (s, 3H), 1.73 (s, 3H), 20.024 (s, 9H). 13C NMR
(75 MHz, CDCl3): 1.00 (ꢀ3), 33.40, 55.32, 71.25, 113.85 (ꢀ2), 121.78,
126.03 (ꢀ2), 134.02, 159.75.
2-(Trimethylsilyoxy)-2-(40-bromophenyl) Propanenitrile
The title compound was obtained (69.20 mg, conversion 20%). The physical
and spectral data were identical to those previously reported for this
compound.[22] 1H NMR (300 MHz, CDCl3): d 7.42–7.46 (d, J ¼ 13.5 Hz,
2H), 7.31–7.35 (d, J ¼ 12.6 Hz, 2H), 1.74 (s, 3H), 20.002 (s, 9H). 13C
NMR (75 MHz, CDCl3): 1.00 (ꢀ3), 33.42, 71.02, 115.87, 121.07, 122.66,
126.30 (ꢀ2), 131.73 (ꢀ2), 141.19. HRMS: calcd. for C12H16SiNOBr:
297.01845, found: 297.1875.
2-(Trimethylsilyoxy)-2-(40-chlorophenyl) Propanenitrile
The title compound was obtained (0.14 g, conversion 56%). The physical
and spectral data were identical to those previously reported for this
compound.[22] 1H NMR (300 MHz, CDCl3): d 8.13–8.16 (m, 2H), 7.61–
7.64 (m, 2H), 7.33–7.36 (m, 2H), 1.75 (s, 3H), 0.12 (s, 9H). 13C NMR
(75 MHz, CDCl3): 0.96 (ꢀ3), 33.42, 70.96, 121.12, 126.00, 128.75 (ꢀ2),
134.50 (ꢀ2), 140.64.

Cyanosilylation of Prochiral Ketones
2623
2-(Trimethyloxy)-2-(40-nitrophenyl) Propanenitrile
The title compound was obtained (0.21 g, conversion 79%). The physical and
spectral data were identical to those previously reported for this compound.[22]
1H NMR (300 MHz, CDCl3): d 7.46–7.49 (m, 2H), 7.29–7.33 (m, 2H), 1.78
(s, 3H), 0.09 (s, 9H).
REFERENCES
1. Luo, M.; Ma, H. Z.; Su, Q. D. Asian J. Chem. 2003, 15, 497–499.
2. Yu, X. Y.; Jin, G. X.; Hu, N. H.; Weng, L. H. Organometallics 2002, 21, 5540.
3. Tang, G. R.; Jin, G. X.; Weng, L. H. J. Organomet. Chem. 2004, 689, 678.
4. Qian, C. T.; Wang, I. C. Tetrahedron 2000, 56, 7193.
5. Qian, C. T.; Wang, I. C. Tetrahedron. 2000, 41, 2203.
6. Luo, M. J. Mol. Catal. A 2004, 227, 183–186.
7. Masumoto, S.; Suzuki, M.; Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2002, 43,
8647–8651.
8. Yabu, K.; Masumoto, S.; Kana, M.; Curran, D. P.; Shibasaki, M. Tetrahedron Lett.
2002, 43, 2923–2926.
9. Hamashima, Y.; Sawada, D.; Kanai, M.; Shibasaki, M. J. Am Chem. Soc. 1999,
121, 2641–2642.
10. Hamashima, Y.; Kanai, M.; Shibasaki, M. Tetrahedron Lett. 2001, 42, 691–694.
11. Masumoto, S.; Yabu, K.; Kana, M.; Shibasaki, M. Tetrahedron Lett. 2002, 43,
2919–2922.
12. Tian, S.-K.; Hong, R.; Deng, L. J. Am. Chem. Soc. 2003, 125, 9900–9901.
13. Deng, H. B.; Isler, M. R.; Snapper, M. L.; Hoveyda, A. H. Angew. Chem., Int. Ed.
Engl. 2002, 41, 1009.
14. Chen, F.; Feng, X.; Qin, B.; Zhang, G.; Jiang, Y. Org. Lett. 2003, 5, 949–952.
15. Chen, F. X.; Qin, B.; Feng, X. M.; Zhang, G. L.; Jiang, Y. Z. Tetrahedron 2004, 59,
10449–10460.
16. He, B.; Chen, F. X.; Lin, Y.; Feng, X. M.; Zhang, G. L. Tetrahedron Lett. 2004, 45,
5465–5467.
17. Keith, J. M.; Jacobsen, E. N. Org. Lett. 2004, 6, 153–155.
18. Luo, M.; Ma, H. Z.; Su, Q. D. Asian J. Chem. 2002, 14, 1469–1473.
19. Luo, M.; Ma, H. Z.; Su, Q. D. Chinese J. Struct. Chem. 2003, 22, 643–646.
20. Hamashima, Y.; Kanai, M.; Shibasaki, M. J. Am. Chem Soc. 2000, 122,
7412–7413.
21. He, B.; Chen, F. X.; Li, Y.; Feng, X. M.; Zhang, G. L. Eur. J. Org. Chem. 2004, 22,
4657–4666.
22. Ryu, D. H.; Corey, E. J. J. Am. Chem. Soc. 2005, 127, 5384–5387.