A novel bifunctional Ti(IV) complex catalyzed asymmetric silylcyanation of aldehydes

Article abstract of DOI:10.1002/hc.20652

A novel Lewis acid and Lewis base bifunctional Ti(IV) complex was formed in situ upon the treatment of ligand (R)-3,3′-bis(diphenylphosphinoyl)-BINOL with Ti(PrO-i)₄, which is proven to be an efficient catalyst in the asymmetric trimethylsilylc

Full text of DOI:10.1002/hc.20652

Heteroatom Chemistry
Volume 22, Number 1, 2011
A Novel Bifunctional Ti(IV) Complex Catalyzed
Asymmetric Silylcyanation of Aldehydes
Hongying Tang and Zhongbiao Zhang
Tianjin Key Laboratory of Water Environment and Resources, Tianjin Nomal University, Tianjin,
300387, People’s Republic of China
Received 9 July 2010; revised 26 September 2010
LiI), in the presence of a chiral ligand, such as
ABSTRACT:
A novel Lewis acid and Lewis
Schiff bases [9–18], dihydroxy compounds [19–21],
bis-oxazolines [22,23], amides [24–31], and phos-
phorus compounds [32–36]. Lately, chiral Lewis
bases coordinating to and activating silylated nucle-
ophiles have been introduced into this field [32,37].
Therefore, it is rational to design a new bifunc-
tional asymmetric catalyst consisting of Lewis acid
and Lewis base moieties, which activate both elec-
trophiles and nucleophiles at defined positions si-
multaneously. Using this concept, we report here the
titanium complex formed in situ upon the treatment
of ligand (R)-3,3ꢀ-bis(diphenylphosphinoyl)-BINOL
(L1) as a novel bifunctional catalyst in asymmetric
trimethylsilylcyanation of aldehydes. We envisioned
that the titanium would work as a Lewis acid to acti-
vate the carbonyl group, and the oxygen atom of the
phosphine oxide would function as a Lewis base to
activate the silylated nucleophiles.
base bifunctional Ti(IV) complex was formed
in situ upon the treatment of ligand (R)-3,3ꢀ-
bis(diphenylphosphinoyl)-BINOL with Ti(PrO-i)4 ,
which is proven to be an efficient catalyst in
the asymmetric trimethylsilylcyanation of aldehydes.
The corresponding cyanohydrins were obtained in
high to excellent chemical yield (83–93%) albeit
with moderate enantioselectivities (up to 51% enan-
ꢁ
C
tiomeric excess).
2010 Wiley Periodicals, Inc. Het-
eroatom Chem 22:31–35, 2011; View this article online
at wileyonlinelibrary.com. DOI 10.1002/hc.20652
INTRODUCTION
Optically active cyanohydrins are important inter-
mediates in organic synthesis for the synthesis of
a variety of valuable classes of chiral compounds,
such as α-amino acids, α-hydroxy carboxylic acids,
β-amino alcohols, vicinal diols, and α-hydroxy ke-
tones. Many efficient approaches have been reported
for their preparation by biochemical and chemical
methods [1–8]. In the latter case, the most impor-
tant one is the asymmetric silylcyanation of alde-
hydes with trimethylsilyl cyanide catalyzed by a
Lewis acid center (TiV, AlIII, VV, LaIII, MgII, and
RESULTS AND DISCUSSION
Following Ishihara’s procedure [38], (R)-3,3ꢀ-
bis(diphenylphosphinoyl)-BINOL (L1) was pre-
pared from commercially available (R)-BINOL in
two steps (Scheme 1). (R)-BINOL in THF was treated
with NaH (2.2 equiv) followed by the dropwise ad-
dition of diphenylphosphinic chloride (2.2 equiv),
to give the corresponding phosphinate (R)-1 quan-
titatively without further purification [39]. The re-
arrangement of (R)-1 upon treatment with lithium
diisopropylamide (10 equiv) in THF at –78◦C gave
L1 as a colorless crystal after being purified by
column chromatography (200–300 mesh, gradient
eluted with dichloromethane/ethyl acetate) [40].
Correspondence to: Z. Zhang; e-mail: ttxyx27@yahoo.com.cn
and zhangzhongbiao@hotmail.com.
Contract grant sponsor: Tianjin Natural Science Foundation
and Research Fund for Young Scientist, Tianjin Normal Univer-
sity of China.
Contract grant number: 08JCYBJC26900 and 52LX29.
2010 Wiley Periodicals, Inc.
c
ꢀ
31

32 Tang and Zhang
O
TABLE 2 Screening the Loading of Catalysts in the Asym-
metric Trimethylsilylcyanation of Benzaldehyde Catalyzed by
L1 /Ti(IV)
PPh2
NaH (2.2 equiv)
O
Ph2P(=O)Cl (2.2 equiv)
LDA (10 equiv)
OH
OH
OH
OH
OPPh2
OPPh2
O
OH
OSiMe3
THF, r.t., 3 h
> 99%
THF, –78oC, 3 h
51%
O
H+/H2O
L1/Ti(OPr-i)4
CN
*
PPh2
O
CN
*
H + Me3SiCN
(R)-BINOL
(R)-1
L1
THF, 0oC
2a
SCHEME 1 Preparation of L1.
Enantiomeric
L1
Ti(IV)
Yield
(%)a
Excess
With ligand L1 in hand, the influence of solvents
and temperature on the reaction was systematically
investigated first by using the reaction of benzalde-
hyde and trimethylsilylcyanide as a model in the
presence of 10 mol% L1 and 20 mol% Ti(PrO-i)4.
The results are presented in Table 1.
Entry (mol%) (mol%)
(%)b
Con.c
1
2
3
4
5
6
10
5
20
30
10
10
20
10
40
60
10
40
91
87
89
90
83
86
51
27
42
35
14
27
R
R
R
R
R
R
As shown in Table 1, the Ti(IV)/L1 catalytic
system exhibited high catalytic activity to the
trimethylsilylcyanation of benzaldehyde. The corre-
sponding cyanohydrin 2a was obtained in good to
excellent yields for all the solvents tested (entries
1–4, 87–93%). However, the solvents had a notable
effect on the enantioselectivity of the reaction. For
example, (R)-mandelonitrile was attained with an
enantioselectivity of 51% enantiomeric excess (ee)
when the reaction was performed in THF (entry 4),
whereas conducting the reaction in chloroform led
to (S)-mandelonitrile as the major isomer almost
with the same level of stereoselectivity (entry 3). Ei-
ther employment of methylene chloride or DMF as
the solvent resulted in total loss of stereocontrol to
afford cyanohydrin 2a in racemic form (entries 1
and 2). So we chose THF as the best solvent for this
reaction in terms of yield and enantioselectivity. By
Con. = Configuration.
aIsolated yield.
bDetermined by GC analysis of acetate of the cyanohydrin on a chiral
column.
c Determined by comparison of the sign of specific rotation value with
the literature [12,16,41].
comparison of the parallel reactions carried out at
different temperatures (20, 0, –20, –40◦C) (entries
4–7), it is demonstrated that 0◦C was the optimal re-
action temperature for the reaction, which gave the
highest enantioselectivity (51% ee, entry 4).
Other factors, such as catalyst loading and the
mole ratio of ligand L1 to Ti(OPr-i)4 influenc-
ing the reaction, were also preliminarily investi-
gated employing the reaction of benzaldehyde and
trimethylsilylcyanide as the model; the results are
presented in Table 2.
As shown in Table 2, although the catalyst load-
ing demonstrates a slight influence on the yield of
the cyanohydrin 2a, either increasing or lowering
the catalyst loading resulted in an obvious decrease
in the enantioselectivity of the reaction. In addition,
the mole ratio of Ligand L1 to Ti(OPr-i)4 was also
found to be an essential factor for this reaction. By
adjusting the mole ratio of L1 to Ti(OPr-i)4 from 1:2
to 1:4 or 1:1, both led to a dramatic decrease in enan-
tiomeric excess of values of the product (entries 1, 5,
and 6).
Based on the above-mentioned results, the re-
action was best conducted in THF at 0◦C in the
presence of the Ti(IV) complex in situ formed from
L1 (10 mol%) and Ti(OPr-i)4 (20 mol%). Under
these optimal reaction conditions, the asymmetric
trimethylsilylcyanation of a set of aldehydes was per-
formed to examine the substrate generality of this
reaction. The results are summarized in Table 3.
As shown in Table 3, the novel Ti(IV)/L1 catalytic
system demonstrated high efficacy in the transfor-
mation of aldehydes to cyanohydrins. In all cases,
TABLE 1 Influence of Solvents and the Reaction Tempera-
ture on the Asymmetric Trimethylsilylcyanation of Benzalde-
hyde
L1/Ti(OPr-i)4
OH
CN
OSiMe3
O
H+/H2O
(10/20 mol%)
*
CN
*
H + Me3SiCN
2a
Enantiomeric
Excess
Temperature Yield
Entry Solvent
(◦C)
(%)a
(%)b
Con.c
1
2
3
4
5
6
7
CH2Cl2
DMF
CHCl3
THF
THF
THF
0
0
0
93
87
90
91
88
90
92
4
0
50
51
0
32
50
S
–
S
R
–
R
R
0
−40
−20
20
THF
Con. = Configuration.
aIsolated yield.
bDetermined by GC analysis of the acetate of the cyanohydrin on a
chiral column.
c Determined by comparison of the sign of specific rotation value with
the literature [12,16,41].
Heteroatom Chemistry DOI 10.1002/hc

A Novel Bifunctional Ti(IV) Complex Catalyzed Asymmetric Silylcyanation of Aldehydes 33
13C NMR were determined with the help of either a
Bruker AMX-300 or a Varian 400 MHz instrument.
Chemical shifts were measured relative to residual
solvent peaks as an internal standard set to δ =
TABLE 3 Asymmetric Trimethylsilylcyanation of Aldehydes
Catalyzed by L1/Ti(OPr-i )4
OH
(1) L1 (10 mol%)/Ti(OPr-i)4 (20 mol%)
Me3SiCN
+
RCHO
2) H+/H2O
R
CN
*
2
1
7.26 and δ = 77.1 ppm (CDCl3) for H and 13C, re-
spectively. Specific rotations were measured with a
Perkin–Elmer 341MC polarimeter. Enantiomeric ex-
cesses were determined with the help of an Agilent
6890 instrument (β- DEX120 column). Elemental
analyses were conducted with a Yanaco CHN Corder
MT-3 automatic analyzer. Melting points were deter-
mined with the help of a T-4 melting point apparatus
and are uncorrected.
Yield
Enantiomeric
Excess (%)b
Entry
R
(%)a
Con.c
1 (2a)
2 (2b)
3 (2c) 2-MeOC6H4
4 (2d) 4-MeOC6H4
5 (2e)
6 (2f)
7 (2g)
8 (2h)
9 (2i)
C6H5
4-ClC6H4
89
88
93
85
90
91
88
85
76
51
22
11
19
27
35
21
5
R
R
R
R
R
R
R
R
R
2-MeC6H4
3-MeC6H4
2-Naphthyl
PhCH2CH2
2-Furyl
Following Ishihara’s procedure [38], (R)-3,3ꢀ-
Bis(diphenylphosphinoyl)-BINOL (L1) was pre-
pared from commercially available (R)-BINOL in
two steps.
9
Con. = Configuration.
aIsolated yield.
(R)-1,1ꢀ -Binaphthalene-2,2ꢀ -bis(diphenylphos-
phinate)((R)-1) [39].31P NMR (δ, CDCl3) 39.89 ppm.
(R)-3,3ꢀ-Bis(diphenylphosphinoyl)-BINOL (L1)
was purified by column chromatography (200–300
mesh, gradient eluted with dichloromethane/ethyl
acetate) [40]. Yield: 51%. mp > 280◦C, [α]D20 +93.2◦
bDetermined by GC analysis of the acetate of the cyanohydrin on a
β-DEX120 column.
c Absolute configuration was assigned by comparison of the sign of
specific rotation value with the literature [12,16,41].
the corresponding products were obtained in good
to excellent yield (76–93%). Whereas, the intro-
duction of either electron-withdrawing (entry 2) or
electron-donating substituents (entries 3–6) on the
benzene ring disfavored the stereocontrol of the re-
action and the ee values decreased markedly. The
best result (51% ee, 2a) was observed in the case
of nonsubstituted benzaldehyde. Aliphatic aldehyde,
3-phenylpropanal, and heteroaromatic aldehydes, 2-
furaldehyde, also worked well in this transformation
to provide cyanohydrins 2h and 2i in good yield al-
beit with quite poor enantioselectivities (Table 3, en-
tries 8 and 9).
1
(c = 1.31, CHCl3). H NMR (δ, CDCl3): 7.22–7.31
(m, 8H, Ar-H), 7.51–7.62 (m, 11H, Ar-H), 7.72–7.84
(m, 11H, Ar-H), 10.58 (broad, 2H, OH). 31P NMR (δ,
CDCl3) 40.69 ppm.
General Procedure for L1/Ti(OPr-i)4-Catalyzed
Asymmetric Silylcyanation of Aldehydes
A solution of L1 (689 mg, 10 mmol%) in THF
(3 mL) was stirred in a Pyrex Schlenk tube at room
temperature under a nitrogen atmosphere. To the
solution, Ti(OPr-i)4 (60 μL, 0.4 mmol) was added
with a syringe pump after the complete dissolution
of L1. This solution was stirred for 2 h at room
temperature then cooled to 0◦C, aldehyde (2 mmol)
and trimethylsilyl cyanide (400 mg, 4 mmol) were
added. After stirring for 24 h at this temperature,
the mixture was poured into a mixture of HCl (1 N,
15 mL) and ethyl acetate (30 mL) and was stirred
for 4 h at room temperature. The organic layer was
washed with distilled water and saturated NaHCO3
(each 10 mL) and dried with anhydrous sodium sul-
fate. The solution was concentrated and purified
by column chromatography (200–300 mesh, gradi-
ent eluted with petroleum ether/ethyl acetate) to
yield the expected cyanohydrin. The pure cyanohy-
drin was converted directly into the corresponding
acetates by treatment with acetic acid anhydride
(2 equiv) and pyridine (2 equiv) in CH2Cl2 (20 mL) at
room temperature for 12 h. The separated organic
layer was washed with 5% H2SO4, distilled water,
and saturated NaHCO3 (each 10 mL), then dried with
CONCLUSION
In summary, asymmetric trimethylsilylcyanation of
aldehydes was realized in the presence of a novel
Lewis acid and Lewis base bifunctional Ti(IV) com-
plex formed in situ upon the treatment of ligand
(R)-3,3ꢀ bis(diphenylphosphinoyl)-BINOL (L1) with
Ti(PrO-i)4. The corresponding cyanohydrins were
obtained in high yield (76–93%) with moderate enan-
tioselectivities (up to 51% ee). Although the results
were unsatisfactory, this concept could provide a
guide for designing new asymmetric catalysts for
the reaction of a variety of nucleophiles, including
silylated ones, with carbonyl compounds.
EXPERIMENTAL
General Methods
All reagents and solvents were commercial grade
1
and purified prior to use when necessary. H and
Heteroatom Chemistry DOI 10.1002/hc

34 Tang and Zhang
anhydrous sodium sulfate and concentrated. The
crude acetate was purified by column chromatogra-
phy (200–300 mesh; petroleum ether/ethyl acetate,
5:1) to provide the acetylated cyanohydrin, which
was used for further chiral GC analysis.
acetate, β-DEX120 column): tR = 155.49 (major),
151.58 (minor) min.
(R)-(–)-2-Hydroxy-4-phenylbutanenitrile
(2h)
[41]. Colorless oil. Yield: 143 mg, 89%. [α]D20
=
1
–0.58 (c = 1.13, CHCl3), 5% ee. H NMR (300 MHz,
CDCl3): δ = 2.15–2.21 (m, 2H, CH2), 2.61 (br s,
1H, OH), 2.83–2.88 (m, 2H, CH2), 4.43 (t, J = 6.9
Hz, 1H, CH), 7.21–7.24 (m, 3 Harom), 7.30–7.34
(m, 2 Harom) ppm. GC (corresponding acetate,
β-DEX120 column): tR = 67.38 (major), 65.59
(minor) min.
(R)-(+)-Mandelonitrile (2a) [41]. Colorless oil.
Yield: 118 mg, 89%. [α]2D0 = +20.6 (c = 1.15, CHCl3),
1
51% ee. H NMR (400 MHz, CDCl3): δ = 3.19 (br s,
1H, OH), 5.53 (s, 1H, CH), 7.43–7.45 (m, 3 Harom),
7.51–7.53 (m, 2 Harom) ppm. GC (corresponding ac-
etate, β-DEX120 column): tR = 23.47 (minor), 22.35
(major) min.
(R)-(–)-2-(Furan-2-yl)-2-hydroxyacetonitrile (2i)
[41]. Colorless oil. Yield: 111 mg, 90%. [α]D20
= –3.91 (c = 1.10, CHCl3), 9% ee. 1H NMR
(400 MHz, CDCl3): δ = 4.31 (br s, 1H, OH),
5.57 (s, 1H, CH), 7.27–7.71 (m, 3 Harom) ppm.
GC (corresponding acetate, β-DEX120 column):
tR = 26.13 (major), 28.53 (minor) min.
(R)-(+)-α-Hydroxy-4-chlorophenylacetonitrile (2b)
[41]. Colorless oil. Yield: 147 mg, 88%. [α]D20
=
+8.9 (c = 1.00, CHCl3), 22% ee. 1H NMR (400
MHz, CDCl3): δ = 4.28 (br s, 1H, OH), 5.52 (s,
1H, OH), 7.43 (d, J = 8.8 Hz, 2 Harom), 7.47 (s, J
= 8.8 Hz, 2 Harom) ppm. GC (corresponding ac-
etate, β-DEX120 column): tR = 66.28 (minor), 63.46
(major) min.
(R)-(+)-α-Hydroxy-2-methoxyphenylacetonitrile
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