Titanium(IV)(salen) and yanadium(V)(salen) complexes derived from C 2- and C1-symmetric diamines for asymmetric cyanohydrin synthesis

Article abstract of DOI:10.1055/s-2008-1077900

Titanium and vanadium salen complexes have been prepared from C ₂- and C₁-symmetric acyclic diamines. All of the complexes catalysed the asymmetric addition of trimethylsilyl cyanide to benzaldehyde and the sense of asymmetric induction was determined by the nature of the substituents. The vanadium complex of a valine-derived diamine gave good results with a range of aromatic and aliphatic aldehydes. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-2008-1077900

2150
LETTER
Titanium(IV)(salen) and Vanadium(V)(salen) Complexes Derived from
C2- and C1-Symmetric Diamines for Asymmetric Cyanohydrin Synthesis
A
symmetric Cyano
u
hydrin Synt
r
hesis
U
s
i
ing
M
etal(sal
N
en) Complexes . Belokon’,b Jamie Hunt,a Michael North*a
a
School of Chemistry and University Research Centre in Catalysis and Intensified Processing, Bedson Building, Newcastle University,
Newcastle upon Tyne, NE1 7RU, UK
Fax +44(870)1313783; E-mail: Michael.north@ncl.ac.uk
A.N. Nesmeyanov Institute of Organo-Element Compounds, Russian Academy of Sciences, Vavilov 28, 119991 Moscow, Russian
Federation
b
Received 9 April 2008
Dedicated to Sir Jack E. Baldwin on the occasion of his 70th birthday
OX
CN
O
1 or 2
Abstract: Titanium and vanadium salen complexes have been pre-
pared from C2- and C1-symmetric acyclic diamines. All of the com-
plexes catalysed the asymmetric addition of trimethylsilyl cyanide
to benzaldehyde and the sense of asymmetric induction was deter-
mined by the nature of the substituents. The vanadium complex of
a valine-derived diamine gave good results with a range of aromatic
and aliphatic aldehydes.
+
XCN
R
R
H
Scheme 1 Asymmetric cyanohydrin synthesis using catalysts 1
and 2
tBu
tBu
Key words: cyanohydrins, homogenous catalysis, asymmetric ca-
talysis, Schiff bases, titanium
tBu tBu
O
O
N
N
O
N
N
Over the last twelve years,1 we have developed titani-
um(IV) 1 and vanadium(V) 2a,b salen complexes derived
from cyclohexanediamine as highly effective catalysts for
the asymmetric addition of trimethylsilyl cyanide,1,2 metal
cyanides1,3 (in the presence of an anhydride), and
cyanoformates1,4 to aldehydes (Scheme 1). Complexes 1
and 2 are active at room temperature; at substrate-to-cata-
lyst ratios of up to 1000:1 and give products with ee >90%
from a range of aromatic aldehydes. Aliphatic aldehydes
are also accepted as substrates, though with reduced enan-
tioselectivities (70–90%) and complex 1 will also catalyse
the asymmetric addition of trimethylsilyl cyanide to ke-
tones.1,5 Complexes 1 and 2 have been commercialized6
by NPIL under the trademark CACHy.
Ti
Ti
O
O
O
tBu tBu
1
tBu
tBu
O
V
N
N
O
tBu
O
tBu
X
tBu
tBu
2a: X = EtOSO3
2b: X = Cl
We have previously reported1,2a the influence on the enan-
tioselectivity of the substituents on the aromatic rings of
catalysts 1 and 2 (Figure 1) and shown that the presence
of 3,5-di-tert-butyl groups gives the highest levels of
asymmetric induction. However, all our previous work in
this area has employed (R,R)-1,2-diaminocyclohexane as
a readily available, C2-symmetrical diamine,7 and in every
case, this resulted in addition of cyanide to the re face of
the aldehyde. In this letter we report the synthesis of salen
ligands derived from a range of acyclic diamines and dis-
cuss their catalytic activity in the asymmetric addition of
trimethylsilyl cyanide to benzaldehyde.
Figure 1 Salen complexes for asymmetric cyanohydrin synthesis
cially available and diamines 3b–e were prepared by a
general literature procedure8 as shown in Scheme 2. Di-
amines 3a–e were condensed with 3,5-di-tert-butylsali-
cylaldehyde to give ligands 4a–e. Ligand 4a has
previously been complexed to titanium(IV) isopropoxide
by Jiang and the resulting in situ prepared complex (20
mol%) used to catalyse the asymmetric addition of trime-
thylsilyl cyanide to benzaldehyde with just 39% ee after a
reaction time of 24 hours at –78 °C.9
Initially, diamines 3a–e were selected as representative
acyclic diamines, chosen to probe any influence of steric
effects within the salen ligand. Compound 3a is commer-
Ligands 4a–e were complexed to titanium tetrachloride to
give dichloride complexes 5a–e. Attempts to convert
complexes 5a–e into bimetallic complexes analogous to
structure 1 were problematic due to competing ligand-hy-
drolysis reactions. Instead, complexes 5a–e were utilised
as precatalysts as they will be converted into bimetallic
complexes in situ by adventitious moisture.2a,10 Ligands
SYNLETT 2008, No. 14, pp 2150–2154
2
2
.0
8
.2
0
0
8
Advanced online publication: 15.07.2008
DOI: 10.1055/s-2008-1077900; Art ID: D11308ST
© Georg Thieme Verlag Stuttgart · New York

LETTER
Asymmetric Cyanohydrin Synthesis Using Metal(salen) Complexes
2151
Table 1 Synthesis of Mandelonitrile Trimethylsilyl Ether Using
Catalysts 5 and 6
allylMgBr or MeLi
–78 to 20 °C
Me
Ph
or tBuMgCl or iPrMgCl
50 °C
R
R
NH
NH
Me
N
Me
Ph
Catalyst (mol%)
5a (0.1)
5a (1.0)
5a (2.0)
5a (10.0)
6a (0.1)
6a (1.0)
6a (2.0)
5b (0.1)
5b (1.0)
5b (2.0)
6b (0.1)
6b (1.0)
6b (2.0)
5c (0.1)
5c (1.0)
5c (2.0)
5c (10.0)
6c (0.1)
6c (1.0)
6c (2.0)
5d (0.1)
Conversion (%)
ee (%) (config)a
52 (S)
38 (S)
31 (S)
20 (S)
68 (S)
80 (S)
77 (S)
26 (R)
26 (R)
26 (R)
28 (R)
20 (R)
20 (R)
4 (R)
N
14–80%
Ph
64
78
89
100
83
100
100
7
Me
Ph
tBu
20% Pd(OH)2/C, EtOH
10 atm H2, 3 d
tBu
3,5-di-tert-butylsalicylaldehyde
(2 equiv), EtOH, reflux, 18 h
R
R
NH2
NH2
R
R
N
N
OH
OH
R = Ph, 100% (1 step)
R = Me, Pr, iPr, tBu
66–87% (2 steps)
tBu
3a–e
3–6a: R = Ph
3–6b: R = Me
3–6c: R = Pr
3–6d: R = iPr
3–6e: R = tBu
tBu
20
34
16
58
66
12
27
42
100
78
80
84
21
37
44
12
55
59
50
62
77
57
72
77
4a–e
TiCl4 (1 equiv)
CH2Cl2, 3 h, r.t.
VOSO4 (1.8 equiv), EtOH,
reflux, O2, 3 h
100%
42–83%
tBu
tBu
tBu
tBu
O
N
N
R
O
O
N
N
R
R
Cl
Ti
Cl
EtOSO3
O
V
6 (R)
O
R
tBu
tBu
14 (R)
30 (R)
53 (R)
62 (R)
62 (R)
2 (R)
tBu
5a–e
tBu
6a–e
Scheme 2 Synthesis of catalysts 5a–e and 6a–e
4a–e were also treated with vanadyl sulfate in the pres-
ence of oxygen to give vanadium(V)(salen) complexes
6a–e. Each of complexes 5 and 6 was evaluated as a cata- 5d (1.0)
2 (R)
lyst for the asymmetric addition of trimethylsilyl cyanide
5d (2.0)
2 (R)
to benzaldehyde under standard conditions.11 The results
of this study are reported in Table 1.
6d (0.1)
6d (1.0)
6d (2.0)
5e (0.1)
5e (1.0)
5e (2.0)
6e (0.1)
6e (1.0)
6e (2.0)
6 (R)
Complex 5a gave only moderate levels of asymmetric in-
duction, comparable to those reported by Jiang for the ti-
tanium(IV) isopropoxide complex of the same ligand.9
However, complex 5a was intrinsically more active than
Jiang’s catalyst as only 1.0 mol% of catalyst was required
to achieve 78% conversion of benzaldehyde into mande-
lonitrile trimethylsilyl ether. The increase in enantioselec-
tivity as the amount of complex 5a is reduced, mirrors the
effect previously seen with the corresponding complex
derived from 1,2-diaminocyclohexane and is indicative of
the in situ formation of a bimetallic complex analogous to
structure 1.2a The corresponding vanadium complex 6a
was intrinsically more enantioselective than titanium
complex 5a as previously observed for complexes 1 and
2.1,2 In addition, in this case the enantioselectivity in-
creased as the amount of catalyst was increased from 0.1
to 1.0 mol% as no in situ hydrolysis is needed to form cat-
alytically active bimetallic species.
23 (R)
23 (R)
27 (R)
50 (R)
41 (R)
26 (R)
52 (R)
56 (R)
a Determined by chiral GC using the conditions reported in refs. 2–4
after conversion of the mandelonitrile trimethylsilyl ether into man-
delonitrile acetate by the method of Kagan.12
The most notable feature of catalysis by complexes 5b–e
is the inversion of enantioselectivity. This is the first time
Synlett 2008, No. 14, 2150–2154 © Thieme Stuttgart · New York

2152
Y. N. Belokon’ et al.
LETTER
7–9a: R = Me (S)
b: R = Bn (S)
c: R = Ph (R)
d: R = iPr (S)
R
N
X
X
M
X
H
R
R
O
O
N
N
R
R
N
O
O
N
N
M
X
H
H
H
tBu
OH OH
tBu
gauche (R pseudo-equatorial)
anti (R pseudo-axial)
tBu
tBu
7a–d
9a–c: 1) VOSO4, THF–EtOH, reflux, 3 h
TiCl4 (1 equiv)
CH2Cl2, 3 h,
r.t.
2) (NH4)2Ce(NO3)6, MeCN, 20 min, r.t.
X
H
O
R
9a: 50%
3) 1 N HCl (aq)
9b: 82%
N
N
9d: VOCl3, THF, 30 min, r.t.
X
O
X
H
H
R
R
N
N
M
M
O
O
9c: 90%
9d: 90%
100%
tBu
tBu
tBu
R
X
H
gauche (∆ cis-β favoured)
anti (Λ cis-β favoured)
tBu
tBu
Figure 2 Gauche and anti conformations of a salen ligand
N
O
N
O
N
O
Cl Ti Cl
V
Cl
(salen)M CN
R
N
O
R
O
tBu
O
CN
H
R
M(salen)
–
O
tBu
8a–d
tBu
9a–d
Figure 3 Stereochemistry inducing transition state for asymmetric
cyanohydrin synthesis using metal(salen) complexes
Scheme 3 Synthesis of catalysts 8a–d and 9a–d
that salen ligands derived from an R,R-diamine have been
found to catalyse the asymmetric addition of cyanide to
the si face of benzaldehyde. The magnitude of the asym-
metric induction using catalysts 5b–d was very low (up to
30%), whilst complex 5e gave more respectable enantio-
selectivities (up to 50%). Both the magnitude and the
sense of asymmetric induction can be explained on the ba-
sis of the diamine unit within the salen ligand, which can
adopt either a gauche or an anti conformation (Figure 2).
Cyclic diamines such as 1,2-diaminocyclohexane can
only adopt the gauche conformation, but for acyclic di-
amines the anti conformation is generally thermodynami-
cally preferred as it minimises steric interactions between
the R groups; and between the R groups and the imine hy-
drogens. Whilst a salen ligand normally prefers to be pla-
nar, it is well known that it can also adopt a cis-b
configuration,13 and this is necessary to allow formation
of the established bimetallic transition state for asymmet-
ric cyanohydrin synthesis in which cyanide is transferred
intramolecularly to the coordinated aldehyde (Figure 3).1
lytically active complexes with both D- and L-
configurations being formed. Crystal structures of copper
and chromium complexes derived from ligand 4e show
that the diamine unit is locked into the anti conformation
to avoid steric interactions between the two tert-butyl
groups.14 Thus in this case, exclusive formation of the L-
configuration of the titanium(salen) unit would be predict-
ed, consistent with the significantly higher asymmetric in-
duction observed with this complex. Similar reasoning
applies to vanadium-based complexes 6b–e, though in
this case catalyst 6c exhibited surprisingly high reactivity
and enantioselectivity and was a better catalyst even than
complex 6e.
To investigate further the influence of substituents within
the diamine unit on the catalytic activity, a series of four
monosubstituted diamine-derived salen ligands 7a–d was
prepared from the corresponding amino acids.15 Ligands
7a–d were then complexed to both titanium and vanadium
to give complexes 8a–d and 9a–d, respectively, as shown
in Scheme 3. Each of complexes 8a–d and 9a–d was then
tested as a catalyst for the asymmetric addition of trimeth-
ylsilyl cyanide to benzaldehyde under the standard
conditions11 (Table 2). All of the titanium complexes
8a–d gave disappointing enantioselectivities and low con-
versions, neither of which was significantly improved by
increasing the catalyst loading [best result 54% ee (S) and
61% conversion using 2 mol% of catalyst 8d]. The vana-
dium(V) complexes 9a–d gave more promising results
with complex 9d giving particularly good asymmetric in-
duction.
When applied to octahedral complexes, with a salen
ligand in the cis-b configuration, the consequence of
changing the conformation of the diamine unit from
gauche to anti is to change the preferred stereochemistry
of the metal(salen) unit from D to L. This is due to the
‘twist’ of the diamine unit, which determines the overall
‘twist’ of the salen ligand (Figure 2). Thus, for complexes
5b–e it appears that the salen ligand prefers to adopt the
anti conformation, resulting in a L-configuration around
the metal centres and thus inverting the enantioselctivity
of asymmetric cyanohydrin synthesis compared to com-
plexes 1, 2, 5a, and 6a.
Complexes 8b–d and 9a–d all catalysed the addition of
cyanide to the re face of benzaldehyde, giving (S)-mande-
lonitrile trimethylsilyl ether. However, complexes 7c and
The low levels of asymmetric induction observed with
catalysts 5b–d may be due to the diamine adopting a mix-
ture of gauche and anti conformations, resulting in cata-
Synlett 2008, No. 14, 2150–2154 © Thieme Stuttgart · New York

LETTER
Asymmetric Cyanohydrin Synthesis Using Metal(salen) Complexes
2153
Table 2 Synthesis of Mandelonitrile Trimethylsilyl Ether Using
Catalysts 8 and 9
Table 3 Synthesis of Cyanohydrin Trimethylsilyl Ethers Using Cat-
alyst 9d (0.1 mol%)
Catalyst (0.1 mol%) Conversion (%)
ee (%) (config)a
30 (R)
4 (S)
Aldehyde
Conversion (%)
ee (%) (config)a
78 (S)
8a
8b
8c
8d
9a
9b
9c
9d
9
3-MeC6H4CHO
4-MeC6H4CHO
4-MeOC6H4CHO
4-F3CC6H4CHO
CyCHO
86
20
27
31
57
24
53
100
90
78 (S)
20 (S)
100
100
91
45 (S)
46 (S)
77 (S)
4 (S)
73 (S)
62 (S)
C8H17CHO
93
73 (S)
32 (S)
Me2CHCHO
Me3CCHO
85
73 (S)
81 (S)
100
45 (S)
a Determined by chiral GC using the conditions reported in refs. 2–4
after conversion of the mandelonitrile trimethylsilyl ether into man-
delonitrile acetate by the method of Kagan.12
a Determined by chiral GC using the conditions reported in ref. 2–4
after conversion of the mandelonitrile trimethylsilyl ether into man-
delonitrile acetate by the method of Kagan.12
8c were derived from (R)-phenylglycine whilst all of the
other complexes were derived from S-amino acids. There
is only limited structural information available on com-
plexes derived from ligands 7a,c and none on the com-
plexes of ligands 7b,d. However, whilst the only crystal
structure of a salen complex derived from phenylgly-
cinamine shows that the phenyl ring adopts a pseudo-
equatorial position,16 crystal structures of vanadi-
um(salen) complexes derived from 2,3-diaminopropane
indicate that the methyl group can adopt either a pseudo-
axial or a pseudo-equatorial position.17 Thus, the inver-
sion of enantioselectivity observed for complexes 7c and
8c may again be due to a change in the preferred confor-
mation of the salen ligand.
Acknowledgment
The authors thank the EPSRC national mass spectrometry service at
the University of Wales, Swansea for recording mass spectra.
References and Notes
(1) For reviews of the development and applications of these
catalysts, see: (a) North, M. Tetrahedron: Asymmetry 2003,
14, 147. (b) Achard, T. R. J.; Clutterbuck, L. A.; North, M.
Synlett 2005, 1828.
(2) (a) Belokon’, Y. N.; Caveda-Cepas, S.; Green, B.;
Ikonnikov, N. S.; Khrustalev, V. N.; Larichev, V. S.;
Moscalenko, M. A.; North, M.; Orizu, C.; Tararov, V. I.;
Tasinazzo, M.; Timofeeva, G. I.; Yashkina, L. V. J. Am.
Chem. Soc. 1999, 121, 3968. (b) Belokon’, Y. N.; North,
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Y. N.; Green, B.; Ikonnikov, N. S.; North, M.; Parsons, T.;
Tararov, V. I. Tetrahedron 2001, 57, 771. (d) Belokon’,
Y. N.; Maleev, V. I.; North, M.; Usanov, D. L. Chem.
Commun. 2006, 4614.
The high catalytic activity of complex 9d can be account-
ed for by the chloride counterion as we have previously
shown that this has a significant influence on the catalytic
activity of the vanadium-based catalysts.2d However, the
high level of asymmetric induction obtained using cata-
lyst 9d was surprising given the poor results obtained with
catalysts 5d, 6d, and 8d which also contain isopropyl sub-
stituents. Therefore, catalyst 9d was screened with eight
other aldehydes as shown in Table 3. With the exception
of the electron-rich 4-methoxybenzaldehyde, all of the ar-
omatic aldehydes studied gave enantioselectivities of 77–
81%. Similarly, with the exception of the sterically hin-
dered pivaldehyde, all of the aliphatic substrates gave a
cyanohydrin derivative with 73% ee. This trend in reactiv-
ity is the same as that previously reported for catalyst
2a.2b,c
(3) Belokon’, Y. N.; Blacker, A. J.; Clutterbuck, L. A.; Hogg,
D.; North, M.; Reeve, C. Eur. J. Org. Chem. 2006, 4609; and
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E.; Nomura, H.; North, M. Chem. Commun. 2006, 1775.
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In conclusion, it has been shown that the nature of the sub-
stituent(s) within the diamine of a salen ligand influences
on which one of the two enantiotopic faces of an aldehyde
the reaction occurs during metal(salen)-catalysed asym-
metric cyanohydrin synthesis. This effect can be traced
back to the preferred configuration of the salen ligand
around the metal ion.
Synlett 2008, No. 14, 2150–2154 © Thieme Stuttgart · New York

2154
Y. N. Belokon’ et al.
LETTER
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(11) Benzaldehyde (0.10 mL, 0.98 mmol) and catalyst 5, 6, 8, or
9 (0.1–10 mol%) were dissolved in freshly distilled CH2Cl2
(5 mL). Trimethylsilyl cyanide (0.14 mL, 1.08 mmol, 1.1
equiv) was added, and the reaction was stirred at r.t.
overnight. The resulting solution was filtered through a pad
of SiO2 using CH2Cl2 as eluent. Solvent was removed in
vacuo to give mandelonitrile trimethylsilyl ether as a yellow
oil. The conversion was determined by 1H NMR
spectroscopy. The mandelonitrile trimethylsilyl ether and
Sc(OTf)3 (1 mol%) were then dissolved in MeCN (5 mL),
and Ac2O (2.0 equiv) was added. The reaction mixture was
stirred at r.t. for 20 min, then filtered through a pad of SiO2
using CH2Cl2 as eluent. Solvent was removed in vacuo to
give mandelonitrile acetate as a yellow oil which was
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Synlett 2008, No. 14, 2150–2154 © Thieme Stuttgart · New York