Chiral bispyridylamide metal complexes as catalysts for the enantioselective addition of TMSCN to aldehydes

Article abstract of DOI:10.1016/j.jorganchem.2004.06.014

The use of (1R,2R)-N,N′-bis(2-pyridinecarboxyamido)-1,2- diphenylethane metal complexes as catalysts for the enantioselective addition of trimethylsilyl cyanide to aldehydes is described. Enantioselectivities up to 70% ee were obtained with a Ti(IV) catal

Full text of DOI:10.1016/j.jorganchem.2004.06.014

Journal of Organometallic Chemistry 689 (2004) 3750–3755
www.elsevier.com/locate/jorganchem
Chiral bispyridylamide metal complexes as catalysts for the
enantioselective addition of TMSCN to aldehydes
a,
a
b
a,
*
Oscar Belda *, Sophie Duquesne , Andreas Fischer , Christina Moberg
a
Department of Chemistry, Organic Chemistry, KTH, Teknikringen 36 Plan 6, SE-100 44 Stockholm, Sweden
b
Department of Chemistry, Inorganic Chemistry, KTH, SE-100 44, Stockholm, Sweden
Received 3 May 2004; accepted 7 June 2004
Available online 10 July 2004
Abstract
The use of (1R,2R)-N,N0-bis(2-pyridinecarboxyamido)-1,2-diphenylethane metal complexes as catalysts for the enantioselective
addition of trimethylsilyl cyanide to aldehydes is described. Enantioselectivities up to 70% ee were obtained with a Ti(IV) catalyst.
Complexes with Zr(IV), Sc(III), Yb(III) and Cu(II) afforded less selective catalysts. For the Zr(IV) complex, a rate and selectivity
enhancement was observed when adding 0.5 equiv. of water with respect to the catalyst. Studies of the metal complexes involved in
1
the reaction were carried out by means of H NMR spectroscopy. A Zr complex was shown by X-ray crystallography to exhibit
distorted octahedral coordination, with the four nitrogen atoms of the doubly deprotonated ligand essentially in one plane.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Pyridylamide; Asymmetric catalysis; Cyanohydrins; Lewis acid
1. Introduction
ring opening of cyclohexene oxide with good enantiose-
lection [5].
The asymmetric addition of cyanide to carbonyl
groups is a topic of major interest because the resulting
enantiomerically enriched cyanohydrins can be trans-
formed into synthetically versatile building blocks with-
out loss of the optical purity [1]. Enzymatic methods,
chiral Lewis bases and chiral Lewis acids have been used
to induce asymmetry in the reaction [2]. Tetradentate sa-
len ligands [3,4] have been much studied in this reaction,
and they have become the basis of some of the most ef-
ficient and versatile asymmetric catalysts for the synthe-
sis of cyanohydrins by means of asymmetric Lewis acid
catalysis. Our group has studied the use of another type
of tetradentate ligands, bispyridylamides, in asymmetric
catalysis and showed that complexes of bispyridylamide
1 (Fig. 1) with group IV metal alkoxides, catalyzed the
Herein, we describe the use and achievements of di-
verse ligand 1–metal complexes in the enantioselective
addition of cyanide to aldehydes.
2. Results and discussion
In order to determine the best conditions for the reac-
tion, we used benzaldehyde as model substrate and
Ti(OiPr)4 as the Lewis acid (Eq. (1)). As for the cyanat-
ing reagent, we employed trimethylsilyl cyanide
(TMSCN), a widely used source of cyanide in reactions
affording silylated cyanohydrins [6,7]
ð1Þ
*
Corresponding authors. Tel.: +4687908128; fax: +4687912333.
E-mail address: belda@kth.se (O. Belda).
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.06.014

O. Belda et al. / Journal of Organometallic Chemistry 689 (2004) 3750–3755
3751
Fig. 1. Structure of bispyridylamide 1.
First we examined the effect of the solvent on the re-
action outcome when using 5 mol% of the catalyst with
respect to benzaldehyde (Table 1) and found that meth-
ylene chloride was a better solvent for the reaction than
THF, EtOAc and toluene, affording the product in al-
most quantitative yield and with 60% ee after 4 h reac-
tion time. In dry acetonitrile, almost complete
conversion was obtained after already 1 h and the prod-
uct displayed also in this reaction 60% ee. When THF or
acetonitrile was used as solvent, the addition of CH2Cl2
was needed in order to dissolve the ligand.
Next, the influence on the reaction of the amount of
catalyst relative to benzaldehyde, the metal to ligand ra-
tio and the concentration of benzaldehyde at the start of
the reaction ([PhCHO]0) was explored. Dichlorometh-
ane was used as solvent, in order to keep the system as
anhydrous as possible (Table 2).
Fig. 2. Variation of the ee with time.
While changes in the metal to ligand ratio had no sig-
nificant effect (entries 1–4), both the relative amount of
catalyst and the initial concentration of benzaldehyde
had a clear effect on the outcome of the reaction. With
1–2.5% mmol of catalyst with respect to benzaldehyde
and an initial concentration of benzaldehyde between
5 and 10 M, trimethylsilyl protected mandelonitrile 2
was obtained in high yield and with 70% ee (entries 5
and 6). Dilution of the reaction mixture had a negative
effect on the enantioselectivity, but yields were still high
(entries 7–9).
Cooling the reaction mixture to 0 ꢁC did not improve
the ee of the cyanohydrin and performing the reaction at
100 ꢁC in a microwave cavity afforded 91% yield of the
product with merely 34% ee.
In the course of this optimization, we observed that
the enantiomeric excess increased with time (Fig. 2). A
possible explanation to this effect might be that a cata-
lyst exhibiting higher enantioselectivity may be formed
from the enantiomerically enriched product.
To test this hypothesis, a small amount of TMS-pro-
tected mandelonitrile (2) with 70% ee was added
together with the catalyst and the reagents in the begin-
ning of the reaction. However, this had no noticeable
effect on the enantioselectivity (Table 3, entry 1). Neither
was any effect observed when 70% ee mandelonitrile was
added instead (entry 2).
Table 1
Effect of the solvent on the reactiona (Eq. (1))
Entry
Solvent
t (h)
Yield (%)b
ee (%)
1
2
3
4
5
CH2Cl2
THF:CH2Cl2 4:1
EtOAc
4
1
1
1
1
>95
15
60
4 (R)
28
60
16
93
CH3CN:CH2Cl2 5:1
Toluene
11
2
a
Reactions were carried out with 5 mol% of Ti(OiPr)4 and 1 and
initial concentration of PhCHO=1 M.
b
GC yields.
Table 2
Effect of the amount of Ti(OiPr)4 and 1 relative to benzaldehyde and the concentration of benzaldehyde in the beginning of the reaction (Eq. (1))
Entry
mol% Ti(iPrO)4
mol% 1
[PhCHO]0 (M)
Yield (%)a
ee (%)
1
2
3
4
5
6
7
8
9
a
5
10
10
5
5
10
5
1
1
>95
>95
60
60
60
60
70
70
66
63
57
1
90
>95
>95 (86)b
>95
>95
10
1
1
1
10
5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
0.5
0.33
0.25
>95
>95
Determined by GC.
Isolated yield.
b

3752
O. Belda et al. / Journal of Organometallic Chemistry 689 (2004) 3750–3755
Table 3
Influence of addition of 2 on the reactiona (Eq. (1))
It has been shown in similar systems, that bimetallic
species, in which Ti activates both TMSCN and the al-
dehyde, are present in the catalytic cycle. The proposed
key step is the intramolecular transfer of cyanide to the
coordinated aldehyde [9]. A similar mechanism, with
dual activation of the nucleophile and the electrophile,
may be operating for our catalyst as well, even though
conclusive evidence cannot be drawn from the NMR
data only.
Entry
mol% Added 2
Yield (%)c
ee (%)
1
2
a
2.16
1.64b
>95
>95
60 (S)
60 (S)
Reactions were carried out with 5 mol% of Ti(OiPr)4 and 1 and
70% ee 2 as additive.
b
Deprotected alcohol from 2.
GC yield.
c
When using the catalytic system for other aldehydes
than benzaldehyde (Eq. (2)), the respective products
were generally obtained with lower enantioselectivity
and reaction times were in the same range or longer
(Table 4).
A second hypothesis was formulated on the supposi-
tion that a more enantioselective catalyst might form as
a result of the decrease in concentration of benzaldehyde
during the reaction. To test this assumption we per-
formed the reaction by slowly adding the substrate,
benzaldehyde, to the reaction mixture in such a way that
the concentration of benzaldehyde was essentially con-
stant during the reaction. The enantiomeric excess of
the product obtained in this way was the same as when
the reaction was run under the standard conditions,
however.
It has been observed in similar systems, that slow dif-
fusion of water into the reaction mixture resulted in an
increase of the ee of the product with time, due to the
formation of a dimeric Ti complex with O-bridges that
was more active and selective in the reaction than the in-
itial monomeric complex [8]. We added small amounts
of water to the reaction mixture but the product 2 was
then obtained with lower ee (30% ee with 0.5% water,
and 0% ee with 1.2% water).
At this point, we decided to study the complexes that
were involved in the reaction by NMR spectroscopy to
see whether the increase in ee with time was caused by
slow formation of different catalytic species. When mix-
ing one equivalent of ligand 1 with one equivalent of
Ti(OiPr)4 no reaction occurred. In order to form a
Ti(IV) complex with ligand 1, TMSCN needed to be
present, as evidenced by these studies. Indeed, TMSCN
reacted with Ti(OiPr)4, as shown by the disappearance
of the 1H NMR signals for the isopropyl groups of
Ti(OiPr)4 at d 1.23 (doublet) and 4.47 ppm (heptet)
and the appearance of new signals at d 1.03 (doublet)
and 3.88 ppm (heptet) as well as new 13C NMR signals
at d 26.2 and 65.1 ppm, replacing the initial signals at d
26.9 and 76.6 ppm. The complex obtained reacted fur-
ther with 1 to give a mixture of complexes: all 1H
NMR signals from the ligand disappeared and a new
set of signals appeared. Particularly significant are the
new low field signals at d 9.99 and 9.81 ppm (ratio
100:17), indicative of complex formation. When these
preformed complexes were used for the cyanation reac-
tion instead of the in situ formed complexes, the final
enantiomeric excess of the product was again 70%. At-
tempts to isolate the complexes by crystallization were
unsuccessful.
ð2Þ
Para-substituted benzaldehydes with an electron-do-
nating group (MeO–) were more reactive and gave the
product with higher enantioselectivity than those having
an electron-withdrawing group (F3C–) (16 vs. 66 h, and
47 vs. 11% ee, entries 2 and 3). This unexpected reactiv-
ity was similar to that previously found by North et al.
[8]. With a bromine in the ortho position a reaction time
of 25 h was required for the reaction to go to completion
and the product was obtained with 24% ee. Aliphatic al-
dehydes, such as valeraldehyde (entry 5), exhibited a re-
activity similar to that of benzaldehyde and the product
had 41% ee. For the more sterically hindered pivalalde-
hyde (entry 6), the reactivity was similar, but the enan-
tiomeric excess of the resulting cyanohydrin was only
12%. In contrast to the results obtained when using alde-
hydes as substrates, acetophenone required a reaction
time of 119 h and the enantiomeric excess of the product
was very low (entry 7).
Next, we turned our attention to other metal alkox-
ides that could form efficient catalysts for the reaction
using 1 as a ligand (Eq. (3)). A first screening was made
using the best conditions found for Ti (Table 5)
ð3Þ
Table 4
Cyanation of other carbonyl compounds (Eq. (2))a
Entry
R
t (h)
Yield (%)b
ee (%)
1
2
3
4
5
6
7
Ph
6
16
66
25
5
>95
>95
>95
>95
>95
>95
>95
70
47
11
24
41
12
8
4-MeOPh
4-F3CPh
2-BrPh
C4H9
tBu
5
119
Acetophenone
a
Reactions were carried out in CH2Cl2 with 1% Ti(OiPr)4 and 1 at
room temperature.
b
GC yield.

O. Belda et al. / Journal of Organometallic Chemistry 689 (2004) 3750–3755
3753
Table 5
Screening of various metals together with ligand 1 as catalysts for the
cyanide addition to benzaldehyde (Eq. (3))a
M(OR)x
mol %
t (h)
Yield (%)b
ee (%)
Zr(OtBu)4
Zr(OtBu)4
Sc(OiPr)3
Yb(OiPr)3
Cu(OAc)2
a
1
10
1
23
4
78
91
3
26
2
3
>95
>95
10
1
16
3
8
1
0
Reactions were carried out in CH2Cl2 at room temperature.
GC-yield.
b
All metal complexes catalyzed the reaction, with the
exception of the Cu(II) complex [10], but very low or
no asymmetric induction was observed. Despite these
disappointing results, we decided to optimize the reac-
tion conditions for Zr. We had previously found that ad-
dition of a small amount of a secondary amine was
crucial for the reactivity and enantioselectivity induced
by the catalyst in the ring opening of cyclohexene oxide
with trimethylsilyl azide [5]. Adding a small amount of
pyrrolidine to the reaction mixture did not improve the
reactivity or enantioselectivity of the catalyst (Table 6)
for the addition of TMSCN to benzaldehyde catalyzed
by 1 and Zr(OtBu)4, however. On the other hand, addi-
tion of 0.5 equiv. of H2O proved to increase both the re-
activity and enantioselectivity. This effect has previously
been observed in the asymmetric aldol reaction catalyzed
by Zr–binol complexes [11]. The reason for the improve-
ment in reactivity and enantioselectivity seemed to be the
formation of a more active symmetric dimeric or oligo-
meric catalyst containing oxygen bridges.
By heating an equimolar mixture of Zr(tBuO)4 and 1
in benzene, a complex could be crystallized. The X-ray
structure showed a monomeric Zr complex with a dis-
torted octahedral coordination, in which the two pyri-
dine nitrogen atoms and the two deprotonated amide
nitrogen atoms were almost coplanar and the two alkox-
ide groups above and below the plane, respectively
(O3–Zr–O4 =124.1ꢁ, Table 7) (Fig. 3). This mode of co-
ordination is typical for the deprotonated ligands [12]. It
is interesting to note that the C–O–Zr angles are close to
linear, 169.7ꢁ and 174.1ꢁ, indicating multiple bonding of
the alkoxide groups [13].
Fig. 3. X-ray structure of [Zr(OtBu)2(1R,2R)-N,N0-bis(2-pyridinecar-
boxyamido)-1,2-diphenylethane].
Table 7
Selected bond lengths (pm) and bond angles (ꢁ) for Zr complex with
ligand 1
Bond lengths
Bond angles
Zr1–O3
Zr1–O4
Zr1–N1
Zr1–N2
Zr1–N3
Zr1–N4
192.4(8)
192.2(7)
234(1)
O3–Zr1–O4
O4–Zr1–N3
O3–Zr1–N3
O4–Zr1–N2
O3–Zr1–N2
O3–Zr1–N1
N3–Zr1–N1
N2–Zr1–N1
O4–Zr1–N4
O3–Zr1–N4
N3–Zr1–N4
N2–Zr1–N4
N1–Zr1–N4
124.1(3)
105.7(4)
118.8(3)
120.6(3)
70.6(3)
84.3(3)
138.1(3)
69.5(3)
82.9(3)
82.9(3)
69.2(3)
137.7(3)
152.4(3)
220.7(8)
220.6(9)
236.5(8)
The conformation of the ligand, with pseudo-axial
orientation of the phenyl rings, was found to be similar
to that of the previously described Cu(II) complex [10].
In the presence of TMSCN, benzaldehyde and water,
a mixture of complexes formed, as shown by NMR
spectroscopy. One of these, with e.g., a doublet at
10.12 ppm originating from an amide proton, was iden-
tical to a dimeric complex previously observed in the
Table 6
Screening of additives
Entry
mol% 1
mol% Zr(tBuO)4
Remarks
t (h)
Yield (%)a
ee (%)
1
2
3
4
5
6
a
1
10
10
10
10
10
1
10
10
10
10
10
–
23
4
78
91
3
26
26
29
56
7
–
1 mol% C4H8NH
1 mol% H2O
5 mol% H2O
10 mol% H2O
5
>95
>95
>95
>95
3
2
2
GC yield.

3754
O. Belda et al. / Journal of Organometallic Chemistry 689 (2004) 3750–3755
ring opening of epoxides with trimethylsilyl azide [5].
Whether activation of both the nucleophile and the car-
bonyl group took place using the Zr catalyst is not clear.
Attempts to isolate dimeric or oligomeric Zr complex of
1 by crystallization were unsuccessful.
extracts dried (Na2SO4). Evaporation of the solvent af-
forded a solid that was recrystallized from EtOH to give
the pure product as white needles. Yield: 0.81 g, 80%.
Spectroscopic data were in agreement with that pub-
lished previously for the compound [10].
4.3. General procedure for the addition of TMSCN to
benzaldehyde catalyzed by Ti
3. Conclusions
A study of the asymmetric addition of TMSCN to al-
dehydes catalyzed by metal alkoxides and (1R,2R)-
N,N0-bis(2-pyridinecarboxyamido)-1,2-diphenylethane
as ligand was carried out. The products were obtained in
good yields but with modest enantioselectivity (11–70%
ee).
Ti and Zr alkoxide complexes with bispyridylamide 1
as ligand were prepared and studied by NMR spectros-
copy and X-ray crystallography. When the catalyst was
formed from Ti(OiPr)4 and ligand 1, evidence for
dimeric or oligomeric species was obtained. From
Zr(OtBu)4 and 1, a monomeric complex was isolated
and characterized in the solid state as well as in solution.
The active catalyst was, however, composed of dimeric
or oligomeric complexes which were formed more
efficiently when adding a small amount of water.
Further applications of 1 in asymmetric catalyzed re-
actions are under study.
TMSCN (0.39 mL, 2.95 mmol) and benzaldehyde
(0.25 mL, 2.46 mmol) were added to a solution of 1
(10.5 mg, 0.025 mmol) and Ti(OiPr)4 (7.5 lL, 0.025
mmol) in CH2Cl2 (0.5 mL) and the reaction was moni-
tored by GC. The reaction mixture was diluted with
Et2O and the product purified by column chromatogra-
phy on SiO2 using Et2O as eluent. Spectroscopic data of
the product were identical to that previously reported in
the literature [14]. The absolute configuration was as-
signed by means of optical rotation [15].
4.4. General procedure for the addition of TMSCN to
benzaldehyde catalyzed by Zr with H2O as additive
TMSCN (95 lL, 0.71 mmol) and benzaldehyde (60
lL, 0.59 mmol) were added to a solution of 1 (25 mg,
0.059 mmol) and Zr(OtBu)4 (23 lL, 0.059 mmol) in a
0.06 M solution of H2O in CH2Cl2 (0.5 mL, 0,03 mmol
H2O) and the reaction was monitored by GC.
4. Experimental
5. X-ray crystallography
4.1. General
5.1. Structural analysis of 1–Zr(OtBu)2
Benzaldehyde and Ti(OiPr)4 were distilled and stored
under Ar. Zr(OtBu)4, Sc(OiPr)3 and Yb(OiPr)3 (Strem)
were stored in a drybox and used as received. All sol-
vents were carefully dried according to standard proce-
dures before use and handled under N2. All catalytic
reactions were run in dried glassware under a dry Ar at-
mosphere using standard techniques. Yields were deter-
mined by GC using naphthalene as external standard
unless otherwise noted. The enantiomeric excess of the
cyanohydrins was determined by GC on a Chiraldex
G-TA 30 m·0.25 mm column.
Crystals of 1–Zr(OtBu)2 were prepared as follows:
Zr(OtBu)4 (25 lL, 0.064 mmol) was added to a suspen-
sion of ligand 1 (22.5 mg, 0.054 mmol) in dry benzene
(0.5 mL) at 70 ꢁC under an Ar atmosphere. The result-
ing solution was cooled to room temperature over a pe-
riod of 5 h. The resulting hygroscopic crystals were
sealed into capillaries in a nitrogen-filled glove box. Dif-
fraction data were collected at 297 K on a Bruker-Non-
ius Kappa CCD diffractometer. The structures were
solved using Direct Methods [SHELXS97] [16] and re-
fined on F2 with anisotropic thermal parameters for all
non-H atoms [SHELXL97] [17]. H atoms were refined
on calculated positions using a riding model. Crystal
data for 1–Zr(OtBu)2: sum formula C34H36N4O4Zr, cell
4.2. Preparation of (1R,2R)-N,N0-bis(2-pyridinecarbox-
amide)-1,2-diphenylethane (1)
˚
A suspension of picolinic acid (0.59 g, 4.76 mmol)
and 1,10-carbonyldiimidazole (CDI) (0.92 g, 5.70 mmol)
in THF (5 mL) was heated to 50 ꢁC for 1 h (evolution of
gas was observed). Then (1R,2R)-1,2-diphenyldiamino-
ethane (0.50 g, 2.38 mmol) was added at once at 50 ꢁC
and the mixture stirred vigorously at room temperature
(formation of a precipitate). The mixture was then
extracted (CH2Cl2/H2O) and the combined organic
constants a=9.1450(13), b=10.291(2), c=34.450(5) A,
3
V=3242.1(9) A , Z=4, qcalc =1.344 g/cm3, orthorhom-
˚
bic, space group P212121 (No. 19), T=297 K, radiation
˚
Mo Ka, k=0.71073 A, absorption coefficient 0.381
mmꢀ1, h range 4.5ꢁ<h<20.8ꢁ, number of measured re-
flections 10214, number of unique reflections 3312,
Rint =0.0857, number of refined parameters 388, residu-
als R1 =0.0662 (2649 reflections with I>2r(I))

O. Belda et al. / Journal of Organometallic Chemistry 689 (2004) 3750–3755
3755
wR2 =0.142 (all reflections), goodness-of-fit 1.166, flack
parameter x=ꢀ0.07(10), difference electron density
peak/hole 0.65/ꢀ0.36.
[6] W.C. Groutas, D. Felker, Synthesis (1980) 861.
[7] We realized that the quality of the TMSCN used was very
important for the outcome of the reaction. When dark-coloured
TMSCN from Lancaster was used the reaction times were longer,
the products were obtained in low ee:s (10–46% ee) and the
reproducibility was low. On the other hand, when we used
TMSCN from Acros or Aldrich, the reproducibility was high
whereas the ee:s of the products varied slightly between two
different batches of the reagent supplied by Acros (batch 1
afforded the product 2 in 70% ee and batch 2 in 60% ee under the
best conditions found). No significant differences were observed in
the 1H NMR spectra of the reagent from the different suppliers.
The reagent from Lancaster and batch 1 from Acros were both
dark-colored liquids, whereas the reagent from batch 2 Acros and
Aldrich were colorless liquids. Since Fe–CN complexes are dark-
colored, we decided to analyze by ICP whether any Fe was present
in the various reagents and found that the reagents did not contain
Fe above 0.08 ppm. We decided to use TMSCN from Aldrich kept
under N2, and the experiments were run several times in order to
verify their reproducibility.
6. Supplementary material
Crystallographic data for the structural analysis have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 237475 for 1–Zr(OtBu)2. These
data can be obtained free of charge via The Director,
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK
(fax: +44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk
or http://www.ccdc.cam.ac.uk/conts/retrieving.html).
Acknowledgement
[8] Y.N. BelokonÕ, S. Caveda-Cepas, B. Green, N.S. Ikonnikov,
V.N. Khrustalev, V.S. Larichev, M.A. Moscalenko, M. North,
C. Orizu, V.I. Tararov, M. Tasinazzo, G.I. Timofeeva, L.V.
Yashkina, J. Am. Chem. Soc. 121 (1999) 3968.
This work was supported by the Swedish Foundation
for Strategic Research. The Swedish Research Council
(VR) is acknowledged for funding of the single-crystal
diffractometer.
[9] Y.N. BelokonÕ, B. Green, N.S. Ikonnikov, V.S. Larichev, B.V.
Lokshin, M.A. Moscalenko, M. North, C. Orizu, A.S. Peregudov,
G.I. Timofeeva, Eur. J. Org. Chem. (2000) 2655.
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