Asymmetric addition of trimethylsilylcyanide to N-benzylimines catalyzed by recyclable chiral dimeric V(V) salen complex

Article abstract of DOI:10.1016/j.jorganchem.2010.01.018

Chiral dimeric vanadium (V) salen complex (10 mol%) derived from 5,5-Methylene di-[(S,S)-{N-(3-tert-butyl salicylidine)-N′-(3′,5′-di-tert-butyl salicylidene)]-1,2-cyclohexanediamine] with vanadyl suphate followed by auto oxidation was used as efficient catalyst for enantioselective Strecker reaction of N-benzylimines with TMSCN at -30 °C. Excellent yield (92%) of α-aminonitrile and high chiral induction was achieved (ee up to 94%) in case of 2-methoxy substituted N-benzylimines in 10 h. The catalytic system worked well up to four cycles with retention of enantioselectivity.

Full text of DOI:10.1016/j.jorganchem.2010.01.018

Journal of Organometallic Chemistry 695 (2010) 1133–1137
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Asymmetric addition of trimethylsilylcyanide to N-benzylimines catalyzed by
recyclable chiral dimeric V(V) salen complex
*
Noor-ul H. Khan , S. Saravanan, Rukhsana I. Kureshy, Sayed H.R. Abdi, Arghya Sadhukhan, Hari C. Bajaj
Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute (CSMCRI), Council of Scientific and Industrial Research (CSIR), G.B. Marg,
Bhavnagar - 364 002, Gujarat, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Chiral dimeric vanadium (V) salen complex (10 mol%) derived from 5,5-Methylene di-[(S,S)-{N-(3-tert-
butyl salicylidine)-N0-(30,50-di-tert-butyl salicylidene)]-1,2-cyclohexanediamine] with vanadyl suphate
followed by auto oxidation was used as efficient catalyst for enantioselective Strecker reaction of N-ben-
Received 2 December 2009
Received in revised form 8 January 2010
Accepted 14 January 2010
Available online 22 January 2010
zylimines with TMSCN at ꢀ30 °C. Excellent yield (92%) of
a-aminonitrile and high chiral induction was
achieved (ee up to 94%) in case of 2-methoxy substituted N-benzylimines in 10 h. The catalytic system
worked well up to four cycles with retention of enantioselectivity.
Ó 2010 Published by Elsevier B.V.
Keywords:
Strecker reaction
N-Benzylimines
Dimeric V(V) salen complex
Enantioselective
Recyclable
1. Introduction
dimeric V(V) salen complex for the asymmetric addition of TMSCN
to various N-benzylimines.
The catalytic asymmetric Strecker reaction represents simple
and efficient method for the synthesis of optically pure -amino
acid derivatives [1], nucleic acids [2], various nitrogen and sulfur
containing heterocycles and pharmaceuticals [3]. The classical
Strecker reaction reported in 1850 comprises a condensation of
an aldehyde, ammonia and a cyanide source, followed by the
a
2. Results and discussion
Chiral vanadium (V) salen complex 1 (Fig. 1) was synthesized by
the reaction of ligand 5,5-methylene di-[(S,S)-{N-(3-tert-butyl sali-
cylidine)-N0-(30,50-di-tert-butyl salicylidene)]-1,2-cyclohexanedi-
amine] with vanadyl sulfate by the reported method [9a]. Our
systemic study for the optimization of the reaction condition for
the Strecker reaction (Fig. 2) was initiated by the use of 2-methoxy
imine as a model substrate with TMSCN as a source of cyanide
using chiral dimeric V(V) salen complex 1 as a recyclable catalyst.
As the temperature plays a crucial role to achieve high chiral
induction in asymmetric catalysis, we first varied the reaction tem-
perature in a stepwise manner from room temperature (RT) to
ꢀ40 °C (Table 1, entries 1–5). On decreasing the temperature from
hydrolysis of the resulting
sical Strecker reaction yields racemic products, which after hydro-
lysis yield -amino acids in racemic form [4]. Asymmetric version
a-amino nitrile [1a]. However, the clas-
a
of this synthetically very useful reaction reportedly used a variety
of chiral catalyst such as organocatalysts [5], Jacobsen’s Schiff base
complexes [6], bi-functional catalyst [7] and metal based catalysts
[4,8]. Among these, Jacobsen salen-metal complexes have emerged
as an efficient catalyst for the asymmetric Strecker reaction [8a].
Since chiral ligands are expensive, the recycling of chiral catalyst
is of immense value. In this context, our group has reported di-
meric salen complexes as recyclable chiral catalysts for various
asymmetric organic transformations [9]. These dimeric complexes
due to relatively high molecular weight are less soluble in solvents
like hexane and hence can be easily precipitated out from the reac-
tion mixture in a post-work up step. Previously we have reported
dimeric (V) salen complex as an efficient catalyst for the cyanation
of aldehydes [9a]. Herein, we are extending the application of
RT to ꢀ30 °C (entries 1–5), the yield of
a-aminonitrile remained
nearly same, however there was an increase in reaction time and
improvement in enantioselectivity. A further decrease in the reac-
tion temperature from –30 to –40 °C adversely affects the reactiv-
ity and enantioselectivity (entry 6). Hence, ꢀ30 °C was taken as
optimum reaction temperature for the present protocol for the
synthesis of a-aminonitrile. Next, to find out the optimum catalyst
loading, we carried out the Strecker reaction with 5, 10 and
20 mol% of the catalyst (Table 1, entries 4, 6, and 7) at ꢀ30 °C. In
the case of 5 mol% catalyst loading 89% yield with 82% ee was
achieved within 10 h. When we increase the catalyst loading from
* Corresponding author. Tel.: +91 0278 2567760; fax: +91 0278 2566970.
E-mail address: khan251293@yahoo.in (N.-u.H. Khan).
0022-328X/$ - see front matter Ó 2010 Published by Elsevier B.V.
doi:10.1016/j.jorganchem.2010.01.018

1134
N.-u.H. Khan et al. / Journal of Organometallic Chemistry 695 (2010) 1133–1137
100
SO4Et
SO4Et
80
H
H
H
H
O
V
O
V
N
N
N
N
O
60
40
20
0
% Yield
% ee
H2
C
O
O
O
O
O
H
H
H
H
Fig. 1. Structure of catalyst 1.
1
2
3
4
No. of Cycles
100
Fig. 3. Recycling of the catalyst 1.
80
60
40
20
0
% Yield
% ee
was obtained with dichloromethane. However, on using chloro-
form, acetonitrile, THF as a solvent although the product yield
was 72–88% but the reaction took non chiral path. The best results
in terms of yield and ee of the product for the model reaction was
achieved with toluene as a solvent (Table 1, entry 4). In all catalytic
runs the 1S, 2S chiral dimeric V(V) salen complex gives the R-form
of
a-aminonitrile.
After optimizing the reaction conditions (Table 1, entry 4) this
catalytic protocol was implemented on various N-benzylimines
in order to see the generality of the catalytic system and the results
are summarized in Table 2. Both the electron donating and electron
withdrawing substituent present on imine had some effect on the
asymmetric Strecker reaction. N-Benzylimines with no substituent
gave 89% isolated yield with 80% ee. The electron donating (–Me,
–OMe) groups at 2- and 4-positions of N-benzylimines gave higher
yields as well as ee (Table 2, entries 2, 4, 5, 7) than in the cases
where these substituents were on 3-position (Table 2, entries 3,
6). Among the methyl and methoxy substituent, the methoxy
substituent showed better results in terms of product yield and ee
(Table 2, entries 4, 7). The presence of electron withdrawing group
on the aromatic ring of imines gave moderate yield and ee (Table 2,
entries 8, 9). Furthermore, on conducting the same reaction with
N-benzylimines derived from aliphatic aldehydes and benzyl
amine we could achieve a poor yield (65%) and ee (22%) in the case
of crotonaldehyde (Table 2, entry 11) whereas with trimethyl acet-
aldehyde better yield (78%) and ee (75%) was achieved (Table 2,
entry 10).
Cat. (mol%)
10 10 10 10 10
5
20 10 10 10 10
10 16 10 10 10 10 10 10
-20 -30 -40 -30 -30 -30 -30 -30 -30
Time(h)
Temp.(oC)
4
7
0
8
27
Fig. 2. Optimization of the reaction conditions for Strecker reaction of N-
benzylimines with TMSCN.
5 to 10 mol% there was significant improvement in the yield (92%)
and ee (94%) of the product, however a further increase in the cat-
alyst loading (20 mol%) was inconsequential. Having optimized the
reaction temperature (ꢀ30 °C) and catalyst loading (10 mol%), next
we optimized reaction medium bearing in mind that the reactivity
and enantioselectivity of the Strecker reaction is strongly depen-
dent on the nature of the solvent used [8a]. The solvents e.g., tolu-
ene, dichloromethane, chloroform, acetonitrile and THF (Table 1,
entries 4, 8–11) were screened for their effect on the yield and
ee of the product. A good yield (90%) with moderate ee (38%)
Table 1
Data for the optimization of reaction conditions.a
CN
CH N
Catalyst 1
C
H
N
H
+
TMSCN
µ
Water 20
l
OMe
OMe
Entry
Catalyst loading (mol%)
Temp (°C)
Time (h)
Solvent
Yieldb (%)
Eec (%)
1
2
3
4
5
6
7
8
10
10
10
10
10
5
20
10
10
10
10
RT
0
4
7
8
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
CH2Cl2
CHCl3
93
89
90
92
86
89
91
90
88
78
72
28
64
76
94
92
82
92
38
ꢀ20
ꢀ30
ꢀ40
ꢀ30
ꢀ30
ꢀ30
ꢀ30
ꢀ30
ꢀ30
10
16
10
10
10
10
10
10
9
10
11
Racemic
Racemic
Racemic
CH3CN
THF
a
b
c
All reactions were carried out using 1.5 equiv of TMSCN.
Isolated yield.
Ee were determined using chiral OD-H column.

N.-u.H. Khan et al. / Journal of Organometallic Chemistry 695 (2010) 1133–1137
1135
Table 2
Enantioselective addition of TMSCN to various N-benzylimines catalyzed by dimeric V(V) salen complex 1.a
CN
CH N
Catalyst 1(10 mol%)
C
H
N
H
R
+
TMSCN
R
-30 o
l
C
µ
Water 20
S. no.
Imine
C6H5CH@NCH2Ph
Yieldb (%)
Eec (%)
1
2
3
4
5
6
7
8
89
82
68
75
90
78
92
88
72
78
65
80
83
76
79
89
60
94
75
62
75
22
4-MeC6H4CH@NCH2Ph
3-MeC6H4CH@NCH2Ph
2-MeC6H4CH@NCH2Ph
4-MeOC6H4CH@NCH2Ph
3-MeOC6H4CH@NCH2Ph
2-MeOC6H4CH@NCH2Ph
4-FC6H4CH@NCH2Ph
4-ClC6H4CH@NCH2Ph
(CH3)3CCH@NCH2Ph
CH3CH@CHCH@NCH2Ph
9
10
11
a
b
c
Reaction carried out at ꢀ30 °C in toluene using 10 mol% of catalyst 1 using 1.5 equiv of TMSCN.
Isolated yield.
Ee were determined using chiral OD and OD-H column.
time use of the same catalyst is given in Table 3. The activity of
the recycled catalysts gradually decreased upon successive use
possibly due to some physical loss of the catalyst with retention
of enantioselectivity (Fig. 3).
H2O
O
Si
CN
+
CH
N
V
R
H
C
HCN
N
R
3. Conclusion
O
In conclusion chiral dimeric vanadium (V) salen complex was
used for the asymmetric addition of TMSCN to N-benzylimines.
Catalytic
Cycle
O
V
V
Excellent yield (92%) of
a-aminonitrile and high chiral induction
was achieved (ee up to 94%) in case of 2-methoxy substituted
N-benzylimines. Besides, V(V) salen complex turned to be the most
efficient recyclable system reported so far in the literature.
H
C
N
R
CN
H-CN
C
H
N
H
4. Experimental
R
4.1. Materials and methods
Scheme 1. Proposed catalytic cycle for asymmetric Strecker reaction.
NMR spectra were obtained with a Bruker F113 V spectrometer
(500 MHz and 125 MHz for 1H and 13C, respectively) and are refer-
enced internally with TMS. FTIR spectra were recorded on Perkin
Elmer Spectrum GX spectrophotometer in KBr window. High-reso-
lution mass spectra were obtained with a LC–MS (Q-TOF) LC
(Waters), MS (Micromass) instruments. Optical rotations were
measured with a Digipol 781 Automatic Polarimeter Rudolph
Instruments. Enantiomeric excess (ee) were determined by HPLC
(Shimadzu SCL-10AVP) using Daicel Chiralpak OD-H and OD chiral
columns with 2-propanol/hexane as eluent. For the product
purification flash chromatography was performed using silica gel
100–200 mesh, Vanadyl sulphate penta hydrate(VOSO4.5H2O)
purchased from s.d. Fine-Chem Limited Mumbai (India). TMSCN,
Benzaldehyde, 2-methoxy benzaldehyde, 3-methoxy benzaldehyde,
4-methoxy benzaldehyde, 4-fluoro benzaldehyde, 4-chlorobenzal-
dehyde, trimethylacetaldehyde, crotonaldehyde (Aldrich Chemicals)
2-methyl benzaldehyde, 3-methyl benzaldehyde, 4-methyl benzal-
dehyde, benzylamine (Merck Chemicals) and used as received. The
recyclable dimeric V(V) salen complex 1 derived from 5,5-Methy-
lene di-[(S,S)-{N-(3-tert-butyl salicylidine)-N0-(30,50-di-tert-butyl
salicylidene)]-1,2-cyclohexanediamine] with vanadyl suphate fol-
lowed by auto oxidation was synthesized by our previously re-
ported method [9a]. All the solvents were dried using standard
procedures [10], distilled and stored under nitrogen.
Scheme 1 depicted the probable catalytic cycle for the enantio-
selective Strecker reaction, where the imine was polarized by the
weak interaction through the lone pair of electrons present on it
with metal center of the catalyst followed by the nucleophilic addi-
tion of cyanide ion (HCN was generated by the reaction of H2O and
TMSCN) to the polarized carbon of imines, resulting in the forma-
tion of the product and liberated the catalyst to takes part in an-
other catalytic cycle.
Catalyst recycling studies were carried out by precipitating the
catalyst by the addition of hexane to the post catalytic reaction
mixture. The recovered catalyst was then charged with fresh sub-
strate and reactants and the catalytic run was conducted in the
similar manner as in the case of fresh catalyst. The data of four-
Table 3
Catalyst recycling data for the enantioselective addition of TMSCN to 2-methoxy
substituted N-benzylimine.
Run
1
2
3
4
Time (h)
Yield
Ee
10
92
94
10
89
92
12
84
93
14
83
91

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N.-u.H. Khan et al. / Journal of Organometallic Chemistry 695 (2010) 1133–1137
4.1.1. Synthesis of complex 1
7.1–7.5 (m, 9H). CHIRALCEL OD column, hexane/2-propanol = 95:5,
The complex 1 was synthesized by the reported procedure [9a].
The solution of 5,5-Methylene di-[(S,S)-{N-(3-tert-butyl salicyli-
dine)-N0-(30,50-di-tert-butyl salicylidene)]-1,2-cyclohexanediamine]
(0.250 g, 0.252 mmol) was dissolved in a mixed solvent system-
ethanol:CH2Cl2 (3:2, 15 ml) to which an aqueous solution of vana-
dyl sulphate penta hydrate VOSO4ꢁ5H2O (0.128 g, 0.504 mmol in
2 ml water) was added drop-wise at room temperature in an inert
atmosphere. The resulting solution was refluxed for 4 h and then
cooled to room temperature with an extended stirring for 12 h
while opening the side arm of the reaction flask for aerial oxida-
tion. Solvent was completely evaporated from the reaction mixture
and the residue was dissolved in CH2Cl2 (10 ml), washed with
water (3 ꢂ 5 ml). The organic layer was dried over anhydrous
Na2SO4, filtered and evaporated to give dark green dimeric V(V)
complex.
flow rate 0.5 ml/min, tr1(minor) = 24.89 min, tr2(major) = 34.50 min.
4.1.4.5. N-Benzyl (S)-2-amino-(4-methoxyphenyl) acetonitrile (Table
2, entry 5). 1H NMR (500 MHz, CDCl3) d = 1.62 (br s, 1H), 3.81 (s,
3H), 3.95 (d, J = 13.0 Hz, 1H), 4.06 (d, J = 13.0 Hz, 1H), 4.69 (s,
1H), 6.92(d, J = 8.5 Hz, 2H), 7.3–7.5 (m, 7H). CHIRALCEL OD column,
hexane/2-propanol = 95:5, flow rate 0.5 ml/min, tr1(minor) = 37.34
min, tr2(major) = 41.19 min.
4.1.4.6. N-Benzyl (S)-2-amino-(3-methoxyphenyl) acetonitrile (Table
2, entry 6). 1H NMR (500 MHz, CDCl3) d = 1.84 (br s, 1H), 3.82 (s,
3H,), 3.95 (d, J = 13.0 Hz, 1H), 4.04 (d, J = 13.0 Hz, 1H), 4.72 (s,1H),
7.3–7.5 (m, 9H). CHIRALCEL OD column, hexane/2-propanol = 95:5,
flow rate 0.5 ml/min, tr1(minor) = 38.62 min, tr2(major) = 42.22 min.
4.1.4.7. N-Benzyl (S)-2-amino-(2-methoxyphenyl) acetonitrile (Table
2, entry 7). 1H NMR (500 MHz, CDCl3) d = 2.0 (br s, 1H), 3.77 (s, 3H),
3.86 (d, J = 13.0 Hz, 1H), 3.99 (d, J = 13.0 Hz, 1H), 4.72 (s, 1H),
6.84(d, J = 8 Hz, 2H) 7.2–7.4 (m, 7H). CHIRALCEL OD-H column,
hexane/2-propanol = 99:1, flow rate 0.8 ml/min, tr1(minor) =
57.56 min, tr2(major) = 59.42 min.
4.1.2. Typical experimental procedure for the synthesis of
N-benzylimines [8a]
To a stirred solution of the aldehyde (1 equiv) and anhydrous
magnesium sulfate (2 g) in dichloromethane (10 ml), under nitro-
gen at room temperature, was added benzyl amine (1 equiv). The
reaction mixture was stirred for 21 h, then the magnesium sulfate
was removed by filtration and the solvent removed in vacuum to
give the N-benzyl imine as a pale yellow oil or solid. The imines
were sufficiently pure for use without further purification.
4.1.4.8. N-Benzyl (S)-2-amino-(4-chlorophenyl) acetonitrile (Table 2,
entry 8). 1H NMR (500 MHz, CDCl3) d = 1.6 (br s, 1H), 3.95 (d,
J = 13.0 Hz, 1H), 4.05 (d, J = 13.0 Hz, 1H), 4.72 (s, 1H), 7.2–7.5 (m,
9H). CHIRALCEL OD column, hexane/2-propanol = 95:5, flow rate
0.5 ml/min, tr1(minor) = 30.63 min, tr2(major) = 34.77 min.
4.1.3. Typical experimental procedure for addition of TMSCN to
N-benzylimines
The chiral V(V) dimeric salen complex (10 mg, 0.009 mmol) was
dissolved in dry toluene (3 ml) and the solution was cooled to
ꢀ30 °C under N2 atmosphere. To the cooled solution N-benzyli-
mine (0.09 mmol) was added which was followed by the addition
of TMSCN (0.75 equiv) in a drop wise manner over 12 min with
4.1.4.9. N-Benzyl (S)-2-amino-(4-fluorophenyl) acetonitrile (Table 2,
entry 9). 1H NMR (500 MHz, CDCl3) d = 1.78 (br s, 1H), 3.88 (d,
J = 13.0 Hz, 1H), 3.98 (d, J = 13.0 Hz, 1H), 4.64 (s, 1H), 6.9–7.4 (m,
9H). CHIRALCEL OD column, hexane/2-propanol = 95:5, flow rate
0.5 ml/min, tr1(minor) = 35.26 min, tr2(major) = 39.17 min.
stirring. To this stirred solution, H2O (20
additional quantity of TMSCN (0.75 equiv) over
l
l) was added and an
period of
a
4.1.4.10. N-Benzyl (S)-2-amino-3,3-dimethyl butanonitrile (Table 2,
entry 10). 1H NMR (500 MHz, CDCl3) d = 1.18 (s, 9H), 3.03 (s, 1H),
3.88 (d, J = 13.0 Hz, 1H), 4.01 (s, 1H), 7.2–7.4 (m, 5H). CHIRALCEL
OD column, hexane/2-propanol = 95:5, flow rate 0.5 ml/min,
tr1(minor) = 34.82 min, tr2(major) = 36.38 min.
30 min. The reaction was monitored on TLC using hexane/ethyl
acetate (90/10) as eluent. The product was purified by flash column
chromatography on silica gel (eluted with hexane/ethyl acetate =
90:10). The purified products were characterized by 1H were in
agreement with the reported values [8a].
Acknowledgements
4.1.4. Characterization data
4.1.4.1. N-Benzyl (S)-2-amino-phenylacetonitrile (Table 2, entry
1). 1H NMR (500 MHz, CDCl3) d = 1.86 (br s, 1H), 3.95 (d,
J = 13.0 Hz, 1H), 4.06 (d, J = 13.0 Hz, 1H), 4.76 (s, 1H), 7.3–7.6 (m,
10H) HPLC analysis: CHIRALCEL OD column, hexane/2-propanol =
95:5, flow rate 0.8 ml/min, tr1(minor) = 29.06 min, tr2(major) =
31.50 min.
N.H. Khan and S. Saravanan are thankful to DST and CSIR Net-
work project on Catalysis for financial assistance and also thankful
to Analytical Science Discipline for providing instrumentation
facility.
Appendix A. Supplementary material
4.1.4.2. N-Benzyl (S)-2-amino-(4-methylphenyl) acetonitrile (Table 2,
entry 2). 1H NMR (500 MHz, CDCl3) d = 1.83 (1H, br s), 2.36 (3H, s),
3.95 (d, J = 13.0 Hz, 1H), 4.06 (d, J = 13.0 Hz, 1H), 4.71 (s, 1H), 7.2–
7.4 (m, 9H). CHIRALCEL OD column, hexane/2-propanol = 95:5,
flow rate 0.5 ml/min, tr1(minor) = 26.60 min, tr2(major) = 29.05 min.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2010.01.018.
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