A polymer-bound oxidovanadium(IV) complex prepared from an L-cysteine-derived ligand for the oxidative amination of styrene

Article abstract of DOI:10.1002/ejic.200700662

The ligand H₂sal-cys (I) derived from salicylaldehyde and Lcysteine has been covalently bonded to chloromethylated polystyrene cross-linked with 5% divinylbenzene. Upon treatment with [VO(acac)₂] in dimethylformamide (DMF) the polystyrene-bound ligand PS-H₂sal-cys (II) gave the oxidovanadium(IV) complex, PS-[VO(sal-cys)·DMF] (1). The corresponding neat complex, [VO(sal-eta)]₂ (2), has also been prepared similarly in methanol. These complexes have been characterised by IR, electronic, EPR spectroscopic studies, magnetic susceptibility measurements and thermal as well as scanning electron micrographs studies. Complex [VO-(sal-eta)]₂ exhibits a medium intensity band at 980 cm ^-1 in the IR spectrum due to ν(V=O) stretch. Broad features of the EPR spectrum for the neat complex along with magnetic susceptibility studies suggest the presence of antiferromagnetic exchange interaction between two vanadium centers in close proximity. Both complexes catalyze the oxidative amination of styrene, in mild basic conditions, with secondary amines (diethylamine, imidazole, and benzimidazole) and gave a mixture of two aminated products in good yields. Amongst the two aminated products, the anti-Markovnikov product is favored over the Markovnikov one due to the steric hindrance posed by the secondary amines. The polymeranchored heterogeneous catalyst is free from leaching during catalytic action and recyclable. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200700662

FULL PAPER
DOI: 10.1002/ejic.200700662
A Polymer-Bound Oxidovanadium(IV) Complex Prepared from an
Derived Ligand for the Oxidative Amination of Styrene
L
-Cysteine-
Mannar R. Maurya,*[a] U. Kumar,[a] Isabel Correia,[b] Pedro Adão,[b] and
João Costa Pessoa[b]
Keywords: Polymer-anchored vanadium complex / Heterogeneous catalysis / Oxidative amination of styrene / EPR spec-
troscopy
The ligand H2sal-cys (I) derived from salicylaldehyde and
L
-
susceptibility studies suggest the presence of antiferromag-
netic exchange interaction between two vanadium centers in
close proximity. Both complexes catalyze the oxidative amin-
ation of styrene, in mild basic conditions, with secondary
amines (diethylamine, imidazole, and benzimidazole) and
gave a mixture of two aminated products in good yields.
Amongst the two aminated products, the anti-Markovnikov
product is favored over the Markovnikov one due to the ste-
ric hindrance posed by the secondary amines. The polymer-
anchored heterogeneous catalyst is free from leaching during
catalytic action and recyclable.
cysteine has been covalently bonded to chloromethylated
polystyrene cross-linked with 5% divinylbenzene. Upon
treatment with [VO(acac)2] in dimethylformamide (DMF) the
polystyrene-bound ligand PS-H2sal-cys (II) gave the oxido-
vanadium(IV) complex, PS-[VO(sal-cys)·DMF] (1). The corre-
sponding neat complex, [VO(sal-eta)]2 (2), has also been pre-
pared similarly in methanol. These complexes have been
characterised by IR, electronic, EPR spectroscopic studies,
magnetic susceptibility measurements and thermal as well
as scanning electron micrographs studies. Complex [VO-
(sal-eta)]2 exhibits a medium intensity band at 980 cm–1 in
the IR spectrum due to ν(V=O) stretch. Broad features of the
EPR spectrum for the neat complex along with magnetic
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
Nonactivated alkenes are generally resistant toward nu-
cleophilic attack of amines, but upon complexation to an
electrophilic transition metal, olefins become susceptible to-
ward nucleophiles.[1] The subsequent addition of the amine
depends on the steric nature of the olefin and amine. The
nucleophilic nitrogen normally reacts in such a way that the
least-hindered metal alkyl complex is formed (Markovnikov
addition), but steric factors of the amine substituents may
dominate the reaction favoring the anti-Markovnikov prod-
ucts.[1]
Introduction
Metal-catalyzed addition of nucleophilic amines to al-
kenes, called amination, represents one of the important
routes to synthesize nitrogen-based organic molecules.[1]
The amination may proceed in the presence or absence of
oxygen and the processes, called oxidative amination or hy-
droamination, respectively, are both highly useful. On the
basis of regioselectivity, Markovnikov or anti-Markovnikov
products may be produced (Scheme 1).
Catalytic intramolecular amination of alkenes may also
result if a secondary amine group is present on the alkene
itself at an appropriate position (e.g. Scheme 2). Efficient
catalytic intramolecular hydroamination of alkenes has also
been developed.[1–7] Normally catalysts based on late transi-
tion-metal complexes (e.g. of Pd, Pt, Ru, Rh etc.) or lantha-
nides are used for aminations. Though these complexes are
less sensitive towards oxygen, literature cites reports on hy-
droamination[1–7] as well as oxidative aminations.[8–21] Since
complexes of early transition metals, e.g. vanadium, are fre-
quently used as oxidation catalysts, they are potential can-
didates for oxidative aminations. However, no oxidative ami-
nation using vanadium complexes as catalysts has been re-
ported in the literature. We, therefore, decided to prepare
oxidovanadium(IV) complexes and use them as catalysts for
the oxidative amination of styrene.
Scheme 1.
[a] Department of Chemistry, Indian Institute of Technology
Roorkee,
Roorkee 247667, India
E. mail: rkmanfcy@iitr.ernet.in
[b] Centro Química Estrutural, Instituto Superior Técnico – TU
Lisbon,
Av. Rovisco Pais, 1049-001, Lisboa, Portugal
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M. R. Maurya, U. Kumar, I. Correia, P. Adão, J. Costa Pessoa
FULL PAPER
Scheme 2.
In this work the synthesis and characterization of a poly-
mer-anchored oxidovanadium(IV) complex of the Schiff
base derived from salicylaldehyde and cysteine [H2sal-cys I;
Scheme 3] is reported. We demonstrate the catalytic per-
formance of the complex, PS-[VO(sal-cys)·DMF] (1) [PS-
H2sal-cys (II), PS = polystyrene backbone; Scheme 3] in the
oxidative amination of styrene with secondary amines such
as diethylamine, imidazole, and benzimidazole in mild basic
conditions. Covalent bonding to polystyrene provides extra
stability to the catalyst and makes the catalytic reaction
heterogeneous in nature. The neat complex [VO(sal-eta)]2
[H2sal-eta (III) is the Schiff base derived from salicylalde-
hyde and 2-aminoethanethiol; Scheme 3] having similar do-
nor atoms has also been prepared and its catalytic activity
is compared with that of the polymer-anchored complex.
Scheme 4. Synthetic procedure for II. PS represents the polystyrene
support.
The anchored ligand, PS-H2sal-cys reacts with [VO-
(acac)2] in DMF at ca. 90 °C to give the polymer-anchored
blackish green oxidovanadium(IV) complex, PS-[VO(sal-cys)·
DMF] (1); Scheme 5.
Scheme 5. Synthetic procedure for 1.
Similarly, the reaction of [VO(acac)2] with an equimolar
amount of neat ligand, H2sal-eta (III), in refluxing acetoni-
trile yielded the neat oxidovanadium(IV) complex, [VO-
(sal-eta)]2 (2); see Equation (1).
Scheme 3.
(1)
Complex 2 exhibits an effective magnetic moment of
1.47 µB at 298 K, which is subnormal compared with the
expected value of 1.73 µB for d1(S = ½) systems. We have
also studied the magnetic susceptibility of the complex as
a function of temperature in the 90–298 K range. Figure 1
presents the plots of magnetic moment (µB) and magnetic
Results and Discussion
Synthesis, Solid-State Characterization, and Reactivity
The elemental analyses data of the ligands and com-
plexes are presented in Table 1. The spectroscopic charac-
1
terization (IR, H and 13C NMR) of the ligands, H2sal-cys
corr
susceptibility (χMcorr) vs. temperature for 2. The χM
(I) and H2sal-eta (III), used in the present study, confirm
their structure. The chloromethylated polystyrene, cross-
linked with 5% divinylbenzene, reacts with H2sal-cys in
DMF in the presence of triethylamine and ethyl acetate to
give the polymer-anchored ligand PS-H2sal-cys (II).
Scheme 4 summarizes the synthetic procedure. During this
process the –COOH group of cysteine reacts with the –
CH2Cl group of polystyrene. The remaining chlorine con-
tent of 1.6% (0.46 mmol Cl g–1 of resin in PS-H2sal-cys)
suggests ca. 91% loading of the ligand.
shows a slight dependence on temperature and the observed
magnetic moments vary in the range 1.47–1.19 µB. The Cu-
rie–Weiss plot [1/χMcorr vs. T] shown in Figure 2 is a straight
line with a Weiss constant of –60 K. These results indicate
an antiferromagnetic interaction, probably due to a dimeric
Table 1. Physico-chemical data of the solid compounds.
Compound
Color
Observed[a] (%)
C
H
N
S
V
PS-H2sal-cys (II) orange
PS-[VO(sal-cys)· blackish 68.78
72.59
8.14
7.82
3.56
4.01
5.96
5.40
–
8.01
DMF] (1)
green
H2sal-eta (III)
yellow
60.23
6.25
7.63
17.18
–
(59.66) (6.07) (7.73) (17.67)
44.31 3.94 5.64 12.81
[VO(sal-eta)]2 (2) green
20.0
(43.90) (3.65) (5.69) (13.00) (20.73)
[a] Calculated values for neat ligand and complex are given in pa-
renthesis.
Figure 1. Plot of the magnetic moment (µB) and magnetic suscep-
tibility (χmcorr) vs. temperature for [VO(sal-eta)]2 (2).
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Polymer-Bound Oxidovanadium(IV) Complex
structure of the complex. Syamal and Kale[22,23] have sug- the formation of a complex with fixed geometry. Because
gested an antiferromagnetic interaction through dimeriza- of poor loading of the metal complex it was not possible to
tion in the related complex [VO(sal-ea)]2 (H2sal-ea is the obtain accurate information on the morphological changes
Schiff base derived from salicylaldehyde and 2-amino- in terms of orientation of ligands coordinated to the metal
ethanol) based on the magnetic susceptibility data and EPR ion. A scanning electron micrograph of complex 1 after
studies, and later confirmed by a single-crystal X-ray dif- keeping it in acetonitrile for 2 h has also been recorded and
fraction study.[24] We have also previously observed dimer- is reproduced in Figure 3(c). Upon swelling the beads the
ization of [VO(sal-ohyba)] (H2sal-ohyba is the Schiff base opening of the pores can be visualized.
derived from salicylaldehyde and o-hydroxylbenzyl-
amine).[25]
TGA Study
Thermogravimetric analysis under an oxygen atmosphere
show the high stability of PS-[VO(sal-cys)·DMF] (1) up to
ca. 175 °C. The first weight loss starting just above 175 °C is
possibly due to coordinated DMF. Thereafter the complex
decomposes in several steps. Quantitative measurement of
weight loss at various stages was not possible due to their
overlap. However, the stability of final residues at ca. 700 °C
suggests the formation of V2O5. The metal content calcu-
lated from the final residues was close to the one obtained
by atomic absorption spectrometry.
IR Spectral Study
corr
Figure 2. Curie–Weiss plot [1/χm
vs. T] for [VO(sal-eta)]2 (2).
A partial list of selected IR absorption bands of ligands
and complexes is given in Table 2. The absence of peaks at
1264 and 673 cm–1 due to the –CH2Cl group of chlorometh-
ylated polystyrene in the polymer-anchored ligand suggests
the covalent bonding of chloromethylated polystyrene to
the ligand.[26] A sharp band due to ν(C=O) of carboxylate
appears at ca. 1665 cm–1 in the polymer-anchored ligand
and complex. The ν(C=N) (azomethine) stretch of PS-
H2sal-cys shows up at 1632 cm–1. This peak shifts to lower
wave numbers upon coordination (1619 cm–1) indicating the
coordination of the azomethine nitrogen atom. Similarly,
III exhibits this band at 1632 cm–1, which is shifted to
1620 cm–1 in 2. The coordination of the phenolic oxygen
atom is indicated by the absence of the band centered at ca.
3400 cm–1 in the complexes. It was not possible to identify
bands confirming the coordination of DMF in 1 due to
the strong absorption of ν(C=O) in the same region. Both
complexes also exhibit sharp strong bands at 980 cm–1 due
to ν(V=O).
Scanning Electron Micrograph Study
Scanning Electron Micrographs (SEM) for single beads
of pure chloromethylated polystyrene, PS-H2sal-cys (II) and
PS-[VO(sal-cys)·DMF] (1) were obtained to observe the
morphological changes. Images of II and 1 are reproduced
in Figure 3. As expected, the pure polystyrene bead has a
smooth and flat surface, while the anchored ligand and
complex show slight roughening of the top layer. This
roughening is more intense in 1 possibly due to the interac-
tion of vanadium with the anchored ligand that results in
Table 2. Selected IR spectroscopic data (wavenumbers, cm–1).[a]
Compound
ν(OH)
ν(C=N)
ν(V=O)
PS-H2sal-cys (II)
PS-[VO(sal-cys)·DMF] (1)
H2sal-eta (III)
3400 (br)
–
3400 (br)
–
1632
1619
1632
1620
–
980 (sh, s)
–
980 (sh, s)
[VO(sal-eta)]2 (2)
[a] br = broad, sh = sharp, s = strong.
UV/Vis Spectroscopic Studies
Figure 3. Scanning electron micrographs (SEM) of (a) PS-H2sal-
cys (II), (b) PS-[VO(sal-cys)·DMF] (1), and (c) PS-[VO(sal-cys)·
DMF] (1) (after swelling); magnification ca. 200.
As expected, broader bands are observed in the case of
the polymer-anchored complex in nujol as compared to the
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M. R. Maurya, U. Kumar, I. Correia, P. Adão, J. Costa Pessoa
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neat complex, and some relevant data are presented in magnetically diluted VIVO complexes and show a slight
Table 3. The electronic spectrum of H2sal-eta (III) in meth- rhombic distortion, which can be seen in the perpendicular
anol exhibits three UV bands at 215, 255, and 316 nm. lines MI = 7/2 and 5/2, indicating a small distortion of the
These bands are assigned to φǞφ*, πǞπ*, and nǞπ* tran- square pyramidal geometry. The resolved EPR pattern
sitions, respectively. The band at 253 nm is also observed in indicates that the vanadium centers are well dispersed
the spectrum of [VO(sal-eta)]2 (2), but the 215 nm band in the polymer matrix. The spectra were simulated and
could not be located. The broad transition at ca. 350 nm in the spin Hamiltonian parameters obtained are gx = 1.980,
complex 2 is possibly due to merging of a LMCT band with gy = 1.975, gz = 1.938, Ax = 63.1ϫ10–4 cm–1, Ay
the nǞπ* intraligand band. In addition, 2 in DMF displays 66.5ϫ10–4 cm–1, Az = 173.3ϫ10–4 cm–1.
two weak broad bands at 536 and 605 nm due to d–d transi-
tions, confirming the presence of a vanadium(IV) center.
The polymer-anchored ligand PS-H2sal-cys (II) in nujol is
similar to that of III, with only slight deviation in their
positions (Figure 4). Complex PS-[VO(sal-cys)·DMF] exhi-
bits a broad featured band at 395 nm possibly due to a
LMCT band in addition to the other three intraligand
bands.
=
Table 3. Electronic spectra of ligands and complexes.
Compound
Solvent
λmax [nm]
H2sal-eta
H2sal-cys
[VO(sal-eta)]2
PS-H2sal-cys
PS-[VO(sal-cys)·DMF]
acetonitrile
acetonitrile
dichloromethane
nujol
316, 255, 215
350, 272, 218
605, 535, 350, 253
323, 260, 231
395, 321, 242, 230
Figure 5. Powder EPR spectra of samples of the fresh and spent
catalysts PS-[VO(sal-cys)·DMF] (1).
nujol
The EPR spectrum of DMSO solutions of the neat com-
plex 2 was measured at 77 K. Immediately after dissolution
the spectrum shows the presence of a broad band typical
of aggregated molecules, possibly due to the presence of
dimeric species (Figure 6). Such a dimeric structure has
been confirmed for example for [VO(sal-ea)]2 (H2sal-ea is
the Schiff base derived from salicylaldehyde and 2-amino-
ethanol) by a single-crystal X-ray diffraction study.[24] After
some time under air the broad band progressively disap-
pears and the final spectrum shows axial symmetry. The
spin Hamiltonian parameters obtained by simulation of the
experimental spectrum are: gЌ = 1.976, gʈ = 1.948, AЌ
=
59.0ϫ10–4 cm–1 and Aʈ = 165.0ϫ10–4 cm–1. The value of
Aʈ can be estimated (Aʈest) using the additivity relationship
proposed by Chasteen[27] with estimated accuracy of
Ϯ3ϫ10–4 cm–1. This value can be correlated to the number
and type of binding groups in the equatorial position, since
each donor group has a specific contribution to the
Aʈ and the sum of contributions of the four equatorial
Figure 4. Electronic spectra of (a) PS-(H2sal-cys) (II) and (b) PS-
[VO(sal-cys)·DMF] (1) recorded in nujol.
EPR Spectroscopic Study
The EPR spectra of samples of the neat and polymer-
anchored complexes were obtained from “frozen” solutions
(77 K) and powder, respectively. The anisotropy is not tot-
ally averaged away by motion of the molecules and nearly
axial EPR spectra were obtained.
After carrying out the catalytic reactions the polymer-
anchored catalyst 1 was filtered, washed and dried, and its
EPR spectrum was recorded under the same conditions as
that of the freshly prepared PS complex. Both spectra are
shown in Figure 5. The spectra of the fresh and spent cata-
lyst are identical indicating that the environment and coor-
dination geometry of the vanadyl ion were retained after
Figure 6. EPR spectra of frozen solutions (77 K) containing 5 m
of [VO(sal-eta)]2 (2) dissolved in DMSO immediately after dissol-
the catalytic reaction. Both spectra are characteristic of ution and after 24 h.
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Polymer-Bound Oxidovanadium(IV) Complex
groups gives Aʈest that may be compared with the Aʈ 0.040 g) were considered at a styrene to amine ratio of 1:2
observed. If we consider an equatorial binding mode in- under the above reaction conditions, and the results are
volving [O–phenolate, Nimine, R–S–, DMSO] the Aʈest
=
given in Figure 8. As can be seen in the figure, 0.020 g of
153.9ϫ10–4 cm–1,[28] which is too low. Therefore the coordi- catalyst gave only 29.5% conversion while 0.030 g and
nation should involve [Ophenolate, Nimine, 2 ϫ DMSO]equatorial 0.040 g of catalyst loadings have shown a comparable con-
and the thiolate group, if coordinated, should be in the axial version of 73 and 76%, respectively. Thus, 0.030 g of cata-
position.
lyst was found to be adequate to carry out the reaction
for maximum production. The temperature of the reaction
mixture has also influence on the performance of the cata-
lyst. As shown in Figure 9, running the reaction at 75 °C
gave a much better yield of the reaction products than at
50 or 65 °C.
Catalytic Oxidative Amination of Styrene
Polymer-anchored complex, PS-[VO(sal-cys)·DMF] (1)
catalyzes the oxidative amination of styrene with dieth-
ylamine, imidazole, and benzimidazole in the presence of
oxygen and triethylamine, to give the corresponding en-
amines, Scheme 6. While HPLC gave an idea about percent
conversion of styrene only, gas chromatography identified
the two types of products formed, along with the percent
conversion of styrene, and GC-MS confirmed their identi-
ties. Both types of products were finally separated by column
liquid chromatography using chloroform as the eluent in
1
all cases. Their H NMR spectroscopic data, given in the
experimental section, confirmed the formation of Markov-
nikov and anti-Markovnikov products. We did not observe
any polymerization of styrene during the catalytic reaction.
In order to optimize the catalytic activity of 1, the following
parameters were studied in detail considering diethylamine
as a representative substrate:
(i) effect of the amount of amine (mol of amine per mol of
styrene),
(ii) effect of the amount of catalyst (mol of catalyst per mol
of styrene),
Figure 7. Effect of styrene to diethylamine ratio on the oxidative
amination of styrene. Reaction conditions: styrene (1.04 g,
10 mmol), PS-[VO(sal-cys)·DMF] (0.030 g) and triethylamine
(6.06 g, 60 mmol) in 20 mL of CH3CN at 75 °C.
Under the optimized conditions, i.e. styrene (1.04 g,
(iii) temperature of the reaction mixture.
10 mmol), catalyst PS-[VO(sal-cys)·DMF] (0.030 g), trieth-
Reactions were carried out at three different styrene to ylamine (6.06 g, 60 mmol), CH3CN (20 mL) and tempera-
diethylamine molar ratios of 1:1, 1:1.5, and 1:2 using a fixed ture (75 °C), the oxidative amination of styrene, catalyzed
amount of styrene (1.04 g, 10 mmol), catalyst (0.030 g), and by 1, with imidazole and benzimidazole (20 mmol each) has
triethylamine (6.06 g, 60 mmol) in 20 mL of CH3CN at also been carried out, and the progress of the formation
75 °C. Samples were periodically analyzed up to 4 h. As of aminated products as a function of time is presented in
illustrated in Figure 7, a maximum of 32% conversion has Figure 10. It is clear from the Figure that diethylamine gave
been achieved after 4 h of reaction time at a styrene to a maximum of 73% conversion, followed by imidazole with
amine molar ratio of 1:1. Increasing this ratio to 1:1.5 led 56%, and benzimidazole gave only 24% conversion. The
to a conversion of ca. 45% while 1:2 molar ratios showed a enamines formed do not hydrolyze into the corresponding
maximum of ca. 73% conversion. In another experiment, carbonyl and amine compounds under the influence of
three different catalyst loadings (viz. 0.020 g, 0.030 g, and water produced during catalytic reactions. However, its
Scheme 6. a: N,N-Diethyl-1-phenylethenamine, b: N,N-diethyl-1,2-phenylethenamine, c: 1-(1-phenylvinyl)-1H-imidazole, d: 1-styryl-1H-
imidazole, e: 1-(1-phenylvinyl)-1H-benzimidazole and f: 1-styryl-1H-benzimidazole.
Eur. J. Inorg. Chem. 2008, 577–587
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Figure 8. Effect of the amount of catalyst PS-[VO(sal-cys)·DMF]
on the oxidative amination of styrene with diethylamine. Reaction
conditions: styrene (1.04 g, 10 mmol), styrene/amine (1:2) and tri-
ethylamine (6.06 g, 60 mmol) in 20 mL of CH3CN at 75 °C.
Figure 10. Comparison of the activity of PS-[VO(sal-cys)·DMF] for
different substrates. Reaction conditions: styrene (1.04 g, 10 mmol),
PS-[VO(sal-cys)·DMF] (0.030 g) and triethylamine (6.06 g,
60 mmol), secondary amine (20 mmol) in 20 mL of CH3CN at
75 °C.
products. This can be rationalized taking into account the
steric hindrance imposed by the secondary amine, which
decreases the formation of the Markovnikov products.
Thus, benzimidazole, having the highest steric hindrance,
resulted in the formation of 1-(1-phenylvinyl)-1H-benzimid-
azole in lower yield. Within the anti-Markovnikov products
the selectivity follows the order: 1-styryl-1H-benzimidazole
(81%) Ͼ 1-styryl-1H-imidazole (74%) Ͼ N,N-diethyl-1,2-
phenylethenamine (71%). This is again in accordance with
the order of decreasing steric hindrance.
We have also tested the catalytic activity of the neat com-
plex 2, a close analogue of 1 in terms of coordination envi-
ronment, in the oxidative amination of styrene taking the
same mol concentration as used for 1. Its performance for
various aminated products after 4 h of reaction is also pre-
sented in Table 4. It is clear from the Table that the neat
complex exhibits slightly lower conversion than the an-
Figure 9. Effect of the temperature on the oxidative amination of
styrene with diethylamine. Reaction conditions: styrene (1.04 g,
10 mmol), PS-[VO(sal-cys)·DMF] (0.030 g) and triethylamine
(6.06 g, 60 mmol) in 20 mL of CH3CN.
presence only slightly affected the conversion. The elimi- chored analogue for all reactions. The improvement in the
nation of water from the reaction mixture by using dry mo- catalytic activity of the anchored complex may be due to
lecular sieves (4 Å) improved the conversion by ca. 2%.
uniform distribution of metal centers on the polymer ma-
Table 4 presents data on conversion, turn over frequency trix, and/or an increased availability of styrene molecules
(TOF) of the catalysts, and selectivity of the Markovnikov which may adsorb on the polymer, close to the catalyst.
and anti-Markovnikov products obtained. It is clear from The carboxylate group on the α-position of nitrogen has
the Table that the selectivity of N,N-diethyl-1,2-phenyle- no influence on the catalytic reaction as it is bound to the
thenamine, 1-styryl-1H-imidazole, and 1-styryl-1H-benz- polymer.
imidazole, i.e. the yield of the anti-Markovnikov products
The selectivity of the Markovnikov aminated products is
(see Scheme 6), are higher than those of N,N-diethyl-1- slightly higher for the neat complex than for the anchored
phenylethenamine, 1-(1-phenylvinyl)-1H-imidazole, and 1- one. However, this effect is rather marginal and may be due
(1-phenylvinyl)-1H-benzimidazole, i.e. the Markovnikov to additional steric hindrance posed by polymeric support
Table 4. Conversion and reaction products using neat 2 and anchored 1 catalysts.
Entry
Catalyst
% Conversion
TOF [h–1]
% Yield
Markovnikov product
anti-Markovnikov product
1
2
3
4
5
6
1
1
1
2
2
2
diethylamine
imidazole
benzimidazole
diethylamine
imidazole
73.2
56.3
24.1
65.3
47.4
21.3
39
30
13
21
14
7
28.8
25.9
18.9
32.6
26.9
20.7
71.2
74.1
81.1
67.4
73.1
79.3
benzimidazole
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Polymer-Bound Oxidovanadium(IV) Complex
to anchored catalyst which might restrict the Markovnikov presence of a small amount of triethylamine, i.e. under the
product formation. The calculated turn-over frequency for reaction conditions applied for oxidative amination, and
all the reactions with anchored catalyst are higher than monitored the resulting changes by electronic absorption
those carried out using the neat complex. Besides, its sta- spectroscopy. Thus, the addition of one drop of 0.5-molar
bility and recycle ability make the anchored catalyst better styrene solution in CH2Cl2 (complex 2/styrene molar ratio
than the neat analogue.
of ca. 1:10) to 5 mL of ca. 10–4 molar solutions of 2 in
dichloromethane resulted in the spectral changes with time
presented in Figure 11 (a). The intensity of the 353 nm
band slowly decreases after the addition of styrene, while
Possible Routes to Amination
When carrying out catalytic reactions with Pt-, Pd-, the intensity of the 319 nm band increases. The band at
Rh-, and Ru- etc. -based catalysts, Müller and Beller[1] sug- 277 nm only gains intensity. The position and optical densi-
gested that a possible route for oxidative amination is ties of the bands due to d-d transitions remain constant;
through intermediate complex formation of styrene with therefore the main features of the coordination geometry
the catalyst. This intermediate then couples with the amine around the vanadium center do not change much (cf. inset
to give another aminoalkyl intermediate complex, followed of part a of Figure 11). Rather similar spectral changes have
by oxidative cleavage of the M–C bond in the presence of also been observed when a solution of 2 in dichlorometh-
O2. As our catalytic system is based on an oxidovanadium- ane, also containing a small amount of triethylamine was
(IV) complex and no vanadium(IV)–styrene interaction is treated with either diethylamine (complex 2/diethylamine
reported in the literature, we have explored this possibility molar ratio of ca. 1:10; Figure 11, b), or a mixture of sty-
by studying the interaction of the neat vanadium complex, rene and diethylamine (Figure 11, c). However, visible
[VO(sal-eta)]2 with styrene and/or diethylamine by spectro- changes in d-d bands have been observed on passing oxygen
scopic techniques: UV/Vis absorption and EPR.
through a solution of 2 containing one drop of 0.5-molar
styrene solution and a small amount of triethylamine in
CH2Cl2 (complex 2/styrene molar ratio of ca. 1:10; Fig-
ure 11, d). Here, both d-d bands slowly disappear indicating
the oxidation of oxidovanadium(IV) complex to an oxido-
vanadium(V) species in the presence of styrene. A similar
UV/Vis Spectral Studies
We started by treating the neat vanadium complex,
[VO(sal-eta)]2 (2) with styrene in dichloromethane in the
Figure 11. Change with time of the UV/Vis spectra of ca. 5 mL of a 10–4  solution of [VO(sal-eta)]2 (2) in dichloromethane, in the
presence of a small amount of triethylamine, after the addition of: (a) one drop of 0.5-molar styrene solution (complex 2/styrene molar
ratio of ca. 1:10), (b) one drop of 0.5-molar diethylamine solution (complex 2/diethylamine molar ratio of ca. 1:10), and (c) one drop of
0.5-molar styrene solution and one drop of 0.5-molar diethylamine solution (again complex 2: styrene: diethylamine molar ratio of ca.
1:10:10) and (d) one drop of 0.5-molar styrene solution (complex 2/styrene molar ratio of ca. 1:10) in the presence of oxygen. Spectra
were recorded in 15 min intervals. With triethylamine no such changes were observed in the absorption spectra of solutions of 2.
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FULL PAPER
observation has also been observed on passing oxygen high-field region of the EPR spectra of frozen solutions
through a solution of 2 containing one drop of 0.5-molar containing 5 m of [VO(sal-eta)]2 and 1, 2, 3, and 10 molar
diethylamine solution and a small amount of triethylamine equivalents of the substrates. Upon addition of styrene, di-
in CH2Cl2 (complex 2/diethylamine molar ratio of ca. 1:10). ethylamine or both, the broad band indicative of the pres-
It is important to note that passing of oxygen for days ence of the dimeric form of [VO(sal-eta)]2 decreases its in-
through a CH2Cl2 solution of 2 had no effect on the visible tensity, and a monomeric form corresponding to [VO-
spectra.
(sal-eta)], possibly with
a binding mode involving
The interpretation of the changes shown in Figure 11 is [Ophenolate, Nimine, 2 ϫ DMSO]equatorial, becomes the main
not straightforward. However, the changes described above species detected. In the solutions where diethylamine is
hint towards the participation of styrene or diethylamine in added (Figure 13, b), no change in the EPR spectra was
the process of oxidation of oxidovanadium(IV) to oxidova- detected that may support its equatorial coordination.
nadium(V) in the presence of oxygen, followed by elimi- However, its coordination trans to the oxo-oxygen atom
nation of the aminated product(s). It is possible that the cannot be ruled out.
catalytic and redox processes operating involve coordinated
styrene and/or amine. However, it is not possible at this
stage to comment further on the exact nature of these pro-
cesses and we are pursuing further theoretical and experi-
mental studies in this field. With other metal complexes
similar types of interactions have been discussed in the lit-
erature.[1]
EPR Spectral Studies
In order to find evidence for the intermediate species
formed during the catalytic cycle, the neat complex [VO-
(sal-eta)]2 was dissolved in DMSO containing 2 equiv. of
triethylamine and different amounts of styrene and/or di-
Figure 12. EPR spectra of frozen solutions containing the neat
ethylamine were added. Only after addition of ca. 10 equiv.
complex 2 (5 m) dissolved in DMSO, with and without 10 equiv.
of styrene. The presence of a new species is clearly seen in the high-
of styrene is a new species clearly detected by EPR, this
being shown in Figures 12 and 13. Figure 13 shows the and low-field regions of the spectrum printed in gray.
Figure 13. High-field region of the EPR spectra of frozen DMSO solutions (77 K) containing 5 m of complex 2 and 1, 2, 3, or 10 equiv.
of styrene and/or diethylamine. (a) Complex 2 + styrene, (b) complex 2 + diethylamine, (c) complex 2 + styrene + diethylamine.
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Polymer-Bound Oxidovanadium(IV) Complex
Scheme 7. Proposed overall mechanism for the oxidative amination of styrene. Only a schematic representation of [VO(sal-eta)]2 (2) is
shown.
The parameters for the new species formed (gray spec-
Whether the secondary amine coordinates in an axial po-
trum in Figure 12) are totally different from those of the sition to the vanadium center is not clear. Assuming this
complex in the absence of such an excess of styrene, and intermediate indeed forms, the selectivity could eventually
correspond to a higher Aʈ value. The spin Hamiltonian pa- be explained by a higher stability of the conformer that
rameters were obtained by simulation: gЌ = 1.979, gʈ = would lead to the anti-Markovnikov product. Alternatively,
1.945, AЌ = 60.4ϫ10–4 cm–1, and Aʈ = 168.9ϫ10–4 cm–1. the selectivity may be more easily explained on the basis
These results clearly show that only in the presence of sty- of the aminoalkyloxidovanadium species. Two distinct such
rene a clear change occurs in the EPR spectra, resulting intermediates may form: one corresponding to the Markov-
from a change in the coordination environment of the vana- nikov product, and the other corresponding to the anti-
dium center, and these changes can only be reasonably ex- Markovnikov product, the latter involving less steric hin-
plained by assuming the coordination of styrene. To the drance, thereby being favored.
best of our knowledge there is no data in the literature for
The formation of the final product is obtained by elimi-
the contribution of a coordinated olefin to Aʈ. This evi- nation of aminated species through cleavage of the vana-
dence for the coordination of styrene to vanadium supports dium–carbon bond. Because of the steric crowd present on
the mechanism for the reaction proposed in Scheme 7.
the secondary amine, the formation of the second interme-
diate species becomes more favorable and hence the yield
of the second product (see parts b, d, or f in Scheme 6) is
higher than the yield of the first product (see parts a, c,
or e in Scheme 6). The whole mechanism is illustrated in
Scheme 7. It is clear from the mechanism that the steric
crowd present on the secondary amine will direct the final
product. Thus, the anti-Markovnikov product will be fa-
vored over the Markovnikov product. Our experimental re-
sults (vide supra) also support this mechanism.
Proposed Mechanism of the Catalytic Reaction
On the basis of the spectroscopic (EPR and electronic)
results, it is reasonable to propose that the oxidovanadium-
(IV) activates the olefin via complex formation possibly
through π-bonding of the alkene group followed by its oxi-
dation to an oxidovanadium(V) species. Complex formation
of olefin through π-bonding causes partial shifting of the
electron density from the double bond to the empty d-or-
bital of the vanadium followed by polarization of the
double bond. While the dioxo complex interacts with sec-
ondary amine through hydrogen bonding and come close
Conclusions
A polymer-anchored oxidovanadium(IV) complex, PS-
to styrene, the polarization facilitates the nucleophilic at- [VO(sal-cys)·DMF] of the Schiff base derived from the reac-
tack of secondary amines to styrene. Because of the close tion of salicylaldehyde and -cysteine (H2sal-cys) has been
proximity of two hydrogen atoms and the oxygen of the prepared and characterized. The catalytic activity of PS-
oxo group of the dioxidovanadium(V) species, the reductive [VO(sal-cys)·DMF] for the oxidative amination of styrene
removal of water and formation of an oxidovanadium(IV) with secondary amines such as diethylamine, imidazole, and
intermediate species may result immediately. This process is benzimidazole in mild basic conditions gave a mixture of
fast and is not observable when all reagents are taken to- two aminated products in good yields. The steric hindrance
gether and monitored by either EPR or electronic absorp- present on the secondary amine directs the formation of the
tion spectroscopy.
final product and the anti-Markovnikov products have been
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M. R. Maurya, U. Kumar, I. Correia, P. Adão, J. Costa Pessoa
FULL PAPER
phenolic OH), 8.09 (d, 1 H, aromatic), 7.06–7.34 (m, 1 H, aro-
matic), 6.76–6.81 (m, 2 H, aromatic), 5.65, 5.84 (s, 1 H, -CH=N-,
geometrical isomers), 3.21, 3.34 (br., 1 H each, -CH2) ppm. 13C
NMR ([D6]DMSO): δ = 173.4 (carboxylic), 156.4 (azomethine),
155.1 (phenolic), 129.5, 128.6, 119.5, 119.2, 116.2 (aromatic), 65.2,
38.6 (alkyl chain) ppm. IR and 1H NMR spectroscopic data match
well with the literature values.
obtained in major yield. The corresponding neat complex,
[VO(sal-eta)]2 (H2sal-eta is the Schiff base derived from sali-
cylaldehyde and 2-aminoethanethiol) has also been pre-
pared. The catalytic activity of the neat complex has also
been found to be good. This study presents the first report
of an oxidovanadium(IV) complex for oxidative aminations.
The formation of possible intermediates during catalytic ac-
tion have been explored by titrating solutions of the neat
complex [VO(sal-eta)]2 with styrene, diethylamine, or a mix-
ture of styrene and diethylamine in the presence of a small
amount of triethylamine, following the titrations by EPR
spectroscopy. The EPR data gave evidence for the coordina-
tion of styrene to vanadium, therefore supporting a mecha-
nism for catalysis involving activation of the olefin via com-
plex formation followed by the attack by the amine. UV/
Vis experiments in the presence of O2 indicate the formation
of a dioxidovanadium(V) species, suggesting the participa-
tion of this species in the final redox steps that lead to the
formation of the enamine products.
PS-H2sal-cys (II): Chloromethylated polystyrene (4.0 g) was sus-
pended in DMF (30 mL) for 2 h. A solution of H2sal-cys (13.44 g,
60 mmol) dissolved in 40 mL of DMF was added to the above sus-
pension followed by triethylamine (5.05 g, 50 mmol) in ethyl acetate
(50 mL). The reaction mixture was heated at 90 °C for 20 h with
mechanical stirring in an oil bath. After cooling to room tempera-
ture, the yellow resin beads were separated by filtration, washed
thoroughly with hot DMF followed by hot methanol, and dried in
an air oven at 120 °C.
H2sal-eta (III): Ligand III was prepared following the literature
procedure. A solution of 2-aminoethanethiol hydrochloride (1.13 g,
10 mmol) dissolved in methanol (15 mL) was treated with KOH
(0.56 g, 10 mmol) and stirred for 1 h. After filtration, it was added
to a solution of salicylaldehyde (1.22 g, 10 mmol) dissolved in
10 mL of methanol and stirred for 3 h. After reducing the volume
of the solvent to ca. 10 mL, it was kept in a refrigerator where a
yellow solid slowly precipitated over 1 week. This was filtered,
washed with a small amount of cold methanol and dried in vacuo;
Experimental Section
Materials and Methods: Analytical reagent grade 2-aminoethane-
thiol hydrochloride (Aldrich Chemicals Co., USA), -cysteine,
imidazole, benzimidazole (S. D. Fine Chemicals, India), styrene
(Acros Organics, USA), V2O5, (Loba Chemie, India), acetylacetone
(E. Merck, India), aqueous 30% H2O2, salicylaldehyde (Ranbaxy,
India), and other chemicals were used as obtained. [VO(acac)2][29]
and H2sal-cys[30] were prepared according to the methods reported
in the literature.
1
yield 82%. H NMR ([D6]DMSO): δ = 13.31 (br., 1 H, phenolic-
OH), 8.57 (s, 1 H, -CH=N-), 7.44–6.87 (m, 4 H, aromatic), 3.88 (s,
2 H, -CH2-), 3.08 (s, 2 H, -CH2-) ppm. 13C NMR ([D6]DMSO): δ
= 167.4 (azomethine), 160.9 (phenolic), 132.9, 132.2, 119.3, 119.1,
116.9 (aromatic), 57.2, 38.9 (alkyl chain) ppm.
PS-[VO(sal-cys)·DMF] (1): The polymer-anchored ligand PS-
H2sal-cys (2.0 g) was allowed to swell in DMF (20 mL) for 2 h and
to this a solution of [VO(acac)2] (5.30 g, 20 mmol) in DMF (50 mL)
was added and the reaction mixture was heated at ca. 90 °C in an
oil bath for 15 h with slow mechanical stirring. The blackish green
beads of PS-[VO(sal-cys)·DMF] were filtered off, washed with hot
DMF followed by hot methanol, and dried in an air oven at 120 °C.
Physical and Spectroscopic Studies: Elemental analyses of the li-
gands and complexes were obtained with an Elementar model Va-
rio-EL-III. IR spectra were recorded as KBr pellets with a Nicolet
NEXUS Aligent 1100 FT-IR spectrometer. Electronic spectra of
polymer-anchored compounds were recorded in Nujol with a Shi-
madzu 1601 UV/Vis spectrophotometer by layering the sample
mull inside of one of the cuvettes while keeping the other one lay-
ered with Nujol as reference. Other ligands and complexes were
recorded in methanol. 1H NMR spectra were obtained with a
Bruker 500 MHz spectrometer in [D6]DMSO. The EPR spectra
were recorded with a Bruker ESP 300E X-band spectrometer. For
the polymer-anchored complex samples the spectra were measured
at room temperature and for the neat complex samples in DMSO
the EPR spectra were measured at 77 K (with glasses made by
freezing the solutions in liquid nitrogen). The spin Hamiltonian
parameters were obtained by simulation of the spectra with the
computer program of Rockenbauer and Korecz.[31] The magnetic
susceptibility of oxidovanadium(IV) complex was measured at
298 K with a Vibrating Sample Magnetometer model 155, using
nickel as the standard. Diamagnetic corrections were carried out
using Pascal’s increments.[32] Thermogravimetric analyses of the
complexes were carried out using a TG Stanton Redcroft STA 780
under an oxygen atmosphere. Scanning Electron Micrographs
(SEM) of catalysts were recorded with a Leo instrument model 435
VP. The samples were coated with a thin film of gold to prevent
surface changing and to protect the surface material from thermal
damage by the electron beam.
[VO(sal-eta)]2 (2): A solution of H2sal-eta (0.36 g, 2 mmol) in ace-
tonitrile (20 mL) was stirred with [VO(acac)2] (0.53 g, 2 mmol) dis-
solved in acetonitrile (20 mL) for 4 h. The precipitated green solid
of [VO(sal-eta)]2 was filtered off, washed with acetonitrile, and
dried in vacuo; yield 65%.
Oxidative Amination of Styrene: The oxidative amination of styrene
was carried out in a 50-mL two-necked reaction flask fitted with
a septum and water circulated condenser. The polymer-anchored
complex was allowed to swell in acetonitrile for 2 h prior to use in
each experiment. In a typical reaction, styrene (2.08 g, 20 mmol),
40 mmol secondary amine (diethylamine, 2.92 g; imidazole, 2.72 g
or benzimidazole 4.81 g) and triethylamine (6.06 g, 60 mmol) were
mixed in 20 mL of acetonitrile and the reaction mixture was heated
at 75 °C with the continuous bubbling of oxygen through a syringe.
Catalyst, PS-[VO(sal-cys)·DMF] (0.030 g) was added to the reac-
tion mixture and stirred for 4 h. The progress of the reaction was
monitored by withdrawing small amounts of the reaction mixture
at different time intervals and analyzing them quantitatively by
HPLC [with an Agilent 1100 series model equipped with a Hyper-
sil® BDS C18 (250ϫ4.6 mm, 5 µ) column (30:70, water/acetoni-
trile)]. Finally products were identified quantitatively using a Ther-
moelectron gas-chromatograph with an HP-1 capillary column
(30 mϫ0.25 mmϫ0.25 µm) and FID detector. The identities of the
products were confirmed by GC-MS (Perkin–Elmer, Clarus 500).
The effect of various parameters such as temperature, amount of
Synthesis of Ligands and Complexes
H2sal-cys (I): Ligand H2sal-cys was prepared as reported in the
literature.[30] Yield 70%. 1H NMR ([D6]DMSO): δ = 4.22 (br., 1 H,
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Polymer-Bound Oxidovanadium(IV) Complex
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Characterization of Aminated Products
Aminated Products with Diethylamine: (i) 1H NMR ([D6]DMSO)
(N,N-diethyl-2-phenylethanamine): δ = 7.5 (d, 1 H, =N-CH=),
7.47–7.26 (m, 5 H, aromatic), 5.37 (d, 1 H, -CH=), 3.51 (q, 4 H,
1
-CH2-), 0.99 (t, 6 H, CH3) ppm. (ii) H NMR ([D6]DMSO) (N,N-
diethyl-1-phenylethanamine): δ = 7.48–7.26 (m, 5 H, aromatic),
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-CH3) ppm.
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styryl-1H-imidazole): δ = 8.09 (s, 1 H, imidazole ring =CH-), 7.71–
7.26 (m, 5 H, aromatic), 7.35 (d, 2 H, imidazole ring-CH=CH-),
1
6.37 (d, 1 H, =CH-), 5.64 (d, 1 H, =N-CH=) ppm. (ii) H NMR
([D6]DMSO) [1-(1-phenylvinyl)-1H-imidazole]: δ = 8.10 (s, 1 H,
imidazole ring =CH-), 7.71 (d, 2 H, imidazole ring-CH=CH-),
7.47–7.07 (m, 5 H, aromatic), 5.63, 5.25 (d, 1 H each, =CH-) ppm.
1
Aminated Products with Benzimidazole: (i) H NMR ([D6]DMSO)
(1-styryl-1H-benzimidazole): δ = 8.28 (s, benzimidazole ring =CH-
), 7.69–7.23 (m, 9 H, aromatic), 6.21 (d, 1 H, =CH-), 5.84 (d, 1 H,
1
=N-CH=) ppm. (ii) H NMR ([D6]DMSO) [1-(1-phenylvinyl)-1H-
benzimidazole]: δ = 8.11 (s, benzimidazole ring =CH-), 7.47–7.26
(m, 9 H, aromatic), 6.28, 5.63 (d, 1 H each, =CH-) ppm.
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Acknowledgments
Financial support from University Grant Commission and Depart-
ment of Science and Technology, Government of India, New Delhi
is gratefully acknowledged. The authors also wish to thank for fin-
ancial support from Fundo Europeu de Desenvolvimento Regional
(FEDER), Fundação para a Ciência e Tecnologia, and Programa
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Published Online: November 27, 2007
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