Asymmetric aerobic oxidation of α-hydroxy acid derivatives catalyzed by reusable, polystyrene-supported chiral N-salicylidene oxidovanadium tert-leucinates

Article abstract of DOI:10.1002/adsc.201100062

The direct immobilization of two different C-5-propargyl ether-modified, chiral N-salicylidene vanadyl(V) tert-leucinates onto 4-azidomethyl-substituted polystyrene by click chemistry was examined. Among the eight different solvents investigated, the resulting polystyrene-supported catalysts promote the asymmetric, aerobic oxidation of α-hydroxy (thio)esters and amides with enantioselectivities of up to 99% ee (selectivity factor up to 41) in chloroform. These polystyrene-supported catalysts can be readily recovered by filtration and reused for at least four consecutive runs without discernible loss of reactivity and enantioselectivity.

Full text of DOI:10.1002/adsc.201100062

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DOI: 10.1002/adsc.201100062
Asymmetric Aerobic Oxidation of a-Hydroxy Acid Derivatives
Catalyzed by Reusable, Polystyrene-Supported Chiral
N-Salicylidene Oxidovanadium tert-Leucinates
Santosh B. Salunke,b N.Seshu Babu,a and Chien-Tien Chena,*
a
Department of Chemistry, National Tsing-Hua University, Hsinchu, Taiwan
Fax : (+886) 3-573-9240; phone: (+886) 3-573-3363; e-mail: ctchen@mx.nthu.edu.tw
Department of Chemistry, National Taiwan Normal University, Taipei, Taiwan
b
Received: January 24, 2011; Published online: May 17, 2011
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201100062.
Abstract: The direct immobilization of two differ-
ent C-5-propargyl ether-modified, chiral N-salicyli-
dene vanadyl(V) tert-leucinates onto 4-azidomethyl-
substituted polystyrene by click chemistry was ex-
amined. Among the eight different solvents investi-
gated, the resulting polystyrene-supported catalysts
promote the asymmetric, aerobic oxidation of a-hy-
droxy (thio)esters and amides with enantioselectivi-
ties of up to 99% ee (selectivity factor up to 41) in
chloroform. These polystyrene-supported catalysts
can be readily recovered by filtration and reused
for at least four consecutive runs without discerni-
ble loss of reactivity and enantioselectivity.
vanadyl(V) carboxylates[6] and alocholates,[7] respec-
tively, for the preparation of optically enriched a-hy-
droxy carboxylic acid derivatives. The oxidative kinet-
ic resolution strategy has also shown its potential in
the syntheses of biologically relevant a-hydroxy phos-
phonates,[8] a-hydroxy ketones[9], enantioenriched
cyclic ethers[10] and an antitumor agent – octalactin
A.[11] Moreover, chiral vanadyl(V) complexes are also
an effective catalysts in asymmetric sulfide oxida-
tion[12] and oxidative coupling of 2-naphthols.[13] In ad-
dition, these complexes acts as a DNA photocleavage
agent[14] and metal ion-specific (e.g., K+ and Ag+)
transporters.[15]
Although these homogenous transition metal com-
plexes offer high levels of reactivity and enantioselec-
tivity, their applications in the chemical and pharma-
ceutical industry remain somewhat limited due to
their relatively high cost, difficulties in separation and
contamination of metal complexes or metal salts
either in waste or the end products. Consequently,
their immobilization on solid supports has attracted
Keywords: click chemistry; direct immobilization;
enantioselective aerobic oxidation; recyclable cata-
lysts; vanadyl compounds
Transition metal complex-catalyzed asymmetric aero- enormous interest, as this allows for facile separation
bic oxidations[1] have recently emerged as highly effi- of the heterogeneous catalysts from products, enables
cient methods to obtain optically pure alcohols along efficient recovery and reuse of catalysts and minimiz-
with kinetic resolution, especially in cases where es pollution occurring from metal complexes.[16] De-
asymmetric reduction of ketones could not provide spite these options and numerous applications of
sufficiently high enantioselectivity. Significant prog- chiral vanadium complexes, only a few approaches for
ress in this area has been made with a variety of their immobilization have been described.[17] Among
chiral metal catalysts.[2] In particular, enantiomerically these, the Jones group reported polymer-supported,
pure a-hydroxy carboxylic acid and mandelic acid de- tridentate vanadyl complexes derived from salicylal-
rivatives are ubiquitous building blocks in enantiose- dehydes and optically active a-amino alcohols.[17d]
lective synthesis and the synthesis of biologically Preimmobilization of the Schiff base ligands by a co-
active compounds such as semi-synthetic penicillin, polymerization method followed by in-situ introdcu-
cephalosporin, anti-thrombotic and anti-obesity tion of the vanadyl(V) units led to the resulting heter-
agents.[3] With the extension of interest in new organic ogeneous catalysts for oxidative kinetic resolution of
transformations by using vanadyl species,[4,5] we and a-hydroxy esters with good to moderate enantioselec-
Tosteꢀs group have developed highly enantioselective tivities albeit with a somewhat limited substrate
aerobic oxidations catalyzed by chiral N-salicylidene scope.
1234
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Asymmetric Aerobic Oxidation of a-Hydroxy Acid Derivatives
Scheme 1. Synthetic design of heterogeneous, PS-supported chiral vanadyl catalysts 1 and 2 by click chemistry.
We report here for the first time, the direct immobi- firmed by the complete disappearanceÀo1 f the typical
ꢀ
lization of chiral vanadyl(V) tert-leucinates on a poly- azide N N stretching band at 2082 cm on IR spec-
styrene (PS) support through the copper(I)-catalyzed troscopy, along with the formation of new characteris-
azide-alkyne cycloaddition[18] reaction (i.e., click tic bands at 1619–1623 cmÀ1 (C=N), 1548–1550 cmÀ1
chemistry[19]) and their applications in the heterogene- (COO) and 982–995 cmÀ1 (V=O) in 1 and 2. The va-
ous asymmetric aerobic oxidation of a-hydroxy acid nadium loadings in catalyst 1 and 2 were found to be
derivatives. We envisaged that click chemistry, an effi- 0.74 mmolgÀ1
G
and
0.78 mmolgÀ1-
cient and mild immobilization technique,[20] should
ACHTUNGTRENNUNG
lead to economical, eco-friendly and recyclable poly- coupled plasma atomic emission spectroscopy (ICP-
mer-supported vanadyl catalysts (1 and 2) without AES).[21] Binding energies of V 2p3/2 for the PSS cata-
any freely dissociated metal complexes. In addition, lysts 1 and 2 were determined to be 517.3 eV and
the anchoring of an azido-bearing polymer at the C-5 518.8 eV, respectively, by X-ray photoelectron spec-
position of the catalysts 7 or 8 through an ether troscopy (XPS), which are attributed to V(V) spe-
spacer should avoid any interference of the polymer cies.[13d,22]
backbone with the catalytic site (Scheme 1).
By following our optimal homogeneous catalytic
Propargyl ether-modified salicylaldehydes 4 and 6 asymmetric aerobic oxidation protocol,[6] we tested
were readily prepared in two steps from 2-tert-butyl- the newly developed heterogeneous PSS catalysts 1
benzene-1,4-diol and 3-tert-butyl-2-hydroxybenzalde- and 2 for the kinetic resolution of racemic benzyl
hyde, respectively, in good yields.[21] They can be mandelate 10 as a model substrate by using molecular
transformed into the corresponding chiral vanadyl(V) oxygen as an oxidant, Table 1. The aerobic oxidation
complexes 7 and 8 by using our own protocol as de- of 10, catalyzed by PSS catalyst 1, was completed in
scribed previously with l-tert-butylglycine and VO- 60 h at 57% conversion (entry 1, Table 1). In marked
ACHTUNGTRENNUNG(SO4). Conversely, azido-substituted resin 9 was pre- contrast, the corresponding analoguous PSS catalyst 2
pared by treating chloromethyl-polystyrene with is more reactive, which effects 51% conversion of the
sodium azide. Alkyne-modified chiral oxidovanadium same substrate within 40 h (entry 2, Table 1). In addi-
complexes were then grafted onto the azido function- tion, the enantioselectivity for the asymmetric oxida-
alized resin by a copper(I)-catalyzed cycloaddition, af- tion of O-benzyl mandelate catalyzed by 2 (krel =10)
fording the immobilized catalysts 1 and 2 (Scheme 2). is significantly higher than that catalyzed by 1 (krel =
Notably, they can also be prepared by first carrying 6). Since the extent of resin swelling highly depends
out the click chemistry followed by vanadyl complex on solvent attributes, we paid much more attention to
formation with similar efficiency. Complete consump- identify an optimal solvent for this reaction by using
tion of the azido-functionalized resin during the cyclo- PSS catalyst 2. After screening several polar coordina-
addition with alkyne-modified catalysts was con- tion (TBME, CH3CN, and THF) and non-polar
Scheme 2. Preparation of PSS catalysts 1 and 2.
Adv. Synth. Catal. 2011, 353, 1234 – 1240
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Table 1. Effect of PSS catalysts 1 or 2 and solvents on the asymmetric aerobic oxidation of racemic benzyl mandalate 10.
[d]
Entry
Catalyst/Solvent
Time [h]
Conversion [%][a]
% ee[b] (Yield[c] [%])
krel
1
2
3
4
5
6
7
8
1/toluene
2/toluene
2/TBME
2/CH3CN
2/THF
2/C6H5Cl
2/CCl4
60
40
80
168
80
48
112
26
50
34
22
57
52
51
44
trace
55
63
56
55
68 (40)
70 (45)
65 (46)
29 (53)
n.d.
82 (42)
75 (34)
90 (40)
79 (42)
88 (41)
96 (41)
6
10
8
3
n.d.
13
6
17
11
16
24
2/CH2Cl2
9
10
11
2/
N
1/CHCl3
2/CHCl3
56
57
[a]
1
Determined by H NMR analysis of the reaction mixture.
[b]
[c]
[d]
Determined by HPLC analysis on a Chiralcel AD-H column.
Isolated, purified material for the alcohol by column chromatography.
k
rel =ln
N
(1Àee)]/ln
G
ACHTUNGTREN(NUNG 1+ee)], where C is conversion and ee is enantiomeric excess.
(chlorobenzene and haloalkanes) solvents, a signifi-
The substrate scopes for the kinetic resolution of
cant improvement in asymmetric induction was ob- mandelates by heterogeneous catalyst 2 with varying
served in chloroform (krel =24, entry 11 in Table 1). ester appendages were further examined (Table 3).
Under the optimal reaction conditions, PSS catalyst 1 The beneficial effect of an O-benzyl substituent (krel =
was found to be less enantioselective (krel =16, 24) over O-methyl (11, krel =12), O-ethyl (12, krel =9),
entry 10 in Table 1) in comparison with PSS catalyst and O-isopropyl (13, krel =11) substituents was ob-
2.
served. This finding prompted us to subsequently re-
The effect of catalyst loadings on enantioselectivity solve S-benzyl and N-benzyl mandelates, 14 and 15,
(Table 2) was further evaluated. It was found that cat- under the optimal conditions. It was found that N-
alyst loading of 2 at 2 mol% can still promote the benzylmandelamide is more enantioselective towards
aerobic oxidation of 10 without significant loss in re- asymmetric oxidation for catalyst 2 (krel =41) followed
activity and selectivity (entry 4, Table 2). Further de- by O-benzyl mandelate, whereas S-benzyl thiomande-
crease in catalyst loading to 1 mol% led to a sluggish late was found to be the least enantioselective (kel =
reaction (80 h) and moderate enantioselectivity (69% 17). Notably, oxidation of N-benzylmandelamide pro-
ee, entry 5 in Table 2).
ceeded to 55% conversion, providing the (R)-enantio-
Table 2. Effect of catalyst loadings on kinetic resolution of 10.
[d]
Entry
Catalyst Loading [mol%]
Time [h]
Conversion [%][a]
% ee[b] (Yield[c] [%])
krel
1
2
3
4
5
5
4
3
2
1
22
22
23
27
80
57
56
55
53
51
96 (41)
94 (42)
91 (42)
83 (43)
69 (46)
24
23
21
17
10
[a]
1
Determined by H NMR analysis of the reaction mixture.
Determined by HPLC analysis on a Chiralcel AD-H column.
[b]
[c]
[d]
Isolated, purified material for the alcohol by column chromatography.
rel =ln (1Àee)]/ln[(1ÀC)(1+ee)], where C is conversion and ee is enantiomeric excess.
[(1ÀC)
k
A
N
N
ACHTUNGTRENNUNG
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Asymmetric Aerobic Oxidation of a-Hydroxy Acid Derivatives
Table 3. Effects of O-, N-, or S-substituents on the asymmetric aerobic oxidation of racemic mandelates by PSS catalyst 2.
[d]
X-R
Time [h]
Conversion [%][a]
% ee[b] (Yield[c] [%])
krel
OCH2Ph (10)
OCH3 (11)
OCH2CH3 (12)
OiPr (13)
SCH2Ph (14)
NHCH2Ph (15)
NHCH(Ph)2 (16)
NHiPr (17)
22
28
28
30
24
27
52
29
57
58
61
62
58
55
55
59
96 (41)
87 (38)
87 (35)
92 (34)
93 (39)
98 (43)
64 (42)
94 (38)
24
12
9
11
17
41
6
16
[a]
1
Determined by H NMR analysis of the reaction mixture.
Determined by HPLC analysis on a Chiralcel AD-H column.
Isolated, purified material for the alcohol by column chromatography.
[b]
[c]
[d]
k
rel =ln
N
(1Àee)]/ln
G
ACHTUNGTREN(NUNG 1+ee)], where C is conversion and ee is enantiomeric excess.
mer of 15 in 43% yield and 98% ee along with N- derivatives examined, the substrate bearing an elec-
benzyl-benzoyl-formamide in 53% isolated yield. In tron-donating para-methyl group 18 (krel =19) is
marked contrast, substrates 16 (krel =6) and 17 (krel = slightly more selective than those with electron-with-
16) bearing N-diphenylmethyl and N-isopropyl appen- drawing groups, para-chloro 19 (krel =15) and para-
dages are less reactive and selective than 15 towards CF3 20 (krel =13). Substrates bearing ortho-methyl 21
asymmetric oxidation under optimal reaction condi- (krel =9) and ortho-chloro 22 (krel =17) groups are less
tions.
reactive (112 and 88 h) than the corresponding para-
To probe further the substrate scope catalyzed by substituted counterparts. The kinetic resolution pro-
PSS catalyst 2, a series of benzyl a-hydroxyamides cess also works well in the cases of a-1-naphthyl- and
bearing different a-aryl and alkyl groups was exam- a-2-thiophenyl-containing analogues, leading to re-
ined under the optimal aerobic oxidation conditions covery of the (R)-enantiomers 23 and 24 in 90% ee
(Table 4). Among the three para-substituted phenyl (krel =13) and 91% ee (krel =14), respectively. The oxi-
Table 4. Effects of a-substituents on the asymmetric aerobic oxidation of racemic N-benzyl a-hydroxy-amides by catalyst 2.
[d]
G/substrate
Time [h]
Conversion [%][a]
% ee[b] (Yield[c] [%])
krel
4-CH3C6H4/18
4-ClC6H4/19
4-CF3C6H4/20
2-CH3C6H4/21
2-ClC6H4/22
36
30
32
112
88
38
22
8
60
62
56
58
64
59
59
56
57
60
97 (37)
97 (35)
85 (42)
80 (39)
99 (33)
90 (39)
91 (35)
86 (41)
91 (40)
44 (38)
19
15
13
9
17
13
14
14
17
3
1-naphthyl/23
2-thiophenyl/24
(E)-PhCH=CH/25
cyclopropyl/26[e]
90
95
ACHTUNGTRENNUNG
(CH3)2CH/27[e]
[a]
[b]
[c]
[d]
[e]
1
Determined by H NMR analysis of the reaction mixture.
Determined by HPLC analysis on a Chiralcel AD-H column.
Isolated, purified material for the alcohol by column chromatography.
k
rel =ln
N
(1Àee)]/ln
G
ACHTUNGTREN(NUNG 1+ee)], where C is conversion and ee is enantiomeric excess.
Reactions are conducted at 408C.
Adv. Synth. Catal. 2011, 353, 1234 – 1240
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Table 5. Recycling experiments for enantioselective aerobic
oxidations of racemic substrates 10, 15 and 23 by PSS-2.
[d]
Entry Cycle Substrate Time Conversion % ee[b]
krel
[h]
[%][a]
(yield[c]
[%])
1
2
3
4
5
6
7
8
1
10
10
10
10
15
15
15
15
23
23
23
23
22
23
26
27
27
30
29
30
38
37
44
47
57
55
59
54
55
56
53
54
59
54
58
61
96 (41)
92 (42)
97 (38)
89 (42)
98 (43)
99 (42)
94 (45)
96 (44)
90 (39)
81 (43)
86 (38)
88 (36)
24
23
21
21
41
41
39
39
13
13
11
10
2[e]
3[e]
4[e]
1
2[e]
3[e]
4[e]
1
9
10
11
12
2[e]
3[e]
4[e]
Figure 1. Photos for mixture containing (a) only PSS catalyst
2 and (b) PSS catalyst 2 and substrate 10 in CHCl3 with stir-
ring.
[a]
[b]
1
Determined by H NMR analysis of the reaction mixture.
Determined by HPLC analysis on a Chiralcel AD-H
column.
[c]
[d]
[e]
Isolated, purified material for the alcohol by column
chromatography.
dation of substrate 25 possessing a trans-cinnamyl
group (krel =14) proceeded at a significantly faster
rate (8 h) than those (30–112 h) of a-aryl analogues
without any intervening epoxidation at the alkene
moiety. Conversely, substrates bearing a-alkyl groups
like 26 and 27 are somewhat inert at ambient temper-
ature under the standard conditions. Nevertheless, the
desired process can be effected at slightly elevated
temperatures, albeit with prolonged reaction times
(90–95 h). Notably, the selectivity factors of their oxi-
dations catalyzed by 2 highly hinge on the steric ef-
fects of the a-alkyl groups. In comparison, the sub-
strate 26 bearing a 28 a-cyclopropyl group (krel =17)
is highly enantioselective as compared to that bearing
an a-isopropyl group 27 (krel =3).
k
rel =ln
N
(1Àee)]/ln
G
ACHTUNGERTN(NUNG 1+ee)], where C is con-
version and ee is enantiomeric excess.
Recycled PSS catalyst 2 from the previous run was used.
Scheme 3. Potential leaching of vanadium by hydrolysis of
PSS-2.
To demonstrate the recyclability and reusability of
PSS catalyst 2, the aerobic oxidation reactions of 10 authentic complex 29 (5 mol%) for the aerobic oxida-
by recovered catalyst were tested. Notably, PSS cata- tion of 10 was found to be completely inactive in
lyst 2 is completely insoluble and floats on the surface CHCl3 for 3 days, thus excluding its potential interfer-
of CHCl3 (Figure 1, a). Nevertheless, it can be fully ence.
dispersed into CHCl3 upon stirring with substrate 10
(Figure 1, b).
To demonstrate the synthetic utility of this proto-
col, a practical scale experiment (5 mmol) was per-
After completion of each cycle, the catalyst 2 was formed (Scheme 4). With 3 mol% of PSS 2, the de-
filtered, washed with CHCl3, and dried under sired (R)-benzyl mandelate 10 was obtained in 39%
vacuum. Notably, the recycled catalyst 2 was reused yield (472 mg) and 96% ee (krel =23). The oxidized
for four consecutive runs on three representative sub- product 10ꢀ (56% yield) was then converted back to
strates (10, 15, and 23) without significant erosion in racemic 10 in 96% yield by treatment with a solution
reactivity and selectivity (Table 5). The vanadium of NaBH4 in MeOH at 08C in 15 min. Repetition of
loading of the recovered catalyst 2 after the 4th cycle the sequences allowed the preparation of optically
was then checked again by ICP-AES MS. About 9% pure (R)-10 in a reasonable quantity. Previously,
loss of vanadium from PSS catalyst 2 was observed in we[6,13] and others[1] have demonstrated that during
the recycling experiments. In principle, the hydrolysis the aerobic oxidation event the molecular oxygen ulti-
of PSS catalyst 2 would lead to PSS salicylaldehyde mately leads to water which has a negligible effect on
28 and tert-leucine-based vanadyl complex 29 the reaction rate. Therefore, the catalytic protocol can
(Scheme 3). Nevertheless, 28 can be readily converted be carried out under an oxygen atmosphere on a
back to PSS-2. Notably, a control experiment using practically large scale.
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Asymmetric Aerobic Oxidation of a-Hydroxy Acid Derivatives
Scheme 4. Practical scale synthesis of (R)-10 by kinetic resolution of 10 and the recycle of 10 by reduction of 10’.
In conclusion, we have documented the first effi- catalyst loading=0.95 mmolgÀ1 of resin; vanadium load-
ing=0.73 mmolgÀ1 of resin.
Data for PSS catalyst 2: FT-IR (KBr): n=3336 (br), 2906,
cient immobilization of C-5-propargyl ether-modified
chiral N-salicylidene vanadyl(V) tert-leucinates onto
azido-functionalized PS by click chemistry. In addi-
tion, we have demonstrated for the first time that the
2850, 1658, 1619 (C=N), 1548 (COO), 1452, 1334, 1206,
1167, 1077, 995 (V=O), 908, 841 cmÀ1; elemental analysis
(%) found: C 69.33, H 7.33, N 5.49; ICP-AES MS (%): V,
resulting PSS catalysts promote aerobic oxidations of
3.98; catalyst loading=0.98 mmolgÀ1 of resin; vanadium
a broad range of a-hydroxy carboxylic acid deriva-
tives smoothly with excellent enantioselectivities in
CHCl3. A practical scale synthesis of optically pure,
mandelic acid derivatives can be made with 3 mol%
loading of PSS-2 in CHCl3. Furthermore, the PSS-
based catalysts can be readily recovered by filtration
and reused without discernible loss of reactivity and
enantioselectivity. The partially hydrolyzed catalyst
observed after four runs can be readily converted
back to the original entity.
loading=0.78 mmolgÀ1 of resin.
Representative Procedure for the Asymmetric
Aerobic Oxidation of Racemic a-Hydroxy Carboxylic
Acid Derivatives by PSS-2
Into a 5-mL, two-necked, round-bottomed flask was placed
PSS catalyst 2 (25.5 mg, 0.025 mmol, 5 mol%) in oxygen-sa-
turated CHCl3 (0.25 mL). A solution of a-hydroxy carboxyl-
ic acid derivative (0.5 mmol, 1 equiv.) in oxygen-saturated
CHCl3 (0.25 mL) was added and the resulting heterogeneous
mixture was vigorously stirred at ambient temperature
under an oxygen atmosphere. The reaction progress was
1
monitored by H NMR spectroscopy for percent conversion.
Experimental Section
Upon reaching 55–64% conversion, the reaction mixture
was filtered through a sintered-glass funnel and the insolu-
ble PSS catalyst 2 was washed with CHCl3 (2ꢂ0.5 mL). The
combined filtrates were concentrated under reduced pres-
sure. The resulting residue was loaded directly on top of an
eluent-filled silica gel column and purified by flash column
chromatography (EtOAc/hexanes). The enantiomeric excess
of the pure, resolved a-hydroxy carboxylic acid derivative
was analyzed by chiral HPLC analysis.
General Procedure for the Synthesis of Polystyrene-
Supported (PSS) Vanadyl Catalysts 1 and 2
(Azidomethyl)polystyrene resin 9 (estimated loading of
azide=1.07 mmolgÀ1, 935 mg, 1 mmol, 1 equiv.) was placed
in 1:1 mixture of anhydrous DMF:THF (20 mL). To this sus-
pension was added propargyl ether-modified vanadyl(V)
methoxide complex 7 or 8 (1.5 mmol, 1.5 equiv.) followed by
N,N-diisopropylethylamine (1.04 mL, 774 mg, 6 mmol,
6 equiv.) and copper(I) iodide (9.5 mg, 0.05 mmol,
0.05 equiv.). The resulting mixture was vigorously stirred at
ambient temperature and the progress of the reaction was
monitored by IR spectroscopy. After having been stirred for
68–70 h, the typical azide band at 2082 cmÀ1 in the IR spec-
trum had completely disappeared. The heterogeneous reac-
tion mixture was then filtered and the solid was sequentially
washed with water (250 mL), DMF (250 mL), THF
(250 mL), THF:MeOH (1:1, 250 mL), MeOH (250 mL) and
THF (250 mL). The solid was then placed in air-saturated
MeOH (20 mL) and the mixture was refluxed under an
oxygen atmosphere for 12 h and then gradually cooled to
ambient temperature. The solid was filtered and dried under
vacuum overnight to afford polystyrene supported chiral va-
nadyl(V) methoxide complex 1 or 2 (i.e., PSS catalyst 1 or
2).
General Procedure for the Recovery of PSS Catalyst
2
Upon reaching 53–56% conversion, the reaction mixture
was filtered through a sintered-glass funnel and the insolu-
ble PSS catalyst 2 was washed with CHCl3 (2ꢂ0.5 mL). The
recovered PSS catalyst 2 was dried under vacuum overnight
and then placed in MeOH (1.0 mL). The resulting suspen-
sion was purged with oxygen gas for 30 min. The insoluble
catalyst was filtered, dried under vacuum for 30 min and
used for the next run.
Acknowledgements
Data for PSS catalyst 1: FT-IR (KBr): n=3384 (br), 1658,
1622 (C=N), 1599, 1547 (COO), 1420, 1338, 1202, 1153,
1045, 982 (V=O), 903, 757 cmÀ1; elemental analysis (%)
found: C 66.51, H, 6.65, N 5.35; ICP-AES MS (%): V, 3.71;
We thank the National Science Council of Taiwan for finan-
cial support of this research.
Adv. Synth. Catal. 2011, 353, 1234 – 1240
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COMMUNICATIONS
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