Expedient immobilization of TEMPO by copper-catalyzed azide-alkyne [3 + 2]-cycloaddition onto polystyrene resin

Article abstract of DOI:10.1002/adsc.200606043

TEMPO was readily grafted by copper(I)-catalyzed azide-alkyne cycloaddition onto polystyrene. Starting with commercially available Merrifield resin (4.3 mmol/g) almost quantitative loading of TEMPO onto the polymer was achieved (≥ 4 mmol/g). The so obtained PS-CLICK-TEMPO allowed the oxidation of alcohols to aldehydes with bleach or molecular oxygen as the terminal oxidant with high yields and selectivity in multiple cycles without loss of activity.

Full text of DOI:10.1002/adsc.200606043

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DOI: 10.1002/adsc.200606043
Expedient Immobilization of TEMPO by Copper-Catalyzed
Azide-Alkyne [3+2]-Cycloaddition onto Polystyrene Resin
Alexandru Gheorghe,a Ai Matsuno,a and Oliver Reisera,*
a
Institut für Organische Chemie, Universität Regensburg, Universitätsstr.31, 93053 Regensburg, Germany
Fax : (+49)-941-943-4121; e-mail: Oliver.Reiser@chemie.uni-regensburg.de
Received: February 7, 2006; Accepted: April 15, 2006
Abstract: TEMPO was readily grafted by copper(I)-
catalyzed azide-alkyne cycloaddition onto polystyr-
ene.Starting with commercially available Merrifield
resin (4.3 mmol/g) almost quantitative loading
of TEMPO onto the polymer was achieved
(ꢀ 4 mmol/g).The so obtained PS-CLICK-TEMPO
allowed the oxidation of alcohols to aldehydes with
bleach or molecular oxygen as the terminal oxidant
with high yields and selectivity in multiple cycles
without loss of activity.
supported oxammonium salts.[8n] These catalysts have
shown good efficiency for the selective oxidation of
primary and secondary alcohols to the corresponding
aldehydes and ketones.Alternatively, polymer bound
co-oxidants have been proposed to overcome limita-
tions associated with the necessity to employ stoichio-
metric amounts of the co-oxidant in solution, requir-
ing additional purification steps.[9]
We report here the simple and efficient preparation
of a new polystyrene-supported TEMPO by using
click chemistry as a practical tagging method and
demonstrate its high activity for the oxidation of alco-
hols using bleach or molecular oxygen as a co-oxi-
dant.
Keywords: catalyst immobilization; oxidation;
oxygen; polymers; polystyrene resin; TEMPO
The catalyst could be prepared in an easy two-step
sequence starting from commercially available 4-hy-
droxy-TEMPO 1 and polystyrene-supported azide 3
The copper-catalyzed[1] azide-alkyne cycloaddition[2] (Scheme 1).[10] Propargylation of 1 was straight-for-
(CuAAC) reaction, coined as a click reaction,[3] has ward giving rise to 2, which set the stage for the sub-
proven to be most powerful for ligating functional sequent copper(I)-catalyzed cycloaddition: following
molecules to supporting scaffolds or to each other.[4] a protocol developed by Gmeiner and co-workers[5a]
Following the seminal contributions of Gmeiner and 6 mol% Cu(I) iodide were found to be optimal to
co-workers,[5] there is a growing awareness that this achieve the smooth formation of 4 as judged by the
reaction can also be used for the synthesis of func- complete disappearance of the typical IR absorption
tional polymers and dendrimers.[6] In addition, the of the azide group (2095 cmÀ1).No co-reductant had
CuAAC, offering wide tolerance for reactive or sensi-
tive groups, should have great potential for the syn-
thesis of heterogeneously immobilized catalysts and
reagents.
Oxidation of alcohols using catalytic amounts of
the stable nitroxyl radical 2,2,6,6-tetramethylpiperi-
dine-1-oxyl (TEMPO) in combination with safe and
easy to handle primary oxidants has received signifi-
cant attention due to the low toxicity of the reagent
and the good chemoselectivity achieved.[7] The de-
mands for inexpensive, environmentally friendly and
renewable polymer-supported (PS) catalysts have led
to the synthesis of several immobilized TEMPO moi-
eties,[8] including silica-supported TEMPO,[8a,b]
MCM-41-supported TEMPO,[8c] sol-gel TEMPO,[8d,e]
PEG-TEMPO,[8f–h] polynorbornene-derived TEMPO,[8i]
fluorous-tagged TEMPO,[8j,k] acid functionalized Fibre-
CatTM [8l] polyamine TEMPO (PIPO)[8m] and polymer- Scheme 1. Synthesis of PS-CLICK-TEMPO 4.
,
1016
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Immobilization of TEMPO
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Table 1. PS-CLICK-TEMPO mediated oxidation of alcohols
to be added, indicating that no oxidation of Cu(I) by
TEMPO had occurred.The estimated catalyst loading
based on elemental analysis and recovered 2 was
4 mmolgÀ1 of nitroxyl radical.This value is very close
to that of the original Merrifield resin (ca.
4.3 mmolgÀ1) showing the high efficiency of this tag-
to carbonyl derivatives.[a]
Entry Alcohol
Conversion Yield
Purity
[%][b]
[%][b]
[%][c]
1
2
benzyl alcohol
>98
4-bromobenzyl al- >98
cohol
99
93
>98
>98
I
ging method.When 3 mol% Cu iodide were em-
ployed, only 50% conversion was achieved, resulting
in a resin 4 with a catalyst loading of 2 mmolgÀ1 of ni-
troxyl radical.
3
4
4-methylbenzyl
alcohol
4-methoxybenzyl >98
>98
98
92
>98
>98
The new heterogeneous “PS-CLICK-TEMPO” 4
proved to be very efficient for the selective oxidation
of aliphatic and benzylic alcohols under Anelliꢁs con-
ditions[7b], using sodium hypochlorite as terminal oxi-
dant and KBr as co-catalyst (Table 1).In all cases 25.
mol% of PS-CLICK-TEMPO 4 was used, allowing
complete conversion of the substrates generally in
30 min.Primary benzylic alcohols (entries 1–5) gave
the corresponding aldehydes with high yields and
complete chemoselectivity, no overoxidation to the
alcohol
2-phenylethanol
1-octanol
5
6
7
8
9
10
>98
>98
>98
>98
>98
>98
95
95
96
99
99
98
>98
95
95
96
>98
>98
1-decanol
1-dodecanol
3-nonanol[d]
cyclohexanol[e]
[a]
Alcohol (1 mmol) in CH2Cl2 (2 mL), KBr (0.3 mmol), PS-
TEMPO (2.5 mol%), NaOCl (1.3 mmol), NaHCO3
(0.2 mmol), 08C.Reaction time =30 min.
respective carboxylic acids was observed.Moreover, 4 [b] Determined by 1H and 13C NMR; >98% indicates that
no starting material or by-products could be detected.
Yields of isolated products.
was also effective for the oxidation of the more chal-
lenging aliphatic alcohols (entries 6–10).Formation of
the aldehydes from primary alcohols (entries 6–8) was
accompanied by minute amounts (<5%) of carboxyl-
ic acids, while oxidation of more demanding secon-
dary alcohols (entries 9 and 10) required somewhat
longer reaction times (1–5 h) to achieve complete
conversion.In all cases, the purity of the isolated al-
dehydes was>95% without the need for purification
by column chromatography.
We next examined the recyclability of our catalyst
(Table 2).PS-CLICK-TEMPO 4 was easily recovered
by filtration and reused for the next cycle without fur-
ther activation in five subsequent runs employing 4-
methylbenzyl alcohol as substrate.Virtually no loss of
activity was observed, and also the chemoselectivity
for the formation of the aldehyde remained very high
in all cases.Only traces of carboxylic acid, being
easily removed by washing the organic phase with
sodium bicarbonate solution, were observed.
[c]
[d] Reaction time=5 h.
[e]
Reaction time=1 h.
Table 2. Recycling of PS-CLICK-TEMPO 4 in the oxidation
of 4-methylbenzyl alcohol.[a]
Run
Conversion [%][b]
Yield [%][c]
Purity [%][b]
1
2
3
4
5
>98
>98
>98
>98
>98
95
94
93
95
92
>98
>98
>98
>98
>98
[a]
Alcohol (3 mmol) in CH2Cl2 (6 mL), KBr (1.0 mmol), PS-
TEMPO (2.5 mol%), NaOCl (3.9 mmol), NaHCO3
(0.6 mmol), 08C.Reaction time =30 min.
[b] Determined by 1H and 13C NMR; >98% indicates that
no starting material or by-products could be detected.
[c]
Yields of isolated products.
In 1984, Semmelhack reported that TEMPO/CuCl
can oxidize allylic and benzylic alcohols to aldehydes
using molecular oxygen.[7a] This method was recently for industrial processes since non-chlorinated solvents
improved with the discovery of several variations of and oxygen as the terminal oxidant are used.PS-
the original Cu/TEMPO catalytic systems.[11,13b] Min- CLICK-TEMPO 4 showed outstanding activity in all
isci et al.showed that the aerobic oxidation of both
experiments with generally employing only 5 mol%
activated and unactivated alcohols can be performed of catalyst loading (Table 3) at reaction times of 3–
under mild conditions using Mn(II)-Co(II) or Mn(II)- 6 h.Only cinnamyl alcohol, which is known to be less
Cu(II) nitrates in acetic acid under ambient pressure reactive,[8h] was cleanly oxidized within 24 h in the
and temperature.[12] Although numerous immobilized presence of 10 mol% catalyst.
TEMPO variants on both organic and inorganic sup-
Also under these conditions PS-CLICK-TEMPO 4
ports were synthesized, only few examples are known could be recycled efficiently.In five consecutive runs
where molecular oxygen is used as terminal oxi- using 4-bromobenzyl alcohol as the substrate the cata-
dant.[8c,h,l,13] We decided to investigate the catalytic lyst was recovered by simple filtration and reused as
properties of PS-CLICK-TEMPO 4 under the condi- such giving high yields of aldehyde in all cases
tions developed by Minisci, being especially attractive (Table 4).
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Alexandru Gheorghe et al.
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Table 3. Aerobic oxidation of alcohols to carbonyl compounds by PS-CLICK-TEMPO.[a]
Entry
Substrate
Time [h]
Conversion [%][b]
Yield [%][c]
Purity [%][b]
1
2
3
4
5
6
benzyl alcohol
3
3
3
6
6
24
>98
>98
>98
>98
>98
>98
98
95
98
97
96
96
>98
>98
>98
95
95
>98
4-bromobenzyl alcohol
4-methylbenzyl alcohol
1-octanol
1-decanol
cinnamyl alcohold
[a] Alcohol (1 mmol) in AcOH (1 mL), Mn
A
G
À1, 25 mg, 5 mol%), 408C.
[b] Determined by 1H and 13C NMR; >98% indicates that no starting material or by-products could be detected.
[c]
Yields of isolated products.
[d] PS-TEMPO (10 mol%).
Table 4. Aerobic oxidation of 4-bromobenzyl alcohol by PS- Propargyl Ether TEMPO (2)
CLICK-TEMPO.Recycling experiment. [a]
To a stirring suspension of NaH (60% in mineral oil,
Run
Conversion [%][b]
Yield [%][c]
Purity [%][b]
850 mg, 22.0 mmol) in dry DMF (100 mL) 4-hydroxy-
TEMPO 1 (3.0 g, 17.44 mmol) was added portionwise at
08C and stirred at room temperature for 30 min.Propargyl
bromide (2.0 mL, 22.0 mmol) was added dropwise at 08C.
The resulting mixture was stirred for 3 h at room tempera-
ture Water (100 mL) was added and the solution was ex-
tracted with EtOAc (5”50 mL).The combined organic
phases were washed with water (10”50 mL) and dried over
MgSO4, filtered, evaporated under reduced pressure and pu-
rified by column chromatography (silica gel, 10% EtOAc in
hexanes) to give the title compound 2 as an orange solid;
yield: 2.47 g (68%); mp 58–598C; MS (PI-EI-MS 70 eV):
m/z=210.2 (M+); anal.calcd for C 12H20NO2: C 68.54, H
9.59, N 6.66; found: C 68.21, H 9.89, N 6.33.
1
2
3
4
5
>98
>98
>98
>98
>96
94
95
93
91
93
>98
>98
>98
>98
96
[a]
Alcohol (3 mmol) in AcOH (3 mL), Mn
(0.6 mmol), Co(NO3)3·6 H2O (0.6 mmol), PS-TEMPO 4
(4.0 mmol/loading, 38 mg, 5 mol%), 408C.
(NO3)2·4 H2O
ACHTREUNG
[b] Determined by 1H and 13C NMR.
[c]
Yields of isolated products.
In conclusion, we have developed an extremely
simple and practical protocol for grafting TEMPO
onto polystyrene resin from readily available and in-
expensive starting materials using the copper(I)-cata-
lyzed alkyne-azide cycloaddition as the ligation
method.The resulting PS-CLICK-TEMPO 4 proved
to be highly effective in the chemoselective oxidation
of alcohols with both bleach and molecular oxygen
under mild conditions.Moreover, it can be easily re-
covered and recycled without any loss of catalytic ac-
tivity.
Azidomethyl Polystyrene (3)
Merrifield resin (2.5 g, 4.3 mmol/g) in 25 mL DMSO was
shaken at 508C with NaN3 (2.45 g, 37.63 mmol) for 2 days.
After being cooled to room temperature, the suspension was
filtered and the resin was washed with MeOH (5”15 mL)
and CH2Cl2 (5”15 mL) to give resin 3; yield: 2.4 g; IR
(KBr): n=2095 cmÀ1
.
PS-CLICK-TEMPO (4)
Method A: To a suspension of Merrifield supported azide 3
(estimated loading 4.3 mmol/g of azide, 1.10 g, 4.76 mmol)
in degassed DCM (100 mL), 4-propargyloxy-TEMPO 2
(1.87 g, 8.58 mmol) and CuI (27 mg, 3 mol%) were added.
The resulting mixture was shaken at room temperature for 3
days.The resin was filtered and washed with CH 2Cl2 (5”
50 mL) and dried under vacuum to afford PS-TEMPO 4
with an estimated TEMPO loading of 2.0 mmol/g; yield.
1.40 g. The loading was determined by measurement the
quantity of unreacted propargyl ether TEMPO 2, elemental
analysis and IR.
Experimental Section
General
DMF was distilled over P2O5 before use.Dichloromethane
was distilled over calcium hydride.The Merrifield resin ( ca.
4.3 mmol/g Cl loading, cross-linked with 2% DVB, 200–400
mesh) was purchased from Fluka.All commercially avail-
able compounds were used as received.
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Adv. Synth. Catal. 2006, 348, 1016 – 1020

Immobilization of TEMPO
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Method B. To a suspension of Merrifield supported azide
(estimated loading 4.3 mmol/g of azide, 570 mg,
Aerobic Oxidation of 4-Bromobenzyl Alcohol by
PS-CLICK-TEMPO; Recycling Experiments
3
2.43 mmol) in degassed THF (50 mL), 4-propargyloxy-
TEMPO 2 (960 mg, 4.57 mmol) and CuI (27 mg, 6 mol%)
were added.The resulting mixture was shaken at room tem-
perature for 5 days.The resin was filtered and washed with
CH2Cl2 (5”50 mL) and dried under vacuum to afford PS-
4-Bromobenzyl alcohol (561 mg, 3 mmol), Mn
A
(15 mg, 0.6 mmol), Co(NO3)3·6H2O (18 mg, 0.6 mmol), PS-
AHCTREUNG
CLICK-TEMPO 4 (4.0 mmolgÀ1 loading, 38 mg, 5 mol%)
and glacial acetic acid (3 mL) were added to a Schlenk flask
and heated at 408C under an oxygen atmosphere.After 4 h
the reaction mixture was filtered, washed with water and
CH2Cl2.The organic layer was washed with water (2”
10 mL), dried over MgSO4 and concentrated under vacuum
to afford 4-bromobenzaldehyde.PS-TEMPO was washed
with water and CH2Cl2 and reused without further purifica-
TEMPO
4
with an estimated TEMPO loading of
4.0 mmol/g; yield: 1.05 g. The loading was determined by
measurement the quantity of unreacted propargyl ether
TEMPO 2, elemental analysis and IR.
General Procedure for the Oxidation of Alcohols by
PS-CLICK-TEMPO/Bleach
tion.Fresh portions of Mn
ACHTREUNG
and Co
run.
ACHTREUNG
An alcohol (1.0 mmol) in 2 mL CH2Cl2, KBr (40 mg,
0.3 mmol) and PS-TEMPO 4 (2.0 mmol/g loading, 13 mg,
2.5 mol%) were added to a round-bottom flask. The reac-
tion mixture was stirred at 08C before addition of 0.8 mL
NaOCl (10%) and NaHCO3 (40 mg, 50 mgmLÀ1 bleach).
The resulting suspension was stirred at 08C for 30 min.The
reaction mixture was filtered, washed with CH2Cl2, dried
over MgSO4 and concentrated under vacuum to afford the
corresponding aldehyde.
Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
schaft (SPP 1179 Organokatalyse) and the Bayerische For-
schungsgesellschaft (fellowshipfor AM).
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