Sustainable Micellar Gold Catalysis - Poly(2-oxazolines) as Versatile Amphiphiles

Article abstract of DOI:10.1002/adsc.201600139

The application of five polymer amphiphiles in the gold-catalyzed allene cycloisomerizations under aqueous micellar conditions is described. The polymers were prepared by ring-opening cationic polymerization based on poly(2-methyl-2-oxazoline) as hydrophilic segment and different hydrocarbon- or fluorocarbon-based hydrophobic segments. The catalytic activity in the gold-catalyzed allene cyclization is strongly dependent on the type of gold precursor, the salt concentration in the bulk aqueous medium, and the concentration of the polymeric amphiphile. Best results were obtained with 2 mol% of gold(III) bromide, 1 mM of amphiphile and 5 M sodium chloride, affording over 80% yield for different heterocyclic products. The catalyst system is also suitable for the dehydrative cyclization of acetylenic diols to furans. Moreover, successful catalyst recycling was demonstrated in three consecutive runs when using optimized extraction conditions.

Full text of DOI:10.1002/adsc.201600139

FULL PAPERS
DOI: 10.1002/adsc.201600139
Sustainable Micellar Gold Catalysis – Poly(2-oxazolines) as
Versatile Amphiphiles
Linda Lempke,a Andrea Ernst,b Fabian Kahl,a Ralf Weberskirch,b,*
and Norbert Krausea,*
a
Faculty of Chemistry and Chemical Biology, Organic Chemistry, TU Dortmund, Otto-Hahn-Str. 6, 44227 Dortmund,
Germany
Fax: (+49)-231-755-3884; phone: (+49)-231-755-3882; e-mail: norbert.krause@tu-dortmund.de
Faculty of Chemistry and Chemical Biology, Polymer Hybrid Systems, TU Dortmund, Otto-Hahn-Str. 6, 44227 Dortmund,
b
Germany
Fax: (+49)-231-755-5363; phone: (+49)-231-755-3863; e-mail: ralf.weberskirch@tu-dortmund.de
Received: February 1, 2016; Revised: March 22, 2016; Published online: April 25, 2016
Dedicated to Prof. Dr. Henning Hopf on the occasion of his 75th birthday.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201600139.
Abstract: The application of five polymer amphi- phiphile. Best results were obtained with 2 mol% of
philes in the gold-catalyzed allene cycloisomeriza- gold(III) bromide, 1 mM of amphiphile and 5M
tions under aqueous micellar conditions is described. sodium chloride, affording over 80% yield for differ-
The polymers were prepared by ring-opening cation- ent heterocyclic products. The catalyst system is also
ic polymerization based on poly(2-methyl-2-oxazo- suitable for the dehydrative cyclization of acetylenic
line) as hydrophilic segment and different hydrocar- diols to furans. Moreover, successful catalyst recy-
bon- or fluorocarbon-based hydrophobic segments. cling was demonstrated in three consecutive runs
The catalytic activity in the gold-catalyzed allene when using optimized extraction conditions.
cyclization is strongly dependent on the type of gold
precursor, the salt concentration in the bulk aqueous Keywords: allene cycloisomerization; gold catalysis;
medium, and the concentration of the polymeric am- green chemistry; micelles; polymer amphiphiles
Introduction
a rapid decomposition of the catalyst to metallic gold.
Here, the use of micellar catalysis opens a versatile al-
Gold catalysis represents a powerful synthetic strat- ternative which was rarely used up to now. Particular-
egy which is particularly useful in terms of efficiency ly vitamin E-based nanomicelles were pointed out as
and selectivity.[1] Since gold catalysts are usually non- an adequate reaction medium for transition metal-cat-
toxic and many reactions take place under mild condi- alyzed coupling reactions[4] and also for gold cataly-
tions, gold catalysis is highly suitable for the develop- sis.[5] Polymeric amphiphiles may represent an alterna-
ment of sustainable synthetic methods. Different ap- tive to low molecular weight surfactants. Their higher
proaches were implemented so far to allow catalyst structural variability and straightforward synthesis en-
recycling and to reduce the amount of organic sol- ables versatile strategies and possibilities of fine
vents. In this context, ionic liquids have to be men- tuning.[6] Previous work has shown that the cationic
tioned because of their excellent solvent properties ring-opening polymerization (CROP) of 2-oxazolines
which enable the transformation of many substrates provides an excellent method to synthesize tailor-
of different polarity.[2] In contrast, the use of water as made polymeric nanoreactors, which have been used
reaction medium is strongly limited. Water-soluble for transition metal-catalyzed transformations.[7] The
gold catalysts allow the conversion of polar com- CROP methodology enables the introduction of func-
pounds with potential of catalyst recycling but hydro- tional groups via the initiator[8] and/or the terminating
phobic reactants are more difficult to convert.[3] How- agent,[9] and the diversity of the monomers allows
ever, if simple gold catalysts are used in water[3d] or in control of the polarity of the polymer from hydrophil-
an organic solvent,[1] recycling is impossible due to ic to hydrophobic and fluorophilic.[10]
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Table 1. Influence of polymeric amphiphile and salt addition
on the gold-catalyzed cycloisomerization of allene 1a.
Here, we show for the first time the effect of five
different poly(2-oxazoline) amphiphiles in the gold-
catalyzed cycloisomerization of functionalized allenes
to the corresponding heterocycles under aqueous mi-
cellar reaction conditions. Of particular interest is the
effect of different salt concentrations on micelle size
and on the catalytic activity. Moreover, we studied
the effect of micelles with a hydrocarbon core vs.
a partly fluorinated core on the gold-catalyzed con-
version of fluorinated and non-fluorinated allenic sub-
strates. The catalyst system was also applied to the de-
hydrative cyclization of acetylenic diols to furans.
Entry
Amphiphile
NaCl
Time[a]
Yield [%]
1
1
3
4
5
6
7
8
P1 (17 mM)
P1 (5 mM)
P1 (17 mM)
P1 (5 mM)
–
–
2 d
2 d
1 d
5 h
84
84
78
80
87
83
84
92
82
80
86
80
3 M
3 M
3 M
3 M
3 M
–
3 M
–
-
P1 (1 mM)
1 h
P1 (0.5 mM)
P1 (0.125 mM)
P2 (1 mM)
20 min
10 min
6 h
30 min
30 min
15 min
5 min
Results and Discussion
To explore the influence of fluorinated and non-fluo-
rinated amphiphiles in micellar gold catalysis, five dif-
ferent block copolymers were prepared (Figure 1).
The amphiphilic polymers P1–P4 were synthesized
via block copolymerization of 2-heptyl-2-oxazoline
and 2-methyl-2-oxazoline using methyl triflate (P1,
P2), butyl triflate (P3), and 4,4,4-trifluorobutyl tosy-
late (P4), respectively, as initiators. P5 was synthe-
sized without the addition of 2-heptyl-2-oxazoline be-
9
P2 (1 mM)
10
11
12
PTS (17 mM)[b]
PTS (10 mM)[b]
PTS (17 mM)[b]
3 M
[a]
Time required for complete conversion of allene 1a.
PTS=polyoxyethanyl-a-tocopheryl sebacate.
[b]
To examine the ability of the amphiphiles P1 to P5
cause of the hydrophobic character of the initiator to promote the gold-catalyzed cycloisomerization of
1H,1H,2H,2H-perfluorodecyl tosylate. All polymeri- functionalized allenes under micellar reaction condi-
zations were terminated by the addition of piperidine. tions, the conversion of substrate 1a into the corre-
The critical micelle concentration was determined for sponding dihydrofuran 2a was chosen as benchmark
polymers P3 (cmc=2·10À6 M) and P5 (cmc= reaction (Table 1). Similar to the typical procedure
1.2·10À6 M) by fluorescence spectroscopy and pyrene for vitamin E-based amphiphiles like polyoxyethanyl-
solubilization experiments and was in the micromolar a-tocopheryl sebacate (PTS),[5] we started with a poly-
range which is typical for such polymers.[7c,d]
mer concentration of 17 mM for P1. With 5 mol% of
AuBr3, long reaction times of 2 days were observed,
even if a lower polymer concentration of 5 mM was
chosen (Table 1, entries 1 and 2). However, prior in-
vestigations with PTS or TPGS have shown that the
addition of NaCl to the micellar solution can be very
beneficial for the process and often leads to an accel-
eration.[5a,b] For this reason, we tested the addition of
NaCl (3M) and found a strong rate increase. Using
17 mM of P1 in a 3M NaCl solution, the cyclization
was finished after 1 day, while lower polymer concen-
trations of 5 and 1 mM resulted in shorter reaction
times of 5 h and 1 h, respectively (Table 1, entries 3, 4
and 5). Even lower concentrations of the amphiphile
(0.5 or 0.125 mM) decreased the reaction times fur-
ther to 20 and 10 min, but the allene was not com-
pletely dissolved anymore (Table 1, entries 6 and 7).
Therefore, the investigations were continued with
a polymer concentration of 1 mM. Under these condi-
tions, P2 was also tested with and without the addi-
tion of NaCl, supporting the hypothesis of a general
salt effect (Table 1, entries 8 and 9). For comparison,
we also tested PTS and found the same concentration
and salt effect on the reactivity (Table 1, entries 10–
12). Compared to the polymeric amphiphiles P1 and
Figure 1. Amphiphilic poly(2-oxazolines) P1–P5 (red: hy-
drophobic part; blue: hydrophilic part).
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Sustainable Micellar Gold Catalysis – Poly(2-oxazolines)
Table 3. Influence of the gold catalyst on the cycloisomeriza-
tion of allene 1a in the presence of amphiphile P2.[a]
P2, the cyclization is much faster in PTS-derived mi-
celles; the latter, however, require in general higher
concentrations and do not offer the structural flexibil-
ity of poly(2-oxazolines). All cyclizations took place
with good to very good yields.
To further examine the salt effect, allene 1a was cy-
clized under constant conditions using P2 as amphi-
phile (2 mol% AuBr3, 1 mM P2, H2O), while increas-
ing the salt concentration systematically. The results
are shown in Table 2. While the product was isolated
Entry [Au]
Time Conversion Yield
[h]
[%][b]
[%]
1
1
3
AuBr3
AuCl3
AuCl
2.5
3
24
quant.
quant.
quant.
80
86
88
4
24
40
nd
Table 2. Influence of the salt concentration on the cycloiso-
merization of allene 1a in the presence of amphiphile P2.a]
5
6
7
Ph3PAuNTf2
Ph3PAuCl[c]
Ph3PAuCl
24
24
24
29
25
0
nd
nd
nd
Entry
NaCl
Time[b]
Yield [%]
[a]
[b]
[c]
1
1
3
4
5
6
1 M
2 M
3 M
4 M
5 M
7 M
2 d
9 h
7 h
5 h
2.5 h
2.5 h
80
91
87
81
80
82
Conditions: 2 mol% [Au], 1 mM P2.
Conversions were determined by GC.
2 mol% AgBF4 were added.
[a]
Conditions: 2 mol% AuBr3, 1 mM P2.
Time required for complete conversion of allene 1a.
[b]
in high yields, the reaction time decreased with higher
NaCl concentrations. With 1M NaCl, a rather long re-
action time of 2 days was observed, whereas a 5 or
7M NaCl solution led to a much shorter time of 2.5 h
(Table 2, entries 1, 5 and 6). When 7M NaCl was
used, the solubility of NaCl in water at room temper-
ature reached the limit and excess NaCl precipitated.
Thus, a 5M NaCl solution was used for further inves-
tigations.
Next, the activity of different gold catalysts was in-
vestigated. Besides AuBr3, also AuCl3, AuCl, John-
phos-gold complex A, Ph3PAuNTf2 and Ph3PAuCl
were tested. A solution of 1 mM P2 and 5M NaCl
was used with 2 mol% of the respective catalyst. As
can be seen in Table 3, the simple gold salts AuBr3
and AuCl3 achieved the shortest conversion times of
2.5 and 3 h, while AuCl required 24 h (Table 3, en-
tries 1–3). All three catalysts gave good yields while
for the other gold complexes incomplete conversion
Scheme 1. Gold-catalyzed cycloisomerization of a-hydroxy-
and a-aminoallenes 1.
Short conversion times from 0.5 to 2.5 h and good
of the allene was observed after 24 h (Table 3, en- yields (80–88%) of cyclization products 2 were ob-
tries 4–6). Ph3PAuCl has to be activated with AgBF4; tained in all cases. Particularly noteworthy is the
without the silver additive no reaction occurs smooth conversion of the solid, rather lipophilic al-
(Table 3, entry 7). In contrast to other cases of gold lenes 1f/g to the corresponding heterocycles; sub-
catalysis in water, this indicates that no dissociation of strates of this type are unreactive in aqueous medium
the chloro gold complex takes place in the micellar in the absence of an amphiphile.[5a] In general, hy-
system.[3f,i,j,11]
droxyallenes bearing a benzyl ether (2b–d) or ester
Under the optimized conditions (2 mol% AuBr3, side chain (2e) react faster than their counterpart
5M NaCl, 1 mM P2, H2O) several a-hydroxy- or ami- with a TBS ether (2a). Due to its high reactivity, we
noallenes 1 were cyclized to the corresponding het- selected allene 1b for further study of the influence of
erocycles 2, allowing a comparison of their reactivity different polymers in the presence or absence of NaCl
(Scheme 1).
(Table 4).
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Table 4. Influence of the polymeric amphiphile on the gold-
catalyzed cycloisomerization of allene 1b.
ample, the diameter of the micelles formed from am-
phiphile P3 rose from 23 nm in sweet water to 101 nm
in salt water (Table 5, entry 3). However, the micelle
size is not the only factor affecting the reactivity. For
example, amphiphile P2 forms smaller micelles than
P1 (Table 5, entry 2 vs. 1) but gives a faster cyclization
of allene 1b (Table 4, entry 2 vs. 1). Likewise, the mi-
celle diameter of P2 with salt is similar to that of P3
without NaCl (Table 5, entry 2 vs. 3), but their reactiv-
ity is quite different (Table 4, entry 2 vs. 3). Most
striking is the low reactivity of fluorinated amphiphile
P4 (Table 4, entry 4), considering the fact that this
forms by far the largest micelles (Table 5, entry 4). It
seems reasonable to assume that (besides the size of
the micelle) the transport of substrate, catalyst, and
product in and out of the micelle, as well as, the solu-
bility of the reactants and the stabilization of the cata-
lyst (possibly as aurate anion), affect the reactivity.
We also examined the influence of the amphiphile
concentration, as well as the presence of substrate
Entry Amphiphile Conversion [%]
(no NaCl)[a]
Conversion [%]
(5M NaCl)[a]
1
2
3
4
5
6
P1
P2
P3
P4
P5
–
58
68
48
35
10
100
99
99
100
78
54
100[b]
[a]
[b]
Determined by GC.
After 10 min.
A general accelerating salt effect was found for all and catalyst, on the size of the micelles formed from
amphiphiles – fluorinated and non-fluorinated ones. amphiphile P2 (Table 6). The concentration of the
Conversions after 30 min reaction time of 10–68% in polymer turned out to be quite important for the re-
the absence of salt, and of 54–100% in the presence activity (cf. Table 1). Surprisingly, this does not corre-
of 5M NaCl were observed. Moreover, the fluorinat- spond to the micelle size, which is not changing when
ed polymers P4 and P5 showed a decreased reactivity, the polymer concentration is increased from 1 to
compared to their non-fluorinated counterparts P1– 10 mM (Table 6, entries 1 and 2). However, decreas-
P3 (Table 4, entries 1–3 vs. 4 and 5). This trend was ing the concentration to 0.5 mM raises the micelle di-
not influenced by the addition of salt. A rapid cycliza- ameter from 21 to 34 nm (Table 6, entry 3). The addi-
tion of allene 1b to dihydrofuran 2b was observed tion of AuBr3 (2 mol%) hardly affects the micelle size
also in the absence of an amphiphile (Table 4, (Table 6, entry 4). In contrast, addition of the allene
entry 6). Under these conditions, the reaction takes 1b causes a strong enlargement of the micelle to
place in the oily layer or droplets of the allene. It 74 nm (Table 6, entry 5) which does not change very
must be noted however, that this procedure is not ap- much when the gold catalyst is added as well (Table 6,
plicable to solid and/or less reactive allenes and does entry 6). TEM pictures show the presence of spherical
not allow a recycling of the catalyst.[5a]
micelles with a dark contrast caused by the loading
To obtain more information about a possible corre- with the gold catalyst. Altogether these data indicate
lation of micelle size and reactivity, we performed dy- a remarkable flexibility of the micellar systems which
namic light scattering (DLS) measurements. In analo- have the ability to adapt to the prevailing conditions.
gy to earlier investigations with PTS,[5a] we found a dis-
In the next set of experiments we were interested
tinct increase of the micelle diameter upon addition how nanomicelles with a hydrocarbon or partially flu-
of NaCl for all five polymers P1–P5 (Table 5). For ex-
Table 6. Influence of the reaction parameters on the micellar
diameter of amphiphile P2.
Table 5. Influence of NaCl on the micellar diameter of poly-
Entry c [mM][a] Addition
Diameter [nm]b)
meric amphiphiles P1-P5.[a]
1
2
3
4
5
6
1
–
–
–
21Æ4
22Æ4
34Æ7
25Æ3
74Æ22
Entry Amphiphile Diameter without Diameter with 5M
10
0.5
1
1
1
NaCl [nm]
NaCl [nm]
[c]
1
2
3
4
5
P1
P2
P3
P4
P5
14Æ4
10Æ2
23Æ4
41Æ4
7Æ3
36Æ3
AuBr3
21Æ4
allene 1b[d]
101Æ22
33Æ5/162Æ11[b]
19Æ2
AuBr3[c] +allene 1b[d] 82Æ51
[a]
Concentration of amphiphile P2 dissolved in a 5M NaCl
solution.
Determined by DLS.
[a]
[b]
[c]
[d]
Diameter determined by DLS of a 1 mM solution of the
amphiphile.
Two maxima were observed.
2.3 mmol/mL=2 mol% relative to 1b.
[b]
0.12 mmolmLÀ1.
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Sustainable Micellar Gold Catalysis – Poly(2-oxazolines)
Table 7. Influence of the polymer on the gold-catalyzed cy-
cloisomerization of fluorinated allene 1g.
Table 8. Recycling of the micellar catalyst solution.
Run
Time[a]
Yield [%]
1
2
3
30 min
2 h
4 h
85
92
90
Entry
Amphiphile[a]
Conversion after
15 min [%][b]
Time[c]
[a]
Required for complete conversion of allene 1g.
1
2
3
4
P2
P3
P4
P5
56
30
36
13
75 min
2 h
90 min
2 d
However, due to the formation of a foam, the extrac-
tion of the cyclization products was not feasible.
Using Et2O for the extraction enables a clear phase
separation, but also caused a strong loss of catalytic
activity by catalyst leaching. However, when we used
a mixture of n-pentane/Et2O (8:2, v/v) the extraction
of the product 2b and recycling the micellar medium
was possible (Table 8). The gradual loss of catalytic
activity caused by the product extraction required an
[a]
Conversion without amphiphile in 5M NaCl after 24 h:
32%.
Conversions were determined by GC.
Until complete conversion of allene 1g.
[b]
[c]
orocarbon-like core affect the conversion of fluorinat- extension of the reaction time in the second and third
ed substrates. Using allene 1g and the polymers P2– run. In doing so, high product yields around 90% and
P5 in the presence of 3 mol% AuBr3, we determined acceptable conversion times of 2 and 4 h in the
the conversion after 15 min, as well as the time re- second and third run were observed (Table 8, entries 2
quired for full conversion (Table 7). Also for this sub- and 3 vs. 1). A possibility to alleviate the reactivity
strate, non-fluorinated amphiphile P2 gave the best decrease would be the addition of more catalyst,[5c]
result with a reaction time of 75 min (Table 7, but this was not examined here.
entry 1). Interestingly, P4 containing a terminal CF3
Finally, we also examined the possibility of per-
group in the hydrophobic polymer segment is slightly forming dehydrating transformations in bulk water
more reactive than the non-fluorinated analogue P3 using the hydrophobic effect of the poly(2-oxazoline)
(Table 7, entry 2 vs. 3). In contrast, P5 with a longer amphiphiles. As a benchmark reaction, we selected
fluorinated chain gave complete conversion only after the gold-catalyzed dehydrative cyclization of acetylen-
2 days. (Table 7, entry 4). The very low reactivity in ic diols to furans which has been carried previously in
the presence of P5 can be explained by the polymer vitamin E-derived nanomicelles (Scheme 2).[5b] In the
structure and the type of fluorocarbon chain in the presence of polymer P2 and 5M NaCl, the four diols
micellar core. The increased size of fluorine versus hy- 3a–d were converted successfully into the correspond-
drogen is known to be responsible for the rigid struc- ing furans 4a–d with good yields of 67–82%. The reac-
ture of fluorocarbons. Study of the dynamic nature of tions required 1–4 h at room temperature, except for
such fluorinated micelles by NMR spectroscopy re- the triphenyl-substituted diol 3c which took 24 h to
vealed a solid-like micellar core at room temperatur- afford product 4c with 67% yield. It should be noted
e[8a,12a] that most likely limits substrate solubilization
while micellar cores composed of hydrocarbon chains
are more dynamic.[8a] Higher reaction temperature[12b]
or a different polymer structure that does not favor
a dense packing of the fluorocarbon chains[12c] might
help to overcome the limitation of such highly fluori-
nated polymer amphiphiles. Without any amphiphile,
the poor solubility of the substrate 1g, which is
a solid, led to a very slow conversion (32% after
24 h).
The possibility of catalyst recycling is one of the
main targets of sustainable chemistry. Therefore, we
tested the catalyst recycling using the unpolar solvents
n-hexane, isohexane or n-pentane for extraction. diols 3 in the presence of polymeric amphiphile P2.
Scheme 2. Gold-catalyzed cycloisomerization of acetylenic
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P4, 2-methyl-2-oxazoline (1.00 g, 11.76 mmol, 25.00 equiv.)
was added and the mixture stirred at 1208C for 2 h. The
polymerization was terminated by the addition of piperidine
(3.00 equiv.) and the mixture was stirred at room tempera-
ture overnight. The solvent was removed under reduced
pressure. The remaining residue was dissolved in chloroform
and K2CO3 was added. The resulting mixture was stirred at
room temperature for 4 h. The solid was removed by filtra-
tion and the polymer was precipitated with cold Et2O. After
centrifugation, the polymer was dialyzed against water and
lyophilized. The polymer was obtained as a white solid.
that the presence of micelles is essential for the reac-
tion;[5b] without the amphiphile, substrate 3a gave
only traces of furan 4a after 2 h.
Conclusions
We have synthesized and applied five different
poly(2-oxazoline)-based amphiphiles with different
compositions and hydrophobic end groups (hydrocar-
bon-like or partly fluorinated). The polymers were
used as amphiphiles in gold-catalyzed cyclization re-
actions of functionalied allenes or alkynes under mi-
cellar reaction conditions. We have found a strong
effect of the hydrophobic micellar core and of the salt
concentration on the catalytic activity. Furthermore,
detailed studies towards the correlation between mi-
celle size and reactivity were performed. Overall, we
present a new approach for fine-tuning of a catalytic
system which also allows catalyst recycling. These in-
vestigations provide an important contribution to
green chemistry and open the door for further studies
of transition metal-catalyzed transformations in nano-
micelles which will benefit from the enormous varia-
bility of amphiphilic polymers.
1
P3: H NMR (500 MHz, CDCl3): d=0.82–0.93 (m, 22H,
CH3,hep/CH3,I), 1.24 (s, 59H, [CH2,hep)4], 2.04–2.10 (m,
52H), 3.18 (brs, 2H, CH2NI), 3.41 (m, 100H,
CH2CH2,backbone).
P4: 1H NMR (500 MHz, CDCl3): d=0.85 (s, 27H,
CH3,hep), 1.25 [s, 74H, (CH2,hep)4], 1.82–1.90 (m, 2H,
CH2F3,I)
2.05–2.12
(m,
75H),
3.41
(m,
132H,
CH2CH2,backbone).
1
P5: H NMR (500 MHz, CDCl3): d=2.06–2.12 (m, 42H),
3.07 (t, 2H, CH2N,I), 3.46 (m, 57H, CH2CH2,backbone). See the
Supporting Information for mass spectral analyses.
Cyclization
Standard
experimental
procedure:
The
substrate
(1.2 mmolmLÀ1) was dissolved in a solution of the respec-
tive polymer (1 mM) in aqueous 5M NaCl. Then the gold
catalyst (2 mol%) was added. The reaction was followed by
TLC or GC. After complete conversion of the substrate, the
mixture was filtered through Celite. To achieve good yields,
thorough washing of the Celite layer with Et2O is necessary.
The organic phase was dried with MgSO4, the solvent was
evaporated under reduced pressure, and the residue was pu-
rified by flash column chromatography.
Experimental Section
Polymer Synthesis
P1 and P2: To a solution of methyl triflate (77.18 mg,
0.471 mmol, 1.00 equiv.) in 5 mL acetonitrile was added 2-
methyl-2-oxazoline (1.00 g, 11.76 mmol, 25.00 equiv.). The
reaction mixture was stirred at 1208C for 2 h. Subsequently,
2-heptyl-2-oxazoline (557 mg, 3.29 mmol, 7.00 equiv.) was
added and the mixture was stirred at 1108C for 4 h. The
polymerization was terminated by the addition of piperidine
(120 mg, 1.41 mmol, 3.00 equiv.) and the mixture was stirred
at room temperature overnight. The solvent was removed
under reduced pressure. The remaining residue was dis-
solved in chloroform and K2CO3 was added. The resulting
mixture was stirred at room temperature for 4 h. The solid
was removed by filtration and the polymer was precipitated
with cold Et2O. After centrifugation, the polymer was dia-
lyzed against water and lyophilized. The polymer was ob-
tained as a white solid.
tert-Butyldimethyl[(5-(tert-butyl)-3-methyl-2,5-dihydrofur-
an-2-yl]methoxysilane (2a): 50 mg (0.176 mmol) of allene
1a, 4.5 mg of polymer P2, 438 mg NaCl, 1.5 mL H2O und
1.5 mg AuBr3 were used following the general procedure.
After 2.5 h complete conversion of the allene was observed.
Isolation and purification on silica gel (SiO2, pentane/Et2O,
10:1) gave dihydrofuran 2a as a colorless oil; yield: 40 mg
1
(0.14 mmol, 80%). H NMR (500 MHz, CDCl3): d=5.42 (m,
1H, CH), 4.56 (m, 1H, CH), 4.37 (m, 1H, CH), 3.68 (d, J=
5.3 Hz, 2H, CH2), 1.75 (s, 3H, CH3), 0.89 (s, 9H, t-Bu), 0.85
(s, 9H, t-Bu), 0.07 (s, 3H, CH3), 0.06 (s, 3H, CH3); 13C NMR
(126 MHz, CDCl3): d=138.4, 122.7, 93.5, 87.8, 66.1, 34.2,
25.9, 25.8, 18.3, 13.0, À5.4, À5.4.
2-(Benzyloxymethyl)-5-(tert-butyl)-3-methyl-2,5-dihydro-
furan (2b): 91 mg (0.352 mmol) of allene 1b, 9 mg of poly-
mer P2, 876 mg NaCl, 3 mL H2O und 3 mg AuBr3 were used
following the general procedure. After 30 min complete con-
version of the allene was observed. Isolation and purifica-
tion on silica gel (SiO2, pentane/Et2O, 10:1) gave dihydrofur-
an 2b as a colorless oil; yield: 75 mg (0.29 mmol, 82%).
1H NMR (500 MHz, CDCl3): d=7.36–7.24 (m, 5H, CHAr),
5.50 (m, 1H, CH), 4.72 (m, 1H, CH), 4.65 (dd, J=31.6,
12.3 Hz, 2H, CH2), 4.48 (m, 1H, CH), 3.60 (dd, J=10.4,
3.4 Hz, 1H, CH2), 3.53 (dd, J=10.5, 4.6 Hz, 1H, CH2), 1.73
(m, 3H, CH3), 0.88 (s, 9H, t-Bu); 13C NMR (126 MHz,
CDCl3): d=138.5, 137.2, 128.3, 127.7, 127.5, 123.3, 93.9, 87.5,
73.4, 71.7, 35.8, 25.5, 12.6.
P1: 1H NMR (500 MHz, CDCl3): d=0.86 (s, 43H,
CH3,hep), 1.28 [s, 114H, (CH2,hep)4], 2.07–2.14 (m, 142H),
2.96/3.05 (m, 3H, CH3,I), 3.44 (m, 214H, CH2CH2,backbone).
P2: 1H NMR (500 MHz, CDCl3): d=0.88 (s, 22H,
CH3,hep), 1.30 [s, 59H, (CH2,hep)4], 2.08–2.15 (m, 62H), 2.96/
3.05 (m, 3H, CH3,I), 3.46 (m, 107H, CH2CH2,backbone). See the
Supporting information for mass spectral analysis.
P3, P4 and P5: To a solution of the initiators butyl tri-
flate (for P3), 1-tosyloxy-4,4,4-trifluorobutane (for P4), or
1H,1H,2H,2H-perfluorodecyl tosylate (for P5; 1.00 equiv.)
in 5 mL acetonitrile was added 2-heptyl-2-oxazoline (for P3
and P4) or 2-methyl-2-oxazoline (for P5; 10.00 equiv.). The
reaction mixture was stirred at 1108C for 4 h. For P3 and
1496
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Sustainable Micellar Gold Catalysis – Poly(2-oxazolines)
2-(Benzyloxymethyl)-5-butyl-3-methyl-2,5-dihydrofuran
(2c): 92 mg (0.352 mmol) of allene 1c, 9 mg of polymer P2,
876 mg NaCl, 3 mL H2O und 3 mg AuBr3 were used follow-
ing the general procedure. After 30 min complete conver-
sion of the allene was observed. Isolation and purification
on silica gel (SiO2, pentane/Et2O, 10:1) gave dihydrofuran
2c as a colorless oil; yield: 81 mg (0.31 mmol, 88%).
1H NMR (400 MHz, CDCl3): d=7.36–7.26 (m, 5H, CHAr),
5.50 (m, 1H, CH), 4.83 (m, 1H, CH), 4.76 (m, 1H, CH),
4.60 (dd, J=21.7, 12.3 Hz, 2H, CH2), 3.59 (dd, J=10.4,
3.4 Hz, 1H, CH2), 3.52 (dd, J=10.4, 4.8 Hz, 1H, CH2), 1.71
(m, 3H, CH3), 1.57–1.50 (m, 2H, CH2), 1.37–1.28 (m, 4H,
CH2), 0.90 (t, J=6.9 Hz, 3H, CH3); 13C NMR (101 MHz,
CDCl3): d=138.4, 135.8, 128.3, 127.6, 127.4, 125.8, 86.8, 85.5,
73.4, 71.6, 36.1, 27.3, 22.8, 14.1, 12.6.
2-(Benzyloxymethyl)-3-methyl-5-phenyl-2,5-dihydrofuran
(2d): 99 mg (0.352 mmol) of allene 1d, 9 mg of polymer P2,
876 mg NaCl, 3 mL H2O und 3 mg AuBr3 were used follow-
ing the general procedure. After 1 h complete conversion of
the allene was observed. Isolation and purification on silica
gel (SiO2, pentane/Et2O, 10:1) gave of dihydrofuran 2d as
colorless oil; yield: 85 mg (0.30 mmol, 86%). 1H NMR
(500 MHz, CDCl3): d=7.40–7.26 (m, 10H, CHAr), 5.82 (m,
1H, CH), 5.62 (m, 1H, CH), 5.00 (m, 1H, CH), 4.66 (dd,
J=11.7, 29.8 Hz, 2H, CH2), 3.71 (dd, J=3.4, 10.5 Hz, 1H,
CH2), 3.64 (dd, J=4.6, 10.6 Hz, 1H, CH2), 1.80 (m, 3H,
CH3); 13C NMR (126 MHz, CDCl3): d=142.5, 138.4, 136.4,
128.4, 128.3, 127.6, 127.6, 127.5, 126.3, 126.0, 87.7, 87.3, 73.5,
71.4, 12.5.
P2, 876 mg NaCl, 3 mL H2O und 3 mg AuBr3 were used fol-
lowing the general procedure. After 2.5 h complete conver-
sion of the allene was observed. Isolation and purification
on silica gel (SiO2, pentane/Et2O, 10:1) gave dihydrofuran
2g as a colorless oil; yield: 83 mg (0.28 mmol, 80%).
1H NMR (400 MHz, CDCl3): d=7.78 (s, 1H, CHAr), 7.74 (s,
2H, CHAr), 5.86 (m, 1H, CH), 5.51 (m, 1H, CH), 4.79–4.73
(m, 1H, CH2), 4.69–4.64 (m, 1H, CH2), 1.85 (m, 3H, CH3);
13C NMR (101 MHz, CDCl3): d=145.5, 138.2, 131.7 (q,
J
C,F =33.3 Hz), 126.4 (m), 123.7 (q, JC,F =272.7 Hz), 122.7,
121.5 (m), 86.2, 78.8, 12.2.
5-Butyl-2,3-dimethylfuran (4a): 60 mg (0.352 mmol) of
diol 3a, 9 mg of polymer P2, 876 mg NaCl, 3 mL H2O und
3 mg AuBr3 were used following the general procedure.
After 2 h complete conversion of the diol was observed. Iso-
lation and purification on silica gel (SiO2, pentane/Et2O,
10:1) gave furan 4a as
a colorless oil; yield: 45 mg
1
(0.28 mmol, 80%). H NMR (600 MHz, CDCl3): d=5.75 (s,
1H, CHAr), 2.53 (t, J=7.6 Hz, 2H, CH2), 2.16 (s, 3H, CH3),
1.90 (s, 3H, CH3), 1.61–1.50 (m, 2H, CH2), 1.40–1.35 (m,
2H, CH2), 0.93 (t, J=7.4 Hz, 3H, CH3); 13C NMR
(151 MHz, CDCl3): d=153.5, 145.1, 114.1, 107.7, 30.4, 27.7,
22.3, 13.8, 11.2, 9.9.
2,3-Dimethyl-5-phenylfuran (4b): 67 mg (0.352 mmol) of
diol 3b, 9 mg of polymer P2, 876 mg NaCl, 3 mL H2O und
3 mg AuBr3 were used following the general procedure.
After 1 h complete conversion of the diol was observed. Iso-
lation and purification on silica gel (SiO2, pentane/Et2O,
10:1) gave furan 4b as
a colorless oil; yield: 48 mg
1
[5-(tert-Butyl)-3-methyl-2,5-dihydrofuran-2-yl]methyl ace-
tate (2e): 75 mg (0.352 mmol) of allene 1e, 9 mg of polymer
P2, 876 mg NaCl, 3 mL H2O und 3 mg AuBr3 were used fol-
lowing the general procedure. After 1 h complete conver-
sion of the allene was observed. Isolation and purification
on silica gel (SiO2, pentane/Et2O, 10:1) gave dihydrofuran
2e as a colorless oil; yield: 60 mg (0.28 mmol, 80%).
1H NMR (400 MHz, CDCl3): d=5.50 (m, 1H, CH), 4.73 (m,
1H, CH), 4.44 (m, 1H, CH), 4.19 (dd, J=11.8, 2.9 Hz, 1H,
CH2), 4.07 (dd, J=11.8, 5.0 Hz, 1H, CH2), 2.05 (d, J=
0.7 Hz, 3H, CH3), 1.72 (m, 3H, CH3), 0.85 (s, 9H, t-Bu);
13C NMR (101 MHz, CDCl3): d=171.1, 135.8, 124.2, 94.1,
86.0, 65.4, 35.7, 25.5, 20.9, 12.4.
2-(Benzyloxymethyl)-5-butyl-3-methyl-1-tosyl-2,5-dihy-
dro-1H-pyrrole (2f): 144 mg (0.352 mmol) of allene 1f, 9 mg
of polymer P2, 876 mg NaCl, 3 mL H2O und 3 mg AuBr3
were used following the general procedure. After 2 h com-
plete conversion of the allene was observed. Isolation and
purification on silica gel (SiO2, pentane/Et2O, 10:1) gave di-
(0.28 mmol, 80%). H NMR (500 MHz, CDCl3): d=7.62 (d,
J=8.3 Hz, 2H, CHAr), 7.36 (t, J=7.6 Hz, 2H, CHAr), 7.21 (t,
J=7.4 Hz, 1H, CHAr), 6.46 (s, 1H, CHAr), 2.30 (s, 3H, CH3),
2.00 (s, 3H, CH3); 13C NMR (126 MHz, CDCl3): d=150.9,
147.3, 131.2, 128.5, 126.5, 123.1, 116.1, 108.3, 34.1, 22.4.
2,3,5-Triphenylfuran (4c): 111 mg (0.352 mmol) of diol 3c,
9 mg of polymer P2, 876 mg NaCl, 3 mL H2O und 3 mg
AuBr3 were used following the general procedure. The reac-
tion was stopped after 24 h. Isolation and purification on
silica gel (SiO2, pentane/Et2O, 10:1) gave furan 4c as white
1
solid; yield: 70 mg (0.24 mmol, 67%). H NMR (500 MHz,
CDCl3): d=7.83–7.79 (m, 2H, CHAr), 7.69–7.65 (m, 2H,
CHAr), 7.54–7.50 (m, 2H, CHAr), 7.48–7.41 (m, 4H, CHAr),
7.40–7.28 (m, 5H, CHAr), 6.86 (m, 1H, CHAr); 13C NMR
(101 MHz, CDCl3): d=152.5, 147.9, 134.3, 131.1, 130.5,
128.7, 128.7, 128.6, 128.4, 127.5, 127.5, 127.3, 126.1, 124.5,
123.8, 109.4.
2,3-Diphenylfuran (4d): 84 mg (0.352 mmol) of diol 3d,
9 mg of polymer P2, 876 mg NaCl, 3 mL H2O und 3 mg
AuBr3 were used following the general procedure. After 4 h
complete conversion of the diol was observed. Isolation and
purification on silica gel (SiO2, pentane/Et2O, 10:1) gave
furan 4d as a white solid; yield: 64 mg (0.29 mmol, 82%).
1H NMR (500 MHz, CDCl3): d=7.46 (m, 2H, CHAr), 7.43
(m, 1H, CHAr),7.36–7.33 (m, 2H, CHAr), 7.31–7.27 (m, 2H,
CHAr), 7.23–7.19 (m, 3H, CHAr), 7.18–7.16 (m, 1H, CHAr),
7.48 (m, 1H, CHAr); 13C NMR (126 MHz, CDCl3): d=148.6,
141.5, 134,4, 131.2, 128.7, 128.6, 128.4, 127.5, 127.1, 126.3,
122.3, 114.0.
hydro-1H-pyrrole 2f as
a colorless oil; yield: 120 mg
1
(0.29 mmol, 83%). H NMR (400 MHz, CDCl3): d=7.75 (d,
J=8.3 Hz, 2H, CHAr), 7.32–7.24 (m, 3H, CHAr), 7.22–7.14
(m, 4H, CHAr), 5.40 (m, 1H, CH), 4.57 (m, 1H, CH), 4.46
(m, 1H, CH), 4.33 (d, J=12.2 Hz, 1H, CH2), 4.18 (d, J=
12.2 Hz, 1H, CH2), 4.00 (dd, J=10.5, 3.2 Hz, 1H, CH2), 3.68
(dd, J=10.5, 1.9 Hz, 1H, CH2), 2.34 (s, 3H, CH3), 1.98–1.90
(m, 1H, CH2), 1.76–1.68 (m, 1H, CH2), 1.67 (s, 3H, CH3),
1.30–1.10 (m, 4H, CH2), 0.85 (t, J=7.2 Hz, 3 H CH3);
13C NMR (101 MHz, CDCl3): d=142.4, 139.4, 138.1, 134.9,
129.1, 128.1, 127.3, 127.3, 126.7, 124.8, 72.6, 69.9, 67.9, 67.2,
34.1, 26.5, 22.6, 21.3, 14.0, 13.6.
Recycling: The reaction was performed as described
above with 0.352 mmol of allene 1b in 2 mL polymer solu-
tion (1 mM P2, 5M NaCl, 2 mL H2O). After complete con-
version, the product was extracted with a mixture of n-pen-
2-[3,5-Bis(trifluormethyl)phenyl]-4-methyl-2,5-dihydrofur-
an (2g): 104 mg (0.352 mmol) of allene 1g, 9 mg of polymer
Adv. Synth. Catal. 2016, 358, 1491 – 1499
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Linda Lempke et al.
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mixture for 5–10 min, and removal of the organic layer with
a pipette. This procedure was repeated until no more than
traces of the product remained in the micellar phase (detec-
tion by TLC). The combined organic phases were dried with
MgSO4, the solvent was evaporated under reduced pressure,
and the residue was purified by flash column chromatogra-
phy. The micellar catalyst solution was exposed to reduced
pressure to remove traces of the organic solvent. Then it
was reused by addition of another portion of the substrate.
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Acknowledgements
We thank Prof. Dr. Jçrg Tiller (TU Dortmund) for providing
the devices for DLS and fluorescence measurements. We also
thank Monika Meuris (Group Prof. Jçrg Tiller) for the TEM
measurements. Financial support by SusChemSys (which is
co-financed by the Regional Development Fund (Investing in
Your Future) of the European Union and the state of North
Rhine-Westphalia), the European Metrology Research Pro-
gramme (EMRP, project IND15 SurfChem) and the Adolf-
Martens-Fund is gratefully acknowledged. The EMRP is
jointly funded by the EMRP participating countries within
EURAMET and the European Union.
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