Asymmetric C-C bond formation with L-prolinol derived chiral catalysts immobilized on polymer fibers

Article abstract of DOI:10.1016/j.tetasy.2003.10.021

N-(4-Vinylbenzyl)-α,α-diphenyl-L-prolinol was immobilized on polyethylene fibers by electron beam induced pre-irradiation grafting using styrene as a co-monomer. The resulting polymer supported catalyst P2 was utilized in asymmetric C-C bond forming react

Full text of DOI:10.1016/j.tetasy.2003.10.021

Tetrahedron:
Asymmetry
Tetrahedron: Asymmetry 15 (2004) 231–237
Asymmetric C–C bond formation with L-prolinol derived
chiral catalysts immobilized on polymer fibers
a
b
Sylvestre Degni,a, Carl-Eric Wilen and Reko Leino
*
ꢀ
a
ꢁ
ꢁ
ꢁ
ꢁ
Laboratory of Polymer Technology, Abo Akademi University, FIN-20500 Abo, Finland
b
Department of Organic Chemistry, Abo Akademi University, FIN-20500 Abo, Finland
Received 17 September 2003; accepted 1 October 2003
Abstract—N-(4-Vinylbenzyl)-a,a-diphenyl-
diation grafting using styrene as a co-monomer. The resulting polymer supported catalyst P2 was utilized in asymmetric C–C bond
forming reactions and its performance compared with those of previously prepared fibrous cross-linked a,a-diphenyl- -prolinol P1
L-prolinol was immobilized on polyethylene fibers by electron beam induced pre-irra-
L
and homogeneous model compounds. The fibrous catalyst yields similar enantioselectivities in the asymmetric addition of dieth-
ylzinc to benzaldehyde as its homogeneous reference reaction and can be recycled with minimal loss of activity. When P2 was used
as the catalyst in the addition of phenylacetylene to benzaldehyde, the corresponding propargylic alcohol was obtained with
enantiomeric excesses of up to 91%.
ꢀ 2003 Published by Elsevier Ltd.
1. Introduction
benzaldehyde reacts with diethylzinc to give racemic
1-phenylpropanol in ca. 15% yield after 18 h.5a Accord-
ingly, catalysts supported on PS operate in a competitive
situation and unless a sufficient proportion of the cata-
lytic sites are readily accessible, the reaction paths
yielding racemic products will have a significant contri-
bution to the overall reaction. Consequently, in these
cases, relatively low enantioselectivities may be observed.
Several chiral ligands have proven to be successful in the
catalytic enantioselective nucleophilic additions of dial-
kylzincs1 and phenylacetylenes2;3 to aldehydes, some of
the most studied, single step, asymmetric carbon–carbon
bond forming reactions. The strategy of attaching such
chiral ligands to polymeric supports has been widely
used to prepare heterogeneous, immobilized catalysts
for enantioselective conversions.4 This approach is ad-
vantageous compared to the classical homogeneous
version due to: (1) the ease of separation of the expen-
sive chiral catalyst from the reaction system, and hence
the possibility of reutilizing the catalyst for successive
reactions, (2) convenient operation in flow reactors or
flow membrane reactors for continuous production, and
(3) for the development of environmentally safe pro-
cesses for the production of fine chemicals.
A significant problem associated with the conventional
PS-based supports is their low mechanical strength and
often poor thermo-oxidative stability. In a previous
paper, we described the facile anchoring of chiral
TADDOL and amino alcohol ligands on mechanically
stable and chemically inert polyethylene (PE) fibers by
electron beam (EB) induced pre-irradiation graft co-
polymerization.7 Chiral styrenic ligand derivatives such
as 1 (Fig. 1) were successfully immobilized on the fibrous
support to generate the corresponding polymeric cata-
lysts, which were then employed in the addition of di-
ethylzinc to benzaldehyde. For example, the fibrous
catalyst P1 gave 1-phenylpropanol in 36% yield and
62:38 ratio of the (R)- and (S)-enantiomers.7 The catalyst
was successfully recycled without significant losses in
activity and enantioselectivity.
Various chiral b-amino alcohols supported on polysty-
rene (PS) have been used as catalysts5 and earlier work
has been covered in several reviews.6 The successful de-
sign of supported catalysts for the addition of dialkyl-
zincs to aldehydes is a particular challenge: In the absence
of added catalysts, these reactions take place slowly to
give a racemic product. For example, in toluene at 20 ꢁC,
The low yield and only moderate stereocontrol obtained
with P1 are possibly associated with the cross-linking
nature of the styrenic ligand monomer 1 resulting in the
active sites being embedded in the surrounding polymer
* Corresponding author. Tel.: +358-2-2154866; fax: +358-2-2154992;
e-mail: sdegni@abo.fi
0957-4166/$ - see front matter ꢀ 2003 Published by Elsevier Ltd.
doi:10.1016/j.tetasy.2003.10.021

232
S. Degni et al. / Tetrahedron: Asymmetry 15 (2004) 231–237
ported chiral catalysts, architecture, and properties of
the support, likewise, play an important role in deter-
mining the catalytic performance.
The previously reported monomeric ligand 1 contains
two styryl functionalities, thus potentially resulting in a
cross-linked polymeric network when subjected to rad-
ical polymerization. Such architecture may result in a
limited accessibility of the chiral active sites, which in
turn may have contributed to the fairly low yields and
enantioselectivities observed in the previous study using
P1.7 We were thus interested in the opportunity to
compare cross-linking versus pendant immobilization of
chiral amino alcohols to recyclable fibrous supports and
the possible benefits in the site accessibility obtained
with the latter.
OH
OH
N
N
H
H
1
P1
Figure 1. Previously described monomeric and fibrous chiral amino
alcohols 1 and P1.
matrix. As a continuation of the previous study, we re-
port here the immobilization of the pendantly substi-
tuted chiral amino alcohol 2 (Fig. 2) on PE fibers using
the previously described EB grafting technique. The
corresponding polymeric catalyst P2 together with sev-
eral homogeneous reference catalysts was then
employed in the addition of diethylzinc and phenyl-
acetylene to benzaldehyde and the results of the former
reaction compared with those obtained in our previous
study.7
The styrenic chiral amino alcohol 2 and the reference
catalyst 3 were prepared in good yield from a,a-di-
phenyl-L-prolinol 4 (Fig. 3) and 4-vinylbenzylchloride
following a literature procedure.9 Compound 2 was then
immobilized on polyethylene fibers according to the
previously described EB pre-irradiation grafting proce-
dure7 to obtain the polymeric chiral catalyst P2 with a
loading of 0.4 mmol/g as confirmed by elemental nitro-
gen analysis. Next, the activity and selectivity of P2 was
investigated in the addition of diethylzinc to benzalde-
hyde at 0 ꢁC and ambient temperature using a catalyst
loading of 5 mol %. The monomeric chiral amino alco-
hols 1–5 were studied for comparison. The results are
presented in Table 1 together with those previously
obtained with P1.7
OH
OH
N
N
H
N
OH
OH
N
N
2
P2
OH
H
Figure 2. Monomeric and fibrous chiral amino alcohols 2 and P2
described in the present work.
3
4
5
2. Addition of diethylzinc to benzaldehyde
Figure 3. Chiral amino alcohols 3–5.
The addition of diethylzinc to benzaldehyde in the
presence of chiral amino alcohol catalysts yields a mix-
ture of (R)- and (S)-1-phenylpropanols (Scheme 1). The
predominant enantiomer is determined by the configu-
ration of the chiral catalyst. Enantioselectivity of the
addition is thus highly dependent on the structure of the
ligand, especially the substitution pattern of the nitrogen
atom5a;8 and the carbinol carbon. In the case of sup-
As evident from Table 1, the pendantly supported cat-
alyst P2 is superior to the cross-linked catalyst P1 in
terms of reaction yield. In addition, the enantioselec-
tivities obtained with P2 are slightly higher than those
obtained with P1. Overall the performance of P2 is very
similar to those of its homogeneous N-vinylbenzyl and
N-benzyl substituted analogues 2 and 3. The enantio-
selectivities of all studied N-substituted catalysts are
slightly lower than those obtained with the homoge-
neous N–H prolinols 1 and 4. This is in accordance with
previous observations by others.10 Thus, addition of
Et2Zn to benzaldehyde in the presence of 5 mol % P2
gave (R)-1-phenylpropanol in 98% yield and 38%
enantiomeric excess. The polymeric catalyst was suc-
cessfully recycled for two subsequent runs without sig-
nificant losses in activity and enantioselectivity. The
O
OH
5 mol % amino alcohol
toluene
H
(R or S)
+ 2.5 equiv. Et2Zn
Scheme 1. Addition of diethylzinc to benzaldehyde catalyzed by chiral
amino alcohols.

S. Degni et al. / Tetrahedron: Asymmetry 15 (2004) 231–237
233
Table 1. Addition of diethylzinc to benzaldehyde in the presence of homogeneous and heterogeneous chiral amino alcohols
Entry
Ligand
PhCHO
(mmol/ml)
Polymer (g)
Loading
(mmol/g)
Temperature
(ꢁC)
Yielda (%)
Eed (R)%
Reference
1
1
0.38
0.42
0.42
0.38
0.2
0
0
0
0
0
0
0
0
0
0
0
25
0
0
70b
76
44
44
68
86
24
20
22
38
40
36
34
46
25
7
2
2
Quant.c
Quant.c
97
This work
This work
7
3
3
4
4
5
5
89b
This work
7
6
P1
P1e
P1f
P2
P2e
P2f
P2
3–Li
P2–Li
0.52
0.56
0.49
0.35
0.35
0.42
0.42
0.4
1
0.21
0.21
0.21
0.4
36
33
7
0.8
0.7
0.53
0.5
0.48
0.53
7
8
30
98
7
9
This work
This work
This work
This work
This work
This work
10
11
12
13
14
0.4
0.4
95
96
Quant.c
0.4
97
85
0.4
0.45
0.4
a Estimated by GC after 24 h.
b Isolated yield.
c No benzaldehyde detected by GC or 1H NMR.
d Determined by chiral GC.
e First regeneration of the polymeric catalyst.
f Second regeneration.
reaction temperature (0 ꢁC vs 25 ꢁC) has only little effect
on the performance of P2 in its present application. The
effect of reaction time on the yield approximated of 1-
phenylpropanol using P2 and regenerated P2 as cata-
lysts in the three successive runs is displayed in Figure 4.
nately, under identical reaction conditions, the perfor-
mance of the polymeric analogue P2–Li was slightly
inferior to the homogeneous system producing an
85:5:10 ratio of 1-phenylpropanol (ee ¼ 25%), benzyl
alcohol and unreacted benzaldehyde as determined by
GC. Thus, contrary to some literature reports, in com-
parison with P2, the use of the lithium salt method does
not result in any improvements in terms of yield or
enantioselectivity of the polymeric catalyst. One possible
explanation may be related to the presence of ÔinertÕ salt
effects on the stereochemical outcome of this reaction.13
100
90
80
70
60
50
40
30
20
10
0
3. Addition of phenylacetylene to benzaldehyde
0
5
10
15
20
25
30
Time (hrs.)
Due to the promising results obtained in the addition of
Et2Zn to benzaldehyde we decided to extend the appli-
cation of P2 to other types of enantioselective trans-
formations. Thus, we undertook a preliminary study of
phenylacetylene addition to benzaldehyde in the pres-
ence Zn(OTf)2 using P2 as catalyst (Scheme 2).
Figure 4. The effect of reaction time on the yield of 1-phenylpropanol
using P2 and regenerated P2 in the addition of diethylzinc to benz-
aldehyde. Symbols: (r) First run, (j) First regeneration, (N) Second
regeneration (entries 9–11 in Table 1).
Two additional experiments were carried out using the
lithium salt of the chiral amino alcohols 3 and P2
(entries 13 and 14 in Table 1). As described previously
by Corey et al.,11 enantioselectivity in the addition of
diethylzinc to benzaldehyde may in some cases be im-
proved by the use of the lithium salt derivatives of the
chiral b-amino alcohol ligands. Inspired by earlier
work,12 the homogeneous and polymeric ligands 3 and
P2 were treated with an equimolar amount of butyl
lithium and employed in the alkylation of benzaldehyde.
Thus, when benzaldehyde (1 equiv) was treated with
diethylzinc (2.5 equiv) in the presence of 3–Li (0.1 equiv)
in toluene at 0 ꢁC for 22 h, a 97:1.6:1.4 ratio of 1-phe-
nylpropanol (46% ee), benzyl alcohol and unreacted
benzaldehyde was detected by GC analysis. Unfortu-
1.1 eq. Zn(OTF)2
OH
O
1.2 eq. Et3N
(R or S)
1.2 eq. amino alcohol
H
toluene
R
H
R
+
Scheme 2. Addition of phenylacetylene to benzaldehyde catalyzed by
chiral amino alcohols.
The obtained chiral secondary propargylic alcohols are
versatile building blocks for asymmetric synthesis and
their utility as key starting materials in a number of
applications has been demonstrated in previous works

234
S. Degni et al. / Tetrahedron: Asymmetry 15 (2004) 231–237
by other authors.14 A number of ligands have been
employed as chiral mediators in the enantioselective
alkynylation of aromatic aldehydes.2a;b The relationship
between ligand structure, enantioselectivity and catalytic
activity in this reaction has been investigated by utilizing
various substituted chiral amino alcohols.2a In these
reports, some problems were encountered: (a) Systems
with good enantioselectivity required the use of large
quantities of the chiral auxiliary;3 (b) Formation of
considerable amounts of side products and only mode-
rate enantioselectivities for the desired products.2;3;15
case the fibrous catalyst was successfully recycled with-
out significant losses in catalyst performance. The low
yields obtained with the fibrous catalyst are possibly
associated with poorer interaction between the reagents
in the polymer matrix and the related site accessibility
issues and/or the low swelling of the polymer in the
solvent employed.
4. Scanning electron microscope studies
In our preliminary investigation, the chiral amino
alcohol 5 (Fig. 3) was prepared16 and used as the model
catalyst. To our knowledge, compound 5 has not been
applied previously for this transformation. In order to
find suitable conditions for the application of P2 in
combination with a Zn reagent, we first turned to the
procedure described by Moore et al. using Ti(OiPr)4 for
activation of the catalyst system.17 Using this procedure
with 5, 1-phenylpropanol was obtained in low yield
without observable formation of the target propargylic
alcohol even at prolonged reaction times. Based on these
observations, we suspected that diethylzinc is not reac-
tive enough for in situ generation of the zinc phenyl-
acetylide intermedite, which according to the two step
reaction mechanism16 adds to benzaldehyde to produce
the chiral secondary propargylic alcohol. Next, follow-
ing the procedure described by Carreira and co-work-
ers3a we were able to successfully carry out this reaction
using P2 in combination with Zn(OTf)2 and Et3N.
Compounds 3 and 5 were used as reference catalysts.
The results are summarized in Table 2.
In order to obtain further insight into the morphology
of the fibrous catalysts, a scanning electron microscope
(SEM) study was carried out on P2, the corresponding
unfunctionalized parent fiber P0 and the previously re-
ported catalyst P1. SEM micrographs of the fiber sur-
faces are displayed in Figure 5. Both functionalized
fibers P1 and P2 show clear evidence of a surface layer
when compared to the ungrafted fiber P0. For both P1
and P2, grafting has occurred exclusively on the fiber
surface and not in the space between the individual
fibers. The fibrous morphology is clearly retained in the
grafting experiments.
Figure 5. A scanning electron micrograph (SEM) of the unfunction-
alized parent fiber P0 (left) and the fibrous amino alcohol catalysts P1
(middle) and P2 (right).
Our results compare favorably with the highest enantio-
selectivities reported for the addition of terminal alkynes
to aromatic aldehydes using either stoichiometric or
catalytic amounts of chiral amino alcohols in combi-
nation with Zn(OTf)2.3 As evident from Table 2, graft-
ing of the catalyst to a polymer support significantly
decreased the yield of the reaction, whereas the enantio-
selectivity remained on the same level. Thus, alkynyl-
ation of the benzaldehyde using the homogeneous
catalyst 3 gave the propargylic alcohol in a quantitative
yield with 89% enantiomeric excess, while under identi-
cal reaction conditions a <50% yield and 91% ee was
obtained with the polymeric catalysts P2. Also in this
5. Conclusion
The new fibrous chiral amino alcohol catalyst P2 has
been prepared by electron beam induced preirradiation
grafting of the corresponding monomer onto mechani-
cally stable polyethylene fibers and utilized in the
enantioselective additions of diethyl zinc and phenyl-
acetylene to benzaldehyde. In contrast to catalyst P1
reported in our previous study,7 the pendantly immo-
bilized catalyst P2 provides (R)-1-phenylpropanol in
excellent yield and slightly higher but similarly moderate
enantioselectivity. The higher yield is possibly a conse-
quence of the enhanced site accessibility resulting from
the pendant versus cross-linking immobilization of
the chiral ligand. The scope of the fibrous catalysts
was further broadened by the application of P2 in the
addition of phenylacetylene to benzaldehyde. High
enantioselectivities but poor yields were obtained. The
fibrous catalyst was, in both applications, successfully
recycled twice without significant loss in performance.
Polyethylene fibers in combination with the EB-grafting
technique thus provides an attractive, morphologically
stable, robust, and easily recyclable alternative to the
traditional polystyrene based supports, which com-
monly suffer from poor mechanical and thermal stabil-
ities.
Table 2. Addition of phenylacetylene to benzaldehyde catalyzed by
homogeneous and heterogeneous chiral amino alcohols in combina-
tion with Zn(OTf)2 and Et3N
Entry
Ligand
Time (h)
Yield (%)a
Ee (%)c
1
2
3
4
5
3
15
23
48
48
48
Quant.b
67
45
89
48
91
90
88
5
P2
P2d
P2e
37
30
a Isolated yield after flash chromatography on silica gel.
b Estimated by GC after 15 h (no benzaldehyde detected).
c Determined by HPLC.
d First regeneration of the polymeric catalyst.
e Second regeneration.

S. Degni et al. / Tetrahedron: Asymmetry 15 (2004) 231–237
235
6. Experimental
analysis (average of two runs: 0.57 wt % N, corre-
sponding to a loading of 0.4 mmol ligand per gram of
fiber). IR (KBr, cmꢀ1): 3360, 3082, 3025, 2913, 2848,
1600, 1493, 1472, 1451, 1370, 1029, 757, 748, 698, 694,
6.1. General
All reactions were carried out under argon atmosphere
using flame-dried glassware. Ti(OiPr)4 (Acros Organics,
98%), Et2Zn (1.1 M in toluene, Fluka), Zn(OTf)2 (Acros
Organics, 98%), and phenylacetylene (Acros Organics,
98%) were used as received. The chiral reference amino
alcohol 4 was purchased from Fluka. Synthesis of 1 and
P1 has been described in our previous paper.7 Com-
pounds 2 and 3,9 as well as 513 were prepared according
to literature procedures. Triethylamine was dried by
distillation over KOH. All organic solvents were dried
and distilled under argon prior to use when applicable.
Benzaldehyde (Merck) was distilled prior to use. Tolu-
ene (Aldrich, anhydrous) was purified by passing
through columns containing alumina. All other com-
mercially available chemicals and solvents used were of
puriss p.a. quality, or purified and dried according to
standard methods. TLC: precoated silica gel 60 F254
(Merck); visualization by irradiation with UV light.
Flash chromatography (FC): silica gel 60 (0.04–0.063,
Merck). Infrared (IR) spectra were recorded using a
Perkin Elmer Spectrum 1000 FTIR spectrometer. 1H
and 13C NMR spectra were recorded using a Jeol JNM
A 500 NMR spectrometer operating at 500.13 MHz for
1H and 125.78 MHz for 13C, respectively. The chemical
shifts are expressed in ppm downfield from internal
TMS or referenced against the solvent signal. Polari-
metric analyses were performed using a Perkin Elmer
241 polarimeter. Enantioselectivities were determined
either by capillary gas chromatography (CGC) on an
HP-5 GC (cross-linked 5% PH ME siloxane, column:
15 m · 0.53 mm · 1.5 lm film b-cyclodextrin, 30 m ·
0.25 mm · 1.5 lm film thickness) or by HPLC using an
HP 1090 liquid chromatograph system equipped with a
Chiralcel OD column (Daicel Chemical Industries), 10%
IPA in hexane mixture as mobile phase and detection by
UV–vis detector at 254 nm. GC/MS analyses were per-
formed using an HP-5890 SERIES II gas chromato-
graph equipped with a 5971 A mass selective detector
(column: 30 m · 0.25 mm · 0.25 lm HP-1MS). Elemental
analyses were obtained from the Microanalytical Service
of Micro Kemi AB, Uppsala (Sweden).
538 cmꢀ1
.
6.3. General procedures for the addition of diethylzinc
to benzaldehyde in the presence of chiral amino
alcohol catalysts
6.3.1. Homogeneous catalysts. In a typical procedure, to
a solution of 3 (72.1 mg, 0.21 mmol) in toluene (10 mL)
at 0 ꢁC was added dropwise a solution of Et2Zn (9.6 mL,
1.1 M in toluene) during a period of 30 min. The mixture
was slowly warmed to ambient temperature and stirred
for another 30 min. After cooling to 0 ꢁC, benzaldehyde
(0.4 mL, 4.21 mmol) was slowly added for 5 min, and the
reaction mixture was then stirred at ambient tempera-
ture for 24 h. The reaction was then quenched by addi-
tion of 2% cold aqueous HCl. After extraction with
dichloromethane (2 · 20 mL) the combined organic ex-
tracts were washed with brine (10 mL), dried over
magnesium sulfate, and evaporated to dryness.
6.3.2. Polymeric catalyst. As described in the previous
paper,7 the calculated amount of the polymeric catalyst
P2 (0.53 g, loading 0.4 mmol/g, 0.21 mmol) was sus-
pended in toluene (15 mL) and stirred for 1 h at ambient
temperature. The solvent was stripped off in vacuo to
remove traces of water. The catalyst was then suspended
again in toluene (10 mL) and treated with a solution
Et2Zn (9.6 mL, 1.1 M in toluene) added dropwise at 0 ꢁC
during a period of 30 min. The reaction mixture was
slowly warmed to ambient temperature and stirred for
an additional 30 min. After cooling to 0 ꢁC, benzalde-
hyde (0.4 mL, 4.21 mmol) was slowly added for a period
of 5 min, and the resulting reaction mixture then left to
stir for 24 h at ambient temperature. The reaction was
then quenched by the addition of 2 M HCl (10 mL) and
stirred for 90 min at ambient temperature. The poly-
meric catalyst was filtered on a frit and washed with
water (20 mL), THF (10 mL), and diethyl ether (50 mL).
The filtrate was extracted with diethyl ether (2 · 50 mL)
and the combined organic phases washed with brine
(50 mL), dried over magnesium sulfate, and evaporated
to dryness.
6.2. Fiber supported chiral catalyst P2
Cut polyethylene fibers (0.7 Dtex) were irradiated in a
nitrogen atmosphere to a total dose of 100 kGy using an
Electrocurtain electron accelerator (Energy Science Inc.)
operating at an acceleration voltage of 175 kV. Imme-
diately after the irradiation, 5 g of the polyethylene
fibers were immersed in a nitrogen purged solution of
1.85 g of the monomer 2, 7.1 g styrene, and 0.14 g divi-
nylbenzene in 35 g ethanol. The reaction was allowed to
proceed under nitrogen for 2 h at 60 ꢁC. Next 75 mL of
degassed water was added and the temperature in-
creased to reflux for 6 h. The grafted fiber was separated
by filtration, washed twice with ethanol, and dried in
vacuum. The gravimetrical grafting yield was calculated
to 90%. Loading of P2 was confirmed by nitrogen
6.3.3. Lithium salts. A solution of the chiral ligand 3 or a
suspension of the polymeric catalyst P2 (0.2 mmol,
10 mol %) in toluene was treated dropwise with an
equimolar amount of n-BuLi (2.5 M solution in hexanes)
at )40 ꢁC. After stirring for 15 min, a solution of Et2Zn
(5 mmol, 1.1 M in toluene) was added for a period of
20 min and the mixture was then allowed to warm up to
0 ꢁC. Next, benzaldehyde (2 mmol) was added and the
reaction mixture was stirred for 20 h at 0 ꢁC. The reac-
tion was quenched by the addition of 5M HCl (10 mL)
and extracted with diethyl ether (3 · 20 mL). The com-
bined organic phases were washed with NaHSO3
(20 mL), NaHCO3 (20 mL), brine (50 mL), and dried

236
S. Degni et al. / Tetrahedron: Asymmetry 15 (2004) 231–237
over magnesium sulfate. Evaporation to dryness afford-
ed the crude products.
acetone, and dichloromethane followed by drying under
high vacuum (oil pump) at ambient temperature for
several hours.
6.4. General procedures for the addition of phenyl-
acetylene to benzaldehyde in the presence of chiral
amino alcohol catalysts
Acknowledgements
This work was supported by the Academy of Finland
and Smoptech Ltd. S.D. is grateful to Svenska Tekniska
Vetenskapsakademin for a postgraduate fellowship. The
authors thank Robert Peltonen, Kenneth Ekman, and
Mats Sundell (Smoptech Ltd) for supplying the polymer
fibers and for carrying out the pre-irradiation grafting
6.4.1. Homogeneous catalysts. A 50 mL round bottom
flask was charged with Zn(OTf)2 (200 mg, 0.55 mmol,
1.1 equiv) and 3 (212 mg, 0.6 mmol, 1.2 equiv) and
purged with argon for 30 min. To this mixture was then
added toluene (2 mL) and triethylamine (61 mg, 85 lL,
1.2 equiv). The resulting mixture was stirred for 2 h at
ambient temperature followed by the addition of phen-
ylacetylene (61.3 mg, 66 lL, 0.6 mmol) in one portion.
Next, after stirring for 15 min, benzaldehyde (53 mg,
51 lL, 0.5 mmol) was added in one portion. After an
appropriate reaction time, the mixture was quenched by
addition of saturated aq NH4Cl solution (5 mL). The
mixture was poured into a separation funnel containing
diethyl ether (10 mL). The layers were separated and the
aqueous phase was extracted with diethyl ether
(3 · 10 mL). The combined organic layers were washed
with brine (10 mL), dried over magnesium sulfate, and
concentrated in vacuo. Purification of the residue by
flash column chromatography on silica gel (pentane/
diethyl ether 95:5) afforded the propargylic secondary
alcohol.12;13 The enantiomeric ratio of the product was
determined by HPLC (Chiralcel OD, 10% iPrOH in
hexane, 254 nm); t, 13.228 min (major enantiomer),
22.469 min (minor enantiomer).
€
experiments. Paivi Pennanen, Markku Reunanen, and
Andrei Pranovich are thanked for their skillful assis-
tance with NMR, MS, and HPLC analyses. SEM ana-
lyses were performed by Clifford Ekholm.
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6.4.2. Polymeric catalyst. A 50 mL RB flask was charged
with Zn(OTf)2 (200 mg, 0.55 mmol, 1.1 equiv) and P2
(0.757 g, loading 0.4 mmol/g, 0.303 mmol, 1.2 equiv),
and kept in vacuo (0.03 mbar) for 12 h at ambient tem-
perature. Next, the flask was purged with argon for
30 min followed by the addition of toluene (8 mL), and
triethylamine (31 mg, 43 lL, 1.2 equiv). The resulting
mixture was stirred for 19 h at ambient temperature
before addition of phenylacetylene (31 mg, 33 lL,
0.303 mmol) in one portion. After stirring for 30 min,
benzaldehyde (27 mg, 25 lL, 0.252 mmol) was added in
one portion. After an appropriate reaction time, the
mixture was quenched by the addition of saturated aq
NH4Cl solution (10 mL). The polymeric catalyst was
filtered on a frit and washed with water (20 mL), THF
(10 mL), and diethyl ether (50 mL). The filtrate was ex-
tracted with diethyl ether (2 · 50 mL) and the combined
organic phases washed with brine (50 mL), dried over
magnesium sulfate, and evaporated to dryness. Purifi-
cation of the residue by flash column chromatography
on silica gel (pentane/diethyl ether 95:5) afforded the
propargylic secondary alcohol. The enantiomeric ratio
of the product was determined by HPLC.
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