Asymmetric sulfoxidation of thioethers with hydrogen peroxide in water mediated by platinum chiral catalyst

Article abstract of DOI:10.1002/adsc.200505030

Easy stereoselective oxidation of prochiral aryl alkyl sulfides 2 to the corresponding sulfoxides can be achieved in water-surfactant medium with inexpensive hydrogen peroxide mediated by the chiral platinum diphosphine complex {[(R)-BINAP]Pt(μ-OH)}₂

Full text of DOI:10.1002/adsc.200505030

FULL PAPERS
Asymmetric Sulfoxidation of Thioethers with Hydrogen Peroxide
in Water Mediated by Platinum Chiral Catalyst
Alessandro Scarso, Giorgio Strukul*
`
`
Department of Chemistry, Universita Ca Foscari di Venezia, Dorsoduro 2137, 30123 Venice, Italy
Fax: (þ39)-041-234-8517, e-mail: strukul@unive.it
Received: January 21, 2005; Accepted: March 21, 2005
Abstract: Easy stereoselective oxidation of prochiral phase separation, (ii) catalyst loading as low as 1%
aryl alkyl sulfides 2 to the corresponding sulfoxides mol, (iii) good yields, sulfoxide 3 to sulfone 4 ratio
can be achieved in water-surfactant medium with in- up to 200:1 and enantioselectivities up to 88%, (iv)
expensive hydrogen peroxide mediated by the chiral mild experimental conditions.
platinum diphosphine complex {[(R)-BINAP]Pt(m-
OH)}2(BF4)2 (1). Remarkable key features of general
interest are (i) easy isolation of the products from cat- Keywords: asymmetric catalysis; oxidation; platinum;
alyst by simple diethyl ether/water-surfactant two sulfoxides; surfactants; water chemistry
Introduction
gen peroxide and, as a test reaction, we chose the asym-
metric sulfoxidation of prochiral thioethers. Chiral sulf-
In asymmetric catalysis, in addition to the classical tar- oxides find applications as important auxiliaries in
gets such as high enantioselectivity and yield as well as asymmetric synthesis[14] and in the pharmaceutical in-
easy procedures, the use of safe and environmentally dustry.[15,16] Many catalytic processes have been devel-
friendly reagents and mild conditions are stringent re- oped for their preparation, usually based on catalysts
quirements.[1–3] Last but not least, easy isolation of the comprising early transition metal complexes.[17] Only
enantioenriched product is also advisable. Other open few of these processes are highly stereoselective towards
challenges are the replacement of organic and especially a broad range of alkyl aryl and dialkyl thioethers, but
chlorinated solvents with water[4,5] which, in many cas- they make use of stoichiometric amounts of chiral li-
es,[6–9] has been demonstrated to enhance selectivity gands.[18] Other systems are catalytic with loadings as
and asymmetric induction compared to the same pro- low as 2% mol but employing alkyl or aryl hydroperox-
cesses carried out in organic media.
ides as primary oxidant which produce alcohols as by-
To exploit the many advantages of water as reaction products. Moreover, in most of these examples low tem-
medium, like low cost, safety and environmental accept- peratures were required, always in chlorinated sol-
ability, the major handicap to be overcome is the gener- vents.[15] More recently, hydrogen peroxide has attracted
ally low solubility of organic substrates (and of most the attention of many groups and it has been successfully
metal complex catalysts) and their possible sensitivity employed as oxidant with good chemical yields and
to the acidity and nucleophilicity of this solvent. Among enantioselectivity[19] in catalytic sulfoxidation processes,
the different strategies employed to induce solubiliza- but still in common organic chlorinated solvents.
tion in water, besides the use of organic cosolvents, the
Herein we report an easy to accomplish catalytic
employment of surfactants above the critical micellar asymmetric sulfoxidation process (Scheme 1) in which
concentration (CMC) plays a crucial role.[10] This ap- the dimeric {[(R)-BINAP]Pt(m-OH)}2(BF4)2 complex
proach has been applied to some asymmetric catalytic (1)[20,21] with low loading activates 35% hydrogen perox-
reactions such as hydrogenation.
ide in water-surfactant solutions towards aryl alkyl sul-
In oxidation processes,[11,12] hydrogen peroxide is a fides. To the best of our knowledge this is the first exam-
highly appealing oxidant due to its low cost, high atom ple of a catalytic asymmetric oxidation in water.[22]
economy[13] and safe handling. Moreover, water is the
only by-product of its reduction and this makes its use
very attractive in the development of “green” oxidation Results and Discussion
processes.
We thus strived towards this goal by seeking an asym- The effect of different surfactants on the reaction
metric catalytic oxidation system in water with hydro- (Scheme 1) is summarized in Table 1. If the reaction is
Adv. Synth. Catal. 2005, 347, 1227–1234
DOI: 10.1002/adsc.200505030
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Alessandro Scarso, Giorgio Strukul
FULL PAPERS
Scheme 1. {[(R)-BINAP]Pt(m-OH)}2(BF4)2 (1)-catalyzed asymmetric sulfoxidation of prochiral aryl alkyl sulfides 2 with hydro-
gen peroxide in water with surfactants.
carried out in dichloromethane (Table 1, entries 1 and tioselectivities increase ((Table 1, entries 5 to 7).
2), with a loading as low as 1% mol, a fast conversion Among the possible surfactants, sodium dodecyl sulfate
with good chemical yields but rather low enantioselec- (SDS) is responsible for the highest enantioselectivity
tivity is observed. When switching the reaction medium and therefore the effect of its concentration was thor-
to water-surfactant, 1 is able to convert thioanisole (2a) oughly investigated.
into the corresponding sulfoxide 3a with comparable
The action of the surfactant above its CMC is to solu-
yields, higher sulfoxide 3a to sulfone 4a ratio and better bilize in water the substrate and catalyst 1 (as demon-
enantioselectivities,[23] depending on the type of surfac- strated by the NMR spectra reported in Figure 1) thus
tant used (Table 1, entries 5, 6, 7). Under the latter con- allowing intimate contact between the two. If no surfac-
ditions the reactant-products mixture is separated from tant is employed, the reaction is sluggish (one order of
the catalyst by simple extraction with diethyl ether in magnitude slower) and not stereoselective. Above
which the catalyst itself is not soluble.
8 mM, SDS aggregates in water forming micelles, and
Cationic and neutral surfactants (Table 1, entries 3 up to 75 mM (Figure 2) an increase of the sulfoxide ee
and 4) led to low [3]/[4] ratios, as well as low yields in was observed.[24] When the SDS concentration was fur-
sulfoxide with no enantioselectivity. This behavior is ther increased to 300 mM an almost linear decrease of
likely due to low solubilization of the catalyst 1 by cati- ee down to formation of racemic 3a was observed.
onic and neutral micelles. On the contrary with anionic
Figure 2 shows also a concomitant influence of the
surfactants the bis-cationic catalyst 1 is much more solu- SDS concentration on the initial rate of the reaction.
ble and, as a consequence, yields, [3]/[4] ratios and enan- The initial increase of activity due to a better catalyst
Table 1. Catalytic enantioselective oxidation of aryl methyl sulfides with hydrogen peroxide mediated by 1.
Entry R1
R2
Time [h] Yield[a] [%] [3]/[4] ratio ee[b] [%] Abs. Conf.[c] Solvent
1
2
3
4
5
6
7
C6H5 (2a)
p-NO2-C6H4 (2b) CH3 24
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
C6H5 (2a)
CH3 8.5
99
41
16
20
85
82
98
80
25
31
56
>200
180
>200
16
26[d]
5
R-(þ)
R-(þ)
S-(À)
–
R-(þ)
R-(þ)
R-(þ)
CH2Cl2
CH2Cl2
[e]
À
CH3 24
CH3 24
CH3 24
CH3 24
CH3 24
H2O CTABr
0
H2O-Triton-X100[f]
[g]
À
29
30
40
H2O C12H25(C6H5)SO3Na
[h]
À
H2O C12H25SO3Na
[i]
À
H2O C12H25SO4Na
General conditions: substrate:H2O2 : 1¼100: 100: 1; [substrate]¼0.75 mmol, [H2O2]¼0.75 mmol, [1]¼0.0075 mmol, solvent
3 mL, room temperature in air.
[a]
Yield in sulfoxide determined by GC (column HP-5).
Enantiomeric excess determined by CSP-GC (column Lipodex-E).
Absolute configuration determined by optical rotations and comparison of the retention orders with known literature data.
Enantiomeric excess determined by integration of the H NMR spectrum with (R)-BINOL at 253 K in CDCl3.
Cetyltrimethylammonium bromide (168 mM, 1 mM in micelles).
Polyoxyethylene(10)isooctyl phenyl ether (150 mM, 1 mM in micelles).
Sodium dodecylbenzenesulfonate (63 mM, 1 mM in micelles).
Sodium dodecylsulfonate (58 mM, 1 mM in micelles).
[b]
[c]
[d]
[e]
[f]
[g]
[h]
[i]
1
Sodium dodecyl sulfate SDS (75 mM, 1 mM in micelles).
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Asymmetric Sulfoxidation of Thioethers
FULL PAPERS
Figure 2. Effect of [SDS] on both enantiomeric excess (trian-
gles) and initial rate (circles) for the asymmetric sulfoxida-
tion of thioanisole (2a) with hydrogen peroxide [substrate
2a:H2O2 :1¼100:100:1;
[2a]¼0.75 mmol,
[H2O2]¼
0.75 mmol, [1]¼0.0075 mmol, solvent water 3 mL, sodium
dodecyl sulfate SDS (75 mM, 1 m M in micelles), room tem-
perature].
[4] ratio) giving the maximum ee at an intermediate val-
ue, while the conversion remains unchanged.
This overall behavior seems to suggest that the mi-
celleꢁs average size[25] and the catalyst amount affect
heavily both catalyst positioning and substrate approach
to the active species. To further investigate this point and
explore the synthetic scope of this oxidation procedure
the catalytic enantioselective oxidation of different
aryl alkyl sulfides with hydrogen peroxide in water-
1
Figure 1. NMR spectra: 31P{1H} (top) and H NMR spectra
(bottom) of {[(R)-BINAP]Pt(m-OH)}2(BF4)2 (1). [1]¼
2.5 mM, [SDS]¼75 mM in H2O/D2O (75/25).
and substrate solubilization is followed by a decrease SDS solution catalyzed by 1 was studied (Table 3).
down to a minimum corresponding to the highest enan- With solid substrates diethyl ether (generally used to
tioselectivity. This may be no coincidence as it is known extract the products) has to be employed from the begin-
that, in general, lower activities allow a better discrimi- ning with the purpose of solubilizing the substrate and
nation between the two diastereomeric transition states. promoting catalyst-substrate interaction. Aqueous
Also the substrate to catalyst ratio (Table 2) appears to phase dissolves catalyst and oxidant, while the ether
have an effect on the enantioselectivity (and on the [3]/ phase extracts the organic reagent and products.
Table 2. Catalytic enantioselective oxidation of thioanisole (2a) with hydrogen peroxide in water-SDS solution mediated by 1
at different molar ratios.
Entry
Yield[a] [%]
[3]/[4] ratio
ee[b] [%]
Abs. Conf.[c]
Molar ratios
1
2
3
4
98
98
99
99
>200
>200
115
33
40
34
26
R-(þ)
R-(þ)
R-(þ)
R-(þ)
2a :H2O2 : 1¼50: 50: 1
2a :H2O2 : 1¼100: 100: 1
2a :H2O2 : 1¼200: 200: 1
2a :H2O2 : 1¼1000: 1000: 1
90
General conditions: [1]¼0.0075 mmol, solvent water 3 mL, sodium dodecyl sulfate SDS (75 mM, 1 mM in micelles), room
temperature in air , reaction time 24 h.
[a]
Yield in sulfoxide determined by GC (column HP-5).
Enantiomeric excess determined by CSP-GC (column Lipodex-E).
Absolute configuration determined by optical rotations and comparison of the retention orders with known literature data.
[b]
[c]
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Table 3. Catalytic enantioselective oxidation of aryl alkyl sulfides 2 with hydrogen peroxide in water-SDS solution mediated by
1.
Entry
R1
R2
Time [h]
Yield[a] [%]
[3]/[4] ratio
ee[b] [%]
Abs. Conf.[c]
1
2
3
4
5
6
7
8
C6H5 (2a)
CH3
CH3
Bn
CH3
CH3
CH3
CH3
CH3
24
24
48
24
24
24
48
48
98
99
75
96
99
87
68
63
>200
32
19
40
R-(þ)
R-(þ)
n.d.
R-(þ)
R-(þ)
R-(þ)
R-(þ)
R-(þ)
2-naphthyl (2h)
C6H5 (2c)
34[d]
24[d]
22[d]
31
À
p-CH3O C6H4 (2d)
97
p-CH3-C6H4 (2e)
p-Cl-C6H4 (2f)
>200
>200
21
48
p-CN-C6H4 (2g)
p-NO2-C6H4 (2b)
63
90
88[d]
General conditions: substrate:H2O2 : 1¼100: 100: 1; [substrate]¼0.75 mmol, [H2O2]¼0.75 mmol, [1]¼0.0075 mmol, solvent
water 3 mL, sodium dodecyl sulfate SDS (75 mM, 1 mM in micelles), room temperature in air.
[a]
Yield in sulfoxide determined by GC (column HP-5).
Enantiomeric excess determined by CSP-GC (column Lipodex-E).
Absolute configuration determined by optical rotations and comparison of the retention orders with known literature data.
Enantiomeric excess determined by integration of the H NMR spectrum with (R)-BINOL at 253 K in CDCl3.
[b]
[c]
[d]
1
The stereoselective oxidation is hardly sensitive to the the chiral diphosphine ligand on the product enantiose-
steric hindrance between the thioether substituents R1 lectivity, observing a remarkable positive non-linear ef-
and R2 as observed by comparing 2a and 2-methylsulfa- fect (þ)-NLE (Figure 3) which, according to the general
nylnaphthalene (2 h) (entries 1 and 2 in Table 3), while, principle suggested by Kagan,[27] supports the dimeric
as most other examples reported in the literature,[15] it nature of the catalytically active species.
does reflect the steric unbalance between R1 and R2 (en-
It is therefore likely that hydrogen peroxide is activat-
tries 1 and 3 in Table 3). Additionally, it is much more ed by substitution of the hydroxide bridging group of the
affected by the electronic properties of the substrates catalyst to give a hydroperoxy species, with a bimetallic
(Table 3, entries 7 and 8 versus entry 1).
framework, with subsequent electrophilic oxidation of
Decreasing the electron-donating properties of the the substrates. This view, supported by the experimental
substituents in the aryl moiety of aryl methyl sulfides evidence reported in Figure 3, seems peculiar for the
(Table 3) causes both a decrease of yield, confirming present oxidation reaction and could be due to the spe-
the highly electrophilic oxidation character of the cata- cific medium employed. In fact, it contrasts with what is
lytic system, and a concomitant increase in the ee with observed in dichloroethane solution with the same class
the highest value (88%) observed with p-O2N-C6H4-
SCH3 (2b). The selectivity of the process is very high,
ranging from [3]/[4]ꢀ200 for electron-rich substrates,
to 20 for electron-poor ones.
The Lewis acid character of the complex[26] favors rac-
emization of the sulfoxides. This effect is more evident
with the most electron-poor substrates which are known
to be sensitive to acids.[18] For example, if the reaction
with 2b is carried out in dichloromethane solution (Ta-
ble 1, entry 2), the initial 55% ee decreases to 26% after
24 h. This negative effect can be relieved in micellar me-
dia by employing diethyl ether in which 1 is not soluble.
This allows us to extract the sulfoxides from the micelles,
thus limiting the direct contact between catalyst and
product and allowing isolation of the enantioenriched
sulfoxide by simple two-phase separation. It has to be
stressed once again that the higher initial stereoselectiv-
ity observed in aqueous media compared to dichlorome-
thane is probably a consequence of the unique proper-
Figure 3. Positive non linear effect (þ)-NLE for the oxidation
ties of water as solvent, in particular to its “hydrophobic
of 2g with hydrogen peroxide catalyzed by 1 in diethyl ether/
effect”[6] that allows a closer contact between substrate
water-SDS biphasic system {substrate 2g:H2O2 :1¼
and catalyst.
100:100:1; [2g]¼0.75 mmol, [H2O2]¼0.75 mmol, [1]¼
In order to get more insight into the stereoselective
0.0075 mmol, solvent water 3 mL, sodium dodecyl sulfate
process, we explored the effect of the optical purity of SDS (75 mM, 1 mM in micelles), room temperature}.
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Figure 4. Postulated model for the approach of thioanisole
(2a) to the dimeric hydroperoxo active species. In particular,
it is worth noting the proximity between the aromatic groups
of the substrate and the complex and between the peroxidic
oxygen and the sulfur atom of the thioether.
Figure 5. Portion of the 2D NOESY spectrum of {[(R)-BI-
NAP]Pt(m-OH)}2(BF4)2 (1), [1]¼2.5 mM, [SDS]¼75 mM in
H2O/D2O (75/25), showing the cross-peak between the cata-
lyst and the second methylene of SDS.
of complexes in the Baeyer–Villiger oxidation of ke-
tones,[19] where the catalytically active species was rec-
ognized to be monomeric.
On the basis of the above observations, it is possible to
propose a molecular model for the approach of the sub- ated by channeling through the aliphatic chains of the
strate to the catalyst like in Figure 4 in which the aromat- aggregated surfactant in which the sulfide itself is dis-
ic phenyl rings of (R)-BINAP present in the dimeric solved. This view is also in general agreement with the
complex form a shallow pocket. Here the thioether micelle size effect reported in Figure 2.
can be accommodated driven by p–p interactions which
The dimeric structure of the catalytically active spe-
are more pronounced for electron-withdrawing sub- cies and the electrophilic nature of the oxidation suggest
stituents on the substrate (see Table 3, entry 7 and 8, a possible catalytic cycle as reported in Scheme 2. The
the highest enantioselectivity is observed with the lower pKa of hydrogen peroxide allows a facile acid-
most electron-poor substrates). The peroxidic oxygen base reaction with exchange of the hydroxy bridging
is placed below the sulfur atom of the substrate which moiety with the hydroperoxidic anion.[29] Metal activa-
can attack the oxygen leading to the sulfoxide. This mod- tion of the peroxy oxygen occurs, which is subsequently
el accounts for the observed prevailing enantiomer in all attacked by the sulfur atom of the thioanisole driven
cases tested.
close to the complex by p–p and metal-sulfur coordina-
The above observed modest effect on the ee found tion. This proposal is quite similar to the classical mech-
with either 2a or 2 h, seems to be an indication that the anism of electrophilic oxidation with hydroperoxides
presence of both substrate and 1 inside a cavity such as suggested by Sharpless[30] and Kagan[18a] many years
a micelle is unlikely. In fact, as was found by Thomas ago. The neutral sulfoxide product is then released and
and co-workers,[28] albeit for different enantioselective the hydroxy bridging ligand is restored for the next cat-
transformations, confining reactants and catalyst in the alytic cycle.
cavity of a zeolite results in strong steric effects. It may
The synthetic methodology reported here represents
also be suggested that, the active species being bis-cati- a viable way for carrying out asymmetric sulfoxidation
onic, the latter is likely to interact with the micelle on the in water. Catalyst loading is low, yields and sulfoxide/sul-
external part where the negatively charged sulfate fone selectivity are from good to excellent (no other
groups are located. This is confirmed by a 2D-NOESY products are formed) and, as was previously reported
experiment (Figure 5) in which clear cross-peaks are de- for other asymmetric transformations,[6–9] the aqueous
tected between the naphthyl moiety of the dimeric cata- medium allows a significant improvement in the asym-
lyst and the second methylene of SDS (75 mM in H2O/ metric induction, compared to the use of organic sol-
D2O solution), suggesting the close proximity between vents, although ees are from moderate to good, with
the aromatic surfaces of the catalyst and the hydrocar- only one case in the>80% range. From this point of
bon chains of the surfactant, due to hydrophobic interac- view, in the literature, catalysts capable of a better enan-
tions. The approach of the substrate (from inside the mi- tioselective performance do exist, but they invariably
celle) to the catalyst (outside the micelle) may be medi- work in organic (chlorinated) solvents and require ei-
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Scheme 2. Possible catalytic mechanistic cycle for the oxidation of aryl alkyl sulfides with hydrogen peroxide mediated by 1 in
water-SDS.
ther a higher loading, or different oxidants, or low tem- and hence, upon recycling, much of its activity and enan-
peratures with consequent slow down of rates.[15]
So far, the synthetic scope of this protocol seems to be
limited to alkyl aryl sulfides with a pronounced steric un-
balance between the two substituents. This is relatively
common and, indeed, catalysts capable of promoting
the asymmetric oxidation of dialkyl or diaryl sulfides
are only few and operate at the expenses of other impor-
tant synthetic parameters.[18]
One major advantage of the present protocol is the
possibility to use commercial hydrogen peroxide solu-
tions as the oxidant, i.e., the most environmentally
friendly peroxy oxidant one can think of.
The use of surfactants seems to be of quite general ap-
plicability, as micelles provide an interface that allows
the catalyst (that is insoluble both in water and in diethyl
ether) to contact the substrate and promote its oxida-
tion. In doing so, they allow the use of water as the reac-
tion medium also with “ordinary” transition metal cata-
lysts, thereby avoiding the need to modify the catalyst
with hydrophilic functional groups. Micelles also play
a pivotal role in the approach between catalyst and sub-
strate (depending on their size) that has a strong influ-
ence on the enantioselectivity of the system.
tioselectivity are lost.
Conclusion
In summary, we have developed the first example of
asymmetric catalytic sulfoxidation in water. Additional
key features towards possible synthetic applications are
(i) easy isolation of the products from the catalyst by
simple diethyl ether/water SDS two phase separation,
(ii) use of green and inexpensive hydrogen peroxide as
oxidant, (iii) catalyst loading as low as 1% mol, (iv)
good yields, sulfoxide/sulfone selectivities up to 200
and enantioselectivities up to 88%, (v) use of mild exper-
imental conditions. Extension to a broader range of sub-
strates, to the use of more oxidation-resistant catalysts
and further investigation on the effect of surfactant on
the catalytic activity are underway.
Experimental Section
From a practical point of view, the possibility to sepa-
rate the products from the catalyst and the exhausted ox-
idant by simple diethyl ether/water-SDS separation is
also an important advantage. Products can be easily iso-
lated by simple solvent evaporation and, in principle, the
catalyst could be reutilized by simple addition of fresh
substrate and oxidant. Unfortunately, the catalyst re-
General
Diethyl ether was freshly distilled prior to use and water was
purified according to the milliQ technique. Hydrogen peroxide
(35% Aldrich) as well as compounds 2a, 2c, 2d, 2e, 2f, 2g, 2b
(Aldrich) are commercial products and were used without fur-
ther purification. 1H NMR, and 31P{1H} NMR spectra were re-
corded at 298 K, unless otherwise stated, on a Bruker
ported here is rather sensitive to hydrogen peroxide AVANCE 300 spectrometer operating at 300.15 and
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Asymmetric Sulfoxidation of Thioethers
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121.50 MHz, respectively. d values in ppm are relative to SiMe4
and 85% H3PO4. The 2D-NOESY experiment was acquired
with a spectrum width of 10 ppm, a relaxation delay d1 of 1 s,
using 2 K data points in the t2 dimension and 512 data points
in the t1 dimension, with subsequent weighting with the sine-
bell function using 160 scans for each t1 increment. The mixing
time d8 employed was 0.4 s. GLC measurements were taken on
a Hewlett-Packard 5890A gas chromatograph equipped with
an FID detector (gas carrier He). All reactions were monitored
on a 25 m HP-5 capillary column. Enantiomeric excess was de-
termined as reported in Tables 1 to 3.
References and Notes
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Sulfide 2 h was prepared by alkylation with methyl iodide of
the corresponding 2-naphthalenethiol and purified by flash
chromatography on silica. NMR spectroscopic data (1H, 13C
NMR) and mass analysis are in agreement with literature
data.[31] The chiral complex {[(R)-BINAP]Pt(m-OH)}2(BF4)2,
was prepared following the procedure reported in the litera-
ture.[20]
Partially resolved 1 complexes employed for the non-linear
effect study (NLE) were prepared starting from mixtures of
[(R)-BINAP]PtCl2 and [(S)-BINAP]PtCl2.[20] The Pt precur-
sors were dissolved in the proper enantiomeric ratio in wet ace-
tone (25 mL) and dichloromethane (25 mL) at room tempera-
ture and treated with 2 equivalents per Pt atom of a standar-
dized solution of AgBF4 in acetone. The reaction mixture
was stirred under nitrogen for 2 h and then the solid AgCl
formed was filtered off. After concentration, the solution was
treated with diethyl ether to give a pale yellow solid, which
was filtered off and dried under vacuum; yield: 90–95%.
˘
[10] S. Tas¸cioglu, Tetrahedron 1996, 52, 11113–11152.
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Acknowledgements
This workwas supported by MIUR (Rome) and the University
of Venice. AS thanks the University of Venice for a fellowship
and GS gratefully acknowledges Johnson Matthey Ltd for the
loan of platinum.
Adv. Synth. Catal. 2005, 347, 1227–1234
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ꢀ 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1233

Alessandro Scarso, Giorgio Strukul
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