Application of a Grubbs-Hoveyda metathesis catalyst noncovalently immobilized by fluorous-fluorous interactions

Article abstract of DOI:10.1002/hlca.200690080

A Grubbs-Hoveyda metathesis catalyst bearing a tris(perfluoroalkyl)silyl tag for efficient noncovalent attachment to fluorous silica gel (FSG) was synthesized and employed in ring-closing metathesis (RCM) reactions in CH ₂Cl₂. After

Full text of DOI:10.1002/hlca.200690080

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Helvetica Chimica Acta – Vol. 89 (2006)
Application of a Grubbs–Hoveyda Metathesis Catalyst Noncovalently
Immobilized by Fluorous–Fluorous Interactions
by Florian Michalek, and Willi Bannwarth*
Institut für Organische Chemie und Biochemie, Albert-Ludwigs-Universität Freiburg, Albertstrasse 21,
D-79104 Freiburg
(phone: +49761203-6073; fax: +49761203-8705; e-mail: willi.bannwarth@organik.chemie.uni-
freiburg.de)
A Grubbs–Hoveyda metathesis catalyst bearing a tris(perfluoroalkyl)silyl tag for efficient noncova-
lent attachment to fluorous silica gel (FSG) was synthesized and employed in ring-closing metathesis
(RCM) reactions in CH2Cl2. After the reaction, a solvent switch to a polar system allowed for recovery
of the catalyst by filtration and its reuse. The approach was demonstrated for a number of different sub-
strates. Furthermore, it was shown that the application of this catalytic system yielded products with low
ruthenium content.
Introduction. – Homogeneous catalytic reactions are widely used in organic synthe-
sis. Their major drawback however, is the difficult separation of the metal complexes
from the products after the reaction. Immobilization techniques are a method to over-
come this problem [1]. One of these technologies is the application of perfluoro-tagged
catalysts in perfluorinated solvents [2]. These solvents are immiscible with most organic
solvents at room temperature and hence, allow reactions to be performed under fluo-
rous biphasic conditions [3]. Thus, the perfluoro-tagged catalyst can be recovered after
the reaction by a simple liquid-liquid extraction and reused in consecutive runs. Despite
the advantages offered by the approach, the needed fluorous solvents are expensive
and environmentally persistent [4].
For this reason, Curran and Luo developed the so-called light fluorous technology
[5]. In this method, the reaction is carried out in an organic solvent, and the perfluoro-
tagged compound is afterwards separated from the products by solid-phase extraction
on fluorous silica gel (FSG). One advantage of this technique is that the tagged mole-
cules require lower F-content than normally needed for fluorous biphasic applications.
In this respect, Matsugi and Curran have recently reported a light fluorous Grubbs–
Hoveyda catalyst and demonstrated its use in ring-closing metathesis (RCM) reactions
applying different substrates [6].
As an alternative approach to omit perfluorinated solvents, we have used FSG as
support for the noncovalent immobilization of perfluorinated catalysts to be applied
in organic solvents. After the reaction, the catalyst can be removed by a simple filtra-
tion step and can be reapplied to further reactions. This technology was successfully
demonstrated for Pd-mediated Suzuki and Sonogashira couplings [7].
Perfluoro-tagged compounds can exhibit strong interactions with perfluorinated
stationary phases. These interactions depend on the chain length of the perfluoro entity
ꢀ 2006 Verlag Helvetica Chimica Acta AG, Zürich

Helvetica Chimica Acta – Vol. 89 (2006)
1031
and especially on the number of the perfluoro tags, the reason for this being coopera-
tivity. In HPLC experiments, we could corroborate these effects by investigating and
comparing retention times of different perfluoro tag-modified compounds on different
FSGs [8]. The results led to our preference for (C8F17CH2CH2)3Si tags for the noncova-
A
lent immobilization of ligands and complexes on FSG. These tags have the further
advantage that they can be easily introduced into aromatic systems which are rampant
in many ligands of catalysts. We have been able to demonstrate this in a recent report
on the synthesis of perfluoro-tagged salen ligands, binap, and the styrene derivative 1
[9].
Results and Discussion. – According to Scheme 1, the ligand 1 was transformed into
the perfluoro-tagged Grubbs–Hoveyda metathesis catalyst 3 by reacting it with Grubbs
catalyst 2 (second generation) [10]. In this reaction, the addition of (trifluoromethyl)-
benzene (BTF) was necessary to ensure homogeneous conditions. After the reaction,
catalyst 3 was isolated by column chromatography in pure form.
Scheme 1. Synthesis of the Perfluoro-Tagged Catalyst 3
i) CuCl, CH2Cl2/BTF, reflux, 4 h, 48%.
The FSG used for the noncovalent attachment of catalyst 3 is shown in Fig. 1. For
the attachment, catalyst 3 was dissolved in Et2O in which the FSG 4 had been sus-
A
pended. Removal of the solvent yielded the supported catalyst as a greenish, free-flow-
ing powder with a loading value of 5 mmol/g.
Fig. 1. Fluorous silica gels (FSG) 4 for the noncovalent immobilization
of catalyst 3
Preliminary experiments had revealed that 3 shows a high solubility even in rela-
tively polar solvents like MeOH. For this reason, we envisaged a so-called solvent
switch which we had previously successfully employed in a multistep synthesis starting

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Helvetica Chimica Acta – Vol. 89 (2006)
from (C8F17CH2CH2)3
ACHTREUNG
envisaged that this approach would be implemented in such a way that the reaction
would be performed in CH2Cl2 and after reaction, the solvent would be evaporated,
with subsequent washings with a polar solvent to remove the product from the immo-
bilized catalyst. Incidentally, Matsugi and Curran also described the solvent switch
approach for their light fluorous metathesis catalyst [6]. The reaction was carried out
in CH2Cl2 and, thereafter, the catalyst was re-adsorbed on the support material in
MeOH/H2O 4 :1.
We had found independently that MeOH/H2O 4 :1 was suitable for our perfluoro-
tagged catalyst since washing steps with this mixture (same amounts as in the metathe-
sis experiments) on the immobilized catalyst (FSG 4a or 4b) had led to a leaching of Ru
of only 1% into the solvent mixture. A two-fold repetition resulted in the same amount
of released ruthenium. We also tested the leaching of the catalyst when immobilized on
unmodified silica gel. The result showed a slightly higher leaching of 3% which can still
be regarded as very low. This was a clear indication that insolubility of the catalyst in
MeOH/H2O 4 :1 was the main driving force for the adsorption to the support, and
that fluorous-fluorous interactions contributed only marginally.
Next, we examined the influence of the support material during actual catalytic
reactions. Equal amounts of catalyst were immobilized on FSGs 4a and 4b and also
on unmodified silica gel. As a benchmark test, the RCM of 5a leading to 6a was carried
out with 1 mol-% of catalyst corresponding to 1 mg of catalyst on 100 mg of silica gel or
FSG (Scheme 2). As shown in Fig. 2, the nature of the support had, as expected, only a
minor impact on the conversion rates in the different cycles. All conversions were in the
same range, with FSG 4b showing slightly better results than the other supports. The
results with standard silica gel as support are in contrast to Matsugiꢁs and Curranꢁs
observation that their light fluorous catalyst gave ꢂinferiorꢁ results [6]. The generally
reduced conversion in runs 2 and 3 are attributed to decomposition of the catalyst
since the decrease is much higher than the leaching values that were found in the initial
washing experiments. It is known that Grubbs catalysts are especially prone to decom-
position in the presence of MeOH and H2O [12]. The conversions after recycling were
by no means comparable to our covalently immobilized catalyst [13].
Scheme 2. Benchmark Ring-Closing Metathesis (RCM) Reaction
To get a better understanding on the dependency of recycling on the amount of cat-
alyst employed, we immobilized different amounts of 3 on 4b to test consecutive RCMs
with 5a as substrate and 5, 2.5, 1 and 0.2 mol-% of catalyst. In contrast to Matsugi and
Curran, we did not correct the loss of catalyst during the different runs [6]. This means
that the same amount of substrate was used in all runs. As expected, the higher the cat-
alyst loading, the more consecutive runs were possible (Fig. 3). With 5 and 2.5 mol-% of
3, we were able to perform four runs with high conversions and then, we observed a
drop in activity for both amounts. A catalyst loading of 1 mol-% showed a comparable

Helvetica Chimica Acta – Vol. 89 (2006)
1033
~
&
Fig. 2. Recycling experiments with 3 immobilized on different supports: FSG 4a ( ), FSG 4b ( ),
*
)
unmodified silica gel (
&
^
Fig. 3. Recycling experiments with different catalyst amounts: 5 mol-% ( ), 2.5 mol-% ( ), 1 mol-%
~
^
( ), and 0.2 mol-% (
)
conversion only in the first two runs. With 0.2 mol-% of 3, no decent conversion was
observed even in the first run. The accumulated turnover numbers are summarized
in Table 1.
Table 1. Turnover Numbers (TON) Obtained in Consecutive RCM of 5a with Different Catalyst Amounts
Catalyst amount [mol-%]
Number of runs
TONa)
5
7
95
2.5
6
169
1
5
294
0.2
3
110
a) The values are accumulated TON.
Next, we extended our study to the RCM of several a,w-dienes employing 1 mol-%
of catalyst 3 (Table 2). After 2 h, the conversion of all substrates was complete. For the

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Helvetica Chimica Acta – Vol. 89 (2006)
Table 2. RCM of Different a,w-Dienes with Catalyst 3
Substrate
Product
Conversion [%] run 1, run 2a)
>98, >98
>98, >98
>98, 92
>98, 84
>98, 10
>98, 55c)
>98, 16
>98, 62c)
a) Conversions determined by 1H-NMR. b) E=COOEt. c) 2.5 mol-% of 3.
formation of 6a–d, high conversions were obtained after recycling but not for the for-
mation of 6e and 6f. These RCMs were repeated with 2.5 mol-% of 3, but even then
only moderate conversions were observed in the second run.
In general, a major limitation of metathesis reactions with Grubbs catalysts is the
high leaching of Ru into the product. Several groups reported that applying the Hov-
eyda-type catalyst, the leaching of Ru could be decreased substantially [14]. We our-
selves reported recently that performing olefin metathesis in supercritical carbon diox-
ide results in a Ru content in the product as low as 20 ppm [15]. Due to the low solu-
bility of 3 in MeOH/H2O, we envisaged that in this case too, we would be able to reduce
the leaching strongly. So, we performed the RCM of 5a on a larger scale and then deter-
mined the Ru in the crude product by atomic absorption spectroscopy (AAS). A clear
trend was not observed. The lowest Ru-content in the product, 86 ppm, was found with
3 on FSG 4b. With 3 on FSG 4a, 137 ppm, and with 3 on normal silica gel, 119 ppm of
Ru were found.

Helvetica Chimica Acta – Vol. 89 (2006)
1035
Conclusion. – We prepared a (C8F17CH2CH2)3Si-modified Grubbs–Hoveyda olefin
G
metathesis catalyst, which was highly active in ring-closing metathesis reactions. The
catalyst was noncovalently immobilized on fluorous silica gel and applied in CH2Cl2
as solvent. After the reaction, a solvent switch to MeOH/H2O was applied to re-attach
the catalyst to the FSG, which allowed its separation from the product by filtration. The
thus isolated supported catalyst could be reused in further cycles. This was demon-
strated in RCMs for a number of different substrates. Furthermore, the aqueous
workup yielded products of high purity. We found only 86–137 ppm of Ru in the
crude product.
We would like to thank Degussa-Hüls (Rheinfelden, Germany), Clariant (Gendorf, Germany), and
Grace (Worms, Germany) for generous gifts of chemicals. Also we would like to thank Dr. M. Keller, Mrs.
M. Schonhard, and Mr. F. Reinbold for recording NMR spectra, Mr. C. Warth and Dr. J. Wçrth for record-
ing mass spectra, Mr. E. Hickl for performing elemental analyses, and Mrs. S. Hirth-Walter for performing
atomic absorption spectra.
Experimental Part
General. All reagents were obtained from Aldrich, Fluka, or Lancaster and were of the highest purity
available. CH2Cl2 was dried over CaH2. The solvents used for the catalytic reactions and the workup were
reaction-grade solvents. FSGs 4a and 4b were prepared based on silica gel (100–300 mm particle size, 500
Š pore size, 70–90 m2/g specific surface) obtained from Grace as described earlier [7]. Column chroma-
tography (CC): commercially available MN silica gel 60 (0.063–0.2 mm/70–230 mesh) ASTM for CC
from Baker. HPLC: Agilent-1100 system with binary pump, sample changer, column oven, and diode
array detector.
NMR Spectra at 300, 400, and 500 MHz (1H) and at 100.6 and 125.7 MHz (13C); chemical shifts d in
ppm rel. to Me
4Si (=0 ppm) for 1H and rel. to CHCl3 (=77.0 ppm) for 13C, resp., J in Hz. MS: TSQ-700
U
mass spectrometer (EI, CI, ESI); in m/z (rel. %).
[1,3-Bis(2,4,6-trimethylphenyl)imidazolidin-2-ylidene]dichloro{2-(isopropoxy-kO)-5-[tris(3,3,4,4,5,
5,6,6,7,7,8,8,9,9,10,10,10-Heptadecafluorodecyl)silyl]benzylidene-kC}ruthenium (3). Grubbs catalyst 2
(109 mg, 128 mmol) was dissolved under Ar in anh. CH2Cl2 (8 ml). Then, 1 (130 mg, 85.0 mmol) dissolved
in degassed BTF (3 ml) and CuCl (13 mg, 131 mmol) were added, and the mixture was heated for 4 h at
608 (oil bath). After cooling to r.t., the mixture was filtered over silica gel and the filtrate was purified by
CC (cyclohexane ! cyclohexane/CH2Cl2 1:1): 3 (80.0 mg, 48%). Green solid. 1H-NMR (500 MHz,
CDCl3): 1.05–1.09 (m, (C8
A
CH2
CH2)3
U
G
G
N
2.36–2.45 (m, 6 Me); 4.18 (s, NCH2CH2N); 4.91 (sept., J=6.1, Me2
AHCTREUNG
6.99 (d, J=1.5, 1 arom. H); 7.05 (s, 4 arom. H); 7.52 (dd, J=8.1, 1.5, 1 arom. H); 16.53 (s, Ru=
CHAr). 13C-NMR (125.7 MHz, CDCl3): 0.1; 1.6; 20.9; 21.2; 22.2; 25.3; 25.5; 25.6; 27.0; 75.9; 113.9;
116.1; 118.0; 120.8; 124.0; 127.4; 128.4; 129.5; 130.3; 134.7; 139.0; 145.3; 154.4; 210.3; 295.1. EI-MS:
2000 (3), 1999 (6), 1998 (13), 1997 (19), 1996 (31), 1995 (29), 1994 (37, M+), 1993 (29), 1992.4 (19),
1992.0 (16), 1991 (10), 1990 (5), 1920 (2), 1919 (3), 1918 (5), 1917 (6), 1916 (9), 1915 (79), 1914 (6),
1913 (4), 1912 (3), 1883 (7), 1882 (16), 1881 (18), 1880 (33), 1879 (33), 1878 (31), 1877 (31), 1876 (17),
1875 (12), 1874 (11), 1873 (6), 1530 (6), 1518 (18), 1475 (14), 1457 (6), 940 (5), 939 (5), 620 (7), 619
(7), 499 (12), 445 (4), 444 (9), 443 (11), 442 (21), 441 (20), 440 (22), 439 (20), 438 (14), 437 (9), 436 (8),
435 (4), 409 (10), 408 (31), 407 (44), 406 (75), 405 (96), 404 (100), 403 (88), 402 (71), 401 (43), 400
(42), 399 (34), 398 (24), 397 (17), 396 (12), 395 (12), 394 (9), 393 (8), 392 (10), 391 (12), 390 (13), 389
(19), 388 (13), 387 (7), 386 (6), 385 (6), 363 (13), 345 (15), 339 (29), 308 (6), 307.0 (22), 306.9 (17), 305
(37), 304 (50), 303 (63), 301 (26), 299 (6), 297 (17), 296 (7), 295 (43), 289 (12), 287 (9), 281 (7), 278 (9),
275 (7), 245 (7), 244 (12), 243 (9), 242 (38), 239 (7), 231 (9), 217 (6), 216 (20), 215 (25), 214 (82), 213
(9), 204 (7). ESI-MS (pos.): 2000 (100), 1999 (86), 1998 (65), 1997 (46), 1996 (33), 1995 (24), 1994 (25,
M+), 1993 (7), 1992, (5), 1991 (4), 1985 (7), 1984 (6), 1983 (10), 1982 (10), 1981 (11), 1980 (15), 1979

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Helvetica Chimica Acta – Vol. 89 (2006)
(11), 1978 (21), 1977 (13), 1976 (11), 1975 (7), 1964 (4), 1963 (5), 1962 (10), 1961 (14), 1960 (12), 1959 (18),
1958 (14), 1957 (14), 1956 (11), 1955 (10), 1954 (7), 1953 (7), 1928 (3), 1927 (6), 1926 (9), 1925 (14), 1924
(12), 1923 (13), 1922 (10), 1921 (5), 1920 (4), 1919 (2), 1918 (1), 1904 (1), 1903 (4), 1902 (5), 1901 (6), 1900
(8), 1899 (6), 1898 (4), 1897 (3), 1896 (2), 1895 (1), 1894 (2), 765 (5), 764 (14), 681 (4), 680 (10), 663 (6), 549
(7), 548 (24), 475 (6), 439 (11), 438 (6), 437 (19), 436 (12), 435 (11), 434 (11), 422 (4), 407 (5), 405 (6), 307 (5).
GeneralProcedure for the Recycling Experiments. The catalyst 3 loaded on FSG 4b was placed in a
reaction tube, and olefin 5a was added from a 0.05M stock soln. in CH2Cl2 (1 ml, 50 mmol). The mixture
was shaken for 2 h at 608 (oil bath). Then, a sample of the mixture was taken to determine the conversion
by HPLC. After this, Et
obtained by washing the silica gel with MeOH/H2O 4 :1 (5”1 ml). After the washing with MeOH/H2O
4 :1, the silica gel was dried by washing with Et2O and was reused.
A
AHCTREUNG
GeneralProcedure for the Ring-Closing Metathesis. To a 0.05M stock soln. of the a,w-diene in CH2Cl2
(1 ml, 50 mmol) was added the catalyst noncovalently immobilized on FSG 4b. The mixture was shaken
for 2 h at 608 (oil bath). After cooling to r.t., Et
The RCM product was obtained by washing the silica gel with MeOH/H2O 4 :1 (5”1 ml). After the wash-
ing with MeOH/H2O 4 :1, the silica gel was dried by washing with Et2O and was reused.
2,5-Dihydro-1-[(4-methylphenyl)sulfonyl]-1H-pyrrole (6a) [16]. 1H-NMR (CDCl3): 2.42 (s,
MeC6H4); 4.12 (s, 4 H, CH2CH=CHCH2); 5.65 (s, CH2CH=CHCH2); 7.32 (mAA’BB’, Japp. =8.1, 2 arom.
A
AHCTREUNG
AHCTREUNG
H); 7.72 (mAA’BB’, Japp. =8.1, 2 arom. H). 13C-NMR (CDCl3): 21.6; 54.9; 125.5; 127.5; 129.8; 134.4;
143.5. MS: 223 (50, M+), 155 (52), 91 (92), 86 (13), 84 (20), 68 (100), 65 (24), 41 (24).
2,3,4,7-Tetrahydro-1-[(4-methylphenyl)sulfonyl]-1H-azepine (6b) [17]. 1H-NMR (CDCl3): 1.80 (m, 2
H); 2.18 (m, 2 H); 2.41 (s, MeC6H4); 3.39 (t, J=6.1, NCH2CH2); 3.83 (d, J=4.5, NCH2CH=CH); 5.64 (dt,
J=10.6, 5.1, HꢀC(5) or HꢀC(6)); 5.77 (dt, J=10.9, 5.3, HꢀC(6) or HꢀC(5)); 7.28 (mAA’BB’, Japp =8.2, 2
arom. H); 7.68 (mAA’BB’, Japp =8.1, 2 arom. H). 13C-NMR (CDCl3): 21.5; 26.9; 31.0; 46.4; 49.7; 126.7; 127.3;
129.6; 133.0; 136.5; 143.1. MS: 251 (100, M+), 236 (7), 184 (85), 155 (40), 96 (87), 91 (37), 69 (35), 67 (30),
41 (24).
1
Cyclopent-3-ene-1,1-dicarboxylic Acid Diethyl Ester (6c) [18]. H-NMR (CDCl3): 1.25 (t, J=7.1, 2
MeCH2); 3.01 (s, CH2
C
G
[(Cyclohex-2-en-1-yloxy)methyl]benzene (6d) [19]. 1H-NMR (CDCl3): 1.51–1.59 (m, 1 H);
1.71–1.89 (m, 3 H); 1.91–1.99 (m, 1 H); 2.02–2.10 (m, 1 H); 3.93–3.98 (m, HꢀC(1)); 4.55 (dAA’
,
J=12.0, 1 H, CH2O); 4.61 (dAA’, J=12.0, 1 H, CH2O); 5.79–5.89 (m, HꢀC(2), HꢀC(3)); 7.24–7.37
(m, 5 arom. H). 13C-NMR (CDCl3): 19.3; 25.3; 28.4; 70.0; 72.2; 127.4; 127.6; 127.8; 128.3; 130.9; 139.1.
MS: 188 (5, M+), 130 (9), 97 (48), 91 (100), 84 (8), 81 (13), 79 (13), 77 (7), 69 (22), 65 (11), 55 (10), 41 (15).
[(Cyclopent-2-en-1-yloxy)methyl]benzene (6e) [20]. 1H-NMR (CDCl3): 1.82–1.89 (m, 1 H, CH2(4));
2.12–2.19 (m, 1 H, CH2(4)); 2.23–2.30 (m, 1 H, CH2(5)); 2.47–2.55 (m, 1 H, CH2(5)); 4.51 (d, JAB =11.7,
1 H, CH2O); 4.55 (d, JAB =11.7, 1 H, CH2O); 4.67 (m, HꢀC(1)); 5.88–5.91 (m, HꢀC(3)); 6.01–6.04 (m,
HꢀC(2)); 7.24–7.37 (m, Ph). 13C-NMR (CDCl3): 29.8; 31.1; 70.6; 84.5; 127.4; 127.8; 128.3; 130.8; 135.7;
138.9. MS: 192 (12, [M+NH4]+), 175 (4, [M+H]+), 157 (10), 126 (21), 108 (17), 91 (36), 84 (100), 67 (7).
1
2-Phenyl-3,6-dihydro-2H-pyran (6f) [21]. H-NMR (CDCl3): 2.20–2.44 (m, 2 H); 4.33–4.39 (m, 2
H); 4.56 (dd, J=10.0, 3.8, HꢀC(2)); 5.77–5.85 (m, 1 H); 5.88–5.96 (m, 1 H); 7.25–7.41 (m, Ph). 13C-
NMR (CDCl3): 32.9; 66.6; 75.7; 124.5; 125.9; 126.4; 127.5; 128.4; 142.6. MS: 160 (16, M+), 105 (100),
77 (18), 54 (75).
REFERENCES
[1] C. C. Tzschucke, C. Markert, W. Bannwarth, S. Roller, A. Hebel, R. Haag, Angew. Chem. 2002, 114,
4136; Angew. Chem., Int. Ed. 2002, 41, 3964; J. Yoshida, K. Itami, Chem. Rev. 2002, 102, 3693; D. J.
Cole-Hamilton, Science (Washington, D.C.) 2003, 299, 1702.
[2] I. T. Horvµth, J. Rµbai, Science (Washington, D.C.) 1994, 266, 72; D. P. Curran, Angew. Chem. 1998,
110, 1230; Angew. Chem., Int. Ed. 1998, 37, 1174; I. T. Horvµth, Acc. Chem. Res. 1998, 31, 641; J. A.
Gladysz, D. P. Curran, Tetrahedron 2002, 58, 3823.

Helvetica Chimica Acta – Vol. 89 (2006)
1037
[3] R. L. Scott, J. Am. Chem. Soc. 1948, 70, 4090; J. H. Hildebrand, D. R. F. Cochran, J. Am. Chem. Soc.
1949, 71, 22; R. L. Scott, J. Phys. Chem. 1958, 62, 136.
[4] A. R. Ravishankara, S. Solomon, A. A. Turnipseed, R. F. Warren, Science (Washington, D.C.) 1993,
259, 194; D. J. Wuebbles, J. M. Calm, Science (Washington, D.C.) 1997, 278, 1090.
[5] D. P. Curran, Z. Luo, J. Am. Chem. Soc. 1999, 121, 9069; Q. Zhang, Z. Luo, D. P. Curran, J. Org.
Chem. 2000, 65, 8866; D. P. Curran, Synlett 2001, 1488.
[6] M. Matsugi, D. P. Curran, J. Org. Chem. 2005, 70, 1636.
[7] C. C. Tzschucke, C. Markert, H. Glatz, W. Bannwarth, Angew. Chem. 2002, 114, 4678; Angew.
Chem., Int. Ed. 2002, 41, 4500; C. C. Tzschucke, W. Bannwarth, Helv. Chim. Acta 2004, 87, 2882.
[8] H. Glatz, C. Blay, H. Engelhardt, W. Bannwarth, Chromatographia 2004, 59, 567.
[9] V. Andrushko, D. Schwinn, C. C. Tzschucke, F. Michalek, J. Horn, C. Mçssner, W. Bannwarth, Helv.
Chim. Acta 2005, 88, 936.
[10] J. S. Kingsbury, J. P. A. Harrity, P. J. Bonitatebus Jr., A. H. Hoveyda, J. Am. Chem. Soc. 1999, 121,
791; S. B. Garber, J. S. Kingsbury, B. L. Gray, A. H. Hoveyda, J. Am. Chem. Soc. 2000, 122, 8168.
[11] D. Schwinn, W. Bannwarth, Helv. Chim. Acta 2002, 85, 255.
[12] T. M. Trnka, J. P. Morgan, M. S. Sanford, T. E. Wilhelm, M. Scholl, T.-L. Choi, S. Ding, M. W. Day,
R. H. Grubbs, J. Am. Chem. Soc. 2003, 125, 2546; M. B. Dinger, J. C. Mol, Organometallics 2003, 22,
1089; M. B. Dinger, J. C. Mol, Eur. J. Inorg. Chem. 2003, 2827.
[13] F. KoÅ, F. Michalek, L. Rumi, W. Bannwarth, R. Haag, Synthesis 2005, 3362.
[14] M. Mayr, B. Mayr, M. R. Buchmeiser, Angew. Chem. 2001, 113, 3957; Angew. Chem., Int. Ed. 2001,
40, 3839; D. Fischer, S. Blechert, Adv. Synth. Catal. 2005, 347, 1329; J. C. Conrad, H. H. Parnas, J. L.
Snelgrove, D. E. Fogg, J. Am. Chem. Soc. 2005, 127, 11882.
[15] F. Michalek, D. Mädge, J. Rühe, W. Bannwarth, Eur. J. Org. Chem. 2006, 577.
[16] A. Fürstner, A. F. Hill, M. Liebl, J. D. E. T. Wilton-Ely, Chem. Commun. 1999, 601.
[17] M. S. Visser, N. M. Heron, M. T. Didiuk, J. F. Sagal, A. H. Hoveyda, J. Am. Chem. Soc. 1996, 118,
4291.
[18] D. F. Taber, K. J. Frankowski, J. Org. Chem. 2003, 68, 6047.
[19] S. E. Denmark, J. P. Edwards, J. Org. Chem. 1991, 56, 6974.
[20] J. J. Tufariello, A. C. Bayer, J. J. Spadaro Jr., J. Am. Chem. Soc. 1979, 101, 3309.
[21] B. Schmidt, J. Chem. Soc., Perkin Trans. 1 1999, 2627.
Received January 24, 2006