On the Enantioselective Rhodium-Catalyzed Enyne Cyclization

Article abstract of DOI:10.1002/adsc.200303086

Several chiral ligands were tested in the enantioselective rhodium-catalyzed enyne cyclization, but none gave results comparable to the BINAP ligand. Among the tested catalyst precursors the soon to be commercially available [(COD)Rh(H₂O)₂

Full text of DOI:10.1002/adsc.200303086

FULL PAPERS
On the Enantioselective Rhodium-Catalyzed Enyne Cyclization
a,
a
b
A. Stephen K. Hashmi, * Patrick Haufe, Andreas Rivas Nass
a
Institut f¸r Organische Chemie, Universit‰t Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany
Fax: ( ‡ 49)-711-685 4321, e-mail: hashmi@hashmi.de
b
Umicore AG & Co. KG, Precious Metals Chemistry, Rodenbacher Chaussee 4, 63403 Hanau-Wolfgang, Germany
(
formerly OMG AG & Co. KG)
Received: May 1, 2003; Accepted: July 22, 2003
Abstract: Several chiral ligands were tested in the was proved to have an (E)-configuration. A new
enantioselective rhodium-catalyzed enyne cyclization, product, the 1,3-diene 16 was observed at higher
but none gave results comparable to the BINAP temperatures. In the presence of an (E)-configurated
ligand. Among the tested catalyst precursors the soon double bond in the starting material, the reaction
to be commercially available [(COD)Rh(H O) ]CF SO
completely failed.
2
2
3
3
complex is an interesting alternative to the [(COD)-
RhCl] because it did not need the activation with
2
silver ions. A new stereogenic and 1,2-disubstituted Keywords: alkenes; alkynes; asymmetric catalysis;
double bond was formed in the product 12, the latter cyclization; homogeneous catalysis; rhodium
Introduction
latest publication the work focused on BINAP 6, which
delivered the best and most reliable results.[ One
8]
The transition metal-catalyzed enyne cyclization, the general drawback was the relatively high amount of
[
1]
intramolecular version of the Alder-ene-reaction, has catalyst needed (in most cases 10 mol % of rhodium).
become an important tool in organic synthesis. The Furthermore, only (Z)-olefins were used as substrates,
catalyst allows this transformation under mild condi- certainly a significant synthetic limitation. On the other
tions at moderate temperatures with substrates that, for hand, in related reactions that proceed through similar
[
9]
electronic reasons, would need very high reaction intermediates, (E)-olefins reacted readily. In preced-
temperatures in a concerted, uncatalyzed reaction. ing publications it was also mentioned that the reaction
Initially, this was developed with palladium catalysts can in principle be conducted with only 0.4 mol% of
by Trost et al.[ In the meantime, many other groups catalyst and that only (Z)-olefins react.
2]
[8]
[10]
have contributed and several other metals have proven
[
3]
to be catalytically active for this transformation. In
2
these reactions from a planar sp -carbon in the starting
material 1, a tetrahedral stereogenic center is generated
in the product 2. There have been several efforts for a
catalytic enantioselective version of this reaction in the
past years.[ While these used palladium catalysts, so We now wanted to test whether other ligands and
far the most convincing synthetic results come from catalyst precursors allow a significant reduction of the
Zhang et al.×s recent and most beautiful work utilizing amount of catalyst and still give a good ee, and briefly re-
4,5]
[
6,7,8]
rhodium catalysts.
address the question of the use of (E)-olefins.
Results and Discussion
First we wanted to gather some experience with the
rhodium catalysts, thus we prepared the (Z)-enyne 11
from phenylacetylene 7 and commercially available (Z)-
They could achieve excellent enantioselectivities pent-2-en-1-ol 10. Subjecting 11 to 10 mol % of
(
sometimes above 99% ee) and good to excellent [(COD)RhCl] , rac-BINAP and AgSbF readily deliv-
2
6
[6,7,8]
yields.
Typical ligands they used initially were ered the cyclization product rac-12 in a satisfying 85%
Tunaphos 3, BICPO 4 and (R,R)-Me-DuPhos 5. In the yield (Table 1, entry 1). Then we switched to enantio-
Adv. Synth. Catal. 2003, 345, 1237 ± 1241
DOI: 10.1002/adsc.200303086
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1237

FULL PAPERS
A. Stephan K. Hashmi et al.
Table 1. Different precursors and ligands for the conversion of 11 to 12.
Entry
Catalyst precursor
Ligand
Ag salt
Temp.
ee (by GC)
Isolated yield
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
[(COD)RhCl]2
[(COD)RhCl]2
[(COD)RhCl]2
[(COD)RhCl]2
[(COD)RhCl]2
[(COD)RhCl]2
[(COD)RhCl]2
[((R,R)-Depyphos)Rh(COD)]BF4
rac-BINAP
(R)-BINAP
dppe
(R,R)-MeDuphos
(R,R)-Depyphos
TaniaPhos
MandyPhos
±
(R)-BINAP
(R)-BINAP
(R)-BINAP
(R)-BINAP
(R)-BINAP
(R,R)-Depyphos
TaniaPhos
MandyPhos
-
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
±
±
±
±
±
AgBF4
AgBF4
AgBF4
AgBF4
±
rt
rt
rt
rt
rt
rt
rt
rt
±
85%
69%
> 99%
[
[
[
[
[
a]
a]
a]
a]
a]
±
±
±
±
±
±
±
±
±
±
75%
> 99%
76%
93%
[(C
2R hO] BD F)4
508C
508C78.5%
808C
rt
rt
rt
rt
rt
rt
1
1
1
1
1
1
1
1
1
1
2
[(NBD) Rh]BF
4%
2
4
[(COD)Rh(acac)]
±
[a]±
82%
90%
[(COD)Rh(H O) ]CF SO
> 99%
> 99%
2
2
3
3
[(COD)RhCl]2
[
[
[
[
a]
a]
a]
a]
[(COD)RhCl]2
[(COD)RhCl]2
[(COD)RhCl]2
±
±
±
±
±
±
±
±
[(COD)Rh(H O) ]CF SO
2
2
3
3
3
[
b]
[(COD)Rh(H O) ]CF SO
(R)-BINAP
±
±
±
808C
rt
rt
±
±
2
2
3
± [
a]
a]
[((R,R)-Depyphos)Rh(COD)]BF4
[(COD)RhCl]2
AgSbF6
AgSbF6
±
±
[
±
All reactions were conducted with 10 mol % of catalyst in DCE.
[
[
a]
b]
Only starting material could be isolated.
Formation of the 1,3-diene 16 in 16% yield.
merically pure (R)-BINAP (entry 2) that delivered an (entry 8). With (R)-BINAP and [(COD) Rh]BF , a
2
4
ee of more than 99% (with the same configuration as 93% yield and 99% ee were observed (entry 9), while
Zhang et al.)[ as determined by chiral GC(direct
11]
[(NBD) Rh]BF (entry 10) provided a low 4% yield
2 4
comparison with rac-12). In both products the config- with a 78.5% ee, and [(COD)Rh(acac)] gave no
uration of the newly formed stereogenic double bond is conversion at all (entry 11). Only the new [(COD)Rh-
[
13,14]
(
E) as clearly deduced from the 15.2 Hz coupling (H O) ]CF SO
delivered an 82% yield with more
2
2
3
3
constant of the two vicinal hydrogen atoms at this 1,2- than 99% ee (entry 12).
[
12]
disubstituted double bond.
Then going back to the [(COD)RhCl] dimer as a
2
catalyst precursor and an activation with the easier to
handle silver tetrafluoroborate instead of the extremely
hygroscopic hexafluoroantimonate did not change any-
thing: with (R)-BINAP again an ee higher than 99% and
a 90% yield were obtained while with (R,R)-Depyphos,
TaniaPhos and MandyPhos the reaction failed once
more (entries 13 ± 16).
With the ligands dppe, (R,R)-Me-Duphos 5, (R,R)-
Depyphos 13, TaniaPhos 14 and MandyPhos 15, no
reaction was observed under the same conditions
(
entries 3 ± 7).
Control experiments confirmed that [(COD)Rh-
Then we switched to cationic catalyst precursors for (H O) ]CF SO without a diphosphane ligand failed to
2
2
3
3
the (R)-BINAP complexes which do not need the convert 11 (entry 17), with (R)-BINAP at 808Ca new
activation by silver(I). [((R,R)-Depyphos)Rh- side-product, the quite unusual 1,3-diene 16 was ob-
COD)]BF delivered a 76% yield and a 75% ee tained (entry 18). The structure is easily deduced from
(
4
1238
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Enantioselective Rhodium-Catalyzed Enyne Cyclization
FULL PAPERS
the NMR data, nicely showing the 1-propenyl moiety Conclusion
with the methyl proton resonance at 1.84 ppm and a
vicinal coupling of 6.6 Hz, one vinyl proton resonance at None of the tested ligands gave results comparable to
5
6
.52 ppm possessing as one coupling constant the same the BINAP ligand, but the soon to be commercially
[
16]
.6 Hz and the last vinyl proton resonance at 6.57 ppm. available [(COD)Rh(H O) ]CF SO complex, which
2
2
3
3
The (E)-configuration was deduced from a coupling does not need the activation with silver ions, is an
constant of 15.7 Hz between the two vicinal hydrogen interesting alternative to the [(COD)RhCl] as a catalyst
2
[
15]
atoms at that double bond. The carbon NMR shows precursor. In the product 12 apart from the new
no more alkyne resonances, there are now two methyl- stereogenic center also a stereogenic 1,2-disubstituted
ene group resonances at 76.6 and 78.6 ppm for the double bond is formed, the latter was proved to have an
carbon atoms next to the oxygen atoms and the double (E)-configuration. At higher temperatures the 1,3-diene
bond and, most significant, one methylene group at 16 was observed as a new product. Efforts to conduct the
3
1.7 ppm for the benzylic-allylic methylene group. reaction with an (E)-configurated double bond in the
Suitable methylene resonances are also found in the starting material were unsuccessful.
proton NMR (4.52 ppm/4.59 ppm for the methylene
groups between oxygen and the double bond in the
heterocycle and 3.54 ppm for the benzylic-allylic meth-
ylene group).
Experimental Section
3-Phenyl-2-propyn-1-ol (8)
The substance was prepared by a general procedure for related
substances given in ref.[ Under a nitrogen atmosphere
17]
1
0.0 mL (9.30 g, 91.1 mmol) phenylacetylene (7) in 80.0 mL
of absolute THF were cooled to À 788C. Then 56.9 mL n-BuLi
91.1 mmol, 1.6 M in hexanes) were added dropwise. The
solution was allowed to warm to 08Cand then 3.60 g
120 mmol) paraformaldehyde are added. After the solution
had warmed to room temperature it finally was heated to 458C
(
All efforts for a significant reduction of the amount of
catalyst (at least one order of magnitude) did not deliver
any product even after longer reaction times. We did not
(
try heating, as mentioned above an increase of temper- for90minutes. After coolingto room temperature, thereaction
mixture was poured into a solution of 25 g ammonium chloride
ature should lead to a different and achiral product.
Compared to other precursors for the catalyst, at in 250 mL of water. After separation of the phases the aqueous
phase was extracted with ether five times, dried over MgSO4,
room temperature [(COD)Rh(H O) ]CF SO shows
only about 1/8 of the activity of the [(COD)RhCl]2/
2
2
3
3
filtered and the solvent removed under vacuum. A subsequent
column chromatography furnished 8 as a yellow oil; yield:
AgSbF system but is clearly the most active catalyst
6
1
0.7 g (81.0 mmol, 90%). The spectroscopic data were in
among the precursors that do not need an activation by a
silver salt (entries 8 ± 12). It is interesting that the
rhodium-Depyphos complex is catalytically active (al-
beit with low enantioselectivity, entry 8), while efforts
for the in situ generation with either AgSbF or AgBF
[17]
accordance with the literature.
R (PE:EE, 2:1) ˆ 0.30;
f
1
H NMR (300 MHz, CDCl ): d ˆ 1.69 (t, J ˆ 6.2 Hz, 1H), 4.50
3
(
d, J ˆ 6.1 Hz, 2H), 7.30 ± 7.34 (m, 3H), 7.42 ± 7.46 (m, 2H).
6
4
were unsuccessful (entries 5 and 14). As the chloro-(1,5-
cyclooctadiene)-rhodium(I) itself is inactive (entry 20),
maybe the in situ formation of the diphosphane complex
is hindered. Futhermore, the Depyphos ligand might co-
ordinate more strongly to the silver ion und and thus not
co-ordinate to rhodium. Evidence for this was obtained
3
-Phenyl-2-propynyl Bromide (9)
The substance was prepared by a procedure from ref.[19] 4.84 g
(
36.7 mmol) 8 in 40 mL of absolute ether were cooled to 08C.
Then 0.5 mL pyridine and 1.40 mL (3.98 g, 14.7 mmol) PBr3
were added and stirring was continued for 4 hours at 08C. Then
the mixture was poured into NaHCO solution, extracted with
ether three times and dried over Na SO . After filtration and
removal of the solvent under vacuum 9 was obtained as a
yellow oil; yield: 4.14 g (21.2 mmol, 58%). R (PE:Et O,
3
when silver ions (AgBF ) were added to the rhodium-
4
2
4
Depyphos complex; this led to an inhibition of the
reaction (entry 19).
f
2
1
Trying to use a 3:1 mixture of the (E)- and the (Z)- 50:1) ˆ 0.43; H NMR (300 MHz, CDCl ): d ˆ 4.17 (s, 2H),
3
7
.28 ± 7.46 (m, 5H).
isomer of the known 17 was completely unsuccessful,
not even the (Z)-portion of the mixture reacted under
several conditions in the presence of the (R)-BINAP
ligand.
(
2Z)-(3-Pent-2-enyloxy-prop-1-ynyl)benzene (11)
The substance was prepared by a combination of two
procedures.[
10) in THF were cooled to 08C. Then 19.4 mg (81.0 mmol)
NaH were added. When no more gas was evolved, 158 mg
20,21]
82.0 mL (699 mg, 812 mmol) cis-2-penten-1-ol
(
Adv. Synth. Catal. 2003, 345, 1237 ± 1241
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¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPERS
A. Stephan K. Hashmi et al.
‡
(
810 mmol) 3-phenyl-2-propyn-1-yl bromide (9) in THF were
(%) ˆ 200 (7) [M ], 105 (94), 77 (72); anal. calcd. for C H O
14
16
added. Stirring was continued at 08Cfor one hour, then the
solution was allowed to warm to room temperature, washed
with water and the aqueous phase extracted with ether three
times. The combined organic phases were dried over MgSO4,
filtered and the solvent removed under vacuum. Column
chromatography on silica gel afforded 11 as a pale yellow oil;
yield: 76.3 mg (381 mmol, 47%). R (PE:Et O, 50:1) ˆ 0.20; IR
(200.28): C83.96, H 8.05; found: C81.38, H 7.89.
3-Benzyl-4-propenyl-2,5-dihydrofuran (16)
4
0.0 mg (200 mmol) of the enyne 11 in 2.0 mL DCE were
degassed in a Schlenk tube. Then 7.9 mg (199 mmol, 10 mol %)
[(COD)Rh(H O) ]CF SO and 12.5 mg (201 mmol) (R)-BI-
f
2
À1
1
(
(
neat): nÄ ˆ 2964 cm, 2933, 2236, 1072, 692 cm ; H NMR
2
2
3
3
NAP were added and the mixture was heated to 808Cfor 16
hours. Column chromatography (PE:DCM; 1:1) furnished 16
as a yellow oil; yield: 6.5 mg (32 mmol, 16%). R (PE:DCM;
CDCl , 500 MHz): d ˆ 1.00 (t, J ˆ 7.6 Hz, 3H), 2.14 (ddq,
3
J ˆ 7.5 Hz, 7.5 Hz, 1.5 Hz, 2H), 4.20 (d, J ˆ 6.75 Hz, 2H), 4.37
(
s, 2H), 5.53 (dtt, J ˆ 11.0, 6.75, 1.5 Hz, 1H), 5.66 (dtt, J ˆ
1.0 Hz, 7.37 Hz, 1.4 Hz, 1H), 7.29 ± 7.36 (m, 3H), 7.43 ± 7.46
f
1
:1) ˆ 0.14; IR (film): nÄ ˆ 3026, 2912, 2840, 1455, 1168, 1064,
1
À1
1
13
757, 700, 533 cm ; H NMR (300 MHz, CDCl ): d ˆ 1.84 (d,
(
(
(
m, 2H); C NMR (CDCl , 75 MHz): d ˆ 14.3 (q), 21.0 (t), 58.0
3
3
J ˆ 6.6 Hz, 3H), 3.54 (s, 2H), 4.52 ± 4.54 (m, 2H), 4.79 ± 4.81 (m,
t), 65.4 (t), 85.4 (s), 86.2 (s), 122.8 (s), 123.2 (d), 124. 9 (d), 128.7
d, 2C), 132.1 (d, 2C), 136.9 (d); MS [EI (‡), 70 eV]: m/z (%) ˆ
2
H), 5.52 (ddt, J ˆ 15.7 Hz, 6.6 Hz, 1.0 Hz, 1H), 6.57 (d, J ˆ
1
3
‡
15.7 Hz, 1H), 7.14 ± 7.31 (m, 5H); C NMR (125 MHz, CDCl ):
2
00 (0.13) [M ], 129 (35), 115 (100); anal. calcd. for C H O
3
1
4
16
d ˆ 19.2 (q), 31.7 (t), 76.6 (t), 78.6 (t), 122.2 (d), 126.3 (d), 128.3
(
200.28): C83.83, H 7.58; found: C82.10, H 7.90.
(
d), 128.7 (d, 2C), 128.9 (d, 2C), 131.4 (s), 132.4 (s), 139.0 (s);
‡
MS [EI (‡), 70 eV]: m/z (%) ˆ 200 (23) [M ], 109 (89), 84 (97);
HRMS [EI (‡), 70 eV]: calcd. for C H O: 200.1201; found:
1
4
16
General Procedure for Rhodium-Catalyzed Reactions 200.1201.
The enyne was placed in a Schlenk tube, 2.0 mL solvent (DCE,
DCM or C D ) were added and the solution was degassed by
6
6
freeze-thaw cycles. Depending on the precursor, now two
different procedures were used:
Acknowledgements
Procedure A: The rhodium cation was prepared in situ by the
addition of a silver salt at room temperature.
Procedure B: A diphosphane ligand was either already
coordinated to rhodium or becomes attached by ligand
exchange at 508Cor 80 8C.
A. S. K. H. thanks the Fonds der Chemischen Industrie and the
Karl-Ziegler Stiftung for their support of their work.
Gas chromatography (GC): the determination of the References and Notes
enantiomeric excess (ee) was done with chiral GCby
comparison with the racemic sample in the GCfacility of the
Institut f¸r Organische Chemie at the Universit‰t Stuttgart on
an instrument from Firma Mega/Split with a chiral column
[1] K. Alder, F. Pascher, A. Schmitz, Ber. Dtsch. Chem. Ges.
1943, 76, 27 ± 53.
[2] B. M. Trost, Acc. Chem. Res. 1990, 23, 34 ± 42; B. M.
Trost, M. J. Krische, Synlett 1998, 1 ± 16.
Bondex u n b; carrier gas H , 0.4 bar; temperature program:
2
1
008C/70 min isotherm/18Cper minute up to 120 8C/ then up
to 2008C.
Retention times for the racemic sample of 12: enantiomer
:86.03 minutes; enantiomer 2:87.35 minutes.
[
[
[
3] C. Aubert, O. Buisine, M. Malacria, Chem. Rev. 2002,
02, 813 ± 834.
4] M. Hatano, M. Terada, K. Mikami, Angew. Chem. 2001,
13, 255 ± 259; Angew. Chem. Int. Ed. 2001, 40, 249 ± 253.
5] A. Goeke, M. Sawamura, R. Kuwano, Y. Ito, Angew.
Chem. 1996, 108, 686 ± 687; Angew. Chem. Int. Ed. Engl.
1
1
1
1
996, 35, 662 ± 663.
3
-Benzylidene-4-propenyltetrahydrofuran (12)
[
6] P. Cao, X. Zhang, Angew. Chem. 2000, 112, 4270 ± 4272;
Angew. Chem. Int. Ed. Engl. 2000, 39, 4106 ± 4108.
7] A. Lei, J. P. Waldkirch, M. He, X. Zhang, Angew.
Chemie. 2002, 114, 4708 ± 4711; Angew. Chem. Int. Ed.
Engl. 2002, 41, 4526 ± 4529.
8] A. Lei, M. He, S. Wu, X. Zhang, Angew. Chem. 2002,
1
3
4
0.0 mg (200 mmol) of the 11 and 2.0 mL dichloroethane were
degassed in Schlenk tube. Then 5.0 mg (10 mmol)
(COD)RhCl]2 and 12.5 mg (201 mmol) (R)-BINAP were
a
[
[
added. After stirring for one minute AgSbF was added. The
reaction was monitored by TLC. Column chromatography
6
[
(
PE:EE:DCM, 30:1:1) furnished 12 as a yellow oil; yield:
14, 3607 ± 3610; Angew. Chem. Int. Ed. Engl. 2002, 41,
457 ± 3460.
2
3
6
6
7.5 mg (140 mmol, 69%,>99% ee). Rf (PE:EE:DCM,
0:1:1) ˆ 0.23; IR (neat): nÄ ˆ 2938, 1473, 1375, 1061, 893,
À1
1
[9] B. Wang, P. Cao, X. Zhang, Tetrahedron Lett. 2000, 41,
041 ± 8044.
10] P. Cao, B. Wang, X. Zhang, J. Am. Chem. Soc. 2000, 122,
490 ± 6491.
32 cm ; H NMR (500 MHz, CDCl ): d ˆ 1.78 (dd, J ˆ
3
8
.5 Hz, 1.6 Hz, 3H), 3.43 ± 3.51 (m, 2H), 4.08 (dd with the
[
[
same coupling constant, thus pseudo-t, J ˆ 7.1 Hz, 7.1 Hz, 1H),
.61 (dt, J ˆ 14.4 Hz, 2.2 Hz, 1H), 4.75 (d, J ˆ 14.4 Hz, 1H), 5.35
ddq, J ˆ 15.2 Hz, 8.2 Hz, 1.6 Hz, 1H), 5.67 (dq, J ˆ 15.2 Hz,
6
4
(
11] The absolute configuration is still unknown; in analogy
to Zhang et al.[ (R)-BINAP delivered the ( ± )-product.
8]
6
(
.5 Hz, 1H), 6.25 ± 6.26 (m, 1H), 7.15 ± 7.35 (m, 3H), 7.35 ± 7.38
m, 2H); 13C NMR (125 MHz, CDCl ): d ˆ 18.0 (q), 49.7 (d),
[12] Zhang et al. reported that both double bonds of the
product possess the (E)-configuration.[ They based
their assignment on unpublished NOE data. In principle
3
7]
7
1
0.3 (t), 72.6 (t), 121.6 (d), 126.6 (d), 127.9 (d, 2C), 128.5 (d, 2C),
28.8 (d), 129.2 (d), 137.3 (s), 144.5 (s); MS [EI (‡), 70 eV]: m/z
1240
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
Adv. Synth. Catal. 2003, 345, 1237 ± 1241

Enantioselective Rhodium-Catalyzed Enyne Cyclization
FULL PAPERS
this is indeed needed for the exocyclic double bond, but
might be ambiguous for the other double bond since the
hydrogen atoms involved also show a scalar coupling.
Here the vicinal coupling constant between the two
hydrogen atoms on the double bond is probably the most
direct way to retrieve this information.
[16] Contact address: Umicore, AG & CoKG Precious Met-
als Chemistry, P. O. Box 1351, 63457 Hanau-Wolfgang,
Germany.
[17] L. Brandsma, Preparative Acetylenic Chemistry, 2nd.
edn., Elsevier Science, Amsterdam, 1988, pp. 24 ± 25 and
81 ± 82.
[
[
13] A. Rivas Nass, (to Umicore), DE 103 39 790.6.
14] J. W. Bats, A. Rivas Nass, A. S. K. Hashmi, Acta Cryst.
Ser E 2003, submitted.
[18] G. T. Crisp, P. D. Turner, K. A. Stephens, J. Organomet.
Chem. 1998, 570, 219 ± 224.
[19] T. Ooi, Y. Hokke, E. Tayama, K. Maruoka, Tetrahedron
2001, 57, 135 ± 144.
[20] Y. Liu, Y. Inoue, T. Hakushi, Bull. Chem. Soc. Jpn. 1990,
63, 3044 ± 3046.
[21] B. P. Czech, D. A. Babb, B. Son, R. A. Bartsch, J. Org.
Chem. 1984, 49, 4805 ± 4810.
[
15] In the rhodium-catalyzed reactions similar stereogenic
double bonds have been reported to form with (E)-
configuration[ , but the data provided there only
mentioned multiplets for the vinylic protons, no exact
proof was published.
7,9]
Adv. Synth. Catal. 2003, 345, 1237 ± 1241
asc.wiley-vch.de
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1241