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Chiral Heterocylic Carbenes in Asymmetric Homogeneous Catalysis.

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Chiral Heterocylic Carbenes in Asymmetric
Homogeneous Catalysis**
on tantalum or zirconium were used as molecular precursors. In
these cases, however, alkyl groups are detached during the grafting process as a consequence of prot~Iysis.'~]
In our case, all
alkyl groups first remain at the metal since the acidic surface
silanols attack the nitridomolybdenum moiety, whereas only in
the second step an I-elimination occurs, yielding the corresponding alkylidene species. Further work in this area is in
progress.
Wolfgang A. Herrmann,* Lukas J. Goossen,
Christian Kocher, and Georg R. J. Artus
According to preliminary investigations, N-heterocyclic carbenes of the imidazole, pyrazole, and triazole type act as controlling ligands in organometallic homogeneous catalysis.['**]
The key advantage of these ligands appears to be that they do
not dissociate from the metal centers, particularly in the case of
electron-rich catalytically active metals like rhodium o r palladium."] In contrast to the well-established phosphanes, an excess of ligand is not needed-an important criterion for future
industrial applications. In this publication it is shown that chiral
N-heterocyclic carbenes can be prepared conveniently; moreover, they form organometallic complexes for asymmetric homogeneous catalysis.
For the chiral imidazolium salts l a , b a ring closure synthesis
starting from inexpensive materials was needed,[31which can be
applied broadly and which proceeds without racemization. The
C,-symmetric derivatives la,b with chiral centers directly connected to the ligand core were obtained in one step and without
racemization from chiral amines, glyoxal, and formaldehyde
according to Equation (1). The identity of l a was confirmed by a
Received: May 27. 1996 [Z 89781El
German version: Angew Chem. 1996. 108. 2978-2980
Keywords: catalysis - complexes with nitrogen ligands . molybdenum compounds - polymerizations * ring-opening metatheses
polymerizations
[ l ] App1ii.d Hon7o~eiieou.s Curul~si.~
by Urgunometullic Complu.res (Eds.: B.
Cornils. W. A. Herrmann), VCH, Weinheim, 1996.
[2] a) K . J. Ivin, U/e/in Metuthesis, Academic Press. London 1983. b) Recent review: J MoI. in ref. [I]. p. 318.
[3] a) K . Weiss, G . Lossel. AnRew. Chem. 1989, 101, 7 5 ; Angeit.. Chem. I n / . Ed.
EngI. 1989.28.62 b j V. Dufaud, G. P. Niccolai. J. Thivolle-Cazat, J.-M. Basset,
J. Ani. Chrm. S o c . 1995, 117. 4289.
[4] W A. Herrmann. S Bogdanovic. R. Poli, T. Priermeier. J. A m Cliem. Soc.
1994. 116,4989.
[5] S. Bogdanovic, Ph.D. Thesis. Technische Universitit Miinchen, 1994.
[6] a ) Za: 1 (1 .0mmol) and triphenylsilanol(1 mmol) were placed in a Schlenk tube
under argon and dissolved in T H F (30 mL). The reaction mixture was stirred
at 60 C for 48 h. Removal of the solvent under reduced pressure
and extraction of the residue with n-hexane ( 5 mL) afforded Z a as
a white powder Recrystallization from n-hexane at O'C gave
240 mg
Z a as white crystals in 4 0 % yield. Analysis: ialcd
H
,, C
, H3
(found): C61.68 (61.30). H8.36 (8 40). N2.72 (2.74). SpectroR/'c'
CIscopic data: FT- IR (Nujol) i= 3371 c m - ' : v(N-H). 'H NMR
H\
H
,
NH2
H\
,H
(400 MHz. C,,D,. 25"C): 6 =7.93 (m, 6H). 7.22 (m. 9 H ) . 2.24
+
c=c
c=c
(Mo-CH,/Bu. s. 6 H ) . 0.99 (Mo-CH,C(CH,),, s. 27H) I3C
R
R
R.
h!
Yc ,R
H\
,H
HCI
NMR (100 MHz. C,D,, 25 C): S =138.81, 135.95. 129.50,
c-c
'C'
-3H20
P
\
f/
\\
128.YX. 84.94 (MoCH,tBu). 34.84 (Mo-CH,C(CH,),). 32 59
H3C H
0
0
H "
(MoCH,C(CH,j,).
"Si
NMR (80 MHz. C,D,.
25-C):
6 = 19.5. b) Zb: 1 (1.0 mmol) and 1,1.3.3-tetraphenyldisilox(R,R) - 2a,b
(R,R) - l a , b
0
ane- I.3-diol (0 5 mmol) were placed in a Schlenk tube under arII
a
R
=
c
&
;
b
R=c,,H,
gon and dissolved in T H F (30 mL). The reaction mixture was
-c, H
stirred at 60 C for 4h. Removal of the solvent under reduced
H'
pressure and extraction of the residue with n-hexane (10 mLj afforded Zb as off-white crystals. Recrystallization from n-hexane
at 0 C gave 409 mg of Z b as pale yellow crystals in 77% yield.
Analysis: calcd (found). C61.11 (61.30). H8.36 (8.40). N2.64 (2.74). Spectrocrystal s t r ~ c t u r e . ~
Deprotonation
~]
of the chiral imidazolium
scopic data: FT IR (Nujol) ? = 3371 c m - ' : i,(N-H). ' H N M R (400MHz,
salts was accomplished quantitatively by a novel procedure[51
C,D,,. 75 C). 6 = 8.11 (m. 8 H j . 7 27 (m. 12H). 2.16 (Mo-CH,rBu, s, 12H),
with sodium hydride in liquid ammonia at - 33 oC.[61No side
0.97 (Mo-CH,C(CH,j,. s. 54H). "C N M R (100MHz. C,D,, 25 T ) :
reactions were observed.
d =139.44. 135.40, 129.37. 128 99. 128.01, 85.56 (Mo-CH,rBu). 34.95 (MoCH,C'(CH,),). 32.58 (Mo-CH,C(CH,),). 29SiN M R (80 MHz. C,D,. 25 C):
If the ring closure is performed with racemic amines the mesob = - 45.8.
isomers of la,b and 2a,b are formed as well (NMR). This con[7] Single crystal.; from n-hexane solution at 0 C. Crystal data: C,,H,,Mo,firms that the synthesis is free from racemization up to the step
NLOJSi2(1061.4), monoclinic, space group P2,:c (no. 14); n =11.653(1),
of the free carbenes 2a,b. A possible configurational lability of
h = 1 0 3 0 5 ( 2 j . ~ = 2 4 . 4 9 6 ( 3 ) A , p = 9 7 . l S ( l r , 2 = 4 . V=5751(1)A3,
P,,~,,, = 1 226 gcm-3. p = 5.2 cm-', Mo,, radiation. -80 C ; Enraf-Nonius
2a,b because of the high acidity of the a-protons could not be
CAD4.o) scan. 10681 recorded reflections, of which 8929 with I>O were used
excluded.
in the refinement. The structure solution was achieved by direct methods.
The large tendency of the N-heterocyclic carbenes toward
R = 0.04. Rii = 0.032. residual electron density + 0.44/- 0.36 e k 3
c ~ m p l e x a t i o nwas
~ ~ confirmed
~
with these new chiral deriva[XI Partially dehydroxylated silica was used for the grafting study (Degussa.
200 m' g ; pretreatment at 500 .>Cfor 15 h under vacuum).
tives. For example, hexacarbonyltungsten reacts at 25 "C with
[9] J Kress. J A Osborn, G. Schoettel in Adiances m Meto1 Curhene Chemi.stry
the (R,R)-configurated carbenes 2a,b (generated in situ) to form
(Ed : U. Schubert), Kluwer, Dordrecht, 1989
the chiral carbene complexes (R,R)-4a,b as shown in Equa[lo] J.-M Basset i n ref. [I], p. 624.
if
-
.!(I!.
1'
.l.,ll.ll
I
T
~
tion (2). According to an X-ray structure analysis the naphthyl
[*] Prof. Dr. W A. Herrmann. L. J. Goossen. C. Kocher. Dr G. J R Artus
Anorganisch-chemisches lnstitut der Technischen Universitit Munchen
Lichtenbergstrasse 4, D-85747 Garching (Germany)
Fax: Int. code +(89)289-13473
e-mail . herrmanniu arthur.anorg.chemie.tu-muenchen.de
[**I Heterocyclic Carbenes, Part 9. This work received generous support from the
Deutsche Forschungsgemeinschaft, the Fonds der Chemischen lndustrie (PhD
scholarship to L.J.G.). the Bdyerische Forschungsstiftung (Bayerischer For-
schungsverbund Katalyse. FORKAT). and the Volkswagenstiftung. Part 8:
W. A. Herrmann. M. Elison, 0 Runte. G. R. J. Artus. J. Urgunomet. ClTem.
1995. 501. C1 -C4.
Anfeu'. C'hmi. I n / Ed. End. 1996. 35. N a 23/24
8 VCH
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> 90% chem. yield
> 30% ODt vield
(R, R) - 5a
(R, R) - 5b
derivative 2b contains two independent molecules in the asymmetric unit of the unit cell, which are distinguished by different
conformations of the carbene ligand in relation to the M(CO),
fragment (Fig. 1).I8]
The average carbene-tungsten distance C1W1 is as long as expected (2.303 A), and both the bond lengths
and IR spectrum indicate that TI backbonding is not significant
with the N-heterocyclic ligand.['] Starting from (S,S)-la the
nickel complex (S,S)-3a was synthesized in two steps."'] The
absolute configuration was confirmed by a crystal structure.r41
tive conversions and optical inductions higher than 30 % were
achieved.
The complexes Sa,b are active in hydrosilylation without an
induction period and even at low temperatures (Fig. 2). The
enantiomeric excesses are independent of the catalyst concentration as well as of the degree of conversion. All observations lead
to the conclusion that the chiral N-heterocyclic carbenes remain
bound to the metal center, in particular because a dissociated
carbene ligand would not be stable under the conditions of the
catalysis. The lifetime of the catalysts is remarkably high; even
after reaction times of more than two weeks the catalysts
showed no signs of decomposition.
100
L
''.A
-
- _- - -_- _
--_ --_ --_
Fig. I . Crystal structure (PLATON drawing) of one of the two independent molecules of the chiral tungsten complex (R,R)-4b.The thermal ellipsoides are drawn at
the 50 YOprobability level, and unimportant H atoms are omitted for clarity. Selected bond lengths [A] and angles [ '] (values in square brackets correspond to the
second independent molecule in the unit cell): W l - C l 2.308(7), [2.298(7)] N l - C l
1.359(9), [1.36(1)] N l - C l 4 1.373(9), [1.349(9)] N2-CI 1.356(9) [1.37(1)], N2-Cl5
1.37(1) [1.36(1)], C14-Cl5 1.34(1) [1.33(1)]; C1-N1-C14 111.8(6) [112.9(6)], C1N2-Cl5 lll.O(6) [111.6(6)], Nl-Cl-N2 103.5(6) [101.9(6)], NI-Cl4-Cl5 106.0(6)
[106.3(6)]. N2-Cl5-Cl4 107(7) [107.3(6)].
The rhodium complexes Sa,b were obtained according to
Equation (3) in 8 8 % yield.["] They are stable in air both as
solids and in solution, and their thermal stability is also remarkable (decomposition at 202 "C (Sa) and 210 "C (Sb)). When heated in solution they do not lose the heterocyclic carbene ligand
even at temperatures higher than 100°C (NMR study). Thus,
the new complex type has excellent properties for an application
in asymmetric synthesis. This was confirmed in the first tested
application, the hydrosilylation of acetophenone according to
Equation (4): With complex Sb as a catalyst, almost quantita2806
0 VCH
Verlagsgesellschaft mhH, 0-69451 Weinheim,1996
0
20
f[hI
-
40
60
Fig. 2. Catalytic hydrosilylation of acetophenone: conversion vs. time. xAp=
acetophenone concentration. Conditions:
-2O"C, 1 % 5b;
2O"C, 1% 5b;
mO'C, 1 % 5 b ; ~ - 2 0 ' C . O , 1 % 5 b .
These particular ligands were selected, because they do not
contain any functional groups; decomposition products of the
chiral ligands, for example the imiazolium salts la,b, can therefore not coordinate to the catalytic center. All optical induction
is lost if the catalyst is purposely degraded by addition of acid
during the catalysis. A mixture of [Rh,(cod),CI,] and l b shows
high activity in hydrosilylation; however, no enantiomeric enrichment results. Addition of l b alone also does not influence
the optical induction of the catalysts Sa,b. The optical induction
is distinctly temperature-dependent and decreases at higher temperatures (Table 1). However, no significant solvent dependence
could be observed.
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Table 1. Hydrosil!lation of acetophenone with diphenylsilane in T H F with 5b as
catalyst
(cat) [Oh]
1
0.2
0.1
1
1
1
1 [a1
0.1 [b]
1
1 [CI
T [ C]
- 20
- 20
- 20
- 34
0
20
- 20
- 20
- 20
- 20
I
Id
6d
12 d
2d
4h
l h
24 h
6d
7h
24 h
Conv. [%I
ee[%]
TON
90
90
90
90
90
90
90
90
60
60
26
26
26
32
12
<5
26
24
26
27
90
450
900
90
90
90
90
900
60
60
[a] Addition of 3 equiv Ib. [b] Containing 5 YO[Rh,(cod),CI,]. [c] Methyl naphthyl
ketone instead of acerophenone.
Only chiral chelate complexes of rhodium lead to higher
enantiomeric excesses in the catalytic hydrosilylation. For example, Rh/glucophinite yields 65 YO ee,[''] Rh/MPFA 49 YO
ee,1131
and Rh/Pybox (without excess ligand) 83 % ee.Il4]
In spite of as yet unsatisfactory ee values, N-heterocyclic carbenes indeed appear to be controlling ligands well suitable for
asymmetric homogeneous catalysis. Because the metal-carbon
bond is very stable, no ligand excess is needed. The inexpensive
synthesis and the structural variety of these types of ligands is
another advantage. A significant improvement in the optical
yields can be anticipated from further modified, donor-functionalized ligands.
Experimental Procedure
1a.b: A 500 mL round-bottomed flask was charged with ( R ) - or (S)-1-phenylethylamine (1 1.9 g, 0.10 mol) in toluene (100 mL). Under vigorous stirring and slight
cooling paraformaldehyde (3.00 g. 0.10 mol) was added. After 30 min another
~
equivalent of the amine was added at O'C. Under constant cooling 3 . 3 HCI
(30 mL. 0.10 moll was slowly added. After removal of the cold bath 40% aqueous
glyoxal solution was slowly added (14.5 mL. 0.10 mol). The mixture was stirred for
12 h at 35-40'C For workup diethyl ether (100 mL) and saturated Na,CO, solution (50 mL) were added, the phases separated, and the aqueous layer washed with
diethyl ether. After removal of the volatiles in vacuo the residue was taken up in
CH,Cl, (1 50 mL). dried over MgSO,, and filtered. The solvent was then evaporated
off, and the yellow residue was washed with dtethyl ether, which transformed it into
a light yellow extremely hygroscopic powder. Yield: 24.5 g (79%). The naphthyl
derivative Ib was prepared similarly from (R)-naphthylethylamine in 77% yield.
Za,b: The imidazolium salts 1a.b (10.0 mmol) were deprotonated in a mixture of
T H F (20 mL) and ammonia (100 mL)at -33'C with NaH (10.8 mmol). The starting materials were not completely soluble, but during the deprotonation a clear
yellow solution was formed. After evaporation of the ammonia the volume was
adjusted to 40 mL with T H F The 0 . 2 5 ~solution obtained this way was used
without further workup. NMR spectroscopic characterization (100 MHz, (THF/
CD,NO,): d[C(cdrbene)] = 211.2 @a), 210.5 (2b).
3a,b: Hexacarbonyltungsten (880 mg, 2.5 mmol) or tetracarbonylnickel (426 mg.
2.5 mmol) were treated with solutions of the free carbenes 2a or 2b (2.5 mmol).
After chromatography the residues were crystallized from methylene chloride
Compounds 3a (629 g. 63%, colorless crystals) and 4a (945 mg, 63%, yellow crystals) were obtained in good yield, and 4b in moderate yield (525 mg, 29%. yellow
crystals).
5a,b: [Rh,(cod),CI,] (200 mg, 0.40 mmol) in T H F ( 5 mL) was treated with a freshly
produced carbene solution (see 2 , 3.3 mL, 0.8 mmol). After 1 h the solvent was
removed in vacuo. the residue taken up in CH,CI,, and the resulting mixture
filtered. Compounds 5a.b were purified by precipitation from n-pentane/CH,CI,
mixtures. Yield. 327 mg 5 s (79%) and 441 mg 5b (71 % ) a s yellow powders.
Typical catalytic hydrosilylation: A vial equipped with magnetic stirrer and septum
was charged with (R.R)-5b (25mg. 1 mol%), acetophenone (0.5 mL, 4.2 mmol).
T H F (1 mL). and dr-n-butyl ether (100 mg as internal standard). The solution was
cooled to - 20' C, and diphenylsilane (0.8mL, 4 2 mmol) was added by syringe.
Thereafter. methanol (1 mL) and a catalytic amount of p-tolylsulfonic acid were
added to hydrolyze the silyl ether (20 min). The 1-phenylethanol was separated
from the silyl ethers by kugelrohr distillation. and its purity was determined by
NM R spectroscopy The enantiomeric excesses were determined by gas chromatography (Lipodex A column, 8O'C isorhermal)
-
Keywords: asymmetric catalysis * carbene complexes homogeneous catalysis hydrosilylations . rhodium compounds
-
W. A. Herrmann, M. Elison, J. Fischer, C. Kocher. G R. J. Artus. Angew.
Chem. 1995, 107. 2602-2605; Angew. Chem. Int. Ed. E n ~ l1995. 34. 23712374.
W. A. Herrmann, M. Elison, J. Fischer, C. Kocher, DE4447068 (Hoechst AG),
1994; EP0719758, 1996: DE4447067 (Hoechst AG). 1994. EP0719753, 1996;
DE47066 (Hoechst AG). 1994; EP0721953. 1996.
A. A. Griednev. I. M Mihaltseva, Syntk. Commun. 1994. 24. 1547-1555.
W. A. Herrmann, L. J Goossen. G . R. J. Artus, Orgunonirrullic.~,in print.
W. A. Herrmann. C. Kocher. L. J. Goossen. G . R. J. Artus. Cheni. Eur. J. 1996.
2, 1627-1636.
(S.S)-2a: l3C NMR (100 MHz, THF/CD,NO,): 6 = 711.2 (C:). 144 3
(phenyl-CR). 127.1 (p-phenyl-CH). 126.6 (phenyl-CH), 117.8 ( N C H = ) , 59.5
(NCH), 22.3 (CH,) (S.S)-Zb: "C N M R (400 MHz,CDCI,): 6 = 210.5 (C:).
134.4. 131.8. 129 3, 129.0. 126.9. 126.3, 125.9, 124.4. 124 3 (naphthyl), 118.7
(NC=). 56.4 (NCH). 22.2 (CH,).
a) K. Ofele. W. A. Herrmann. D . Mihalios, M. Elison. E. Herdtweck, E.
Scherer. J. Mink. J: Organomet. Chem. 1993. 459, 177- 184; b) M. Regitz.
Angew. Cheni. 1996. 122. 791-794; Angew. Chem. Inr. Ed. EngI. 1996, 35.
725 - 728.
M , = 700.39, yellow plate
Crystal structure analysis of 4b (C,,H,,N,O,W),
0.05x0.15x0.15 mm3. monoclinic, space group P2,. N =12.481(2).
h = 12.349(1), c = 19.073(2) A, h = 100.14(1)', V = 2893.8 A3. Z = 4. pFnird=
1.61 gem-,. p = 41.2cm-'. T = -1OO.O(3)-C, measured with IPDS (STOE)
with Mo,, radiation, 240 pictures in the range 0' < rp < 360 AGO= 1S ' . 5 min
exposure. distance between detector and crystal 80 mm. range 1 42' < O
<24.2 '; 32746 reflections measured, 7 beyond the dynamic region, 0 overlapping, 2690 systematic absences or reflections with negative intensity. 8569
independent; R,,,,, = 0 06, Lorentzian polarization and empirical absorption
corrections performed with IPDS-Software, 7699 reflections with I > 1 .Ori(l)
used for refinement, 720 parameters. 10.7 reflections pet- parameter. Flack
parameter -0 02(1). Chebychev polynomial weighting. hydrogen atomscalculated, residual electron density 1.71 e A - ' and -2.28 e k 3 . G O F = 0.814,
R = ~ ( l I F o l- l F c J ) / ~ ~ F=0.035,
o[
R,= Ew(IFol - lFLl)' ~ w F ~ J=0.040.
"'
Several physically doubtful thermal displacement parameters were obtained
from refinement of 4b. Unsatisfactorily corrected absorption effects cannot be
excluded as a reason. However, there is some evidence that 4b is contaminated
with a small amount of 4a. The resulting unresolvable disorder in the crystal
of 4b may well cause meaningless displacement parameters. Crystallographic
data (excluding structure factors) for the structure reported in this paper
have been deposited with the Cambridge Crystallographic Data Centre as
supplementary publication no. CCDC-179-122 Copies of the data can be
obtained free of charge on application to The Director. CCDC. 12 Union
Road, Cambridge CB2 1EZ. UK (fax: int. code +(1223)336-033; e-mail
techedei chemcrys c a m x uk).
Trunsirion Merul Carhene Comple.xes (Eds.: U. Schubert. H. Fischer. P. Hofmann. K. Weiss. K.-H. Dotz, E R. Kreissl), VCH, Weinheim 1983.p. 137-143
3a: ' H N M R (400 MHz. CDCI,): 6 =7.4-7.2 (overlapping multipletts. 10H.
phenyl-H). 6.75 (s, 2H, HC=), 6.08 (4, 'J(H,H) = 7 Hz. ZH. NCH), 1.73 (d.
'J(H,H) = 7 Hz, 6H, CH,); "C N M R (100 MHz, CDCI,): ii = 197.8 (CO),
189.2(CN,). 141.0,1?8.6,127.7,126.7(Ph-C). 118.4(NC=).58.5(NCH).20 8
(CH,); IR ( T H F ) i ( C 0 ) = 2050,1968 cm-'. 4a: ' H N M R (400 MHz, C,D,).
6 =7.14-7.29 (m, 10H. Ph-H): 6.48 (9, '4H.H) = 3 Hz. 2H, NCH), 6.28 (s.
2H. HC=), 1.55 (d. 'J(H,H) = 6.5 Hz. 6H, CH,); " C NMR (100 MHz.
C,D,): 6 = 200.9 (trans-CO), 198.5 (ci.s-CO). 180.3 (CN,), 141.2 (p-PhC).
129.4(PhC). 128.6(PhCR), 127.1 (PhC). 120.4(=CH).60 9(CH),21 7(CH3);
1R (THF) i = 2060, 1927cm-'.
5b: 'H N M R (400 MHz, CDCI,): 6 = 8.97 (d, 'J(H.H) = 9 Hz. 1H. naphthylH), 8.58 (d. 'J(H,H) = 9 Hz. 1H. naphthyl-H), 8.0-7.0(m. 12H, naphthyl-H),
7.22(q.3J(H,H) = 7 Hz.2H7NCH).7.01 (m.2H.HC=),4.X(m,2H,cod-CH),
4.19(m. 1H.cod-CH), 3.39(m. lH,cod-CH).2.5-1.0(m. XH,cod-CH,), 2.03
(d. 'J(H,H) = 7 Hz, 3H, CH,), 1 84(d, 'J(H,H) = 7 Hz. 3H. CH,); " C NMR
(100 MHz. CDCI,). 6 = 183.5 (d, iJ(i"3Rh,C) = 51 Hz. CN,), 141.0. 136.4,
133.8, 131.0, 130.1. 129.0, 128.8, 128.5, 127.7, 127.2. 1267. 126.1, 125.3. 125.1,
124.9. 123.8. 123.7. 121.4 (naphthyl-C). 119.3 (NCH), 118 6 (NCH), 98.4 (d.
'J('O'Rh.C) = 7 Hz. cod-CH). 97.6 (d. 'J("'Rh,C) = 7 Hz. cod-CH). 70 0 (d,
'J("'Rh.C) = 5 Hz, cod-CH), 66.5 (d, 'J("'Rh,C) = 5 Hz. cod-CH). S i . 2
(NCH), 55.2 (NCH). 32.5, 32.0 (cod-CH,), 29.2 (CH,). 27 2 (CH,), 23.1.22.8
(cod-CH,).
T H. Johnson. K. C. Klein. S. Thomen. J: Mof. Cutal. 1981. 12. 37-40.
I.Ojima, K. Hirai in Asymmetric Synthesis. @)I.5 (Ed J. I). Morrison), Academic Press, 1985, p 103-113.
[14] H. Brunner. Methoden der Organischen Chemie (Houben- Weyl). 4th ed.
1952-. Vol. E 21, Thieme. Stuttgart. 1995. p. 4074-4081.
.
Received: June 27, 1996 [Z9260IE]
German version: Angeu. Chem. 1996,108. 2980-2982
Angc(w. Chcm. Inr. Ed. Ennl. 1996, 35, No. 23/24
J; VCH Verlagsgesellschafr mhH, D-69451 Weinheim,1996
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