Generation of reactive species by one-electron reduction of Fischer-type carbene complexes of group 6 metals and their use for carbon - Carbon bond formation

Article abstract of DOI:10.1002/chem.200390112

Carbon - carbon bond-forming reactions mediated by one-electron reduction of Fischer-type carbene complexes of Group 6 metals were investigated. In the case of aryl- or silylcarbene complexes of tungsten, the anion radical species generated by one-electro

Full text of DOI:10.1002/chem.200390112

FULL PAPER
Generation of Reactive Species by One-Electron Reduction of Fischer-Type
Carbene Complexes of Group 6 Metals and Their Use for Carbon Carbon
Bond Formation
Kohei Fuchibe[a, c] and Nobuharu Iwasawa*[b]
Abstract: Carbon carbon bond-forming reactions mediated by one-electron re-
duction of Fischer-type carbene complexes of Group 6 metals were investigated. In
the case of aryl- or silylcarbene complexes of tungsten, the anion radical species
generated by one-electron reduction smoothly underwent addition reaction to ethyl
acrylate. One-electron reduction of a,b-unsaturated carbene complexes afforded
biscarbene complexes by dimerization of the corresponding anion radical species at
the position g to the metal center. In contrast, one-electron reduction of chromium
phenyl- or alkylcarbene complexes gave, via carbonyl insertion, a-methoxyacylchro-
mate complexes, which further underwent conjugate addition to various electron-
poor olefins to give the corresponding a-methoxyketones.
Keywords:
chromium ¥ radical ions ¥ reduction
¥ tungsten
carbene ligands
¥
R
R
Introduction
δ– δ+
(OC)5M
~
O
OMe
OMe
Since their first synthesis reported by Fischer and Maasbˆl,[1]
Fischer-type carbene complexes of Group 6 metals have
attracted much interest, not only from complex chemists but
also from organic chemists due to their unique characteristics.
In particular, over the last two decades, these carbene
complexes have found increasing use as synthetic reagents
by exploiting their unique reactivities,[2] and various useful
reactions, including benzannulation[2a] and ketene forma-
tion,[2b] have been developed.
One of the most important properties of these complexes is
their high electrophilicity. As the Fischer-type carbene com-
plexes of Group 6 metals usually contain five carbonyl ligands
which behave as strong p acids, the carbon metal double
bond is highly polarized, so that the carbene carbon atom is
electrophilic (Figure 1). The carbon metal double bond
Figure 1. Similarity of Fischer-type carbene complexes to esters. M ˆ
Group 6 metal.
exhibits similar properties to the carbonyl group, and alkoxy-
substituted Fischer-type carbene complexes behave like
esters.
For example, various nucleophiles such as amines[3] and
organolithium reagents[4] perform nucleophilic attack on the
carbene carbon atom of complexes 1 to give tetrahedral
intermediates 2, which usually expel methanol to afford
nucleophile-substituted carbene complexes 3, which are
further converted to demetalated products should they be
unstable.[2f, 4d] Furthermore, alkyl-substituted carbene com-
plexes 4 are readily deprotonated with various bases to give
stable anionic species 5,[5] which react like ester enolates with
electrophiles such as alkyl halides (or pseudohalides)[6] and
aldehydes[7] to give a-substituted carbene complexes 6
(Scheme 1).
[a] Dr. K. Fuchibe
Department of Chemistry, Graduate School of Science
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
[b] Prof. N. Iwasawa
It has also been reported that when phenylcarbene
complexes 7 (M ˆ Cr, W) were treated with Na/K in THF/
HMPA at À788C, one-electron reduction of 7 took place to
yield the stable anion radical 8, which corresponds to ketyl
radical formation from carbonyl compounds (Scheme 2).[8]
Although these anion radical species possess a unique
structure in which a carbon-centered radical is substituted by
a pentacarbonylmetal moiety, they have never been employed
in further carbon carbon bond forming reactions.
Department of Chemistry
Tokyo Institute of Technology
2-12-1 O-okayama, Meguro-ku, Tokyo 152-8551 (Japan)
Fax : (‡81) 3-5734-2746
E-mail: niwasawa@chem.titech.ac.jp
[c] Dr. K. Fuchibe
Present address:
Department of Chemistry, Faculty of Science
Gakushuin University
1-5-1 Mejiro, Toshima-ku, Tokyo 171-8588 (Japan)
Chem. Eur. J. 2003, 9, No. 4
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905

N. Iwasawa and K. Fuchibe
FULL PAPER
–Nu
R
(OC)5M
OMe
R
R
H3O+
–
(OC)5M
Nu
OMe
(OC)5M
– MeOH
Nu
1
3
2
:B
R
R
R
H
E+
E
–
(OC)5M
(OC)5M
(OC)5M
OMe
OMe
OMe
4
5
6
Scheme 1. Characteristic behavior of Fischer-type carbene complexes of
Group 6 metals. M ˆ Group 6 metal.
Ph
Ph
–
Na/K
+
m
(OC)5M
(OC)5M
THF–HMPA, –78 °C
OMe
OMe
7
8
Scheme 2. One-electron reduction of phenylcarbene complexes with alkali
metals. M ˆ Group 6 metal, m ˆ Na or K.
Here we report details of our investigations on the
development of carbon carbon bond forming reactions
utilizing the novel reactive species generated by samarium(ii)
iodide-mediated one-electron reduction of Fischer-type car-
bene complexes of Group 6 metals.[9]
Scheme 4. Proposed mechanism for formation of trans-stilbene. The
cation [SmI2] is omitted for clarity.
‡
olefin as a radical acceptor (Scheme 5). When 9 was treated
with 2.5 molar amounts of samarium(ii) iodide in the presence
of an equimolar amount of ethyl acrylate and 5.0 molar
amounts of methanol at À788C in THF, the colors of
Results and Discussion
One-electron reduction of tungsten aryl- or silylcarbene
complexes and their coupling reaction with ethyl acrylate:
We first examined the possibility of using samarium(ii) iodide
as a one-electron reductant[10] for Fischer-type carbene
complexes. When the phenylcarbene tungsten complex 9
was treated with 2.0 molar amounts of samarium(ii) iodide in
the presence of 5.0 molar amounts of methanol at À788C in
THF, the characteristic red color of 9 disappeared within a few
minutes and trans-stilbene was formed in 60% yield
(Scheme 3).
2.5 molar amounts of SmI2
Ph
Ph
5.0 molar amounts of MeOH
–
+
(OC)5W
(OC)5W
CO2Et
OMe
OMe
x molar amount(s)
9
10
OMe
Ph
+
+
Ph
CO2Et
Ph
Ph
CO2Et
15
16 (E/Z=1/1)
x=1.0
x=6.1
9%
38%
9%
30%
2.0 molar amounts of SmI2
34%
–
Ph
5.0 molar amounts of MeOH
‡
Ph
Scheme 5. Coupling of 9 with ethyl acrylate. The cation [SmI2] is omitted
for clarity.
(OC)5W
Ph
THF, –78 °C
OMe
60%
9
Scheme 3. One-electron reduction of phenylcarbene complex 9.
samarium(ii) iodide and 9 disappeared within a few minutes,
and methyl ether 15 and olefin 16 were obtained, albeit in low
yields (9% each). These coupling products were accompanied
by trans-stilbene in 30% yield, but this stilbene formation was
completely suppressed by using an excess of ethyl acrylate
(6.1 molar amounts), to give 15 and 16 in 38 and 34% yield,
respectively.
Scheme 6 shows a plausible mechanism for this reaction.
Radical anion 10, generated by one-electron reduction of 9,
undergoes radical addition to ethyl acrylate. The resulting
radical anion intermediate 17 is further reduced by samar-
ium(ii) iodide and then protonated by methanol to give
alkyltungsten intermediate 18. Methyl ether 15 is produced by
protonation of the carbon tungsten bond of the intermediate
The reaction is presumed to proceed as follows (Scheme 4):
one-electron reduction of 9 gives anion radical 10, which
dimerizes to give dianion intermediate 11. Dianion 11 is then
protonated at the oxygen atom of the methoxyl group, and
elimination of methanol gives monoanion intermediate 12,
which leads to anion radical intermediate 13 by a reduction/
elimination (path A) or by an elimination/reduction (path B).
The dimerized intermediate 13 is further reduced to give
alkenylditungsten intermediate 14, and protonation of 14 with
methanol yields trans-stilbene.
As samarium(ii) iodide was effective in reducing 9, we next
examined the reaction in the presence of an electron-poor
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Reactive Species from Metal Carbene Complexes
905 914
On the basis of these results, we now believe that methyl
ether 15 is produced by protonation of the carbon tungsten
bond of the intermediate 18 by the methanol in the reaction
mixture, while olefin 16 is produced by deprotonation of the
unstable carbene intermediate 19,[5] followed by direct pro-
tonation of the carbon tungsten bond of the resulting
alkenyltungsten intermediate 20, not by the methanol in the
reaction medium, but by the water employed for workup
(Scheme 8).[11]
SmI2
MeOH
–
Ph
Ph
CO2Et
(OC)5W OMe
–
(OC)5W
(OC)5W
THF
Ph
CO2Et
OMe
OMe
10
17
9
–
OMe
Ph
(OC)5W OMe
SmI2, MeOH
MeOH
Ph
CO2Et
CO2Et
18
15
– MeOH
W(CO)5
–
H (D)
W(CO)5
H2O
W(CO)5
Ph
CO2Et
–OMe
(D2O)
Ph
CO2Et
Ph
CO2Et
Ph
CO2Et
H
Ph
CO2Et
20
16
19
16
19
Scheme 6. A plausible mechanism for the formation of 15 and 16. The
cation [SmI2] is omitted for clarity.
‡
Scheme 8. Mechanism for formation of 16. The cation [SmI2] is omitted
for clarity.
‡
18, while the olefin 16 is thought to be produced by 1,2-
hydrogen migration of the unstable carbene intermediate 19,
which is generated from 18 by elimination of methanol.
To confirm the mechanism described above, deuterium-
labeling experiments were carried out. First, we carried out
the reaction in the presence of CH3OD as a deuterium source
and performed workup with H2O(pH 7 phosphate buffer,
Scheme 7). The methyl ether 15 was deuterated at the position
a to the ethoxycarbonyl group and a to the methoxyl group
(15', 95 and 84% D, respectively), while the E and Z isomers
of olefin 16 were deuterated only at the position a to the
ethoxycarbonyl group ((E)-16' and (Z)-16', 100 and 95% D,
respectively).
This reaction is applicable to arylcarbene complexes
bearing electron-withdrawing or electron-donating groups at
the para position of the phenyl group.
One-electron reduction of p-bromophenylcarbene complex
21a went to completion within a few minutes at À788C to give
methyl ether 22a in 72% yield without formation of the olefin
23a (Scheme 9).
Br
2.5 molar amounts of SmI2
5.0 molar amounts of MeOH
+
CO2Et
THF, –78°C
(OC)5W
6.1 molar amounts
OMe
21a
2.5 molar amounts of SmI2
Ph
OMe
5.0 molar amounts of CH3OX
THF, –78°C
Y2O
CO2Et
+
(OC)5W
CO2Et
CO2Et
(quench)
OMe
Br
Br
5.9 molar amounts
9
22a 72%
23a –
Scheme 9. Reaction of p-bromophenylcarbene complex 21a.
(84%D)
D (95%D)
D (100%D)
Ph D (95%D)
CO2Et
MeO
D
CH3OX=CH3OD
Y2O=H2O
+
+
+
+
The reaction of p-methoxyphenylcarbene complex 21b was
slow in THF, but use of the mixed solvent THF/DMPU (9/1)
enabled the reaction to proceed smoothly, and in this case
olefin 23b was obtained as the major product (82% yield)
accompanied by methyl ether 22b (8% yield) (Scheme 10).[12]
Ph
CO2Et
Ph
CO2Et
15' 42%
(E)-16' 21%
(Z)-16' 13%
OMe
D (74%D)
CO2Et
Ph
CH3OX=CH3OH
Y2O=D2O
Ph
CO2Et
Ph
D
CO2Et
OMe
(no deuteration)
(70% D)
(Z)-16" 11%
3.8 molar amounts of SmI2
5.0 molar amounts of MeOH
15 33%
(E)-16" 13%
+
CO2Et
THF–DMPU (9/1), –78°C
Scheme 7. Deuterium-labeling experiments.
(OC)5W
OMe
6.2 molar amounts
21b
On the other hand, when the same reaction was carried out
in the presence of CH3OH and workup was performed with
D2O, the (E)- and (Z)-16 were deuterated at the position a to
the phenyl group ((E)-16'' and (Z)-16'', 74 and 70% D,
respectively), while the methyl ether 15 was not deuterated at
all.
OMe
CO2Et
+
CO2Et
MeO
MeO
23b 82% (E/Z=7/3)
22b 8%
Scheme 10. Reaction of p-methoxyphenylcarbene complex 21b.
Chem. Eur. J. 2003, 9, No. 4
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907

N. Iwasawa and K. Fuchibe
FULL PAPER
The effect of the para substituent on the product distribu-
tion can be explained as follows (Figure 2): the bromine atom
in intermediate 24a retards elimination of methanol and
makes protonation of the carbon tungsten bond of 24a
relatively faster to give the methyl ether 22a as the sole
product. On the other hand, the methoxyl group at the para
position in 24b accelerates elimination of methanol, and this
results in selective formation of olefin 23b.
Reductive dimerization of a,b-unsaturated carbene com-
plexes: synthesis of biscarbene complexes: We next examined
one-electron reduction of a,b-unsaturated carbene complexes
in the expectation that dimerization of anion radical species at
the position g to the metal center would give novel biscarbene
complexes.[15]
When the trans-propenylcarbene tungsten complex 28a was
treated with 1.1 molar amounts of samarium(ii) iodide in the
presence of 5.0 molar amounts of methanol at À788C in THF
(Scheme 12), reduction went to completion within a few
minutes to give the desired biscarbene complex 29a in 94%
yield. Thus, dimerization of 30a selectively occurred at the
position g to the metal center.[16, 17]
–
–
(OC)5W OMe
(OC)5W OMe
[SmI2]+
[SmI2]+
CO2Et
CO2Et
Br
MeO
24a
24b
1.1 molar amounts of SmI2
–
Figure 2. The substituent effect of a bromine atom and a methoxyl group
5.0 molar amounts of MeOH
(OC)5W
at the para position on the intermediate 24a or 24b.
(OC)5W
THF, –78 °C to RT
OMe
OMe
30a
28a
Reaction of silylcarbene complex 25 also proceeded
smoothly, and vinylsilane derivative 26 was obtained in 75%
yield exclusively as the E isomer (Scheme 11). Probably, the
bulky silyl group accelerates elimination of methanol from
intermediate 27.
–
(CO)5W
MeO
(CO)5W
MeOH
OMe
OMe
MeO
–
W(CO)5
W(CO)5
29a 94% (d.r.=3/1)
31a
Scheme 12. One-electron reduction of trans-propenylcarbene complex
‡
28a. The cation [SmI2] is omitted for clarity.
–
SiMePh2
MeO W(CO)5
SmI2, MeOH
+
(OC)5W
CO2Et
THF, –78°C
Ph2MeSi
CO2Et
OMe
25
Various kinds of a,b-unsaturated carbene complexes,
including b,b-dimethyl-substituted carbene complex 28b,
yielded the corresponding biscarbene complexes in good
yields (Table 1). Chromium complexes were also suitable for
this reaction.[18]
Furthermore, dienylcarbene complex 28 f also underwent
reductive dimerization at the terminal position to give
biscarbene complex 29 f in 93% yield (Scheme 13).
27
W(CO)5
– MeOH
Ph2MeSi
CO2Et
Ph2MeSi
CO2Et
26 75% (E only)
Scheme 11. Reaction of silylcarbene complex 25 with ethyl acrylate.
‡
The cation [SmI2] is omitted for clarity. 25/ethyl acrylate/SmI2/MeOH ˆ
1/22/3/5.
Table 1. Synthesis of biscarbene complexes.
R1 R2
1.1 molar amounts of SmI2
However, a similar reaction of the phenylcarbene complex
9 with methyl crotonate failed, and only trans-stilbene was
obtained in 52% yield. This suggests that the rate of addition
of anion radical 10 is greatly decreased by the presence of the
b-methyl group of the crotonate.[13] Furthermore, reaction of a
butylcarbene complex of tungsten (with a butyl group instead
of the phenyl group in 9) with ethyl acrylate also failed, and no
addition products were obtained. Samarium(ii) iodide was
consumed in the reduction of ethyl acrylate when the reaction
mixture was warmed to room temperature, and the carbene
complex was recovered in 56% yield. Thus, the present
method for the generation and reaction of tungsten-contain-
ing radical species is limited to aryl- and silylcarbene
complexes.
(OC)5M
R1
5.0 molar amounts of MeOH
OMe
MeO
(OC)5M
R2
OMe
THF, –78°C to RT
R1 R2
M(CO)5
28a–e
29a–e
M
R1
R2
28
Yield of 29 [%] (d.r.)
W
W
W
Cr
Cr
Me
Me
Ph
Me
Me
H
Me
H
a
b
c
94 (3/1) (29a)
83 (29b)
67 (1/1) (29c)
60 (29d)
Me
H
d
e
72 (4/1) (29e)
These samarium(ii) iodide-mediated one-electron reduc-
tions of Fischer-type carbene complexes can be regarded as an
umpolung of the carbene complexes. The carbene carbon
atom of Fischer-type carbene complexes of Group 6 metals is
highly electrophilic. One-electron reduction of the complexes
makes the carbene carbon atom a nucleophilic radical center,
which preferably adds to electron-poor olefins.[14]
Scheme 13. Reductive dimerization of dienylcarbene complex 28 f.
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Reactive Species from Metal Carbene Complexes
905 914
Although various kinds of biscarbene complexes have
previously been synthesized,[19] and recently some of them
have been utilized as synthetic reagents,[20] general routes to
this type of biscarbene complexes are still limited.[21] This
reductive dimerization of a,b-unsaturated carbene complexes
can be utilized as a general, efficient, and easy method for the
synthesis of symmetrical biscarbene complexes.
Formation of a-methoxyketone 36 suggests that insertion of
a carbonyl ligand into the anion radical 37 or its reduced form
38 took place to give a-methoxyacylchromate complex 39
competitively with the direct addition of 37 to the acrylate
(Scheme 16). It is presumed that 39 then undergoes conjugate
addition to ethyl acrylate to give the a-methoxyketone 36.[22]
Furthermore, functionalized carbene complexes can be
prepared by reductive coupling of a,b-unsaturated carbene
complexes with electron-poor olefins. When 28b or 28d was
treated with 2 molar amounts of samarium(ii) iodide in the
presence of a large excess of an electron-poor olefin and 5
molar amounts of methanol, the functionalized carbene
complexes 32 34 were obtained in good yields (Scheme 14).
Ph
Ph
CO2Et
CO2Et
16
Ph
Ph
SmI2
–
+
(OC)5Cr
(OC)5Cr
OMe
OMe
OMe
35
37
15
carbonyl insertion
SmI2, MeOH
2 molar amounts of SmI2
5 molar amounts of MeOH
+
EWG
(OC)5M
THF, –78°C to RT
OMe
OMe
–
Ph
H
–
Cr(CO)4L
Cr(CO)5
Ph
28b (M=W) or
28d (M=Cr)
MeO
10–30 molar amounts
O
38
L=solvent
(OC)5M
(OC)5M
MeO
carbonyl insertion
SmI2, MeOH
OMe
+
MeO
EWG
M(CO)5
M=W, EWG=CO2Et
M=W, EWG=CN
M=Cr, EWG=CN
32 67%
33 75%
34 83%
29b 18%
–
–
OMe
OMe
–
CO2Et
Cr(CO)4L
CO2Et
Ph
Ph
Scheme 14. Reductive coupling of 28b or 28d with electron-poor olefins.
EWG ˆ electron-withdrawing group.
O
O
39
36
Scheme 16. Supposed mechanism for reaction of the chromium complex
‡
35. The cation [SmI2] is omitted for clarity.
Generation of a-methoxyacylchromate complexes by one-
electron reduction of chromium carbene complexes: As
described in Scheme 5, one-electron reduction of tungsten
phenylcarbene complex 9 yielded anion radical 10, which
underwent addition to ethyl acrylate. We next examined the
one-electron reduction of chromium phenylcarbene complex
35 and found that an a-methoxyacylchromate complex was
produced by carbonyl insertion.
Complex 35 was treated with 2.1 molar amounts of
samarium(ii) iodide in the presence of 6.1 molar amounts of
ethyl acrylate and 5.0 molar amounts of methanol at À788C in
THF. The rate of reduction was slower than that of tungsten
complex 9, but 35 was completely consumed after a few hours
to give the olefin 16 and the methyl ether 15 in 60 and 8%
yield, respectively. Interestingly, formation of a small amount
of a-methoxyketone 36 was detected in this reaction of
chromium complex 35 (Scheme 15).
As acylchromate complexes are known to be stable,[23] we
examined the possibility of adding the acrylate after reduction
of the carbene complex 35 (Scheme 17). Thus, complex 35 was
5.5 molar amounts
2.1 molar amounts of SmI2
Ph
CO2Et
5.0 molar amounts of MeOH
(OC)5Cr
THF, –78°C, overnight
–78°C to RT
OMe
35
OMe
OMe
CO2Et
Ph
CO2Et
Ph
Ph
CO2Et
O
36 75 %
15 –
16 –
Scheme 17. Selective formation of 36.
2.1 molar amounts of SmI2
treated with 2.1 molar amounts of samarium(ii) iodide in the
presence of 5.0 molar amounts of methanol at À788C in THF,
and after stirring overnight the reaction mixture was treated
with 5.5 molar amounts of ethyl acrylate and warmed to room
temperature. Usual workup of the reaction mixture gave the
desired a-methoxyketone 36 in 75% yield without formation
of 15 or 16.[24]
Thus, chromium carbene complex 35 underwent carbonyl
insertion, while the corresponding tungsten complex 9 gave
trans-stilbene in 60% yield when treated with samarium(ii)
Ph
5.0 molar amounts of MeOH
+
(OC)5Cr
CO2Et
THF, –78°C
OMe
6.1 molar amounts
35
OMe
OMe
CO2Et
+
+
Ph
CO2Et
Ph
O
Ph
CO2Et
16 60% (E/Z=1/4)
15 8%
36 10 %
Scheme 15. Reaction of phenylcarbene complex of chromium 35.
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N. Iwasawa and K. Fuchibe
FULL PAPER
Table 2. Preparation of a-methoxyketones.
iodide in the absence of ethyl acrylate. This characteristic
difference between tungsten and chromium complexes is
probably due to the difference in electronegativity of these
metals. As chromium is more electropositive than tungsten
(Pauling electronegativities: Cr 1.6, W 1.7), and migratory
insertion of a carbonyl ligand is facilitated by the nucleophi-
licity of the migrating alkyl group, chromium complexes are
expected to be more prone to carbonyl insertion than the
corresponding tungsten complexes.[25]
SmI2–HMPA complex
OMe
O
R1
–
5.0 molar amounts of MeOH
Cr(CO)4L
R1
(OC)5Cr
THF, –78°C
(SmII/40, 42, 43=4.5)
OMe
40 (R1=nBu)
42 (R1=sBu)
43 (R1=Me)
OMe R2
R2
EWG
EWG
R1
–78°C to RT
O
41a–h
We next applied this reaction to alkyl-substituted carbene
complexes. Butylcarbene complex 40 was, however, not
reduced by samarium(ii) iodide even under reflux in THF,
probably due to the electron-donating property of the butyl
group. This inertness was overcome by using samarium(ii)
iodide HMPA complex as the one-electron reductant.[26] As
shown in Scheme 18, samarium(ii) iodide HMPA complex
reduced 40 smoothly at À788C, and treatment of the resulting
solution with ethyl acrylate afforded the corresponding a-
methoxyketone 41a in 85% yield.
R1
R2
a-methoxyketone
Yield of 41 [%] (d.r.)
85 (41a)
EWG
OMe
CO2Et
nBu (40)
CO2Et
nBu
O
OMe
CO2Me
nBu (40)
nBu (40)
nBu (40)
nBu (40)
sBu (42)
sBu (42)
Me (43)
46 (81/19) (41b)
61 (53/47) (41c)
69 (41d)
CO2Me
nBu
O
OMe CO2Me
CO2Me
MeO2C
CO2Me
nBu
nBu
nBu
sBu
sBu
OMe
nBu
–
SmI2–HMPA complex
O
Cr(CO)4L
(OC)5Cr
nBu
THF, –78°C
(SmII/40=4.5)
OMe
OMe
O
CN
CN
40
O
OMe
OMe
CO2Et
O
O
CO2Et
51 (51/49) (41e)
65 (55/45) (41 f)
56 (56/44) (41g)
55 (41h)
nBu
–78°C to RT
O
O
OMe
41a 85%
CO2Et
CN
CO2Et
Scheme 18. Reaction of butylcarbene complex 40 with SmI2-HMPA
complex.
O
OMe
CN
Table 2 shows the generality of this reaction. The a-
methoxyacylchromate complex derived from butylcarbene
complex 40 reacted not only with acrylate esters but also with
acrylonitrile and cyclopentenone to give the corresponding a-
methoxyketones. The reaction could also be applied to sec-
butyl- and methylcarbene complexes (42 and 43).
O
OMe
CO2CH2CH2Ph
CO2CH2CH2Ph
O
Sˆderberg et al. reported that acyl(pentacarbonyl)chro-
mate complexes undergo conjugate addition to electron-poor
olefins under thermal (refluxing in THF) or photochemical
conditions.[22] It is believed that liberation of one carbonyl
ligand from the complex is necessary for olefin coordination
to occur. On the other hand, our reaction proceeded between
À788C and room temperature. This is probably due to facile
coordination of the electron-poor olefins to chromium, as
only a weakly coordinating solvent molecule occupies the
vacant site generated by the carbonyl insertion.[27]
complexes gave a-methoxyacylchromate complexes by car-
bonyl insertion, which further underwent conjugate addition
to electron-poor olefins. Furthermore, a,b-unsaturated car-
bene complexes gave dimerized biscarbene complexes in
good yields. Thus, the anion radical species of Fischer-type
carbene complexes exhibit distinctive behavior that depends
on the nature of the central metal atom and the substituent on
the carbene carbon atom.[30]
Experimental Section
Conclusion
General: All reactions were carried out under an argon atmosphere. All
materials were purified by distillation or recrystallization before use. NMR
spectra were recorded on Bruker DRX500 (500 MHz for 1H, 125 MHz for
13C), JEOL Lambda400 (400 MHz for 1H, 100 MHz for 13C), JEOL AL400
(400 MHz for 1H, 100 MHz for 13C), or JEOL Lambda300 (300 MHz for
1H, 75 MHz for 13C) instruments. Chemical shifts d for 1H are referenced to
residual chloroform (d ˆ 7.24 ppm) as internal standard. Chemical shifts d
for 13C are referenced to a solvent signal (CDCl3, d ˆ 77.0 ppm) as internal
standard. IR spectra were recorded on JASCOFT/IR-200 or JASCOFT/
The anion radical species generated by one-electron reduction
of Fischer-type carbene complexes of tungsten and chromium
are suitable for carbon carbon bond-forming reactions. In
the case of aryl- or silylcarbene complexes of tungsten, the
anion radicals underwent addition to electron-poor olefins. In
contrast, anion radicals derived from chromium carbene
910
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chem. Eur. J. 2003, 9, No. 4

Reactive Species from Metal Carbene Complexes
905 914
IR460PLUS instruments. High-resolution mass spectrometry (HRMS) was
conducted with 70 eV electron impact ionization on a JEOL JMS-SX102A
instrument with perfluorokerosene as standard. Flash column chromatog-
raphy and preparative thin layer chromatography (preparative TLC) were
conducted on silica gel (Merck Kieselgel 60 Art 7734 and Wako gel B-5F,
respectively).
Reaction of (p-bromophenyl)carbene complex 21a with ethyl acrylate: The
reaction was carried out in a similar manner as for the reaction of tungsten
phenylcarbene complex 9.
Ethyl 4-(4-bromophenyl)-4-methoxybutanoate (22a): 1H NMR (300 MHz,
CDCl3, 258C): d ˆ 1.22 (t, 3J(H,H) ˆ 7.1 Hz, 3H; CH3), 1.85 2.12 (m, 2H;
CH2), 2.34 (t, 3J(H,H) ˆ 7.7 Hz, 2H; CH2), 3.18 (s, 3H; CH3), 4.05 4.13 (m,
3H; CH, CH2), 7.14 (d, 3J(H,H) ˆ 11.0 Hz, 2H; ArH), 7.46 ppm (d,
3J(H,H) ˆ 11.0 Hz, 2H; ArH); 13C NMR (75 MHz, CDCl3, 258C): d ˆ
14.2, 30.4, 33.0, 56.8, 60.3, 82.1, 121.4, 128.3, 131.6, 140.7, 173.2 ppm; IR
Preparation of carbene complexes: Aryl-, alkyl-, and silylcarbene com-
plexes and a,b-unsaturated carbene complexes, except for 28c and 28 f,
were prepared according to reference [28]. Complexes 28c and 28 f were
prepared according to the method reported by Aumann and Heinen.[7b]
(neat): nÄ ˆ 2931, 1732 (C O), 1108, 823 cmÀ1; elemental analysis (%) calcd
ˆ
for C13H17O3Br (301.2): C 51.84, H 5.69; found C 51.90, H 5.91.
Preparation of samarium(ii) iodide: Samarium(ii) iodide was prepared from
freshly distilled diiodomethane and samarium metal.[10b]
Reaction of p-methoxyphenylcarbene complex 21b with ethyl acrylate: A
THF solution of samarium(ii) iodide (8.0 mL, 0.085m, 0.68 mmol) was
added to a THF solution (3.5 mL) of the tungsten p-methoxyphenylcar-
bene complex 21b (84 mg, 0.18 mmol), ethyl acrylate (112 mg, 1.12 mmol),
methanol (35 mL), and DMPU (0.35 mL) at À788C. The blue color of
samarium(ii) iodide disappeared immediately, and the reaction mixture
became heterogeneous. The reaction mixture was stirred for 30 min at this
temperature and quenched with pH 7 phosphate buffer. The products were
extracted with ethyl acetate four times, and the combined organic phase
was dried over anhydrous magnesium sulfate. After evaporation of the
solvent, the crude product was purified by preparative TLC (hexane/ethyl
acetate 8/1) to give ethyl (Z)-4-(4-methoxyphenyl)but-3-enoate ((Z)-23b,
9.9 mg, 25%), ethyl (E)-4-(4-methoxyphenyl)but-3-enoate ((E)-23b,
22.6 mg, 57%), and ethyl 4-methoxy-4-(4-methoxyphenyl)butanoate
(22b, 3.6 mg, 8%).
One-electron reduction of tungsten phenylcarbene complex
9 with
samarium(ii) iodide: A THF solution of samarium(ii) iodide (6.4 mL,
0.10m, 0.64 mmol) was added to a THF solution (1.5 mL) of 9 (140 mg,
0.315 mmol) and methanol (50 mL) at À788C. After the mixture was stirred
for 2 h, the reaction was quenched with pH 7 phosphate buffer at this
temperature. The product was extracted with ethyl acetate four times, and
the combined organic phase was dried over anhydrous magnesium sulfate.
After evaporation of the solvent, the crude product was purified by
preparative TLC (hexane) to give trans-stilbene (18 mg, 60%). The
spectral data of the product were in complete agreement with those of
the authentic sample, which is commercially available.
Reaction of tungsten phenylcarbene complex 9 with ethyl acrylate: A THF
solution of samarium(ii) iodide (6.8 mL, 0.10m, 0.68 mmol) was added to a
THF solution (3.8 mL) of 9 (120 mg, 0.270 mmol), methanol (53 mL), and
ethyl acrylate (162 mg, 1.62 mmol) at À788C. After the mixture was stirred
for 30 min, the reaction was quenched with pH 7 phosphate buffer at this
temperature. The products were extracted with ethyl acetate four times,
and the combined organic phase was dried over anhydrous magnesium
sulfate. After evaporation of the solvent, the crude product was purified by
preparative TLC (hexane/ethyl acetate 8/1) to give ethyl 4-methoxy-4-
phenylbutanoate (15, 24 mg, 38%) and ethyl 4-phenylbut-3-enoate (16,
17 mg, 33%, E/Z ˆ 47/53).
Ethyl (Z)-4-(4-methoxyphenyl)but-3-enoate ((Z)-23b): 1H NMR
(400 MHz, CDCl3, 258C): d ˆ 1.25 (t, 3J(H,H) ˆ 7.2 Hz, 3H; CH3), 3.31
(dd, 3J(H,H) ˆ 7.2 Hz, 4J(H,H) ˆ 1.6 Hz, 2H; CH2), 3.80 (s, 3H; CH3), 4.15
(q, 3J(H,H) ˆ 7.2 Hz, 2H; CH2), 5.79 (dt, 3J(H,H) ˆ 7.2, 12.0 Hz, 1H; CH),
6.55 (d, 3J(H,H) ˆ 12.0 Hz, 1H; CH), 6.86 (d, 3J(H,H) ˆ 9.0 Hz, 2H; ArH),
7.20 ppm (d, 3J(H,H) ˆ 9.0 Hz, 2H; ArH); 13C NMR (100 MHz, CDCl3,
258C): d ˆ 14.2, 34.2, 55.3, 60.7, 113.7, 121.8, 129.2, 129.9, 131.4, 158.6,
171.9 ppm; IR (neat): nÄ ˆ 2980, 1735 (C C), 1610, 1515, 1180 cmÀ1; HRMS
ˆ
‡
‡
calcd for C13H16O3 (220.26): 220.1100 [M] ; found: 220.1078 [M] .
Ethyl 4-methoxy-4-phenylbutanoate (15): 1H NMR (300 MHz, CDCl3,
258C): d ˆ 1.22 (t, 3J(H,H) ˆ 7.2 Hz, 3H; CH3), 1.89 2.11 (m, 2H; CH2),
Ethyl (E)-4-(4-methoxyphenyl)but-3-enoate ((E)-23b): 1H NMR
(400 MHz, CDCl3, 258C): d ˆ 1.26 (t, 3J(H,H) ˆ 7.2 Hz, 3H; CH3), 3.19
(dd, 3J(H,H) ˆ 7.2 Hz, 4J(H,H) ˆ 1.2 Hz, 2H; CH2), 3.78 (s, 3H; CH3), 4.15
(q, 3J(H,H) ˆ 7.2 Hz, 2H; CH2), 6.14 (dt, 3J(H,H) ˆ 7.2, 16.0 Hz, 1H; CH),
6.41 (d, 3J(H,H) ˆ 16.0 Hz, 1H; CH), 6.82 (d, 3J(H,H) ˆ 9.2 Hz, 2H; ArH),
7.29 ppm (d, 3J(H,H) ˆ 9.2 Hz, 2H; ArH); 13C NMR (100 MHz, CDCl3,
258C): d ˆ 14.2, 38.5, 55.3, 60.7, 113.9, 119.6, 127.4, 129.7, 132.7, 159.1,
3
3
2.35 (t, J(H,H) ˆ 7.4 Hz, 2H; CH2), 3.23 (s, 3H; CH3), 4.09 (q, J(H,H) ˆ
7.2 Hz, 2H; CH2), 4.13 (dd, 3J(H,H) ˆ 5.7, 7.8 Hz, 1H; CH), 7.24 7.36 ppm
(m, 5H; ArH); 13C NMR (75 MHz, CDCl3, 258C): d ˆ 14.2, 30.6, 33.1, 56.7,
60.3, 82.8, 126.6, 127.7, 128.4, 141.5, 173.5 ppm; IR (neat): nÄ ˆ 2982, 1735
(C O), 1106, 702 cmÀ1; HRMS calcd for C13H18O3 (222.27): 221.1178 [M À
ˆ
‡
‡
H] ; found: 221.1179 [M À H] .
171.8 ppm; IR (neat): nÄ ˆ 2985, 1735 (C C), 1606, 1511, 1250 cmÀ1
;
ˆ
Ethyl 4-phenylbut-3-enoate (16): 1H NMR spectra were in complete
elemental analysis (%) calcd for C13H16O3 (220.3): C 70.89, H 7.32; found:
C 70.92, H 7.54.
agreement with those in the literature.[29]
Deuterium-labeling experiments: The spectral data of the compounds 15',
16', and 16'' are given below. The percentages of deuterium incorporation
were determined by the relative intensity of the peaks versus the methyl
signal of the methyl ether moiety for 15' and versus the methylene signal of
the ethyl ester moiety for 16' and 16''.
Reaction of methyldiphenylsilylcarbene complex 25 with ethyl acrylate:
The reaction was carried out in a similar manner to the reaction of tungsten
phenylcarbene complex 9 using 3.0 molar amounts of samarium(ii) iodide
and 22 molar amounts of ethyl acrylate.
Ethyl (E)-4-(methyldiphenylsilyl)but-3-enoate (26): 1H NMR (500 MHz,
1
3
15': H NMR (300 MHz, CDCl3, 258C): d ˆ 1.22 (t, J(H,H) ˆ 7.2 Hz, 3H;
CH3), 1.89 2.11 (m, 2H; CH2), 2.32 2.39 (m, 1.05H; CHD, 95% D), 3.23
(s, 3H; CH3), 4.08 4.16 (m, 2.16H; CH2 ‡ CD, 84% D), 7.24 7.36 ppm (m,
5H; ArH).
3
CDCl3, 258C): d ˆ 0.62 (s, 3H; CH3), 1.23 (t, J(H,H) ˆ 7.5 Hz, 3H; CH3),
3.21 (dd, 3J(H,H) ˆ 6.5 Hz, 4J(H,H) ˆ 1.5 Hz, 2H; CH2), 4.13 (q, 3J(H,H) ˆ
7.5 Hz, 2H; CH2), 6.10 (dt, 3J(H,H) ˆ 18.5 Hz, 4J(H,H) ˆ 1.5 Hz, 1H; CH),
3
6.22 (dt, J(H,H) ˆ 6.5, 18.5 Hz, 1H; CH), 7.32 7.39 (m, 6H; ArH), 7.50
(E)-16': 1H NMR (300 MHz, CDCl3, 258C): d ˆ 1.28 (t, 3J(H,H) ˆ 7.2 Hz,
7.55 ppm (m, 4H; ArH); 13C NMR (125 MHz, CDCl3, 258C): d ˆ À3.9,
3
3H; CH3), 3.16 3.22 (m, 1H; CHD, 100% D), 4.17 (q, J(H,H) ˆ 7.2 Hz,
14.2, 42.2, 60.6, 127.8, 129.3, 130.5, 134.8, 136.3, 141.6, 171.1 ppm; IR (neat):
2H; CH2), 6.30 (dd, 3J(H,H) ˆ 7.1, 11.8 Hz, 1H; CH), 6.50 (d, 3J(H,H) ˆ
nÄ ˆ 2980, 1734 (C C), 1430, 1112 cmÀ1; elemental analysis (%) calcd for
ˆ
11.8 Hz, 1H; CH), 7.21 7.40 ppm (m, 5H; ArH).
C19H22O2Si (310.5): C 73.50, H 7.14; found: C 73.20, H 7.19.
(Z)-16': 1H NMR (300 MHz, CDCl3, 258C): d ˆ 1.26 (t, 3J(H,H) ˆ 7.2 Hz,
3H; CH3), 3.29 3.38 (m, 1.05H; CHD, 95%D), 4.17 (q, 3J(H,H) ˆ 7.2 Hz,
2H; CH2), 5.90 (dd, 3J(H,H) ˆ 6.3, 7.9 Hz, 1H; CH), 6.64 (d, 3J(H,H) ˆ
7.9 Hz, 1H; CH), 7.23 7.41 ppm (m, 5H; ArH).
Reductive dimerization of a,b-unsaturated carbene complexes: A typical
procedure is described for tungsten trans-propenylcarbene complex (28a).
A solution of samarium(ii) iodide (4.0 mL, 0.089m, 0.36 mmol) in THF was
added to a solution of the tungsten trans-propenylcarbene complex 28a
(116 mg, 0.284 mmol) in THF (3.0 mL) and methanol (56 mL) at À788C.
After stirring for 15 min at this temperature, the reaction mixture was
exposed to an oxygen atmosphere (1 atm) for a few minutes at À788C to
quench remaining samarium(ii) iodide. When the blue color of samarium(ii)
iodide had disappeared, the oxygen atmosphere was replaced by argon, and
the reaction mixture warmed to room temperature. The solvent was
evaporated and the reaction mixture was filtered using a small amount of
silica gel and dichloromethane. Purification by column chromatography
1
3
(E)-16'': H NMR (400 MHz, CDCl3, 258C): d ˆ 1.28 (t, J(H,H) ˆ 7.2 Hz,
3
3
3H; CH3), 3.17 (d, J(H,H) ˆ 6.7 Hz, 2H; CH2), 4.17 (q, J(H,H) ˆ 7.2 Hz,
2H; CH2), 6.24 6.37 (m, 1H; CH), 6.50 (d, 3J(H,H) ˆ 11.8 Hz, 0.26H; CD,
74% D), 7.21 7.40 ppm (m, 5H; ArH).
1
3
(Z)-16'': H NMR (400 MHz, CDCl3, 258C): d ˆ 1.26 (t, J(H,H) ˆ 7.2 Hz,
3
3
3H; CH3), 3.32 (d, J(H,H) ˆ 7.5 Hz, 2H; CH2), 4.17 (q, J(H,H) ˆ 7.2 Hz,
3
2H; CH2), 5.83 5.94 (m, 1H; CH), 6.64 (d, J(H,H) ˆ 7.9 Hz, 0.30H; CD,
70% D), 7.23 7.41 ppm (m, 5H; ArH).
Chem. Eur. J. 2003, 9, No. 4
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0947-6539/03/0904-0911 $ 20.00+.50/0
911

N. Iwasawa and K. Fuchibe
FULL PAPER
(hexane/ethyl acetate 10/1) yielded biscarbene complex 29a (110 mg, 94%)
as a yellow solid.
ature. Evaporation of the solvent and purification by column chromatog-
raphy (hexane/ethyl acetate 10/1) gave coupling product 33 (104 mg, 75%)
as a yellow oil.
1,6-Dimethoxy-3,4-dimethylhexa-1,6-diylidenebis(pentacarbonyltungs-
ten(0)) (29a, 3:1 mixture of diastereomers): 1H NMR (400 MHz, CDCl3,
5-Cyano-1-methoxy-3,3-dimethylpent-1-ylidene(pentacarbonyl)tungs-
3
258C): d ˆ 0.80 (d, J(H,H) ˆ 6.8 Hz, 6H  0.75; CH3), 0.83 (d, 3J(H,H) ˆ
ten(0) (33): 1H NMR (400 MHz, CDCl3, 258C): d ˆ 0.99 (s, 6H; CH3), 1.72
3
3
6.8 Hz, 6H Â 0.25; CH3), 2.03 2.15 (m, 2H; CH), 2.96 3.25 (m, 4H; CH2),
4.61 (s, 6H Â 0.75; CH3), 4.62 ppm (s, 6H Â 0.25; CH3); 13C NMR
(100 MHz, CDCl3, 258C): d ˆ 15.4, 17.0, 36.2, 36.9, 68.5, 69.8, 70.5, 196.9,
(t, J(H,H) ˆ 8.0 Hz, 2H; CH2), 2.26 (t, J(H,H) ˆ 8.0 Hz, 2H; CH2), 3.19
(s, 2H; CH2), 4.64 ppm (s, 3H; CH3); 13C NMR (100 MHz, CDCl3, 258C):
d ˆ 12.5, 27.5, 36.6, 38.1, 70.5, 73.4, 119.9, 197.2, 203.1, 339.7 ppm; IR (neat):
nÄ ˆ 2962, 2070, 1913, 1451, 1259 cmÀ1; elemental analysis (%) calcd for
C14H15O6NW (477.1): C 35.24, H 3.17, N 2.94; found: C 35.51, H 3.31, N 2.87.
202.7, 337.1, 337.4 ppm; IR (neat): nÄ ˆ 2061, 1908 (br), 1446, 1273, 665 cmÀ1
;
elemental analysis (%) calcd for C20H18O12W2 (817.9): C 29.37, H 2.22;
found: C 29.67, H 2.46.
5-Ethoxycarbonyl-1-methoxy-3,3-dimethylpent-1-ylidene(pentacarbonyl)-
tungsten(0) (32): 1H NMR (400 MHz, CDCl3, 258C): d ˆ 0.95 (s, 6H; CH3),
1.22 (t, 3J(H,H) ˆ 8.0 Hz, 3H; CH3), 1.62 (t, 3J(H,H) ˆ 8.0 Hz, 2H; CH2),
Spectral data of compounds listed in Table 1:
1,6-Dimethoxy-3,3,4,4-tetramethylhexa-1,6-diylidenebis(pentacarbonyl-
tungsten(0)) (29b): 1H NMR (400 MHz, CDCl3, 258C): d ˆ 0.99 (s, 12H;
CH3), 3.29 (s, 4H; CH2), 4.64 ppm (s, 6H; CH3); 13C NMR (100 MHz,
CDCl3, 258C): d ˆ 22.6, 44.2, 69.8, 70.5, 197.4, 203.3, 343.2 ppm; IR (neat):
nÄ ˆ 2070, 1920 (br), 1450, 1255, 570 cmÀ1; elemental analysis (%) calcd for
C22H22O12W2 (846.0): C 31.23, H 2.62; found: C 31.25, H 2.83.
3
3
2.21 (t, J(H,H) ˆ 8.0 Hz, 2H; CH2), 3.18 (s, 2H; CH2), 4.09 (q, J(H,H) ˆ
8.0 Hz, 2H; CH2), 4.61 ppm (s, 3H; CH3); 13C NMR (100 MHz, CDCl3,
258C): d ˆ 14.1, 27.8, 29.7, 36.8, 37.8, 60.4, 70.4, 74.1, 173.7, 197.4, 203.3,
À1
ˆ
341.0 ppm; IR (neat): nÄ ˆ 2960, 2070, 1920, 1740 (C C), 1450, 1260 cm
;
elemental analysis (%) calcd for C16H20O8W (588.1): C 36.66, H 3.85;
found: C 36.91, H 4.03.
1,6-Dimethoxy-3,4-diphenylhexa-1,6-diylidenebis(pentacarbonyltungs-
ten(0)) (29c, 1:1 mixture of diastereomers): 1H NMR (500 MHz, CDCl3,
258C): d ˆ 2.82 (dd, 2J(H,H) ˆ 16.0 Hz, 3J(H,H) ˆ 3.0 Hz, 2H  0.5), 3.35
(m, 2H Â 0.5), 3.42 3.50 (m, 2H), 3.66 3.72 (m, 2H Â 0.5), 3.79 3.87 (m,
2H Â 0.5), 4.29 (s, 6H Â 0.5; CH3), 4.37 (s, 6H Â 0.5; CH3), 6.85 6.86 (m,
4H Â 0.5; ArH), 7.01 7.04 (m, 2H Â 0.5; ArH), 7.07 7.10 (m, 4H Â 0.5;
ArH), 7.20 7.23 (m, 6H Â 0.5; ArH), 7.30 7.33 ppm (m, 4H Â 0.5; ArH);
13C NMR (125 MHz, CDCl3, 258C): d ˆ 48.1, 48.5, 68.6, 69.7, 70.0, 70.2,
126.5, 127.2, 127.9, 128.0, 128.3, 128.8, 140.7, 141.5, 197.0, 202.8, 334.1,
335.3 ppm; IR (neat): nÄ ˆ 2920, 2075, 1910 (br), 1455, 1260 cmÀ1; elemental
analysis (%) calcd for C30H22O12W2 (942.1): C 38.24, H 2.35; found: C 38.53,
H 2.62.
5-Cyano-1-methoxy-3,3-dimethylpent-1-ylidene(pentacarbonyl)chromi-
um(0) (34): 1H NMR (400 MHz, CDCl3, 258C): d ˆ 0.87 (s, 6H; CH3), 1.61
(t, 3J(H,H) ˆ 7.8 Hz, 2H; CH2), 2.15 (t, 3J(H,H) ˆ 7.8 Hz, 2H; CH2), 3.25 (s,
2H; CH2), 4.74 ppm (s, 3H; CH3); 13C NMR (100 MHz, CDCl3, 258C): d ˆ
12.6, 27.4, 36.6, 38.1, 67.7, 71.6, 119.8, 215.7, 222.4, 365.6 ppm; IR (neat): nÄ ˆ
2964, 2063, 1923, 1455, 1261 cmÀ1; HRMS calcd for C14H15O6NCr (345.27):
‡
‡
345.0304 [M] ; found: 345.0288 [M] .
One-electron reduction of chromium phenylcarbene complex 35:
A
solution of samarium(ii) iodide (14.0 mL, 0.10m, 1.40 mmol) in THF was
added to a THF solution (6.0 mL) of chromium phenylcarbene complex 35
(204 mg, 0.654 mmol) and methanol (129 mL) at À788C. The mixture was
stirred overnight at this temperature, and then a solution of ethyl acrylate
(361 mg, 3.60 mmol) in THF was added. When addition was complete, the
reaction mixture was warmed to room temperature and stirred for an
additional hour. The reaction was quenched with pH 7 phosphate buffer at
room temperature, and the product extracted with ethyl acetate. The
combined organic phase was dried over anhydrous magnesium sulfate, and
the solvent evaporated. The crude product was filtered using a small
amount of silica gel and dichloromethane and purified by preparative TLC
(hexane/ethyl acetate 4/1) to give ethyl 5-methoxy-4-oxo-5-phenylpenta-
noate (36, 123 mg, 75%) as a colorless oil.
1,6-Dimethoxy-3,3,4,4-tetramethylhexa-1,6-diylidenebis(pentacarbonyl-
chromium(0)) (29d): 1H NMR (400 MHz, CDCl3, 258C): d ˆ 0.96 (s, 12H;
CH3), 3.42 (s, 4H; CH2), 4.83 ppm (s, 6H; CH3); 13C NMR (100 MHz,
CDCl3, 258C): d ˆ 22.4, 44.2, 67.7, 68.5, 216.3, 223.2, 369.8 ppm; IR (neat):
nÄ ˆ 2060, 1992, 1901 (br), 1449, 1249 cmÀ1; elemental analysis (%) calcd for
C22H22O12Cr2 (582.4): C 45.37, H 3.81; found: C 45.07, H 3.86.
1,6-Dimethoxy-3,4-dimethyhexa-1,6-diylidenebis(pentacarbonylchromi-
um(0)) (29e, 4:1 mixture of diastereomers): 1H NMR (400 MHz, CDCl3,
258C): d ˆ 0.72 0.81 (m, 6H; CH3), 1.95 2.10 (m, 2H; CH), 3.10 3.38 (m,
4H; CH2), 4.79 (s, 6H Â 0.2; CH3), 4.80 ppm (s, 6H Â 0.8; CH3); 13C NMR
(100 MHz, CDCl3, 258C): d ˆ 15.1, 16.8, 35.8, 36.4, 66.9, 67.8, 68.2, 216.30,
216.33, 222.96, 222.98, 364.3, 364.6 ppm; IR (neat): nÄ ˆ 2060, 1936 (br),
1445, 1259, 664 cmÀ1; elemental analysis (%) calcd for C20H18O12Cr2
(554.3): C 43.33, H 3.27; found: C 43.61, H 3.56.
Ethyl 5-methoxy-4-oxo-5-phenylpentanoate (36): 1H NMR (300 MHz,
3
3
CDCl3, 258C): d ˆ 1.18 (t, J(H,H) ˆ 7.2 Hz, 3H; CH3), 2.48 (t, J(H,H) ˆ
6.8 Hz, 1H; CH2), 2.49 (t, 3J(H,H) ˆ 6.3 Hz, 1H; CH2), 2.68 2.89 (m, 2H;
CH2), 3.37 (s, 3H; CH3), 4.06 (q, 3J(H,H) ˆ 7.2 Hz, 2H; CH2), 4.70 (s, 1H;
CH), 7.30 7.37 ppm (m, 5H; ArH); 13C NMR (75 MHz, CDCl3, 258C): d ˆ
14.1, 27.6, 32.4, 57.3, 60.6, 89.0, 127.0, 128.6, 128.8, 135.9, 172.6, 207.0 ppm;
(E,E)-1,10-Dimethoxy-5,6-diphenyldeca-3,7-diene-1,10-diylidenebis(pen-
tacarbonyltungsten(0)) (29 f, 1:1 mixture of diastereomers): 1H NMR
(500 MHz, CDCl3, 258C): d ˆ 3.56 (dd, 2J(H,H) ˆ 15.0 Hz, 3J(H,H) ˆ
6.0 Hz, 2H ‡ 2H  0.5; CH, CH2), 3.70 (dd, 2J(H,H) ˆ 14.5 Hz,
3J(H,H) ˆ 7.0 Hz, 2 H  0.5; CH2), 3.84 (dd, 2J(H,H) ˆ 15.0 Hz, 3J(H,H) ˆ
7.0 Hz, 2H  0.5; CH2), 3.91 (dd, 2J(H,H) ˆ 15.0 Hz, 3J(H,H) ˆ 7.0 Hz,
2H Â 0.5; CH2), 4.35 (s, 6H Â 0.5; CH3), 4.53 (s, 6H Â 0.5; CH3), 5.09
IR (neat): nÄ ˆ 2980, 1732 (C C), 1202, 702 cmÀ1; HRMS calcd for C14H18O4
ˆ
‡
‡
(250.28): 250.1205 [M] ; found: 250.1215 [M] .
One-electron reduction of chromium alkylcarbene complexes: A typical
procedure is described for the reaction of chromium butylcarbene complex
40 with ethyl acrylate.
3
3
(quintet, J(H,H) ˆ 7.0 Hz, 2H  0.5; CH), 5.31 (quintet, J(H,H) ˆ 7.0 Hz,
A THF solution of samarium(ii) iodide HMPA complex (prepared by
mixing HMPA (0.43 mL, 2.5 mmol) with a THF solution of samarium(ii)
iodide (8.5 mL, 0.10m, 0.85 mmol) at room temperature) was added to a
THF solution (2.7 mL) of chromium butylcarbene complex 40 (55 mg,
0.19 mmol) and methanol (38 mL) at À788C. The mixture was stirred for
20 min at this temperature and then a THF solution (1.5 mL) of ethyl
acrylate (113 mg, 1.13 mmol) was added. When addition was complete, the
reaction mixture was warmed to room temperature and stirred for an
additional hour. The reaction was quenched with pH 7 phosphate buffer,
and the product extracted with ethyl acetate. The combined organic phase
was dried over anhydrous magnesium sulfate and purified by column
chromatography (hexane/ethyl acetate 4/1) to give ethyl 5-methoxy-4-
oxononanoate (41a, 36 mg, 85%) as a colorless oil.
Ethyl 5-methoxy-4-oxononanoate (41a): 1H NMR (300 MHz, CDCl3,
258C): d ˆ 0.87 (t, 3J(H,H) ˆ 6.8 Hz, 3H; CH3), 1.23 (t, 3J(H,H) ˆ 7.1 Hz,
3H; CH3), 1.18 1.40 (m, 4H; CH2CH2), 1.61 (t, 3J(H,H) ˆ 6.8 Hz, 2H;
CH2), 2.56 (t, 3J(H,H) ˆ 6.6 Hz, 2H; CH2), 2.71 2.89 (m, 2H; CH2), 3.35 (s,
3H; CH3), 3.59 (t, 3J(H,H) ˆ 6.3 Hz, 1H; CH), 4.10 ppm (q, 3J(H,H) ˆ
7.1 Hz, 2H; CH2); 13C NMR (75 MHz, CDCl3, 258C): d ˆ 13.9, 14.2, 22.5,
3
2H  0.5; CH), 5.53 (dd, J(H,H) ˆ 5.5, 14.0 Hz, 2H  0.5; CH), 5.85 (dd,
3J(H,H) ˆ 6.5, 14.0 Hz, 2H  0.5; CH), 6.92 7.32 ppm (m, 10H; ArH);
13C NMR (125 MHz, CDCl3, 258C): d ˆ 54.7, 67.6, 68.0, 70.3, 123.7, 124.3,
126.1, 126.4, 128.1, 128.2, 128.35, 128.39, 136.0, 136.4, 142.38, 142.40, 197.07,
197.11, 203.0, 203.2, 332.1, 332.3 ppm; IR (neat): nÄ ˆ 2956, 2069, 1915, 1450,
1245 cmÀ1; HRMS calcd for C34H26O12W2 (994.15): 994.0444 [M] ; found:
‡
‡
994.0490 [M] .
Reaction of a,b-unsaturated carbene complexes with electron-poor olefins:
A typical procedure is described for the reaction of tungsten isobutenyl-
carbene complex 28b with acrylonitrile.
A THF solution of samarium(ii) iodide (6.1 mL, 0.10m, 0.61 mmol) was
added to a solution (3.0 mL) of tungsten isobutenylcarbene complex 28b
(123 mg, 0.291 mmol), acrylonitrile (462 mg, 8.71 mmol), and methanol
(57 mL) at À788C. The reaction mixture was stirred overnight at this
temperature and then exposed to an oxygen atmosphere (1 atm) to quench
remaining samarium(ii) iodide. After a few minutes, the blue color of
samarium(ii) iodide had disappeared, and the oxygen atmosphere was
replaced with argon, and the reaction mixture warmed to room temper-
912
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Reactive Species from Metal Carbene Complexes
905 914
27.2, 27.5, 31.8, 32.4, 58.2, 60.6, 87.2, 172.7, 211.5 ppm; IR (neat): nÄ ˆ 2958,
1.13 1.27 (m, 1H), 1.35 1.45 (m, 1H Â 0.44), 1.47 1.56 (m, 1H Â 0.56),
1.69 1.78 (m, 1H), 2.52 2.59 (m, 2H; CH2), 2.74 2.97 (m, 2H; CH2), 3.33
(d, 3J(H,H) ˆ 6.8 Hz, 1H  0.56; CH), 3.34 (s, 3H  0.56; CH3), 3.36 (s,
3H  0.44; CH3), 3.46 ppm (d, 3J(H,H) ˆ 5.1 Hz, 1H  0.44; CH); 13C NMR
(75 MHz, CDCl3, 258C): d ˆ 11.08, 11.11, 11.5, 14.2, 14.7, 24.8, 25.6, 33.9,
34.5, 37.3, 38.0, 59.0, 59.4, 90.3, 91.3, 119.0, 128.4, 209.3, 209.7 ppm; IR
1733 (C C), 1716 (C C), 1205, 1099 cmÀ1; HRMS calcd for C12H22O4
ˆ
ˆ
‡
‡
(230.30): 185.1177 [M À EtO] ; found: 185.1179 [M À EtO] .
Spectral data of the compounds listed in Table 2:
Methyl 5-methoxy-3-methyl-4-oxononanoate (41b, 81:19 mixture of dia-
stereomers): 1H NMR (400 MHz, CDCl3, 258C): d ˆ 0.88 (t, 3J(H,H) ˆ
7.1 Hz, 3H; CH3), 1.10 (d, 3J(H,H) ˆ 7.3 Hz, 3H  0.81; CH3), 1.11 (d,
3J(H,H) ˆ 7.4 Hz, 3 H  0.19; CH3), 1.23 1.44 (m, 4H; CH2CH2), 1.52 1.62
(m, 1H; CH2), 1.67 1.78 (m, 1H; CH2), 2.30 (dd, 2J(H,H) ˆ 17.1 Hz,
3J(H,H) ˆ 5.7 Hz, 1H  0.81; CH2), 2.31 (dd, 2J(H,H) ˆ 16.8 Hz, 3J(H,H) ˆ
5.4 Hz, 1H  0.19; CH2), 2.76 (dd, 2J(H,H) ˆ 16.8 Hz, 3J(H,H) ˆ 9.0 Hz,
1H  0.19; CH2), 2.79 (dd, 2J(H,H) ˆ 17.1 Hz, 3J(H,H) ˆ 9.3 Hz, 1H  0.81;
CH2), 3.17 3.45 (m, 1H; CH), 3.36 (s, 3H Â 0.19; CH3), 3.38 (s, 3H Â 0.81;
CH3), 3.63 (s, 3H; CH3), 3.76 (dd, 3J(H,H) ˆ 3.9, 8.1 Hz, 1H  0.81; CH),
3.84 ppm (dd, 3J(H,H) ˆ 4.4, 7.6 Hz, 1H  0.19; CH); 13C NMR (100 MHz,
CDCl3, 258C): d ˆ 13.9, 16.9, 22.5, 22.6, 27.4, 27.7, 29.7, 30.8, 31.8, 36.4, 37.6,
37.8, 38.0, 51.6, 51.7, 58.1, 58.4, 86.0, 86.4, 172.5, 172.7, 213.5, 214.1 ppm; IR
ˆ
(neat): nÄ ˆ 2966, 2250, 1720 (C C), 1462, 1096; elemental analysis (%)
calcd for C10H17NO2 (183.2): C 65.56, H 9.35, N 7.65; found C 65.26, H 9.35,
N 7.36.
Phenethyl 5-methoxy-4-oxohexanoate (41 h): 1H NMR (300 MHz, CDCl3,
258C): d ˆ 1.30 (d, 3J(H,H) ˆ 7.2 Hz, 3H; CH3), 2.59 (t, 3J(H,H) ˆ 6.8 Hz,
3
2H; CH2), 2.80 2.85 (m, 2H; CH2), 2.93 (t, J(H,H) ˆ 7.1 Hz, 2H; CH2),
3.37 (s, 3H; CH3), 3.76 (q, 3J(H,H) ˆ 7.2 Hz, 1H; CH), 4.29 (t, 3J(H,H) ˆ
7.1 Hz, 2H; CH2), 7.16 7.33 ppm (m, 5H; ArH); 13C NMR (75 MHz,
CDCl3, 258C): d ˆ 17.0, 27.4, 31.9, 35.0, 57.5, 65.0, 82.6, 126.5, 128.4, 128.8,
ˆ
137.7, 172.6, 211.0 ppm; IR (neat): nÄ ˆ 2940, 1736 (C C), 1168, 702;
elemental analysis calcd (%) for C15H20O3 (248.3): C 68.16, H 7.63; found: C
67.86, H 7.55.
(neat): nÄ ˆ 2957, 1738 (C C), 1715 (C C), 1200, 1100 cmÀ1; elemental
analysis (%) calcd for C12H22O4 (230.3): C 62.58, H 9.63; found: C 62.58, H
9.83.
ˆ
ˆ
Dimethyl 2-(2-methoxyhexanoyl)succinate (41c, 53:47 mixture of diaster-
eomers): 1H NMR (300 MHz, CDCl3, 258C): d ˆ 0.86 (m, 3H; CH3), 1.10
1.40 (m, 4H; CH2CH2), 1.50 1.80 (m, 2H; CH2), 2.70 (dd, 2J(H,H) ˆ
17.0 Hz, 3J(H,H) ˆ 6.0 Hz, 1H  0.47; CH2), 2.79 (dd, 2J(H,H) ˆ 17.4 Hz,
3J(H,H) ˆ 6.0 Hz, 1H  0.53; CH2), 2.90 (dd, 2J(H,H) ˆ 17.0 Hz, 3J(H,H) ˆ
8.4 Hz, 1H  0.47; CH2), 2.92 (dd, 2J(H,H) ˆ 17.4 Hz, 3J(H,H) ˆ 8.1 Hz,
1H Â 0.53; CH2), 3.33 (s, 3H Â 0.53; CH3), 3.36 (s, 3H Â 0.47; CH3), 3.64 (s,
6H  0.53; CH3), 3.69 (s, 6H  0.47; CH3), 3.74 (t, 3J(H,H) ˆ 6.0 Hz, 1H Â
0.53; CH), 3.83 (dd, 3J(H,H) ˆ 6.0, 9.0 Hz, 1H  0.47; CH), 4.21 (dd,
3J(H,H) ˆ 6.0, 8.1 Hz, 1H  0.53; CH), 4.24 ppm (dd, 3J(H,H) ˆ 6.0, 8.4 Hz,
1H  0.47; CH); 13C NMR (75 MHz, CDCl3, 258C): d ˆ 13.8, 22.4, 22.5,
26.8, 27.3, 30.7, 31.2, 31.9, 32.2, 49.7, 49.9, 52.0, 52.5, 52.6, 58.4, 58.5, 86.4,
86.5, 168.7, 169.1, 171.3, 171.6, 205.4, 205.9 ppm; IR (neat): nÄ ˆ 2955, 1740
Acknowledgement
We are grateful to Professor Koichi Narasaka (University of Tokyo) for
helpful discussions during this work. This research was partly supported by
a Grant-in-Aid for Scientific Research from the Ministry of Education,
Science, Sports and Culture of Japan and the Toray Science Foundation.
[1] E. O. Fischer, A. Maasbˆl, Angew. Chem. 1964, 76, 645; Angew. Chem.
Int. Ed. Engl. 1964, 3, 580.
[2] a) W. D. Wulff in Comprehensive Organometallic ChemistryII , Vol. 12
(Eds.: E. W. Abel, F. G. A. Stone, G. Wilkinson), Pergamon, New
York, 1995, p. 469; b) L. S. Hegedus in Comprehensive Organometallic
ChemistryII , Vol. 12 (Eds.: E. W. Abel, F. G. A. Stone, G. Wilkinson),
Pergamon, New York, 1995, p. 549; c) F. J. Brown, Prog. Inorg. Chem.
1980, 27, 1; d) W. D. Wulff in Comprehensive Organic Synthesis, Vol. 5
(Eds.: B. M. Trost, I. Fleming), Pergamon Press, New York, 1991,
p. 1065; e) K. H. Dˆtz, Angew. Chem. 1984, 96, 573; Angew. Chem. Int.
Ed. Engl. 1984, 23, 587; f) F. Z. Dˆrwald, Metal Carbenes in Organic
Synthesis, Wiley-VCH, Weinheim, 1999, p. 13; g) D. F. Harvey, D. M.
Sigano, Chem. Rev. 1996, 96, 271; h) L. S. Hegedus, Tetrahedron 1997,
53, 4105.
(C C), 1724 (C C), 1437, 1162 cmÀ1; HRMS calcd for C13H22O6 (274.31):
ˆ
ˆ
‡
‡
243.1233 [M À MeO] ; found: 243.1223 [M À MeO] .
1
5-Methoxy-4-oxononanenitrile (41d): H NMR (300 MHz, CDCl3, 258C):
d ˆ 0.86 (t, 3J(H,H) ˆ 7.1 Hz, 3H; CH3), 1.20 1.40 (m, 4H; CH2CH2),
1.52 1.72 (m, 2H; CH2), 2.56 (t, 3J(H,H) ˆ 6.8 Hz, 2H; CH2), 2.81 2.99
(m, 2H; CH2), 3.34 (s, 3H; CH3), 3.60 ppm (t, 3J(H,H) ˆ 6.2 Hz, 1H; CH);
13C NMR (75 MHz, CDCl3, 258C): d ˆ 11.1, 13.8, 22.4, 26.9, 31.4, 33.4, 58.3,
ˆ
86.8, 119.0, 209.2 ppm; IR (neat): nÄ ˆ 2935, 2250, 1721 (C C), 1465,
‡
1099 cmÀ1; HRMS calcd for C10H17O2N (183.25): 183.1259 [M] ; found:
‡
183.1259 [M] .
[3] For the reaction of Fischer-type carbene complexes of Group 6 metals
with amines, see: a) U. Klabunde, E. O. Fischer, J. Am. Chem. Soc.
1967, 89, 7141; b) E. O. Fischer, B. Heckl, H. Werner, J. Organomet.
Chem. 1971, 28, 359; c) R. Aumann, P. Hinterding, Chem. Ber. 1993,
126, 421; d) K. Wei˚, E. O. Fischer, Chem. Ber. 1973, 106, 1277.
[4] For the nucleophilic addition of organolithium reagents to Fischer-
type carbene complexes of Group 6 metals, see: a) C. P. Casey, T. J.
Burkhardt, J. Am. Chem. Soc. 1973, 95, 5833; b) E. O. Fischer, W.
Held, F. R. Krei˚l, A. Frank, G. Huttner, Chem. Ber. 1977, 110, 656;
c) E. O. Fischer, W. Held, J. Organomet. Chem. 1976, 112, C59; d) N.
Iwasawa, M. Saitou, Chem. Lett. 1994, 231. Organolithium reagents,
including lithium enolates, also undergo 1,4- or 1,6-addition to alkenyl-
or arylcarbene complexes. For a recent review of these conjugate
3-(2-Methoxyhexanoyl)cyclopentan-1-one (41e, 50:50 mixture of diaster-
eomers): 1H NMR (500 MHz, CDCl3, 258C): d ˆ 0.85 0.90 (m, 3H; CH3),
1.27 1.39 (m, 4H; CH2CH2), 1.61 1.69 (m, 2H; CH2), 1.92 2.04 (m, 1H;
cyclopentanone), 2.16 2.48 (m, 5H; cyclopentanone), 3.34 (s, 3H Â 0.50;
CH3), 3.37 (s, 3H Â 0.50; CH3), 3.55 3.63 (m, 1H; cyclopentanone), 3.66
3.70 ppm (m, 1H; CH); 13C NMR (125 MHz, CDCl3, 258C): d ˆ 13.8, 22.47,
22.48, 26.1, 26.5, 27.0, 27.3, 30.9, 31.3, 37.46, 37.48, 40.7, 41.2, 42.7, 42.8, 58.2,
58.3, 86.81, 86.83, 213.1, 213.3, 216.50, 216.51 ppm; IR (neat): nÄ ˆ 2956, 1746
(C C), 1713 (C C), 1103 cmÀ1; elemental analysis (%) calcd for C12H20O3
(212.3): C 67.89, H 9.50; found C 67.69, H 9.60.
ˆ
ˆ
Ethyl 5-methoxy-6-methyl-4-oxooctanoate (41 f, 55:45 mixture of diaster-
eomers): 1H NMR (500 MHz, CDCl3, 258C): d ˆ 0.82 (d, 3J(H,H) ˆ 7.4 Hz,
3H  0.55; CH3), 0.848 (d, 3J(H,H) ˆ 6.8 Hz, 3H  0.45; CH3), 0.849 (t,
3J(H,H) ˆ 7.5 Hz, 3 H  0.55; CH3), 0.88 (t, 3J(H,H) ˆ 7.5 Hz, 3H  0.45;
¬
¬
ƒ
addition reactions, see: e) J. Barluenga, J. Floretz, F. J. Fananas, J.
Organomet. Chem. 2001, 624, 5.
3
CH3), 1.19 1.29 (m, 1H; CH), 1.22 (t, J(H,H) ˆ 7.2 Hz, 3H; CH3), 1.37
[5] The ethylcarbene complex of chromium has pKa ˆ 8 in THF and
pKa ˆ 12 in water: a) C. P. Casey, R. L. Anderson, J. Am. Chem. Soc.
1974, 96, 1230; b) J. R. Gandler, C. F. Bernasconi, Organometallics
1989, 8, 2282.
[6] For reactions of anions of Fischer-type carbene complexes with alkyl
halides (or pseudohalides), see: a) Y.- C. Xu, W. D. Wulff, J. Org.
Chem. 1987, 52, 3263; b) J. S¸ltemeyer, K. H. Dˆtz, H. Hupfer, M.
Nieger, J. Organomet. Chem. 2000, 606, 26.
[7] For reactions of anions of Fischer-type carbene complexes with
carbonyl compounds, see: a) W. D. Wulff, S. R. Gilbertston, J. Am.
Chem. Soc. 1985, 107, 503; b) R. Aumann, H. Heinen, Chem. Ber. 1987,
120, 537; c) H. Wang, R. P. Hsung, W. D. Wulff, Tetrahedron Lett. 1998,
39, 1849.
1.45 (m, 1H Â 0.45; CH), 1.49 1.57 (m, 1H Â 0.55; CH), 1.69 1.79 (m, 1H;
CH), 2.54 (t, 3J(H,H) ˆ 6.8 Hz, 2H  0.55; CH2), 2.55 (t, 3J(H,H) ˆ 6.6 Hz,
2H  0.45; CH2), 2.69 2.88 (m, 2H; CH2), 3.29 (d, 3J(H,H) ˆ 7.1 Hz, 1H Â
0.55; CH), 3.33 (s, 3H Â 0.55; CH3), 3.37 (s, 3H Â 0.45; CH3), 3.47 (d,
3J(H,H) ˆ 5.0 Hz, 1H  0.45; CH), 4.10 ppm (q, 3J(H,H) ˆ 7.2 Hz, 2H;
CH2); 13C NMR (125 MHz, CDCl3, 258C): d ˆ 11.1, 11.6, 14.1, 14.7, 24.4,
24.8, 25.8, 27.4, 27.6, 33.0, 33.5, 33.9, 37.2, 37.8, 58.8, 59.1, 60.55, 60.57, 90.3,
ˆ
91.7, 172.7, 173.3, 211.4, 211.5 ppm; IR (neat): nÄ ˆ 2965, 1737 (C C), 1517,
‡
1205, 1097 cmÀ1; HRMS calcd for C12H22O4 (230.30): 185.1178 [M À EtO] ;
‡
found: 185.1180 [M À EtO] .
5-Methoxy-6-methyl-4-oxooctanenitrile (41g, 56:44 mixture of diastereom-
ers): 1H NMR (300 MHz, CDCl3, 258C): d ˆ 0.80 0.93 (m, 6H; CH3),
Chem. Eur. J. 2003, 9, No. 4
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
0947-6539/03/0904-0913 $ 20.00+.50/0
913

N. Iwasawa and K. Fuchibe
FULL PAPER
[8] P. J. Krusic, U. Klabunde, C. P. Casey, T. F. Block, J. Am. Chem. Soc.
1976, 98, 2015.
[9] For our previous communication, see: K. Fuchibe, N. Iwasawa, Org.
Lett. 2000, 2, 3297.
[24] We previously reported that the a-methoxyketone 36 was accompa-
nied by small amounts of methyl ether 15 and olefin 16 under similar
conditions.[9] However, re-examination of this reaction afforded the a-
methoxyketone 36 without formation of 15 or 16.
[10] a) J. L. Namy, P. Girard, H. B. Kagan, Nouv. J. Chim. 1977, 1, 5; b) P.
Girard, J. L. Namy, H. B. Kagan, J. Am. Chem. Soc. 1980, 102, 2693.
[11] The alkenyltungsten intermediate 20 was not protonated by methanol
at À788C. Even when the reaction was carried out in the presence of
CH3OD and stirred overnight, no incorporation of deuterium into the
olefin 16 was observed.
[12] DMPU ˆ N,N'-dimethylpropyleneurea. When the reaction was car-
ried out in THF, about 50% yield of olefin 23b and about 50%
recovery of complex 21b were obtained even after stirring overnight.
DMPU is known to increase the rate of reduction with samarium(ii)
iodide: T. Mukhopadhyay, D. Seebach, Helv. Chim. Acta 1982, 65, 385.
[13] This result is consistent with the fact that the rate of addition of
cyclohexyl radical to methyl crotonate is as little as one hundredth of
that to methyl acrylate: B. Giese, Angew. Chem. 1983, 95, 771; Angew.
Chem. Int. Ed. Engl. 1983, 22, 753.
[14] In another type of umpolung, a dianionic species prepared from a
carbene complex of chromium by reduction with potassium 1-meth-
ylnaphthalenide underwent nucleophilic addition to carbon dioxide:
S. Lee, N. J. Cooper, J. Am. Chem. Soc. 1990, 112, 9419.
[15] a,b-Unsaturated esters are reduced by samarium(ii) iodide to give
azipates, products dimerized at the position b to the carbonyl group: J.
Inanaga, Y. Handa, T. Tabuchi, K. Otsubo, M. Yamaguchi, T.
Hanamoto, Tetrahedron Lett. 1991, 32, 6557.
[25] The lower bond energy of the chromium carbon single bond may be
responsible for this difference. The strength of the metal methyl
bond in [Cp(CO)3CrMe] was estimated to be about 55 70% of that in
the corresponding tungsten complex: D. C. McKean, G. P. McQuillan,
A. R. Morrisson, I. Torto, J. Chem. Soc. Dalton Trans. 1985, 1207.
Facile insertion of a carbonyl ligand into the carbon metal double
bond of carbene complexes to give ketene complexes is well known
for chromium, but not so common for tungsten.[2b, h] For example, the
reaction of phenyl- or alkenylchromium carbene complexes with
alkynes usually gives hydronaphthoquinone or hydroquinone deriv-
atives, respectively, via carbon monoxide insertion (Dˆtz benzannu-
lation reaction), whereas the similar reaction with the corresponding
tungsten complexes usually gives indene or cyclopentadiene deriva-
tives. These differences are usually explained by the lower bond
energy of the chromium CObond relative to the tungsten CO
bond: W. D. Wulff, B. M. Bax, T. A. Brandvold, K. S. Chan, A. M.
Gilbert, R. P. Hsung, Organometallics 1994, 13, 102; and ref. [2g]. In
our case, however, it might not be appropriate to apply this difference
in bond energy of the metal carbonyl bond, as the carbonyl ligand is
not liberated from the complexes.
[26] HMPA ˆ N,N,N',N',N'',N''-hexamethylphosphoramide. HMPA is
known to increase the rate of reduction: a) K. Otsubo, J. Inanaga,
M. Yamaguchi, Tetrahedron Lett. 1986, 27, 5763; b) J. Inanaga, M.
Ishikawa, M. Yamaguchi, Chem. Lett. 1987, 1485; c) M. Shabangi,
R. A. Flowers, II, Tetrahedron Lett. 1997, 38, 1137.
[16] It is not clear whether 29a is produced by direct radical coupling of
30a or by radical addition of 30a to 28a.
ˆ
[17] The reaction mixture was warmed to room temperature when
reduction was complete. The yellow-green solution changed in color
to red, and this suggests that protonation of the dianion intermediate
31a by methanol took place.
[18] One-electron reduction of a (2-phenylethynyl)carbene complex of
tungsten gave a complex mixture of products.
[27] Hydride reduction of the carbene complex [Ph(EtO)C Cr(CO)5] with
N-methyl-1,4-dihydropyridine yielded the a-ethoxyacylchromate
complex, which underwent conjugate addition to cyclopentenone to
give an a-ethoxyketone under similar conditions to our reaction: a) H.
Rudler, A. Parlier, B. Martin-Vaca, E. Garrier, J. Vaissermann, Chem.
¬
Commun. 1999, 1439; b) H. Rudler, A. Parlier, T. Durand-Reville, B.
[19] M. A. Sierra, Chem. Rev. 2000, 100, 3591.
Martin-Vaca, M. Audouin, E. Garrier, V. Certal, J. Vaissermann,
Tetrahedron 2000, 56, 5001.
[20] E. Kuester, L. S. Hegedus, Organometallics 1999, 18, 5318.
[21] For synthesis of biscarbene complexes in which the carbene carbon
atoms are tethered by an alkyl chain, see: a) A. Geisbauer, S. Mihan,
W. Beck, J. Organomet. Chem. 1995, 501, 61; b) D. W. Macomber, M.-
H. Hung, A. G. Verma, Organometallics 1988, 7, 2072; and ref. [20].
[22] a) B. C. Sˆderberg, D. C. York, T. R. Hoye, G. M. Rehberg, J. A.
Suriano, Organometallics 1994, 13, 4501; b) B. C. Sˆderberg, D. C.
York, E. A. Harriston, H. J. Caprara, A. H. Flurry, Organometallics
1995, 14, 3712.
[28] W. D. Wulff, W. E. Bauta, R. W. Kaesler, P. J. Lankford, R. A. Miller,
C. K. Murray, D. C. Yang, J. Am. Chem. Soc. 1990, 112, 3642.
[29] R. M. Gerkin, B. Rickborn, J. Am. Chem. Soc. 1967, 89, 5850.
[30] Note added in proof (26. 12. 2002): After submission of this manu-
script, a paper by Sierra and co-workers appeared in which the
reductive dimerization of a,b-unsaturated Fischer carbene complexes
by using C8K as a one-electron reductant was described: M. A. Sierra,
¬
¬
¬
ƒ
P. Ramırez-Lopez, M. Gomez-Gallego, T. Lejon, M. J. Mancheno,
[23] Acylchromate complexes produced by the reaction of hexacarbonyl-
chromium with alkyl- or aryllithium reagents are isolable in the form
of their tetramethylammonium salts: E. O. Fischer, A. Maasbˆl,
Chem. Ber. 1967, 100, 2445.
Angew. Chem. 2002, 114, 3592; Angew. Chem. Int. Ed. 2002, 41, 3442.
Received: September 2, 2002 [F4384]
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