Reactions of novel reactive species generated by samarium(II)-mediated one-electron reduction of fischer-type carbene complexes

Article abstract of DOI:10.1021/ol006367q

(matrix presented) Two types of novel reactive species were generated by one-electron reductions of Fischer-type carbene complexes. In the case of tungsten complexes, one-electron reductions generated anion radical species, which added to electron-deficient olefins to give addition products. In the case of chromium complexes, carbonyl insertion occurred to give acyl chromate complexes, which underwent 1,4-addition to various electron-deficient olefins.

Full text of DOI:10.1021/ol006367q

ORGANIC
LETTERS
2
000
Vol. 2, No. 21
297-3299
Reactions of Novel Reactive Species
Generated by Samarium(II)-Mediated
One-Electron Reduction of Fischer-Type
Carbene Complexes
3
†
Kohei Fuchibe and Nobuharu Iwasawa*
Department of Chemistry, Tokyo Institute of Technology, 2-12-1 O-okayama,
Meguro-ku, Tokyo 152-8551, Japan
niwasawa@chem.titech.ac.jp
Received July 22, 2000
ABSTRACT
Two types of novel reactive species were generated by one-electron reductions of Fischer-type carbene complexes. In the case of tungsten
complexes, one-electron reductions generated anion radical species, which added to electron-deficient olefins to give addition products. In
the case of chromium complexes, carbonyl insertion occurred to give acyl chromate complexes, which underwent 1,4-addition to various
electron-deficient olefins.
It is well-known that the pentacarbonyl metal moiety of the
Fischer-type carbene complexes of group 6 metals acts as a
strongly electron-withdrawing group and that the carbon-
metal double bond of the complexes behaves like a carbonyl
species generated by samarium(II)-mediated reduction of
Fischer-type carbene complexes of group 6 metals.
First, one-electron reduction of the tungsten phenyl carbene
complex 1a was examined. When a THF solution of complex
1a was treated with 2.5 molar amounts of samarium diiodide3
in the presence of methanol at -78 °C, trans-stilbene 2 was
obtained in about 60% yield (Scheme 1).
1
group. One would therefore expect that one-electron reduc-
tion of the Fischer-type carbene complexes would generate
metal-containing anion radical species, which could be
employed for further carbon-carbon bond formations. In
fact, on one-electron reduction of a chromium phenyl carbene
complex with Na/K, Casey et al. observed by ESR an anion
radical species, which was stable at -78 °C in a highly
Scheme 1
2
diluted solution. This species, however, has not been
employed in further carbon-carbon bond-forming reactions.
In this Letter, we report the reactions of two types of reactive
†
Department of Chemistry, Graduate School of Science, The University
of Tokyo.
(
1) (a) Brown, F. J. Prog. Inorg. Chem. 1980, 27, 1. (b) Wulff, W. D. In
When the same reaction was carried out in the presence
of ethyl acrylate, methyl ether 3 and olefin 4 were obtained
ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.; Pergamon
Press: New York, 1991; Vol. 5, p 1065. (c) Wulff, W. D. In ComprehensiVe
Organometallic Chemistry II; Abel, A. W., Stone, F. G. A., Wilkinson, G.,
Eds.; Pergamon: New York, 1995; Vol. 12, p 469.
(
2) Krusic, P. J.; Klabunde, U.; Casey, C. P.; Block, T. F. J. Am. Chem.
(3) (a) Namy, J. L.; Girard, P.; Kagan, H. B. NouV. J. Chim. 1977, 1, 5.
(b) Molander, G. A.; Harris, C. R. Chem. ReV. 1996, 96, 307.
Soc. 1976, 98, 2015.
1
0.1021/ol006367q CCC: $19.00 © 2000 American Chemical Society
Published on Web 09/19/2000

in 38% and 33% yields, respectively.4 p-Bromophenyl
carbene complex 1b could also be employed in this reaction
(
methanol gives methyl ether 3. On the other hand, elimina-
8
tion of samarium methoxide from 8 gives unstable carbene
complex 9, which is deprotonated by the eliminated sa-
marium methoxide to give alkenyl tungsten species 10.
Protonation of 10 on quenching gives olefin 4.9
Scheme 2).5
Next, one-electron reduction of the chromium phenyl
carbene complex 11 was examined. When 11 was treated
with samarium diiodide in the presence of ethyl acrylate and
methanol, carbonyl-inserted product 12 was obtained in 8%
yield, along with the methyl ether 3 and olefin 4 in 10%
and 60% yields, respectively (Scheme 4, Table 1). Further-
Scheme 2
Scheme 4
Our proposed reaction mechanism is shown in Scheme 3:
One-electron reduction of the complex 1 gives the expected
anion radical species 5. In the absence of ethyl acrylate, the
more, the yield of ketone 12 increased to 41% when ethyl
acrylate was added after the treatment of carbene complex
Scheme 3
1
1 with samarium diiodide.
Table 1. Reduction of Chromium Phenyl Carbene Complex
We propose ketone 12 to be derived from acyl chromate
complex 16 (Scheme 5): One-electron reduction of carbene
Scheme 5
anion radical 5 is converted to anionic species 6 by
protonation and further one-electron reduction. Elimination
of samarium methoxide from the anionic species 6 gives
unstable carbene complex 7, which dimerizes to give trans-
stilbene 2.6
,7
In the presence of ethyl acrylate, the anion radical species
5
undergoes radical addition to the acceptor to give, after
one-electron reduction and protonation, alkyl tungsten species
. Protonolysis of the carbon-tungsten bond of 8 by
8
(
4) When ethyl acrylate was added after the treatment of carbene complex
a with samarium diiodide, trans-stilbene 2 was obtained in about 60%
yield. This indicates that the anion radical species 5 is short-lived.
5) When the tungsten butyl carbene complex and ethyl acrylate were
treated in the same manner as for the phenyl carbene complex, no reaction
took place at -78 °C. When the mixture was slowly warmed to rt, reduction
of ethyl acrylate took place while the carbene complex remained intact.
1
complex 11 gives anion radical species 13. Because of the
weaker bond energy of the carbon-chromium bond com-
pared to the carbon-tungsten bond, carbonyl insertion takes
place at the stage of the anion radical species 13 or its one-
electron reduced and protonated species 15 to give acyl
(
(6) Generation of trans-stilbene 2 from the tungsten benzylidene complex
is described in the following paper: Fischer, H.; Zeuner, S.; Ackermann,
K. J. Chem. Soc., Chem. Commun. 1984, 684.
10
chromate complex 16. The resulting acyl chromate complex
3298
Org. Lett., Vol. 2, No. 21, 2000

1
6 undergoes 1,4-addition to the electron-deficient olefin to
between -78 °C and rt. This is probably due to the facile
coordination of electron-deficient olefins to chromium
enabled by carbonyl insertion generating a coordinatively
unsaturated species.
11
give ketone 12.
The reaction was further examined by employing several
chromium alkyl carbene complexes (Scheme 6, Table 2).
In summary, the Fischer-type carbene complexes of group
6
metals were reduced with samarium(II) diiodide to give
two types of reactive species depending on the metal center
of the carbene complexes. In the case of the aryl tungsten
complexes, an anion radical species was generated by one
electron-reduction, and this underwent radical addition to an
electron-deficient olefin. In the case of the chromium
complexes, carbonyl insertion took place within the reduced
species to give acyl chromate complexes, which underwent
Scheme 6
1
,4-addition to electron-deficient olefins.
Acknowledgment. We are grateful to Professor Koichi
Narasaka (the Universityof Tokyo) and Dr. Hidehiro Sakurai
Osaka University) 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 Founda-
tion.
Although one-electron reduction of the alkyl carbene com-
plexes did not proceed with samarium diiodide alone, the
samarium diiodide-HMPA complex was found to reduce
them at -78 °C.12 As summarized in Table 2, the reaction
of n-butyl, s-butyl, and methyl carbene complexes with
various electron-deficient olefins proceeded to give the
corresponding addition products in moderate to good yields.
S o¨ derberg et al. reported that acyl(pentacarbonyl)-
chromate complexes react with electron-deficient olefins in
a 1,4-manner under thermal (refluxing in THF) or photo-
chemical conditions.11 It is believed that liberation of one
carbonyl ligand from the complex is necessary for olefin
coordination. On the other hand, our reaction proceeded
(
Supporting Information Available: General procedures
for the samarium(II)-mediated reactions and the spectroscopic
data of the products. This material is available free of charge
via the Internet at http://pubs.acs.org.
OL006367Q
(7) It is also possible that dimerization of anion radical species 5 occurs
first, and that subsequent elimination of a methoxy samarium species gives
dinuclear carbene complex 19. One-electron reduction of each carbene
moiety followed by protonation could give trans-stilbene 2.
Table 2. 1,4-Addition of Acyl Chromate Complexes to
Electron-Deficient Olefins
1
8/%
R1
EWG
R2
R3
(diastereomer ratio)a
(
8) When methanol-d4 was employed, the position R to the methoxy
group of 3 was 79% deuterated.
9) When methanol-d4 was employed, the position R to the phenyl group
of 4 was not deuterated. However, when the reaction was quenched with
D2O, the position was 76% and 70% deuterated for the E- and Z- isomers,
respectively. These results suggest that 10 was not protonated by methanol
in the reaction mixture.
n-Bu
n-Bu CN
CO2Et
H
H
H
H
H
Me
85
69
46
(
n-Bu
n-Bu
n-Bu
s-Bu
s-Bu
CO2Me
(81:19)
C(O)(CH2)2
H
51
(50:50)
61
(10) (a) Lee, I.; Cooper, N. J. J. Am. Chem. Soc. 1993, 115, 4389. (b)
(cyclopentenone)
Rudler, H.: Parlier, A.; D.-R e´ ville, T.; M.-Vaca, B.; Audouin, M.; Garrier,
E.; Certal, V.; Vaissermann, J. Tetrahedron 2000, 56, 5001.
CO2Me
H
H
H
H
CO2Me
(
(
(
53:47)
65
55:45)
56
56:44)
55
(11) (a) S o¨ derberg, B. C.; York, D. C.; Hoye, T. R.; Rehberg, G. M.;
CO2Et
H
H
H
Suriano, J. A. Organometallics 1994, 13, 4501. (b) S o¨ derberg, B. C.; York,
D. C.; Harriston, E. A.; Caprara, H. J.; Flurry, A. H. Organometallics 1995,
14, 3712. See also: (c) Barluenga, J.; Rodr ´ı guez, F.; Fan a˜ n a´ s, F. J. Chem.
CN
Eur. J. 2000, 6, 1930. (d) Sakurai, H.; Narasaka, K. Chem. Lett. 1994, 2017,
and ref 10b.
(12) A total of 2.8 equiv of HMPA based on samarium diiodide was
employed. Use of more or less HMPA resulted in a decrease of the yields
of the products. Concerning the effect of HMPA, see: Inanaga, J.; Ishikawa,
M.; Yamaguchi, M. Chem. Lett. 1987, 1485. Shabangi, M.; Flowers, R.
A., II. Tetrahedron Lett. 1997, 38, 1137.
Me
CO2(CH2)2Ph
a
1
Determined by integration of H NMR. Relative stereochemistry was
not determined.
Org. Lett., Vol. 2, No. 21, 2000
3299