Cobalt-catalyzed trimethylsilylmethylmagnesium-promoted radical alkenylation of alkyl halides: A complement to the Heck reaction

Article abstract of DOI:10.1021/ja061417t

A cobalt complex, [CoCl₂(dpph)] (DPPH = [1,6- bis(diphenylphosphino)hexane]), catalyzes an intermolecular styrylation reaction of alkyl halides in the presence of Me₃SiCH₂MgCl in ether to yield β-alkylstyrenes. A variety of alkyl halides including alkyl chlorides can participate in the styrylation. A radical mechanism is strongly suggested for the styrylation reaction. The sequential isomerization/styrylation reactions of cyclopropylmethyl bromide and 6-bromo-1-hexene provide evidence of the radical mechanism. Crystallographic and spectroscopic investigations on cobalt complexes reveal that the reaction would begin with single electron transfer from an electron-rich (diphosphine)bis(trimethylsilylmethyl)cobalt(II) complex followed by reductive elimination to yield 1,2-bis(trimethylsilyl)ethane and a (diphosphine)cobalt(I) complex. The combination of [CoCl ₂(dppb)] (DPPB = [1,4-bis(diphenylphosphino)butane]) catalyst and Me₃SiCH₂-MgCl induces intramolecular Heck-type cyclization reactions of 6-halo-1-hexenes via a radical process. On the other hand, the intramolecular cyclization of the prenyl ether of 2-iodophenol would proceed in a fashion similar to the conventional palladium-catalyzed transformation. The nonradical oxidative addition of carbon(sp²)-halogen bonds to cobalt is separately verified by a cobalt-catalyzed cross-coupling reaction of alkenyl halides with Me₃SiCH₂MgCl with retention of configuration of the starting vinyl halides. The cobalt-catalyzed intermolecular radical styrylation reaction of alkyl halides is applied to stereoselective variants. Styrylations of 1-alkoxy-2-bromocyclopentane derivatives provide trans-1-alkoxy-2-styrylcyclopentane skeletons, one of which is optically pure.

Full text of DOI:10.1021/ja061417t

Published on Web 05/28/2006
Cobalt-Catalyzed Trimethylsilylmethylmagnesium-Promoted
Radical Alkenylation of Alkyl Halides: A Complement to the
Heck Reaction
Walter Affo,† Hirohisa Ohmiya,† Takuma Fujioka,† Yousuke Ikeda,†
Tomoaki Nakamura,† Hideki Yorimitsu,*,† Koichiro Oshima,*,† Yuki Imamura,‡
Tsutomu Mizuta,‡ and Katsuhiko Miyoshi‡
Contribution from the Department of Material Chemistry, Graduate School of Engineering,
Kyoto UniVersity, Kyoto-daigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan, and Department
of Chemistry, Graduate School of Science, Hiroshima UniVersity, Kagamiyama 1-3-1,
Higashi-Hiroshima, Hiroshima 739-8526, Japan
Received March 1, 2006; E-mail: yori@orgrxn.mbox.media.kyoto-u.ac.jp; oshima@orgrxn.mbox.media.kyoto-u.ac.jp
Abstract: A cobalt complex, [CoCl2(dpph)] (DPPH ) [1,6-bis(diphenylphosphino)hexane]), catalyzes an
intermolecular styrylation reaction of alkyl halides in the presence of Me3SiCH2MgCl in ether to yield
â-alkylstyrenes. A variety of alkyl halides including alkyl chlorides can participate in the styrylation. A radical
mechanism is strongly suggested for the styrylation reaction. The sequential isomerization/styrylation
reactions of cyclopropylmethyl bromide and 6-bromo-1-hexene provide evidence of the radical mechanism.
Crystallographic and spectroscopic investigations on cobalt complexes reveal that the reaction would begin
with single electron transfer from an electron-rich (diphosphine)bis(trimethylsilylmethyl)cobalt(II) complex
followed by reductive elimination to yield 1,2-bis(trimethylsilyl)ethane and a (diphosphine)cobalt(I) complex.
The combination of [CoCl2(dppb)] (DPPB ) [1,4-bis(diphenylphosphino)butane]) catalyst and Me3SiCH2-
MgCl induces intramolecular Heck-type cyclization reactions of 6-halo-1-hexenes via a radical process.
On the other hand, the intramolecular cyclization of the prenyl ether of 2-iodophenol would proceed in a
fashion similar to the conventional palladium-catalyzed transformation. The nonradical oxidative addition
of carbon(sp2)-halogen bonds to cobalt is separately verified by a cobalt-catalyzed cross-coupling reaction
of alkenyl halides with Me3SiCH2MgCl with retention of configuration of the starting vinyl halides. The cobalt-
catalyzed intermolecular radical styrylation reaction of alkyl halides is applied to stereoselective variants.
Styrylations of 1-alkoxy-2-bromocyclopentane derivatives provide trans-1-alkoxy-2-styrylcyclopentane
skeletons, one of which is optically pure.
Introduction
have been made to increase the utility. Despite such devotions
to the reaction during the last three decades, the major limitation
The Heck reaction is among the most powerful carbon-
carbon bond formation reactions in organic synthesis.1,2 It is
applied to various fields of chemical science, ranging from
syntheses of chemicals of biological interest to those of high-
performance functional organic materials. The scope and
limitations have been fully investigated, and considerable efforts
unsolved so far is that one cannot use alkyl halides having
hydrogen at the â position to the halide atom as substrates. The
alkylpalladiums formed from such halides normally undergo
â-hydride elimination more rapidly than insertion of alkenes
(Scheme 1a). Although there are some reports on the palladium-
catalyzed Heck reaction of alkyl halides, the reactions employ
iodomethane, R-haloacetate,3 benzyl halide,4 and 1-bromoada-
mantane,5 which have no detachable â-hydrogens. An additional
difficulty stems from the much slower oxidative addition of alkyl
halides due to the lack of proximal π-systems.6
† Kyoto University.
‡ Hiroshima University.
(1) (a) Mizoroki, T.; Mori, K.; Ozaki, A. Bull. Chem. Soc. Jpn. 1971, 44, 581.
(b) Heck, R. F.; Nolley, J. P., Jr. J. Org. Chem. 1972, 37, 2320-2322.
(2) For review, see: (a) Bra¨se, S.; de Meijere, A. In Metal-catalyzed Cross-
coupling Reactions; Diederich, F., Stang, P. J., Eds; Wiley-VCH: Wein-
heim, 1998; Chapter 3. (b) Link, J. T.; Overman, L. E. In Metal-catalyzed
Cross-coupling Reactions; Diederich, F., Stang, P. J., Eds.; Wiley-VCH:
Weinheim, 1998; Chapter 6. (c) Bra¨se, S.; de Meijere, A. In Metal-catalyzed
Cross-coupling Reactions, 2nd ed.; de Meijere, A., Diederich, F., Eds;
Wiley-VCH: Weinheim, 2004; Chapter 5. (d) Heck, R. F. Org. React. 1982,
27, 345-390. (e) Beletskaya, I. P.; Cheprakov, A. V. Chem. ReV. 2000,
100, 3009-3066. (f) Crips, G. T. Chem. Soc. ReV. 1998, 27, 427-436. (g)
de Meijere, A.; Meyer, F. E. Angew. Chem., Int. Ed. Engl. 1994, 33, 2379-
2411. (h) Cabri, W.; Candiani, I. Acc. Chem. Res. 1995, 28, 2-7. (i) Beller,
M.; Riermeier, T. H.; Stark, G. In Transition Metals for Organic Synthesis;
Beller, M., Bolm, C., Ed.; Wiley-VCH: Weinheim, 1998; Volume 1,
Chapter 2.13. (j) Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E., Ed.; Wiley: New York, 2002; Vol. 1, Chapter IV.2.
To overcome these difficulties, Heck-type reactions of alkyl
halides with alkenes mediated by transition metals other than
(3) Glorius, F. Tetrahedron Lett. 2003, 44, 5751-5754.
(4) (a) Wu, G. Z.; Lamaty, F.; Negishi, E. J. Org. Chem. 1989, 54, 2507-
2508. (b) Pan, Y.; Jiang, X.; Hu, H. Tetrahedron Lett. 2000, 41, 725-727.
(5) (a) Hori, K.; Ando, M.; Takaishi, N.; Inamoto, Y. Tetrahedron Lett. 1987,
28, 5883-5886. (b) Bra¨se, S.; Waegell, B.; de Meijere, A. Synthesis 1998,
148-152. (c) Narahashi, H.; Yamamoto, A.; Shimizu, I. Chem. Lett. 2004,
33, 348-349 and references cited therein.
(6) Negishi, E. In Handbook of Organopalladium Chemistry for Organic
Synthesis; Negishi, E., Ed.; Wiley: New York, 2002; Vol. 1, Chapter II.3.1.
9
8068
J. AM. CHEM. SOC. 2006, 128, 8068-8077
10.1021/ja061417t CCC: $33.50 © 2006 American Chemical Society

Cobalt-Catalyzed Alkenylation of Alkyl Halides
A R T I C L E S
Table 1. Cobalt-Catalyzed Styrylation of Alkyl Halidesa
Scheme 1
entry
R
−X 1
time/h
temp/
°
C
3
yield/%
1
2
3
4
5
6
7
8
9
nC6H13CH(CH3)Br
nC12H25Br
nC12H25Br
Ad-Brb
8
8
3
8
8
3
3
3
3
3
3
3
20
20
35
20
20
35
35
35
35
35
35
35
3b
3c
3c
3d
3e
3e
3c
3c
3d
3a
3f
73
76
71
87
11
67
57
74
90
84
55c
53
tC4H9Br
tC4H9Br
nC12H25I
nC12H25Cl
Ad-Clb
10
11
12
cC6H11Cl
CH3I
CH2dCH(CH2)3Br
3g
a Conditions: R-X (1, 1.5 mmol), styrene (2a, 1.0 mmol), the Grignard
reagent (2.5 mmol), CoCl2 (0.05 mmol), and DPPH (0.06 mmol). b Ad )
1-adamantyl. c p-Chlorostyrene was used instead of styrene.
palladium have been attracting increasing attention. However,
most of the variants are not satisfactory with regard to yield
and/or reaction conditions. Nickel-catalyzed reaction of alkyl
bromides with styrene afforded the desired products in moderate
yields.7 Cobaloxime-catalyzed zinc-mediated similar transforma-
tions required photolysis and resulted in unsatisfactory yields.8
Very recently, it was found that titanocene-catalyzed butyl-
magnesium bromide-promoted styrylation of alkyl bromides and
some chlorides is very effective in attaining satisfactory yields.9
We independently found that a cobalt salt catalyzes inter-
molecular styrylation of alkyl halides in the presence of
trimethylsilylmethylmagnesium chloride.10 Here we report the
full details and application of the cobalt-catalyzed Heck-type
reactions.11
Our approach to an alkyl version of the Heck reaction is
outlined in Scheme 1b. We have been interested in the synthetic
use of single electron transfer from electron-rich cobalt com-
plexes to alkyl halides.12 Compared with the conventional
oxidative addition of alkyl halides to palladium, the cobalt-
mediated single electron transfer process is reasonably rapid to
generate the corresponding alkyl radical and halide anion. The
radical generated does not suffer from â-hydride elimination
and adds to an activated alkene, styrene, for instance, to yield
a new stabilized radical. The new radical is trapped by a cobalt
complex, and subsequent â-hydride elimination provides the
desired Heck-type product.
Results and Discussion
Styrylation of Alkyl Halides. A mixture of bromocyclo-
hexane (1a, 1.5 mmol) and styrene (2a, 1.0 mmol) was treated
with trimethylsilylmethylmagnesium chloride (1.0 M ethereal
solution, 2.5 mmol) in ether in the presence of [CoCl2(dpph)]
(DPPH ) 1,6-bis(diphenylphosphino)hexane). The reaction
mixture was stirred for 8 h at 20 °C to afford â-cyclohexyl-
styrene (3a) in 86% yield (eq 1). Heating the reaction mixture
(7) Lebedev. S. A.; Lopatina, V. S.; Petrov, E. S.; Beletskaya, I. P. J.
Organomet. Chem. 1988, 344, 253-259.
(8) (a) Branchaud, B. P.; Detlefsen, W. D. Tetrahedron Lett. 1991, 32, 6273-
6276. Transformation with a stoichiometric amount of cobaloximes: (b)
Tada, M.; Okabe, M. Chem. Lett. 1980, 201-204. (c) Branchaud, B. P.;
Yu, G.-X. Organometallics 1993, 12, 4262-4264. (d) Ali, A.; Gill, G. B.;
Pattenden, G.; Roan, G. A.; Kam, T.-S. J. Chem. Soc., Perkin Trans. 1
1996, 1081-1093 and references therein. (e) Iqbal, J.; Bhatia, B.; Nayyar,
N. K. Chem. ReV. 1994, 94, 519-564.
at reflux improved the yield of 3a up to 91% and the reaction
time to 3 h. A trace amount of dimerized product 3a′ was the
only detectable byproduct, which was readily separable from
3a upon purification on silica gel.
(9) (a) Terao, J.; Watabe, H.; Miyamoto, M.; Kambe, N. Bull. Chem. Soc.
Jpn. 2003, 76, 2209-2214. (b) Terao, J.; Kambe, N. Yuki Gosei Kagaku
Kyokaishi 2001, 59, 1044-1051.
The reaction proceeded similarly in the absence of light,
which eliminates the possibility of a photoinduced reaction.
Cobalt(II) bromide, instead of cobalt chloride, served as well.
The amount of the Grignard reagent also affected the yield of
3a. Use of 1.5 or 3.5 equiv of the Grignard reagent led to a
decreased yield of 3a. PhMe2SiCH2MgCl also effected the
alkylation reaction (78% yield of 3a at 35 °C). Trialkylsilyl-
methyl Grignard reagents were essential to obtain the desired
product. The reaction with phenyl, methyl, ethyl, and neopentyl
Grignard reagents provided traces of 3a. Cyclohexene was
detected mainly. The effect of the ligand deserves discussion
(vide infra).
(10) Ikeda, Y.; Nakamura, T.; Yorimitsu, H.; Oshima, K. J. Am. Chem. Soc.
2002, 124, 6514-6515.
(11) Cobalt-catalyzed Heck reactions of aryl halides with alkenes were
reported: (a) Gomes, P.; Gosmini, C.; Ne´de´lec, J.-Y.; Pe´richon, J.
Tetrahedron Lett. 2002, 43, 5901-5903. (b) Gomes, P.; Gosmini, C.;
Pe´richon, J. Tetrahedron 2003, 59, 2999-3002. (c) Amatore, M.; Gosmini,
C.; Pe´richon, J. Eur. J. Org. Chem. 2005, 989-992.
(12) (a) Wakabayashi, K.; Yorimitsu, H.; Oshima, K. J. Am. Chem. Soc. 2001,
123, 5374-5375. (b) Tsuji, T.; Yorimitsu, H.; Oshima, K. Angew. Chem.,
Int. Ed. 2002, 41, 4137-4139. (c) Mizutani, K.; Shinokubo, H.; Oshima,
K. Org. Lett. 2003, 5, 3959-3961. (d) Ohmiya, H.; Yorimitsu, H.; Oshima,
K. Chem. Lett. 2004, 33, 1240-1241. (e) Mizutani, K.; Yorimitsu, H.;
Oshima, K. Chem. Lett. 2004, 33, 832-833. (f) Ohmiya, H.; Tsuji, T.;
Yorimitsu, H.; Oshima, K. Chem.sEur. J. 2004, 10, 5640-5648. (g) Ikeda,
Y.; Yorimitsu, H. Shinokubo, H.; Oshima, K. AdV. Synth. Catal. 2004,
346, 1631-1634. (h) Ohmiya, H.; Wakabayashi, K.; Yorimitsu, H.; Oshima,
K. Tetrahedron 2006, 62, 2207-2213. (i) Ohmiya, H.; Yorimitsu, H.;
Oshima, K. J. Am. Chem. Soc. 2006, 128, 1886-1889. (j) Shinokubo, H.;
Oshima, K. Eur. J. Org. Chem. 2004, 2081-2091. (k) Yorimitsu, H.;
Oshima, K. Pure Appl. Chem. 2006, 78, 441-449.
A variety of alkyl halides were examined (Table 1). Primary
and tertiary alkyl bromides as well as secondary ones underwent
9
J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006 8069

A R T I C L E S
Affo et al.
Table 2. Reaction with Styrene Derivativesa
Table 3. Optimization of Liganda
entry
ligand
yield of 3a/%
1
2
3
4
5
6
7
8
Ph2PCH2PPh2 (DPPM)
Ph2P(CH2)2PPh2 (DPPE)
Ph2P(CH2)3PPh2 (DPPP)
Ph2P(CH2)4PPh2 (DPPB)
Ph2P(CH2)5PPh2 (DPPPEN)
Ph2P(CH2)6PPh2 (DPPH)
Ph2P(CH2)8PPh2 (DPPO)
Ph3P (2 equiv to Co)
<20
<10
<20
52
70
86
entry
2
Ar
4
yield/%
1
2
3
4
5
6
7
8
2b
2c
2d
2e
2f
2g
2h
2i
C6H4-p-Me
C6H4-p-Cl
C6H4-m-Cl
C6H4-o-Cl
4b
4c
4d
4e
4f
4g
4h
4i
87
85
82
85
82
29
95
66
75
60
9
10
EtPh2P (2 equiv to Co)
Ph2PCH2C6H4CH2PPh2 (DPPX)
58
33
C6H4-p-OMe
C6H4-p-CON(CH2Ph)2
C6H4-m-CON(CH2Ph)2
C6H4-m-COOtC4H9
a Conditions: CoCl2 (0.05 mmol), ligand (0.06 mmol), 1a (1.5 mmol),
2a (1.0 mmol), and Me3SiCH2MgCl (2.5 mmol) were used. The reactions
were performed at 20 °C for 8 h.
a Conditions: 1a (1.5 mmol), 2 (1.0 mmol), the Grignard reagent (2.5
mmol), CoCl2 (0.05 mmol), and DPPH (0.06 mmol).
support the generation of an alkyl radical from an alkyl halide.
Furthermore, as described above, the dimerization of benzylic
radicals can rationalize the formation of 3a′. The mechanism
of the cobalt-catalyzed reaction is thus quite different from the
palladium-catalyzed one (vide infra).
the styrylation reaction (entries 1-6). The reaction of tert-butyl
bromide required a higher temperature to attain an acceptable
result (entries 5 and 6). Compared to the use of lauryl bromide,
the use of lauryl iodide resulted in a low yield of 3c (entry 7).
It is worth noting that alkyl chlorides, which are often unreactive
in transition metal-catalyzed reactions, proved to be excellent
alkyl sources in this reaction (entries 8-10). For instance,
treatment of a mixture of lauryl chloride and styrene with Me3-
SiCH2MgCl in ether at reflux furnished 3c in 74% yield under
the [CoCl2(dpph)] catalysis. The reaction with iodomethane
afforded the corresponding product 3f in moderate yield (entry
11). Because the Grignard reagent was used, functional groups
such as ester, phthalimide, and hydroxy groups were not
compatible. A terminal alkenyl moiety survived under the
reaction conditions (entry 12).
The [CoCl2(dpph)]/Me3SiCH2MgCl system efficiently medi-
ates a Heck-type reaction of alkyl halides, including alkyl
chlorides, with styrene derivatives. The procedure is simple,
and the reaction tolerates some functionalities. The reaction
represents a complementary method to the palladium-catalyzed
Heck styrylation reaction.
Mechanistic Investigation: Single Electron Transfer from
Cobalt Complexes. A variety of phosphine ligands were
surveyed to clarify the ligand effect (Table 3). When Ph2P-
(CH2)nPPh2 (n ) 1-4) and triphenylphosphine were employed,
the yields of 3a decreased and significant amounts of (2-
cyclohexylethyl)benzene and 3a′ were obtained (entries 1-4
and 8). As the number of the methylene units between the
phosphorus atoms increased, the yield was dramatically im-
proved (entries 4-6). DPPH having a hexamethylene tether gave
the highest yield. DPPO, 1,8-bis(diphenylphosphino)octane,
seems to have a too long methylene chain (entry 7). The yield
was fair when ethyldiphenylphosphine, electronically equivalent
to DPPH, was employed (entry 9). Diphosphino-p-xylene also
failed to afford 3a in a satisfactory yield (entry 10). Amine
ligands such as tetramethylethylenediamine afforded none of
3a.
The reaction tolerates seemingly labile functionalities (Table
2). Methoxy- and chlorostyrenes were alkylated efficiently in
refluxing ether (entries 2-5). Unfortunately, the para substitution
with a carbamoyl group decreased the yield of the product since
the phenylogous acrylamide 2g is so reactive that uncatalyzed
side reactions took place (entry 6). The meta substituted 2h
underwent alkylation efficiently (entry 7). A tert-butoxycarbonyl
group also was left untouched under the reaction conditions
(entry 8). Unfortunately, the reactions with 1-octene and butyl
vinyl ether resulted in failure. An attempted alkylation of methyl
acrylate resulted in the formation of a complex mixture.
It is unclear whether DPPH having a long tether binds to
one cobalt center intramolecularly or DPPH forms di- or
multinuclear complexes with cobalt during the reaction. To
explore the nature of the Co(dpph) complexes, DPPH was
allowed to react with CoCl2. While attempts to prepare crystals
of cobalt complexes in ethereal solutions failed due to their low
solubility, we could obtain blue single crystals of a macrocyclic
dinuclear complex of [CoCl2(dpph)]2 from acetone (Figure 1).14
The cobalt centers have tetrahedral geometries, wherein the
P-Co-P angles are 106° and the Cl-Co-Cl angles are 111°.
The reaction with cyclopropylmethyl bromide yielded a ring-
opening product, â-(3-butenyl)styrene (5), in 50% yield (eq 2).
In addition, a tetrahydrofuran derivative 7 was obtained when
iodo acetal 6 was employed (eq 3). Ring opening of a
(13) (a) Newcomb, M. In Radicals in Organic Synthesis; Renaud, P.; Sibi, M.,
Eds.; Wiley-VCH: Weinheim, 2001; Vol. 1, Chapter 3.1. (b) Newcomb,
M.; Choi, S. Y.; Horner, J. H. J. Org. Chem. 1999, 64, 1225-1231. (c)
Beckwith, A. L. J.; Glover, S. A. Aust. J. Chem. 1987, 40, 157-173.
(14) The gold, platinum, and silver analogues were reported. (a) Kitagawa, S.;
Kondo, M.; Kawata, S.; Wada, S.; Maekawa, M.; Munakata, M. Inorg.
Chem. 1995, 34, 1455-1465. (b) Kotani, S.; Adachi, T.; Yoshida, T.;
Onitsuka, K.; Sonogashira, K. Chem. Lett. 1994, 1127-1130. (c) Effendy;
di Nicola, C.; Fianchini, M.; Pettinari, C.; Skelton, B. W.; Somers, N.;
White, A. H. Inorg. Chim. Acta 2005, 358, 763-795.
cyclopropylmethyl radical and ring closure of a 5-hexenyl
radical are well-known processes.13 These observations strongly
9
8070 J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006

Cobalt-Catalyzed Alkenylation of Alkyl Halides
A R T I C L E S
Figure 1. One of the two independent molecules of [CoCl2(dpph)]2. Displacement ellipsoids are drawn with 50% thermal ellipsoids. The selected bond
distances (Å) and angles (deg) are Co1-Cl2 2.2143(10), Co1-Cl1 2.2254(9), Co1-P1 2.3595(9), Co1-P2 2.3865(10), Cl2-Co1-Cl1 111.13(4), Cl2-
Co1-P1 110.97(4), Cl1-Co1-P1 108.67(4), Cl2-Co1-P2 109.45(4), Cl1-Co1-P2 110.24(4), P1-Co1-P2 106.27(3). For further details, see Supporting
Information.
Scheme 2
The lengths of the P-Co and Cl-Co bonds are comparable to
nm arising from the ferrocenyl units. The spectra of the two
cobalt complexes are significantly different from those of
[CoCl2(dppp)] (λmax ) 584, 635, 735 nm in THF),15 [CoCl2-
(dpph)]2 (λmax ) 587, 639, 690 nm in THF), and [CoCl2(tms-
pfp)] (8, λmax ) 439, 605, 646, 706 nm in dichloromethane).
These facts are highly suggestive of the formation of stable [Co-
(CH2SiMe3)2(dpph)]n, where the suffix n means the unidentified
degree of aggregation and is most likely to be 1 or 2, upon
mixing cobalt chloride, DPPH, and 2 equiv of Me3SiCH2MgCl.
The formation of [Co(CH2SiMe3)2(dpph)]n is in stark contrast
to the generation of homoleptic (tetraphosphine)cobalt(0), [Co-
(dppp)2], upon treatment of [CoCl2(dppp)] with 2 equiv of
those of [CoCl2(dppp)].15
Motivated by the success, we tried to obtain a (trimethylsi-
lylmethyl)(dpph)cobalt complex for X-ray crystallographic
analysis. Sequential treatment of CoCl2 with DPPH and with 2
equiv of trimethylsilylmethylmagnesium chloride in ether
resulted in the formation of a greenish yellow solution with some
precipitates of a similar color (Scheme 2). However, no crystals
suitable for X-ray diffraction were available.
The UV-visible spectrum of the greenish yellow supernatant
showed two absorption bands at λmax ) 648 and 782 nm. These
absorption bands are quite similar to those of [Co(CH2SiMe3)2-
(tms-pfp)] (9).16 The cobalt complex 9 showed absorptions at
λmax ) 637 and 764 nm, in addition to absorptions at 450-550
(16) (a) Imamura, Y.; Mizuta, T.; Miyoshi, K.; Yorimitsu, H.; Oshima, K. Chem.
Lett. 2006, 35, 260-261. (b) Mizuta, T.; Imamura, Y.; Miyoshi, K.;
Yorimitsu, H.; Oshima, K. Organometallics 2005, 24, 990-996. (c) Mizuta,
T.; Imamura, Y.; Miyoshi, K. J. Am. Chem. Soc. 2003, 125, 2068-2069.
(d) Imamura, Y.; Mizuta, T.; Miyoshi, K. Organometallics 2006, 25, 882-
886.
(15) Heinze, K.; Muttner, G.; Zsolnai, L.; Schober, P. Inorg. Chem. 1997, 36,
5457-5469.
9
J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006 8071

A R T I C L E S
Scheme 3
Affo et al.
trimethylsilylmethylmagnesium chloride with concomitant lib-
eration of Me3SiCH2CH2SiMe3.16a Decomposition of [Co(CH2-
SiMe3)2(dpph)]n that forms Me3SiCH2CH2SiMe3 proceeded
much more slowly. It is worth noting that the deep red [Co-
(dppp)2] exhibited a spectrum completely different from that
of [Co(CH2SiMe3)2(dpph)]n. Therefore, the styrylation reaction
would start from [Co(CH2SiMe3)2(dpph)]n.
mechanism comprises an initiation step and a catalytic cycle.
The initiation step starts with the reaction of [CoIICl2(dpph)]
with Me3SiCH2MgCl to yield [CoII(CH2SiMe3)2(dpph)]. Al-
though an excess of the Grignard reagent was in the reaction
mixture, the formation of conceivable cobaltate complexes is
unlikely.17 [CoII(CH2SiMe3)2(dpph)] would undergo the single
electron-transfer process to yield [CoI(dpph)]+ and an alkyl
radical R•. The 12-electron cationic complex [CoI(dpph)]+ is
so electron-deficient that the complex can react immediately
with Me3SiCH2MgCl to furnish [CoI(CH2SiMe3)(dpph)].18 The
complex captures the benzylic radical that emerges through the
addition of the alkyl radical to styrene. Productive â-elimination
followed by rapid reductive elimination of tetramethylsilane
would yield [Co0(dpph)]. In the catalytic cycle, the zerovalent
cobalt complex would be instead responsible for the single
electron transfer. The catalytic cycle consists of reactions similar
to those in the initiation step. The only difference is that
reductive elimination does not occur after the single electron
transfer since [Co0(dpph)], the electron donor, has no Me3SiCH2
groups. Instead of [Co0(dpph)], an ate complex, [Co0(CH2-
SiMe3)(dpph)]-MgCl+, which is derived from [Co0(dpph)] and
1 equiv of the Grignard reagent, can effect the single electron
transfer (not shown in Scheme 3). In the alternative case, neutral
[CoI(CH2SiMe3)(dpph)] would be directly produced after the
single electron transfer, capturing the benzylic radical. Also note
that, even with the present mechanistic work, there still remains
Treatment of [CoCl2(tms-pfp)] with 2 equiv of Me3SiCH2-
MgCl followed by an addition of 10 equiv of benzyl bromide
provided 1 equiv of 1,2-diphenylethane and 1,2-bis(trimethyl-
silyl)ethane, along with 8 equiv of benzyl bromide recovered.
No trimethyl(2-phenylethyl)silane was detected. Sequential
treatment of CoCl2 with 1 equiv of DPPH, 2 equiv of Me3-
SiCH2MgCl, and 10 equiv of benzyl bromide led to the same
distribution of products. The formation of 1,2-diphenylethane
would stem from the homo-coupling of benzyl radicals. A
reductive elimination pathway can account for the production
of 1,2-bis(trimethylsilyl)ethane. The absence of the cross-
coupling product, trimethyl(2-phenylethyl)silane, is noteworthy.
Judging from these results, the following electron-transfer
process is most plausible for the initial single electron-transfer
step. The complex [CoII(CH2SiMe3)2(dpph)] (hereafter the suffix
n is omitted for clarity) would transfer a single electron to an
alkyl halide to yield [CoIII(CH2SiMe3)2(dpph)]+ and the radical
anion of the alkyl halide. The cobalt complex would undergo
instant reductive elimination to yield [CoI(dpph)] and 1,2-bis-
(trimethylsilyl)ethane. Meanwhile, the radical anion of the alkyl
halide would fragment into the corresponding alkyl radical and
halide anion.
(17) See the formation of [CoII(CH2SiMe3)2(tmeda)] in the presence of an excess
of Me3SiCH2Li: Hay-Motherwell, R. S.; Wilkinson, G.; Hussain, B.;
Hursthouse, M. B. Polyhedron 1990, 9, 931-937.
(18) The number of CH2SiMe3 groups on the cobalt is likely to be one, yet
zero or two is possible.
On the basis of these observations, we are tempted to propose
a mechanism for the cobalt-catalyzed reaction (Scheme 3). The
9
8072 J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006

Cobalt-Catalyzed Alkenylation of Alkyl Halides
A R T I C L E S
Table 4. Influence of Ligand in the Cobalt-Catalyzed
Scheme 4
Intramolecular Cyclization
entry
L
11a/%
12a/%
1
2
3
4
5
6
7
DPPM
DPPE
DPPP
DPPB
DPPPEN
DPPH
DPPF
58
8
28
74
20
9
8
8
68
84
78
69
81
12a. A higher temperature would facilitate the â-hydride
elimination that produces 11a as well as the single electron-
transfer process. The choice of the ligand was important. Use
of DPPM, DPPP, DPPPEN, DPPH, and DPPF furnished 11a
in lower yields. The reason for the advantage of DPPB over
DPPH in the intramolecular cyclization is not clear. Coordination
of the olefinic part to the cobalt complex upon single electron
transfer24 might occur, leading to the superiority of DPPB.
Surprisingly, 12a was obtained as a major product when DPPE
was employed. Only a small amount of deuterium was
incorporated at the methyl group of 12a upon quenching the
reactions with DCl/D2O. The formation of 12a thus supports a
radical cyclization mechanism, wherein a part of the corre-
sponding oxacyclopentylmethyl radical abstracts a hydrogen
atom from THF25 and the rest was trapped by a cobalt complex
to finally produce 11a (Scheme 4). Use of a trialkylsilylmethyl
Grignard reagent was again essential for a successful reaction.
For example, neopentylmagnesium bromide was inferior to the
trimethylsilylmethylmagnesium reagent and afforded 11a and
12a in 18% and 60% yields, respectively. Other alkyl Grignard
reagents such as butylmagnesium bromide also yielded 12a as
a major product. In the absence of [CoCl2(dppb)], 10a was left
untouched. The corresponding bromide of 10a was inferior to
the iodide 10a (59% of 11a and 20% of 12a).
Table 5 summarizes the results of the cobalt-catalyzed
intramolecular cyclization. Substrates with terminal alkenes
10b-10f participated in the cyclization to yield the correspond-
ing products in excellent yields. In addition to oxacycles, aza-
and carbocycles 11e and 11f were also readily accessible. In
each case of 11b-11f, its saturated analogue was detected in
less than 9% yield. Treatment of 10g with Me3SiCH2MgCl
under the catalysis of [CoCl2(dppb)] furnished bicyclic product
11g. Cyclization onto a trisubstituted alkene resulted in a
moderate yield of the expected isopropenyl-substituted product
11h. Use of DPPP instead of DPPB slightly increased the yield
of 11h to 66%. It is worth noting that small yet significant
amounts of the saturated analogue of 11h were obtained in both
cases (13% with DPPB, 14% with DPPP).
11
a possibility that the reaction proceeds via CoI and CoIII as
observed in vitamin B12- or cobaloxime-mediated reactions.8
Intramolecular Heck-Type Transformation: An Implica-
tion of the Mechanistic Duality of the Oxidative Addition
of Organic Halide to the Cobalt Complex. The intramolecular
Heck reaction of 1-halo-1,5-hexadiene derivatives is a powerful
and reliable method in constructing a diverse range of organic
molecules.19 On the other hand, palladium-catalyzed intra-
molecular cyclization reactions of 6-halo-1-hexene derivatives,
wherein fission of the sp3-hybridized carbon-halogen bond is
required, have rarely been reported.20 Alternatively, cobaloxime-
mediated cyclization allows for the conversion of 6-halo-1-
hexene derivatives into methylenecyclopentanes.21 In most cases,
a stoichiometric amount of cobaloxime and irradiation are
essential. No reports of cobaloxime-catalyzed cyclization of
6-halo-1-hexenes leading to methylenecyclopentanes are known.22
With the success of the intermolecular styrylation of alkyl
halides, we next examined the cobalt-catalyzed intramolecular
cyclization of 6-halo-1-hexenes into methylenecyclopentanes.23
The reaction of iodo acetal 10a was first examined (Table
4). Treatment of 10a with trimethylsilylmethylmagnesium
chloride in refluxing THF under [CoCl2(dppb)] catalysis
provided the desired product 11a in 84% yield, along with a
saturated compound 12a (entry 4). The intramolecular version
required reaction conditions slightly different from those of the
intermolecular reaction. THF was used as a solvent, and a higher
temperature was essential. The reaction at ambient temperature
in THF was slow and yielded a more complex crude mixture.
The reaction in ether at reflux resulted in a lower yield of 11a,
incomplete consumption of 10a, and an increased amount of
(19) (a) Bra¨se, S.; de Meijere, A. In Handbook of Organopalladium Chemistry
for Organic Synthesis; Negishi, E., Ed.; Wiley: New York, 2002; Vol. 1,
Chapter IV.2.2. (b) Nicolaou, K. C.; Sorensen, E. J. Classics in Total
Synthesis; VCH: New York, 1996; Chapter 31. (c) Dounay, A. B.;
Overman, L. E. Chem. ReV. 2003, 103, 2945-2963. Also see ref 2b.
(20) See ref 19b. However, in most cases, there are no hydrogens that are
involved in â-hydride elimination. An exception was the following:
Oppolzer, W.; De Vita, R. J. J. Org. Chem. 1991, 56, 6256-6257.
(21) (a) Pattenden, G. Chem. Soc. ReV. 1988, 17, 361-382. (b) Tada, M. Yuki
Gosei Kagaku Kyokaishi 1998, 56, 544-556. (c) Reference 8e.
Substrates 14 and 16, which securely undergo usual oxidative
addition to palladium, were subjected to the reaction conditions
(eqs 4 and 5).26 The prenyl ether of 2-iodophenol (14a) and
(22) Cobaloxime-catalyzed intermolecular Heck-type reaction under irradiation
was reported. See ref 8a. Vitamin B12-catalyzed reaction of 6-halo-1-hexene
derivatives giving vinylcyclopentanes, not methylenecyclopentanes, was
reported: (a) Busato, S.; Tinembart, O.; Zhang, Z.; Scheffold, R.
Tetrahedron 1990, 46, 3155-3166. (b) Busato, S.; Scheffold, R. HelV.
Chim. Acta 1994, 77, 92-99.
(24) Intramolecular coordination of a tethered alkene to a metal in action often
has a considerable effect on the overall transformation. For instance, see:
Miller, K. M.; Jamison, T. F. J. Am. Chem. Soc. 2004, 126, 15342-15343.
(25) Newcomb, M.; Curran, D. P. Acc. Chem. Res. 1988, 21, 206-214.
(26) For example, Larock reported a palladium-catalyzed cyclization reaction
of 14 leading to isopropyl-substituted benzofuran. Larock, R. C.; Stinn, D.
E. Tetrahedron Lett. 1988, 29, 4687-4690.
(23) A part of this section was communicated: Fujioka, T.; Nakamura, T.;
Yorimitsu, H.; Oshima, K. Org. Lett. 2002, 4, 2257-2259.
9
J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006 8073

A R T I C L E S
Affo et al.
Table 5. Cobalt-Catalyzed Intramolecular Cyclization of 6-Halo-1-hexene Derivativesa
a Conditions: 10 (0.50 mmol), the Grignard reagent (1.5 mmol), CoCl2 (0.05 mmol), DPPB (0.06 mmol), THF, reflux, 5 min. b Substrates having wavy
bonds are 1:1 mixtures of diastereomers. c Diastereomeric ratios of the products are in parentheses.
N,N-diprenyl-2-iodoaniline (16) were cleanly transformed to
isopropenyl-substituted heterocycles 15 and 17, respectively.
The behavior of the cobalt complex depends on the organic
halides, which will be confirmed in the next section.
Stereospecific Cross-Coupling Reaction of Alkenyl Halides
with Silylmethyl Grignard Reagents: Evidence of Substrate-
Dependent Oxidative Addition. The generation of an alkenyl
radical from an alkenyl halide would lead to the loss of the
original stereochemistry of the starting halide through a very
rapid isomerization of the alkenyl radical.27 Hence, a stereospe-
cific cross-coupling reaction of alkenyl halides with trimethyl-
silylmethylmagnesium chloride would be a suitable probe to
verify the nonradical oxidative addition mechanism of an sp2-
hybridized carbon-halogen bond to cobalt (eqs 6 and 7).
Bromide 14b and chloride 14c were less reactive. In the reaction
of 14c, 42% of 14c was recovered, and chlorophenol was
obtained in 20% yield.
In the reaction of 14 and 16, no traces of isopropyl-substituted
products were detected, which is intriguingly different from the
reaction of 10 having an sp3-C-X bond. If the relevant aryl
radical was generated, the subsequent radical cyclization would
form a tertiary alkyl radical. It is unlikely that the tertiary alkyl
radical would be completely trapped by a cobalt complex,
judging from the product distribution of the reactions of halo
acetals 10. Alternatively, it is quite reasonable that aryl halides
14 and 16 undergo oxidative addition to form aryl-cobalt bonds
in a fashion similar to the conventional oxidative addition of
aryl halide to palladium, instead of the generation of the
corresponding aryl radical via a single electron-transfer process.
Treatment of (E)-1-iodo-1-dodecene with trimethylsilylmeth-
ylmagnesium chloride in the presence of a catalytic amount of
anhydrous cobalt(II) chloride in THF at 25 °C afforded (E)-1-
trimethylsilyl-2-tridecene exclusively. On the other hand, (Z)-
(27) (a) Fessenden, R. W.; Shuler, R. H. J. Chem. Phys. 1963, 39, 2147-2195.
(b) Wedegaertner, D. K.; Kopchik, R. M.; Kampmeier, J. A. J. Am. Chem.
Soc. 1971, 93, 6890-6895.
9
8074 J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006

Cobalt-Catalyzed Alkenylation of Alkyl Halides
A R T I C L E S
Table 6. Preparation of Allylsilanesa
a Alkenyl halide (1.0 mmol), Me2RSiCH2MgCl (1.5 mmol), and CoCl2 (0.05 mmol) were employed. b Starting material (46%) was recovered.
1-iodo-1-dodecene solely afforded the corresponding (Z)-
allylsilane at -10 °C. The reaction of (Z)-1-iodo-1-dodecene
at 25 °C gave rise to contamination of (Z)-isomer with (E)-
trimethylsilyl-3-methyl-2-nonene in 74% yield upon treatment
with TMSCH2MgCl (entry 5). Unfortunately, the transformation
of (E)-5-iodo-5-decene resulted in a low yield (entry 6).
However, the reaction proceeded with a retention of configu-
ration. The retention of the stereochemistry was also observed
in the reaction of a stereoisomeric mixture of â-bromostyrene
isomer (∼10%). Use of ligands such as DPPH and triphen-
ylphosphine decreased the efficiency of the reaction, albeit with
no effects on the stereospecificity.
In light of the importance of allylsilanes in organic synthesis,28
a cobalt-catalyzed cross-coupling reaction of alkenyl halides
with trimethylsilylmethylmagnesium halide was investigated
(entry 7). It is worth noting that similar reactions in the presence
(Table 6). It is worth noting that ligandless cobalt(II) chloride
of styrene resulted in no significant changes. The anticipated
is much cheaper than palladium and nickel complexes.29,30 Not
Heck transformation was not observed. A propargylic silane
only alkenyl iodides but also alkenyl bromides and chlorides
was also prepared in a similar fashion (eq 9).
reacted effectively to give the corresponding allylsilanes in good
to excellent yields (entries 1 and 2). Isopropoxydimethylsilyl-
methylmagnesium chloride reacted as well as TMSCH2MgCl
(entry 3). The product enjoyed Tamao-Fleming oxidation31 to
furnish the corresponding allyl alcohol (eq 8). Trisubstituted
Diastereoselectivity in the Styrylation Reaction of 1-Alkoxy-
2-bromocycloalkane Derivatives. The present intermolecular
alkenyl iodide, (E)-1-iodo-2-methyl-1-octene, provided (E)-1-
cobalt-catalyzed Heck-type reaction allows us to use alkyl
halides having hydrogen or heteroatom at the â position to the
(28) (a) Sarkar, T. K. In Science of Synthesis; Fleming, I., Ed.; Georg Thieme
Verlag: Stuttgart, 2002; Vol. 4, Chapter 4.4.40, pp 837-925. (b) Tamao,
K. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I., Eds.;
halide atom. We thus examined the use of alkyl bromides having
stereocenters next to the brominated carbons in the intermo-
lecular styrylation reaction. The reaction inherently posed
interesting questions about the diastereoselectivity of the
intermolecular Heck-type reaction. The selectivity would origi-
nate from the steric reasons, i.e., styrene would approach
preferentially trans to the shielding substituent of the radical.32
We chose 1-alkoxy-2-bromocycloalkane derivatives as sub-
strates because of their ready availability through bromoetheri-
Pergamon Press: Oxford, 1991; Vol. 3, Chapter 2.2, pp 435-480. (c) Miura,
K.; Hosomi, A. In Main Group Metals in Organic Synthesis; Yamamoto,
H., Oshima, K., Ed.; Wiley-VCH: Weinheim, 2004; Vol. 2, Chapter 10.3.
(29) (a) Negishi, E.; Luo, F. T.; Rand, C. L. Tetrahedron Lett. 1982, 23, 27-
30. (b) Hayashi, T.; Konishi, M.; Okamoto, Y.; Kabeta, K.; Kumada, M.
J. Org. Chem. 1986, 51, 3772-3781.
(30) Cobalt-catalyzed alkenylation with organometallic reagents in THF/NMP:
(a) Cahiez, G.; Avedissian, H. Tetrahedron Lett. 1998, 39, 6159-6162.
(b) Avedissian, H.; Be´rillon, L.; Cahiez, G.; Knochel, P. Tetrahedron Lett.
1998, 39, 6163-6166. Cobalt-catalyzed monocoupling reaction of 1,2-
dihaloethylenes with silylmethylmagnesium reagents in THF/NMP to yield
(E)-3-halo-2-propenylsilanes was reported: (c) Kamachi, T.; Kuno, A.;
Matsuno, C.; Okamoto, S. Tetrahedron Lett. 2004, 45, 4677-4679. Cobalt-
catalyzed cross-coupling reaction of aryl halides with organometallic
reagents. (d) Korn, T. J.; Knochel, P. Angew. Chem., Int. Ed. 2005, 44,
2947-2951. (e) Korn, T. J.; Cahiez, G.; Knochel, P. Synlett 2003, 1892-
1894.
(32) (a) Curran, D. P.; Porter, N. A.; Giese, B. Stereochemistry of Radical
Reactions; VCH: Weinheim, 1996; Chapter 3. (b) Giese, B. Angew. Chem.,
Int. Ed. Engl. 1985, 24, 553-565. (c) Giese, B. Angew. Chem., Int. Ed.
Engl. 1989, 28, 969-980. (d) Giese, B.; Heuck, K.; Lenhardt, H.; Luning,
U. Chem. Ber. 1984, 117, 2132-2139. (e) Moufid, N.; Renaud, P. HelV.
Chim. Acta 1995, 78, 1001-1005.
(31) (a) Tamao, K.; Nakajima, T.; Kumada, M. Organometallics 1984, 3, 1655-
1660. (b) Fleming, I.; Henning, R.; Plaut, H. J. Chem. Soc., Chem. Commun.
1984, 29-31.
9
J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006 8075

A R T I C L E S
Affo et al.
fication of cycloalkenes with alcohols and conformational
restriction arising from their cyclic structures. The reaction of
trans-2-(tert-butyldiphenylsiloxy)-1-bromocyclohexane (18) with
p-methoxystyrene provided a mixture of diastereomers, trans-
19 and cis-19, in a ratio of 64:36 (eq 10, Ar means p-
methoxyphenyl group hereafter). We thought that the low
selectivity would result from the exclusive existence of the large
siloxy group at the equatorial position of the cyclohexane ring.
To resolve this problem, we considered the anomeric effect of
cyclic acetals 20, which permit an alkoxy substituent on the
acetal carbon to locate predominantly at the axial position and
hence would give rise to better diastereoselectivities (Scheme
5). To this end, bromo acetals 20a and 20b were exposed to
the cobalt-catalyzed styrylation reaction (eq 11). The reaction
Gratifyingly, the reaction gave the trans-adduct 23c exclusively
(trans/cis > 97:3). The acetal 23c would undergo a variety of
transformations to yield tetrahydrofuran derivatives.12i
Scheme 5
of the isopropyl acetal gave the highest trans/cis ratio, which
is unfortunately 69:31. These unsatisfactory selectivities would
be attributable to the rather flexible nature of six-membered
rings.
Since excellent stereocontrol is usually observed in a five-
membered ring system,32a,33 we thus turned our attention to the
reactions of 22 (eq 12). To our delight, excellent diastereose-
lectivity was observed. An anomeric effect operated in the
reaction of 22b to improve the selectivity compared to the case
of 22a.
Conclusion
The cobalt-catalyzed Heck-type reactions have a high po-
tential of eliminating the possibility of â-elimination, allowing
us to employ alkyl halides as coupling partners. In light of the
importance of the Heck reaction, the alkyl version of the Heck
reaction under the cobalt catalysis would make significant
progress in organic synthesis. Its stereoselective variant is also
available, in which the formation of a radical intermediate is
the key for the stereoselectivity. Mechanistic investigations have
revealed that single electron transfer from the cobalt complexes
[CoII(CH2SiMe3)2(dpph)] and [Co0(dpph)] to alkyl halides
induces the generation of the corresponding radical intermedi-
ates. The generation of radical intermediates is limited in case
alkyl halides were used. Alkenyl and aryl halides undergo
nonradical oxidative addition.
Bromoetherification of 2,3-dihydrofuran with enantiomeri-
cally pure (+)-isomenthol provided trans-bromo acetals (2S,3R)-
22c and (2R,3S)-22c in a ratio of 1:1. Subsequent chromato-
graphic separation of these diastereomers yielded both isomers
in enantiomerically pure forms. Use of other chiral alcohols such
as (-)-menthol and trans-2-phenyl-1-cyclohexanol resulted in
more inefficient separation. Enantiomerically pure (2S,3R)-22c
was subjected to the cobalt-mediated Heck styrylation reaction.
Noteworthy is the generation of a new carbon-cobalt bond
by the capture of a carbon-centered radical that is newly formed
after a certain radical transformation. As demonstrated in the
cobalt-catalyzed reaction, transition-metal-catalyzed radical
(33) Renaud, P. In Radicals in Organic Synthesis; Renaud, P., Sibi, M. P., Eds.;
Wiley-VCH: Weinheim, 2001; Vol. 1, Chapter 4.2.
9
8076 J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006

Cobalt-Catalyzed Alkenylation of Alkyl Halides
A R T I C L E S
reactions can offer novel transformation, by taking advantage
of both a radical reaction and transition metal catalysis.34
Culture, Sports, Science and Technology, Government of Japan.
W.A. acknowledges JSPS for financial support.
Supporting Information Available: Experimental details,
characterization data for new compounds, and UV-visible
spectra of cobalt complexes (PDF), and crystallographic data
of [CoCl2(dpph)]2 (CIF). This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. This work was supported by Grants-in-
Aid for Scientific Research from the Ministry of Education,
(34) (a) Gansa¨uer, A.; Bluhm, H. Chem. ReV. 2000, 100, 2771-2788. (b)
Knochel, P. Synlett 1995, 393-403. (c) Snider, B. B. Chem. ReV. 1996,
96, 339-363. (d) Molander, G. A.; Harris, C. R. Chem. ReV. 1996, 96,
302-338. Also see refs 9b, 12j, 12k, and 21.
JA061417T
9
J. AM. CHEM. SOC. VOL. 128, NO. 24, 2006 8077