Cobalt-catalyzed dimerization of alkenes

Article abstract of DOI:10.1016/j.tetlet.2004.04.085

In the presence of CoX₂(PPh₃)₂/3 PPh ₃ and zinc metal conjugated alkenes (CH₂CHCOOR, CH ₂CHCN, CH₂CHSO₂Ph and CH ₂CHCONEt₂) undergo reductive tail-to-tail dimerization to yield the corresponding saturated linear products. Under similar reaction conditions, vinylarenes (ArCHCH₂) give stereoselective head-to-tail dimerization products, trans-1,3-diarylbut-1-ene, in good to excellent yields.

Full text of DOI:10.1016/j.tetlet.2004.04.085

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 6203–6206
Cobalt-catalyzed dimerization of alkenes
Chun-Chih Wang, Pao-Shun Lin and Chien-Hong Cheng*
Department of Chemistry, National Tsing Hua University, Hsinchu, 300 Taiwan
Received 5 February 2004; revised 31 March 2004; accepted 7 April 2004
Available online 2 July 2004
Abstract—In the presence of CoX2(PPh3)2/3PPh and zinc metal conjugated alkenes (CH2CHCOOR, CH2CHCN, CH2CHSO2Ph
3
and CH2CHCONEt2) undergo reductive tail-to-tail dimerization to yield the corresponding saturated linear products. Under similar
reaction conditions, vinylarenes (ArCHCH2) give stereoselective head-to-tail dimerization products, trans-1,3-diarylbut-1-ene, in
good to excellent yields.
Ó 2004 Elsevier Ltd. All rights reserved.
Transition metal-catalyzed dimerization of olefins is
useful for synthesizing important intermediates for fine
and industrial chemicals.1;2 A widely studied example is
the dimerization of methyl acrylate catalyzed by Rh, Ru,
Pd and Ni complexes.3;4 This regioselective tail-to-tail
dimerization of methyl acrylate coupled with further
hydrogenation and hydrolysis has provided an attractive
alternative route to adipic acid, one of the monomers
used in the production of Nylon-6,6.5;6 In this work, we
wish to report two cobalt-catalyzed reaction modes: (a)
reductive dimerization of conjugated alkenes to yield
directly the saturated linear diesters, offering a more
convenient method to the synthesis of adipic acid (Eq. 1)
and, (b) cobalt-catalyzed head-to-tail dimerization of
vinylarenes in good yields (Eq. 3). To date, few effective
methods in the literature are known on the dimerization
of electron-withdrawing alkenes7;8 and vinylarenes.9;10
produced if no extra water was added. As shown by Eq.
1, the formation of 2a requires water and zinc metal as
the proton source and as the reducing agent, respec-
tively.
To understand the nature of the present catalytic reac-
tion, the catalytic activities of various metal complexes
for the reductive dimerization of butyl acrylate com-
plexes were examined. Both cobalt phosphine complexes
CoI2(PPh3)2, and CoCl2(PPh3)2 show catalytic activities
giving dibutyl adipate (2a) in 68% and 30% yields,
respectively. For cobalt complexes containing bidentate
ligands such as CoI2(dppe), CoCl2(dppe) and
CoI2(dppm) low activities were observed affording 2b in
50%, 28% and 22% yields, respectively. The addition of
extra PPh3 to the CoI2(PPh3)2 system reveals great
influence to the yield of product 2a. The addition of
extra PPh3 to the CoI2(PPh3)2 system first increases the
product yield. However, as the amount of PPh3 in-
creases to more than 3.2 equiv, inhibition of the catalytic
activity and decrease of product yield were observed.
The most efficient catalyst system for this reaction
consists of CoI2(PPh3)2 and 3.2 equiv of extra PPh3
giving the desired product in 90% yield. Nickel and
Butyl acrylate in the presence of CoI2(PPh3)2 (5 mol %),
PPh3 (16 mol %), zinc metal powder and water in ace-
tonitrile at 80 °C undergoes tail-to-tail reductive dimer-
ization to give product 2a (dibutyl adipate) in 90% yield.
The reaction is completely regioselective. No isomer
other than 2a was detected in the reaction mixture in the
1H NMR spectrum. Control reactions reveal that no
reductive dimerization occurred in the absence of a co-
balt complex or zinc metal and only a trace of 2a was
palladium
complexes
such
as
NiBr2(dppe),
NiCl2(PPh3)2, PdCl2(CH3CN) and Pd(dba)2 showed no
catalytic activity for the reductive dimerization under
similar reaction conditions.
The choice of solvent is also vital to the catalytic reac-
tion. Several common solvents were tested for the cat-
alytic reductive dimerization of butyl acrylate. The most
effective solvent is acetonitrile in which the reaction gave
Keywords: Reductive dimerization; Head-to-tail dimerization; Cobalt;
Acrylate; Vinylarene.
* Corresponding author. Tel.: +8-863-572-1454; fax: +8-863-572-4698;
e-mail: chcheng@mx.nthu.edu.tw
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.04.085

6204
C.-C. Wang et al. / Tetrahedron Letters 45 (2004) 6203–6206
Table 1. Results of Co-catalyzed reductive dimerization of conjugated
alkenesa
2a in 90% yield. Dichloromethane, THF and DMF are
less effective providing 2a in 63%, 33% and 26% yields,
respectively, while toluene is totally ineffective giving no
reductive dimerization product. The presence of a stoi-
chiometric amount of water as a proton source is nec-
essary for the catalytic reaction to proceed smoothly. In
general, 0.7–1 mol of water was added to the reaction
solution to achieve the highest yield of 2a. However, the
presence of a large excess of water led to deactivation of
the cobalt complex and the formation of hydrogenation
product of butyl acrylate, butyl propionate.
Entry
Substrate
Yieldb (%)
1
2
3
4
5
6
7
1a R ¼ CO2Bu
2a 90
2b 89
2c 96
2d 97
2e 89
2f 91
2g 62c
1b R ¼ CO2Me
1c R ¼ CO2CH2Ph-4-tBu
1d R ¼ CO2CH2PhO2CH2
1e R ¼ CN
1f R ¼ SO2Ph
1g R ¼ CONEt2
a Reaction conditions: substrate (2.00 mmol), CoI2(PPh3)2 (5 mol %),
Zn (3.0 mmol), H2O (0.7–1.0 mmol), PPh3 (0.16 mmol), CH3CN
(2.0 mL), 80 °C and 24 h.
b Isolated yields.
CoI (PPh ) / PPh
3
2
3 2
R
R
ZnO
Zn
H2O
R
c 1g (2.00 mmol), CoI2(dppe) (5 mol %), Zn (3.0 mmol), H2O (0.4 mmol),
CH3CN (1.0 mL), 1,4-dioxane (1.0 mL), 80 °C and 24 h.
o
CH CN / 80 C
3
2a-2g
1a R = CO Bu
2
1b R =CO Me
2
C(CH3)3
1c R = CO CH
2
2
Table 2. Results of Co-catalyzed dimerization of vinylarenesa
1d R = CO CH
2
2
O
O
Entry
Substrate
Yieldb (%)
1e R = CN
1f R = SO Ph
2
1
3a R ¼ H
4a 88
4b 86
4c 62
4d 94
4e 82
4f 89
4g 91
4h 89
4i 66
4j 74
4k 84
4l 79
1g R = CONEt
2
2
3b R ¼ 4-Ph
3c R ¼ 2-OMe
3d R ¼ 3-OMe
3e R ¼ 4-OMe
3f R ¼ 4-OAc
3g R ¼ 4-NH2
3h R ¼ 4-NPh2
3i R ¼ 2-F
3
ð1Þ
4
5
Similarly, acrylate 1b, 1c and 1d are also reductively
dimerized to give 2b, 2c and 2d in excellent yields. Other
electron-withdrawing alkenes also undergo reductive
tail-to-tail dimerization in the presence of CoI2(PPh3)2,
PPh3, zinc metal powder and water. Thus, acrylonitrile
1e afforded adiponitrile 2e and phenyl vinyl sulfone 1f
gave the corresponding reductive dimer 2f in 89% and
91% isolated yields, respectively. The reductive dimer-
ization can be further extended to N,N-disubstituted
acrylamide. Thus, N,N-diethyl acrylamide 1g in the
presence of CoI2(dppe) in a mixture of acetonitrile and
1,4-dioxane (v/v ¼ 1/1) is reductively dimerized to
afford product 2g in 62% yield (Table 1, entry 7 and Eq.
2). For each of these reductive dimerization reactions
only one isomer with the structure similar to 2a was
observed.
6
7
8
9
10
11
12
3j R ¼ 3-F
3k R ¼ 4-F
3l R ¼ 3-Cl
a Reaction conditions: vinylarene (2.00 mmol), CoCl2(PPh3)2
(10 mol %), Zn (3.0 mmol), EtOH (20 mol %), PPh3 (30 mol %),
CH3CN (2.0 mL), 80 °C and 18 h.
b Isolated yields.
CoCl2(PPh3)2 / PPh3 / Zn
R
R
R
CH3CN / EtOH / 80 oC / 18 h
4a-4l
3a-3l
ð3Þ
O
O
To understand the role of water and to help elucidate
the mechanism of the present catalytic reaction, an
isotope labelling experiment using D2O (99.9%) to re-
place normal water for the reductive dimerization of
vinyl phenyl sulfone 1f was performed. Analysis of
o
CoI (dppe) / Zn / 80 C
2
N
N
N
CH CN /1,4-Dioxane / H O
3
2
O
1g
2g 62%
ð2Þ
product 2f0 by H NMR shows that the two a-protons
1
were 87% deuterated and no deuteration for other pro-
tons in this product was observed. The isotope abun-
dance was determined by 1H NMR integration method.
Under similar catalytic reaction conditions, vinyla-
renes undergo head-to-tail dimerization instead of the
tail-to-tail reductive dimerization observed for acrylates.
Various reaction conditions for this head-to-tail dimer-
ization were investigated. Suitable catalytic reaction
conditions consisted of CoCl2(PPh3)2, PPh3, zinc metal
powder and EtOH. A range of vinylarenes (3a–l) with
different substituents on the aromatic rings were con-
verted to 1,3-diaryl-1-butenes in good to excellent yields
under the reaction conditions (Table 2). The catalytic
reactions appear insensitive to the substituent on the
aromatic ring. Vinylarenes with either an electron-
donating or withdrawing substituent on the aromatic
ring of vinylarenes were dimerized smoothly.
1
In agreement with the H NMR results, the 13C NMR
spectrum of 2f0 isolated from the above labelling study
reveals characteristic deuterium-coupled triplets at
54.9 ppm for the a-carbons.
D
H
SO2Ph
PhO2S
H
D
87%
2 f '
While the mechanism for the reductive dimerization of
acrylate is not clear, a possible pathway that can explain

C.-C. Wang et al. / Tetrahedron Letters 45 (2004) 6203–6206
6205
the observed tail-to-tail coupling of the two acrylates
Co(III)
and the highly regioselective deuteration by D2O of the
product is based on a cobalt(III) hydride as a key
intermediate as shown in Scheme 1. The catalytic reac-
tion is likely initiated by the reduction of the Co(II)
complex to a Co(I) species. Protonation of the latter by
water generated a cobalt(III) hydride. Insertion of an
acrylate molecule into the cobalt-hydride bond gives a
five-membered ring species. Further insertion of acrylate
molecule and protonation provides the expected reduc-
tive dimerization product. A similar pathway involving
a Rh(III) or a Co(III) hydride has been proposed to
catalyze the tail-to-tail dimerization of acrylate cata-
lyzed by rhodium and cobalt complexes.3a;7b;11
Co(III)-H
5
Co(III)
-Co(III)-H
Scheme 2.
(III)
Co
(III)
Co
O
The dimerization of vinylarenes can also be explained
based on an cobalt(III) hydride as the catalytic inter-
mediate. Insertion of the vinyl group into the Co–H
bond with the hydride adding to the terminal carbon
gives intermediate 5 as shown in Scheme 2. Further
insertion of a vinylarene to the Co–C bond in 5 via the
same regiochemistry followed by b-hydrogen elimina-
tion affords the dimerization product and regenerates
the cobalt(III) hydride (Scheme 2).
R
O
O
Ar
R
O
Ar
I
II
Scheme 3.
the saturated linear diesters, dinitrile and disulfone,
offering a convenient alternative method to the synthesis
of adipic acid or adiponitrile that are important mono-
mers for the production of Nylon-6,6. Secondly, we
have disclosed a very effective regio- and stereoselective
cobalt-catalyzed reaction of head-to-tail dimerization of
vinylarenes.
Another possible pathway involving cobaltacyclopen-
tanes I or II as key intermediates (Scheme 3) cannot be
ruled out. Coordination of two acrylates (vinylarenes) to
the Co(I) centre followed by cyclometallation gives
intermediate I (or II). Successive protonation of this
intermediate affords the final product 2. This mechanism
accounts well for the observed regiochemistry of prod-
uct 2 (or 4) and also explain nicely the results of the
foregoing deuterium-labelling experiment.
Acknowledgements
In conclusion, we have developed two unique methods
for coupling of alkenes. One is the reductive dimeriza-
tion of electron-withdrawing olefins catalyzed by cobalt
complexes. This cobalt-catalyzed reaction yields directly
We thank the National Science Council of the Republic
of China (NSC-91-2113-M-007-053) for the support of
this research.
Scheme 1.

6206
C.-C. Wang et al. / Tetrahedron Letters 45 (2004) 6203–6206
6. (a) Thomas, J. M.; Raja, R.; Sankar, G.; Bell, R. G. Acc.
References and notes
Chem. Res. 2001, 34, 191–200; (b) Niu, W.; Draths, K. M.;
Frost, J. W. Biotechnol. Prog. 2002, 18, 201–211; (c) Kiss,
G. Chem. Rev. 2001, 101, 3435–3456.
1. (a) Chauvin, Y.; Olivier, H. In Applied Homogeneous
Catalysis with Organometallic Compounds; Cornils, B.,
Herrmann, W., Eds.; VCH: New York, 1996; Vol. 1, pp
258–268; (b) Skupinska, J. Chem. Rev. 1991, 91, 613–648.
2. (a) Alderson, T.; Jenner, E.; Lindsey, R. V. J. Am. Chem.
Soc. 1965, 87, 5638–5645; (b) Brookhart, M.; White, P. S.;
DiRenzo, G. M. J. Am. Chem. Soc. 1996, 118, 6225–6234;
(c) Small, B. L.; Marcucci, A. J. Organometallics 2001, 20,
5738–5744; (d) Small, B. L. Organometallics 2003, 22,
3178–3183; (e) Mitsudo, T.; Suzuki, T.; Zhang, S.-W.;
Imai, D.; Fujita, K.; Manabe, T.; Shiotsuki, M.; Wata-
nabe, Y.; Wada, K.; Kondo, T. J. Am. Chem. Soc. 1999,
121, 1839–1850.
3. (a) Brookhart, M.; Sabo-Etienne, S. J. Am. Chem. Soc.
1991, 113, 2777–2779; (b) McKinney, R. J.; Colton, M. C.
Organometallics 1986, 5, 1080–1085; (c) Tkatchenko, I.;
Neibecker, D.; Grenouillet, P. Organometallics 1984, 3,
1130–1132; (d) Pertici, P.; Ballantini, V.; Salvadori, P.;
Bennett, M. A. Organometallics 1995, 14, 2565–2569.
4. (a) Barlow, M. G.; Bryant, M. J.; Haszeldine, R. N.;
Mackie, A. G. J. Organomet. Chem. 1970, 21, 215–226; (b)
Wilke, G. Angew. Chem., Int. Ed. Engl. 1988, 27, 185–206;
(c) Trost, B. M.; Toste, F. D.; Pinkerton, A. B. Chem. Rev.
2001, 101, 2067–2096; (d) Oehme, G.; Pracejus, H.
Tetrahedron Lett. 1979, 343–344.
7. (a) Inanaga, J.; Handa, Y.; Tabuchi, T.; Otsubo, K.;
Yamaguchi, M.; Hanamoto, T. Tetrahedron Lett. 1991,
32, 6557–6558; (b) Kanai, H.; Ishii, K. Bull. Chem. Soc.
Jpn. 1981, 54, 1015–1018; (c) Druliner, J. D.; Blackstone,
R. C. J. Organomet. Chem. 1982, 240, 277–283; (d)
Strohmeier, W.; Kaiser, A. J. Organomet. Chem. 1976,
114, 273–279.
8. (a) Arad, Y.; Levy, M.; Vofsi, D. J. Org. Chem. 1969, 34,
3709–3710; (b) Chiusoli, G. P.; Agnes, G.; Cometti, G.
Chem. Commun. 1968, 1515–1516; (c) Baizer, M. H.
Chemtech 1980, 161–164; (d) Danly, D. E. Chemtech 1980,
302–311.
9. (a) Tsuchimoto, T.; Kamiyama, S.; Negoro, R.; Shira-
kawa, E.; Kawakami, Y. Chem. Commun. 2003, 852–853;
(b) Corson, B. B.; Heintzelman, W. J.; Moe, H.; Rousseau,
C. R. J. Org. Chem. 1962, 27, 1636–1640; (c) Matsubara,
C.; Kojima, M. Tetrahedron Lett. 1999, 40, 3439–3442; (d)
Waratuke, S. A.; Johnson, E. S.; Thorn, M. G.; Fanwick,
P. E.; Rothwell, I. P. Chem. Commun. 1996, 2617–2618.
10. (a) Fornes, V.; Garcia, H.; Marti, V.; Fernandez, L.
Tetrahedron 1998, 54, 3827–3832; (b) Rao, V. J.; Prevost,
N.; Ramamurthy, V.; Kojima, M.; Johnston, L. J. Chem.
Commun. 1997, 2209–2210.
11. (a) Schrauzer, G. N.; Weber, J. H.; Beckham, T. M. J. Am.
Chem. Soc. 1970, 92, 7078–7086; (b) Brookhart, M.;
Hauptman, E. J. Am. Chem. Soc. 1992, 114, 4437–4439;
(c) Hauptman, E.; Sabo-Etienne, S.; White, P. S.; Brook-
hart, M.; Garner, M. J.; Fagan, P. J.; Calabrese, J. C.
J. Am. Chem. Soc. 1994, 116, 8038–8060.
5. (a) Parshall, G. W.; Ittel, S. D. Applied Homogeneous
Catalysis, 2nd ed.; Wiley-Interscience: New York, 1992,
p 242; (b) Weissermel, K.; Arpe, H. J. In Industrial
Organic Chemistry; 2nd ed.; Verlag Chemie: Weinheim,
Germany, 1993; (c) McKinney, R. J. U.S. Patent
4,504,674, 1985.