Article
Organometallics, Vol. 29, No. 19, 2010 4299
Scheme 1. Cobalt(I) Complexes Frequently Used for [2þ2þ2]
-20 °C).15 However, during our studies of cobalt-catalyzed
cycloaddition reactions,16 we came across the question of
whether the reactivity of 4 and the relative stability of com-
plexes like 3 or 5 can be combined to comparably stable but
more reactive Co(I) complexes. In addition we were interested
in the question of whether Co(I) complexes containing functio-
nalized olefins can be synthesized, because the direct prepara-
tion was hampered by the use of reactive alkali metals and no
other general synthetic access is known so far.17
In order to avoid the use of gaseous ethylene, we focused
on olefins that are liquids and easily removable from the
reaction mixture but yet stable against the strongly reductive
conditions of the complex preparation and also capable of
providing at least some significant bonding to the metal
center. These efforts led us to the application of trimethylvi-
nylsilane as the ligand of choice (Scheme 2). The convenient
reaction of cobaltocene and potassium in the presence of
trimethylvinylsilane at -78 °C resulted in the formation of 6
as a red compound in high yield (80%).18
Cycloaddition Reactions
special emphasis on cyclotrimerization reactions.10 The early
developments in this area with respect to complex organic
synthesis have been highlighted inter alia with the synthesis of
(()-estrone by Vollhardt et al.11
Today, complexes of the entire periodic table triad con-
sisting of cobalt, rhodium, and iridium have found broad use
in different cyclotrimerization reactions, favorably yielding
substituted arenes, heteroarenes, and biaryls,12 including the
asymmetric synthesis of biaryl compounds.13 In the case of
cobalt-based catalysts still a rather small number of isolable
CpCo(I) compounds is frequently applied (Scheme 1).
From the CpCo(I) complexes shown in Scheme 1, 1-4 have
been well known for a rather long time now, compared to
complex 5, which has been reported only recently by Gandon
et al.14 They exemplify the difficulties in balancing sufficient
stabilization of the catalytically active species on one side, while
on the other these spectator ligands must be labile enough to
liberate the active catalyst complex under preferably mild con-
ditions. If the stabilizing ligands are too tightly bound, these
catalyst complexes require external energy supply by either
heating to elevated temperatures or light or both together, to
efficiently generate the catalytically active species, such as 1-3
and 5. Complex 4 proved to be reactive at much lower tempera-
tures, but requires careful handling under inert conditions and
storage under an atmosphere of ethylene because even storage
under nitrogen or argon can lead to decomposition. Compound
3 can be handled for short periods of time in air, but heat or
irradiation is required to activate this catalyst precursor.
The single-crystal X-ray structure analysis confirmed the
formation of the [CpCo(H2CdCHSiMe3)2] (6) (Figure 1).19
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(15) Recent examples for the use of 4: (a) Sehnal, P.; Stara, I. G.;
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Saman, D.; Tichy, M.; Mısek, J.; Cvacka, J.; Rulısek, L.;
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Chocholousova, J.; Vacek, J.; Goryl, G.; Szymonski, M.; Cısarova, I.;
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Stary, I. Proc. Natl. Acad. Sci. 2009, 106, 13169. (b) Leboeuf, D.; Gandon,
V.; Aubert, C.; Malacria, M. Chem.;Eur. J. 2009, 15, 2129. (c) Amslinger,
S.; Aubert, C.; Gandon, V.; Malacria, M.; Paredes, E.; Vollhardt, K. P. C.
Synlett 2008, 2056. (d) Aubert, C.; Betschmann, P.; Eichberg, M. J.; Gandon,
V.; Heckrodt, T. J.; Lehmann, J.; Malacria, M.; Masjost, B.; Paredes, E.;
Vollhardt, K. P. C.; Whitener, G. D. Chem.;Eur. J. 2007, 13, 7443. (e)
Ampt, K. A. M.; Duckett, S. B.; Perutz, R. N. Dalton Trans. 2007, 2993. (f)
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Hrdina, R.; Stara, I. G.; Dufkova, L.; Mitchel, S.; Císarova, I.; Kotora, M.
Tetrahedron 2006, 62, 968. (g) Goswami, A.; Maier, C.-J.; Pritzkow, H.;
Siebert, W. J. Organomet. Chem. 2005, 690, 3251.
(16) (a) Hapke, M.; Kral, K.; Fischer, C.; Spannenberg, A.; Gutnov,
A.; Redkin, D.; Heller, B. J. Org. Chem. 2010, 75, 3993–4003. (b) Heller,
B.; Redkin, D.; Gutnov, A.; Fischer, C.; Bonrath, W.; Karge, R.; Hapke, M.
Synthesis 2008, 69. (c) Heller, B.; Gutnov, A.; Fischer, C.; Drexler, H.-J.;
Spannenberg, A.; Redkin, D.; Sundermann, C.; Sundermann, B. Chem.;
Eur. J. 2007, 13, 1117. (d) Gutnov, A.; Heller, B.; Fischer, C.; Drexler, H.-J.;
Spannenberg, A.; Sundermann, C.; Sundermann, B. Angew. Chem., Int. Ed.
2004, 43, 3795.
Results and Discussion
€
(17) Habermann, D. Ph.D. Thesis, Universitat Bochum, 1980.
Synthesis and Structure of the CpCo(I) Olefin Complexes.
The reductive abstraction of Cp ligands from transition
metal complexes is a known but rather little used methodol-
ogy. The usefulness was first demonstrated by the prepara-
tion of 4 from cobaltocene using alkali metals in the presence
of ethylene.7 This methodology also gave way for the pre-
paration of other CpCo(I) complexes with unfunctionalized
cyclic diolefins such as 1,5-cyclooctadiene (COD), yielding
compound 3. Since then, the use of 4 was reported in several
cases, partially including low reaction temperatures (up to
(18) Complex 6 has been reported to be synthesized by adaption of
the Jonas protocol, without description of experimental details, and the
NMR data have been provided: Lenges, C. P.; Brookhart, M.; Grant,
B. E. J. Organomet. Chem. 1997, 528, 199.
(19) (a) Crystal data for 6: C15H29CoSi2, Mr = 324.49, triclinic, space
˚
˚
˚
group P1, a = 6.8151(5) A, b = 11.5177(9) A, c = 11.9438(9) A, R =
3
˚
77.994(6)°, β = 75.525(6)°, γ = 87.846(6)°, V = 887.80(12) A , Z = 2,
F
calcd = 1.214 g cm-3, μ = 1.086 mm-1, T = 200 K, 10 000 reflections
3
measured, 3758 independent reflections (Rint = 0.0307), 3187 reflections
observed [I > 2σ(I)], final R indices [I > 2σ(I)]: R1 = 0.0265, wR2 =
0.0676, R indices (all data): R1 = 0.0334, wR2 = 0.0696, 193 refined
parameters. (b) Crystal data for 7: C9H11Cl6CoSi2, Mr = 446.99,
˚
˚
monoclinic, space group C2/c, a = 15.9701(11) A, b = 6.8829(3) A,
˚
3
˚
c = 15.3137(9) A, β = 96.381(5)°, V = 1672.9(2) A , Z = 4, Fcalcd
=
(10) (a) Bonnemann, H. Angew. Chem., Int. Ed. Engl. 1978, 17, 505.
1.775 g cm-3, μ = 2.106 mm-1, T = 200 K, 8882 reflections measured,
€
3
€
(b) Bonnemann, H. Angew. Chem., Int. Ed. Engl. 1985, 24, 248.
1573 independent reflections (Rint = 0.0294), of which 1210 were
observed [I > 2σ(I)], final R indices [I > 2σ(I)]: R1 = 0.0342, wR2 =
0.0845, R indices (all data): R1 = 0.0482, wR2 = 0.0886, 102 refined
parameters. (c) Crystal data for 8: C11H15CoO, Mr = 222.16, mono-
(11) Review: Vollhardt, K. P. C. Angew. Chem., Int. Ed. Engl. 1984,
23, 539.
(12) Recent reviews: (a) Chopade, P. R.; J. Louie, J. Adv. Synth.
Catal. 2006, 348, 2307. (b) Heller, B.; Hapke, M. Chem. Soc. Rev. 2007, 36,
1085. (c) Agenet, N.; Busine, O.; Slowinski, F.; Gandon, V.; Aubert, C.;
Malacria, M. Org. React. 2007, 68, 1. (d) Tanaka, K. Synlett 2007, 1977. (e)
clinic, space group P21/n, a = 11.3734(4), b = 7.4820(3), c = 11.7728(5)
3
A, β = 106.919(3)°, V = 958.45(7) A , Z = 4, Fcalcd = 1.540 g cm-3
,
˚
˚
3
μ = 1.744 mm-1, T = 200 K, 11 675 reflections measured, 2032
independent reflections (Rint = 0.0234), of which 1623 were observed
[I > 2σ(I)], final R indices [I > 2σ(I)]: R1 = 0.0210, wR2 = 0.0461, R
indices (all data): R1 = 0.0314, wR2 = 0.0478, 142 refined parameters.
(d) Crystal data for 9: C11H15Co, Mr = 206.16, monoclinic, space group
ꢀ
Scheuermann nee Taylor, C. J.; Ward, B. D. New J. Chem. 2008, 32, 1850. (f)
ꢀ
Hess, W.; Treutwein, J.; Hilt, G. Synthesis 2008, 3537. (g) Varela, J. A.; Saa,
C. Synlett 2008, 2571. (h) Galan, B. R.; Rovis, T. Angew. Chem., Int. Ed.
2009, 48, 2830, and cited references.
(13) (a) Bringmann, G.; Price Mortimer, A. J.; Keller, P. A.; Gresser,
M. J.; Garner, J.; Breuning, M. Angew. Chem., Int. Ed. 2005, 44, 5384. (b)
Tanaka, K. Chem. Asian J. 2009, 4, 508. (c) Shibata, T.; K. Tsuchikama, K.
Org. Biomol. Chem. 2008, 6, 1317.
˚
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˚
Pn, a = 5.6233(6) A, b = 12.5738(13) A, c = 6.7557(7) A, β =
100.803(8)°, V = 469.20(8) A , Z = 2, Fcalcd = 1.459 g cm-3, μ =
3
˚
3
1.767 mm-1, T = 200 K, 5314 reflections measured, 1911 independent
reflections (Rint = 0.0695), of which 1734 were observed [I > 2σ(I)], final
R indices [I > 2σ(I)]: R1 = 0.0412, wR2 = 0.0917, R indices (all data):
R1 = 0.0454, wR2 = 0.0930, 108 refined parameters.
(14) Geny, A.; Agenet, N.; Iannazzo, L.; Malacria, M.; Aubert, C.;
Gandon, V. Angew. Chem., Int. Ed. 2009, 48, 1810.