Kinetic studies on the cobalt-catalyzed norbornadiene intermolecular Pauson-Khand reaction

Article abstract of DOI:10.1021/om060935+

The kinetics for the cobalt-catalyzed intermolecular Pauson-Khand reaction (PKR) between (trimethylsilyl)acetylene and norbornadiene (NBD) at a constant CO pressure has been studied by means of in situ FT-IR. The rate dependence on catalyst and substrate concentrations was examined, and it was found that the process is -1.9 order with respect to CO pressure, zero order with respect to acetylene, 0.3-1.2 order with respect to NBD, and 1.3 order with respect to the Co₂(CO)₈ catalyst. Catalytic reaction intermediates were examined by their corresponding metal carbonyl IR frequencies. By a one-pot consecutive Pauson-Khand experiment, the NBD-dicobalt hexacarbonyl complex was identified as a catalytically active complex. Co₄(CO)₁₂ was also studied as a catalyst source in the PKR. Analysis of the corresponding reaction intermediates by IR demonstrated that Co₂(CO)₈ and Co₄(CO)₁₂ provide identical intermediate profiles upon reaction with TMSC₂H. The experimental measured kinetics are consistent with the alkene insertion being the rate-limiting step in the catalytic PKR. Finally, the effect of phosphine substitution on the catalyst and the use of Lewis acid additives were shown to have a deleterious effect on the reaction rate.

Full text of DOI:10.1021/om060935+

1134
Organometallics 2007, 26, 1134-1142
Kinetic Studies on the Cobalt-Catalyzed Norbornadiene
Intermolecular Pauson-Khand Reaction
Rafel Cabot, Agust´ı Lledo´, Marc Reve´s, Antoni Riera,* and Xavier Verdaguer*
Unitat de Recerca en S´ıntesi Asime`trica (URSA-PCB), Institute for Research in Biomedicine (IRB),
and Departament de Qu´ımica Orga`nica, UniVersitat de Barcelona, c/Josep Samitier, 1-5,
E-08028 Barcelona, Spain
ReceiVed October 11, 2006
The kinetics for the cobalt-catalyzed intermolecular Pauson-Khand reaction (PKR) between (trim-
ethylsilyl)acetylene and norbornadiene (NBD) at a constant CO pressure has been studied by means of
in situ FT-IR. The rate dependence on catalyst and substrate concentrations was examined, and it was
found that the process is -1.9 order with respect to CO pressure, zero order with respect to acetylene,
0.3-1.2 order with respect to NBD, and 1.3 order with respect to the Co2(CO)8 catalyst. Catalytic reaction
intermediates were examined by their corresponding metal carbonyl IR frequencies. By a one-pot
consecutive Pauson-Khand experiment, the NBD-dicobalt hexacarbonyl complex was identified as a
catalytically active complex. Co4(CO)12 was also studied as a catalyst source in the PKR. Analysis of the
corresponding reaction intermediates by IR demonstrated that Co2(CO)8 and Co4(CO)12 provide identical
intermediate profiles upon reaction with TMSC2H. The experimental measured kinetics are consistent
with the alkene insertion being the rate-limiting step in the catalytic PKR. Finally, the effect of phosphine
substitution on the catalyst and the use of Lewis acid additives were shown to have a deleterious effect
on the reaction rate.
catalytic PKR is well documented. Thus, soft and hard Lewis
bases, such as phosphine oxides,5 sulfides,6 sulfoxides,7 thio-
ureas,8 amines,9 and water,10 have been described to enhance
the catalytic PKR. The pressure of CO used in the catalytic
PKR is also a subject of debate. Initially, efficient catalytic
reactions were described at high temperature and CO pressure
(10-40 atm).11 Later, a PKR at atmospheric CO pressure using
Co2(CO)8 as catalyst was described.12 Several studies argue the
convenience of using high-intensity visible light as a promoter
Introduction
Since its discovery in the 1970s, the Pauson-Khand reaction
(PKR) has attracted major interest from the community of
chemists1 because it provides the most straightforward access
to cyclopentenone compounds, which in turn are valuable
synthetic intermediates in natural product synthesis.2,3 The
reaction was first described to be either promoted or catalyzed
by Co2(CO)8. Enormous research effort has since been devoted
to exploring other metal sources to effect this transformation.4
Despite this effort, dicobalt octacarbonyl has remained the most
simple and convenient choice to run the reaction.
(4) Titanium: (a) Hicks, F. A.; Kablaoui, N. M.; Buchwald, S. L. J.
Am. Chem. Soc. 1999, 121, 5881-5898. (b) Hicks, F. A.; Buchwald, S. L.
J. Am. Chem. Soc. 1999, 121, 7026-7033. Molybdenum: (c) Jeong, N.;
Lee, S. J.; Lee, B. Y.; Chung, Y. K. Tetrahedron Lett. 1993, 34, 4027-
4030. (d) Adrio, J.; Rivero, M. R.; Carretero, J. C. Org. Lett. 2005, 7, 431-
434. (e) Brummond, K. M.; Kerekes, A. D.; Wan, H. J. Org. Chem. 2002,
67, 5156-5163. Ruthenium: (f) Kondo, T.; Suzuki, N.; Okada, T.; Mitsudo,
T. J. Am. Chem. Soc. 1997, 119, 6187-6188. (g) Morimoto, T.; Fuji, K.;
Tsutsumi, K.; Kakiuchi, K. J. Am. Chem. Soc. 2002, 124, 3806-3807.
Iridium: (h) Shibata, T.; Takagi, K. J. Am. Chem. Soc. 2000, 122, 9852-
9853. Rhodium: (i) Jeong, N. Organometallics 1998, 17, 3642-3644. (j)
Jeong, N.; Sung, B. K.; Choi, Y. K. J. Am. Chem. Soc. 2000, 122, 6771-
6772. (k) Mukai, C.; Inagaki, F.; Yoshida, T.; Yoshitani, K.; Hara, Y.;
Kitagaki, S. J. Org. Chem. 2005, 70, 7159-7171.
At present, even when only Co2(CO)8 is used as a metal
source, a myriad of reaction conditions have been developed
to improve the reaction yields and broaden the substrate scope.
The use of Lewis bases as additives in the stoichiometric and
* To whom correspondence should be addressed. E-mail: xverdaguer@
pcb.ub.es (X.V.).
(1) (a) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E.; Foreman,
M. I. J. Chem. Soc., Perkin Trans. 1 1973, 977-981. (b) Khand, I. U.;
Knox, G. R.; Pauson, P. L.; Watts, W. E. J. Chem. Soc., Chem. Commun.
1971, 36.
(2) For selected reviews in the field see: (a) Brummond, K. M.; Kent,
J. L. Tetrahedron 2000, 56, 3263-3283. (b) Buchwald, S. L.; Hicks, F. A.
In ComprehensiVe Asymmetric Catalysis I-III; Springer-Verlag: Berlin,
1999; pp 491-510. (c) Geis, O.; Schmalz, H.-G. Angew. Chem., Int. Ed.
1998, 37, 911-914. (d) Gibson, S. E.; Stevenazzi, A. Angew. Chem., Int.
Ed. 2003, 42, 1800-1810. (e) Ingate, S. T.; Marco-Contelles, J. Org. Prep.
Proced. Int. 1998, 30, 121-143. (f) Schore, N. E. Org. React. 1991, 40,
1-90. (g) Sugihara, T.; Yamaguchi, M.; Nishizawa, M. Chem. Eur. J. 2001,
7, 1589-1595. (h) Gibson, S. E.; Mainolfi, N. Angew. Chem., Int. Ed. 2005,
44, 3022-3037. (i) Laschat, S.; Becheanu, A.; Bell, T.; Baro, A. Synlett
2005, 2547-2570.
(3) For recent examples of Pauson-Khand applications in total synthesis
see: (a) Caldwell, J. J.; Cameron, I. D.; Christie, S. D. R.; Hay, A. M.;
Johnstone, C.; Kerr, W. J.; Murray, A. Synthesis 2005, 3293-3296. (b)
Ishizaki, M.; Niimi, Y.; Hoshino, O.; Hara, H.; Takahashi, T. Tetrahedron
2005, 61, 4053-4065. (c) Tang, Y.; Zhang, Y.; Dai, M.; Luo, T.; Deng,
L.; Chen, J.; Yang, Z. Org. Lett. 2005, 7, 885-888. (d) Winkler, J. D.;
Lee, E. C. Y.; Nevels, L. I. Org. Lett. 2005, 7, 1489-1491.
(5) Billington, D. C.; Helps, I. M.; Pauson, P. L.; Thomson, W.; Willison,
D. J. Organomet. Chem. 1988, 354, 233-242.
(6) Sugihara, T.; Yamada, M.; Yamaguchi, M.; Nishizawa, M. Synlett
1999, 771-773.
(7) Chung, Y. K.; Lee, B. Y.; Jeong, N.; Hudecek, M.; Pauson, P. L.
Organometallics 1993, 12, 220-223.
(8) Tang, Y.; Deng, L.; Zhang, Y.; Dong, G.; Chen, J.; Yang, Z. Org.
Lett. 2005, 7, 593-595.
(9) Sugihara, T.; Yamada, M.; Ban, H.; Yamaguchi, M.; Kaneko, C.
Angew. Chem., Int. Ed. 1998, 36, 2801-2804.
(10) (a) Sugihara, T.; Yamaguchi, M. Synlett 1998, 1384-1386. (b)
Sugihara, T.; Yamaguchi, M.; Nishizawa, M. Chem. Eur. J. 2001, 7, 1589-
1595.
(11) Rautenstrauch, V.; Megard, P.; Conesa, J.; Kuester, W. Angew.
Chem., Int. Ed. Engl. 1990, 29, 1413-1416.
(12) (a) Pagenkopf, B. L.; Livinghouse, T. J. Am. Chem. Soc. 1996, 118,
2285-2286. (b) Belanger, D. B.; O’Mahony, D. J. R.; Livinghouse, T.
Tetrahedron Lett. 1998, 39, 7637-7640.
10.1021/om060935+ CCC: $37.00 © 2007 American Chemical Society
Publication on Web 01/30/2007

Cobalt-Catalyzed Pauson-Khand Reaction
Organometallics, Vol. 26, No. 5, 2007 1135
Scheme 1 . Catalytic Intermolecular PKR Used in the
Present Study
or extra-pure Co2(CO)8 in thermally activated processes.
Recently, commercial grade Co2(CO)8 has been shown to be
active in the catalytic PKR at atmospheric CO pressure in the
presence of cyclohexylamine.13
In contrast to other metal-catalyzed processes, a general
catalytic asymmetric version of the PKR has remained elusive.
Recent developments in this field have focused on intramo-
lecular reactions, many of which are mediated by metals other
than cobalt.4b,h,j Bis(phosphine) and bis(phosphite) dicobalt
carbonyl complexes have been successfully applied to the
intramolecular catalytic PKR.14 However, although in some
instances high enantiomeric excess is achieved, the catalytic
systems are not very active with respect to turnover frequencies
(TOF).
Our research group has a longstanding interest in the
development of enantioselective versions of the PKR. We have
developed several chiral auxiliary based asymmetric PKRs.15
More recently, we examined the use of chiral bidentate (P,S)
ligands in the stoichiometric intermolecular PKR.16 This led to
the effective synthesis of optically pure (+)-exo-4-(trimethyl-
silyl)tricyclo[5.2.1.02,6]-4,8-decadien-3-one (1). In turn, this
product has been shown to be a valuable synthon in the synthesis
of phytoprostanes and, thus, we focused our attention on the
large-scale preparation of 1 in racemic form.17 For this purpose,
we examined the cobalt-catalyzed preparation of 1 (Scheme 1).
To our knowledge, there are no reliable kinetic data on the
catalytic PKR. Here we report a complete kinetic study of Co2-
(CO)8-catalyzed intermolecular PKR between (trimethylsilyl)-
ethyne and norbornadiene by means of in situ FT-IR techniques.
Figure 1. Representative three-dimensional stack plot of IR spectra
collected every 5 min for the catalytic PKR between TMSC2H and
NBD.
at 2 bar. Toluene was the solvent of choice because of its high
boiling point. (Trimethylsilyl)ethyne (TMSC2H) was the limiting
substrate when not stated otherwise. The excess of norborna-
diene (NBD) was fixed at 2.0 equiv for most experiments. Under
the standard reaction conditions (77 °C, 2 bar of CO, [Co2-
(CO)8] ) 12.7 mM), a variation of stirring speed between 100
and 1100 rpm did not affect the initial reaction rates. This
excludes CO mass transport interferences in the resulting kinetic
data.19 The purity of dicobalt octacarbonyl catalyst was a major
concern, since it could greatly affect the kinetic measurements.
To check this, commercial Co2(CO)8 stabilized with hexanes
(10 wt %) was compared with a sublimed catalyst sample.
Reaction rates obtained using the purified catalyst were identical
with those of the commercial sample. Therefore, commercial
Co2(CO)8 without further purification was used in the study.20
In all cases reactions remained homogeneous throughout the
kinetic experiments.
Reaction progress analysis was performed by monitoring the
characteristic carbonyl (CdO) stretching absorbance of product
1 at 1698 cm-1. The carbonyl stretching of 1 was undisturbed
by other IR absorbances in the reaction mixture (Figure 1). From
all spectra collected every 5 min, an absorbance vs time profile
was constructed, which graphically reflects the reaction progress
(Figure 2). Gas chromatography (GC) analysis of the resulting
final reaction mixture showed that the tricyclopentenone 1 was
the major product (90%); however, the corresponding diketones
2a (5.2%) and 2b (4.8%) resulting from double PKR onto NBD
were also present (Figure 3).1a Assessment of the reaction
progress by GC analysis allowed us to determine that depletion
of the limiting substrate (alkyne) from the reaction mixture
brings the production of 1 to an end (Figure 2). GC analysis
confirmed that formation of 2a and 2b is a negligible side
reaction within the first 2 h of the catalytic PKR, and therefore,
it should not disturb initial reaction rate measurements. The
absorbance profile for the buildup of 1 showed that there is no
induction period in the PKR studied here.
Results and Discussion
Kinetic measurements were conducted under constant CO
pressure in a 50 mL medium-pressure glass reactor mounted
with a stainless steel cap and fitted with an immersible in situ
FT-IR SiComp probe. The CO pressure was controlled and
maintained constant in the reaction vessel by means of a low-
pressure manometer. To obtain data for the representative
reaction conditions in the synthesis of 1 and to avoid catalyst
decomposition, the working CO overpressure18 was fixed
between 1 and 4.0 bar; however, most reactions were performed
(13) Krafft, M. E.; Bon˜aga, L. V. R.; Hirosawa, C. J. Org. Chem. 2001,
66, 3004-3020.
(14) (a) Hiroi, K.; Watanabe, T.; Kawagishi, R.; Abe, I. Tetrahedron
Lett. 2000, 41, 891-895. (b) Gibson, S. E.; Kaufmann, K. A. C.; Loch, J.
A.; Steed, J. W.; White, A. J. P. Chem. Eur. J. 2005, 11, 2566-2576.
(15) (a) Verdaguer, X.; Moyano, A.; Perica`s, M. A.; Riera, A.; Bernardes,
V.; Greene, A. E.; Alvarez-Larena, A.; Piniella, J. F. J. Am. Chem. Soc.
1994, 116, 2153-2154. (b) Marchueta, I.; Montenegro, E.; Panov, D.; Poch,
M.; Verdaguer, X.; Moyano, A.; Perica`s, M. A.; Riera, A. J. Org. Chem.
2001, 66, 6400-6409.
The PKRs of norbornadiene (NBD) and norbornene are
known to be highly stereoselective, affording almost exclusively
exo cycloadducts, such as the one depicted in Scheme 1.1a
Analysis of the final reaction mixtures by HPLC revealed that
variable amounts of the corresponding endo cyclopentenone as
a minor product could be observed, depending on the reaction
(16) (a) Sola`, J.; Riera, A.; Verdaguer, X.; Maestro, M. A. J. Am. Chem.
Soc. 2005, 127, 13629-13633. (b) Verdaguer, X.; Moyano, A.; Perica`s,
M. A.; Riera, A.; Maestro, M. A.; Mahia, J. J. Am. Chem. Soc. 2000, 122,
10242-10243. (c) Verdaguer, X.; Lledo´, A.; Lopez-Mosquera, C.; Maestro,
M. A.; Perica`s, M. A.; Riera, A. J. Org. Chem. 2004, 69, 8053-8061.
(17) Iqbal, M.; Evans, P.; Lledo´, A.; Verdaguer, X.; Perica`s, M. A.; Riera,
A.; Loeffler, C.; Sinha, A. K.; Mueller, M. J. ChemBioChem 2005, 6, 276-
280.
(19) The stirring speed was set to 1100 rpm for all of the kinetic
measurements. A cross-shaped magnetic stirrer was used throughout the
present study. For the effect of stirring speed on asymmetric hydrogenations
see: Sun, Y.; Landau, R. N.; Wang, J.; LeBlond, C.; Blackmond, D. G. J.
Am. Chem. Soc. 1996, 118, 1348-1353.
(18) Unless stated otherwise, throughout the present study the working
CO pressure will be expressed in bar above atmospheric pressure.
(20) Sublimed Co2(CO)8 samples are far more sensitive to moisture and
oxygen than the corresponding commercial hexanes-stabilized samples.

1136 Organometallics, Vol. 26, No. 5, 2007
Cabot et al.
Figure 2. Reaction profile of the IR spectra collected every 5 min
for the catalytic PKR (black squares). Conditions: [TMSC2H]0 )
0.41 M, [NBD]0 ) 0.84 M, CO pressure 2 bar, 77 °C, [Co2(CO)8]
) 14.3 mM, 2 bar of CO, 77 °C. In addition, product formation
and acetylene depletion as determined by GC analysis are shown.
Figure 4. Reaction profile dependence on catalyst loading.
Conditions: [TMSC2H]0 ) 0.42 M, [NBD]0 ) 0.84 M, CO pressure
2 bar, 77 °C. [Co2(CO)8]: (black curve) 16.9 mM; (blue curve)
12.7 mM; (red curve) 8.5 mM; (green curve) 4.2 mM. Inset: plot
of log (initial rate) vs log [Co2(CO)8], providing a slope of 1.35.
Figure 3. Compounds resulting from double cyclization on NBD
detected in the catalytic intermolecular PKR.
conditions (vide infra). In the present study, both stereoisomers
were considered as a whole the product of the reaction.
The first issue we addressed was whether the present catalytic
system undergoes catalyst deactivation. To verify this, we used
reaction progress kinetic analysis tools. Blackmond and co-
workers showed that representation of the instantaneous reaction
rates vs the limiting substrate concentration is an excellent
method to determine whether a catalytic system undergoes
catalyst deactivation or product inhibition.21 Thus, we proceeded
to measure the reaction profile at two initial alkyne concentra-
tions, [TMSC2H]0 ) 0.42, 0.30 M, with the same excess of
NBD, [NBDexcess] ) 0.42 M. A graphical representation of the
reaction rate vs the alkyne concentration for these two experi-
ments provides two straight lines superimposed on each other
(see Figure 1-S in the Supporting Information).22 This observa-
tion implies that under the present experimental conditions
catalyst deactivation and product inhibition can be ruled out.
Thus, we can assert that Co2(CO)8 alone is a convenient catalyst
source for the catalytic intermolecular PKR.
Reaction order with respect to catalyst concentration was
studied with four catalyst loadings: i.e., 1, 2, 3, and 4 mol %
at 77 °C and 2 bar of CO. The corresponding reaction profiles
are shown graphically in Figure 4. The Co2(CO)8 concentration
greatly affected reaction rates. While the reaction at 4 mol %
of catalyst loading took 5 h to reach completion (black curve),
the reaction took around 15 h (red curve) when the loadings
were reduced to half (2 mol %). To find the corresponding
reaction order with respect to Co2(CO)8, a log/log plot of the
initial reaction rates vs the catalyst concentration was con-
structed.23 Linear fitting of data provided a slope of 1.35 with
an R factor of 0.99 (Figure 4).
Figure 5. Reaction profile dependence on NBD concentration.
Conditions: [TMSC2H]0 ) 0.42 M, [Co2(CO)8] ) 12.7 mM, CO
pressure 2 bar, 77 °C. [NBD]0: (black curve) 0.42 M; (red curve)
0.84 M; (green curve) 1.26 M; (blue curve) 1.68 M; (orange curve)
2.10 M; (pink curve) 3.07 M; (dark green curve) 4.20 M. Inset:
plot of log (initial rate) vs log [NBD].
The effect of acetylene concentration on the present catalytic
system was studied further. As shown in Figure 2, when an
excess of NBD was used, the reaction profile was suddenly
truncated when TMSC2H, the limiting substrate, was depleted
from the reaction media. In addition, four reactions with identical
[NBD]0 (0.42 M) and distinct [TMSC2H]0 (0.30, 0.42, 0.60,
and 0.79 M) were monitored and afforded identical initial rate
profiles (see Figure 2-S in the Supporting Information). This
behavior is indicative of zero-order (saturation) kinetics with
respect to acetylene concentration.
To check the contribution of alkene concentration, a series
of reactions were made at distinct [NBD]0 while maintaining
the rest of the parameters constant. The reactions were
performed using a range of NBD between 1 and 10 equiv (0.42-
4.20 M) with respect to TMSC2H. To find the order dependence
with respect to [NBD], a log/log plot of the initial reaction rates
vs the alkene concentration was constructed (Figure 5). From a
qualitative point of view, olefin concentration had a positive
effect on the reaction rate of the catalytic PKR. The reaction
(21) Blackmond, D. G. Angew. Chem., Int. Ed. 2005, 44, 4302-4320.
(22) The rate was obtained from fitting concentration-time data to an
exponential curve and differentiating this function to obtain rate-time data.
(23) Birk, J. P. J. Chem. Educ. 1976, 53, 704-707.

Cobalt-Catalyzed Pauson-Khand Reaction
Organometallics, Vol. 26, No. 5, 2007 1137
Table 1. Exo to Endo Ratio of 1 and Percentage of 2a + 2b
Table 2. Exo to Endo Ratio of 1 and Percentage of 2a + 2b
for the Catalytic PKR at Several NBD Concentrations
for the Catalytic PKR at Several CO Pressures
[NBD]0 (M (equiv))
exo:endoa
2a + 2b (%)a,b
abs CO press (bar)
exo:endoa
2a + 2b (%)a,b
0.42 (1)
1.26 (3)
1.68 (4)
4.20 (10)
22:1
17:1
14:1
10:1
14.3
2.8
2.1
2
3
4
6:1
19:1
34:1
3.2
6.2
0.8
a Determined by HPLC analysis of the final reaction mixture. b Percent-
age of areas of 2a and 2b with respect to total 1. No correction based upon
extinction coefficient has been applied.
a Determined by HPLC analysis of the final reaction mixture. b Percent-
age of areas of 2a and 2b with respect to total 1. No correction based upon
extinction coefficient has been applied.
Catalytic Intermediates in the PKR. Analysis of the metal
carbonyl area during the PKR may provide valuable information
on the catalytic species in the reaction media. Here we set up
the reactions by adding the catalyst, Co2CO8 (2069, 2042, 2023,
1849 cm-1 in toluene), to a mixture of TMSC2H and NBD.
From initial moments, three major absorbances were detected
at 2092, 2050, and 2023 cm-1 (Figure 7), which correspond to
the stretching of terminal COs in the Co2(CO)6-TMSC2H
complex 3 (Figure 8). It is well-known that the first step in the
stoichiometric PKR is the formation of the alkyne dicobalt
complex.1a These absorbances were the only visible signals in
the metal carbonyl region throughout the reaction. However,
when the limiting substrate was depleted, another metal carbonyl
compound emerged with absorbances at 2069, 2019 (broad),
and 1826 cm-1 (Figure 7).25 The absorbance observed at 1826
cm-1 is indicative of bridging carbonyl ligands but does not
correspond to the initial Co2(CO)8, as could be expected. Instead,
it corresponded to the dicobalt hexacarbonyl mono-NBD
complex 4.26 This was corroborated experimentally by heating
a mixture of Co2(CO)8 and NBD under a CO atmosphere (1
bar). Under these conditions, the corresponding Co2(CO)6-NBD
complex 4 was observed by in situ FT-IR spectroscopy as the
major metal carbonyl complex in solution. Preparation of 4 and
5 has been described by reaction of Co2(CO)8 and NBD under
nitrogen.27 Under these conditions, the major isolated compound
was Co2(CO)4-NBD2 (5; lit. 2021 s, 1798 s cm-1 in CCl4/
CS2), while the mono-NBD complex 4 (lit. 2083 s, 2030 vs,
1820 s cm-1 in CCl4/CS2) was isolated in low yield.
The cobalt carbonyl complex 4 formed at the end of the
reaction was catalytically competent, as demonstrated in a
consecutive PKR experiment (Figure 9). To this end, the
pressure reactor was charged with TMSC2H and NBD and 20
mol % of Co2(CO)8. The cyclization was performed as usual,
and when the first batch of acetylene was consumed, a new
fraction of TMSC2H and NBD was added. This procedure
allowed the reaction to restart up to four times, as observed in
the ladder-like absorbance profile depicted in Figure 9. For the
same experiment, in the metal carbonyl absorbance area, a
switch between the alkyne complex 3 and the olefin intermediate
4 can be observed graphically (Figure 10).
Figure 6. Initial rates vs absolute CO pressure. Reaction condi-
tions: 70 °C, [Co2(CO)8] ) 11.9 mM, [TMSC2H]0 ) 0.41 M,
[NBD]0 ) 0.82 M. Inset: plot of log (initial rate) vs log PCO
,
providing a slope of -1.93.
order with respect to NBD was not constant in the range of
NBD concentrations assayed and increased for high olefin
concentrations. Curve fitting of data shown in Figure 5 and
differentiation provided reaction orders between 0.3 and 1.2.23
Analysis of the reaction mixtures revealed that NBD concentra-
tion had an effect on the stereoselectivity of the PKR (Table
1). Thus, the ratio of endo adduct increased as the concentrations
of the alkene were augmented, while the amount of subproducts
2a and 2b in the final reaction mixture decreased.
We also performed a series of experiments to determine the
dependence of the intermolecular PKR with respect to CO
pressure. There has been much discussion about the need to
run the reaction under either high or low pressures of CO;12,13
however, to date, no kinetic data on this issue have been
reported. To address this question, four experiments were
performed at CO pressures between 1 and 4 bar and at 70 °C.
CO shows low solubility in organic nonpolar solvents. Under
our experimental conditions, the CO concentration ranged
between 8 and 40 mM.24 Initial reaction rates were measured
and plotted against absolute CO pressure (Figure 6). A negative
order dependence of the reaction rate with respect to CO
pressure was observed. A log/log plot of the initial reaction rates
vs the absolute CO pressure afforded a -1.93 (R ) 0.98) order
dependence with respect to CO pressure (Figure 6). The
influence of the CO pressure on the stereoselectivity of the
cyclization was also examined and was found to be critical
(Table 2). Low CO pressure maximizes the formation of the
minor endo adduct, while higher pressures afforded increased
exo selectivities.
The mixed order dependence with respect to catalyst con-
centration suggests that a dimerization pathway may occur to a
certain degree in the reaction mechanism. Pauson and Khand
described the formation of tetranuclear complex 6 and their
corresponding η6-arene complexes in the stoichiometric PKR.28
Tetracobalt dodecacarbonyl (6) is a catalyst for the PKR.13,29
(25) PKR in heptane provided absorbances for the new intermediate at
2073, 2023, 2011, and 1833 cm-1
.
(26) Under the present reaction conditions, the dicobalt octacarbonyl
complex was never observed by in situ FT-IR as an intermediate in the
catalytic PKR.
(27) Winkhaus, G.; Wilkinson, G. J. Chem. Soc. 1961, 602-605.
(28) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E. J. Chem.
Soc., Perkin Trans. 1 1973, 975-977.
(29) Krafft, M. E.; Bon˜aga, V. R. Angew. Chem., Int. Ed. 2000, 39,
3676-3680.
(24) (a) Lu¨hring, P.; Schumpe, A. J. Chem. Eng. Data 1989, 34, 250-
252. (b) Ja´uregui-Haza, U. J.; Pardillo-Fontdevila, E. J.; Wilhelm, A. M.;
Delmas, H. Latin Am. Appl. Res. 2004, 34, 71-74.

1138 Organometallics, Vol. 26, No. 5, 2007
Cabot et al.
Figure 7. Metal carbonyl zone spectra during the catalytic PKR. The blue line corresponds to complex 3 (2092, 2050, and 2023 cm-1) at
the halfway point of conversion, and the red line corresponds to compound 4 (2069, 2019 (broad), and 1826 cm-1) at the end of the
reaction.
Figure 8.
Figure 9. Reaction profile for a one-pot consecutive catalytic PKR.
Reaction conditions: 80 °C, 2 bar of CO, 20 mol % initial catalyst
loading.
However, there are no data available on the true catalytic species
when Co4(CO)12 is used as a metal source. To shed light on
this matter, a catalytic PKR between TMSC2H and NBD using
6 as a catalyst was set up and monitored by in situ FT-IR (Figure
11). Initial reaction spectra of the metal carbonyl area showed
unreacted Co4(CO)12 (6); however, at 77 °C under 2 bar of CO,
the reaction rapidly evolved to provide the corresponding
dicobalt alkyne complex 3 as a major metal carbonyl species
in the reaction media. From this point, the catalytic PKR reaction
evolved in a manner similar that to when Co2(CO)8 was used
as the metal source, and upon depletion of the alkyne substrate,
the olefin complex 4 emerged as the final metal carbonyl
product. As measured experimentally, TMSC2H alone was
responsible for the breakdown of the tetracobalt cluster into 3.
Figure 10. Terminal metal carbonyl zone (2100-1900 cm-1) for
Reaction of 6 with the alkyne at 50 °C and 2 bar of CO provided
the consecutive catalytic PKR experiment. A switch between
complexes 3 and 4 is observed when the alkyne is depleted.
full conversion to 3 within 45 min, as observed by FT-IR
(Scheme 2). Under identical experimental conditions, reaction
of 6 with NBD provided no conversion to the corresponding
research effort has been devoted to validate the PKR mechanism
alkene complex 4.30
by theoretical methods. Nakamura and Perica`s independently
Kinetics and Mechanism for the Catalytic PKR. In 1985,
reported a complete DFT study for the transformation of I to
Magnus proposed what is now the commonly accepted mech-
V.32 Other authors have focused on the alkene coordination and
anism for the stoichiometric PKR.31 This mechanism describes
alkene insertion steps to explain the regio- and stereoselectivity
the transformation of the alkyne hexacarbonyl complex I into
observed in the PKR.33 For a catalytic cycle to conform to the
the dicobalt cyclopentenone complex V (Scheme 3). Intensive
Magnus mechanism, an alkyne-cyclopentenone exchange
would suffice to regenerate I from V (Scheme 3). This represents
the most straightforward manner to close a catalytic cycle for
(30) Conversion of Co4(CO)12 into 5 by reaction with NBD has been
described; see: Kitamura, T.; Joh, T. J. Organomet. Chem. 1974, 65, 235-
243.
(31) Magnus, P.; Exon, C.; Albaugh-Robertson, P. Tetrahedron 1985,
41, 5861-5869.
(32) (a) Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2001, 123,
1703-1708. (b) Va´zquez, J. Ph.D. Thesis, Universitat de Barcelona, 1999.

Cobalt-Catalyzed Pauson-Khand Reaction
Organometallics, Vol. 26, No. 5, 2007 1139
Scheme 3 . Magnus Mechanism for the Stoichiometric PKRa
Figure 11. Metal carbonyl zone spectra during the catalytic PKR
using Co4(CO)12 as a catalyst. The initial reaction mixture spectrum
(black line) shows absorbances of Co4(CO)12 (2053 and 1860 cm-1
)
and TMSC2H (2034 cm-1). After 1 h of reaction at 77 °C (blue
line), major carbonyl absorbances correspond to the alkyne complex
3 (2092, 2050, and 2023 cm-1).
a CO ligands have been omitted for clarity.
Scheme 2 . Breakdown of the Tetracobalt Dodecarbonyl
Cluster with the Alkyne
Scheme 4 . Stoichiometric PKR of 3
the PKR. From the experimental point of view, however, little
is known about the intermediate species involved in the PKR.
To date, the corresponding dicobalt alkyne hexacarbonyl
complex I is the only intermediate species that has been isolated
and identified in the mechanism.34
of Nakamura.32 However, the influence of the CO pressure and
NBD concentration on the stereoselectivity of the final PKR
adducts is difficult to rationalize with this simple mechanism.
We have shown that low CO pressures and high NBD
concentrations (vide infra) favor the formation of endo cycliza-
tion adducts (up to 14%). In contrast, the stoichiometric thermal
reaction (N2, 77 °C) of 3 with NBD provided only 1% of the
corresponding endo adduct (Scheme 4). The differential behavior
between the stoichiometric and the catalytic reactions reveals
that a second reaction pathway might be in operation in the
catalytic reaction.
Exo and endo PKR adducts arise from stereoselective
coordination of the double bond contained in NBD to the metal
center. Thus, the complex II-exo will provide the adduct 1-exo,
while the complex II-endo is the origin of 1-endo products
(Scheme 5). Although II-endo is energetically less favored that
the corresponding II-exo complex,35 formation of II-endo from
I through CO-alkene ligand exchange is expected to take place
in the same proportion for both the catalytic and the stoichio-
metric reaction conditions. Thus, if we consider only equilibrium
processes between complexes I and II-exo/II-endo, it is difficult
to rationalize the difference in stereoselectivity for these two
processes.
From our experimental data, the rate equation for the catalytic
intermolecular PKR between TMSC2H and NBD is
ν ) k[Co2(CO)8]1.3[NBD]0.3-1.2[CO]-1.9
Thus, the reaction rate is overall zero order with respect to
alkyne concentration, 1.3 order with respect to catalyst con-
centration, between 0.3 and 1.2 order with respect to [NBD],
and -1.9 order with respect to CO concentration. The observed
rate equation suggests that a complex reaction mechanism is in
operation in the catalytic intermolecular PKR of norbornadiene.
Despite this complexity, a few conclusions may be drawn. The
positive dependence on NBD and negative dependence on CO,
along with the observation that the corresponding dicobalt
alkyne complex I acts as a major resting state suggests that the
CO-alkene ligand exchange is an energetically demanding
equilibrium process preceding the rate-limiting step. Thus, the
following alkene insertion to give III would be the rate-limiting
step in the catalytic process. In this scenario, a higher concentra-
tion of NBD and lower concentration of CO are beneficial for
the kinetics of the reaction, since they favor formation of the
key intermediate II.
As we have shown in the consecutive PKR experiment, the
NBD complex 4 is formed quantitatively in the absence of
alkyne and is a catalytically viable intermediate in the regenera-
tion of 3 (I in Scheme 5). Formation of 4 under CO pressure
discloses the tendency of NBD to act as a bidentate chelating
ligand. Most interestingly, only double endo coordination allows
NBD to work as a bidentate ligand. This double coordination
of NBD will be facilitated under low [CO] and high [NBD].
These experimental data are consistent with the overall
catalytic cycle shown in Scheme 3 and the theoretical studies
(33) (a) Robert, F.; Milet, A.; Gimbert, Y.; Konya, D.; Greene, A. E. J.
Am. Chem. Soc. 2001, 123, 5396-5400. (b) de Bruin, T. J. M.; Milet, A.;
Robert, F.; Gimbert, Y.; Greene, A. E. J. Am. Chem. Soc. 2001, 123, 7184-
7185. (c) de Bruin, T. J. M.; Milet, A.; Greene, A. E.; Gimbert, Y. J. Org.
Chem. 2004, 69, 1075-1080.
(34) Dicobalt norbornene alkyne bis(diphenylphosphino)methane com-
plexes have been detected by electrospray ionization mass spectroscopy;
see: Gimbert, Y.; Lesage, D.; Milet, A.; Fournier, F.; Greene, A. E.; Tabet,
J.-C. Org. Lett. 2003, 5, 4073-4075.
(35) Endo coordination of NBD is energetically disfavored by ap-
proximately 4.0 kcal/mol, as determined by DFT (B3LYP/LACVP) calcula-
tions.

1140 Organometallics, Vol. 26, No. 5, 2007
Scheme 5 . Feasible Ligand Exchange Processes in the Catalytic PKR of Norbornadienea
Cabot et al.
a Metal carbonyl ligands have been omitted for clarity.
species [Co(CO)3L2]+ and [Co(CO)4]-.38 This phenomenon is
known to be reversible, and it may occur to a certain degree in
the presence of NBD. Nevertheless, none of these species were
observed by FT-IR analysis of the reaction mixtures. The
intrinsic sensitivity threshold for this technique allows only
detection of major catalytic species; minor catalytic components
that may have a major kinetic effect could not be detected.
Dicobalt Phosphine Derivatives. Hexacarbonyl alkyne com-
plexes and monophosphine complexes of Co2(CO)8 have been
described as catalyst substitutes in the PKR.39,40 Among these,
phosphine complexes are of particular interest, since the
phosphorus ligand remains attached to the metal complex
throughout the catalysis. This is an interesting feature for the
development of asymmetric versions of this process. In par-
ticular, dicobalt complexes modified with chiral diphosphines
have been used successfully in the catalytic asymmetric
intermolecular PKR.14 The most significant drawback for these
systems is the lack of reactivity, which has been attributed to a
decreased CO dissociation caused by the presence of phosphine
ligands.41 In this context, we synthesized several dicobalt
phosphine complexes and studied how the electronic and steric
effects of the phosphine moiety influence the catalytic inter-
molecular PKR.
Table 3. Synthesis of Monophosphine Dicobalt Complexes
entry
no.
yield pro-
(%) duct
L2
PR3
conditions
1
2
3
4
5
2 CO
2 CO
2 CO
PPh3
P(OPh)3
THF, room temp
THF, room temp
84
48
60
70
22
7
8
9
10
11
P(OC6H3(But)2)3 hexane, room temp
TMSC2H P(C6H3(CF3)2)3
TMSC2H PCy3
toluene, 60 °C
DME, 60 °C
These are exactly the conditions in which the highest ratio of
the endo adduct is observed. Although no direct evidence
confirms 4 as a catalytically significant intermediate, our
experimental results suggest that involvement of 4 in the
catalytic process might be the origin of the endo adduct. From
4, reaction with the alkyne could provide II-endo/endo, which
after CO uptake would yield II-endo (Scheme 5). Thus, within
the catalytic cycle, formation of 4 could account for the
formation of II-endo and ultimately the endo PKR adduct. This
second reaction pathway is only possible for the catalytic process
and could explain the difference in stereoselectivity between
the stoichiometric and the catalytic reactions.
The synthesis of dicobalt monophosphine complexes is
described in Table 3. Co2(CO)7PR3 complexes 7-9 were
prepared by following small variations of procedures described
in the literature, which gave good to excellent yields (Table 3,
entries 1-3).39 In contrast, direct reaction of Co2(CO)8 with
P(C6H3(CF3)2)3 and PCy3 led to decomposition of the dicobalt
complex. In these two cases, the corresponding dicobalt
pentacarbonyl TMSC2H complexes 10 and 11 were synthesized
as dicobalt monophosphine surrogates (Table 3, entries 4 and
5). With the phosphine complexes in hand, their efficiency in
the catalytic PKR was examined at 80 °C and 2 bar of CO
(Figure 12). On the basis of the reaction profiles in Figure 12,
Finally, there is no direct evidence to explain the mixed order
dependence with respect to [Co2(CO)8]. As for most metal-
catalyzed transformations, first-order kinetics with respect to
catalyst concentration should be expected for the mechanism
depicted in Scheme 4.21 A feasible explanation could be that a
dimerization process occurs to a certain degree in the reaction.
Cobalt carbonyl complexes are postulated to be involved in
higher order bimolecular reaction mechanisms, as in the case
of cobalt-catalyzed hydroformylation.36 The Co2(CO)8 complex
dimerizes to afford the Co4(CO)12 cluster (6) under thermal
activation.37 Another possibility is the breakup of the bimetallic
catalyst into smaller fragments. In the presence of phosphine
ligands Co2(CO)8 suffers disproportionation into the ionic
(38) Szabo, P.; Fekete, L.; Bor, G.; Nagy-Magos, Z.; Marko, L. J.
Organomet. Chem. 1968, 12, 245-248.
(36) For examples of bimetallic hydroformylation mechanisms see: (a)
van Boven, M.; Alemdaroglu, N. H.; Penninger, J. M. L. Ind. Eng. Chem.
Prod. Res. DeV. 1975, 14, 259-264. (b) Azran, J.; Orchin, M. Organo-
metallics 1984, 3, 197-199. (c) Li, C.; Widjaja, E.; Garland, M. J. Am.
Chem. Soc. 2003, 125, 5540-5548.
(37) (a) Ungvary, F.; Marko, L. Inorg. Chim. Acta 1970, 4, 324-326.
(b) Ungvary, F.; Marko, L. J. Organomet. Chem. 1974, 71, 283-286.
(39) (a) Comely, A. C.; Gibson, S. E.; Stevenazzi, A.; Hales, N. J.
Tetrahedron Lett. 2001, 42, 1183-1185. (b) Gibson, S. E.; Johnstone, C.;
Stevenazzi, A. Tetrahedron 2002, 58, 4937-4942.
(40) Belanger, D. B.; Livinghouse, T. Tetrahedron Lett. 1998, 39, 7641-
7644.
(41) van Leuven, P. W. N. M. Homogeneous Catalysis; Kluwer
Academic: Dordrecht, The Netherlands, 2004.

Cobalt-Catalyzed Pauson-Khand Reaction
Organometallics, Vol. 26, No. 5, 2007 1141
Table 4. Product Distribution for Catalytic PKR Performed
in the Presence of Additives
additive
1-exo (%)
1-endo (%)
2a (%)
2b (%)
exo:endo
88.2
92.3
89.3
88.7
90.8
6.3
2.3
6.3
7.0
6.0
3.8
4.0
3.2
3.3
2.3
1.7
1.4
1.2
1.0
0.9
14:1
40:1
14:1
13:1
15:1
TMTU
H2O
DME
CyNH2
were performed at 70 °C and 3 mol % of Co2(CO)8 in toluene
and 0.5 bar of CO overpressure.42 Reaction profiles when using
TMTU (18 mol %), water (17 mol %), DME (12 mol %), and
CyNH2 (18 mol %) were compared with a reaction with no
additives (Figure 13). Recorded data are shown up to 11 h,
which is the time the reaction lacking any additive takes to reach
completion. From this graphic, we can conclude that the addition
of Lewis base additive had a deleterious effect on the reaction
rate (Figure 13). Thus, cyclohexylamine produced the largest
adverse effect on the reaction rate, while TMTU provided only
a slight decrease with respect to Co2(CO)8 alone. Product
selectivity for reactions in the presence of additives provides
further data for analysis (Table 4). Reactions using water, DME,
or CyNH2 provided a product distribution similar to that of
dicobalt octacarbonyl alone. However, TMTU afforded higher
exo/endo selectivity, up to 40:1. This observation indicates that,
under these experimental conditions, TMTU may effectively
bind to the catalyst while other additives produce only catalyst
decomposition. Thus, we can conclude that Lewis base additives
do not produce an effective rate increase in the intermolecular
PKR of NBD. However, their presence may either enhance
product selectivity or improve product isolation.
Figure 12. Reaction profile using a range of cobalt catalysts.
Reaction conditions: [cat] ) 11.9 mM, [TMSC2H]0 ) 0.41 M,
[NBD]0 ) 0.82 M, 80 °C, 2 bar of CO.
Conclusion
Reaction progress analysis performed by in situ FT-IR has
allowed the study of the kinetics of a cobalt-catalyzed inter-
molecular PKR for the first time. Thus, rate dependences with
respect to the individual substrates were calculated, and we
observed that the reaction was -1.9 order with respect to CO
pressure, zero order with respect to TMSC2H, between 0.3 and
1.2 order with to respect to NBD, and 1.3 order with respect to
Co2(CO)8. IR analysis of the metal carbonyl intermediates in a
consecutive catalytic PKR experiment revealed two major
reaction intermediates: the corresponding dicobalt hexacarbonyl
alkyne complex 3, which is the major catalyst resting state, and
the dicobalt hexacarbonyl NBD complex 4, which can be
observed in the absence of the alkyne. These experimental
observations suggest that the corresponding alkene insertion is
the rate-limiting step in the catalytic process studied here. A
detailed ligand exchange process between the dicobalt carbonyl
species is proposed to account for the difference in stereose-
lectivity between the stoichiometric and the catalytic process.
The use of Co4(CO)12 as a catalyst source in the PKR by FT-
IR was also studied, and we have shown that upon reaction
with TMSC2H both Co2(CO)8 and Co4(CO)12 provide identical
intermediate profiles. We also examined the use of phosphine-
modified catalysts and the addition of Lewis bases. Both
strategies had a deleterious effect on the reaction kinetics with
respect to the use of dicobalt octacarbonyl alone.
Figure 13. Reaction profile upon addition of distinct Lewis base
additives. Reaction conditions: 70 °C, [Co2(CO)8] ) 11.9 mM,
[TMSC2H]0 ) 0.41 M, [NBD]0 ) 0.82 M, 0.5 bar of CO.
we can draw a major conclusion: the introduction of a
phosphine ligand on Co2(CO)8 has a deleterious effect on the
reaction rate with respect to the parent system. Among the
phosphine complexes studied, electron-rich PCy3 presented the
lowest reaction rate, while electron-deficient P(C6H3(CF3)2)3
provided the fastest rate. The electronic effects observed are in
agreement with the notion that CO dissociation is an energeti-
cally demanding step in the catalytic cycle. Electron-rich
phosphines hamper CO dissociation because of metal-CO back-
bonding, while electron-deficient phosphines show the opposite
behavior.41
Other Lewis Base Additives. Nucleophilic substances have
been used as additives in either the stoichiometric or the catalytic
PKR. It has been postulated that Lewis base additives facilitate
the dissociative step, in which a carbonyl ligand is replaced by
the incoming olefin. Among the additives used for this purpose,
perhaps the most widely studied are cyclohexylamine, water,
and dimethoxyethane.9,10 Tetramethylthiourea (TMTU) has
recently been used as a promoter for the catalytic intramolecular
PKR.8 In some instances, the use of additives are claimed to
allow the catalytic PKR to run at atmospheric CO pressure.8
To see whether these substances have truly a positive rate
enhancement in the catalytic intermolecular PKR, we tested
several additives and monitored the reaction by FT-IR. Reactions
Experimental Section
Reagents and Methods. Solvents were distilled from appropriate
reagents prior to use. Co2(CO)8 stabilized with hexane (10 wt %)
(42) A low overpressure of CO was selected to mimic reactions at
atmospheric pressure (balloon pressure).

1142 Organometallics, Vol. 26, No. 5, 2007
Cabot et al.
was purchased from Fluka and used without further purification.
All reactions were conducted under a nitrogen or argon atmosphere
using standard Schlenk techniques. Silica gel used for filtration and
flash chromatography of cobalt complexes was previously washed
with Et2O. In some dicobalt alkyne complexes, due to a deficient
relaxation of the carbon cluster atoms, the corresponding signals
were not observed in the 13C NMR spectra. Medium pressure
reaction kinetic measurements were recorded on a ReactIR 4000
from Mettler-Toledo with an immersible ATR SiComp probe fitted
onto a 50 mL Miniclave glass reactor from Buchi AG. Dicobalt
heptacarbonyl complexes 7 and 8 were synthesized by following
reported experimental procedures.40
δ 57.01 (s) ppm. MS (FAB+) : m/z 1026 (M+, 5%), 970 ([M -
2CO]+, 50%), 914 ([M - 4CO]+, 100%).
Dicobalt Pentacarbonyl Complex of (Trimethylsilyl)acetylene
and Tricyclohexylphosphine (11). A Schlenk tube was charged
with DME (2 mL) and the dicobalt hexacarbonyl complex of Me3-
SiCCH (440 mg, 1.15 mmol). A solution of tricyclohexylphosphine
(204 mg, 0.73mmol) in DME (4 mL) was added to this mixture.
The system was purged with argon, and the reaction mixture was
heated to 60 °C. After 4 h, the resulting mixture was purified by
flash chromatography (SiO2/hexane) to yield 101 mg (22%) of 11
as a red oil that solidifies upon standing. IR (KBr): νmax 1950 s,
1
1972 s, 1991 vs, 2049 vs cm-1. H NMR (400 MHz, CDCl3): δ
Dicobalt Heptacarbonyl Complex of Tris(2,4-di-tert-butylphe-
nyl)phosphate (9). A Schlenk tube was charged with dicobalt
octacarbonyl (490 mg, 1.29 mmol). A solution of tris(2,4-di-tert-
butylphenyl) phosphite (701 mg, 1.06 mmol) in hexane (12 mL)
was added to this solid. The system was purged with argon, and
the reaction mixture was stirred at room temperature overnight,
which resulted in a brown precipitate. The solid was filtered and
washed with cold hexane. The solid was dried under vacuum to
afford 633 mg (60%) of 9 as a brown powder. IR (KBr): νmax
0.27 (s, 9H), 1.26-1.96 (m, 33H), 5.81 (d, JP ) 4 Hz, 1H) ppm.
13C NMR (100 MHz, C6D6): δ 2.5 (s, CH3), 26.6 (s, CH2), 27.7
(d, JP ) 9 Hz, CH2), 27.8 (d, JP ) 9 Hz, CH2), 29.7 (d, JP ) 12
Hz, CH2), 37.6 (d, JP ) 16 Hz, CH), 70.6 (s, Cq), 82.7 (s, CH),
203.2, 208.5, 209.7 (br s, CO) ppm. 31P NMR (121 MHz, C6D6):
δ 63.0 (s) ppm. MS (FAB+): m/z 636 (M+, 6%), 608 ([M - CO]+,
65%).
Pauson-Khand Reaction Monitored by IR Spectroscopy. In
a typical experiment, the glass reactor was charged in this order:
with toluene (15 mL), (trimethylsilyl)acetylene (1.4 mL, 9.6 mmol),
norbornadiene (2.0 mL, 19.2 mmol), and Co2(CO)8 (109 mg, 0.287
mmol). Extra toluene (5 mL) was used to wash out the catalyst
from the addition funnel. The reactor was immediately sealed,
purged with CO, charged to the designated CO pressure, and
immersed in a water bath at the appropriate temperature. At this
point, the spectrometer was set to collect one spectrum every 5
min. Profiles at 1698 cm-1 were collected after baseline correction
and analyzed.
1
1982 vs, 2003 s, 2058 w cm-1. H NMR (400 MHz, C6D6): δ
1.10 (s, 27H), 1.60 (s, 27H), 6.95 (s, 3H), 7.50 (s, 3H), 7.33 (d, J
) 7 Hz, 3H) ppm. 13C NMR (75 MHz, CDCl3): δ 30.7 (s, CH3),
31.4 (s, CH3), 34.5 (s, Cq), 35.4 (s, Cq), 120.0 (s, o-Ar CH), 124.3
(s, m-Ar CH), 125.1 (s, p-Ar CH), 139.0 (s, o-Ar Cq), 147.1 (s,
p-Ar Cq), 148.9 (s, Cq-O), 201.8 (br s, Co(CO)). 31P NMR (121
MHz, CDCl3): δ 155.8 (s, 1P) ppm.
Dicobalt Pentacarbonyl Complex of (Trimethysilyl)acetylene
and Tris(3,5-bis(trifluoromethyl)phenyl)phosphine (10). A Schlenk
tube was charged with toluene (1 mL) and the dicobalt hexacarbonyl
complex of Me3SiCCH (86 mg, 0.22 mmol). A solution of tris-
(3,5-bis(trifluoromethyl)phenyl)phosphine (107 mg, 0.15 mmol) in
toluene (4 mL) was added to this mixture. The system was purged
with argon, and the reaction mixture was heated to 60 °C. After 4
h, the resulting mixture was purified by flash chromatography (SiO2/
hexane) to yield 119 mg (70%) of 10 as a red oil that solidifies
Acknowledgment. This work was supported by the MEC
(No. CTQ2005-00623). X.V. thanks the DURSI for “Distincio´
per a la Promocio´ de la Recerca Universita`ria”. Rafel Prohens
and the “Plataforma de Qu´ımica Fina” are kindly acknowledged
for support in FT-IR technology. We thank Prof. A. Moyano
and Prof. P. Cabot for insightful comments on the manuscript.
1
upon standing. IR (film): νmax 2012 vs, 2071 vs cm-1. H NMR
(400 MHz, CDCl3): δ 0.18 (s, 9H), 5.58 (d, JP ) 8 Hz, 1H), 7.86
(d, J ) 10 Hz, o-Ar H, 6H), 8.06 (s, p-Ar H, 3H) ppm. 13C NMR
(100 MHz, C6D6): δ 0.7 (s, CH3), 80.8 (d, JP ) 6 Hz, Cq), 87.0
(d, JP ) 4 Hz, CH), 122.9 (d, JF ) 272 Hz, CF3), 125.3 (s, p-Ar
CH), 132.6 (d, JP ) 10 Hz, m-Ar CH), 133.2 (dd, JP ) 9 Hz, JF )
34 Hz, m-Ar C), 137.4 (d, JP ) 34 Hz, ipso-Ar C) ppm. 19F NMR
(376 MHz, CDCl3): δ 63.6 (s) ppm. 31P NMR (121 MHz, C6D6):
Supporting Information Available: Graphical representations
of the reaction rate vs alkyne concentration (Figure 1-S) and reaction
profile dependence on alkyne concentration (Figure 2-S) and IR
and 1H and 13C NMR spectra for compounds 9-11. This material
is available free of charge via the Internet at http://pubs.acs.org.
OM060935+