Cobalt-Alkyne Complexes with Diphosphine Ligands as Mechanistic Probes for the Pauson-Khand Reaction

Article abstract of DOI:10.1021/om990311m

Enantiomerically pure (3,3-dimethylbutyne)[(R)-BINAP]Co₂(CO)₄ (6a) was obtained via thermal ligand exchange in 27% yield as a single diastereomer. X-ray crystal structure analysis of 6a revealed a basal bridged coordination of the BINAP ligand and a distorted geometry of the central Co₂C₂ core. Comparison of 6a with the structurally related (phenylacetylene)dppm)Co₂(CO)₄ (7) revealed that the geometric distortion is mainly caused by the BINAP ligand. Due to the replacement of basal CO ligands anti to the tert-butyl group in 6a by the diphosphine, the Pauson-Khand reaction of 6a with norbornene is completely suppressed. Comparison with (3,3-dimethylbutyne)(dppm)Co₂(CO)₄ (6b), (3,3-dimethylbutyne)(dppe)Co₂(CO)₄ (6c), 7, (3,3-dimethylbutyne)Co₂(CO)₆ (8a), and (3,3-dimethylbutyne)(PPh₃)Co₂(CO)₅ (8b) showed a retardation of the reaction rate caused by phosphines. The mechanistic consequences for the Pauson-Khand reaction employing chiral phosphine ligands are discussed.

Full text of DOI:10.1021/om990311m

Organometallics 1999, 18, 3859-3864
3859
Coba lt-Alk yn e Com p lexes w ith Dip h osp h in e Liga n d s a s
Mech a n istic P r obes for th e P a u son -Kh a n d Rea ction
Volker Derdau, Sabine Laschat,* and Ina Dix
Institut fu¨r Organische Chemie der Technischen Universita¨t Braunschweig, Hagenring 30,
D-38106 Braunschweig, Germany
Peter G. J ones
Institut fu¨r Anorganische und Analytische Chemie der Technischen Universita¨t Braunschweig,
Hagenring 30, D-38106 Braunschweig, Germany
Received April 29, 1999
Enantiomerically pure (3,3-dimethylbutyne)[(R)-BINAP]Co2(CO)4 (6a ) was obtained via
thermal ligand exchange in 27% yield as a single diastereomer. X-ray crystal structure
analysis of 6a revealed a basal bridged coordination of the BINAP ligand and a distorted
geometry of the central Co2C2 core. Comparison of 6a with the structurally related
(phenylacetylene)dppm)Co2(CO)4 (7) revealed that the geometric distortion is mainly caused
by the BINAP ligand. Due to the replacement of basal CO ligands anti to the tert-butyl
group in 6a by the diphosphine, the Pauson-Khand reaction of 6a with norbornene is
completely suppressed. Comparison with (3,3-dimethylbutyne)(dppm)Co2(CO)4 (6b), (3,3-
dimethylbutyne)(dppe)Co2(CO)4 (6c), 7, (3,3-dimethylbutyne)Co2(CO)6 (8a ), and (3,3-
dimethylbutyne)(PPh3)Co2(CO)5 (8b) showed a retardation of the reaction rate caused by
phosphines. The mechanistic consequences for the Pauson-Khand reaction employing chiral
phosphine ligands are discussed.
Since its discovery in 1973 the synthetic utility of the
Pauson-Khand reaction as a powerful tool for the
construction of cyclopentenones has been demonstrated
in many cases.1 Besides the classical Co2(CO)8-mediated
reaction, recent results showed that several other
transition-metal carbonyl complexes can be successfully
employed for the cocyclization.2 The issue of stereose-
lective cyclopentenone formation has been addressed in
several ways. The most common approach uses chiral
auxiliaries that are attached to either the alkene or
alkyne moiety.3 This methodology has been applied both
intra- and intermolecularly. A very elegant catalytic and
highly enantioselective method for the intramolecular
cocyclization in the presence of enantiomerically pure
titanocene complexes was recently introduced by Buch-
wald.4 In a different approach chiral amine N-oxides
derived from alkaloids were employed in the intermo-
lecular Pauson-Khand reaction to remove selectively
one carbon monoxide from the prochiral cobalt alkyne
(1) Reviews: (a) Schore, N. E. Org. React. 1991, 40, 1-90. (b)
Pauson, P. L. Tetrahedron 1985, 41, 5855-5860. (c) J eong, N. In
Transition Metals for Organic Synthesis; Bolm, C., Beller, M., Eds.;
Wiley-VCH: New York, 1998; p 560-577. (d) Pauson, P. L. In
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Eds.; Springer-Verlag: Berlin, 1987; p 233. (e) Schore, N. E. In
Comprehensive Organic Synthesis; Trost, B. M., Ed.; Pergamon Press:
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955-958; Angew. Chem., Int. Ed. Engl. 1998, 37, 911-915.
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10.1021/om990311m CCC: $18.00 © 1999 American Chemical Society
Publication on Web 08/14/1999

3860 Organometallics, Vol. 18, No. 19, 1999
Derdau et al.
Ch a r t 1
Ch a r t 2
complex.5 However, this procedure requires a large
excess of the chiral amine N-oxide. Alternatively, Pau-
son, Brunner, and Kerr prepared enantiomerically pure
cobalt alkyne complexes bearing a chiral glyphos ligand
(Chart 1) via ligand exchange.6,7 Intermolecular Pau-
son-Khand reactions of these modified cobalt complexes
proceeded with high enantioselectivities. Unfortunately,
tedious chromatographic separation of the diastereo-
meric cobalt complexes was a prerequisite for their
application in the cocyclization. Despite its limitation,
this route seemed to be particularly attractive for
further investigations of enantioselective Pauson-
Khand reactions. We expected that the problems as-
sociated with the use of a chiral monodentate phosphine
ligand could be avoided if an enantiomerically pure C2-
symmetric diphosphine ligand is used instead. For
successful realization of this concept two additional
requirements have to be met. The diphosphine must
have a bite angle that is large enough to form a bridged
cobalt alkyne complex rather than a chelated one, to
minimize the number of possible diastereomeric com-
plexes. Earlier reports by Bonnet,8 Bird,9 and Cullen10
have shown that either apical or basal carbon monoxide
ligands of the cobalt alkyne complex were replaced by
the phosphine, depending on the type of phosphine
ligand (Chart 2). For example, monophosphines such as
triphenylphosphine are usually found in the apical
position (1).8,10 Diphosphines with small bite angles,
such as f6fos, prefer the basal positions, thus forming
the chelated complex 2.10 The apical-basal chelated
geometry 3 has not been observed until now. Diphos-
phines with an increased bite angle and a more flexible
tether such as dppm or dppe prefer a basal bridged
orientation (4a ,b);9,10 diphosphines such as dppb with
a very large bite angle gave the apical bridged species
5.
and the following issues were addressed. (1) Does the
ligand exchange reaction of cobalt alkyne complexes
with BINAP proceed with high diastereoselectivity? (2)
Can these cobalt complexes be used for the cocycliza-
tion? (3) What are the mechanistic consequences? With
regard to the glyphos-mediated cocyclization, our ex-
periments also should determine which of the two
different cobalt atoms in 1, the glyphos-bound Co(CO)2
or the Co(CO)3 moiety, undergoes alkene insertion. The
results toward this end are described below.
Resu lts a n d Discu ssion
The (3,3-dimethylbutyne)[(R)-BINAP]Co2(CO)4 com-
plex 6a was prepared by thermal ligand exchange
reaction from (3,3-dimethylbutyne)Co2(CO)6 (8a ) and
(R)-BINAP in refluxing THF.13 After workup and re-
crystallization from ether, brown needles were obtained
in 27% yield. According to HPLC analysis of the crude
product only a single diastereomer (g98% de) was
formed. Two broad singlets at 48.5 and 36.5 ppm in the
31P NMR spectrum of 6a indicated the presence of two
diasterotopic tetracoordinated phosphorus atoms. Crys-
tals of 6a were suitable for X-ray crystal structure
determination (see below).14
For comparison of the spectroscopic data, cobalt
complexes with the achiral diphosphines dppm and dppe
were prepared. (3,3-dimethylbutyne)(dppm)Co2(CO)4
(6b) and (3,3-dimethylbutyne)(dppe)Co2(CO)4 (6c) were
obtained as described for 6a in 80% and 39% yield,
respectively. Both 6b and 6c were formed as single
diastereomers (g98% de), as confirmed by HPLC analy-
For proper stereochemical control of the Pauson-
Khand reaction with unsymmetrical alkynes the ligand
must be attached selectively to either one of the two
basal positions (R2 > R1), i.e. basal anti (4a ) or basal
syn (4b), or the apical position (5).11 For our investiga-
tions (R)-BINAP was chosen as a suitable diphosphine12
(5) (a) Kerr, W. J .; Kirk, G. G.; Middlemiss, D. Synlett 1995, 1085-
1086. (b) Derdau, V.; Laschat, S.; J ones, P. G. Heterocycles 1998, 45,
1445-1453.
(11) For a detailed discussion of the mechanism of the Pauson-
Khand reaction see ref 1f.
(12) Reviews on BINAP and related systems: (a) Noyori, R.; Takaya,
H. Acc. Chem. Res. 1990, 23, 345-350. (b) Pu, L. Chem. Rev. 1998,
98, 2405-2494.
(13) For photolytic decomplexations see: (a) Anderson, J . C.; Taylor,
B. F.; Viney, C.; Wilson, G. J . J . Organomet. Chem. 1996, 519, 103-
106. (b) Gordon, C. M.; Kiszka, M.; Dunkin, I. R.; Kerr, W. J .; Scott, J .
S.; Gebicki, J . J . Organomet. Chem. 1998, 554, 147-154.
(14) We have deposited atomic coordinates for the structures of 6a
and 7 with the Cambridge Crystallographic Data Centre. The coordi-
nates can be obtained on request from the Director, Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ,
U.K., upon quoting the reference number CCDC 133169/133170.
(6) (a) Bladon, P.; Pauson, P. L.; Brunner, H.; Eder, R. J . Organomet.
Chem. 1988, 355, 449-454. (b) Brunner, H.; Niedernhuber, A. Tetra-
hedron: Asymmetry 1990, 1, 711-714.
(7) (a) Hay, A. M.; Kerr, G. G.; Middlemiss, D. Organometallics 1995,
14, 4986-4988. (b) Kerr, W. J .; Kirk, G. G.; Middlemiss, D. J .
Organomet. Chem. 1996, 519, 93-101.
(8) Bonnet, J .-J .; Mathieu, R. Inorg. Chem. 1978, 17, 1973-1977.
(9) Bird, P. H.; Fraser, A. R.; Hall, D. N. Inorg. Chem. 1977, 16,
1923-1931.
(10) Chia, L. S.; Cullen, W. R.; Franklin, M.; Manning, A. R. Inorg.
Chem. 1975, 14, 2521-2526.

Cobalt-Alkyne Complexes with Diphosphine Ligands
Organometallics, Vol. 18, No. 19, 1999 3861
sis. Because of the presence of two equivalent phospho-
rus atoms of 6b,c, 31P NMR spectra showed in each case
only one broad singlet at 42.8 and 38.6 ppm, respec-
tively. Thus, coordination of the more flexible ligand
dppe with a larger bite angle than dppm resulted in a
high-field shift of the 31P NMR signal. Unfortunately,
no crystals suitable for X-ray crystal structure deter-
mination could be obtained from 6b or 6c. Therefore,
the corresponding phenylacetylene derivative (phenylace-
tylene)(dppm)Co2(CO)4 (7) was prepared in a similar
way in 50% yield as a single diastereomer.18 Again the
31P NMR spectrum displayed only one broad signal at
43.4 ppm. In case of 7 X-ray crystal structure determi-
nation was successful (see below).
The X-ray data provide an unambiguous answer to
the question whether apical bridged or basal bridged
complexes (5 or 4a ,b) were formed.14 As shown in Figure
1a the basal bridged species was found for BINAP
complex 6a . The two phosphorus atoms are oriented anti
with respect to the tert-butyl group, thus avoiding steric
hindrance by the bulky alkyne substituent. The data
clearly indicate that the apical Co-CO bonds (Co1-C2
) 1.767(2) Å, Co2-C4 ) 1.768(2) Å) are shorter than
the basal ones (Co1-C1 ) 1.795(2) Å, Co2-C3 ) 1.788-
(2) Å).
A similar anti basal bridged coordination of the dppm
ligand was observed for complex 7 (Figure 2). Again the
apical Co-CO bonds (Co2-C11 ) 1.771(2) Å, Co1-C10
) 1.773(2) Å) are slightly shorter than the basal ones
(Co2-C12 ) 1.791(1) Å, Co1-C9 ) 1.784(2) Å). This
implies that apical Co-CO bonds in the BINAP complex
6a and dppm complex 7 are stronger than the corre-
sponding basal Co-CO bonds (see below).
The differences between apical and basal Co-CO
bond lengths in 6a and 7 are less than in the complexes
(tolane)Co2(CO)4(dppm)9 (Co-COapical ) 1.70, 1.71 Å,
Co-CObasal ) 1.78, 1.76 Å) and (tolane)Co2(CO)615 (Co-
COapical ) 1.72, 1.71 Å, Co-CObasal ) 1.80, 1.79 Å). Both
apical and basal bonds in 6a and 7 are approximately
0.2 Å shorter than the corresponding Co-CO bonds in
(di-tert-butylacetylene)Co2(CO)616 (Co-COapical ) 1.786,
1.786 Å, Co-CObasal ) 1.816, 1.803 Å). Whereas the
alkyne bond length (1.34 ( 0.01 Å) seems to be quite
robust to structural changes, the Co-Co bond length
varies to a much greater extent in the above-mentioned
cobalt clusters. The following distances were observed:
2.5007(4) Å for 6a and 2.4858(4) Å for 7 as compared to
2.47 Å for (tolane)Co2(CO)6, 2.459 Å for (tolane)Co2(CO)4-
(dppm), and 2.316 Å for (di-tert-butylacetylene)Co2(CO)6.
Analysis of the X-ray data concerning close van der
Waals contacts revealed an unsymmetric structure for
6a ;17 the amount of steric hindrance is different for the
two apical carbon monoxides C2-O2 and C4-O4 (Fig-
ure 1b). A similar difference was found between the two
F igu r e 1. (a, top) X-ray crystal structure of BINAP
complex 6a . Selected bond lengths (Å) and angles (deg):
Co(1)-C(2) ) 1.767(2), Co(1)-C(1) ) 1.795(2), Co(1)-C(6)
) 1.9708(19), Co(1)-C(5) ) 1.979(2), Co(1)-P(1) ) 2.2349-
(6), Co(1)-Co(2) ) 2.5007(4), Co(2)-C(4) ) 1.768(2), Co-
(2)-C(3) ) 1.788(2), Co(2)-C(5) ) 1.952(2), Co(2)-C(6) )
1.9684(19), Co(2)-P(2) ) 2.2741(6), C(5)-C(6) ) 1.331(3),
C(6)-C(7) ) 1.519(3); C(2)-Co(1)-C(1) ) 102.76(10), C(2)-
Co(1)-C(6) ) 103.06(8), C(1)-Co(1)-C(6) ) 101.65(9),
C(2)-Co(1)-C(5) ) 95.84(9), C(1)-Co(1)-C(5) ) 140.26-
(9), C(2)-Co(1)-P(1) ) 94.32(6), C(1)-Co(1)-P(1) ) 101.78-
(7), P(1)-Co(1)-Co(2) ) 100.81(2), C(4)-Co(2)-C(3) )
93.32(10), C(4)-Co(2)-C(5) ) 115.51(9), C(3)-Co(2)-C(5)
) 137.79(9), C(4)-Co(2)-C(6) ) 97.62(8), C(3)-Co(2)-C(6)
) 110.28(9), C(4)-Co(2)-P(2) ) 94.42(7), C(3)-Co(2)-P(2)
) 115.69(7), P(2)-Co(2)-Co(1) ) 115.987(18), C(5)-C(6)-
C(7) ) 138.52(19). (b, bottom) CPK model of 6a viewed
along the O1-C1 bond.
(15) Sly, W. G. J . Am. Chem. Soc. 1959, 81, 18-20.
(16) Cotton, F. A.; J amerson, J . D.; Stults, B. R. J . Am. Chem. Soc.
1976, 98, 1774-1779.
(17) The distortion of the central Co2C2 core from tetrahedral
symmetry in (phenylacetylene)Co2(CO)6 and related complexes was
studied by J ohnson. According to FMO calculations Co-CH interac-
tions are dominant over Co-CPh interactions with regard to the total
cluster bonding. Consequently, Co-CH bonds are shorter than Co-
CPh bonds. See: Housecroft, C. E.; J ohnson, B. F. G.; Khan, M. S.;
Lewis, J .; Raithby, P. R.; Robson, M. E.; Wilkinson, D. A. J . Chem.
Soc., Dalton Trans. 1992, 3171-3178.
basal carbon monoxides C1-O1 and C3-O3. The steri-
cally most hindered CO ligands are C2-O2 and to a
lesser extent C4-O4 (distances between atoms (Å): C2-
C7, 3.769; C2-C8, 3.357; C2-C10, 4.693; C4-C7, 3.369;
C4-C9, 3.358; C4-C10 3.492). The distance between
the basal CO ligands and the bulky tert-butyl group
(18) Billington, D. C.; Helps, I. M.; Pauson, P. L.; Thomson, W. T.;
Wilson, D. J . Organomet. Chem. 1988, 354, 233-242.

3862 Organometallics, Vol. 18, No. 19, 1999
Derdau et al.
F igu r e 3. Reaction rate of the Pauson-Khand reaction
employing (3,3-dimethylbutyne)Co2(CO)5L (8a ,b) and nor-
bornene under various conditions: (1) 1.00 equiv of 8a (L
) CO), 1.5 equiv of norbornene, toluene, 80 °C; (2) 1.00
equiv of 8a (L ) CO), 1.5 equiv of norbornene, CH2Cl2, 6
equiv of NMO, room temperature; (3) 1.00 equiv of 8b (L
) PPh3), 1.5 equiv of norbornene, toluene, 80 °C. For
further details see the Experimental Section.
F igu r e 2. X-ray crystal structure of dppm complex 7.
Selected bond lengths (Å) and angles (deg): Co(1)-C(2) )
1.9513(15), Co(1)-C(1) ) 1.9632(17), Co(1)-C(9) ) 1.7841-
(17), Co(1)-C(10) ) 1.7733(18), Co(1)-P(1) ) 2.2330(5), Co-
(1)-Co(2) ) 2.4858(4), Co(2)-C(11) ) 1.7711(18), Co(2)-
C(12) ) 1.7909(18), Co(2)-C(1) ) 1.9469(16), Co(2)-C(2)
) 1.9742(15), Co(2)-P(2) ) 2.2244(5), C(1)-C(2) ) 1.350-
(2), C(2)-C(3) ) 1.461(2); C(10)-Co(1)-C(9) ) 98.99(8),
C(10)-Co(1)-C(1) ) 101.22(7), C(10)-Co(1)-C(2) ) 98.45-
(7), C(9)-Co(1)-C(1) ) 140.70(7) C(9)-Co(1)-C(2) ) 103.50-
(7), C(10)-Co(1)-P(1) ) 101.41(6), C(9)-Co(1)-P(1) )
111.80(5), P(1)-Co(1)-Co(2) ) 96.925(16), C(11)-Co(2)-
C(12) ) 99.84(8), C(11)-Co(2)-C(1) ) 100.24(7), C(12)-
Co(2)-C(1) ) 142.90(8), C(11)-Co(2)-C(2) ) 101.14(7),
C(12)-Co(2)-C(1) ) 142.90(8), C(11)-Co(2)-P(2) ) 99.57-
(6), C(12)-Co(2)-P(2) ) 107.93(6), P(2)-Co(2)-Co(1) )
96.870(16), C(1)-C(2)-C(3) ) 142.72(15).
decomposition of the cobalt alkyne complex occurred.
Obviously the presence of the diphosphine totally sup-
presses the reaction of the alkyne complex with nor-
bornene. This outcome is in agreement with results by
Pauson18 and J eong,19 who observed a significant re-
tardation of the rate (or at least a decrease of the yield)
when employing phosphine-substituted cobalt carbonyl
complexes for inter- and intramolecular Pauson-Khand
reactions.
To compare the strong effect of the diphosphines on
the reaction rate with the corresponding monophos-
phines, qualitative rate experiments with the parent
(3,3-dimethylbutyne)Co2(CO)6 (8a ) and the monophos-
phine-substituted analogue (3,3-dimethylbutyne)(PPh3)-
Co2(CO)5 (8b) were undertaken. As can be seen in
Figure 3, the NMO-promoted reaction of complex 8a
with norbornene is 1.1 times faster than the thermal
reaction. In both cases a final conversion of 70% was
achieved. However, the phosphine-substituted complex
8b slowed the reaction by a factor of 4. Furthermore,
the final conversion of 8b could not be increased above
40%.20 The decreased reaction rate of the phosphine
complex is probably attributable to the replacement of
a carbon monoxide by a poorer π-acceptor ligand (i.e.
PPh3). thus increasing the back-bonding between cobalt
and the remaining carbon monoxides. This should lead
to a retardation of the initial decarbonylation step in
the Pauson-Khand reaction. According to the mecha-
nistic scheme proposed for the cocyclization,1f the carbon
monoxides in the basal positions anti to the larger
substituent R2 are usually supposed to be most prone
differs by 0.2-0.4 Å, with C1-O1 much closer to the
tert-butyl than C3-O3 (distances between atoms (Å):
C1-C7, 3.674; C1-C10, 3.654; C3-C7, 4.080; C3-C10,
3.874). These structural features should have important
implications for the reactivity (vide infra).
The greater geometric distortion of the central Co2C2
core in 6a as compared to that in 7 can also be deduced
from Co-P bond lengths. Whereas two different Co-P
bond lengths (Co2-P2 ) 2.2741(6) Å, Co1-P1 ) 2.2349-
(6) Å) were found for BINAP complex 6a , the dppm
complex 7 has two Co-P bonds of almost equal length
(Co2-P2 ) 2.2244(5) Å, Co1-P1 ) 2.2330(5) Å). The
increased symmetry of 7 is in good agreement with the
data observed for (tolane)Co2(CO)4(dppm)9 (Co2-P2 )
2.210 Å, Co1-P1 ) 2.215 Å).
Surprisingly, BINAP complex 6a was completely
unreactive in the attempted Pauson-Khand reaction
with norbornene (Scheme 1). Neither thermal conditions
(refluxing in THF or toluene) nor activation by 6 equiv
of N-methylmorpholine N-oxide (NMO, CH2Cl2, room
temperature) gave the desired product. Only starting
material could be recovered.
Similar results were observed with complexes 6b,c
(and 7) containing dppm or dppe as ligands. When a
refluxing mixture of (3,3-dimethylbutyne)(dppm)Co2-
(CO)4 (6b) and an excess of norbornene (10 equiv) in
toluene was treated with 10 equiv of NMO, only
(19) J eong, N.; Hwang, S. H.; Lee, Y.; Chung, Y. K. J . Am. Chem.
Soc. 1994, 116, 3159-3160.
(20) One might argue that thermal reaction of 8b yields PPh3 as a
byproduct, which is presumably
a potent inhibitor, so that the
maximum level of conversion should not exceed 50%. However,
experiments by Pauson and Brunner6a have shown that thermal
cocyclization of a mixture of (phenylacetylene)Co(CO)6 and glyphos
proceeded with much higher yield as compared to the reaction of
preformed, chromatographically pure (phenylacetylene)(glyphos)Co-
(CO)5. This means that PPh3 only inhibits the reaction when it is
directly bound to the cobalt center. In addition, it retards the reactivity
of the neighboring cobalt center.

Cobalt-Alkyne Complexes with Diphosphine Ligands
Organometallics, Vol. 18, No. 19, 1999 3863
Sch em e 1
two enantiotopic cobalt atoms. Work toward this goal
is currently in progress in our laboratories.
Exp er im en ta l Section
All reactions were carried out under argon by using stan-
dard Schlenk techniques. Solvents were dried and deoxygen-
ated by standard procedures. Analytical TLC was performed
on precoated Macherey-Nagel Polygram SIL G/UV254 plates
(0.25 mm thickness), and products were visualized with a
solution of phosphomolybdic acid in EtOH (5%, v/v). Flash
chromatography22 was carried out with Merck silica gel 60
(230-400 mesh). NMR spectra were performed at 400 MHz
(1H), 100 MHz (13C), and 81 MHz (31P). Melting points were
uncorrected. IR spectra: Nicolet 5DXC FT-IR spectrometer.
Optical rotations: 1 dm cells, 1 mL capacity, room tempera-
ture. Mass spectra were obtained at 70 eV; NH3 was used as
reactant gas for CI spectra. GC analysis: HP5-fused silica
capillary column (i.d. 0.32 mm, length 30 m), a temperature
program was run from 80 °C at a rate of 8 °C min-1 up to 280
°C. (3,3-dimethylbutyne)Co2(CO)6 (8a) and (3,3-dimethylbutyne)-
Co2(CO)5(PPh3) (8b) were prepared as described in refs 23 and
24.
Gen er a l P r oced u r e for th e P r ep a r a tion of Coba lt
Alk yn e P h osp h in e Com p lexes. To a solution of dicobalt
octacarbonyl (342 mg, 1.00 mmol) in THF (5 mL) was added
dropwise alkyne (1.50 mmol), and the resulting red-brown
solution was stirred for 30 min at room temperature, until the
evolution of carbon monoxide ceased. Then phosphine (1.00
mmol) was added and the solution was heated for several
hours at 60 °C until TLC indicated complete conversion. After
the mixture was cooled to room temperature, the solvent was
removed in vacuo and the crude product was purified by flash
chromatography on SiO2 and recrystallization from ether.
(3,3-Dim eth ylbu tyn e)[(R)-BINAP ]Co2(CO)4 (6a ). Puri-
fication by flash chromatography (hexanes/ethyl acetate 10:
1) yielded brown crystals (40 mg, 0.04 mmol, 27%): mp 150
°C dec; [R]2D2 198° (c ) 0.05 in CH2Cl2); 1H NMR (400 MHz,
CDCl3) δ 7.90-6.10 (m, 32H, aryl CH), 5.62 (s, br, 1H, CH),
1.22 (s, 9H, CH3); 13C NMR (100 MHz, CDCl3) δ 208.7, 207.8,
204.9, 203.8 (CO), 140.8 (d, J ) 37.2 Hz, aryl CH), 139.2 (d,
J C,P ) 40.9 Hz, aryl CH), 138.0 (d, J C,P ) 31.0 Hz, aryl Cq),
135.0-134.0 (m), 132.9-131.9 (m), 130.0 (d, J C,P ) 15.0 Hz),
129.3-129.2 (m), 128.6-125.4 (m, 37C, aryl CH), 118.4 (Co-
C), 82.5 (Co-CH), 36.6 [C(CH3)3], 32.8 [C(CH3)3]; 31P NMR (81
MHz, CDCl3) δ 48.5 (s, br), 36.5 (s, br); IR (KBr) 2009, 1978,
1950, 1933 cm-1; UV (CH3CN) λmax (log ꢀ) 196 (1.45) nm. Anal.
Calcd for Co2C54H42O4P2‚(C2H5)2O: C, 68.92; H, 5.19. Found:
C, 69.20; H, 5.36.
to undergo decarbonylation and subsequent coordina-
tion of the alkene. The inertness of the cobalt BINAP
complex 6a and the corresponding complexes 6b,c with
dppm and dppe ligands toward the reaction conditions
strongly supports this mechanism.21 The only carbon
monoxide ligands in complex 6a that might be accessible
for the Pauson-Khand reaction are the basal coordi-
nated C1-O1 and C3-O3. However, the insertion step
is very sensitive toward steric hindrance, and thus
insertion from a basal position such as C3-O3 (or C1-
O1) is disfavored due to steric interactions with the tert-
butyl group. In contrast, the other two basal positions
are occupied by the phosphine ligand and, thus, the
Pauson-Khand reaction is completely suppressed. As
a consequence the shutdown of the cocylization pathway
is caused by the decrease of the reaction rate due to the
phosphine and the coordination of the bidentate ligand
at the “wrong” position, i.e., the basal anti instead of
basal syn position.
These results also have important implications on the
mechanism of the glyphos-containing cobalt complexes
mentioned earlier. Thus, the cocyclization of (alkyne)-
(glyphos)Co2(CO)5 with alkenes presumably proceeds at
the cobalt center that does not contain the glyphos
ligand, because this center would react much more
slowly. The Co(CO)2(glyphos) moiety operates as a chiral
“bystander” that directs the steric outcome in the
neighborhood.
X-ray crystal structure analysis of 6a ‚2C4H10O: C62H62
-
Co2O6P2, Mr ) 1082.92, crystal size 0.31 × 0.14 × 0.09 mm,
monoclinic, space group P21, a ) 16.413(2) Å, b ) 10.3967(14)
Å, c ) 17.101(2) Å, â ) 106.770(5)°, V ) 2794.0(7) Å3, Fcalcd
)
1.287 Mg m-3, T ) 143(2) K, Z ) 2, λ ) 0.710 73 Å; Bruker
SMART 1000 CCD diffractometer, 33 528 reflections collected
to θ ) 30°, 16 020 independent reflections, 650 refined
parameters, R1 ) 0.0346, wR2 ) 0.0692; programs used,
SHELXL-97. The structure contains two molecules of diethyl
ether, one of which is disordered over two sites. For details
see ref 14.
Con clu sion
Although (R)-BINAP forms cobalt alkyne complex 6a
with high diastereoselectivity, 6a cannot be used for the
Pauson-Khand reaction because of exchange of the
“wrong”, basal syn oriented carbon monoxide ligand
together with an overall decrease of the reaction rate.
To circumvent the problems discussed above, a chelating
or nonchelating phosphine is required that should bind
with high enantio- and diastereoselectivity to one of the
(3,3-Dim eth ylbu tyn e)(d p p m )Co2(CO)4 (6b). Flash chro-
matography (hexanes/ethyl acetate 6:1) yielded a red-brown
solid (560 mg, 80%): mp 150 °C dec; 1H NMR (400 MHz,
CDCl3) δ 7.56-7.06 (m, 20H, aryl CH), 5.50 (s, br, 1H, CH),
3.62 (s, br, 1H, PCH2P), 3.11 (s, br, 1H, PCH2P), 1.36 [s, 9H,
(22) Still, W. C.; Kahn, M.; Mitra, A. J . Org. Chem. 1978, 43, 2923-
2925.
(23) Varadi, G.; Vizi-Orosz, A.; Vastag, A.; Palyi, G. J . Organomet.
Chem. 1976, 108, 225-233.
(24) Aime, S.; Milone, L.; Rossetti, R.; Stanghellini, P. L. Inorg.
Chim. Acta 1977, 22, 135-137.
(21) Billington and Pauson reported
a 5% yield of 1,3,3a,6a-
tetrahydro-5-phenyl-4H-cyclopenta[c]furan-4-one upon thermal reac-
tion of 7 with 2,5-dihydrofuran.18

3864 Organometallics, Vol. 18, No. 19, 1999
Derdau et al.
X-ray crystal structure analysis of 7‚1/2C4H10O: C39H33
C(CH3)3]; 13C NMR (100 MHz, CDCl3) δ 208.0 (CO), 204.4 (CO),
139.2 (dd, J C,P ) 19.8/19.8 Hz, 2C), 136.8 (dd, J C,P ) 19.4/19.5
Hz, 2C, aryl Cq), 132.1 (dd, J C,P ) 6.3/6.3 Hz, 4C), 131.7 (dd,
J C,P ) 6.3/6.3 Hz, 4C), 129.5 (d, J C,P ) 5.8 Hz, 4C), 128.3 (dd,
J C,P ) 4.6/4.7 Hz, 4C), 128.2 (dd, J C,P ) 4.6/4.7 Hz, 4C, aryl
CH), 119.3 (d, J C,P ) 17.0 Hz, 1C, Co-Cq), 72.5 (Co-CH), 42.4
(dd, J C,P ) 19.3/19.5 Hz, 1C, PCH2P), 36.9 [C(CH3)3], 33.4
[C(CH3)3]; 31P NMR (81 MHz, CDCl3) δ 42.4 (s, br); IR (KBr)
2017, 1983, 1955 cm-1; UV (CH3CN) λmax (log ꢀ) 196 (1.49) nm;
MS (EI) m/z 696 (M, 1), 668 (16), 640 (4), 612 (28), 584 (100),
528 (9), 502 (24), 443 (6), 424 (7), 348 (18), 321 (11), 302 (10),
251 (14), 243 (10), 183 (32), 152 (5), 108 (8), 78 (6). Anal. Calcd
for Co2C35H32O4P2: C, 60.36; H, 4.63. Found: C, 61.12; H, 5.08.
(3,3-Dim eth ylbu tyn e)(d p p e)Co2(CO)4 (6c). Flash chro-
matography (hexanes/ethyl acetate 15:1) yielded a red-brown
solid (280 mg, 39%): mp 150 °C dec; 1H NMR (400 MHz,
CDCl3) δ 7.67-7.01 (m, 20H, aryl CH), 5.41-5.28 (m, 1H, CH),
2.18-1.93 (m, 4H, PCH2CH2P), 1.35 [s, 9H, C(CH3)3]; 13C NMR
(100 MHz, CDCl3) δ 207.1 (CO), 206.1 (CO), 139.2-138.8 (m),
137.8-137.5 (m, 4C, aryl Cq), 132.6-132.5 (m), 131.3-131.2
(m), 129.6-129.2 (m), 128.6-128.2 (m, 20C, arom. CH), 110.4
(Co-Cq), 73.1 (Co-CH), 36.2 [C(CH3)3], 33.6 [C(CH3)3], 24.9
(CH2), 24.7 (CH2); 31P NMR (81 MHz, CDCl3) δ 38.6 (s, br); IR
(KBr) 2010, 1981, 1958, 1923 cm-1; UV (CH3CN) λmax (log ꢀ)
196 (1.76) nm; MS (CI) m/z 711 (M + 1, 0.2), 568 (4), 540 (2),
345 (12), 263 (100), 237 (34), 206 (76), 189 (46), 176 (13), 78
(10). Anal. Calcd for Co2C36H34O4P2: C, 60.86; H, 4.82.
Found: C, 60.45; H, 5.08.
-
Co2O4.5P2, Mr ) 753.45, crystal size 0.31 × 0.14 × 0.09 mm,
monoclinic, space group C2/c, a ) 39.765(4) Å, b ) 12.232(2)
Å, c ) 14.6698(10) Å, â ) 92.303(10)°, V ) 7129.8(15) Å3, Fcalcd
) 1.404 Mg m-3, T ) 143(2) K, Z ) 8, λ ) 0.710 73 Å; Bruker
SMART 1000 CCD diffractometer, 40 338 reflections collected
to θ ) 29.5°, 9950 independent reflections, 430 refined
parameters, R1 ) 0.0294, wR2 ) 0.0754; programs used,
SHELXL-97. The structure contains one molecule of diethyl
ether, disordered over an inversion center. For details see ref
14.
P a u son -Kh a n d Rea ction s of Com p lexes 8a ,b a n d
Nor bor n en e. R u n 1. To a solution of 8a (1.00 mmol) and
anthracene (178 mg, 1.00 mmol, as internal standard) in
toluene (20 mL) was added norbornene (141 mg, 1.50 mmol),
and the reaction mixture was heated to 80 °C. Samples were
taken after 2, 5, 10, 30, 60, 120, and 240 min, respectively,
and analyzed by capillary GC.
Ru n 2. To a solution of 8a (1.00 mmol) and anthracene (178
mg, 1.00 mmol, as internal standard) in CH2Cl2 (20 mL) were
added norbornene (141 mg, 1.50 mmol) and NMO (703 mg,
6.00 mmol), and the reaction mixture was stirred at room
temperature. Samples were taken and analyzed as described
for run 1.
Ru n 3. To a solution of 8b (1.00 mmol) and anthracene (178
mg, 1.00 mmol) in toluene (20 mL) was added norbornene (141
mg, 1.50 mmol), and the reaction mixture was heated to 80
°C. Samples were taken and analyzed as described for run 1.
(P h en yla cetylen e)(d p p m )Co2(CO)4 (7). Flash chroma-
tography (hexanes/ethyl acetate 6:1) yielded a red-brown solid
(360 mg, 50%):18 mp 120 °C dec; 1H NMR (400 MHz, CDCl3) δ
7.63-7.23 (m, 25H, aryl CH), 5.78 (s, br, 1H, CH), 3.58 (s, br,
1H, PCH2P), 3.08 (s, br, 1H, PCH2P); 13C NMR (100 MHz,
CDCl3) δ 207.0 (CO), 203.2 (CO), 136.8-135.2 (m, 4C, aryl Cq),
132.2-131.5 (m), 129.9-129.3 (m), 128.4-127.9 (m), 126.1 (s,
Ack n ow led gm en t. Generous financial support by
the Deutsche Forschungsgemeinschaft (Gerhard Hess
prize for S.L.) and the Fonds der Chemischen Industrie
is gratefully acknowledged.
26C, aryl CH), 109.4 (Co-Cq), 73.5 (Co-CH), 41.4 (dd, J C,P
)
Su p p or tin g In for m a tion Ava ila ble: Tables and figures
giving X-ray crystal structure results for 6a and 7. This
material is available free of charge via the Internet at
http://pubs.acs.org.
15.3/15.5 Hz, 1C, PCH2P); 31P NMR (81 MHz, CDCl3) δ 43.4
(s, br); IR (KBr) 2023, 1992, 1968 cm-1; UV (CH3CN) λmax (log
ꢀ) 196 (1.64) nm; MS (CI) m/z 717 (M + 1, 1), 689 (5), 661 (1),
390 (2), 359 (6), 303 (7), 263 (100), 201 (57), 78 (7). Anal. Calcd
for Co2C37H28O4P2: C, 62.03; H, 3.94. Found: C, 62.05; H, 4.43.
OM990311M