Pendant alkenes promote cobalt-cobalt bond cleavage in (alkyne)(binap) tetracarbonyldicobalt(0) complexes

Article abstract of DOI:10.1021/om070022v

Heating (alkyne)(binap)tetracarbonyldicobalt(0) complexes bearing pendant alkenes leads to cleavage of the cobaltcobalt bond and generation of the mononuclear hydride (binap)(CO)₂CoH at 45-55°C.

Full text of DOI:10.1021/om070022v

1578
Organometallics 2007, 26, 1578-1580
Pendant Alkenes Promote Cobalt-Cobalt Bond Cleavage in
(Alkyne)(binap)tetracarbonyldicobalt(0) Complexes
Susan E. Gibson,*,† Karina A. C. Kaufmann,† Peter R. Haycock,† Andrew J. P. White,†
David J. Hardick,‡ and Matthew J. Tozer‡
Department of Chemistry, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K.,
and MediVir UK Ltd., Chesterford Research Park, Little Chesterford, Essex C10 1XL, U.K.
ReceiVed January 9, 2007
Summary: Heating (alkyne)(binap)tetracarbonyldicobalt(0) com-
plexes bearing pendant alkenes leads to cleaVage of the cobalt-
cobalt bond and generation of the mononuclear hydride
(binap)(CO)2CoH at 45-55 °C.
The cobalt-mediated Pauson-Khand reaction (the coupling
of an alkyne, an alkene, and carbon monoxide to form a
cyclopentenone) is an attractive carbon-carbon bond-forming
reaction that has found many applications in organic synthesis1
since it was first reported over 30 years ago.2 Recently, there
has been much interest in developing the asymmetric and
catalytic aspects of the reaction, and significant progress has
been made in each of these areas. Combination of catalysis and
asymmetric induction in the cobalt-catalyzed reaction has been
achieved by Hiroi, who employed the bis-phosphane binap,3
and Buchwald, who used a binaphthyl-derived phosphite.4
Experimental evidence for the currently accepted mechanism
of the stoichiometric cobalt Pauson-Khand reaction (PKR),
which was proposed by Magnus in 1985 and involves the
stepwise construction of the product cyclopentenone on a series
of dicobalt complexes,5 has remained scarce, although compu-
tational studies have provided interesting insights.6
Figure 1. (binap)hexacarbonyldicobalt(0).
Figure 2. Interconversion of the diastereoisomers of complex 2.
Recently we discovered that binap reacts with cobalt carbonyl
sources typically used in the PKR such as octacarbonyldicobalt-
(0) to form complex 1, in which binap is chelated to one of the
two cobalt atoms (Figure 1).7 Complex 1 catalyzed the PKR of
a standard substrate, N-(prop-2-enyl)-N-(prop-2-ynyl)-p-tolu-
enesulfonamide.7
Figure 3. Substrates for 31P NMR study.
* To whom correspondence should be addressed. Fax: (+44) 207-594-
5804. Tel: (+44) 207-594-1140. E-mail: s.gibson@imperial.ac.uk.
† Imperial College London.
Subsequently we found that complex 1 reacted with a range
of alkynes to form mixtures of two diastereoisomeric complexes,
as exemplified by 2a and 2b (Figure 2; structures confirmed
by X-ray crystallography). In the case of 2, the two isomers
were separable and 31P NMR studies revealed that they
interconverted without decomposition at 75 °C.8
We wish to report herein the results of experiments performed
with alkyne complexes 3-6 (Figure 3), which reveal that
cobalt-cobalt bond cleavage readily occurs in the presence of
a pendant alkene, suggesting that monocobalt species may be
formed during the course of cobalt-mediated PKRs.
To obtain 3, octacarbonyldicobalt(0) and binap were reacted
together in THF. After addition of N-(prop-2-enyl)-N-(prop-2-
ynyl)-p-toluenesulfonamide,9 the reaction mixture was heated
at 40 °C for 2 h. Column chromatography of the resulting
product mixture gave a brown solid.10 The 31P NMR spectrum
‡ Medivir UK Ltd.
(1) For recent reviews on various aspects of the PKR, see: (a) Gibson,
S. E.; Mainolfi, N. Angew. Chem., Int. Ed. 2005, 44, 3022. (b) Laschat, S.;
Becheanu, A.; Bell, T.; Baro, A. Synlett 2005, 2547. (c) Bonaga, L. V. R.;
Krafft, M. E. Tetrahedron 2004, 60, 9795. (d) Blanco-Urgoiti, J.; Anorbe,
L.; Perez-Serrano, L.; Dominguez, G.; Perez-Castells, J. Chem. Soc. ReV.
2004, 33, 32. (e) Gibson, S. E.; Stevenazzi, A. Angew. Chem., Int. Ed.
2003, 42, 1800. (f) Brummond, K. M.; Kent, J. L. Tetrahedron 2000, 56,
3263.
(2) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E. J. Chem.
Soc. D 1971, 36.
(3) (a) Hiroi, K.; Watanabe, T.; Kawagishi, R.; Abe, I. Tetrahedron Lett.
2000, 41, 891. (b) Hiroi, K.; Watanabe, T.; Kawagishi, R.; Abe, I.
Tetrahedron: Asymmetry 2000, 11, 797.
(4) Sturla, S. J.; Buchwald, S. L. J. Org. Chem. 2002, 67, 3398.
(5) Magnus, P.; Principe, L. M. Tetrahedron Lett. 1985, 26, 4851.
(6) (a) Yamanaka, M.; Nakamura, E. J. Am. Chem. Soc. 2001, 123, 1703.
(b) Bruin, T. J. M.; Milet, A.; Robert, F.; Gimbert, Y.; Greene, A. E. J.
Am. Chem. Soc. 2001, 123, 7184. (c) Pericas, M. A.; Balsells, J.; Castro,
J.; Marchueta, I.; Moyano, A.; Riera, A.; Vazquez, J.; Verdaguer, X. Pure
Appl. Chem. 2002, 74, 167. (d) Gimbert, Y.; Lesage, D.; Milet, A.; Fournier,
F.; Greene, A. E.; Tabet, J.-C. Org. Lett. 2003, 5, 4073. (e) de Bruin, T. J.
M.; Milet, A.; Greene, A. E.; Gimbert, Y. J. Org. Chem. 2004, 69, 1075.
(7) Gibson, S. E.; Lewis, S. E.; Loch, J. A.; Steed, J. W.; Tozer, M. J.
Organometallics 2003, 22, 5382.
(8) Gibson, S. E.; Kaufmann, K. A. C.; Loch, J. A.; Steed, J. W.; White,
A. J. P. Chem. Eur. J. 2005, 11, 2566.
(9) Oppolzer, W.; Pimm, A.; Stammen, S. B.; Hume, W. E. HelV. Chim.
Acta 1997, 80, 623.
10.1021/om070022v CCC: $37.00 © 2007 American Chemical Society
Publication on Web 02/27/2007

Communications
Organometallics, Vol. 26, No. 7, 2007 1579
Figure 5. 31P NMR spectra of complex 4 at 30 and 75 °C.
Figure 4. 31P NMR spectra of complex 3 at 30, 45, 50, and 60
°C.
of the solid in DME contained two stronger resonances at δ 59
and 44, associated with one diastereoisomer of 3, and two
weaker resonances at δ 53 and 40, associated with a second
diastereoisomer (Figure 4, 30 °C). These observations correlated
well with those for other (alkyne)(binap)tetracarbonyldicobalt-
(0) complexes synthesized previously.8
Complex 3 proved to be relatively unstable, and so it was
decided to probe its chemistry by 31P NMR spectroscopy. The
complex was dissolved in CO-saturated DME and its spectrum
monitored at 5 °C intervals as the temperature was raised above
30 °C (see Figure 4 for key spectra). The four resonances of 3
dominated the spectrum until 45 °C, but at 50 °C two new
resonances at δ 53.7 and 43.6 had appeared. At 60 °C the four
resonances associated with the alkyne complex 3 had disap-
peared, leaving only the resonances at δ 54 and 43. This
situation remained unchanged up to 75 °C and on lowering the
temperature back to 30 °C.
Figure 6. 31P NMR spectra of complex 5 at 30, 50, 55, and 70
°C.
Having discovered that the effect of temperature on complex
3 differed dramatically from its effect on complex 2 (Figure
2), we probed the role of the pendent alkene using complexes
4-6. The required enynes were synthesized by drawing on
standard protocols8,11 and converted to the desired complexes
4-6 as outlined above for complex 3.10 As described for 3, the
31P NMR spectrum of each complex in CO-saturated DME was
monitored between 30 and 75 °C and afterward at 30 °C. A
clear dependence on the presence and nature of the pendant
alkene was observed. In the case of 4, which is identical with
3 except for the absence of the alkene, the spectrum remained
unchanged throughout the sequence (Figure 5). In the case of
5, which bears a relatively electron-rich alkene, the dominant
new peak at δ 54 started to appear at 55 °C (Figure 6), while
Figure 7. 31P NMR spectra of complex 5 at 30, 40, 45, and 60
°C.
for 6, with a relatively electron-poor alkene, the resonance at δ
54 started to emerge at 45 °C (Figure 7).
(10) Octacarbonyldicobalt(0) (75 mg, 0.220 mmol) and (()-2,2′-bis-
(diphenylphosphino)-1,1′-binaphthalene (140 mg, 0.225 mmol) were dis-
solved in THF (15 mL) and stirred at room temperature for 30 min. The
mixture was heated to 40 °C for 30 min. N-(Prop-2-enyl)-N-(prop-2-ynyl)-
p-toluenesulfonamide9 (56 mg, 0.226 mmol) was added, and the reaction
mixture was stirred at 40 °C for 2 h. The mixture was cooled to room
temperature and was then absorbed onto neutral alumina (grade II). The
brown solid was loaded onto a chromatography column (silica) and eluted
under nitrogen, first with hexane/ethyl acetate (19:1) to remove unreacted
octacarbonyldicobalt(0) and then with hexane/ethyl acetate (7:3) to collect
3 as a brown solid (109 mg, 45%). IR (CHCl3, cm-1): νCO 2043 (s), 1980
(s), 1939 (s). 31P NMR (160 MHz, DME): δ 59.4, 53.2, 44.0, 40.0 (CoPPh2-
Ar), -14.6 (binap; <5%). MS (FAB): m/z (%) 1017 (M+ - 3CO, 11),
989 (M+ - 4CO, 40), 681 [Co(binap)+, 100], 437 [(binap - PPh2)+, 65].
(11) (a) Krafft, M. E.; Bonaga, L. V. R.; Wright, J. A.; Hirosawa, C. J.
J. Org. Chem. 2002, 67, 1233. (b) Watanuki, S.; Mori, M. Organometallics
1995, 14, 5054.
The 31P NMR spectrum of (binap)hexacarbonyldicobalt(0)
(1) in DME contains a single resonance at δ 43,7 and thus it
was assumed that complex 1 was formed during the NMR
sequences described above. An explanation for the resonance
at δ 54 was provided by a combination of X-ray crystallography
and a further NMR experiment.12 Brown crystals observed in
the NMR tube after the NMR experiment on complex 3 were
examined by X-ray crystallography, which revealed that they
were the cobalt hydride structure 7 (Figure 8).13 Repetition of
our standard NMR sequence in d8-THF with complex 3 led to
a final 31P NMR spectrum recorded at 30 °C containing
resonances at δ 53 and 43. The 1H NMR spectrum of this sample
contained a broad resonance at δ -11.6, characteristic of a

1580 Organometallics, Vol. 26, No. 7, 2007
Communications
Scheme 1. Cleavage of the Cobalt-Cobalt Bond of 9 by the
Two-Electron-Donor Ligand Triphenylphosphane
Scheme 2. Proposed Pathway to the Anion of 7 and How It
May Relate to PK Activity
Figure 8. Molecular structure of hydride 7.
cobalt-hydride species, thus providing a correlation between
the solid-state structure and the solution observations.
The results described above reveal that pendent alkenes
promote cobalt-cobalt bond cleavage in (alkyne)(binap)-
tetracarbonyldicobalt(0) complexes. This may be the result of
a direct interaction between the alkene and the cobalt-cobalt
bond, as postulated below, or an indirect effect resulting from
interactions occurring in a downstream intermediate arising from
Pauson-Khand activity. Given that two-electron-donor ligands
have been documented to break the cobalt-cobalt bond in
dicobalt carbonyl species, e.g., octacarbonyldicobalt(0) is readily
transformed into the ionic species 10 presumably via nucleo-
philic attack of triphenylphosphane on complex 9 (Scheme 1),14
it seems reasonable to postulate that similar nucleophilic
displacements may occur in complexes 3, 5, and 6 driven by
the pendant alkenes.
Thus, to explain the results of the NMR experiments described
herein, we propose that carbon monoxide induces the formation
(12) Complex 3 (35 mg) was dissolved in CO-saturated DME (0.8 mL)
and syringed via a filter needle into a WILMAD screw-cap (with a PTFE/
silicone septum) NMR tube filled with carbon monoxide; 31P NMR spectra
were recorded between 30 and 75 °C (see Figure 4). The NMR tube was
then cooled to 30 °C, and the 31P NMR spectrum was recorded at this
temperature. 31P NMR (160 MHz, DME): δ 53.4, 43.2 (CoPPh2Ar), -14.2
(binap; <5%). When the NMR tube was stored in the freezer, small brown
crystals appeared in the NMR tube and these were identified as hydride 7
by X-ray crystallography.13 The early stages of the experiment were
subsequently repeated in d8-THF. When the temperature was lowered to
of the η2-alkyne complex 11 from complex 3 and that this is
followed by the pendant alkene displacing the anion of 7, as
depicted in Scheme 2.
How the observed cobalt-cobalt bond cleavage relates to
Pauson-Khand activity is yet to be determined,15 but it may
be speculated that the cleavage competes with carbon monoxide
loss and alkene binding to give complex 12, which in turn
proceeds to the cyclopentenone product of the PKR and dicobalt
complex 1. Whether or not this speculation is correct, it is clear
that cobalt-cobalt bond cleavage is a facile process in com-
plexes of relevance to the cobalt-mediated Pauson-Khand
reaction and that due consideration of this property should aid
our understanding of this important synthetic process.
1
30 °C, the 31P and H NMR spectra were recorded. 31P NMR (160 MHz,
1
d8-THF): δ 52.9, 42.6. H NMR (400 MHz, d8-THF): δ - 11.6 (broad,
Co-H).
(13) Crystal data for 7: C46H33CoO2P2, Mr ) 738.59, monoclinic, P21/n
(No. 14), a ) 15.0501(7) Å, b ) 14.7142(7) Å, c ) 17.0825(8) Å, â )
110.517(4)°, V ) 3543.0(3) Å3, Z ) 4, Dc ) 1.385 g cm-3, µ(Mo KR) )
0.615 mm-1, T ) 173 K, brown needles, Oxford Diffraction Xcalibur 3
diffractometer, 12 274 independent measured reflections, F2 refinement, R1
) 0.094, wR2 ) 0.114, 5717 independent observed reflections (|Fo| > 4σ-
(|Fo|), 2θmax ) 65°), 464 parameters. CCDC 619746.
Acknowledgment. We thank Medivir UK for generous
studentship support (K.A.C.K.)
(14) (a) Hieber, W.; Freyer, W. Chem. Ber. 1958, 91, 1230. (b) Szabo,
P.; Fekete, L.; Bor, G.; Nagy-Magos, Z.; Marko, L. J. Organomet. Chem.
1968, 12, 245. (c) Comely, A. C.; Gibson, S. E.; Hales, N. J.; Johnstone,
C.; Stevenazzi, A. Org. Biomol. Chem. 2003, 1, 1959.
Supporting Information Available: Text and figures givng
detailed experimental procedures and compound characterization
data and a CIF file giving crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
(15) It is of note in this context, however, that monitoring a 1:1:3 mixture
of octacarbonyldicobalt(0), binap, and N-(prop-2-enyl)-N-(prop-2-ynyl)-p-
toluenesulfonamide in CO-saturated DME at 75 °C by 31P NMR spectros-
copy gave resonances at δ 54 and 43 which simplified after 2 h to a single
resonance at δ 43.
OM070022V