The photochemistry of (μ2-RC2H)Co2(CO)6 species (R=H or C6H5), important intermediates in the Pauson-Khand reaction

Article abstract of DOI:10.1016/s0022-328x(99)00374-5

The photochemistry of (μ₂-RC₂H)Co₂(CO)₆ (R=H or C₆H₅) has been investigated by both time-resolved and steady-state techniques. Pulsed excitation in cyclohexane solution with λ_exc

Full text of DOI:10.1016/s0022-328x(99)00374-5

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 588 (1999) 195–199
The photochemistry of (m2-RC2H)Co2(CO)6 species (R=H or
C6H5), important intermediates in the Pauson–Khand reaction
Sylvia M. Draper a, Conor Long b,*, Bronagh M. Myers b
a Department of Chemistry, Uni6ersity of Dublin, Trinity College, Dublin 2, Ireland
b Inorganic Photochemistry Centre, Dublin City Uni6ersity, Dublin 9, Ireland
Abstract
The photochemistry of (m2-RC2H)Co2(CO)6 (R=H or C6H5) has been investigated by both time-resolved and steady-state
techniques. Pulsed excitation in cyclohexane solution with uexc=355 nm causes CoꢀCo bond homolysis while photolysis at
uexc=532 nm results in CO loss. Steady-state photolysis (uexc\500 nm) in the presence of suitable trapping ligands (L) produced
the monosubstituted complexes (m2-RC2H)Co2(CO)5(L) (L=C5H5N or PPh3) in high yields. © 1999 Elsevier Science S.A. All
rights reserved.
Keywords: Cobalt; Carbonyl; Photochemistry; Flash photolysis; Pauson–Khand
1. Introduction
particularly if the olefin substrates undergo thermally
induced rearrangements. Consequently, techniques that
reduce the thermal demands of this process will signifi-
cantly expand the range of its application.
We are currently interested in investigating the possi-
bility of using photochemical techniques to promote the
Pauson–Khand reaction [1], i.e. the cocylisation of an
alkyne, an alkene and carbon monoxide (Reaction 1).
The products of the Pauson–Khand reaction can be
highly regio- and stereo-chemically pure, explaining its
enormous potential in the synthesis of biologically im-
portant molecules such as hirsutic acid [2],
prostaglandins [3], trans-dihydrojasmonate [4], ste-
modin [5], and (9)-pentalene [6], amongst others.
The carbonyl functionality of the product enone is
usually supplied by a metal carbonyl fragment, gener-
ally derived from dicobalt octacarbonyl, although alter-
native sources such as tungsten [7], and iron [8] carbonyl
compounds have also been successfully used. Many
reactions to date require that the metal carbonyl frag-
ment be supplied in stoichiometric amounts, which is
undoubtedly a significant problem for commercialisa-
tion of the Pauson–Khand process. Consequently, there
is growing interest in the development of systems which
use only catalytic amounts of the metal carbonyl com-
pound [9]. Recently it has been shown that visible light
effectively promotes catalytic Pauson–Khand reactions
[10], however the precise role the photon plays in the
reaction remains uncertain.
In the case of the cobalt system, and based on studies
of the regio- and stereo-chemical control of the prod-
ucts, it has been proposed that decarbonylation of an
initally formed (m2-alkyne)Co2(CO)6 species, is a prereq-
uisite to the overall reaction sequence. Confirmation of
this comes from the work of Krafft and co-workers [11]
who have successfully trapped such a CO-loss species by
a sulphur atom of a thio-substituent on the alkyne,
(Reaction 1)
However, the high temperatures required for the Pau-
son–Khand process is a considerable disadvantage,
* Corresponding author. Tel.: +353-1-7048001: fax: +353-1-
7048002.
E-mail address: conor.long@dcu.ie (C. Long)
0022-328X/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0022-328X(99)00374-5

196
S. M. Draper et al. / Journal of Organometallic Chemistry 588 (1999) 195–199
Table 1
reaction particularly when thermally sensitive substrates
are required.
This paper reports the results of our investigation
The wCO band positions (cm−1; 91 cm−1) in pentane solution for the
dicobalt carbonyl compounds in this investigation
into the photochemistry of
a
range of (m2-
Compound
wCO
alkyne)Co2(CO)6 compounds by both steady-state and
laser flash photolysis techniques and demonstrates the
importance of correct selection of excitation wave-
lengths in promoting the desired CO-loss process.
(m2-C6H5C2H)Co2(CO)6
(m2-C2H2)Co2(CO)6
2094, 2056, 2033, 2029, 2014(sh)
2100, 2060, 2035, 2029, 2018(sh)
2069, 2018, 2005, 1994, 1966,
(m2-C6H5C2H)Co2(CO)5
(pyridine)
(m2-C6H5C2H)Co2(CO)5(PPh3) 2065, 2016, 2006, 1997, 1972
(m2-C6H5C2H)Co2(CO)4(PPh3)2 2021, 2004, 1972, 1949
2. Results and discussion
(m2-C6H5C2H)Co2(CO)4
2018, 2002, 1967, 1942
(pyridine)2
(m2-C2H2)Co2(CO)5(pyridine)
(m2-C2H2)Co2(CO)4(pyridine)2 2021, 2004, 1970, 1941
(m2-C2H2)Co2(CO)5(PPh3)
(m2-C2H2)Co2(CO)4(PPh3)2
Table 1 contains the band positions in the carbonyl
stretching region for the compounds used in this study.
Typical UV–vis spectra for the dicobalt hexacarbonyl
compounds are presented in Fig. 1. In general these
compounds absorb across a broad range of wavelengths
up to 630 nm providing the photochemist with a wide
choice of excitation wavelengths. Preliminary experi-
ments were conducted using broad-band photolysis
techniques in order to identify which spectral region
produced the desired CO-loss process.
2064, 2019, 2002, 1996, 1968
2069, 2017, 2009, 1998, 1975
2028, 1982, 1965, 1948
yielding an isolable pentacarbonyl complex amenable to
spectroscopic and structural analyses.
Conveniently, the (m2-alkyne)Co2(CO)6 intermediates
are readily isolated, and while their thermal chemistry is
well known [12], few detailed studies of their photo-
chemical properties have appeared to date [13]. Of
particular importance, in this regard, is a recent matrix
isolation study, which demonstrated that CO-loss oc-
curs following short-wavelength (uexc=250 nm) photol-
ysis of (m2-alkyne)Co2(CO)6 [14]. This work
demonstrated that (m2-alkyne)Co2(CO)6 is photochemi-
cally inert when irradiated with uexc=350 nm. How-
ever, it would not be feasible to use photons of
wavelengths less than 300 nm in the Pauson–Khand
reaction because of the risk of photochemical damage
to the unsaturated substrates.
A fuller investigation, in particular of the wave-
length-dependent nature of the photochemistry of (m2-
alkyne)Co2(CO)6 compounds, might provide both
further evidence for the importance of the CO-loss
process to the Pauson–Khand reaction, and present an
alternative and more attractive means of promoting the
2.1. Steady-state photolysis of (v2-C6H5C2H)Co2(CO)6
in the presence of trapping ligands
Broad-band photolysis of (m2-C6H5C2H)Co2(CO)6
with uexc\340nm in the presence of a trapping ligand
L (L=C5H5N or PPh3) produced the monosubstituted
(m2-C6H5C2H)Co2(CO)5(L) species. Product assignment
in each case was based on a comparison of the wCO
bands with those of authentic samples of the appropri-
ate monosubstituted hexacarbonyl species (cf. Table 1).
Prolonged photolysis of (m2-C6H5C2H)Co2(CO)5(PPh3)
however, also produced the disubstituted species (m2-
C6H5C2H)Co2(CO)4(PPh3)2. Qualitatively similar re-
sults were obtained in experiments using uexc\400 nm,
however, in general the photolysis times required were
longer under similar experimental conditions owing to
the lower optical density of the parent hexacarbonyl
compound within this spectral range. These results
clearly demonstrate that CO-loss can be achieved under
photochemical conditions, and these results prompted a
fuller investigation of this system using monochromatic
light sources.
2.2. Laser flash photolysis of (v2-C6H5C2H)Co2(CO)6
in cyclohexane solution (uexc=355 nm)
Pulsed photolysis (uexc=355 nm) of (m2-
RC2H)Co2(CO)6 (R=C6H5 or H) in cyclohexane solu-
tion results in a depletion of the absorption of the
hexacarbonyl compound within the rise-time of the
monitoring system (ꢀ 20 ns). The depletion is followed
by a rapid recovery of the absorption to the pre-irradi-
ation level. This behaviour is observed at all monitoring
Fig. 1. The UV–vis spectra of (m2-RC2H)Co2(CO)6 (R=H, —,
conc.=1.6×10−3 M; R=C6H5, - - - - -, conc.=3.2×10−4 M) in
pentane solution at room temperature.

S. M. Draper et al. / Journal of Organometallic Chemistry 588 (1999) 195–199
197
wavelengths where the parent hexacarbonyl has signifi-
cant absorbance.
The recovery of the depleted absorption followed
first-order kinetics, and this provided an observed first-
order rate constant of 4×107 s−1 at 298 K. Addition
of CO to the solution had no effect on the rate of
recovery of the depleted absorption. This is strong
evidence that the depletion of the parent absorption
was not the result of CO-loss. No transient absorption
signals were detected out to monitoring wavelengths of
600 nm, indicating that the photoproduct has a lower
extinction across the entire accessible spectral range
when compared to that of the parent compound. Such
observations have precedence in the literature [15].
A series of experiments was conducted in which a
trapping ligand (C5H5N) was added to the photolysis
solution in a 20-fold excess over the parent carbonyl
compound (typical concentration of parent com-
pound=1×10−5 M). Again the only spectral change
observed was depletion of the parent hexacarbonyl
which recovered with an observed rate constant identi-
cal to that measured in the presence of CO. This
demonstrated that the intermediate produced does not
react with pyridine. However, an IR spectrum recorded
after flash photolysis experiments confirmed the pres-
ence of (m2-RC2H)Co2(CO)5(C5H5N). Clearly the pho-
tochemical changes observed were the result of
excitation by the monitoring lamp rather than the laser
output. This result is consistent with the recently pub-
lished results of matrix isolation studies, which demon-
strated that no photochemistry results from irradiation
with uexc=350 nm [14].
Fig. 2. A typical transient absorption signal observed at 400 nm
following pulsed photolysis of (m2-C6H5C2H)Co2(CO)6 in pentane
solution with uexc=532 nm at 298 K in the presence of CO (conc.=
9×10−3 M).
trations of photoproducts. The kobs was linearly depen-
dent on the concentration of CO, yielding the
second-order rate constant (k2=1.2×106 dm3 mol−1
s−1 at 298 K) as the slope.
Experiments were then conducted in the presence of
a trapping ligand (PPh3). Again a transient absorption
was observed with umax at 400 nm assigned to (m2-
C6H5C2H)Co2(CO)5(solvent). The lifetime of this tran-
sient species depended on the concentration of added
PPh3, and again the analysis of this dependence yields
an estimate of the second order rate constant for the
reaction of (m2-C6H5C2H)Co2(CO)5(solvent) with PPh3
of 3.0×106 dm3 mol−1 s−1 at 298 K. Examination of
the resulting solution by IR spectroscopy confirmed the
presence of (m2-C6H5C2H)Co2(CO)5(PPh3).
To explain the depletion of the parent absorption
observed in these experiments, following laser excita-
tion, without resulting photochemical change, we pro-
pose that homolytic cleavage of the cobaltꢀcobalt bond
occurs, which rapidly undergoes efficient recombina-
tion. Consequently further time-resolved experiments
were conducted using the second harmonic of the Nd-
YAG fundamental at 532 nm.
2.4. Steady-state photolysis of (v2-RC2H)Co2(CO)6
(R=C6H5 or H) in alkane solution
Steady-state experiments using visible light (uexc
500 nm) in the presence of a trapping ligand (L) gave
\
2.3. Laser flash photolysis of (v2-RC2H)Co2(CO)6 in
cyclohexane solution with uexc=532 nm
Pulsed photolysis of (m2-RC2H)Co2(CO)6 (R=C6H5
or H) with uexc=532 nm resulted in the formation of a
transient species which absorbs with a umax at 400 nm.
The position of this absorption compared well to that
observed in matrix isolation experiments on the same
system [14]. Addition of CO ([CO]=9×10−3 M) re-
duced the lifetime of the transient species while its yield
was not affected. A typical transient signal is presented
in Fig. 2. This behaviour is typical of the reaction of
CO-loss intermediates. Under these conditions the sys-
tem is fully reversible i.e. repeated exposure to the laser
radiation did not result in the build-up of high concen-
Fig. 3. Difference IR spectra obtained following broad-band photoly-
sis (uexc\500 nm) of (m2-C2H2)Co2(CO)6 in cyclohexane, acquired at
10, 30, 120, 300 and 600 s, respectively. The negative bands, indicate
depletion of the parent absorptions; while the positive bands are
assigned to (m2-C2H2)Co2(CO)5(PPh3) (see text).

198
S. M. Draper et al. / Journal of Organometallic Chemistry 588 (1999) 195–199
4.2. Equipment
IR spectra were recorded on a Perkin–Elmer 2000
FTIR spectrometer, solution cells were fitted with NaCl
windows (d=0.1 mm), and spectroscopic grade cyclo-
1
hexane or pentane was used. H- and 13C-NMR spectra
were obtained using a Bruker model AC400 spectrome-
ter and were calibrated with respect to the residual
proton resonances of the solvent or with an internal
TMS standard. UV spectra were measured on a
Hewlett–Packard 8453A photodiode array spectrome-
ter using 1 cm quartz cells. Photolysis was performed
using an Applied Photophysics medium-pressure Xe arc
lamp (275 W). Wavelength selection was achieved using
Corning cut-off filters.
The laser system used was a pulsed Nd:YAG laser
capable of generating the second, third or fourth har-
monic at 532, 355 or 266 nm as required, with typical
energies of 150, 30, and 45 mJ per pulse, respectively.
The pulse duration was approximately 10 ns. The ap-
paratus has been described in detail elsewhere with the
excitation and monitoring beams arranged in a cross-
beam configuration [16]. All samples were prepared for
laser flash photolysis in a degassing bulb attached to a
fluorescent cell and protected from light. The solutions
were subjected to three cycles of a freeze–pump–thaw
procedure to 10−2 torr. The solution, at room tempera-
ture, was then subjected to a dynamic vacuum, a pro-
cess which has been shown to remove traces of water
[17]. The required atmosphere of Ar or CO was then
admitted to the sample cell.
Scheme 1. (i) uexc=350 nm in alkane solvent, (ii) ~1 =2.5×10−8 s at
2
298 K, (iii) uexc\400 nm, (iv) k2=1.2×106 dm3 mol−1 s−1 at 298
K for L=CO (carbonyl ligands are indicated thus — for clarity).
rise exclusively to the monosubstituted pentacarbonyl
complexes (m2-RC2H)Co2(CO)5(L) (L=C5H5N or
PPh3). Fig. 3 shows the appearance of new bands and
the continual depletion of parent bands upon photoly-
sis of (m2-RC2H)Co2(CO)6 in the presence of PPh3.
While reducing the excitation wavelengths (uexc\340
nm) increased the yield of the substituted product,
short wavelength photolysis also increased the yield of
the disubstituted derivatives particularly for L=PPh3.
3. Summary and conclusions
This work demonstrates the importance of the cor-
rect selection of excitation wavelength in inducing the
photochemical decarbonylation of (m2-RC2H)Co2(CO)6
complexes. Excitation at 355 nm does not result in
CO-loss and the time resolved behaviour can be inter-
preted in terms of a CoꢀCo bond cleavage followed by
a rapid reformation. In this respect, the compound acts
as a ‘photon sink’ in this region of the spectrum.
Long-wavelength excitation at 532 nm results in the
desired CO-loss process and consequently facilitates the
next step in the Pauson–Khand reaction. This explains
why Livinghouse et al. [10] observed the photochemical
promotion of the Pauson–Khand reaction following
visible light photolysis.
4.3. Synthesis of (v2-alkyne)hexacarbonyldicobalt
complexes
The (m2-alkyne)Co2(CO)6 complexes were prepared
by the standard method, which involves the slow addi-
tion of the alkyne to a degassed solution of Co2(CO)8 in
pentane [18]. The solution was stirred overnight at
room temperature, following which the solvent was
removed under reduced pressure yielding the dark red
products.
The overall photochemistry is summarised in Scheme
1.
Spectroscopic and analytical data for (m2-
1
C2H2)Co2(CO)6: H-NMR (CDCl3): 6.01(s, 2H, CꢀH)
ppm. 13C-NMR (CDCl3): 59.358 (CꢀH), 198.99 (CꢀO)
ppm. UV(C5H12): 346, 447 nm. M.p. 13–13.5°C. Yield
95%. Anal. Calc. C 30.80%, H 0.65%. Found C 31.07%,
H 0.65%. The wCO data are presented in Table 1.
4. Experimental
4.1. Reagents
Co2(CO)8 (Fluka Chemicals), the alkynes phenyl-
acetylene (Aldrich Chemical) and acetylene gas (Air
Products), triphenylphosphine (Aldrich Chemical),
Spectroscopic
and
analytical
data
for
(m2-
C6H5C2H)Co2(CO)6: 1H-NMR (CD3CN): 6.66(1H, s,
ꢁCꢀH), 7.36–7.59(5H, m, Ar CH) ppm. 13C-NMR
(CDCl3): 128.1(s), 128.85(s), 130.21(s), (C6H5) ppm.
UV(C5H12): 352, 422, 536 nm. M.p. 52–53oC. Anal.
Calc. C 43.33%, H 1.56%. Found C 43.06%, H 1.65%.
The wCO data are presented in Table 1.
pyridine, cyclohexane, hept-1-ene, and pentane (spec-
.
troscopic grade, Aldrich Chemical) were used as re-
ceived. All manipulations involving Co2(CO)8 were
conducted under an argon atmosphere.

S. M. Draper et al. / Journal of Organometallic Chemistry 588 (1999) 195–199
199
2 (1977) 295. (g) F.L. Bowden, A.B.P. Lever, Organomet. Chem.
Rev. 3 (1968) 227.
[2] (a) P. Magnus, C. Exon, P. Albough-Robertson, Tetrahedron 41
(1985) 5861. (b) P. Magnus, L. Principle, Tetrhedron Lett. 26
(1985) 4851. (c) C. Exon, P. Magnus, J. Am. Chem. Soc. 105
(1983) 2477.
[3] (a) L. Daalman, R.F. Newton., P.L. Pauson, R.G. Taylor, A.
Wadsworth, J. Chem. Res. (S) (1984) 344. (b) H.J. Jeffer, P.L.
Pauson, J. Chem. Res. (M) (1983) 2201.
[4] V. Rautenshrauch, P. Megard, J. Conesa, W. Ku¨ster, Angew.
Chem. Int. Ed. Engl. 29 (1990) 1413.
4.4. Synthesis of (v2-RC2H)Co2(CO)5(C5H5N) and
(v2-RC2H)Co2(CO)5(PPh3)
The thermal syntheses of (m2-RC2H)Co2(CO)5(L)
complexes where L=PPh3 or pyridine were carried out
according to the method of Manning and co-workers
[19]. Equimolar amounts of the desired ligand and
(m2-RC2H)Co2(CO)6 were added to benzene (5 cm3) and
brought to reflux temperature for 4 h yielding dark red
solutions. The solvent was then removed under reduced
pressure, followed by chromatography on silica gel.
The unreacted starting materials were removed by elu-
tion with petroleum ether. The products were eluted by
diethylether–petroleum ether (1:1 v/v). Evaporation of
the solvent in an Ar stream yielded the crystalline
products. Typical yields were in the range 90–95%. IR
wCO data for these compounds are provided in Table 1.
UV (m2-C2H2)Co2(CO)5(PPh3) (C5H12): umax 322, 376,
and 506 nm.
[5] J. Germanas, C. Aubert, K.P.C. Vollhardt, J. Am. Chem. Soc.
113 (1991) 4006.
[6] N.E. Shore, E.G. Rowley, J. Am. Chem. Soc. 110 (1988) 5224.
[7] (a) T.R. Hoye, J.A. Suriano, J. Am. Chem. Soc. 115 (1993) 1154.
(b) T.R. Hoye, J.A. Suriano, Organometallics 11 (1992) 2044.
[8] (a) A.J. Pearson, R.A. Dubbert, J. Chem. Soc. Chem. Commun.
(1991) 202. (b) A.J. Pearson, R.A. Dubbert, Organometallics 13
(1994) 578. (c) A.J. Pearson, R.A. Dubbert, Organometallics 13
(1994) 1656.
[9] (a) N.Y. Lee, Y.K. Chung, Tetrahedron Lett. 37 (1996) 3145. (b)
S.C. Berk, R.B. Grassman, S.L. Buckwald, J. Am. Chem. Soc.
116 (1994) 8593. (c) B.M. Trost, Science 254 (1991) 1471. (d)
B.M. Trost, Angew. Chem. Int. Ed. Engl. 34 (1995) 259. (e) N.
Jeong, S.H. Hwang, Y.W. Lee, Y.S. Lim, J. Am. Chem. Soc. 119
(1997) 10549. (f) N. Yeong, S.H. Hwang, Y.W. Lee, Y.K.
Chung, J. Am. Chem. Soc. 116 (1994) 3159. (g) B.Y. Lee, Y.K.
Chung, N. Leong, Y. Lee, S.H. Hwang, J. Am. Chem. Soc. 116
(1994) 8793.
4.5. Steady-state photochemical experiments:
photochemical synthesis of (v2-RC2H)Co2(CO)5(L)
(L=C5H5N or PPh3)
[10] B.L. Pagenkopt, T. Livinghouse, J. Am. Chem. Soc. 118 (1996)
2285.
[11] E. Krafft, I.L. Scott, R.H. Romero, S. Feilbelmann, C.E. Van
Pest, J. Am. Chem. Soc. 115 (1993) 7199.
The relevant alkyne compound was dissolved in
cyclohexane together with a four-fold excess of the
desired trapping ligand L. The solution was purged
with argon gas for 15 min, before being transferred to
an IR solution cell for irradiation with light of selected
wavelengths. The reaction was then followed by IR
spectroscopy
[12] (a) I.S. Chia, W.R. Cullen, M. Franklin, A.R. Manning, Inorg.
Chem. 14 (1975) 2521. (b) R.F. Heck, J. Am. Chem. Soc. 85
(1963) 657. (c) J.J. Bonnet, R. Mathieu, Inorg. Chem. 17 (1978)
1973. (d) M. Arewgoda, B.H. Robinson, J. Simpson, J. Am.
Chem. Soc. 105 (1983) 1893. (e) A. Avey, G.F. Nieckaez, D.R.
Tyler, Organometallics 14 (1995) 2790. (f) G. Varadi, A. Vizi-
Orosz, S. Vastag, G. Palyi, J. Organomet. Chem. 108 (1976) 225.
(g) M.E. Krafft, Tetrahedron Lett. 29 (1988) 999. (h) W.G. Sly,
Inorg. Chem. 81 (1959) 18. (i) L.M. Bower, R. Mathieu, Inorg.
Chem. 17 (1978) 1973.
Acknowledgements
[13] J.C. Anderson, B.F. Taylor, C. Viney, E.J. Wilson, J.
Organomet. Chem. 519 (1996) 103.
[14] C.M. Gordon, M. Kiszka, I.R. Dunkin, W.J. Kerr, J.S. Scott, J.
Gebicki, J. Organomet. Chem. 554 (1998) 147.
The authors would like to thank Enterprise Ireland
(formally Forbairt) for financial assistance.
[15] J.R. Knorr, T.L. Brown, J. Am. Chem. Soc. 115 (1993) 4087.
[16] B.S. Creaven, M.W. George, A.G. Ginsburg, C. Hughes, J.M.
Kelly, C. Long, I. McGrath, M.T. Pryce, Organometallics 12
(1993) 3127.
[17] C.J. Breheny, J.M. Kelly, C. Long, S. O’Keeffe, M.T. Pryce, G.
Russell, M.M. Walsh, Organometallics 17 (1998) 3690.
[18] (a) V.Varghese, M. Saha, N. Nicholas, Org. Synth. 67 (1988)
141. (b) H.W. Sternberg, H. Greenfield, R.A. Friedel, J. Wotiz,
R. Markby, I. Wender, J. Am. Chem. Soc. 76 (1954) 1457.
[19] L.S. Chia, W.R. Cullen, M. Franklin, A.R. Manning, Inorg.
Chem. 14 (1975) 2521.
References
[1] (a) I. Ojima, M. Tzamarioudaki, Z. Li, J. Donovan, Chem. Rev.
96 (1996) 635. (b) N.E. Schore, in: L.A. Paquette (Ed.), Organic
Reactions, vol. 40, 1991. (c) N.E. Schore, in: P.M. Trost (Ed.),
Comprehensive Organic Synthesis, vol. 5, Pergamon, Oxford,
1991, p. 1037. (d) P.L. Pauson, Organometallics in: A. de
Meijere, H. tom Dieck (Eds.), Organic Synthesis 2, Springer–
Verlag, Berlin, 1987, p. 234. (e) P.L. Pauson, Tetrahedron 41
(1985) 5855. (f) P.L. Pauson, I.U. Khand, Ann. N.Y. Acad. Sci.
.