Isomerization of 2-methyl-3-butyn-2-ol into prenal: Mechanistic observations on tricomponent catalyst systems of the type Ti(OR)4-CuCl-R'CO2H. Crystal structure of [CuCl(Me2C(OH)C≡CH)]4

Article abstract of DOI:10.1016/S0277-5387(00)00441-1

The mechanism of the title catalytic system was investigated, in particular by synthesizing several compounds that model the postulated metal-oxo-propargyloxo intermediates, such as [Ti(μ-O)(O-Bu')(O₂CTol)]₂ (Tol = p-MeC₆H

Full text of DOI:10.1016/S0277-5387(00)00441-1

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 1693–1698
Isomerization of 2-methyl-3-butyn-2-ol into prenal: mechanistic
observations on tricomponent catalyst systems of the type
Ti(OR)4–CuCl–R%CO2H. Crystal structure of
[CuCl(Me2C(OH)CꢀCH)]4
Christian Y. Lorber *, Marie-The´re`se Youinou, Jacky Kress, John A. Osborn 1
Laboratoire de Chimie des Me´taux de Transition et de Catalyse, UMR du CNRS 7513, Uni6ersite´ Louis Pasteur, Institut Le Bel,
4 rue Blaise Pascal, 67000 Strasbourg, France
Received 28 February 2000; accepted 22 May 2000
Abstract
The mechanism of the title catalytic system was investigated, in particular by synthesizing several compounds that model the
postulated metal–oxo–propargyloxo intermediates, such as [Ti(m-O)(OꢁBut)(O2CTol)]2 (Tol=p-MeC6H4), Ti(OCMe2CꢀCH)4
and [CuCl(Me2C(OH)CꢀCH)]4. The molecular structure of [CuCl(Me2C(OH)CꢀCH)]4 was determined by X-ray crystallography
and heterobimetallic Ti–Cu complexes comprising the 2-methyl-3-butyn-2-oxo fragment are reported. © 2000 Elsevier Science
Ltd. All rights reserved.
Keywords: a-Acetylenic alcohols; Isomerization; Rearrangement; Heterobimetallic titanium–copper systems
1. Introduction
alcohols such as 2-methyl-3-butyn-2-ol (1,1-dimethyl-
propargyl alcohol) can be achieved homogeneously in
the presence of mixtures of titanium alkoxides, copper
(I) chloride and carboxylic acids. Despite the absence of
an oxo ligand, these tricomponent catalyst systems are
even more efficient than the previous oxo catalysts
(Scheme 2).
a,b-Ethylenic carbonyl compounds such as prenal
(3-methyl-2-butenal) are important intermediates in the
manufacture of fragrances, carotenoids or vitamins [1],
and their synthesis via isomerization of a-acetylenic
alcohols has been known for many years [2,3]. This
type of reaction was initially catalysed in protic media,
as first shown by Meyer and Schuster [2], before oxo
complexes of early transition metals such as vanadium,
rhenium and molybdenum were found to be efficient
catalysts [4–6]. The oxo ligands were supposed to play
a key mechanistic role as receptors of migrating propar-
gyl groups, in the [3,3] sigmatropic rearrangement of
metal–oxo–propargyloxo entities (Scheme 1).
The mechanism suggested for the operation of these
new systems is summarized in Scheme 3 [7]. By analogy
More recently however, Chabarde`s has shown [7]
that efficient and selective isomerization of a-acetylenic
Scheme 1.
Scheme 2.
* Current address: Laboratoire de Chimie de Coordination, UPR
du CNRS 8241, 205 route de Narbonne, 31077 Toulouse Cedex,
France. Fax: +33-5-6155-3003.
E-mail address: lorber@lcc-toulouse.fr (C.Y. Lorber).
1 Deceased 23 April, 2000. Address further correspondence to C.Y.
Lorber.
0277-5387/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved.
PII: S0277-5387(00)00441-1

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C.Y. Lorber et al. / Polyhedron 19 (2000) 1693–1698
ter; absorption bands are listed in reciprocal cm as
weak (w), medium (m), or strong (s). 200 MHz 1H
NMR and 50 MHz 13C{1H} NMR spectra were
recorded on a Bruker SY200 instrument; chemical shifts
(l) are given in parts per million relative to TMS.
2.1. Experimental procedure for catalytic isomerisation
of 2-methyl-3-butyn-2-ol
In a typical experiment, 2-methyl-3-butyn-2-ol (8.41
g, 100 mmol), n-nonane (3.41 g) (internal standard for
GC analysis), crotonic acid (1.47 g, 17 mmol), copper(I)
chloride (150 mg, 1.5 mmol), Ti(OꢁPri)4 (42 ml, 1.4
mmol) and o-dichlorobenzene (13 ml) were introduced
into a round-bottom flask fitted with a reflux con-
denser, under an atmosphere of argon. The mixture was
heated to 100°C, after 1 h reaction, GC analysis of the
reaction mixture indicated 98% conversion of 2-methyl-
3-butyn-2-ol into prenal.
Scheme 3.
2.2. Preparation and characterization of
[Ti(v-O)(OꢁBut)(O2CTol)]2 (1)
with the metal–oxo catalysts, it involves initial forma-
tion of a titanium propargyloxo complex a via reaction
of the carboxylic acid and titanium precursor. Copper
chloride, which is necessary for activity, activates the
propargyloxide ligand by interaction with the carbon–
carbon triple bond b. Such an interaction would favour
the [3,3] sigmatropic rearrangement leading to c. Deco-
ordination of CuCl from the allenyloxo ligand of c
would then give d, and displacement of the rearranged
ligand by a further molecule of propargyl alcohol,
followed by its spontaneous isomerization, yields the
a,b-ethylenic carbonyl product and regenerates a.
In this paper, we wish to discuss the initial steps of
this mechanism on the basis of reactivity and model
studies.
Under an atmosphere of argon, a solution of
Ti(tOꢁBu)4 (2.50 g, 7.35 mmol) and TolCO2H (1.00 g,
36.72 mmol) (Tol=p-MeC6H4) in 20 ml of dry C6H6
was refluxed for 9 h. After removal of the volatiles
under vacuum and extraction of the residue in pentane
(10 ml), the solution was kept at −20°C overnight. The
resulting white precipitate was filtered and redissolved
in 10 ml of pentane, and the solution was maintained at
−20°C for a few hours. The solution was then filtered,
and the white solid was dried in vacuo to give 1.20 g of
1 (yield 60%). Anal. Calc. for C24H32O8Ti2 (1): C, 52.96;
H, 5.93. Found: C, 54.50; H, 5.62%. IR (nujol mull,
cm−1) 1607 s (wCꢂC); 1526 s (was COO); 1412 s (ws COO);
1237 s, 1130 s, 1000 s (wCꢁO); 869 s; 764 s (wTiꢁO); 628 m;
1
592 m; 464 m. H NMR (200 MHz, C6D6; l (ppm))
8.18 (d, 3JHaꢁHb=8 Hz, 2H, Ha Tol); 6.76 (d, 3JHaꢁHb=8
t
2. Experimental
Hz, 2H, Hb Tol); 1.86 (s, 3H, MeTol); 1.49 (s, 9H, Bu).
13C{1H} NMR (50 MHz, CDCl3; l (ppm)) 155.5
(CO2H); 144.5, 130.0, 129.0, 127.1 (Caromatic); 31.0
(CMe3); 31.1 (CMe3); 21.9 (MeTol).
All manipulations were carried out under an atmo-
sphere of argon using standard dry box, vacuum line,
or Schlenk tube techniques. Solvents were distilled un-
der nitrogen in the presence of a suitable drying agent.
Copper(I) chloride (Strem) was purified via a known
method [14]. Titanium tetraisopropoxide and titanium
tetratertiobutoxide (Aldrich) were purified by fractional
distillation. Carboxylic acids were dried under vacuum
over P2O5. 2-Methyl-3-butyn-2-ol (Aldrich) was
refluxed over CaH2, fractionally distilled, and stored
2.3. Preparation and characterization of
Ti(OCMe2CꢀCH)4 (2)
To a solution of TiCl4 (556 mg, 2.93 mmol) in
pentane (20 ml) was added dropwise at −30°C a
solution of 2-methyl-3-butyn-2-ol (1.11 g, 13.2 mmol)
and triethylamine (1.30 g, 13.2 mmol) in pentane (20
ml), under an atmosphere of argon. The mixture was
stirred overnight (−20°C) and then filtered to give a
colorless solution. The volatiles were removed under
vacuum to yield the product as pale yellow oil (880 mg,
79%). Further purification was achieved by vacuum
,
over molecular sieves (4 A) in a dry box. Elemental
analyses were performed by the CNRS Microanalytical
Laboratory at Strasbourg or Lyon, France. Infrared
(IR) spectra were recorded as nujol mulls on a Perkin–
Elmer 597 or a Perkin–Elmer 1650 (FTIR) spectrome-

C.Y. Lorber et al. / Polyhedron 19 (2000) 1693–1698
1695
1
distillation. Anal. Calc. for C20H28O4Ti (2): C, 63.16; H,
7.42. Found: C, 63.17; H, 7.44%. IR (nujol mull, cm−1
sion into prenal: H NMR (200 MHz, CDCl3): l 9.95
3
)
(d, JHꢁH=8 Hz, 1H, CHO); 5.89 (m, 1H, ꢂCH); 2.17
4
4
a b
3290 s (wꢀCꢁH); 2090 w (wCꢀC); 1210 s, 1130 s, 1000 s
(d, JH ꢁH=1 Hz, 3H, Mea); 1.98 (d, JH ꢁH 1 Hz, 3H,
1
(wCꢁO). H NMR (200 MHz, C6D6; l (ppm)) 2.10 (s,
Meb).
2.7. Reaction between CuCl and 2
1H, ꢀCꢁH); 1.67 (s, 6H, Me). 13C{1H} NMR (50 MHz,
C6D6; l (ppm)) 89.2 (CꢀCH); 76.3 (CꢁO); 70.0 (ꢀCH);
33.2 (Me).
CuCl (104 mg, 1.06 mmol) was added to a solution
of 2 (400 mg, 1.06 mmol) in 15 ml CH2Cl2. The mixture
was stirred at 20°C for 1 h and then filtered to eliminate
unreacted CuCl. Removal of volatiles under vacuum
yielded orange oil (450 mg). 1H NMR (200 MHz,
CD2Cl2) l 2.69 (s, 1H, ꢀCH); 1.65 (s, 6H, Me) and
peaks from residual 2. IR (nujol mull, cm−1) 3190 s
(wꢀCH); 1940 m (wCꢀC) and bands from residual 2. The
reaction was also followed in situ by 1H NMR in
CDCl3.
2.4. Preparation and characterization of
[CuCl(Me2C(OH)CꢀCH)]4 (3)
Under an atmosphere of argon, 500 mg of freshly
purified CuCl was stirred at about 50°C with 2 ml of
2-methyl-3-butyn-2-ol until a clear solution was ob-
tained. After hot filtration over celite, the colorless
filtrate was allowed to cool slowly to room temperature
to give, after a few days, colorless crystals of 3, which
were suitable for X-ray analysis. Crystals were washed
with pentane (3×2 ml) and dried under vacuum (yield
45%). Anal. Calc. for C20H32O4Cl4Cu4 (3): C, 32.80; H,
3. Results and discussion
4.40. Found: C, 32.82; H, 4.32%. IR (nujol mull, cm−1
)
3500–3200 m (wOH); 3180 s (wꢀCꢁH); 1940 m (wCꢀC); 1205
s, 1160 s (wCꢁO); 240 w (wCuꢁClꢁCu). H NMR (200 MHz,
3.1. Formation of titanium–oxo functions
1
C6D6; l (ppm)) 2.22 (s, 1H, ꢀCꢁH); 1.57 (s, 1H, OH);
1.31 (s, 6H, Me). 13C{1H} NMR (50 MHz, C6D6; l
(ppm)) 89.54 (CꢀCH); 70.07 (ꢀCH); 64.70 (CꢁO); 31.35
(Me).
To investigate the initiation step of the mechanism,
we first measured the influence of the nature of the
carboxylic acid on the catalytic efficiency of a given
pair of titanium and copper compounds. Using a mix-
ture of Ti(OꢁPri)4 and CuCl (1:1), isomerization of
2-methyl-3-butyn-2-ol (50 equivalents) at 100°C, in the
presence of five equivalents of various carboxylic acids,
reaches the following conversion percentages after 30
min: 3,3-dimethylacrylic acid (pKa=5.12), 98%; cro-
tonic acid (pKa=4.69), 97%; p-methoxybenzoic acid
(pKa=4.49), 88%; p-t-butylbenzoic acid (pKa=4.39),
82%; p-toluic acid (pKa=4.37), 72%; acetic acid
(pKa=4.75), 48%; benzoic acid (pKa=4.19), 42%; p-
chlorobenzoic acid (pKa=3.97), 10%; p-nitrobenzoic
acid (pKa=3.41), 3%. The reaction rates therefore de-
crease with decreasing pKa values. This implies that, if
carboxylic acids are necessary for the formation of
titanium oxo species, too strong an acid will be less
favourable; the lower activity obtained with acetic acid
demonstrates that other factors contribute.
2.5. Crystal structure determination
Crystal data were obtained at −100°C using a
Philips PW1100/16 diffractometer with Cu Ka
,
graphite-monochromated radiation (u=1.5418 A). The
scan width (°) in the q/2q mode used was Dq=0.80+
0.14 tan q. A total of 953 hkl reflections were recorded
within 3BqB52°, and 823 reflections having I\3|(I)
were used for determining and refining the structure.
The structure was determined using direct methods and
refined using full-matrix least-squares to R(F)=0.039
and Rw(F)=0.056, GOF=1.478, maximum electronic
density=0.765 eA−3, 145 variables. The Nonius Open-
,
Molen/Alpha package [15] was used for all calculations.
Selected crystal data for 3: C10H16O2Cl2Cu2, Mr=
366.22, orthorhombic, space group Pba2, a=9.170(2),
On the other hand, we have found that the reaction
of Ti(OꢁBut)4 with p-toluic acid (benzene, 80°C) yields
significant amounts of a bridged oxo species which
could be separated and formulated as {Ti(m-
O)(OꢁBut)(O2CTol)}2 (1), and whose structure is pro-
posed in Scheme 4 on the basis of spectroscopic data.
This confirms that titanium alkoxides can be effectively
convert into oxo species in the presence of carboxylic
acids [8,9]. Complex 1 is analogous to earlier com-
pounds such as {Ti(m-O)(m-O2CR)2}2 [8b–c, 9e] and
catalyses the rearrangement of 2-methyl-3-butyn-2-ol
into prenal as efficiently as its titanium tetraalkoxide
precursor. However, analogous substrates or reactants
3
,
,
b=18.544(5), c=8.188(2) A, V=1392(1) A , Z=4,
D
calc=1.75 g cm−3, v=7.231 mm−1
.
2.6. Reaction between TiCl4 and 3
In an NMR tube containing 15 mg of 3 (0.014 mmol)
in 0.45 ml CDCl3 were added 10 ml of TiCl4 (0.055
mmol). The solution became pink after a few minutes
1
at 20°C. H NMR (200 MHz, CDCl3): l 2.80 (s, 1H,
ꢀCH); 1.87 (s, 6H, Me). When the solution was kept at
room temperature for 24 h, it turned blue. GC analysis
1
and H NMR spectroscopy showed complete conver-

1696
C.Y. Lorber et al. / Polyhedron 19 (2000) 1693–1698
Scheme 4.
lead to less clear-cut results. For example, addition of
crotonic acid to Ti(OꢁPri)4 only affords mixtures of
carboxylato and i-propyloxo complexes, in which the
presence of an oxo function could not be firmly
established.
3.2. Formation of titanium–propargyl oxide complexes
In species a, a 1,1-dimethyl-propargyloxo ligand
bound to titanium is also involved. This would arise
from alkoxide exchange between OR ligands initially
present in Ti(OR)4 and 2-methyl-3-butyn-2-ol. Such an
exchange process has literature precedent [10] and, with
R=i-Pr, we obtained a mixture of products of general
Fig. 1. ORTEP diagram of compound 3. Hydrogen atoms are omitted.
,
Selected bond distances (A): Cu(1)ꢁCl(1)=2.263(3), Cu(1)ꢁCl(2)=
2.286(2), Cu(2)ꢁCl(1)=2.258(2), Cu(2)ꢁCl(2’)=2.266(3), Cu(1)ꢁ
C(1)=2.01(1),
Cu(1)ꢁC(2)=2.037(8),
Cu(2)ꢁC(6)=1.97(1),
Cu(2)ꢁC(7)=2.021(9), C(1)ꢁC(2)=1.19(1), C(6)ꢁC(7)=1.18(1). Se-
lected bond angles (°): Cl(1)ꢁCu(1)ꢁCl(2)=105.9(1), C(1)ꢁCu(1)ꢁ
C(2)=34.2(3),
C(6)ꢁCu(2)ꢁC(7)=34.3(4),
C(1)ꢁC(2)ꢁC(3)=
formula Ti(OꢁPri)4−n(OCMe2CꢀCH)n (n=0–4).
A
161.1(9), C(6)ꢁC(7)ꢁC(8)=164(1).
well-defined propargyl oxide complex was synthesized
by reacting four equivalents of 2-methyl-3-butyn-2-ol
with TiCl4 in the presence of triethylamine, leading to
Ti(OCMe2CꢀCH)4 (2) in good yield. The bulkiness of
the dimethylpropargyloxo ligands probably prevents
association by means of oxygen bridges, allowing a
monomeric geometry similar to that of Ti(OꢁBut)4 [10]
to be proposed for this compound (Scheme 4). Reac-
tion of 1,1-dimethylpropargyl alcohol to form the te-
traalkoxide 2 is revealed by a chemical shift variation of
3.3. Formation of copper(I)–chloride complexes of
2-methyl-3-butyn-2-ol
The role of the copper chloride component of the
catalyst system would be to interact with, and hence
activate, the carbon–carbon triple bond of a-acetylenic
alcohols, as in species b (Scheme 3). The affinity of
copper (I) complexes for alkynes, resulting in the for-
mation of p-adducts, is well-established [11], and we
have verified that various propargylic alcohols effec-
tively coordinate through their triple bond to copper
(I). Complexes such as [CuCl(2-butyn-1-ol)]4,
[Cu(CF3CO2)(2-methyl-3-butyn-2-ol)]2, and [Cu(bipy)-
(2-methyl-3-butyn-2-ol)]PF6 were appropriately synthe-
sized [5b]. In particular, dissolution of CuCl into 2-
methyl-3-butyn-2-ol, followed by filtration and slow
crystallization, yielded colorless, air-sensitive crystals of
{CuCl(Me2C(OH)CꢀCH)}4 (3). The 1H NMR spectrum
of 3 in C6D6 showed the terminal acetylenic proton of
the 2-methyl-3-butyn-2-ol shifted downfield on coordi-
nation, from 2.02 to 2.22 ppm (and from 2.42 to 2.60
ppm in CDCl3), whereas the methyl signal is hardly
shifted. This feature characterizes bonding of the car-
bon–carbon triple bond to copper, which is further
defined by the shift of the infrared CꢀC stretching band
from 2110 cm−1 for the free ligand to 1940 cm−1 for 3.
1
1HMe from 1.32 to 1.67 ppm, and of HCꢀCH from 2.02
to 2.10 ppm (in C6D6), the infrared wCꢀC being only
slightly modified. It is worth noting that complex 2 is as
active as other Ti(OR)4 catalysts, and that, in the
absence of 1,1-dimethylpropargyl alcohol, its ligands
isomerize as expected into prenal (100°C, Ti:Cu:acid=
1:1:5).
Further comparison of the ability of various titanium
complexes to catalyse the rearrangement of 2-methyl-3-
butyn-2-ol has led to following qualitative sequence
(CuCl:crotonic
acid:2-methyl-3-butyn-2-ol=1:5:50,
100°C, 30 min reaction):
Ti(OꢁBut)4:Ti(OꢁPri)4\TiCl(OꢁPri)3:TiO(acac)2
\TiCl2(OꢁPri)2\Cp2TiCl2\TiCl3(OꢁPri). Such a re-
sult is consistent with rapid alkoxide exchange on tita-
nium, and completes earlier results [7] by showing, in
particular, the negative effect of chloro ligands.

C.Y. Lorber et al. / Polyhedron 19 (2000) 1693–1698
1697
signals from respectively 1.54 and 2.60 ppm to 1.87 and
2.80 ppm, and by the disappearance of the hydroxyl
proton. Such downfield displacements are similar to
those observed on conversion of 1,1-dimethylpropargyl
alcohol into 2, suggesting that the four hydroxy func-
tions of the copper tetramer have reacted, each with
one molecule of TiCl4, to yield the bimetallic complex
[CuCl(Me2C(OTiCl3)CꢀCH)]4 (4). However, the degree
of association in 3 may be modified during the course
of reaction, especially if chlorides bridge titanium cen-
ters. A simple representation of a monomeric unit of 4
is given in Scheme 5.
Scheme 5.
The crystal structure of 3 has been determined by
single-crystal X-ray diffraction. An ORTEP drawing is
shown in Fig. 1, along with selected interatomic dis-
tances and angles. The molecular structure consists of
two Cu2Cl2(C5H8O)2 units crystallographically related
by a twofold symmetry axis, yielding an overall te-
trameric arrrangement of four copper atoms lying at
the vertices of a slightly distorted tetrahedron. The two
metal centers of the dimeric units are linked through a
nearly symmetrical chloride bridge, with Cu(1)ꢁCl(1)=
On the other hand, addition of one equivalent of
CuCl to 2 in CDCl3 results in the emergence of a new
1,1-dimethylpropargyl oxide spectrum in which the
acetylenic proton signal is shifted downfield to 2.69
ppm, although features of 2 are still observed. This
suggests that a fraction of the carbon–carbon triple
bonds of 2 coordinates to CuCl, as depicted in 5 for
instance (Scheme 5). The spectrum of the uncomplexed
propargyl oxide ligands of 5 would be identical to that
of 2, itself certainly still present. The infrared spectrum
of the orange oil obtained after removal of all volatiles
under vacuum, showing two wCꢀC bands at 2100 and
1940 cm−1, confirms this conclusion.
,
2.263(3) and Cu(2)ꢁCl(1)=2.258(2) A. The two dimeric
units are associated similarly one to each other, with
,
Cu(1)ꢁCl(2)=2.286(2) and Cu(2)ꢁCl(2%)=2.266(3) A,
leading to an eight-membered ring possessing a boat
configuration. Each copper atom is further bound to
the carbon–carbon triple bond of a 2-methyl-3-butyn-
2-ol molecule, thus adopting an approximate trigonal
planar geometry, as already observed in other cop-
per(I)–alkyne compounds [12]. The copper–alkyne in-
teraction is weak, as revealed by the short
Unfortunately, neither 4 nor 5 was stable enough to
be isolated. Interestingly, 4 decomposed at room tem-
perature within 24 h with liberation of prenal, mimick-
ing not only the structure of intermediate b, but also its
reactivity.
In conclusion, our results are consistent with the
mechanistic proposal of Scheme 3. The respective role
of titanium, copper and carboxylic acid in the catalytic
system has been confirmed, and the observation of
heterobimetallic Ti–Cu complexes of 2-methyl-3-butyn-
2-ol, in particular, is unprecedented to our knowledge.
,
carbon–carbon bonds of 1.19(1) and 1.18(1) A, but
strong enough to deviate the propargyl skeleton from
linearity by 18.9(9) and 16(1)°.
This shows how coordination of CuCl to the car-
bon–carbon triple bond of a catalytic intermediate such
as b may facilitate subsequent rearrangement step
(Scheme 3) by favoring nucleophilic attack of the tita-
nium–oxo function on the terminal acetylenic carbon:
besides making the triple bond more reactive, it also
brings the terminal acetylenic carbon closer to the oxo
ligand [13].
4. Supplementary data
Further details of the structure investigation have
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 141001. Copies of this infor-
mation may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK (fax: +44-1223-336-033; e-mail: deposit@
ccdc.cam.ac.uk or www: http://www.ccdc.cam.ac.uk).
3.4. Formation of heterobimetallic Ti–Cu–propargyl
oxide complexes
In order to model more precisely species b, we at-
tempted to synthesize titanium–propargyl oxide com-
plexes such as 2 in which the carbon–carbon triple
bond of the propargyl oxide ligand would be bound to
copper (I) chloride as in 3. Thus, addition of four
equivalents of TiCl4 to tetrameric 3 was followed by H
NMR in CDCl3. Rapid reaction occurs, as revealed by
the significant shift of the methyl and acetylenic proton
Acknowledgements
We thank Dr P. Chabarde`s and Dr C. Mercier for
helpful discussions and we gratefully acknowledge the
support of this work by the Centre National de la
Recherche Scientifique and Rhoˆne-Poulenc.
1

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C.Y. Lorber et al. / Polyhedron 19 (2000) 1693–1698
Chem. Soc., Dalton Trans. (1994) 3533. (e) N.W. Alcock, D.A.
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