F. Simal, L. Wlodarczak, A. Demonceau, A. F. Noels
FULL PAPER
PerkinϪElmer FTϪIR 1720 X spectrometer with a selected resolu-
tion of
PerkinϪElmer 8500 gas chromatograph equipped with a FID and
an internal integrator (column, RSL-150, 0.32 mm ϫ 30 m; injector
temperature, 290 °C; detector temperature, 290 °C; temperature
program, 60 °C, 10 °C·minϪ1, 220 °C, 15 min).
ing properties that are much more pronounced than those
of Cp, again supporting the assumption of the formation
of a 16-electron-species in the key step. It is therefore likely
that the efficiency of ruthenium complexes 1Ϫ3 might be
connected more to the strength of the RuϪP bond,[25] and
hence to the releasing ability of triphenylphosphane, than
to the formation of ring-slipped η3-intermediates.
2
cmϪ1
. Ϫ GLC analyses were performed on a
General Procedure for the Kharasch Addition: The ruthenium com-
plex (0.03 mmol) was placed in a glass tube containing a bar mag-
net and capped with a three-way stopcock. The reactor was purged
of air (three vacuum/nitrogen cycles) and a solution containing
toluene (4 mL), dodecane (0.25 mL), CCl4 (13 mmol), and alkene
(9 mmol) was added. The reaction mixture was then heated in a
thermostated oil bath (see tables and figures for details). Conver-
sion and yield are based on the olefin, and were determined by GC
using dodecane as internal standard. The Kharasch adducts were
characterized by comparison with literature data.
A
coordinatively unsaturated 16-electron ruthenium
center thus having been generated, pseudo-oxidative addi-
tion of the carbonϪhalogen bond could then occur,
yielding both the 17-electron ruthenium(III) species and the
radical R•, which would eventually add to the olefin or,
most probably, recombine with the halogen, according to
the relative position of the equilibrium outlined in
Scheme 1. At that stage, the redox potentials of complexes
1Ϫ3 would be of utmost importance. Cyclic voltammetry
measurements for [RuCl(η5-ligand)(PPh3)2] in dichlorome-
thane indicated that indenyl or pentamethylcyclopentadi-
enyl complexes are oxidized at lower potentials than cyclo-
pentadienyl complexes.[17] It is therefore feasible that penta-
methylcyclopentadienyl and indenyl, acting as electron res-
ervoirs toward the metal fragment [RuCl(PPh3)2] in 2 and 3,
Typical Decomposition Experiment: The ruthenium complex
(0.015 mmol) was weighed out in a Wilmad screw-cap 10 mm
NMR tube, which was then purged of air (three vacuum/nitrogen
cycles) before addition of [D8]toluene (0.5 mL) and toluene (2 mL).
The NMR tube was shaken to dissolve the complex, and the t0 31P
NMR spectrum was then recorded at 293 K. Tetrachloromethane
(14.5 µL, 0.15 mmol, 10 equiv. relative to ruthenium complex) was
respectively, or [RuCl(PPh3)] in the transient species, favor then added, and the NMR tube was shaken. The decomposition
reaction was monitored by 31P NMR at various temperatures (see
rutheniumϪphosphorous bond rupture or stabilize the 16-
captions for further details) to simulate Kharasch addition. Each
electron intermediate. The cyclic voltammetry results are in
complex’s decomposition was repeated at least twice independently
agreement with this interpretation, since the easier oxida-
(on different days) to establish reproducibility. 31P NMR:
tion of the pentamethylcyclopentadienyl and indenyl com-
[RuCl(Cp)(PPh3)2]: δ ϭ 39.87 (s); [RuCl(Cp*)(PPh3)2]: δ ϭ 41.05
plexes suggests higher electron density at the metal, a phe-
(s); [RuCl(Ind)(PPh3)2]: δ ϭ 47.62 (s); PPh3, δ ϭ Ϫ5.00 (s).
nomenon facilitating the pseudo-oxidative addition of the
carbonϪhalogen bond on the metal center.
Acknowledgments
We gratefully acknowledge the assistance of the ‘‘Fonds National
de la Recherche Scientifique’’ (F.N.R.S.), Brussels, and the ‘‘Con-
Conclusions
`
seil de la Recherche’’ (University of Liege) for the purchase of ma-
Air-stable and readily available [RuCl(Cp*)(PPh3)2] and
[RuCl(Ind)(PPh3)2] are the best ruthenium-based catalyst
precursors found so far for promotion of the addition of
jor instruments. We also thank Prof. S. P. Nolan (University of
New Orleans, Louisiana) for helpful comments on the synthesis
of complex 2, and Dr. W. Baratta (University of Udine, Italy) for
CCl4 across olefins, at temperatures as low as 40 °C. A two- stimulating discussions.
step mechanism, in which a phosphane ligand disengage-
[1]
ment occurs prior to the activation of the halogenated com-
pound by the unsaturated ruthenium center, is suggested.
Although a detailed understanding of the reaction mechan-
ism (including the factors that favor ATRA over ATRP)
must await further study, further improvements with this
family of ruthenium(II) initiators can be expected. Various
stereoelectronic variations of the ligands are now under in-
vestigation, together with further study of the mechanism.
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[2] [2a]
H. Matsumoto, T. Nakano, Y. Nagai, Tetrahedron Lett.
[2b]
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[3]
[4]
[5]
Experimental Section
[6] [6a]
M. Sawamoto, M. Kamigaito, Chemtech. 1999, 29, 30Ϫ38.
General: All reactions were performed under an atmosphere of dry
nitrogen, using standard Schlenk and vacuum-line techniques. All
reagents and solvents were dried, distilled, and stored under nitro-
gen at Ϫ20 °C, according to standard procedures.[26] Complexes 1/
4, and 3/5 were purchased from Aldrich and Stem, respectively, and
used as received. Complexes 2[27] and 6[3] were prepared according
to published procedures. NMR spectra were recorded on a Bruker
AM 400 spectrometer. Ϫ Infrared spectra were measured on a
[6b]
Ϫ
Ϫ
K. Matyjaszewski, Chem. Eur. J. 1999, 5, 3095Ϫ3102.
[6c]
K. Matyjaszewski, in Education in Advanced Chemistry,
Mechanistic Aspects of Molecular Catalysis (Ed.: B. Marciniec),
Wydawnictwo Poznanskie, Poznan-Wroclaw 1999, 6, 95Ϫ105.
M. Kato, M. Kamigaito, M. Sawamoto, T. Higashimura, Mac-
romolecules 1995, 28, 1721Ϫ1723.
H. Takahashi, T. Ando, M. Kamigaito, M. Sawamoto, Macro-
molecules 1999, 32, 3820Ϫ3823.
[7]
[8]
2694
Eur. J. Org. Chem. 2001, 2689Ϫ2695