A cationic 1-(2-methylpyridine)phosphole cymene ruthenium chloride complex as an efficient catalyst in the transfer hydrogenation of ketones

Article abstract of DOI:10.1021/om030065l

Reaction of 2,5-diphenylphospholide anion with 2-chloromethylpyridine affords the 1-(2-methylpyridine)phosphole ligand. The corresponding cationic Ru-(cymene)Cl chelate complex catalyzes the hydrogen transfer process of ketones with very high TON and TOF numbers.

Full text of DOI:10.1021/om030065l

1580
Organometallics 2003, 22, 1580-1581
A Ca tion ic 1-(2-Meth ylp yr id in e)P h osp h ole Cym en e
Ru th en iu m Ch lor id e Com p lex a s a n Efficien t Ca ta lyst in
th e Tr a n sfer Hyd r ogen a tion of Keton es
Claire Thoumazet, Mohand Melaimi, Louis Ricard, Franc¸ois Mathey,* and
Pascal Le Floch*
Laboratoire “He´teroe´le´ments et Coordination”, UMR CNRS 7653, De´partement de Chimie,
Ecole Polytechnique, 91128 Palaiseau Cedex, France
Received J anuary 28, 2003
Sch em e 1
Summary: Reaction of 2,5-diphenylphospholide anion
with 2-chloromethylpyridine affords the 1-(2-methylpy-
ridine)phosphole ligand. The corresponding cationic Ru-
(cymene)Cl chelate complex catalyzes the hydrogen trans-
fer process of ketones with very high TON and TOF
numbers.
Due to the presence of two very different binding sites,
mixed P, N-chelate ligands possess unusual electronic
properties that make them very attractive in both
coordination chemistry and catalysis.1 These ligands,
which usually show a pronounced hemilabile character,
have found interesting applications in some catalytic
processes of recognized importance such as reduction,2
allylic alkylation,3 C-C coupling (Heck reaction),4 and
olefin/CO copolymerization.5 As part of a large research
program aimed at incorporating diverse phosphorus
heterocycles in mixed heteroditopic chelates and poly-
dentate ligands,6 we decided to explore the synthesis of
1-phospholyl derivatives of 2-methylpyridine. Herein,
we report on the synthesis, the X-ray crystal structure,
and the catalytic activity in the transfer hydrogenation
of ketones of the cationic cymene RuCl complex of one
of these new ligands.
Among different possible candidates for the phosphole
subunit, we selected the 2,5-diphenylphospholyl ligand,
whose P-functional derivatives show a very good stabil-
ity toward air oxidation. Furthermore, the precursor of
these P-functional compounds, the 1,2,5-triphenylphos-
phole, is cheap and easily available on a multigram scale
from the simple reaction of dichlorophenylphosphine
with 1,4-diphenyl-1,3-butadiene.7 Synthesis of the 1-(2-
methylpyridine)-2,5-diphenylphosphole ligand 3 was
carried out following a classical approach that involves
the reaction of the 2,5-diphenylphospholide anion 2 with
2-chloromethylpyridine in THF at room temperature.
Interestingly, we found that when the easily available
1,1′-bis(2,5-diphenylphosphole)8 1 is used as starting
precursor of anion 2, no chromatographic separation is
needed to isolate 3. Following this procedure, ligand 3
was obtained in 80% yield as an air-stable yellow solid
which can be stored without special precautions. The
formulation of 3 was conventionally established by NMR
and mass spectroscopies and elemental analysis (Scheme
1).
As a preliminary study to explore the coordination
behavior of 3, we deliberately focused our work on the
synthesis of chelate-based (η6-arene)ruthenium com-
plexes. Reaction of 3 with half an equivalent of [Ru(η6-
C10H14)Cl2] did not produce the cationic chelate complex
but the monodentate complex 4, which results from the
simple cleavage of the dimer (eq 1). Additionnal heating
in different solvents did not allow the displacement of
the chloride ligand. Though 4 was fully characterized
(1) See for example: (a) Fritz, H. P.; Gordon, I. R.; Schwarzhans,
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10.1021/om030065l CCC: $25.00 © 2003 American Chemical Society
Publication on Web 03/21/2003

Communications
Organometallics, Vol. 22, No. 8, 2003 1581
Ta ble 1. Tr a n sfer Hyd r ogen a tion of Keton es Usin g
Com p lex 5 a s Ca ta lysta
entry
substrate
yield (%)b
TONd
TOFe
1
2
3
4
4-heptanone
1,3-diphenylacetone
cyclohexanone
syn-2,5-dimethyl-
cyclohexanone
acetophenone
12 (43)c 2.40 × 106 160.0 × 103
19 (55)c 3.80 × 106 253 × 103
100
100
20.0 ×106 1.33 × 106
20.0 × 106 1.33 × 106
5
6
7
8
9
90 (100)c 18 × 106
1.20 × 106
2-acetylepyridine
benzophenone
87 (100)c 17.4 × 106 1.16 × 106
77 (100)c 15.4 × 106 1.026 × 106
4-bromoacetophenone 67 (100)c 13.4 × 106 893 × 103
4-methoxyaceto-
phenone
60 (93)c 12.0 × 106 800 × 103
10 4,4′-bismethoxybenzo- 0 (40)c
8 × 106e
133 × 103f
phenone
a
Conditions: substrate (3 mmol), catalyst 5 (5 × 10-6 mol %),
15 mL (0.1 M KOH in i-PrOH), 90 °C, 15 h. bYields were
F igu r e 1. ORTEP view of one molecule of complex 5.
Ellipsoids are scaled to enclose 50% of the electron density.
The numbering is arbitrary and different from that used
in the assignment of NMR spectra.
determined by 1H NMR or GC. Yield after 60 h. dTON ) mol
c
e
f
product/mol catalyst. TOF ) TON/reaction time. TON and TOF
calculated for t ) 60 h.
of a highly efficient bis-carbene rhodium(III) complex
(max. TON ) 19 × 103 in the conversion of cyclohex-
anone into cyclohexanol).12
Typically, complex 5 was reacted at 90 °C with a
ketone in the presence of KOH as deprotonating agent
and i-PrOH, which serves both as solvent and proton
source. Interestingly, we found that 5 is highly active
in this hydrogenation process (eq 3). Thus, in all
by NMR techniques and elemental analysis, its formu-
lation was definitively established by X-ray crystal-
lography (see Supporting Information available). To
force coordination of the pyridine moiety, complex 4 was
reacted with the chloride abstractor AgBF4. In this way,
complex 5 was quantitatively formed and isolated as an
air-stable orange solid. This complex can also be syn-
thesized in a more straightforward way by reacting 3
with the ruthenium dimer in the presence of AgBF4 (eq
2). Complex 5 was fully characterized, and its X-ray
crystal structure is presented in Figure 1. As can be
seen, the piano-stool geometry of 5 is very classical and
does not deserve further comment.
transformations studied, only a low loading of catalyst
(5 × 10-6 mol %) was needed to reach very important
TON and TOF numbers. As can be seen in Table 1,
aliphatic and aromatic ketones can be readily converted
to the corresponding secondary alcohols. The highest
TON and TOF numbers were obtained in the conversion
of cyclic ketones (total conversion, TON ) 20 × 106, TOF
) 1.33 × 106 mol h-1). From these data, it is also evident
that the electron richness of the aromatic ring strongly
influences the rate of conversion, electron rich rings
being less reactive (see entries 9 and 10). However, even
in these two cases, TONs are higher than those usually
obtained using other catalysts.
Arene ruthenium complexes are attracting a continu-
ing interest in homogeneous catalysis.9 A remarkable
application is their use in the transfer hydrogenation
of ketones.10 Therefore, as a preliminary test for the
catalytic activity of complex 5, we turn our attention to
this process.11 Though efforts are now focusing on the
enantioselective version of this very useful reaction,
recent works have showed that the design of highly
stable catalysts able to provide high TON and TOF
numbers is still a motivating challenge. In this respect,
Crabtree and et al. have recently reported on the use
In summary, we devised an easy access to a new type
of mixed phosphole-pyridine ligands. Preliminary ex-
periments indicate that their cationic Ru-(cymene)
complexes display a promising catalytic activity in the
transfer hydrogenation of ketones. We are now pursuing
our studies on these systems in exploring the synthesis
of functional derivatives such as tridentate species and
the introduction of a chiral center. These results will
be reported in due course.
Ack n ow led gm en t. The authors thank the CNRS
and the Ecole Polytechnique for supporting this work.
(9) For recent references, see: (a) Ku¨cu¨kbay, H.; Cetinkaya, B.;
Guesmi, S.; Dixneuf, P. H. Organometallics 1996, 15, 2434. (b) Fidalgo,
E. G.; Plasseraud, L.; Su¨ss-Fink, G. J . Mol. Catal. 1998, 132, 5. (c)
Bruneau, C.; Dixneuf, P. H. Acc. Chem. Res. 1999, 32, 311. (d)
Weissman, H.; Song, X.; Milstein, D. J . Am. Chem. Soc. 2001, 123,
337. (e) Boxwell, C. J . Dyson, P. J .; Ellis, D. E.; Welton, T. J . Am. Chem.
Soc. 2002, 124, 9334. (f) Su¨ss-Fink, G.; Faure, M.; Ward, T. R. Angew.
Chem., Int. Ed. 2002, 41, 99.
(10) See for example: (a) Noyori, R.; Hashiguchi, S. Acc. Chem. Res.
1997, 30, 97. (b) Fujii, A.; Hashiguchi, S. Uematsu, N.; Ikariya, T.;
Noyori, R. J . Am. Chem. Soc. 1996, 118, 2521.
(11) Noyori, R.; Ohkuma, T. Angew. Chem., Int. Ed. 2001, 40, 40,
and references therein.
Su p p or tin g In for m a tion Ava ila ble: Text detailing the
preparation of compounds 3-5, catalysis protocol, and char-
acterization of products formed and tables of X-ray crystal-
lographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
OM030065L
(12) (a) Albrecht, M.; Crabtree, R. H.; Mata, J .; Peris, E. Chem.
Commun. 2002, 32. (b) Albrecht, M.; Miecznikowski, Samuel, A.; Faller,
J . W.; Crabtree, R. H. Organometallics 2002, 21, 3596.