η5-Rhodium(I) complexes of a λ4- phosphinine anion: Syntheses, X-ray crystal structures, and application in the catalyzed hydroformylation of olefins

Article abstract of DOI:10.1021/om049261y

2,6-Bis(trimethylsilyl)-4,5-diphenylphosphinine 1 and 2,3,5,6- tetraphenylphosphinine 2 react with tert-butyllithium to afford the expected λ⁴-phosphinine anions 3 and 4. These anions react with [Rh(COD)Cl]₂ to yield the corresponding η⁵-complexes 5 and 6, in which coordination occurs through the anionic carbocyclic π-system of the ring. Both complexes were fully characterized by conventional spectroscopic techniques, and their X-ray crystals structures were recorded. The catalytic activity of 5 and 6 was tested in the hydroformylation of olefins. Good conversion yields and turnover frequencies were obtained in the hydroformylation of styrene and cyclohexene under mild conditions with low catalyst loading. The hydroformylation of styrene occurs with a high regioselectivity (93/7) in favor of the branched isomer. Interestingly, catalyst 6 catalyzes the transformation of 2,3-dimethyl-2-butene into 3,4-dimethylpentanal through a tandem isomerization/hydroformylation process.

Full text of DOI:10.1021/om049261y

508
Organometallics 2005, 24, 508-513
η5-Rhodium(I) Complexes of a λ4-Phosphinine Anion:
Syntheses, X-ray Crystal Structures, and Application in
the Catalyzed Hydroformylation of Olefins
Audrey Moores, Nicolas Me´zailles, Louis Ricard, and Pascal Le Floch*
Laboratoire “He´te´roe´le´ments et Coordination”, UMR CNRS 7653, Ecole Polytechnique,
91128 Palaiseau Cedex, France
Received September 23, 2004
2,6-Bis(trimethylsilyl)-4,5-diphenylphosphinine 1 and 2,3,5,6-tetraphenylphosphinine 2
react with tert-butyllithium to afford the expected λ4-phosphinine anions 3 and 4. These
anions react with [Rh(COD)Cl]2 to yield the corresponding η5-complexes 5 and 6, in which
coordination occurs through the anionic carbocyclic π-system of the ring. Both complexes
were fully characterized by conventional spectroscopic techniques, and their X-ray crystals
structures were recorded. The catalytic activity of 5 and 6 was tested in the hydroformylation
of olefins. Good conversion yields and turnover frequencies were obtained in the hydro-
formylation of styrene and cyclohexene under mild conditions with low catalyst loading.
The hydroformylation of styrene occurs with a high regioselectivity (93/7) in favor of the
branched isomer. Interestingly, catalyst 6 catalyzes the transformation of 2,3-dimethyl-2-
butene into 3,4-dimethylpentanal through a tandem isomerization/hydroformylation process.
Introduction
phosphaalkene could be successfully employed in a
series of catalytic transformations of significant syn-
thetic value. Another important molecule in this series
is the phosphinine, the phosphorus equivalent of pyri-
dine.11 Although many synthetic routes, which allow the
preparation of numerous functional derivatives, have
been devised, these ligands were essentially employed
in coordination chemistry and more especially for the
stabilization of highly reduced species.12 Only a few
articles are relative to their use in homogeneous cataly-
sis. Undoubtedly the most significant report was made
by the group of Breit in collaboration with the BASF
firm. Rhodium(I) complexes of 2,4,6-trisubstituted phos-
phinines were found to be active catalysts in the
hydroformylation of olefins under relatively mild condi-
tions.13,14 Importantly, some of these complexes were
Low-coordinated phosphorus ligands possess very
specific electronic properties that markedly differ from
that of their nitrogen counterparts and classical tertiary
phosphines.1 Obviously, the intrinsic high reactivity of
the PdC double bonded systems implies that, in prac-
tice, only kinetically (sterically crowded) or thermody-
namically stabilized (conjugation, aromaticity) ligands
can be employed in catalysis. Recent works by many
groups, including ours, have definitively demonstrated
that such compounds should find important applica-
tions. Thus, phospha- and diphosphaferrocenes already
proved to be valuable systems in different catalytic
processes such as ring opening of epoxides,2 enantiose-
lective hydrogenation,3 Suzuki cross-coupling,4 Miyaura
synthesis of boronic esters,5 enantionselective isomer-
ization of allyl alcohols,6 asymmetric allylic alkylation,7
and cycloaddition.8 Importantly, the groups of Yoshifuji,
Ozawa, and Ito9 and Brookhardt10 also showed that
cationic palladium complexes of sterically hindered
(9) (a) Minami, T.; Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa, F.;
Yoshifuji, M. Angew. Chem., Int. Ed. 2001, 40 (23), 4501. (b) Ozawa,
F.; Okamoto, H.; Kawagishi, S.; Yamamoto, S.; Minami, T.; Yoshifuji,
M. J. Am. Chem. Soc. 2002, 124 (37), 10968-10969. (c) Ozawa, F.;
Ishiyama, T.; Yamamoto, S.; Kawagishi, S.; Murakami, H.; Yoshifuji,
M. Organometallics 2004, 23, 1698-1707. (d) Liang, H. Z.; Ito, S.;
Yoshifuji, M. Org. Lett. 2004, 6 (3), 425-427.
(10) Daugulis, A.; Brookhart, M.; White, P. S. Organometallics 2002,
21 (26), 5935-5943.
(11) Le Floch, P. In Phosphorus-Carbon Heterocyclic Chemistry: The
Rise of a New Domain; Mathey, F., Ed.; Pergamon: Palaiseau, 2001;
pp 485-533.
(12) (a) Me´zailles, N.; Avarvari, N.; Maigrot, N.; Ricard, L.; Mathey,
F.; Le Floch, P.; Cataldo, L.; Berclaz, T.; Geoffroy, M. Angew. Chem.,
Int. Ed. 1999, 38 (21), 3194-3197. (b) Rosa, P.; Me´zailles, N.; Ricard,
L.; Mathey, F.; Le Floch, P. Angew. Chem., Int. Ed. 2000, 39 (10), 1823.
(c) Choua, S.; Sidorenkova, H.; Berclaz, T.; Geoffroy, M.; Rosa, P.;
Me´zailles, N.; Ricard, L.; Mathey, F.; Le Floch, P. J. Am. Chem. Soc.
2000, 122 (49), 12227-12234. (d) Me´zailles, N.; Rosa, P.; Ricard, L.;
Mathey, F.; Le Floch, P. Organometallics 2000, 19 (16), 2941-2943.
(e) Rosa, P.; Me´zailles, N.; Ricard, L.; Mathey, F.; Le Floch, P.; Jean,
Y. Angew. Chem., Int. Ed. 2001, 40 (7), 1251.
(13) Breit, B.; Winde, R.; Harms, K. J. Chem. Soc., Perkin Trans. 1
1997 (18), 2681-2682.
(14) Breit, B.; Winde, R.; Mackewitz, T.; Paciello, R.; Harms, K.
Chem.-Eur. J. 2001, 7 (14), 3106-3121.
(1) (a) Dillon, K. B.; Mathey, F.; Nixon, J. F. Phosphorus: The
Carbon Copy; Wiley: Chichester, 1998. (b) Me´zailles, N.; Mathey, F.;
Le Floch, P. In Progress in Inorganic Chemistry; John Wiley and
Sons: New York, 2001; Vol. 49, p 455.
(2) Garrett, C. E.; Fu, G. C. J. Org. Chem. 1997, 62 (13), 4534-
4535.
(3) Qiao, S.; Fu, G. C. J. Org. Chem. 1998, 63 (13), 4168-4169.
(4) (a) Sava, X.; Ricard, L.; Mathey, F.; Le Floch, P. Organometallics
2000, 19 (23), 4899-4903. (b) Rosa, P.; Sava, X.; Me´zailles, N.; Melaimi,
M.; Ricard, L.; Mathey, F.; Le Floch, P. Phosphorus Sulfur Silicon
Relat. Elem. 2002, 177 (6-7), 1529-1532.
(5) Melaimi, M.; Mathey, F.; Le Floch, P. J. Organomet. Chem. 2001,
640 (1-2), 197-199.
(6) Tanaka, K.; Qiao, S.; Tobisu, M.; Lo, M. M. C.; Fu, G. C. J. Am.
Chem. Soc. 2000, 122 (40), 9870-9871.
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(8) Shintani, R.; Fu, G. C. J. Am. Chem. Soc. 2003, 125 (36), 10778-
10779.
10.1021/om049261y CCC: $30.25 © 2005 American Chemical Society
Publication on Web 01/08/2005

η5-Rhodium(I) Complexes
Organometallics, Vol. 24, No. 4, 2005 509
able to achieve an isomerization prior to hydrofor-
mylation of tetrasubstituted olefins such as 2,3-dimeth-
yl-2-butene to form 3,4-dimethylpentanal.14 Finally, an
η6-phosphinine Fe(0) complex was used for the catalyzed
synthesis of functional pyridines from alkynes and
nitriles.15
Why are phosphinines so rarely employed in cataly-
sis? Many experiments and attempts performed in our
laboratories have shown that, in some cases, phos-
phinine complexes are highly sensitive toward nucleo-
philic attack on the phosphorus atom. Although this
reactivity has not been fully rationalized, one may
propose that coordination on a relatively electron-poor
metal fragment induces a significant decrease of the
aromatic character and the concomitant increase of the
positive charge at phosphorus. Therefore non-kinetically
protected phosphinines are not good candidates for
catalytic purposes. An illustration was given by the
reaction of a neutral platinum(II) 2,2′-biphosphinine
complex with traces of alcohol.16
Thus their use in catalysis would probably be re-
stricted to very specific transformations that avoid the
presence of nucleophilic and/or strongly basic reagents.
An interesting way to circumvent the limitations dis-
cussed above is to exploit the reactivity of the ring in
the synthesis of highly functionalized phosphinine-based
derivatives. In fact, it is well known that phosphinines
react with various reagents to give a variety of mono-
cyclic and bicyclic structures.11 Recently, Breit and
colleagues have shown that a barrelene ligand, which
is available through a [4+2] cycloaddition of benzyne
with a 2,4,6-triaryl-substituted phosphinine, could be
employed as ligand in the rhodium-catalyzed hydro-
formylation of olefins.17,18 Three years ago, we launched
a large program aiming at exploring the potential of λ4-
phosphinine anions19 as ligands. We found that these
molecules could yield interesting complexes depending
on both the nature of the substitution scheme of the ring
and the nature of the metal fragment. Thus, when two
ancillary groups are present at the periphery of the ring,
η1-P-complexes can be formed.20,21 Some of these com-
plexes have found interesting applications in catalysis.
In the absence of chelating ligands, η2- or η5-complexes
are favored. Thus η2-(P-C) Pt(II) and Pd(II) complexes
have recently been synthesized and structurally char-
acterized.22 Some π-complexes with Fe(II) had been
previously evidenced by Ma¨rkl, Dimroth, and Massa,23
and more recently we showed that lithium salts of the
anions also adopt the η5-bonding mode.24 Other com-
plexes of these anions featuring slightly different coor-
dination mode were also isolated.25
Herein we report on the successful synthesis of η5-
Rh(I) complexes of λ4-phosphinine anions and on their
use in the catalyzed hydroformylation of olefins.
Results and Discussion
For this study, the 2,6-bis(trimethylsilyl)-3,5-bis-
(diphenylphosphinine) 1 and 2,3,5,6-tetraphenylphos-
phinine 2 were chosen for their availability and their
resistance toward air and moisture. Additionally, these
two ligands present very different substitution schemes
especially regarding the steric bulk around phosphorus.
Reactivity of phosphinines toward nucleophiles is now
well established,19,24 and we found that t-BuLi reacts
readily at the phosphorus atom of 1 and 2 in THF at
-78 °C. The complete formation of λ4-phosphinine
anions 3 and 4, respectively, was confirmed by 31P NMR
spectroscopy. As previously shown, the formation of the
λ4-phosphinine anion results in a very important upfield
shift (from 269.4 ppm in 1 to -25.9 ppm in 3 and from
206.0 ppm in 2 to -9.0 in 4). Anion 3 was also
structurally characterized as its [Li(Et2O)2] salt (see
Supporting Information). Anions 3 and 4 were reacted
with half an equivalent of [Rh(COD)Cl]2 at room tem-
perature in THF to provide complexes 5 and 6 respec-
tively. After one night of stirring at room temperature
in the case of 5, and a few minutes for 6, the reaction
was complete as attested by the change in color (from
intense pink to orange in the case of 5, from dark blue
to orange for 6). In 31P NMR spectroscopy, the formation
of both complexes was evidenced by the appearance of
a doublet at 17.41 ppm for 5 (1JRhP ) 7.3 Hz) and -4.67
ppm for 6 (1JRhP ) 9.1 Hz). The low magnitude of these
1JRhP coupling constants strongly suggested that η5-
1
coordination had occurred, a large JRhP coupling con-
stant being expected for η1-complexes (for example, a
value of 103.4 Hz was measured for an η1-phosphinine
complex of rhodium21). Both species were isolated pure
as yellow and bright orange powders, respectively, and
1
fully characterized by H and 13C NMR and elemental
(15) Le Floch, P.; Knoch, F.; Kremer, F.; Mathey, F.; Scholz, J.;
Scholz, W.; Thiele, K. H.; Zenneck, U. Eur. J. Inorg. Chem. 1998 (1),
119-126.
analysis (Scheme 2). Importantly, complexes 5 and 6
were found to be air-stable, and no apparent decomposi-
tion was observed after exposure for several weeks.
Suitable crystals of 5 and 6 were obtained, and their
X-ray structures were determined. Views of one mol-
ecule of 5 and 6 are presented in Figures 1 and 2,
respectively, and significant metric parameters (bond
distances and angles) are listed below. As expected from
31P NMR spectroscopy, 5 and 6 feature an η5-coordina-
tion of the ligand to the metal, coordination occurring
(16) Le Floch, P.; Carmichael, D.; Mathey, F. Phosphorus Sulfur
Silicon 1993, 77, 255.
(17) Breit, B.; Fuchs, E. Chem. Commun. 2004, 6, 694-695.
(18) For synthesis of free barrelenes, see: (a) Markl, G.; Lieb, F.
Angew. Chem., Int. Ed. 1968, 7 (9), 733. (b) Ashe, A. J.; Gordon, M. D.
J. Am. Chem. Soc. 1972, 94 (21), 7596. For their sulfurated version
see: (c) Alcaraz, J. M.; Mathey, F. J. Chem. Soc., Chem. Commun.
1984, 8, 508-509. For their complexes, see: (d) Alcaraz, J. M.; Mathey,
F. Tetrahedron Lett. 1984, 25 (2), 207-210. (e) Markl, G.; Beckh, H.
J. Tetrahedron Lett. 1987, 28 (30), 3475-3478. (f) Me´zailles, N.; Ricard,
L.; Mathey, F.; Le Floch, P. Eur. J. Inorg. Chem. 1999, 12, 2233-2241.
(19) (a) Ashe, A. J. I.; Smith, T. W. Tetrahedron Lett. 1977, 407-
410. (b) Ma¨rkl, G.; Merz, A. Tetrahedron Lett. 1968, 3611-3614. (c)
Ma¨rkl, G.; Merz, A. Tetrahedron Lett. 1971, 1215-1218. (d) Ma¨rkl,
G.; Martin, C.; Weber, W. Tetrahedron Lett. 1981, 22, 1207-1210.
(20) (a) Doux, M.; Bouet, C.; Me´zailles, N.; Ricard, L.; Le Floch, P.
Organometallics 2002, 21 (13), 2785-2788. (b) Doux, M.; Me´zailles,
N.; Melaimi, M.; Ricard, L.; Le Floch, P. Chem. Commun. 2002, 15,
1566-1567. (c) Doux, M.; Me´zailles, N.; Ricard, L.; Le Floch, P. Eur.
J. Inorg. Chem. 2003, 3878-3894. (d) Dochnahl, M.; Doux, M.; Faillard,
E.; Ricard, L.; Le Floch, P. Eur. J. Inorg. Chem. 2004, in press.
(21) Doux, M.; Me´zailles, N.; Ricard, L.; Le Floch, P. Organometallics
2003, 22, 4624-4626.
(22) Moores, A.; Mezailles, N.; Ricard, L.; Jean, Y.; Le Floch, P.
Organometallics 2004, 23 (12), 2870-2875.
(23) (a) Ma¨rkl, G.; Martin, C. Angew. Chem., Int. Ed. 1974, 13 (6),
408-409. (b) Dave, T.; Berger, S.; Bilger, E.; Kaletsch, H.; Pebler, J.;
Knecht, J.; Dimroth, K. Organometallics 1985, 4 (9), 1565-1572. (c)
Baum, G.; Massa, W. Organometallics 1985, 4 (9), 1572-1574.
(24) Moores, A.; Ricard, L.; Le Floch, P.; Me´zailles, N. Organome-
tallics 2003, 22, 1960-1966.
(25) (a) Nief, F.; Fischer, J. Organometallics 1986, 5 (5), 877-883.
(b) Lehmkuhl, H.; Paul, R.; Kruger, C.; Tsay, Y. H.; Benn, R.; Mynott,
R. Liebigs Ann. Chem. 1981, 6, 1147-1161.

510 Organometallics, Vol. 24, No. 4, 2005
Moores et al.
Figure 2. ORTEP view of one molecule of complex 6.
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. Relevant distances (Å)
and bond angles (deg): Rh1-C1, 2.420(2); Rh1-C2, 2.336(2);
Rh1-C3, 2.208(2); Rh1-C4, 2.226(2); Rh1-C5, 2.282(2);
P1-C1, 1.834(2); P1-C5, 1.830(2); P1-C6, 1.896(2); C1-
C2, 1.408(3); C2-C3, 1.439(3); C3-C4, 1.426(3); C4-C5,
1.422(3); C5-P1-C1, 96.5(1); C5-P1-C6, 105.5(1); C1-
P1-C6, 106.3(1); C2-C1-P1, 118.9(2); C1-C2-C3, 121.8(2);
C4-C3-C2, 126.5(2); C5-C4-C3, 118.8(2); C4-C5-P1,
121.6(2); C5-Rh1-C1, 71.05(7); P1-C1-Rh1, 88.91(8).
Figure 1. ORTEP view of one molecule of complexe 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. The complex co-
crystallized with one molecule of hexane, which as been
omitted for clarity. Note that the molecule contains a
crystallographic mirror plane. Relevant distances (Å) and
bond angles (deg): Rh1-C1, 2.387(2); Rh1-C2, 2.298(2);
Rh1-C3, 2.217(3); P1-C1, 1.831(2); P1-C6, 1.912(3); Si1-
C1, 1.902(2); C1-C2, 1.416(3); C2-C3, 1.431(3); C1-P1-
C1′, 98.3(2); C1-P1-C6, 105.4(1); C2-C1-P1, 118.4(2);
C1-C2-C3, 121.4(2); C2-C3-C2′, 126.1(3); P1-C1-Si1,
116.3(1); P1-C1-Rh1, 90.6(1); C1-Rh-C1′, 70.9(1); C2-
Rh1-C2′, 67.4(1); Si1-C1-Rh1, 118.2(1).
Scheme 3
Scheme 1
substituents at the R-position of phosphorus. This
probably accounts for the good resistance of the two
complexes toward air oxidation.
These two complexes were then evaluated as catalysts
in the hydroformalytion of alkenes. The experimental
conditions employed are close to those reported by Breit
et al. with λ3-phosphinines as ligands.13,14 One major
difference, however, is found in the metal-to-ligand
ratio: indeed, 5-20 equiv of ligand per equivalent of
[Rh(CO)2acac] was used by Breit et al. In our experi-
ments, as the catalyst is preformed, metal/ligand ratio
has to be 1:1. To draw a precise comparison between
the two catalytic systems, the substrate/metal ratios
were chosen as close as possible to those used by Breit.
First, the classical hydroformylation of styrene was
tested in toluene with complexes 5 and 6 as catalyst
(Scheme 3, Table 2). In this reaction, two products can
be obtained, the linear 3-phenylpropanal and the
branched 2-phenylpropanal, the latter being the most
valuable one. Complexes 5 and 6 showed an interesting
activity, and a good regioselectivity was obtained in both
cases using the ratio Rh:ligand:styrene ) 1:1:200, at
temperatures from 25 to 40 °C and under a pressure of
20 bar. Complex 6 proved to be more active than 5. A
conversion of 20.3% was reached after 4 h at 40 °C for
complex 5, whereas with 6, 24.2% of the substrate was
converted at room temperature. Turnover frequencies
for 5 and 6 were 10.2 h-1 at 40 °C and 16.1 h-1 at 25
°C, respectively. One may reasonably propose that this
difference results from the steric crowding induced by
the silyl groups in 5. Regioselectivity was also enhanced
on going from 5 to 6. A selectivity of 93% in favor of the
branched derivative was recorded for 6, whereas 89%
Scheme 2
through the pentadienyl fragment of the phosphinine
backbone. In both structures, relatively long C-C bond
distances (from 1.416(3) to 1.431(3) Å in 5 and from
1.408(3) to 1.439(3) Å in 6) account for the delocalization
of the negative charge in the carbocyclic system. These
data are similar to those recorded in η5-lithium com-
plexes and free anions (cryptated species).24 As expected,
the phosphorus atom in 5 and 6 is sp3-hybridized (∑
angles ) 309.1° in 5 and 308.3° in 6) and the lone pair
is still available though sterically protected by the
presence of the t-Bu group at phosphorus and the two

η5-Rhodium(I) Complexes
Organometallics, Vol. 24, No. 4, 2005 511
Table 1. Crystal Data and Structural Refinement Details for Structures of Compounds 5 and 6
5
6
cryst size [mm]
empirical formula
molecular mass
cryst syst
space group
a [Å]
0.20 × 0.18 × 0.18
0.20 × 0.14 × 0.12
C
41H64PRhSi2
C41H42PRh
746.98
668.63
monoclinic
monoclinic
P21/m
P21/n
8.447(5)
8.6680(10)
b [Å]
24.218(5)
19.3790(10)
c [Å]
9.788(5)
18.8010(10)
R [deg]
90.00
90.00
â [deg]
100.410(5)
91.8700(10)
γ [deg]
90.00
90.00
V [Å]3
1969.4(16)
3156.5(4)
Z
2
4
calcd density [g‚cm-3
]
1.260
1.407
abs coefficient [cm-1
2θmax [deg]
]
0.562
0.621
30.02
29.95
F(000)
index ranges
796
1392
-11 11; -34 15; -10 10
-12 12; -24 27; -26 26
no. of reflns collected/indep
no. of reflns used
Rint
6822/5022
14 887/9120
6967
0.0296
4182
0.0184
abs corr
0.8959 min., 0.9056 max.
0.8859 min., 0.9292 max.
403
17
0.0400 /0.1033
1.021
1.273(0.094)/-1.193(0.094)
no. of params refined
no. of reflns/params
final R1a/wR2 [I>2σ(I)]b
goodness-of-fit on F2
235
17
0.0409/0.1111
1.092
diff peak/hole [e‚Å-3
]
0.894(0.084)/-0.688(0.084)
2
2
a R1 ) ∑|Fo| - |Fc||/∑|Fo|. b wR2 ) (∑w||Fo| - |Fc|| /∑w|Fo| )1/2
.
Table 2. Catalysis Results from Hydroformylation
of Styrene with 20 bar CO/H2 (1:1) in Toluene
(co ) 0.42 M), with 0.5% of Catalyst
Scheme 4
temp
(°C)
time
(h)
conversion
(%)
TOF
(h-1
catalyst
)
b/l
5
5
6
6
40
40
25
40
4
24
3
20.3
94.0
10.2
7.8
16.1
89/11
89/11
93/7
Scheme 5
24.2
20
100.0
93/7
Table 3. Catalysis Results from Hydroformylation
of Cyclohexene and 2,3-Dimethylbut-2-ene with
Catalyst 6, 20 bar CO/H2 (1:1) in Toluene
(co ) 2.36 M), at 90 °C
only a low loading of catalyst was needed (ratio 6:cy-
clohexene ) 1:4160). Furthermore the TOF was found
to be rather good (648 h-1). This performance can be
compared to the 1959 h-1 obtained by Breit’s using 40
bar of pressure and a Rh:ligand ratio of 1:20.14 Note
that, recently, a Rh(I) complex featuring a 1-phospha-
barrelene as ligand was also successfully employed in
this transformation.17 A high TOF of 11 429 h-1 was
measured, with a pressure of 10 bar at 120 °C using
the same Rh:ligand ratio of 1:20 and a higher initial
concentration (co ) 3.56 M vs 2.36 M in the present case
and with Breit’s phosphinine ligand14). In the case of
the 2,3-dimethylbut-2-ene, isomerization takes place
prior to hydroformylation, which then, in turn, yields
3,4-dimethylpentanal as final product. This reaction is
difficult to perform and requires, at the industrial scale,
very high pressures and temperatures.27 A quite prom-
ising result was obtained using 20 bar CO/H2 (1:1) in
toluene at 90 °C with the ratio 6:substrate ) 1:1000. A
TOF of 4 h-1 was obtained with a conversion of 18.5%
after 47 h. Under much more drastic conditions (60 bar,
100 °C) phosphinine-based catalysts reached a TOF of
catalyst
time conversion TOF
substrate
cyclohexene
2,3-dimethylbut-2-ene
charge (%)
(h)
(%)
(h-1
)
0.024
0.1
4
62.2
18.5
647.9
4
47
was obtained for 5. These results can be compared to
those collected by Breit for the same reaction. At the
same temperature, better turnover frequency and re-
gioselectivity were measured (TOF ) 28.7 h-1, selectiv-
ity ) 95%).13 However a high ligand loading with respect
to the metal (Rh:ligand ) 1:5) had to be used with
phosphinines. A comparison can also be drawn with the
dirhodium(I) bisimidazolium carbene complex of Peris
and Fernandez. A turnover frequency of 16.7 h-1 was
recorded under more drastic conditions (40 °C, 80 bar),
with a Rh:ligand ratio of 400. This catalyst showed a
higher selectivity (99%) than our system.26
Catalyst 6 was thus tested in the hydroformylation
of more substituted substrates, namely, cyclohexene and
2,3-dimethylbut-2-ene. In the first case, very good
results were obtained. Indeed, in 4 h, 62.2% of the
substrate was converted to cyclohexanecarbaldehyde
under mild conditions (20 bar of pressure, 90 °C) and
118 h-1 14
Complex 6 exhibits an interesting catalytic
.
activity in the hydroformylation of styrene, cyclohexene,
and 2,3-dimethylbut-2-ene under very mild conditions.
Its performances compare with those of phosphinine-
(26) Poyatos, M.; Uriz, P.; Mata, J. A.; Claver, C.; Fernandez, E.;
Peris, E. Organometallics 2003, 22 (3), 440-444.
(27) Gresham, W. F.; Brooks, R. E. (DuPont de Nemours&Co) US
2.497.303, 1950.

512 Organometallics, Vol. 24, No. 4, 2005
Moores et al.
Solvent peaks are used as internal reference relative to Me4Si
based systems. To further understand what type of
intermediate was involved in the catalysis and to be
sure that our system was not a λ3-phosphinine-complex
precursor, a THF solution of 6 was placed for half an
hour in the hydroformylation conditions (CO/H2, 1:1, 20
bar). The yellow solution turned red. The 31P NMR
spectrum consisted of a broad peak at 45 ppm, account-
ing for the formation of fluxional compounds. The same
experiment was performed with only CO (20 bar) and
only H2 (20 bar), giving the same result in the first case,
the complex being unaltered by H2 even after 3 h under
pressure. In all circumstances, no formation of λ3-
phosphinine was observed. At this point, the question
of the stability of our system toward ligand elimination
can be discussed, as the ratio ligand/metal is very low
compared to literature. Interestingly, the regioselectivity
of catalyst 6 in the case of the styrene hydroformylation
is not as good as the one obtained with a unmodified
rhodium catalyst (Rh4(CO)12), which reaches 98% for the
branched product at 20 °C.28 The 93% obtained in the
case of 6 is an undubious proof that the ligand is still
present on the metal center during the catalysis. In
general, hydroformylation catalysts feature two-electron-
donor ligands such as phosphine or carbene. Complex
6 is a rare example of π-complex-based catalysts. Indeed,
during the past decade, Alper et al. developed an
original zwitterionic Rh(I) catalyst featuring the tet-
raphenylborate anion coordinated in an η6-fashion to the
metal center. These catalysts also proved to be very
active in the hydroformylation of olefins, such as
styrene, and alkynes.29
1
for H and 13C chemical shifts (ppm); 31P chemical shifts are
relative to a 85% H3PO4 external reference. Coupling constants
are given in hertz. The following abbreviations are used: s,
singlet; d, doublet; t, triplet; m, multiplet; bs, broad singlet.
tert-Buthyllithium in ether solution was purchased from
Aldrich. Phosphinines 1 and 2 were prepared following pro-
cedures described in refs 30 and 31. Elemental analyses were
performed by the “Service d’analyses du CNRS” at Gif sur
Yvette. Hydroformylation reactions were performed in a
stainless steel autoclave fitted with a glass container, equipped
with a magnetic stirrer, and heated by an oil bath. Hydrogen
and carbon monoxide were purchased from Air Liquide.
Synthesis of 5. To a solution of phosphinine 1 (200 mg,
0.509 mmol) in THF (5 mL) was added a solution of t-BuLi in
ether (300 µL, 0.509 mmol, 1.7 M) at -78 °C. The solution
turned from colorless to bright pink. The solution was warmed
to room temperature, and completion of the reaction was
checked by 31P NMR. [RhCODCl]2 (0.26 mmol, 126 mg) was
added to the mixture at room temperature. The solution was
left stirring for one night. The solution obtained was then
orange. Solvent was removed in vacuo. Salts were removed
by extraction and filtration in hexane (3 × 5 mL) over Celite.
Solvent was removed in vacuo, and the compound was isolated
as a yellow powder (285 mg, 85%). Anal. Calcd for C35H50-
PRhSi2: C, 63.61; H, 7.63. Found: C, 63.79; H, 7.84. 1H NMR
(C6D6): 0.24 (s, 18 H, Si-(CH3)3), 1.03 (d, 3JHP ) 12.0 Hz, 9 H,
P-C-(CH3)), 1.85, 2.18 (2 m, 8 H, CH2 of COD), 4.32 (m, 4 H,
CH of COD), 5.49 (s, 1 H, H4), 7.16-7.23, 7.45-7.49 (2 m, 10
3
H, CH of phenyls). 13C NMR (C6D6): 3.41 (d, JCP ) 8 Hz, Si-
2
(CH3)3), 27.83 (d, JCP ) 17 Hz, P-C-(CH3)3), 31.57 (s, CH2 of
2
1
COD), 39.03 (d, JCP ) 36 Hz, P-C-(CH3)3), 59.4 (d, JCP ) 54
Hz, C2-TMS), 73.63 (s, CH of COD), 95.13 (d, 3JCP ) 6 Hz, C4H),
128.01, 128.14, 130.03 (3 s, CH of phenyls), 128.73 (s, Cipso of
phenyls), 143.54 (s, C3-Ph). 31P NMR (C6D6): -17.21 (d,
2JPRh ) 7.3 Hz).
Conclusion
Synthesis of 6. To a solution of phosphinine 2 (200 mg,
0.5 mmol) in THF (5 mL) was added a solution of t-BuLi in
ether (300 µL, 0.5 mmol, 1.7 M) at -78 °C. The solution turned
from colorless to dark blue. The solution was warmed to room
temperature, and completion of the reaction was checked by
31P NMR. [RhCODCl]2 (0.25 mmol, 124 mg) was added to the
mixture at room temperature. The solution became green and
then orange within a few minutes. Solvent was removed in
vacuo. Salts were removed by extraction and filtration in
dichloromethane (3 × 5 mL) over Celite. Solvent was removed
in vacuo, and the compound was isolated as an orange powder
(264 mg, 88%). Anal. Calcd for C41H42PRh: C, 73.65; H, 6.33.
In conclusion, we have synthesized and fully charac-
terized the first η5-coordinated Rh(I) complexes of λ4-
phosphinine anions. These new complexes behave as
active and regioselective catalysts in the hydroformyl-
ation of olefins. Importantly, the difficult hydroformyl-
ation of a tetrasubstituted alkene into the corresponding
aldehyde was performed under very mild conditions.
Further studies will focus on the possible mechanism
of this transformation as well as on the evaluation of
these new type of catalysts in other processes. These
studies are currently in progress in our laboratories, and
results will be reported in due course.
1
3
Found: C, 73.72; H, 6.60. H NMR (CDCl3): 0.67 (d, JHP
)
10.5 Hz, 9 H, P-C-(CH3)3), 1.91, 2.06 (2 m, 8 H, CH2 of COD),
4
3.88 (m, 4 H, CH of COD), 5.32 (d, JHP ) 10.5 Hz, 1 H, H4),
Experimental Section
6.95-7.04, 7.18-7.30 (2 m, 10 H, CH of phenyls). 13C NMR
(CDCl3): 27.70 (d, 2JCP ) 13 Hz, P-C-(CH3)3), 31.33 (bs, CH2 of
COD), 42.34 (d, 2JCP ) 39 Hz, P-C-(CH3)3), masked by solvent
All reactions were routinely performed under an inert
atmosphere of argon or nitrogen by using Schlenk and glovebox
techniques and dry deoxygenated solvents. Dry THF and
hexanes were obtained by distillation from Na/benzophenone.
Dry dichloromethane was distilled on P2O5 and dry toluene
on metallic Na. Nuclear magnetic resonance spectra were
recorded on a Bruker AC-200 SY spectrometer operating at
1
1
peak (CH of COD), 80.23 (d, JCP ) 17 Hz, JCRh ) 4.0 Hz,
3
2
C2-Ph), 87.91 (d, JCP ) 5 Hz, C4H), 119.91 (dd, JCP ) 6 Hz,
1JCRh ) 3 Hz, C3-Ph), 125.03 (d, JCP ) 2 Hz, CmetaH of
4
R-phenyls), 127.19 (s, CparaH of R-phenyls), 127.32, 128.01,
130.60 (3 s, CH of â-phenyls), 131.28 (d, 3JCP ) 12 Hz, Cortho
H
)
)
2
1
300.0 MHz for H, 75.5 MHz for 13C, and 121.5 MHz for 31P.
of R-phenyls), 140.82 (s, Cipso of â-phenyls), 143.26 (d, JCP
24 Hz, Cipso of R-phenyls). 31P NMR (CDCl3): -3.53 (d, 2JPRh
9.0 Hz).
(28) (a) Lazzaroni, R.; Raffaelli, A.; Settambolo, R.; Bertozzi, S.;
Vitulli, G. 1989, 50 (1), 1-9. (b) Lazzaroni, R.; Settambolo, R.; Caiazzo,
A. In Rhodium Catalyzed Hydroformylation; Van Leeuwen, P., Claver,
C., Eds.; Kluwer Academic Publishers: Dordrecht, 2000; pp 15-33.
(29) (a) Amer, I.; Alper, H. J. Am. Chem. Soc. 1990, 112 (9), 3674-
3676. This complex was also used as a precatalyst for hydroformylation
of functionalized alkenes and alkynes. (b) Alper, H.; Zhou, J. Q. J. Org.
Chem. 1992, 57 (13), 3729-3731. (c) Totland, K.; Alper, H. J. Org.
Chem. 1993, 58 (12), 3326-3329. (d) Lee, C. W.; Alper, H. J. Org.
Chem. 1995, 60 (3), 499-503. (e) Van den Hoven, B. G.; Alper, H. J.
Org. Chem. 1999, 64 (26), 9640-9645. (f) Van den Hoven, B. G.; Alper,
H. J. Org. Chem. 1999, 64 (11), 3964-3968.
X-ray Structural Determination. Yellow blocks of com-
plex 5 crystallized by slow evaporation of a saturated solution
in hexanes. Orange needles of complex 6 were obtained by
diffusing hexanes into a dichloromethane solution of the
(30) Avarvari, N.; Le Floch, P.; Mathey, F. J. Am. Chem. Soc. 1996,
118 (47), 11978-11979.
(31) Doux, M.; Me´zailles, N.; Ricard, L.; Le Floch, P. Eur. J. Inorg.
Chem. 2003, 687.

η5-Rhodium(I) Complexes
Organometallics, Vol. 24, No. 4, 2005 513
complex 6. Data were collected on a Nonius Kappa CCD
diffractometer using a Mo KR (λ ) 0.71070 Å) X-ray source
and a graphite monochromator at 150 K. Experimental details
are described in Table 1. The crystal structures were solved
using SIR 9732 and SHELXL-97.33 ORTEP drawings were
made using ORTEP III for Windows.34 These data can be
obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html [or from the Cambridge Crystallographic Data
Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (in-
ternat.) +44-1223/336-033; e-mail: deposit@ccdc.cam.ac.uk].
General Procedure for Styrene Hydroformylation. To
a solution of styrene (243 µL, 2.11 mmol) in toluene (5 mL)
was added the catalyst 5 or 6 (7 mg, 0.01 mmol) under
nitrogen. After complete dissolution of the complex, the
solution was placed in the high-pressure apparatus, which was
then charged with CO (10 bar) and then H2 (20 bar). After
reaction time, the autoclave was cooled to room temperature
and depressurized, then the reaction mixture was analyzed
by GC with internal standard and correction factors.
(3 mL) was added the catalyst 6 (1.9 µmol, 64 µL of a solution
of the complex in toluene, c ) 0.03 mol‚L-1) under nitrogen.
The solution was placed in the high-pressure apparatus, which
was then charged with CO (10 bar) and then H2 (20 bar). After
reaction time, the autoclave was cooled to room temperature
and depressurized, then the reaction mixture was analyzed
by GC with internal standard and correction factors.
General Procedure for 2,3-Dimethylbut-2-ene Hydro-
formylation. To a solution of 2,3-dimethylbut-2-ene (3.57 mL,
30 mmol) in toluene (3 mL) was added the catalyst 6 (20 mg,
0.03 mmol) under nitrogen. The solution was placed in the
high-pressure apparatus, which was then charged with CO
(10 bar) and then H2 (20 bar). After reaction time, the
autoclave was cooled to room temperature and depressurized,
then the reaction mixture was analyzed by GC with internal
standard and correction factors. Aldehyde selectivity g 99%,
regioselectivity g 99%.
Acknowledgment. The CNRS and the Ecole Poly-
technique are thanked for supporting this work.
General Procedure for Cyclohexene Hydroformyla-
tion. To a solution of cyclohexene (810 µL, 8 mmol) in toluene
Supporting Information Available: Tables of crystal
data, atomic coordinates and equivalent isotropic displacement
parameters, bond lengths and bond angles, anisotropic dis-
placement parameters, and hydrogen coordinates, and ORTEP
views of one molecule of compounds 3, 5, and 6. This material
is available free of charge via the Internet at http://pubs.acs.org.
(32) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giaco-
vazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R.
SIR97, an integrated package of computer programs for the solution
and refinement of crystal structures using single-crystal data.
(33) Sheldrick, G. M. SHELXL-97; Universita¨t Go¨ttingen, Go¨ttingen,
Germany, 1997.
(34) Farrugia, L. J. ORTEP-3; Department of Chemistry, University
of Glasgow, 19XX
OM049261Y