Synergistic effects of two Si-H groups and a metal center in transition metal-catalyzed hydrosilylation of unsaturated molecules: A mechanistic study of the RhCl(PPh3)3-catalyzed hydrosilylation of ketones with 1,2-bis(dimethylsilyl)benzene

Article abstract of DOI:10.1021/om800151w

Three rhodadisilacyclopentene complexes are synthesized by the reaction of 1,2-bis(dimethylsilyl)-benzene with RhCl(PPh₃)₃ or Rh(H)(PPh₃)₄, and their contributions to the catalytic hydrosilylation of acetone are discussed. Treatment of the rhodium precursor RhCl(PPh₃)₃ or Rh(H)(PPh₃)₄ with 1,2-bis(dimethylsilyl)benzene affords an unstable Rh(V)-trihydride species having a rhodadisilacy-clopentene skeleton, Rh(Me₂SiC ₆H₄SiMe₂)(H)₃(PPh₃) ₂ (1), as a primary product, which is formed by double oxidative addition of 1,2-bis(dimethylsilyl)benzene to the rhodium center. The complex 1 eliminates H₂ upon concentration to quantitatively form a Rh(III)-monohydride complex, Rh(SiMe₂C₆H ₄SiMe₂)(H)(PPh₃)₂ (2). Further oxidative addition of 1,2-bis(dimethylsilyl)benzene to 2 gives a Rh(III)-trisilyl complex, Rh(Me₂SiC₆H₄SiMe ₂)(η¹-HSiMe₂C₆H ₄SiMe₂)(PPh₃) (3), in which there is an agostic interaction between the Si-H bond and the Rh(III) center. Elimination of H ₂ from 1 is reversible, and the most effective method for preparing 1 in solution is found to be treatment of 2 with H₂. The catalytic behavior of these three new rhodadisilacyclic complexes, RhCl(PPh ₃)₃, and Rh(H)(PPh₃)₄, in the hydrosilylation of acetone with 1,2-bis(dimethylsilyl)benzene was studied. The results suggest the important contribution of the trihydride 1 in the synergistic effect of two proximate Si-H bonds, leading to an unusual rate enhancement in the hydrosilylation of acetone with 1,2-bis(dimethylsilyl) benzene.

Full text of DOI:10.1021/om800151w

3502
Organometallics 2008, 27, 3502–3513
“Synergistic Effects of Two Si-H Groups and a Metal Center” in
Transition Metal-Catalyzed Hydrosilylation of Unsaturated
Molecules: A Mechanistic Study of the RhCl(PPh3)3-Catalyzed
Hydrosilylation of Ketones with 1,2-Bis(dimethylsilyl)benzene
Yusuke Sunada,† Yoshiki Fujimura,‡ and Hideo Nagashima*,†,‡
Institute for Materials Chemistry and Engineering and Graduate School of Engineering Sciences,
Kyushu UniVersity, Kasuga, Fukuoka 816- 8580, Japan
ReceiVed February 19, 2008
Three rhodadisilacyclopentene complexes are synthesized by the reaction of 1,2-bis(dimethylsilyl)-
benzene with RhCl(PPh3)3 or Rh(H)(PPh3)4, and their contributions to the catalytic hydrosilylation
of acetone are discussed. Treatment of the rhodium precursor RhCl(PPh3)3 or Rh(H)(PPh3)4 with
1,2-bis(dimethylsilyl)benzene affords an unstable Rh(V)-trihydride species having a rhodadisilacy-
clopentene skeleton, Rh(Me2SiC6H4SiMe2)(H)3(PPh3)2 (1), as a primary product, which is formed
by double oxidative addition of 1,2-bis(dimethylsilyl)benzene to the rhodium center. The complex
1 eliminates H2 upon concentration to quantitatively form a Rh(III)-monohydride complex,
Rh(SiMe2C6H4SiMe2)(H)(PPh3)2 (2). Further oxidative addition of 1,2-bis(dimethylsilyl)benzene to
2 gives a Rh(III)-trisilyl complex, Rh(Me2SiC6H4SiMe2)(η1-HSiMe2C6H4SiMe2)(PPh3) (3), in which
there is an agostic interaction between the Si-H bond and the Rh(III) center. Elimination of H2
from 1 is reversible, and the most effective method for preparing 1 in solution is found to be treatment
of 2 with H2. The catalytic behavior of these three new rhodadisilacyclic complexes, RhCl(PPh3)3,
and Rh(H)(PPh3)4, in the hydrosilylation of acetone with 1,2-bis(dimethylsilyl)benzene was studied.
The results suggest the important contribution of the trihydride 1 in the synergistic effect of two
proximate Si-H bonds, leading to an unusual rate enhancement in the hydrosilylation of acetone
with 1,2-bis(dimethylsilyl)benzene.
Pt, Co, Pd, and Ni act as hydrosilylation catalysts, in which the
type of metal, the valency of the metal, and the auxiliary ligands
used are important factors in determining the rate and selectivity
of the reaction.3
Interestingly, there are several unique catalytic reactions of
hydrosilanes, which are accomplished only when two Si-H
groups are located spatially close together. The first example
of such a reaction reported in the literature is the RhCl(PPh3)3-
catalyzed reduction of nitriles, which cannot be achieved by
conventional hydrosilanes: Corriu and co-workers discovered
that 1,2-bis(dimethylsilyl)benzene reacted successfully with
benzonitrile to afford the corresponding N,N-disilylated ben-
zylamine in low yields (Scheme 1, eq 1).4 The second example
Introduction
The catalytic hydrosilylation of alkenes and alkynes is one
of the most important reactions for the preparation of organo-
silicon compounds in the laboratory as well as on an industrial
scale.1 In general, hydrosilylation is recognized as a useful tool
in organic synthesis. Activation of Si-H bonds by certain
transition metal complexes provides good methods for the
reduction of carbonyl compounds, including the asymmetric
synthesis of chiral silyl ethers from prochiral ketones.2 It is well
known that the rate and selectivity of these reactions are
controlled by appropriate choice of organosilanes and transition
metal catalysts. Thus, various hydrosilanes with a wide variety
of substituents have been used in hydrosilylation, where the
reactivity of the silanes is actually controlled by tuning the steric
and electronic character induced by the substituents of the silicon
atom. Various transition metal compounds including Rh, Ir, Ru,
(3) (a) Spier, J. L.; Webster, J. A.; Barnes, G. H. J. Am. Chem. Soc.
1957, 79, 974. (b) Hitchcock, P. B.; Lappert, M. F.; Warhurst, N.-J. W.
Angew. Chem., Int. Ed. Engl. 1991, 30, 438. (c) Lappert, M. F.; Maskell,
R. K. J. Organomet. Chem. 1984, 264, 217. (d) Tsuji, J.; Hara, M.; Ohno,
K. Tetrahedron 1974, 30, 2143. (e) Uozumi, Y.; Hayashi, T. J. Am. Chem.
Soc. 1991, 113, 9887. (f) Kumada, M.; Kiso, Y.; Umeno, M. J. Chem. Soc.,
Chem. Commun. 1970, 611. (g) Ojima, I.; Fuchikami, T.; Yatabe, M. J.
Organomet. Chem. 1984, 260, 335. (h) Marciniec, B.; Urbaniak, W.; Pawlak,
P. J. Mol. Catal. 1982, 14, 323. (i) Chalk, A. J. J. Organomet. Chem. 1970,
21, 207. (j) Dumont, W.; Poulin, J.-C.; Dang, T.-P.; Kagan, H. B. J. Am.
Chem. Soc. 1978, 25, 8295. (k) Hayashi, T.; Yamamoto, K.; Kumada, M.
Tetrahedron Lett. 1974, 25, 8295. (l) Brunner, H. Synthesis 1988, 645. (m)
Nishiyama, H.; Kondo, M.; Nakamura, T.; Itoh, K. Organometallics 1991,
10, 500. (n) Faller, J. W.; Chase, K. J. Organometallics 1994, 13, 989. (o)
Nishibayashi, Y.; Segawa, K.; Ohe, K.; Uemura, S. Organometallics 1995,
14, 5486. (p) Evans, D. A.; Michael, F. E.; Tedrow, J. S.; Campos, K. R.
J. Am. Chem. Soc. 2003, 125, 3534. (q) Ojima, I.; Kogure, T.; Kumagai,
M. J. Org. Chem. 1977, 42, 1671.
* Corresponding author. E-mail: nagasima@cm.kyushu-u.ac.jp.
† Institute for Materials Chemistry and Engineering.
‡ Graduate School of Engineering Sciences.
(1) (a) Ojima, I. In The Chemistry of Organosilicon Compounds; Patai,
S., Rapport, Z., Eds.; Wiley: New York, 1989. (b) Brook, M. A. Silicon in
Organic, Organometallic, and Polymer Chemistry; Wiley: New York, 2000.
(c) Speier, J. L. AdV. Organomet. Chem. 1979, 17, 407. (d) ComprehensiVe
Handbook on Hydrosilylation; Marciniec, B., Ed.; Pergamon: Oxford, U.K.,
1992. (e) Marciniec, B.; Gulin´ski, J. J. Organomet. Chem. 1993, 446, 15.
(f) Marciniec, B. Silicon Chem. 2002, 1, 155.
(2) (a) Ojima, I.; Hirai, K. Asymmetric Synthesis; Academic Press: New
York, 1985. (b) Nishiyama, H.; Itoh, K. In Catalytic Asymmetric Synthesis,
2nd ed.; Ojima, I., Ed.; VCH: New York, 2000. (c) Hayashi, T. In
ComprehensiVe Asymmetric Catalysis; Jacobsen, E. N., Pflatz, A., Yama-
moto, H., Eds.; Springer: Berlin, 1999.
(4) Corriu, R. J. P.; Moreau, J. J. E.; Pataud-Sat, M. J. Organomet. Chem.
1982, 228, 301.
10.1021/om800151w CCC: $40.75
2008 American Chemical Society
Publication on Web 06/21/2008

RhCl(PPh3)3-Catalyzed Hydrosilylation of Ketones
Organometallics, Vol. 27, No. 14, 2008 3503
was reported by our research group in 1989: the RhCl(PPh3)3-
catalyzed reduction of ketones with 1,2-bis(dimethylsilyl)ben-
zene showed a significant rate enhancement as compared with
the same reaction with phenyldimethylsilane. The reaction was
accompanied by a unique methyl group migration between the
two silicon atoms (eq 2a).5 This unusual rate enhancement was
generally seen in Rh(PPh3)3Cl-catalyzed hydrosilylations of
ketones with the molecule containing two Si-H groups at a
close distance (eq 2b). As a typical example, the hydrosilylation
rate of acetone with Me2HSi(CH2)nSiHMe2 (n ) 2 or 3) is
50-120 times higher than with EtMe2SiH, whereas there is no
difference between the rate of the reaction with Me2HSi(CH2)n-
SiHMe2 (n ) 1 or 4) and that with EtMe2SiH. Our recent work
has revealed that the rate enhancement provided by the
proximity effect of two Si-H groups is now generally seen in
other transition metal-catalyzed reductions of carbonyl com-
pounds as well. Thus, the ruthenium- or platinum-catalyzed
silane reductions of carboxamides and other carbonyl com-
pounds (eq 2c) were achieved successfully when using
Me2HSiOSiHMe2 or polymethylhydrosiloxane (PMHS) as the
hydrosilane.6
1,2-bis(dimethylsilyl)benzene with benzene under platinum
catalysis resulted in replacement of a silicon hydride by a phenyl
group (eq 4). Such dehydrogenative reactions via C-H bond
activation did not occur with phenyldimethylsilane.8 It is of
interest that 1,2-bis(dimethylsilyl)benzene is not the only silane
effective for the reaction shown in eq 3. The authors commented
briefly that similar dehydrogenative arylation also took place
with 1,1,3,3-tetramethyldisiloxane. Iridium-catalyzed oligomer-
ization of 1,1,3,3-tetramethyldisiloxane, reported by Curtis et
al., may belong to the same category of reactions, in which
redistribution of pentamethyldisiloxane was significantly slow
under the same conditions (eq 5).9
The “proximity effect of dual Si-H groups”, seen in the
above examples, cannot be explained by conventional electronic
and steric considerations of hydrosilanes. Metalladisilacyclic
complexes, which may give a clue to understand the mechanism
and which are shown as A and B in Chart I, are formed by
double oxidative addition of bifunctional silanes to transition
metal complexes followed by dehydrogenation. The intermedi-
ates A and B reasonably explain Tanaka’s catalytic dehydro-
genative double-silylation reactions and Curtis’s dehydrogena-
tive oligomerization. However, they cannot be used to explain
our RhCl(PPh3)3-catalyzed hydrosilylation reactions of ketones
with silanes, because the hydrosilylation of ketones takes place
by way of addition of Si-H across the CdO bond and does
not involve dehydrogenation. Although the RhCl(PPh3)3-
catalyzed hydrosilylation of ketones with reactive bifunctional
silanes such as Me2HSi(CH2)nSiHMe2 (n ) 2) is too rapid to
capture possible catalytic intermediates from the reaction of the
silane with RhCl(PPh3)3, the study of the oxidative addition of
RhCl(PPh3)3 with much less reactive Ph2HSi(CH2)2SiHMe2 or
Ph2HSi(CH2)2SiHPh2 provided NMR evidence suggesting the
Another example of the proximity effect of silanes is seen in
a series of investigations by Tanaka and co-workers, who
realized platinum-catalyzed dehydrogenative double silylations
of alkynes, alkenes, dienes, and aldehydes with 1,2-bis(di-
methylsilyl)benzene (eq 3).7 They also found that treatment of
(5) (a) Nagashima, H.; Tatebe, K.; Ishibashi, I.; Sakakibara, J.; Itoh, K.
Organometallics 1989, 8, 2495. (b) Nagashima, H.; Tatebe, K.; Itoh, K.
J. Chem. Soc., Perkin Trans. 1 1989, 1707. (c) Nagashima, H.; Tatebe, K.;
Ishibashi, I.; Nakaoka, A.; Sakakibara, J.; Itoh, K. Organometallics 1995,
14, 2868.
(6) (a) Hanada, S.; Ishida, T.; Motoyama, Y.; Nagashima, H. J. Org.
Chem. 2007, 72, 7551. (b) Hanada, S.; Motoyama, Y.; Nagashima, H.
Tetrahedron Lett. 2006, 47, 6173. (c) Motoyama, Y.; Mitsui, K.; Ishida,
T.; Nagashima, H. J. Am. Chem. Soc. 2005, 127, 13150.
(7) (a) Uchimaru, Y.; Lautenschlager, H.-J.; Wynd, A. J.; Tanaka, M.;
Goto, M. Organometallics 1992, 11, 2639. (b) Tanaka, M.; Uchimaru, Y.;
Lautenschlager, H.-J. Organometallics 1991, 10, 16. (c) Tanaka, M.;
Uchimaru, Y.; Lautenschlager, H.-J. Chem. Lett. 1992, 45. (d) Tanaka, M.;
Uchimaru, Y.; Lautenschlager, H.-J J. Organomet. Chem. 1992, 428, 1.
(8) Uchimaru, Y.; El Sayed, A. M. M.; Tanaka, M. Organometallics
1993, 12, 2065.
(9) (a) Curtis, M. A.; Green, J. J. Am. Chem. Soc. 1978, 100, 6362. (b)
Green, J.; Curtis, M. A. J. Am. Chem. Soc. 1977, 99, 5176. (c) Curtis, M. A.;
Green, J.; Butler, W. M. J. Organomet. Chem. 1979, 164, 371.

3504 Organometallics, Vol. 27, No. 14, 2008
Sunada et al.
Scheme 1
Chart 1
Ph2HSi(CH2)2SiHPh2 gives a species showing two ill-resolved
signals at -6.3 and -10.6 ppm in an integral ratio of 2:1. The
spectral features of these rhodium species are consistent with
those of the Rh-trihydride species C in Chart 1, formed by
double oxidative addition of bifunctional silanes followed by
replacement of the chloro ligand by a hydride. However, at that
time, extreme moisture sensitivity of these complexes prevented
any further studies, including an X-ray structure determination.
Anticipating the instability of the expected products, we first
carried out NMR studies on the products, formed by careful
treatment of RhCl(PPh3)3 with 1,2-bis(dimethylsilyl)benzene (10
equiv) in C6D6 at room temperature for 1 h under rigorous
exclusion moisture and air. The reaction mixture provided
signals due to two different rhodium hydride species, Rh(Me2-
SiC6H4SiMe2)(H)3(PPh3)2 (1) and Rh(SiMe2C6H4SiMe2)(H)(PPh3)2
(2), in a ratio of 9:1. The rhodium trihydride 1 is unstable and
readily liberates H2 to form 2. In a typical example, 1 was
completely converted to 2, when the C6D6 solution described above
was frozen, degassed, and warmed to room temperature. Interest-
ingly, 2 underwent further oxidative addition with 1,2-bis(dimeth-
ylsilyl)benzene to form a third rhodium species having three Rh-Si
bonds, Rh(Me2SiC6H4SiMe2)(η1-HSiMe2C6H4SiMe2)(PPh3) (3).
Thus, treatment of Rh(PPh3)3Cl with 10 equiv of 1,2-bis(dimeth-
ylsilyl)benzene in C6H6 at room temperature for 1 h, followed by
concentration of the resulting solution, gave a 1:1 mixture of 2
and 3. It is likely that 1 was converted to 2 during the concentration
process, which subsequently underwent further oxidative addition
of 1,2-bis(dimethylsilyl)benzene still present in the reaction mixture
to form 3. In fact, treatment of isolated 2 with 1,2-bis(dimethyl-
silyl)benzene afforded 3 in ca. 68% yield. Formation of 1 from
RhCl(PPh3)3 involves reduction of Rh-Cl to Rh-H with 1,2-
bis(dimethylsilyl)benzene; this is supported by detection of 1-(chlo-
rodimethylsilyl)-2-(dimethylsilyl)benzene in the 1H NMR spectrum.
The hydride precursor RhH(PPh3)4 is also found to be viable as a
precursor of 1, 2, and 3, too.
existence of a Rh(V)H3 metalladisilacyclic species C.5c How-
ever, further mechanistic studies were hampered by the extreme
moisture sensitivity of the formed rhodium species. In this paper,
we report a solution to this problem, namely, by the character-
ization of the reaction products of 1,2-bis(dimethylsilyl)benzene
with RhCl(PPh3)3, which can be isolated successfully under
conditions where moisture is excluded rigorously. Combination
of advanced techniques for the handling of unstable organo-
metallics, their crystallography, and NMR spectroscopy has
enabled us to characterize a rhodadisilacyclopentene 1, similar
to C, and two novel rhodadisilacyclopentenes, 2 and 3 (Chart
1). Their stoichiometric and catalytic reactions with acetone
suggest a role of these intermediates in the catalytic cycle of
the hydrosilylation of ketones.
Results and Discussion
NMR Observation of Three Rhodium Species in the
Oxidative Addition of 1,2-Bis(dimethylsilyl)benzene to
RhCl(PPh3)3. Catalytic hydrosilylation reactions are believed
to be initiated by an oxidative addition of a Si-H moiety to a
transition metal species. It is known that the reaction of
RhCl(PPh3)3 with R3SiH gives the oxidative adduct (R3Si)Rh(H)-
(Cl)(PPh3)2, with a trigonal bipyramidal structure.10 In contrast
to this, we reported earlier5c that treatment of RhCl(PPh3)3 with
Ph2HSi(CH2)2SiHMe2 provides a species showing three unre-
solved Rh-H resonances at -6.7, -7.3, and -10.8 ppm with
an integrated ratio of 1:1:1, whereas the reaction with
Isolation and Characterization of Rh(III) Disilacyclic Com-
plexes Rh(SiMe2C6H4SiMe2)(H)(PPh3)2 (2) and Rh(Me2SiC6-
H4SiMe2)(η1-HSiMe2C6H4SiMe2)(PPh3) (3). Although 1 was
detectable in a solution NMR spectrum, facile elimination of
H2 upon workup prevented the isolation of 1. Attempted
isolation of 1 under various conditions led to exclusive formation
of a mixture of 2 and 3, the ratio of which was controlled by
both the charged ratio of rhodium precursor and 1,2-bis(dim-
ethylsilyl)benzene and the reaction temperature applied. Treat-
ment of 2.5 equiv of 1,2-bis(dimethylsilyl)benzene with either
Rh(PPh3)3Cl or RhH(PPh3)4 at 50 °C for 6 h gave a mixture of
2 and 3, in which 2 was the major product (2:3 > 9:1).
Subsequent recrystallization of the mixture from toluene/hexane
(10) (a) Muir, K. W.; Ibers, J. A. Inorg. Chem. 1970, 9, 440. (b) de
Charentenay, F.; Osborn, J. A.; Wilkinson, G. J. Chem. Soc. A 1968, 787.
(c) Haszeldine, R. N.; Parish, R. V.; Taylor, R. J. J. Chem. Soc. A 1974,
2311. (d) Haszeldine, R. N.; Parish, R. V.; Parry, D. J. Chem. Soc. A 1969,
683.

RhCl(PPh3)3-Catalyzed Hydrosilylation of Ketones
Organometallics, Vol. 27, No. 14, 2008 3505
Figure 1. Molecular structures of 2 (left) and 3 (right) with 50% probability ellipsoid. Hydrogen atoms except for the H(1) atom of 3 were
omitted for clarity.
Table 1. Representative Bond Lengths and Angles for 2 and 3
gave 2 in a pure form in 58% yield. In contrast, 3 was obtained
2
3
as the major product, when the reaction was carried out under
reflux in benzene for 1 h in the presence of an excess amount
of 1,2-bis(dimethylsilyl)benzene. Thus, treatment of 1,2-bis-
(dimethylsilyl)benzene (10 equiv) with Rh(PPh3)3Cl for 1 h
under benzene reflux conditions afforded a mixture of 2 and 3
in a ratio of 2:7, whereas reaction of the silane (10 equiv) with
RhH(PPh3)4 gave a mixture of 2 and 3 in a ratio of 2:5. The
trisilyl complex 3 was alternatively prepared by the reaction of
2 with 1,2-bis(dimethylsilyl)benzene (10 equiv) under reflux in
benzene for 1 h, which afforded a 1:7 mixture of 2 and 3
(Scheme 1).
Bond Lengths (Å)
2.308(2)
Rh-Si(1)
Rh-Si(2)
Rh-Si(3)
Rh-Si(4)
Rh-P(1)
Rh-P(2)
Rh-H(1)
Si-H(1)
2.2918(13)
2.273(2)
2.356(2)
2.345(2)
2.4610(14)
2.3919(13)
2.3305(14)
2.032(8)
1.41(10)
Bond Angles (deg)
84.06(6)
Si(1)-Rh-Si(2)
Si(3)-Rh-Si(4)
P(1)-Rh-P(2)
P(1)-Rh-Si(1)
P(1)-Rh-Si(2)
P(1)-Rh-Si(3)
P(1)-Rh-Si(4)
P(2)-Rh-Si(1)
P(2)-Rh-Si(2)
Rh-H(1)-Si(1)
H(1)-Si(1)-Rh
81.73(6)
Isolation of 2 made possible the complete spectroscopic
assignments and the determination of the molecular structure
by crystallography. In the 31P NMR spectrum of 2, two doublets
appear at 35.7 (JRh-P ) 131.5 Hz) and 26.9 (JRh-P ) 92.1 Hz)
ppm, suggesting the existence of two magnetically nonequivalent
PPh3 ligands. The 1H NMR spectrum of 2 gave a signal due to
a Rh-H at -7.31 ppm as a doublet of doublets. The gNMR
simulation11 of this signal revealed that there is one large trans
P-H coupling (JP-H ) 74 Hz), together with a small Rh-H
and a cis P-H coupling (JP-H ) 24.4 Hz, JRh-H ) 6.4 Hz).
The T1 value of this hydride signal was found to be 926 ms at
room temperature, which is consistent with that of classical
transition metal hydride complexes. The signals due to the SiMe2
groups appeared as two singlets at 0.49 and 0.70 ppm in the 1H
NMR spectrum, whereas two carbon resonances were found to
be at δ ) 8.79, 12.92 in the 13C NMR spectrum. These
correspond to the two symmetric organosilyl moieties with two
magnetically nonequivalent methyl moieties, as would be
observed for the molecular structure described below. The
29Si{1H} NMR spectrum of 2 showed a doublet of doublets at
δ ) 40.4 ppm (d, J ) 35.7, 42.5 Hz). The IR spectrum showed
one strong absorption band derived from the Rh-H moiety at
105.80(4)
107.27(5)
117.81(4)
107.58(5)
165.60(6)
98.87(5)
137.37(5)
103.39(4)
134.96(5)
27.80(5)
adopts a distorted trigonal bipyramidal coordination geometry
with two silicon atoms and one of two phosphorus atoms (P(2))
at the equatorial plane and the other phosphorus atom (P(1)) at
the apical position. The hydride, which should occupy one co-
ordination site of the apical site, was not detected in the Fourier
map; however, its existence is reasonable from the space in the
molecular structure. The Rh atom is out of the plane defined
by Si(1), Si(2), and P(2) by ca. 0.73 Å due to the steric repulsion
between the two PPh3 ligands and that between the apical PPh3
and a methyl group of the SiMe2 moieties nearby. The sum of
the three angles in the equatorial plane is 324.8°. The Rh-Si
bond lengths (2.2918(13), 2.308(2) Å) are comparable to those
found in previously reported five-coordinated trigonal bipyra-
midal rhodium-silyl complexes.12 The bond distance of Rh-P(1)
(2.3919(13) Å) is slightly longer than that of Rh-P(2) (2.3305(14)
Å), presumably due to the strong trans influence between the
P(1)-Rh-H bonds.
1994 cm-1
.
These spectroscopic features are fully consistent with the
molecular structure of 2, which was confirmed by X-ray
diffraction analysis. ORTEP drawings are shown in Figure 1,
and selected bond distances and angles are summarized in Table
1. Complex 2 consists of a five-membered rhodadisilacyclic
skeleton with two phosphorus ligands. The rhodium center
Although 3 was not isolated in pure form, its unequivocal
spectroscopic assignment is possible, and recrystallization gave
(12) (a) Nishihara, Y.; Takemura, M.; Osakada, K. Organometallics
2002, 21, 825. (b) Osakada, K.; Koizumi, T.; Yamamoto, T. Organometallics
1997, 16, 2063.
(11) gNMR Ver. 4.0; Cherwell Scientific Ltd.: Oxford, United Kingdom,
1995.

3506 Organometallics, Vol. 27, No. 14, 2008
Sunada et al.
Scheme 2
a single crystal suitable for X-ray structure determination. In
contrast to the presence of two doublets of doublets of the Rh-P
signals found in 2, 3 showed a single 31P resonance at 14.2
group in 3 was supported unequivocally by the location of the
hydrogen atom in the Fourier map. The Rh · · · (H-Si) distance
(2.032(8) Å) is longer than those of known R-agostic M · · · (H-Si)
complexes (typical values are 1.5-1.8 Å), but close to those in
ꢀ- and γ-agostic M · · · (H-Si) complexes (typical values are
1.8-2.5 Å) reported in the literature.13a In several σ-Si-H
complexes in the literature,13 strong back-donation from the
occupied orbital of the metal to σ* of the Si-H moiety results
in significant elongation of the Si-H bond distances (normally
1.6-1.9 A). The Si(1)-H(1) length (1.41(10) Å) of 3 suggests
the weak δRhfσ*Si-H back-bonding interaction. The Rh-Si(1)
distance of 3.082 Å indicates no direct Rh-Si bonding
interaction, which is often observed in R-agostic M · · · (H-Si)
complexes.13 Two of the three Rh-Si bonds (Rh-Si(3) )
2.356(2), Rh-Si(4) ) 2.345(2) Å) are longer than that of the
third (Rh-Si(2) ) 2.273(2) Å), and the Rh-P bond length
(2.4610(14) Å) is significantly longer than that found in 2 or
those previously reported for trigonal bipyramidal Rh-silyl
phosphine complexes. This may indicate a strong trans influence
of the silyl, phosphorus, and agostic Si-H moieties. Similar
elongation of the Rh-Si and Rh-P bonds in the basal plane
have been reported in square-pyramidal Rh(PMe3)2[SiMe2-
C(Me)dC(Me)SiMe(SiMe3)](SiMe3), in which the Rh-Sibasal
lengths have been reported to be 2.406(2) and 2.346(2) Å, whereas
the shorter Rh-Siapical bond distance was found to be 2.309(2) Å.16
Reversible Reaction of 2 with 1 atm of H2 to Afford
Rh(V) Trihydride Complex 1. As described above, the Rh(V)
trihydride complex Rh(SiMe2C6H4SiMe2)(H)3(PPh3)2 (1) is
unstable and facilely eliminates H2 to form 2; this is a problem
not only for obtaining a single crystal suitable for X-ray structure
determination but also for preparing an NMR sample without
contamination of 2. We have discovered that the elimination
of H2 is reversible and that the best method for preparing a
solution containing 1 in pure form is by treatment of 2 with H2
(1 atm) in C6D6 or toluene-d8 at room temperature. As soon as
the solution is exposed to H2, 1 is quantitatively formed. 1 is
stable enough to carry out lengthy NMR experiments as long
1
ppm (JRh-P ) 85.5 Hz). In the H NMR spectrum, no peak
could be found in the metal-hydride region (-20 to -1 ppm).
Instead, a signal accompanied by a satellite appeared at -0.26
ppm. This significant upfield shift compared with that of 1,2-
bis(dimethylsilyl)benzene (δ ) 4.8 ppm) as well as the coupling
constant of the satellite signal (J29Si–1H ) 112 Hz) indicates an
agostic interaction of the Si-H moiety with the rhodium center.
Typical J29Si–1H values for satellite signals in the presence of
an agostic interaction are 20-140 Hz (R-agostic, 20-80 Hz;
ꢀ-, γ-, and δ-agostic, 80-140 Hz).13 These values are different
from those of H-M-Si species (<20 Hz) and uncoordinated
Si-H groups (150-200 Hz; 188 Hz for 1,2-bis(dimethylsilyl)-
benzene).13 As a closely related example with 3, a δ-agostic
interaction between the transition metal center and the Si-H
moiety was observed in W(CO)4{Ph2PCH2CH2Si(H)R2} (R )
Me, Ph) reported by Schubert and Gilges, in which the J29Si–1H
values are 98.1 Hz for R ) Ph and 95.2 Hz for R ) Me.14
Another related complex with a γ-agostic Rh-H-Si interaction
was reported by Bergman and Brookhart; the J29Si–1H value of
[Cp*Rh(PMe3){o-(HPh2Si)C6H4}](BArF ) (ArF ) 3,5-(CF3)2-
4
C6H3) was reported to be 84 Hz.15 The crystal structure of 3
(vide infra) indicates that eight inequivalent Me groups should
be observed in 1H NMR spectroscopy, in which six 1H
resonances are singlets and another two protons are doublets.
In the actual 1H NMR spectrum measured at room temperature,
only four signals appeared at 0.14, 0.54, 0.58, and 0.63 ppm,
and all of them are singlets. Dynamic behavior was observed
in the 1H NMR spectra at low temperature, in which these four
singlets started to broaden at -70 °C and turned broad at -90
°C. Although 13C NMR measurement at low temperatures was
1
hampered by limited solubility of 3, it is consistent with H
NMR that four signals appeared at 1.48, 1.59, 6.15, and 9.66
ppm in the 13C NMR spectrum at room temperature. A plausible
explanation of these spectroscopic features is the reversible
oxidative addition/reductive elimination process shown in
Scheme 2, which could provide the observation of the methyl
signal due to the H-SiMe2 group to be a singlet and broadening
of the signals at low temperatures. The IR spectrum showed
one strong absorption band derived from the Si-H moiety in 3
at 1966 cm-1, ca. 170 cm-1 lower than that of free 1,2-
bis(dimethylsilyl)benzene (2140 cm-1).
1
as the solution is kept under a hydrogen atmosphere. The H
and 31P NMR resonances show dynamic behavior, typically seen
1
in the seven-coordinated transition metal complexes: the H
NMR spectrum of 1 in toluene-d8 at room temperature shows
only one broad Rh-H signal at around -8.4 ppm, together with
one broad signal at 0.52 ppm due to the Rh-SiMe2 moiety. In
contrast, 1H NMR spectra measured in toluene-d8 at lower than
-70 °C afforded two Rh-H signals at -10.42 (br d, JH-P
)
These data are also consistent with the molecular structure
of 3, shown in Figure 1. The coordination geometry around the
Rh center in 3 is best described as a square-pyramidal arrange-
ment, formed by one phosphorus, two silicon, and one agostic
Si-H moiety in the basal plane, and the remaining silicon (Si(2))
atom is at the apical position. An agostic interaction of the Si-H
133 Hz) and -6.90 (br s) ppm as ill-resolved signals in an
integral ratio of 1:2. The T1 values were estimated to be 2002
ms (δ ) -6.90) and 1906 ms (δ ) -10.42), respectively,
indicating classical hydrides. In the 1H NMR spectrum, the large
H-P coupling of the peak at -10.42 ppm suggests that one
hydride is located at a position trans to one of the phosphorus
ligands. Similarly, the 31P{1H} NMR spectrum at room tem-
perature shows only one very broad signal at around δ ) 36
ppm, whereas two sharp signals appear at 28.4 (JRh-P ) 85.5
(13) (a) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175.
(b) Schubert, U. AdV. Organomet. Chem. 1990, 30, 151.
(14) (a) Schubert, U.; Gilges, H. Organometallics 1996, 15, 2373. (b)
Nikolov, G. I. AdV. Organomet. Chem. 2005, 53, 217.
(15) Taw, F. L.; Bergman, R. G.; Brookhart, M. Organometallics 2004,
23, 886.
(16) Mitchell, G. P.; Tilley, T. D. Organometallics 1996, 15, 3477.

RhCl(PPh3)3-Catalyzed Hydrosilylation of Ketones
Organometallics, Vol. 27, No. 14, 2008 3507
Scheme 3
Hz) and 31.7 (JRh-P ) 98.7 Hz) ppm as doublets due to the
coupling with Rh. The 1H-31P COSY spectrum shows that the
31P NMR signal at δ ) 31.7 ppm correlates to a hydride signal
at δ ) -10.42 ppm. Two magnetically equivalent hydrides (δ1Η
solution when running the catalytic hydrosilylation of acetone
with 1,2-bis(dimethylsilyl)benzene. As reported earlier, the
hydrosilylation of acetone with 1,2-bis(dimethylsilyl)benzene
proceeds smoothly at room temperature in the presence of 1
mol % of RhCl(PPh3)3.5 Direct NMR observation of the
intermediary Rh species in the catalytic reaction was ac-
complished when the reaction of acetone with 1,2-bis(dimeth-
ylsilyl)benzene was carried out in the presence of larger amounts
of RhCl(PPh3)3 (10 mol %) in C6D6 under a nitrogen atmo-
sphere. The results are dependent on the ratios of acetone to
1,2-bis(dimethylsilyl)benzene. With a 1:2 ratio of acetone and
1,2-bis(dimethylsilyl)benzene, two Rh-H signals, due to a 9:1
mixture of 1 and 2, appeared at the initial stage of the reaction.
The ratio did not change even when all of the charged acetone
was consumed. In contrast, the experiment with a 1:1 mixture
of acetone and 1,2-bis(dimethylsilyl)benzene showed that only
the Rh-H signal corresponding to the trihydride 1 was observed
when acetone and 1,2-bis(dimethylsilyl)benzene were still
present in the reaction mixture. After all of the acetone had
been consumed, 1 was converted to 2 within 30 min. In both
31
) -6.90) and one phosphine (δ P ) 28.4 ppm) consequently
reside in the equatorial plane, as shown in Scheme 3, and the
coordination geometry of 1 can best be described as pentagonal
bipyramidal. This structure explains the two singlets at 0.52
and 0.91 ppm due to the two magnetically nonequivalent methyl
moieties on the two magnetically equivalent SiMe2 groups.
Crabtree and co-workers reported an Ir homologue of 1,
Ir(H)3(SiMe2C6H4SiMe2)(PPh3)2, synthesized by the reaction of
Ir(H)5(PPh3)2 with 1,2-bis(dimethylsilyl)benzene, of which the
molecular structure was determined by X-ray diffraction study
to be similar to that of 1 as deduced from our spectroscopic
results;17 the structure has a pentagonal bipyramidal coordination
geometry with one PPh3 and one hydride ligand at the apical
sites, whereas two Si atoms, two hydrides, and one PPh3 are
located in the equatorial plane. The iridium complex also shows
the fluxional behavior in 1H and 31P NMR spectroscopy similar
to that seen for 1.
of the experiments, the trisilyl species 3 was not observed.
Despite a number of studies on metalladisilacyclic complexes
formed by dehydrogenative double oxidative addition of bi-
functional hydrosilanes, to the best of our knowledge, there is
no report suggesting that elimination of H2 is reversible. We
have discovered that the oxidative addition of H2 to 2 is
reversible; facile liberation of 1 mol of H2 from 1 to regenerate
2 was accomplished by evacuating the solution (Scheme 3, eq
1). Interestingly, quantitative formation of 1 was also achieved
by leaving a toluene-d8 solution of 3 stand under a hydrogen
atmosphere in the presence of 1 equiv of PPh3 (Scheme 3, eq
2). The reaction involves the oxidative addition of a H2 molecule
and is a rare example of the hydrogenolysis of a Rh-Si bond,
being accompanied by the dissociation of an equimolar amount
of 1,2-bis(dimethylsilyl)benzene.
Studies on the Rhodadisilacyclopentenes 1, 2, and 3 in
the Catalytic Hydrosilylation of Acetone with 1,2-Bis(di-
methyl)silylbenzene. As described above, the oxidative addition
of a Si-H moiety is believed to be an important primary reaction
in the catalytic hydrosilylation. The following three experiments
were carried out in order to better understand the possible roles
of 1, 2, and 3 in the catalytic hydrosilylation of acetone with
1,2-bis(dimethylsilyl)benzene.
(2) Reaction Profiles of the Catalytic Hydrosilylation.
Complexes 1, 2, 3, Rh(PPh3)3Cl, and Rh(H)(PPh3)4 were used
in the catalytic hydrosilylation of acetone with 1,2-bis(dimeth-
ylsilyl)benzene in C6D6 at 25 °C. The reactions were also
performed in the presence of 2 or 10 equiv. (for Rh cat.) of
additional PPh3. Reaction profiles following the decrease of 1,2-
bis(dimethylsilyl)benzene were determined by the 1H NMR
spectrum, as shown in Figure 2. The results are summarized in
Table 2. The order of the catalytic activity is 2 > 1, 3 >
Rh(PPh3)3Cl > Rh(H)(PPh3)4. Although 3 was not detected as
an intermediate in the above experiment, it has a catalytic
activity similar to 1. In all cases, the reaction rates decreased
in the presence of additional PPh3. As described earlier,5 the
reaction gave not only the expected isopropoxysilane 4 but also
the product 5 resulting from the methyl group migration shown
in Scheme 4. The ratio of 4 to 5 altered within a range of 5:5-
8: 2, being dependent on the reaction conditions. It may be
worthwhile to point out that 4 was formed in a higher ratio,
when an excess of PPh3 was present in the reaction medium.
(3) Stoichiometric Reaction of 2 with Acetone. The highest
catalytic activity of 2 prompted us to examine the stoichiometric
reaction of 2 with acetone in the presence of 2 equiv of PPh3.
Quantitative formation of disilaindane 6 and Rh(H)(PPh3)4 was
confirmed (Scheme 5). The characterization of 6 was unequivo-
cally established by 1H, 13C, C-H COSY, 29Si NMR, and FAB-
(1) Direct Observation of Rhodadisilacyclopentenes in
the Catalytic Hydrosilylation. Among the three rhodium
species reported in this paper, two of them were observed in
1
MS spectroscopies. In the H NMR spectrum of 6, one signal
(17) Loza, M.; Faller, J. W.; Crabtree, R. H. Inorg. Chem. 1995, 34,
2937.
appears at 0.05 ppm as a AB quartet, and the 13C NMR spectrum

3508 Organometallics, Vol. 27, No. 14, 2008
Sunada et al.
Figure 2. Decrease of 1,2-bis(dimethylsilyl)benzene during the hydrosilylation of acetone catalyzed by Rh catalysts in the absence of
additives (left) and in the presence of 2 equiv of additional PPh3 (right).
Table 2. Hydrosilylation of Acetone Catalyzed by Rh Catalysts
oxidative addition of another molecule of 1,2-bis(dimethylsi-
lyl)benzene to form 3. The reaction of 2 with 1,2-bis(dimeth-
ylsilyl)benzene would give a coordinatively saturated Rh(V)
trisilyldihydride species, which is unstable and readily eliminates
H2. Replacement of a PPh3 ligand by an agostic Si-H moiety
leads to the formation of 3. This is also coordinatively
unsaturated (16 e). An intriguing new aspect obtained from these
experiments is the facile oxidative addition and reductive
elimination of 1,2-bis(dimethylsilyl)benzene and/or H2 by way
of the redox behavior of Rh(III) and Rh(V), as shown in Scheme
6. Although a number of studies on the dehydrogenative
formation of metalladisilacycles have been reported,19,20 to our
knowledge, there has been no report on the regeneration of 1,2-
bis(dimethylsilyl)benzene upon contact with H2, as is seen in
the hydrogenation of 3. Formation of 3 takes place by way of
an oxidative addition of three Si-H moieties from two
molecules of 1,2-bis(dimethylsilyl)benzene with the dehydro-
genation of two molecules of H2. There have been few reports
entry
cat.
additive
none
PPh3 (2 equiv)
none
PPh3 (2 equiv)
PPh3 (10 equiv)
none
PPh3 (2 equiv)
PPh3 (10 equiv)
none
time (min)
4:5
1
2
3
4
5
6
7
8
9
1a
1a
2
2
2
3
3
3
18
5:5
8:2
5:5
7:3
7:3
6:4
8:2
7:3
7:3
8:2
1020
<7
74
300
19
733
1217
49
Rh(PPh3)3Cl
Rh(PPh3)4(H)
10
none
104
a Under H2 (1 atm) atmosphere.
contains a singlet at -1.40 ppm, which is split into a triplet
(JC-H ) 120 MHz) under off-resonance conditions. The C-H
COSY experiment shows a correlation between these two
signals, indicating that these signals can be assigned to the
Si-CH2-Si moiety. Three methyl resonances were observed
at 0.22 (s, 3H), 0.31 (s, 3H), and 0.41 (s, 3H) ppm, and the
signals due to the OiPr group appear at 1.08 (d, 3H, CHMe2),
1.09 (d, 3H, CHMe2), and 3.91 (sept, 1H, CHMe2) ppm in the
1H NMR spectrum. These spectral features are similar to those
of 1,1,3,3-tetramethyldisilaindane reported by Barton and co-
workers, which was synthesized by pyrolysis of 1-allyl-2-benzyl-
1,1,2,2-tetramethyldisilane.18a In the 29Si NMR spectrum, two
singlets were confirmed at -35.1 and -44.8 ppm.
(19) Dehydrogenative double oxidative addition of 1,2-disilylbenzenes
and other bifunctional silanes are well-established:(a) Shimada, S.; Tanaka,
M. Coord. Chem. ReV. 2006, 250, 991. (b) Shimada, S.; Tanaka, M.; Shiro,
M. Angew. Chem., Int. Ed. Engl. 1996, 35, 1856. (c) Shimada, S.; Rao,
M. L. N.; Hayashi, T.; Tanaka, M. Angew. Chem., Int. Ed. 2001, 40, 213.
(d) Shimada, S.; Li, Y.-H.; Rao, M. L. N.; Tanaka, M. Organometallics
2006, 25, 3796. (e) Shimada, S.; Tanaka, M.; Honda, K. J. Am. Chem. Soc.
1995, 117, 8289. (f) Shimada, S.; Rao, M. L. N.; Li, Y.-H.; Tanaka, M.
Organometallics 2005, 24, 6029. (g) Shimada, S.; Rao, M. L. N.; Tanaka,
M. Organometallics 1999, 18, 291. (h) Chen, W.; Shimada, S.; Tanaka,
M. Science 2002, 295, 308. (i) Bourg, S.; Boury, B.; Carre´, F. H.; Corriu,
R. J. P. Organometallics 1998, 17, 167. (j) Lee, Y.-J.; Lee, J. -D.; Kim, S.
-J.; Ko, J.; Suh, I. -H.; Cheong, M.; Kang, S. O. Organometallics 2004,
23, 135. (k) Pfeiffer, J.; Kickelbick, G.; Schubert, U. Organometallics 2000,
19, 62. (l) Kang, Y.; Kim, K.; Kang, S.-O.; Ko, J.; Heaton, B. T.; Barkley,
J. V. J. Organomet. Chem. 1997, 532, 79. (m) Loza, M. L.; de Gala, S. R.;
Crabtree, R. H. Inorg. Chem. 1994, 33, 5073. (n) Fink, W. HelV. Chem.
Acta 1976, 59, 606. (o) Fink, W. HelV. Chem. Acta 1974, 57, 1010. (p)
Fink, W. HelV. Chem. Acta 1975, 58, 1464. (q) Corriu, R. J. P.; Moreau,
J. J. E.; Pataund-Sat, M. J. Org. Chem. 1981, 46, 3372. (r) Corriu, R. J. P.;
Moreau, J. J. E.; Pataund-Sat, M. J. Organometallics 1985, 4, 623.
(20) The reaction that may involve the double oxidative addition of two
Si-H bonds to the metal center not followed by elimination of H2 was
reported: Dioumaev, V. K.; Yoo, B. R.; Procopio, L. J.; Carroll, P. J.; Berry,
D. H. J. Am. Chem. Soc. 2003, 125, 8936.
Mechanistic Considerations
Studies on the oxidative addition resulted in synthesis and
characterization of three rhodadisilacyclopentenes, 1, 2, and 3.
The coordinatively saturated Rh(V) disilyltrihydride species 1
is apparently a primary product, which is formed by double
oxidative addition of 1,2-bis(dimethylsilyl)benzene to the co-
ordinatively unsaturated “Rh(H)(PPh3)3” species generated from
ClRh(PPh3)3 or HRh(PPh3)4. Reductive elimination of H2 from
1 gives the Rh(III) disilyl-monohydride complex 2. Resulting
2 is coordinatively unsaturated (16 e) and capable of undergoing
(18) (a) Barton, T. J.; Jacobi, S. A. J. Am. Chem. Soc. 1980, 102, 7979.
(b) Eaborn, C.; Safa, K. D. J. Organomet. Chem. 1981, 2042, 169. (c)
Doszczak, L.; Fey, P.; Tacke, R. Synlett 2007, 5, 753. (d) Doszczak, L.;
Tacke, R. Organometallics 2007, 26, 5722.

RhCl(PPh3)3-Catalyzed Hydrosilylation of Ketones
Organometallics, Vol. 27, No. 14, 2008 3509
Scheme 4
Scheme 5
Scheme 6
on a “triple oxidative addition” of Si-H to transition metals.21
The T1 studies revealed the presence of three “classical hydrides”
in the Rh(V) species 1, whereas NMR and crystallographic
evidence showed the “nonclassical” interaction of a Si-H
species to the Rh(III) center in 3,22 although the Si-H moiety
in 3 could undergo an oxidative addition to the 16 e metal center.
converted with each other with the aid of trace amounts of H2
in the reaction medium. The net catalytic intermediate should
have a minimum number of PPh3 and should be a coordinatively
unsaturated species capable of coordinating acetone. Thus, it is
worthwhile to assume that a 16 e rhodadisilacycle, formed by
dissociation of PPh3 from 1, could be a probable candidate for
the net catalytic species. This may also be in accord with the
fact that the trihydride 1 was observed as a major species in
the catalytic hydrosilylation of acetone in the presence of larger
amounts of RhCl(PPh3)3 (10 mol %) as described above. A
possible mechanism is illustrated in Scheme 7.
Studies on the catalytic hydrosilylation of acetone with 1,2-
bis(dimethylsilyl)benzene showed that the rhodadisilacyclo-
pentenes 1-3 catalyzed the reaction much faster than RhCl(P-
Ph3)3 or RhH(PPh3)4. Addition of PPh3 apparently retarded the
reaction. A consequence of the above-described oxidative
addition studies is that the three rhodadisilacyclopentenes 1-3
are in equilibrium when hydrogen exists in the reaction medium.
Experiments to characterize 1-3 were carried out under strict
exclusion of moisture. However, H2 can be present in catalytic
reactions when chemists are careful to exclude even trace
amounts of water. It is known that hydrosilanes can react with
moisture to form H2 under transition metal catalysis.23 Dehy-
drogenative silylation of acetone to silyl enol ether is also a
potential source of H2 in the reaction medium. Thus, at present,
we consider that the three rhodadisilacyclopentenes are inter-
A detailed mechanism for the catalytic cycle is shown in
Scheme 8. The migratory insertion of acetone followed by
reductive elimination results in conversion of the 18-electron
intermediate C to 14-electron species D. Double oxidative
addition of 1,2-bis(dimethylsilyl)benzene to D leads to formation
of the product 4 and the rhodacycle B through E as the
intermediate. The rate enhancement by 1,2-bis(dimethylsilyl)-
benzene, in which two Si-H groups located in a close distance
suitable for the formation of rhodacycle, can by explained by
two factors. One is the strong trans influence of organosilyl
ligands in A and 1 in Scheme 7, which facilitates dissociation
of PPh3 to generate coordinatively unsaturated species capable
of reacting with acetone. Another is the intramolecular oxidative
addition of the second Si-H group to the rhodium center, i.e.,
the reaction from E to B in Scheme 8, which accelerates the
generation of 4 and B.
(21) Chelate-assisted triple oxidative addition of three Si-H moieties
was reported:(a) Djurovich, P. I.; Safir, A. L.; Keder, N. L.; Watts, R. J.
Inorg. Chem. 1992, 31, 3195. (b) Chen, W.; Shimada, S.; Hayashi, T.;
Tanaka, M. Chem. Lett. 2001, 1096. (c) Chen, W.; Shimada, S.; Tanaka,
M.; Kobayashi, Y.; Saigo, K. J. Am. Chem. Soc. 2004, 126, 8072.
(22) Nonclassical interaction of a Si-H bond of 1,2-bis(dimethylsilyl)-
benzene was seen in several transition metal complexes bearing a η2-(H-
Si) moiety:(a) Delpech, F.; Sabo-Etienne, S.; Daran, J.-C.; Chaudret, B.;
Hussein, K.; Marsden, C. J.; Barthelat, J. -C. J. Am. Chem. Soc. 1999, 121,
6668. (b) Delpech, F.; Sabo-Etienne, S.; Chaudret, B.; Daran, J.-C. J. Am.
Chem. Soc. 1999, 121, 6668. (c) Schubert, U.; Grubert, S. Monatsh. Chem.
1998, 129, 437.
It may be worthwhile to discuss an alternative mechanism
as illustrated in Scheme 9, which could explain the rate of the
catalytic reaction for 2 being greater than for 1. In this
mechanism, coordination of acetone to 2 is followed by
hydrosilylation associated with subsequent oxidative addition
of 1,2-bis(dimethylsilyl)benzene. In the absence of the hydrosi-
lane, disilaindane 6 is formed from intermediate F. The rate
(23) (a) Ison, E. A.; Corbin, R. A.; Abu-Omar, M. M. J. Am. Chem.
Soc. 2005, 127, 11938. (b) Field, L. D.; Messerle, B. A.; Rehr, M.; Soler,
L. P.; Hambley, T. W. Organometallics 2003, 22, 2387.

3510 Organometallics, Vol. 27, No. 14, 2008
Sunada et al.
Scheme 7
Scheme 8
enhancement is explained by the facile intramolecular oxidative
addition of a Si-H group in G, which forms the product 4 and
regenerates the rhodacycle A. However, the process is not very
reasonable when considering the step forming G from F (the
oxidative addition of another molecule of 1,2-bis(dimethylsi-
lyl)benzene to F), a process that is sterically unfavorable.
which the presence of excess PPh3 is unfavorable for the
formation of the second intermediate.24
Conclusion
A special rate enhancement is often seen in the catalytic
hydrosilylation reactions, when bifunctional organosilanes,
in which two Si-H groups are located at a close distance,
are used as the hydrosilane component. The mechanism of
the rate enhancement cannot be explained by conventional
electronic and steric effects of the organosilanes, and special
interactions of the two Si-H groups and the transition metal
should be considered for the interpretation of the phenomena
that should be called “synergistic effects of dual Si-H
groups”. In this paper, we investigated in detail reactions of
two rhodium precursors with 1,2-bis(dimethylsilyl)benzene
As described above, the hydrosilylation of acetone with 1,2-
bis(dimethylsilyl)benzene has a special feature in giving two
isomers, 4 and 5. Isomer 5 is formed by metal-catalyzed
redistribution of the organosilicon compounds, which is often
seen in organosilicon chemistry. The mechanism of the methyl-
group migration is not clear, although pathways through
MdSiR2 intermediates are often proposed.7d In this study, we
have not obtained new results suggesting a clear mechanism.
However, it is worthwhile to point out that the migration of the
methyl group is partly suppressed in the presence of excess PPh3.
This implies that the lifetime of coordinatively unsaturated
species in the catalytic cycle is important for the methyl group
migration. A possible mechanism is shown in Scheme 10, in
(24) One of the reviewers suggested the possibility of disilaindane 6 as
intermediate species to form the methyl migrated product 5. This is excluded
by the RhCl(PPh3)3-catalyzed hydrosilylation of acetone with 1,2-bis(di-
methylsilyl)benzene in the presence of disilaindane 6, in which 6 was proved
to be inert toward the reaction with acetone under the conditions.

RhCl(PPh3)3-Catalyzed Hydrosilylation of Ketones
Organometallics, Vol. 27, No. 14, 2008 3511
Scheme 9
Scheme 10
melting point apparatus. The HRMS spectrum was recorded on
a JEOL Mstation JMS-70 apparatus. Elemental analyses were
performed by a Perkin-Elmer 2400/CHN analyzer. Starting
materials, 1,2-bis(dimethylsilyl)benzene,5c Rh(PPh3)3Cl,25 and
Rh(PPh3)4(H)26 were synthesized by the method reported in the
literature.
and succeeded in synthesizing and characterizing three new
rhodadisilacyclopentenes. In contrast to other metalladisila-
pentenes so far reported, the rhodacycles reversibly react with
H2 by way of a redox reaction between Rh(III) and Rh(V).
Possible involvement of these rhodacycles in the catalytic
hydrosilylation of acetone with 1,2-bis(dimethylsilyl)benzene
is discussed, in which two factors, the strong trans influence
of the organosilyl ligand taking part in generation of a
catalytically active species and the intramoloecular oxidative
addition of the second Si-H group in 1,2-bis(dimethylsilyl)-
benzene, are proposed to be important, as shown in Schemes
7, 8, and 9. The results provide new and interesting aspects
for an explanation of the “synergistic effects of dual Si-H
groups” in the unique rate enhancement in rhodium-catalyzed
hydrosilylations. This may lead to new ideas in exploring
the synergistic effects of dual Si-H groups in other transition
metal-catalyzed hydrosilylation reactions.
Preparation of Rh(SiMe2C6H4SiMe2)(H)3(PPh3)2 (1). In an
NMR tube, complex 2 (16.4 mg, 0.02 mmol) was dissolved in
toluene-d8 (0.4 mL), and the atmosphere was replaced by 1 atm of
H2. After mixing this solution for 10 min, complete formation of
1
complex 1 was observed by H and 31P NMR spectroscopy. The
same compound was alternatively prepared by treatment of 3 with
1
1 equiv of PPh3 using a similar procedure. H NMR (600 MHz
toluene-d8, -70 °C): δ -10.42 (br m, JH-P ) 133 Hz, 1H, Rh-H),
-6.90 (br m, 2H, Rh-H), 0.52 (s, 6H, SiMe2), 0.91 (s, 6H, SiMe2),
6.51-6.64 (br m, 2H, Ph), 6.64-6.81 (br m, 5H, Ph), 6.82-6.96
(br m, 10H, Ph), 6.96-7.03 (br m, 2H, Ph), 7.10-7.29 (br m, 6H,
Ph), 7.36-7.51 (br m, 7H, Ph), 7.72-7.81 (br m, 2H, Ph). 29Si{1H}
NMR (119 MHz, toluene-d8, -70 °C): δ 20.5 (dd, JRh-Si ) 71.2,
JP-Si ) 20.1 Hz). 31P{1H} NMR (242 MHz, toluene-d8, -70 °C):
δ 28.4 (dd, JRh-P ) 85.5, JP-P ) 13.2 Hz), 31.7 (dd, JRh-P ) 98.7,
JP-P ) 13.2 Hz).
Preparation of Rh(SiMe2C6H4SiMe2)(H)(PPh3)2 (2). In a 200
mL Schlenk tube were placed Rh(PPh3)4(H) (2.30 g, 2.0 mmol)
and a benzene solution (100 mL) of 1,2-bis(dimethylsilyl)ben-
zene (486 mg, 2.50 mmol). The resulting solution was stirred
for 6 h at 50 °C, during which the color of the solution turned
orange. The solvent was removed in vacuo, and the residue was
Experimental Section
General Procedures. The manipulation of air- and moisture-
sensitive organometallic compounds was carried out under a dry
argon atmosphere using standard Schlenk tube techniques
associated with a high-vacuum line. All solvents were distilled
over appropriate drying reagents prior to use (toluene, hexane;
Ph2CO/Na, acetone; MS 4A). 1H, 13C, and 31P NMR spectra
were recorded on a JEOL Lambda 600 or a Lambda 400
spectrometer at ambient temperature unless otherwise noted. 1H,
13C, 29Si, and 31P NMR chemical shifts (δ values) are given in
ppm relative to the solvent signal (1H, 13C) or standard
resonances (29Si, external tetramethylsilane; 31P, external 85%
H3PO4). IR spectra were recorded on a JASCO FT/IR-550
spectrometer. Melting points were measured on a Yanaco micro
(25) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. J. Chem.
Soc. A. 1966, 1711. (b) Osborn, J. A.; Wilkinson, G. Inorg. Synth. 1967,
10, 67.
(26) Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. Inorg.
Synth. 1974, 15, 58.

3512 Organometallics, Vol. 27, No. 14, 2008
Sunada et al.
5H, Ph), 7.59-7.68 (m, 2H, Ph). 13C NMR (150 MHz, C6D6,
rt): δ 1.48 (s, SiMe2), 1.59 (s, SiMe2), 6.15 (s, SiMe2), 9.66 (s,
SiMe2), 127.57 (s, Ph), 129.28 (s, Ph), 129.33 (s, Ph), 129.75
(s, Ph), 130.85 (s, Ph), 131.12 (s, Ph), 132.81 (s, Ph), 132.92 (s,
Ph), 132.94 (s, Ph), 134.97 (s, Ph), 135.09 (s, Ph), 135.94 (s,
Ph), 136.08 (s, Ph), 140.80 (s, Ph). 29Si{1H} NMR (119 MHz,
C6D6, rt): δ 11.4 (s, HSiMe2), 37.0 (dd, J ) 37.4, 44.7 Hz),
40.5 (d, J ) 45.6 Hz), 40.6 (d, J ) 45.6 Hz). 31P{1H} NMR
(242 MHz, C6D6, rt): δ 14.2 (d, JRh-P ) 85.5 Hz). IR (KBr):
νSi-H (cm-1) ) 1966 (s).
Preparation of Disilaindane (6). In a 20 mL Schlenk tube, 2
(41 mg, 0.05 mmol) and 2 equiv of PPh3 (26 mg, 0.10 mmol) were
dissolved in benzene (0.5 mL), followed by the addition of acetone
(7.3 µL, 0.10 mmol) at room temperature. The resulting solution
was stirred for 30 min at rt, during which the color of the solution
turned orange, and a precipitate of Rh(H)(PPh3)4 was observed.
The solvent was removed in vacuo, and the residue was extracted
with hexane (3 mL). The hexane solution was concentrated under
vacuum, and a yellow oil of 6 was obtained in 86% yield (11
Table 3. Crystallographic Data for 2 and 3
2
3
empirical formula
fw
C46H47Si2P2Rh
819.89
C38H41Si4P1Rh · 1/2C6H14
795.08
cryst syst
lattice type
space group
a, Å
b, Å
c, Å
monoclinic
primitive
P21/n (#14)
10.7863(11)
17.538(2)
21.737(2)
90
monoclinic
primitive
P21/c (#14)
9.532(4)
38.063(15)
12.290(6)
90
R, deg
ꢀ, deg
101.9792(14)
90
110.466(5)
90
γ, deg
volume, Å3
Z value
4022.4(7)
4
4177(3)
4
Dcalc, g/cm3
F(000)
1.354
1.259
1700.00
5.944
orange, platelet
1660.00
5.873
yellow, platelet
µ(Mo KR), cm-1
cryst color, habit
cryst dimens, mm
0.20 × 0.15 × 0.05 0.10 × 0.05 × 0.02
no. observns (all reflns) 9096
6218
no. variables
refln/param ratio
R (all reflns)
R1 (I > 2.00σ(I))a
wR2 (all reflns)b
GOF
510
480
mg). Anal. Calcd for C13H22O1Si2: C, 62.33; H, 8.85. Found: C,
12
1H2216O128Si2 250.1209,
17.84
0.0849
0.0996
0.3170
0.863
12.95
0.1755
0.0690
0.1755
1.002
0.000
62.65; H, 8.96. HRMS: calcd for
C
13
1
found 250.1213. H NMR (600 MHz, C6D6, rt): δ 0.05 (AB q,
J ) 14.3, 101.7 Hz, 2H, Si-CH2-Si), 0.22 (s, 3H, SiMe2), 0.31
(s, 3H, SiMe2), 0.41 (s, 3H, SiMe(OiPr)), 1.08 (d, J ) 5.5 Hz,
3H, CH(CH3)2), 1.09 (d, J ) 5.5 Hz, 3H, CH(CH3)2), 3.91 (sept,
J ) 5.5 Hz, 1H, CH(CH3)2), 7.23-7.28 (m, 2H, Ph), 7.45-7.53
(m, 1H, Ph), 7.67-7.71 (m, 1H, Ph). 13C NMR (150 MHz, C6D6,
rt): δ -1.40 (s, Si-CH2-Si), 0.69 (s, SiMe2), 0.88 (s, SiMe2),
1.08 (s, SiMe(OiPr)), 26.40 (s, CH(CH3)2), 26.44 (s, CH(CH3)2),
30.54 (s, CH(CH3)2), 129.06 (s, Ph), 129.44 (s, Ph), 132.79 (s,
Ph), 132.99 (s, Ph), 148.86 (s, Ph), 151.22 (s, Ph). 29Si{1H}
NMR (242 MHz, C6D6, rt): δ -35.1 (s, Si(Me)(OiPr)), -44.8
(SiMe2).
max. shift/error in final 0.037
cycle
max. peak in final diff 5.28
1.90
map, e-/Å3
min. peak in final diff
map, e-/Å3
-3.42
-0.89
2
2 2
a R1 ) ∑|Fo| - |Fc|/∑|Fo|. b wR2 ) [∑(w(Fo - Fc ) )/∑(w(Fo2)2)]1/2
.
washed twice with hexane (5 mL). The remaining orange solid
was dissolved in toluene (30 mL) and layered with hexane (60
mL), from which orange crystals of 2 were obtained in 58%
yield (952 mg). Alternatively, the same compound was prepared
by treatment of Rh(PPh3)3Cl with 1,2-bis(dimethylsilyl)benzene
using a similar procedure. Mp: 133 °C (dec). Anal. Calcd for
C46H47P2Si2Rh1: C, 67.30; H, 5.77. Found: C, 67.15; H, 5.65.
1H NMR (600 MHz, C6D6, rt): δ -7.31 (ddd, JH-P ) 6.4, 74
Hz, JH-Rh ) 24.4 Hz, 1H, Rh-H), 0.49 (s, 6H, SiMe2), 0.70 (s,
6H, SiMe2), 6.65-7.06 (br m, 14H, Ph), 7.17-7.30 (br m, 8H,
Ph), 7.31-7.36 (m, 2H, Ph), 7.36-7.56 (br m, 8H, Ph),
7.65-7.72 (m, 2H, Ph). 13C NMR (150 MHz, C6D6, rt): δ 8.79
(s, SiMe2), 12.92 (s, SiMe2), 6.15 (s, SiMe2), 9.66 (s, SiMe2),
129.94 (s, Ph), 131.22 (s, Ph), 134.74 (br s, Ph), 135.26 (br s,
Ph), 136.83 (br s, Ph), 137.16 (br s, Ph), 158.88 (s, Ph). 29Si{1H}
NMR (119 MHz, C6D6, rt): δ 40.4 (dd, J ) 35.7, 42.5 Hz).
31P{1H} NMR (242 MHz, C6D6, rt): δ 26.9 (d, JRh-P ) 92.1
Hz), 35.7 (d, JRh-P ) 131.5 Hz). IR (KBr): νRh-H (cm-1) )
1994 (s).
Reaction Profiles. A mixture of the rhodium complex 1, 2, 3,
RhCl(PPh3)3, or RhH(PPh3)4 (0.02 mmol), hexamethylbenzene
(internal standard, 33 mg, 0.2 mmol), and C6D6 (0.3 mL) was
transferred into a flame-dried NMR tube in a glovebox. Addition
of a solution of 1,2-bis(dimethylsilyl)benzene (39 mg, 0.2 mmol)
in C6D6 (0.2 mL) was followed by the addition of a solution of
acetone (17.6 µL, 0.24 mmol) in C6D6 (0.2 mL). The addition
was carried out carefully to make three phases. The solu-
tion was quickly frozen and connected to a vacuum line. The
tube was evacuated at -78 °C, sealed by flame, quickly warmed,
and shaken several times. The NMR measurement was carried
out periodically at 25 °C in C6D6.
X-ray Data Collection and Reduction. Single crystals of all
complexes were grown from toluene/hexane. X-ray crystal-
lography was performed on a Rigaku Saturn CCD area detector
with graphite-monochromated Mo KR radiation (λ ) 0.71070A).
The data were collected at 123(2) K using ω scans in the θ
range of 3.0° e θ e 27.5° (2) and 3.1° e θ e 27.5° (3). The
data obtained were processed using Crystal-Clear (Rigaku) on
a Pentium computer and were corrected for Lorentz and
polarization effects. The structures were solved by direct
methods27 and expanded using Fourier techniques.28 The non-
hydrogen atoms were refined anisotropically except for the
solvent atoms (hexane for 2). Hydrogen atoms were refined using
the riding model. The final cycle of full-matrix least-squares
refinement on F2 was based on 9096 observed reflections and
Preparation of Rh(Me2SiC6H4SiMe2)(η1-HSiMe2C6H4SiMe2)-
(PPh3) (3). In a 20 mL Schlenk tube were placed complex 2 (410
mg, 0.50 mmol) and a benzene solution (10 mL) of 1,2-bis(di-
methylsilyl)benzene (486 mg, 2.50 mmol). The resulting solution
was stirred for 1 h under benzene reflux conditions, during which
the color of the solution turned dark orange. The solvent was
removed in vacuo, and the residue was washed with hexane (4
mL). The remaining yellow solid was dissolved in toluene (4
mL) and layered with hexane (8 mL), from which orange crystals
of 3 containing ∼13% of 2 were obtained (305 mg, ca. 68%
yield). Alternatively, the same compound was prepared by
treatment of Rh(PPh3)4(H) or Rh(PPh3)3Cl with 1,2-bis(dimeth-
(27) (a) Sheldrick, G. M. SHELX97; 1997. (b) Altomare, A.; Burla, M.
C.; Camalli, M.; Cascarano, G.; Guagliardi, A.; Moliterni, A.; Polidori, G.;
Spagna, R. SIR97; 1999.
(28) DIRDIF99: Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman,
W. P.; de Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-99 program
system; Technical Report of the Crystallography Laboratory; University of
Nijmegen: Nijmegen, The Netherlands, 1999.
1
ylsilyl)benzene using a similar procedure. H NMR (600 MHz,
C6D6, rt): δ -0.26 (br s, JSi-H ) 112 Hz, 1H, Rh-HSi), 0.14
(s, 6H, SiMe2), 0.54 (s, 6H, SiMe2), 0.58 (s, 6H, SiMe2), 0.63
(s, 6H, SiMe2), 6.84-7.00 (m, 8H, Ph), 7.12-7.15 (m, 4H, Ph),
7.16-7.23 (m, 2H, Ph), 7.26-7.32 (m, 2H, Ph), 7.35-7.46 (m,
(29) Cromer, D. T.; Waber, J. T. International Tables for X-ray
Crystallography; Kynoch Press: Birmingham, U.K., 1974; Vol. 4.

RhCl(PPh3)3-Catalyzed Hydrosilylation of Ketones
Organometallics, Vol. 27, No. 14, 2008 3513
461 variable parameters for 2 and 7328 observed reflections and
476 variable parameters for 3. Neutral atom scattering factors
were taken from Cromer and Waber.29 All calculations were
performed using the CrystalStructure30,31 crystallographic soft-
ware package. Details of final refinement as well as the bond
lengths and angles are summarized in the Supporting Informa-
tion, and the numbering scheme employed is also shown in the
Supporting Information, which were drawn with ORTEP with
50% probability ellipsoids.
Acknowledgment. This work was supported by a Grant-
in-Aid for Science Research on Priority Areas (No. 18064014,
Synergy of Elements) from Ministry of Education, Culture,
Sports, Science and Technology, Japan.
Supporting Information Available: 1H, 13C, 29Si, and 31P NMR
data (1, 2, 3, 6), variable-temperature NMR data (1), and details of
crystallographic studies (2 and 3).This material is available free of
charge via the Internet at http://pubs.acs.org.
(30) CrystalStructure 3.8.0: Crystal Structure Analysis Package; Rigaku
and Rigaku/MSC: The Woodlands, TX, 2000-2006.
(31) Carruthers, J. R.; Rollett, J. S.; Betteridge, P. W.; Kinna, D.; Pearce,
L.; Larsen, A.; Gabe, E. CRYSTALS Issue 11; Chemical Crystallography
Laboratory: Oxford, U.K., 1999.
OM800151W