Trans additions of silanes to 1-alkynes catalyzed by ruthenium complexes: Role of in situ formed polynuclear aggregates

Article abstract of DOI:10.1021/om0205243

Role of in situ formed polynuclear aggregates in trans addition of silanes to 1-alkynes catalyzed by ruthenium complexes was investigated. Alkenyl ligands were formed by insertion of 1-alkynes into bridging hydrides. The unusual strereoselectivity of this

Full text of DOI:10.1021/om0205243

Organometallics 2002, 21, 4027-4029
4027
Tr a n s Ad d ition s of Sila n es to 1-Alk yn es Ca ta lyzed by
Ru th en iu m Com p lexes: Role of in Situ F or m ed
P olyn u clea r Aggr ega tes
Marta Mart´ın, Eduardo Sola, Fernando J . Lahoz, and Luis A. Oro*
Departamento de Quı´mica Inorga´nica, Instituto de Ciencia de Materiales de Arago´n,
Universidad de Zaragoza-CSIC, E-50009 Zaragoza, Spain
Received J uly 2, 2002
Ta ble 1. Ad d ition of HSiEt3 to P h CtCH Ca ta lyzed
Summary: (Z)-Alkenyl ligands are formed by insertion
of 1-alkynes into bridging hydrides. The unusual ste-
reoselectivity of this elementary step may account for the
selectivity toward (Z)-alkenylsilanes frequently observed
in 1-alkyne hydrosilylations catalyzed by basic late-
transition-metal complexes.
by [Ru H(XY)(CO)(P R3)2] P r ecu r sor sa
silylated products (%)
PhCdCHSiEt3
time to
100%
PhCt
XY
PR3 T (K) yield (h)
Z
E
CSiEt3
Homogeneous reactions catalyzed by transition-metal
complexes often involve unsaturated metal species with
potentially bridging ligands such as hydrides. These are
propitious features for the formation of polynuclear
compounds,1 which should be regarded as likely com-
ponents of catalytic reactions. When observable, such
polynuclear aggregates usually play just a marginal role
during catalysis, as stable resting states for the cata-
lyst.2 However, the feasibility of some other catalytic
processes seems to rely entirely on the performance of
polynuclear species or transition states.3,4 The catalysis
through these polynuclear compounds can involve sev-
eral types of intermetallic cooperation mechanisms,
which may favor faster reaction rates, and selectivities
different from those obtained through possible mono-
nuclear catalytic cycles.3,5
1
2
3
4
5
Cl
PiPr3 298
PiPr3 298
PiPr3 298
1
1.5
164
100
97
95
31
35
0
2
3
10
38
0
1
1
59
27
acetate
acac
acetate
PPh3
353
353
20
20
Cl, PPh3 PPh3
a
Conditions: solvent, 1,2-dichloroethane; HSiEt3/PhCtCH/
catalyst 100/100/1; [Ru]0 ) 2.23 × 10-3 M.
latter reactions have also provided numerous examples
of another peculiar feature, namely the unusual trans
addition of silanes to 1-alkyne triple bonds to give (Z)-
alkenylsilanes.10 In this communication, which focuses
on ruthenium-catalyzed hydrosilylations of phenylacet-
ylene, we present evidence suggesting that the proclivity
of silanes to yield polynuclear compounds and the
frequent achievement of unusual stereoselectivities in
1-alkyne hydrosilylations are not independent facts.
Actually, they seem to be cause and effect.
The hydrosilylation of alkynes catalyzed by Ru(II)-
phosphine compounds has been thoroughly investi-
gated.11 The selectivity of these reactions has been found
to be dependent on, inter alia, the nature of the catalyst
precursor, as illustrated in Table 1 for some related five-
and six-coordinate Ru(II) complexes in the addition of
HSiEt3 to PhCtCH. It is worth noting that all com-
pounds containing the basic PiPr3 phosphine selectively
produce the (Z)-alkenylsilane, even though their reac-
tion rates are very different, depending on the nature
of the other ancillary ligands. On the other hand, the
catalysts containing PPh3 are less selective, producing
(Z)- and (E)-alkenylsilanes together with products of
dehydrogenative silylation: PhCtCSiEt3 and styrene.
A detailed inspection of the course of these latter
nonselective processes has revealed strong variations
With regard to the abundance of reported examples,
the formation of polynuclear aggregates seems to be
specially favored in reactions involving organosilanes.6,7
Actually, dinuclear species have been recognized to be
the true catalysts in reactions such as dehydrogenative
dimerizations of silanes and hydrosilylations.8,9 These
(1) Venanzi, L. M. Coord. Chem. Rev. 1982, 43, 251-274.
(2) For examples, see: Chaloner, P. A.; Esteruelas, M. A.; J oo´, F.;
Oro, L. A. Homogeneous Hydrogenation; Kluwer: Dordrecht, The
Netherlands, 1993.
(3) Catalysis by Di- and Polynuclear Metal Clusters Complexes;
Adams, R. A., Cotton, F. A., Eds.; Wiley-VCH: New York, 1998.
(4) See for example: (a) Muetterties, E. L. J . Organomet. Chem.
1980, 200, 177-190. (b) Esteruelas, M. A.; Oro, L. A.; Valero, C.
Organometallics 1992, 11, 3362-3369. (c) Broussard, M. E.; J uma, B.;
Train, S. G.; Peng, W. J .; Laneman, S. A.; Stanley, G. G. Science 1993,
260, 1784-1788. (d) Grumbine, S. K.; Tilley, T. D. J . Am. Chem. Soc.
1994, 116, 6951-6952. (e) Torres, F.; Sola, E.; Elduque, A.; Mart´ınez,
A. P.; Lahoz, F. J .; Oro, L. A. Chem. Eur. J . 2000, 6, 2120-2128.
(5) (a) Fryzuk, M. D.; Piers, W. E. Organometallics 1990, 9, 986-
998. (b) Bosnich, B. Inorg. Chem. 1999, 38, 2554-2562. (c) Sola, E.;
Torres, F.; J ime´nez, M. V.; Lo´pez, J . A.; Ruiz, S. E.; Lahoz, F. J .;
Elduque, A.; Oro, L. A. J . Am. Chem. Soc. 2001, 123, 11925-11932.
(6) (a) Braunstein, P.; Knorr, M.; Stern, C. Coord. Chem. Rev. 1998,
178-180, 903-965. (b) Ogino, H.; Tobita, H. Adv. Organomet. Chem.
1998, 42, 223-290. (c) Braunstein, P.; Boag, N. M. Angew. Chem., Int.
Ed. 2001, 40, 2427-2433.
(10) (a) Ojima, I.; Li, Z.; Zhu, J . In The Chemistry of Organic Silicon
Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998;
Vol. 2, pp 1687-1792. (b) Tanke, R. S.; Crabtree, R. H. J . Am. Chem.
Soc. 1990, 112, 7984-7989.
(11) (a) Maruyama, Y.; Yamamura, K.; Sagawa, T.; Katayama, H.;
Ozawa, F. Organometallics 2000, 19, 1308-1318. (b) Maruyama, Y.;
Yamamura, K.; Nakayama, I.; Yoshiuchi, K.; Ozawa, F. J . Am. Chem.
Soc. 1998, 120, 1421-1429. (c) Maddock, S. M.; Rickard, C. E. F.;
Roper, W. R.; Wright, L. J . Organometallics 1996, 15, 1793-1803. (d)
Christ, M. L.; Sabo-Etienne, S.; Chaudret, B. Organometallics 1995,
14, 1082-1084. (e) Esteruelas, M. A.; Herrero, J .; Oro, L. A. Organo-
metallics 1993, 12, 2377-2379.
(7) Osakada, K. J . Organomet. Chem. 2000, 611, 323-331.
(8) (a) Rosenberg, J .; Fryzuk, M. D.; Rettig, S. J . Organometallics
1999, 18, 958-969. (b) Fryzuk, M. D.; Rosenberg, L.; Rettig, S. J .
Organometallics 1996, 15, 2871-2880.
(9) Voskoboynikov, A. Z.; Shestakova, A. K.; Beletskaya, I. P.
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10.1021/om0205243 CCC: $22.00 © 2002 American Chemical Society
Publication on Web 08/31/2002

4028 Organometallics, Vol. 21, No. 20, 2002
Communications
complex [Ru((E)-CHdCHtBu)Cl(CO) (PiPr3)2] (8) was
found to react slowly with an excess of HSiEt3 through
the reaction sequence depicted in eq 2. Complexes 9 and
F igu r e 1. (a) Kinetic profile for the addition of HSiEt3 to
PhCtCH with [RuH(κ2O-O2CtBu)(CO)(PPh3)2] (6) as cata-
lyst precursor, under the conditions detailed in Table 1. T
) 353 K. (b) Dependence of the initial hydrosilylation rate
upon the initial concentration of 1. T ) 298 K.
of the rate and selectivity along the reaction, as shown
in Figure 1a for the catalyst precursor [RuH(κ2O-O2Ct-
Bu)(CO)(PPh3)2] (6). The profile of this reaction suggests
that the initial active species, which mainly catalyzes
dehydrogenative silylation, progressively generates a
different catalyst, which is faster and selectively pro-
duces the (Z)-alkenylsilane. The other PPh3 precursors
in Table 1 have been found to behave in a way similar
to 6, although they require different induction periods
to form the second active species.12 Actually, all com-
pounds in Table 1 can fit within a similar behavior,
assuming that formation of the fast and selective
catalyst from the PiPr3-containing precursors is nearly
immediate.
10 have been characterized by X-ray diffraction methods
(Figure 2). The organic and silylated products also
formed along this reaction sequence have been identified
by GC-MS and NMR methods.
Equation 2 shows noticeable similarities with the
evolution reported for the rhodium complex [RhCl(Pi-
Pr3)2] in the presence of triarylsilanes,7 which involved
the sequential formation of a five-coordinate hydride
silyl compound, a dinuclear complex closely related to
9, and a silylene-bridged dinuclear species. The similar-
ity between both sequential reactions strongly suggests
that reorganization into polynuclear aggregates consti-
tutes a favored alternative for mononuclear silyl com-
plexes that accumulate strong donor ligands. It is worth
noting that several stable PPh3 analogues of the non-
observed silyl complex 11 have been reported,11a-c,13
whereas similar compounds containing both basic silyl
ligands and alkylphosphines remain unknown.
An early reactivity study of precursor 1 under the
conditions of this hydrosilylation reaction revealed the
presence of the alkenyl complex [Ru((E)-CHdCHPh)-
Cl(CO)(PiPr3)2] (7) (eq 1) as the only spectroscopically
The likely formation of polynuclear species suggested
by the aforementioned reactivity also provides satisfac-
tory explanations for the kinetic and stereochemical
features of these catalytic reactions. Figure 1b il-
lustrates the dependence of the initial hydrosilylation
rate upon the initial concentration of precursor 1. At
low precursor concentrations, the experimental data
reveal a second-order dependence, which suggests the
participation during catalysis of dinuclear intermediates
or transition states. The progressive deviation from this
second-order dependence at higher precursor concentra-
tions can be rationalized as the result of reversible
further aggregations of the active species into com-
pounds of higher nuclearity. In the presence of such
aggregates, the concentration of the active species would
depend on the stability constants of the aggregates
rather than on the concentration of the added precursor.
The possible behavior of these proposed aggregates can
be illustrated by compound 10, in which the facile
cleavage of the triply (µ3) bridging hydrides and chlo-
rides has been found to afford unsaturated silylene-
bridged dinuclear fragments. The activation parameters
for this fragmentation process have been estimated by
line-shape analysis of the temperature-dependent 31P-
{1H} NMR spectra of 10 in toluene-d8 as ∆Hq ) 19((1)
kcal mol-1 and ∆Sq ) 22((1) eu.
observable species during catalysis.11e By using this
alkenyl compound as catalyst precursor and the deu-
terated alkyne PhCtCD as substrate, we have evi-
denced that 7 is just a spectator during catalysis, since
the catalytic reaction readily produced the deuterated
(Z)-alkenylsilane without any sign of deuterium incor-
poration into 7. It follows from this observation that the
true catalyst of this hydrosilylation reaction is formed
in amounts not detectable by spectroscopy.
Despite this inconvenience for the identification of the
catalytically active species, we searched for possible
evolutions of 7 under catalytic conditions, by exploring
the reactivity of related alkenyl derivatives. Thus, the
(12) For similar “autocatalytic” kinetic profiles, see: (a) Radu, N.
S.; Tilley, T. D. J . Am. Chem. Soc. 1995, 117, 5863-5864. (b) Fu, P.;
Brard, L.; Li, Y.; Marks, T. J . J . Am. Chem. Soc. 1995, 117, 7157-
7168.
(13) Clark, G. R.; Rickard, C. E. F.; Roper, W. R.; Salter, D. M.;
Wright, L. J . Pure Appl. Chem. 1990, 62, 1039-1042.

Communications
Organometallics, Vol. 21, No. 20, 2002 4029
ordination modes for the bridging alkenyl ligands or,
most likely, the fast exchange between degenerate
structures that contain asymmetric µ,η1:η2 alkenyl
ligands. Interestingly, the existence of an NOE effect
between the 1H NMR signals corresponding to the
vinylic hydrogens indicates an unexpected Z stereo-
chemistry for the bridging alkenyl ligands.
This stereochemistry could arise from an insertion
mechanism comprising the alkyne coordination to one
metal center followed by an exo attack of the hydride
from the neighboring metal atom, in contrast to the endo
attack expected for an insertion into a mononuclear
hydride. Such hydride exo attacks have been previously
proposed in reactions involving dinuclear complexes or
transition states,5a,14 and precedents for similar trans
insertions of alkynes into bridging hydrides are known.15
Complexes 12 and 13 have been found to be inert
toward triethylsilane, which indicates that compounds
9-13 are not true intermediates in the catalytic hy-
drosilylation reaction. Nevertheless, the formation of
such compounds by reaction of the catalyst resting
states (e.g. compounds 7 and 8) with excess silane can
be regarded as a sound indication for the proclivity of
basic catalyst precursors to furnish polynuclear species
under the conditions of catalytic hydrosilylation, even
though the actual polynuclear species formed in the
presence of 1-alkynes are likely to be different from
those described herein, which are formed in the absence
of this substrate. Taking into account that participation
of polynuclear intermediates or transition states can
also provide satisfactory explanations for the observed
kinetic and regioselectivity features of these catalytic
reactions, we believe that the polynuclear mechanistic
approach outlined in this communication could consti-
tute a useful tool for mechanistic understanding and
catalyst improvement.
F igu r e 2. Molecular representations of complexes 9
(above) and 10 (below). The Pr groups of the phosphines
i
and the methyl groups of the silylene bridges have been
omitted for clarity in the structure of 10. Selected inter-
atomic distances (Å) and angles (deg): 9: Ru(1)‚‚‚Ru(2) )
2.9029(8), Ru(1)-P(1) ) 2.3654(18), Ru(1)-Cl(1) ) 2.4145-
(17), Ru(1)-Si(1) ) 2.364(2), Ru(1)-H(1) ) 1.91(4), Ru-
(2)-P(2) ) 2.3772(19), Ru(2)-Cl(1) ) 2.4204(16), Ru(2)-
Si(2) ) 2.3685(19), Ru(2)-H(1) ) 1.80(4); Ru(1)-Cl(1)-
Ru(2) ) 73.80(5), Cl(1)-Ru(1)-C(1) ) 167.4(2), Cl(1)-
Ru(1)-Si(1) ) 104.13(6), P(1)-Ru(1)-Si(1) ) 102.20(7),
Cl(1)-Ru(2)-C(2) ) 163.9(2), Cl(1)-Ru(2)-Si(2) ) 105.18-
(6), P(2)-Ru(2)-Si(2) ) 102.52(7). 10: Ru(1)‚‚‚Ru(2) )
2.8222(10), Ru(3)‚‚‚Ru(4) ) 2.8278(10), Ru(1)-Si(1) )
2.363(3), Ru(1)-Cl(1) ) 2.511(2), Ru(1)-Cl(2) ) 2.686(2),
Ru(1)-H(1) ) 2.19, Ru(2)-Si(1) ) 2.377(3), Ru(2)-Cl(1)
) 2.469(2), Ru(2)-H(1) ) 1.66, Ru(2)-H(2) ) 2.00, Ru-
(3)-Cl(1) ) 2.675(2), Ru(3)-Cl(2) ) 2.511(2), Ru(3)-H(2)
) 2.09, Ru(4)-Cl(2) ) 2.491(2), Ru(4)-H(1) ) 1.62, Ru-
(4)-H(2) ) 1.94; Ru(1)-Si(1)-Ru(2) ) 73.09(8), Ru(1)-Cl-
(1)-Ru(2) ) 69.03(6), Ru(1)-Cl(1)-Ru(3) ) 100.01(8),
Ru(1)-Cl(2)-Ru(3) ) 99.72(8), Ru(1)-Cl(2)-Ru(4) ) 84.60-
(7), Ru(2)-Cl(1)-Ru(3) ) 86.06(7), Ru(3)-Cl(2)-Ru(4) )
68.85(6).
Ack n ow led gm en t. We thank the Plan Nacional de
Investigacio´n, MCYT (Project No. BQU2000-1170).
Su p p or tin g In for m a tion Ava ila ble: Synthesis and char-
acterization data for 6, 8-10, 12, and 13, variable-temperature
NMR data, X-ray information on 9 and 10, and details of
catalytic and kinetic studies. This material is available free
of charge via the Internet at http://pubs.acs.org.
OM0205243
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Compound 10 has been found to react with PhCtCH
and BuCtCH, affording the dinuclear alkenyl com-
plexes 12 and 13, respectively (eq 3). Above 223 K, the
spectroscopic data of these compounds indicate sym-
metric Cs structures, suggesting either symmetric co-
t