A 14-VE platinum phosphabarrelene complex in the hydrosilylation of alkynes

Article abstract of DOI:10.1021/om900067f

A bulky phosphabarrelene ligand is used in the platinum-catalyzed hydrosilylation of alkynes. High regiose- lectivities were achieved under mild conditions from an in situ formed catalyst at low catalyst loadings. Furthermore, a rare 14-VE PtL2 complex wa

Full text of DOI:10.1021/om900067f

2360
Organometallics 2009, 28, 2360–2362
A 14-VE Platinum(0) Phosphabarrelene Complex in the
Hydrosilylation of Alkynes
Matthias Blug, Xavier-Frederic Le Goff, Nicolas Me´zailles, and Pascal Le Floch*
Laboratoire “He´te´roe´le´ments et Coordination”, Ecole Polytechnique, CNRS, 91128 Palaiseau Ce´dex, France
ReceiVed January 28, 2009
Scheme 1. Pt-Catalyzed Hydrosylilation of Alkynes using
Ligand 1
Summary: A bulky phosphabarrelene ligand is used in the
platinum-catalyzed hydrosilylation of alkynes. High regiose-
lectiVities were achieVed under mild conditions from an in situ
formed catalyst at low catalyst loadings. Furthermore, a rare
14-VE PtL2 complex was isolated, which equally proVed to be
a highly reactiVe catalyst precursor.
Sterically crowded tertiary phosphines have found important
applications in the kinetic stabilization of unsaturated low-valent
transition-metal complexes.1-3 Some of these complexes proved
to be among the most active catalysts in organic transformations
of synthetic relevance such as the Pd-catalyzed cross-coupling
reactions.2,4 In some instances, the unsaturation of the metal
coordination sphere could be used for the stabilization of reactive
species.5 Controlling the steric environment of tertiary phos-
phines thus allows us to finely tune the degree of coordinative
unsaturation of a metal center and in turn its reactive behavior,
yet this remains an important challenge. In most cases, bulky
phosphines are available through multistep synthesis from
halogenated derivatives or through metal-catalyzed phosphina-
tion or phosphinylation processes.6,7 In this perspective, low-
coordinated phosphorus species featuring either Ct P or CdP
bonds, which exhibit reactivity close to that of their carbon
counterparts, were shown to behave as efficient precursors of
cyclic phosphines and phosphorus-containing cage compounds.8
Among these, 1-phosphabarrelenes, which are easily prepared
through [4 + 2] classical Diels-Alder reactions between
phosphinines and alkynes, have recently appeared as a very
promising new class of ligands in catalysis for several reasons.9
First, these bicyclic phosphines are well-suited for the stabiliza-
tion of low-valent transition-metal complexes, as they combine
rigidity and a modular steric crowding. Second, they possess
electronic properties significantly different from those of bulky
trialkylphosphines, as they are in fact good π-acceptor ligands.10
Herein we wish to report on the successful use of one of these
phosphabarrelene ligands in the platinum(0)-catalyzed hydro-
silylation of alkynes.11 We also show that the ligand allows the
stabilization of a rare 14-electron Pt(0) species.
* To whom correspondence should be addressed. Tel: +33 1 69 33 44
01. Fax: +33 1 69 33 44 40. E-mail: lefloch@poly.polytechnique.fr.
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Chem. Soc. 2003, 125, 7816–7817. (c) Paul, F.; Patt, J.; Hartwig, J. F. J. Am.
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All our experiments were carried out with ligand 1, which is
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10.1021/om900067f CCC: $40.75
2009 American Chemical Society
Publication on Web 04/01/2009

Communications
Organometallics, Vol. 28, No. 8, 2009 2361
Table 1. Substrate and Silane Screeninga
entry
Pt source
L/Pt
S/Pt
100
1000
10000
100
substrate
PhCCH
PhCCH
PhCCH
PhCCH
PhCCH
PhCCH
PhCCH
1-hexyne
1-hexyne
HCCHCH2OH
3-hexyne
PhCCPh
silane
t (min)
conversn (%)
ꢀ(E)/Rb
1
2
3
4
5
6
7
8
[(COD)PtCl2]
[(COD)PtCl2]
[(COD)PtCl2]
[Pt2(DVDS)3]
[Pt(PCy3)2]
[(COD)PtCl2]
[(COD)PtCl2]
[(COD)PtCl2]
[(COD)PtCl2]
[(COD)PtCl2]
[(COD)PtCl2]
[(COD)PtCl2]
0
2
2
1
2
2
2
2
2
2
2
2
Et3SiH
Et3SiH
Et3SiH
Et3SiH
Et3SiH
PhMe2SiH
Ph3SiH
Et3SiH
PhMe2SiH
Et3SiH
Et3SiH
Et3SiH
60
5
1440
60
5
5
60
5
5
5
100
100
100
85
100
100
0
99
98
99
99
80/20
99/1
92/8
97/3
92/8
99/1
n.d.
99/1
99/1
99/1
100
1000
1000
1000
1000
1000
1000
1000
9
10
11
12
5
5
99
a Conditions: alkyne (1 mmol), silane (1.2 mmol), THF (0.5 mmol). Abbreviations: n.d. ) not determined; S ) substrate; L ) ligand. b The ratio of
isomers was determined by GC; the ꢀ(Z) isomer was not detected.
ylphosphinine at room temperature. Preliminary experiments
were conducted in the case of the silylation of phenylacetylene
with triethylsilane, a transformation which is well documented
and often used as a benchmark (Scheme 1). In a standard
experiment 1 (0.2 equiv) was reacted first with [Pt(COD)Cl2]
(0.1 equiv) in the presence of triethylsilane (120 equiv) in THF
at 60 °C for 2 min prior to the addition of phenylacetylene (100
equiv). The resulting mixture was then placed in a water bath
at 20 °C and the progress of the reaction monitored by GC
analysis. As can be seen in Table 1, entry 2, the presence of
ligand 1 results in an important increase of both the reaction
rate and the selectivity, compared to those for the [Pt(COD)Cl2]-
based catalyst (entry 1), despite a 10 times lower catalyst
loading. A complete conversion with a ꢀ-(E)/R ratio of 99/1
was obtained after only 5 min (entry 2). Note that, at lower
catalyst loadings (0.01%), the catalytic activity decreased while
Figure 1. Molecular structure of 2 in the crystal (ORTEP
representation, thermal ellipsoids set to the 50% probability level,
the regioselectivity was slightly altered (entry 3). As the first
catalytically active species in this transformation involves a Pt(0)
two THF molecules omitted for clarity). Selected bond distances
complex, the combination of 1 equiv of 1 with the Kartstedt
(Å) and angles (deg): Pt1-P1, 2.293(1); Pt1-P2, 2.293(1);
catalyst [Pt2(DVDS)3] was tested. The 16-electron species
Pt1-Cl1, 2.413(2); Pt1-H1, 1.68(1); P1-Pt1-P2, 177.03(5);
formed in situ resulted in a less efficient catalytic system (entry
P1-Pt1-Cl1, 88.52(3).
4). To the best of our knowledge, the combination of 1 and
[Pt(COD)Cl2] turns out to be the most efficient phosphine-based
catalyst reported to date and shows superior selectivity compared
to [Pt(PCy3)2] under our reaction conditions (Table 1, entry 5).
To evaluate the scope of our catalyst, further experiments were
conducted with other silanes and alkynes (see Table 1). Apart
from the case of triphenylsilane, where no reaction was observed
(entry 7), conversions were complete within 5 min and good
regioselectivities were obtained.
In order to get further insight into the mechanism of this
transformation, observation/isolation of intermediates was sought.
The reaction was therefore carried out with higher catalyst
loading (5% [(COD)PtCl2], 10% 1), allowing the direct observa-
tion of Pt species by NMR spectroscopy. As expected from the
first results (cf. Table 1), the reaction of Et3SiH with phenyl-
acetylene is complete within 5 min and a single new platinum
complex is observed in the 31P NMR spectrum (δ -15.56
ppm, s wit sat, JP-Pt ) 3120 Hz), as shown by the presence
of Pt satellites (I ) 1/2, natural abundance 33%). This
complex, 2, was isolated from the reaction mixture (see the
Supporting Information) and fully characterized. In particular,
it is characterized by a high-field resonance in the 1H
spectrum (δ -15.56 ppm, t with sat, JP-H ) 15.3 Hz, JPt-H
) 588.4 Hz), indicating the presence of a hydride ligand,
coupled with two equivalent phosphorus ligands. The X-ray
crystal structure analysis confirmed the complex to be the
trans-[Pt(1)2HCl] complex (Figure 1).
attempt, heating a solution of 2 in a NMR tube in the presence
of triethylsilane in excess (10 equiv) in THF at 60 °C resulted
in the appearance of the new platinum complex 3, characterized
by a signal of weak intensity at δ 5.6 ppm (s with sat, JPPt
)
4644.0 Hz). However, under these conditions, complex 3, which
was only formed in a small amount, could not be isolated.
Supposing that complex 3 could be the corresponding 14-VE
complex which equilibrates with 2 through the oxidative addition
of HCl in solution, experiments were conducted under a stream
of nitrogen. Under these new conditions, the slow but quantita-
tive formation of 3 was observed. This complex could be
prepared in a straightforward manner from the reaction of 1
(2 equiv) with [Pt(COD)Cl2] in the presence of [CoCp2]
(2 equiv) as a reducing agent in acetonitrile as solvent (Scheme
2). The structure of 3 was further confirmed by an X-ray crystal
structure analysis (Figure 2).
Complex 3 is a rare example of a PtL2 complex.3,12 Interest-
ingly, the Pt-P bonds at 2.243(4) and 2.239(1) Å are close to
those of the [(PCy3)2Pt] complex (2.231(4) Å), though the two
ligands display different electronic capacities (note: due to the
higher π-acceptor properties of the phosphabarrelene compared
to a trialkylphosphine, a shorter Pt-P bond length should be
expected). Accordingly, the P-Pt-P bite angles are comparable
in the two complexes (162.62(4)° in 3 vs 160.0(5)° in
[(PCy3)2Pt]) and markedly different from that of the [(P(t-Bu)2-
Ph)2Pt] complex, where agostic interactions between the central
Pt atom and alkyl C-H groups are found. As expected, 3 is
This complex, which was also synthesized independently from
the reaction 1 with [Pt(COD)Cl2] with triethylsilane in THF at
60 °C in good yields (Scheme 2), is a deactivated form of the
catalyst. Indeed, according to 31P NMR spectroscopy, in a first
(12) Arnold, P. L.; Cloke, F. G. N.; Geldbach, T.; Hitchcock, P. B.
Organometallics 1999, 18, 3228–3233.

2362 Organometallics, Vol. 28, No. 8, 2009
Communications
Scheme 2. Syntheses of Complexes 2 and 3
Scheme 3. Reactivity of 3 toward PhCCH and Et3SiH
indeed a very good catalytic precursor and the experiments
reported in Table 1 were successfully reproduced using 0.1%
of catalyst under the same experimental conditions.
In order to get further insight into the initial steps of the
catalytic pathway, complex 3 was reacted with Et3SiH. Surpris-
ingly, no reaction occurred, even at elevated temperature (in a
THF solution at reflux). This result contrasts sharply with the
well-known reactivity of the [Pt(PCy3)2] complex with silanes
between -78 and 0 °C.13 On the other hand, 3 reacted readily
at room temperature with phenylacetylene to afford a new
complex characterized by a AX spin system pattern in 31P NMR
us to conclude that, unlike for other Pt complexes reported so
far, the first step of the catalytic cycle with complex 3 might
involve coordination of the alkyne and not the oxidative addition
of silane on a low-valent Pt(0) complex.
spectroscopy (δ -11.2 ppm, d + sat (JPP ) 35.9 Hz, JPtP
)
3680 Hz), -17.7 ppm, d + sat (JPP ) 35.9 Hz, JPtP ) 3640
Hz)). This characteristic spin pattern of complex 4 correlates
well with the formation of a 16-VE alkyne adduct.14 Importantly,
this complex subsequently reacted with 1 equiv of Et3SiH at
60 °C to yield complex 3 and the trans-triethylstyrylsilane
In summary, we have shown here that bulky phosphabarrelene
ligands act as very efficient ligands in the Pt(0)-catalyzed
hydrosilylation of alkynes under mild conditions. Though some
structural analogy exists between complex 3 and [(PCy3)2Pt0],
a significant difference in reactivity toward Et3SiH is observed.
The basis of this difference resides in the electronic properties
of the ligand (π acceptor). Further studies will now focus on
the reactivity and use of this complex and its derivatives in other
catalytic processes. The use of these new ligands in the
stabilization of other electron-deficient complexes will also be
addressed.
1
according to H and 31P NMR data. Overall, these results led
Acknowledgment. We thank the CNRS (Centre National
de la Recherche Scientifique) and the Ecole Polytechnique
for financial support of this work.
Supporting Information Available: Text, figures, tables, and
CIF files giving full experimental details, references for vinylsilanes,
the 31P NMR spectrum of 4, and crystallographic data for 2 and 3.
This material is available free of charge via the Internet at
http://pubs.acs.org.
OM900067F
Figure 2. Molecular structure of 3 in the crystal (ORTEP re-
presenatation, thermal ellipsoids set to the 50% probability level).
Selected bond distances (Å) and angles (deg): P1-Pt1, 2.239(1);
P2-Pt1, 2.243(1); P1-Pt1-P2, 162.62(4).
(13) Chan, D.; Duckett, S. B.; Heath, S. L.; Khazal, I. G.; Perutz, R. N.;
Sabo-Etienne, S.; Timmins, P. L. Organometallics 2004, 23, 5744–5756.
(14) Ozawa, F.; Mori, T. Organometallics 2003, 22, 3593–3599.