Role of low-valent rhenium species in catalytic hydrosilylation reactions with oxorhenium catalysts

Article abstract of DOI:10.1021/om300654q

The catalytic competency of a Re(III) complex has been demonstrated. In the presence of silane, oxorhenium(V) catalysts are deoxygenated to produce species that are significantly more active than the metal oxo precursors in hydrosilylation reactions. The results presented suggest that, in evaluating mechanisms for catalytic hydrosilylation reactions that involve high-valent metal oxo complexes, the activity of species that may be generated by deoxygenation of the metal with silane should also be systematically investigated as potential catalysts.

Full text of DOI:10.1021/om300654q

Communication
pubs.acs.org/Organometallics
Role of Low-Valent Rhenium Species in Catalytic Hydrosilylation
Reactions with Oxorhenium Catalysts
Jessica L. Smeltz, Paul D. Boyle, and Elon A. Ison*
Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United
States
S
* Supporting Information
ABSTRACT: The catalytic competency of a Re(III) complex has been demonstrated.
In the presence of silane, oxorhenium(V) catalysts are deoxygenated to produce species
that are significantly more active than the metal oxo precursors in hydrosilylation
reactions. The results presented suggest that, in evaluating mechanisms for catalytic
hydrosilylation reactions that involve high-valent metal oxo complexes, the activity of
species that may be generated by deoxygenation of the metal with silane should also be
systematically investigated as potential catalysts.
ver the past decade, several research groups have
investigated the catalytic competency of high-oxidation-
the rhenium(V) acyl intermediate [DAAmRe(O)(C(O)CH3)]
(2), followed by the treatment of 2 with CO to generate 1.2b,4
The catalytic competency of complexes 1−3 for the
hydrosilylation of aldehydes with dimethylphenylsilane was
investigated (Table 1). Complexes 1−3 catalyze the hydro-
silylation of benzaldehyde with neat dimethylphenylsilane at 80
°C. Complex 1 is the most efficient catalyst in the series, and
100% conversion is attained in 2.5 h with 0.1 mol % catalyst
loading (entry 1); 100% conversion is attained in 14 h with
0.01 mol % catalyst loading (entry 2). The Re(V) complexes 2
and 3 are not as active; 98.0% and 83.5% yields are achieved
with 0.1 mol % catalyst loading in 2 and 3 days, respectively
(entries 3 and 4). At room temperature, quantitative conversion
of benzaldehyde to benzyloxydimethyl(phenyl)silane by 1 was
observed in 15 h (entry 5). Complexes 2 and 3, however,
achieved <12% yield after 15 days at room temperature (entries
6 and 7).
Catalyst 1 also performed efficient hydrosilylations with
various other silanes, such as MePh2SiH (entry 8), Et3SiH
(entry 10), and Pr3SiH (entry 11). Hydrosilylation was not
observed when Ph3SiH was utilized as a substrate (entry 9),
presumably due to the steric bulk of this silane. Catalysis of
various para-substituted benzaldehydes with varied function-
alities (CF3, Cl, Me; entries 12−15) was efficient, and reactions
were complete in <2 h.
O
state Re(V) and Re(VII) oxo complexes in hydrosilylation
reactions.1 In these systems, the role of the terminal oxo
ligand(s) has been investigated, and it has been shown that the
oxo ligand either stabilizes the high-valent metal center but
does not participate directly in the catalytic reaction or is
activated with silane to generate a new catalytically active
species.1c,f,i,2 However, in any of these systems, the catalyst may
be deoxygenated by silane to generate a low-valent Re species
that is capable of catalysis. Recently, low-valent Re species not
bearing terminal oxos have also been shown to be efficient
catalysts for hydrosilylation.3
In this paper we provide evidence for the catalytic
competency of a Re(III) complex. We show that, in the
presence of silane, oxorhenium(V) catalysts are deoxygenated
to produce species that are significantly more active than the
metal oxo precursors in hydrosilylation reactions. The results
presented suggest that in evaluating mechanisms for catalytic
hydrosilylation reactions that involve high-valent metal oxo
complexes, the activity of species that may be generated by
deoxygenation of the metal with silane should also be
systematically investigated as potential catalysts.
Recently we reported the synthesis of the rhenium(III)
acetate complex [DAAmRe(CO)(OAc)] (1; DAAm = N,N-
bis(2-arylaminoethyl)methylamine; aryl = C6F5). As shown in
Scheme 1, the reaction proceeds by treating the oxorhenium-
(V) complex [DAAmRe(O)(CH3)] (3) with CO to produce
Under these reaction conditions, catalyst 1 is more efficient
than the previously reported Re(V) complex [Re(O)2(I)-
(PPh3)2] (4)1d (entry 16) and is comparable to [Re(O)-
(hoz)2(NCCH3)]+ (5)1f (entries 17 and 18). Importantly, 1
performs as well as, or better than, the best oxorhenium
catalysts reported thus far. The specific highlights of this
particular catalytic system include: (1) the catalytic system
employs a Re(III) catalyst that does not incorporate a metal
Scheme 1
Received: July 12, 2012
Published: August 23, 2012
© 2012 American Chemical Society
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Communication
with 3 is initially very sluggish (∼24 h). During this time the
reaction mixture undergoes a series of color changes (blue−
green−red). Once the reaction mixture turns red (∼24 h),
rapid catalysis ensues, which implies that 3 is not the
catalytically active species but is transformed in situ by the
substrates to a more active catalyst.
Table 1. Catalytic Hydrosilylation of Aldehydes According to
Eq 1
To investigate the modes of substrate activation with
catalysts 1−3, stoichiometric reactions between the complexes
and the substrates were investigated. Catalysts 1−3 did not
exhibit stoichiometric reactivity with benzaldehyde but did
react stoichiometrically with the dimethylphenylsilane. The
reaction of 1 and 2 with dimethylphenylsilane (Scheme 2)
a
bc
,
entry
cat.
X
R3
time
T (°C)
80
yield (%)
b
1
1
H
Me2Ph
Me2Ph
Me2Ph
Me2Ph
Me2Ph
Me2Ph
Me2Ph
MePh2
Ph3
2.5 h
14 h
2 days
3 days
15 h
15 days
15 days
8 h
94.6
d
b
2
1
H
80
92.7
c
3
2
3
1
2
3
1
1
1
1
1
1
1
1
4
5
H
80
98.0
c
4
H
80
83.5
b
5
H
room temp
93.5
Scheme 2
c
6
H
room temp
11.1
c
7
H
room temp
3.4
c
8
H
80
80
80
80
80
80
80
80
80
80
80
quant
c
9
H
22 h
27 h
53 h
2 h
0
b
10
11
12
13
14
15
16
17
18
H
Et3
86.9
c
H
Pr3
78.6
b
CF3
Cl
Me
OMe
H
Me2Ph
Me2Ph
Me2Ph
Me2Ph
Me2Ph
Me2Ph
Me2Ph
96.3
b
2 h
85.0
b
2 h
98.4
b
2 h
81.6
c
5 h
59.6
c
H
1 h
quant
d
c
5
H
24 h
73.8
a
Reaction conditions: Re (0.00989 mmol), aldehyde (9.89 mmol), and
b
c
silane (11.87 mmol) under an N2 atmosphere. Isolated yield. NMR
yield. Re (0.000989 mmol, 0.01 mol %)
d
resulted in the formation of the dirhenium(II) species 6.
1
Complex 6 was characterized by H and 19F NMR and IR
spectroscopy, as well as X-ray crystallography (Figure 2). The
electron configuration for 6 (σ2π4δ2δ*2) is consistent with a
Re−Re bond order of 3. The Re−Re bond length in 6 is
2.4722(2) Å, which is comparable to those in related dirhenium
complexes reported by Walton and co-workers where the Re−
Re triple bond is weakened by π-back-bonding to the CO π*
orbitals.5 The geometry at each Re center is a distorted trigonal
bypyramid, with the CO and amine nitrogen occupying the
axial planes. The C1−Re1−Re2−C2 dihedral angle is −113.7°
between the two halves of the dimer.
The stoichiometric reaction of 3 with dimethylphenylsilane
was also investigated. As shown in Scheme 3, heating 3 (80 °C)
in the presence of Me2PhSiH resulted in the formation of the
dirhenium complex 7. Complex 7 is isostructural with 6, except
that N2 replaces CO as the terminal ligand. CO stretches are
observed by IR spectroscopy for 6 at 1886 and 1868 cm−1,
while N2 stretches are observed at 2031 and 2015 cm−1 for 7.
An X-ray crystal structure of 7 was also obtained (Figure 2).
The Re−Re bond length in 7 is 2.4447(5) Å.
oxo, (2) the reaction can be performed with low catalyst
loadings and short reaction times, (3) reactions can be
performed without a solvent, and (4) the catalyst is air and
moisture stable and can be easily handled on the benchtop.
In order to compare the catalytic activity of rhenium
complexes in two oxidation states (3 and 2, Re(V); 1, Re(III))
bearing the same DAAm ancillary ligands, the time profile for
product formation by each of the catalysts was monitored by
1H NMR spectroscopy (1, 0.01 mol %; 2 and 3, 0.1 mol %)
(Figure 1). Catalysis with 1 and 2 ensues without an induction
period, with catalyst 1 ∼17 times more active than catalyst 2
(kobs = 0.54 and 0.032 s−1 for 1 and 2, respectively). Catalysis
Catalysis with 6 was investigated and resulted in <5% yield in
5 h. Complex 3 was allowed to react with Me2PhSiH for 24 h to
generate 7 in situ. Once 7 was generated, it was used as the
catalyst for the hydrosilylation of benzaldehyde with
Me2PhSiH. Catalysis with 7 proceeded immediately, unlike
the analogous reaction with catalyst 3; however, catalysis was
significantly slower than with 1 or 2 (Supporting Information).
A plausible mechanism for the reaction of 1 with Me2PhSiH
is depicted in Scheme 4. In this mechanism, Me2PhSiH is
activated by 1 to produce the Re hydride 8. In the absence of
aldehyde, two 8 molecules react to produce H2 and the Re(II)
dimer 6. The high activity observed for catalyst 1 (Re(III)),
which does not contain a terminal oxo, and the facile formation
of 6 and 7 from 2 and 3, respectively, suggest that in the
Figure 1. Time profile for the catalytic hydrosilylation of benzaldehyde
(9.89 mmol) with dimethylphenylsilane (11.87 mmol) with 1 (red
circles; 0.989 μmol, 0.01 mol %); 2 (blue diamonds; 9.89 μmol, 0.1
mol %), and 3 (green squares; 9.89 μmol, 0.1 mol %) at 80 °C under
1
an N2 atmosphere. Yields were obtained by H NMR spectroscopy.
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Figure 2. Thermal ellipsoid (50%) plots of 6 and 7. Hydrogen atoms are omitted, and the C6F5 ligands are represented in wireframe for clarity.
Selected bond lengths (Å) and angles (deg) are as follows. 6: Re1−Re2, 2.4722(2); Re1−C1, 1.880 (3); Re1−N1, 1.9668(19); Re1−N2, 2.242(2).
C1−Re1−Re2, 81.93(7); N1−Re1−Re2, 120.30(6); N2−Re1−Re2, 106.98(5); C1−Re1−N2, 171.07(8); C1−Re1−N1, 95.99(9); N1−Re1−N3,
121.39(8); N1−Re1−N2, 80.28(8). 7: Re1−Re2, 2.4447(5); Re1−N1, 1.9280(16); Re1−N5, 1.9780(17); Re1−N6, 2.1895(16); N1−N2, 1.127(2).
N1−Re1−Re2, 86.81(5); N5−Re1−Re2, 119.94(5); N6−Re1−Re2, 104.25(5); N5−Re1−N7, 122.80(7); N5−Re1−N6, 80.74(6).
Cl, CF3, Me, OMe). As shown in Figure 3, the Hammett plot
obtained from this experiment reveals that electron-with-
drawing groups accelerate the rate of the reaction (ρ = 0.52).
Scheme 3
Scheme 4
Figure 3. Hammett plot for the benzaldehyde competition experi-
ments.
The kinetic isotope effect (KIE) for the hydrosilylation of
benzaldehyde with a 1/1 mixture of Et3Si-H and Et3Si-D was
also investigated according to Scheme 6. A KIE of 1.4 was
observed for the reaction.
Scheme 6
presence of silane the catalytically relevant species when the
oxorhenium(V) complexes 2 and 3 are employed as catalysts are
not high-valent rhenium oxos, but low-valent species that are
generated upon deoxygenation with Me2PhSiH.
In order to investigate the mechanism of the hydrosilylation
reaction catalyzed by 1, several studies were performed. The
relative reactivities of various para-substituted benzaldehyde
substrates toward hydrosilylation were determined by a series
of competition experiments, as depicted in Scheme 5 (X = H,
Scheme 5
Finally, the influence of acetate on the catalytic reaction with
1 was investigated. Exogeneous acetate (20 equiv of AcONBu4
with respect to Re) was added to the catalytic reaction with 1.
The reaction was complete in 2.5 h, similar to the catalytic
reactions that did not employ exogenous acetate. This suggests
that the rate is not dependent on the concentration of acetate.
On the basis of the data presented, the following mechanism
has been proposed for the catalytic hydrosilylation of
benzaldehyde (Scheme 7). (1) Treatment of 1 with Me2PhSiH
results in the formation of the catalytically active Re−H
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Communication
Scheme 7
AUTHOR INFORMATION
Corresponding Author
*E-mail: eaison@ncsu.edu.
■
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge North Carolina State University and the
National Science Foundation via the CAREER Award (CHE-
0955636) for funding.
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complex 8. (2) Complex 8 subsequently reduces benzaldehyde
to yield 9. The observed electronic dependence on the
benzaldehyde substrate (electron-withdrawing groups acceler-
ate the rate) is consistent with the reduction of the CO bond
with the Re−H.6 (3) The reaction of 9 with Me2PhSiH results
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aldehydes. Catalysis with 1 was performed without solvent and
with low catalyst loadings. Importantly, 1 is significantly more
active as a catalyst than the oxorhenium(V) complexes 2 and 3
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are not catalytically relevant, their facile generation from 2 and
3 and the observed high reactivity of the Re(III) complex 1
suggests that when oxorhenium complexes are employed as
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but species that are generated upon deoxygenation with
Me2PhSiH. This work suggests that low-valent Re species
that are generated by the reduction of high-valent metal oxos
may be catalytically relevant and, consequently, their activity
should be systematically investigated.
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ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, tables, and CIF files giving experimental
procedures, crystallographic data for 6 and 7, and spectroscopic
data for organic products. This material is available free of
charge via the Internet at http://pubs.acs.org.
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