Mechanism of the dehydrogenative silylation of alcohols catalyzed by cationic gold complexes: An experimental and theoretical study

Article abstract of DOI:10.1002/chem.201002150

The catalytic activity both of cationic [(XDPP)Au][X] (XDPP=bis-2,5- diphenylphosphole xantphos X=BF₄) and of the isolated gold hydride complex [(XDPP)₂Au₂H][OTf] in the dehydrogenative silylation process is presented. A p

Full text of DOI:10.1002/chem.201002150

DOI: 10.1002/chem.201002150
Mechanism of the Dehydrogenative Silylation of Alcohols Catalyzed by
Cationic Gold Complexes: An Experimental and Theoretical Study
Stꢀphanie Labouille, Aurꢀlie Escalle-Lewis, Yves Jean,* Nicolas Mꢀzailles,* and
†
[a]
Pascal Le Floch
+
Abstract: The catalytic activity both of
cationic [(XDPP)Au][X] (XDPP=bis-
counter anion as a co-catalyst, which
was experimentally confirmed by test-
ing various counterions (X=OTf,
NTf , PF ). Finally, a “Au H ” species
2 6 2
was determined as the intermediate
during the catalytic cycle, which corre-
lates well with the experimental find-
ings on the first example of catalytic
activity of an isolated “AuꢀH” com-
2,5-diphenylphosphole xantphos X=
BF ) and of the isolated gold hydride
4
complex [(XDPP) Au H] AHCUTNGRNEGUN[ OTf] in the
2
2
Keywords: dehydrogenative silyla-
tion · density functional calcula-
tions · gold · gold catalysis · phos-
phane ligands
dehydrogenative silylation process is
presented. A parallel theoretical study
using density functional theory re-
plex.
vealed
a
mechanism involving the
Introduction
Catalytic processes involving gold—gold(0) on supports for
heterogeneous processes, and gold(I) or gold(III) complexes
for homogeneous processes—have been studied in great
AHCTUNGTRENNUNG
[1,2]
detail over the past decades.
The seminal discoveries of
the very high efficiency of the hydrogenation of olefins by
0
[3]
Au in the early 1970s, and of the addition of a C nucleo-
I
phile to a carbonyl moiety catalyzed by a Au complex in
986, truly launched these two parallel research domains.
[4]
1
Scheme 1. The two examples of stable AuꢀH fragments (homometallic
Au Complexes), our cationic complex 3-X, and [(xanthphos)AuCl]-type
complexes B.
Since then, focusing only on homogeneous processes, very
efficient and selective processes have been devised, includ-
[5]
[6]
ing hydrogenation, hydrosilylation, or nucleophilic addi-
[
1,4,7]
tions to p-systems.
tion of alcohols using Au chloride complexes was reported
Recently, the dehydrogenative silyla-
I
rather unreactive, and so far its use as a catalyst has not
been reported.
[8]
by Ito et al. In the above-mentioned processes involving
“
H-Y” (Y=H, BR , SiR ) reagents, AuꢀH intermediates
Following previous work on the stabilization of unsaturat-
ed, low-valent transition-metal fragments using the wide-
bite-angle, electron-accepting ligand XDPP (bis-2,5-diphe-
2
3
[
5c–e,6a,8]
have been postulated
in direct analogy with the same
processes using other transition metals for which such MꢀH
[11,18]
bonds have been observed and fully characterized. Howev-
nylphosphole xantphos 1),
in 2009 we isolated the first
[9]
er, no such species had been isolated for gold until 2008,
AuꢀH complex featuring a phosphine ligand in the form of
+
[12]
when Sadighi et al. reported the first stable AuꢀH complex,
the Au H species, 2-OTf (Scheme 1). The stability of this
2
[10]
A (Scheme 1), featuring an NHC ligand. This complex is
species is undoubtedly linked to our peculiar ligand, as it is
known in the literature that the reaction of [(R P)AuCl]
3
complexes with stoichiometric amounts of hydride sources
[
a] S. Labouille, A. Escalle-Lewis, Prof. Y. Jean, Dr. N. Mꢀzailles,
Prof. P. Le Floch
Laboratoire “Hꢀtꢀroꢀlꢀments et Coordination”
UMR CNRS 7653 (DCPH), Dꢀpartement de Chimie
Ecole Polytechnique
leads to the formation of Au clusters without observation of
[
13,14]
AuꢀH bonds,
and that an excess of hydride sources can
lead to the formation of phosphine-stabilized Au nanoparti-
cles. As far as catalysis is concerned, Ito showed that only
Au chloride complexes, B (Scheme 1), featuring wide-bite-
[15]
9
1128 Palaiseau Cedex (France)
I
Fax : (+33)169-33-44-40
E-mail: nicolas.mezailles@polytechnique.edu
yves.jean@polytechnique.edu
angle ligands of the xantphos (xantphos=4,5-bis(diphenyl-
phosphino)-9,9-dimethylxanthene) type, were efficient pre-
catalysts for dehydrogenative silylation. In 2009, the same
group reported an interesting experimental, mechanistic
[
†] Deceased, March 17, 2010.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002150.
[16]
study, in which they presented convincing evidence for an
2256
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FULL PAPER
1
intermediate binuclear “Au H” species, the structure of
H NMR spectra at regular intervals. Aliquots were taken;
2
which, however, could not be fully ascertained.
the solvent and the silane evaporated, and the signals of the
remaining alcohol versus signals of the silyl ether were inte-
grated. Using stoichiometric amounts of alcohol and silane,
the conversion was 25% within 2 h with 1% of complex 3-
When we began this study, our goals were manifold. First,
we wished to conduct a combined experimental/theoretical
+
ꢀ
study using our XDPP-Au X complex 3-X (Scheme 1), in
order to elucidate the mechanism of the dehydrogenative si-
BF as the catalyst, and was complete within 24 h. As a con-
4
lylation of alcohols. Secondly, we hoped that our isolated
trol, the analogous reaction was performed without a cata-
lyst, and resulted in a small yet significant 4% yield within
2 h. This result shows that the reaction is favorable without
a catalyst, but that is shows slow kinetics. The complex 3-
+
“
Au H ” complex 2-X would be the first “Au-H”-containing
2
complex to be catalytically efficient. The results, presented
herein, exceeded our expectations. Indeed, complex 3-X
proved to be a very efficient catalyst for the transformation,
a result validated through DFT calculations. A mechanism
was postulated and calculated, and the results are in full
accord with experimental observations. A few DFT studies
pertaining to catalytic processes involving cationic metal
BF therefore acts as a true catalyst for this transformation.
4
[
17]
complexes explicitly incorporate the counterion,
two cases, the assistance of the counterion in proton transfer
was observed.
and in
[17e,f]
Our DFT calculations then prompted us
to test various counterions, and the results supported our
+
mechanism. Finally, a Au H intermediate was determined
2
as a minimum in the cycle, which corroborated the fact that
complex 2-X is the first isolated gold hydride species to act
as an efficient catalyst.
Most importantly, the course of the catalytic reaction was
also followed by P NMR spectroscopy, which showed the
3
1
presence of a single signal, corresponding to complex 2-BF ,
4
+
ꢀ
by analogy to the [(1) Au H] ACHUTNRGNENUG[ OTf] (2-OTf), which we re-
2
2
ported previously. In terms of mechanistic considerations,
the observation of such a species points to 2-BF4 being
either a true intermediate in the catalytic cycle, or being a
resting state. The mechanism for this transformation was
therefore studied, with the constraints of rationalizing all
the experimental facts.
Results and Discussion
Experimental catalytic study: Ito et al. recently reported the
dehydrogenative silylation of alcohols catalyzed by a gold(I)
[8,16]
complex B, [AuCl
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(xantphos)].
Interestingly, in their first
report, they showed that only wide-bite-angle ligands pro-
moted the reaction. As mentioned above, in a subsequent
study, the same authors observed a complex featuring a
Theoretical study of the catalytic cycle: To investigate the
[19]
mechanism of this reaction, DFT calculations
were per-
[20]
{
(xantphos) Au H} fragment, but the data did not allow
formed using the B3PW91 functional.
The solvent was
2
2
them to fully elucidate the structure of this species. In the
past, we have demonstrated the superior activity of the
XDPP ligand 1 compared to xantphos, in the allylation of
considered implicitly by performing polarized continuum
model (PCM) single-point calculations on gas-phase opti-
[21,22]
mized geometries.
Complex III-BF4 was used as a
[18]
amines catalyzed by Pd or Pt complexes; this prompted
model for the experimental complex 3-BF . To save comput-
4
I
us to study the XDPP–Au couple as a potential catalyst in
ing time, phenyl substituents on the phosphole ring and
methyl groups on the xanthene backbone were replaced by
H atoms. A view of the optimized geometry is given in
Figure 1, and the main geometrical parameters are reported
in Table 1.
the dehydrogenative silylation process [Eq. (1)].
In 2009, we showed that ligand 1 reacted with one equiva-
lent of [AuCl
mixture of two complexes.
ACHTUNGTRENNUNG( tht)] (tht=tetrahydrothiophene) to form a
[12]
Using this mixture of gold
chloride complexes was not a suitable way to test the cata-
lytic reaction. On the other hand, a single cationic complex,
3
-BF , was obtained from the mixture upon chloride abstrac-
4
tion by the silver salt AgBF4 [Eq. (2)], which was then
tested. Typically, the catalyst, the alcohol, the silane, and the
solvent (dichloroethane) were mixed in a schlenk flask
under an inert atmosphere, and the resulting mixture was
heated at 508C. The reaction was followed by taking
Figure 1. DFT-optimized structure of III-BF
terion BF has been omitted for clarity).
4
, model for 3-BF
4
(the coun-
4
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2257

Y. Jean, N. Mꢀzailles et al.
4
Table 1. Comparison of the DFT-optimized geometry III-BF with the X-
[
a]
ray structure of 3-BF
4
.
Bonds 3-BF [ꢂ] III-BF
4
4
[ꢂ] Angles
P2-Au-P1
Au-P1-C1 109.62(1) 108.2
3-BF
4
[8]
4
III-BF [8]
AuꢀP1 2.299(1)
AuꢀP2 2.298(1)
2.322
2.322
1.834
1.834
1.803
1.805
147.45(4) 147.7
P1ꢀC1
P2ꢀC4
P1ꢀC5
P1ꢀC6
1.816(4)
1.832(4)
1.814(4)
1.814(4)
Au-P2-C4 107.0(1)
108.3
123.2
118.9
P1-C1-C2
C1-C2-O
123.9(3)
117.7(3)
[
a] Experimental structural data are from reference [12].
The agreement between the theoretical and experimental
values for bond lengths and bond angles is excellent, which
validates our choice of combination functional/base. In par-
ticular, differences of less than 0.02 ꢂ are observed for the
most pertinent bond lengths, and the calculated P1-Au-P2
bond angle is overestimated by only 0.28.
Our proposed mechanism, involving the cationic gold
complex III-BF with the explicit counter anion BF , is de-
scribed in Scheme 2. The first step involves a hydride trans-
4
4
Scheme 3. Energy profile in solution (dichloroethane) for the transforma-
tion of III-BF into IV (kcalmol ). Gibbs free energies (kcalmol ) in
4
ꢀ
1
ꢀ1
the gas phase are in parentheses.
step of the catalytic cycle. As can be seen from the transi-
tion-state structure TSIII-BF4-IV given in Figure 2, the hydride
transfer is assisted by the approach of a fluorine atom from
Scheme 2. Computed mechanism for dehydrogenative silylation catalyzed
by 3-BF .
4
Figure 2. View of the transition state TSIII-BF4-IV. Hydrogen atoms (except
the hydride) have been omitted for clarity. Selected bond lengths [ꢂ)
and angles [8]: Au
ꢀ
P1 2.381, AuꢀP2 2.440, AuꢀH 1.732, SiꢀH 1.774, Siꢀ
fer from the silane to the gold center, assisted by the coun-
F1 1.947, BꢀF1 1.562, BꢀF2 1.378; P1-Au-P2 113.2, Au-H-Si 166.7, H-Si-
ter anion, to generate Me SiBF and an intermediate IV. In
3
4
F1 73.7.
a second step, intermediate IV reacts with III-BF to form
4
the dinuclear gold hydride complex II-BF . In a parallel re-
4
action, Me SiBF reacts with the alcohol to yield the desired
the counterion, that is, BF , on the silyl group. Thus, the re-
3
4
4
silyl ether together with the strong acid HBF . In the third
action proceeds through a concerted mechanism involving
the complex, the silane, and the counterion. The involve-
ment of the counterion in this process is one of the major
findings in this study. As a consequence, the counterion had
to be taken into account in our calculations in order to de-
scribe the overall mechanism. The formation of the complex
IV, a hydride gold(I) complex with only one phosphole unit
coordinated to the metal center, appears to be endothermic
4
step, regeneration of the starting complex III-BF is ach-
4
ieved by reaction of II-BF with HF·BF , with the concomi-
4
3
tant liberation of H2.
The energy profile associated with the first step (hydride
transfer from the silane to the cationic complex) is reported
in Scheme 3. Note that at the beginning the cationic com-
plex is interacting with its counterion.
ꢀ
1
The hydride-transfer reaction was found to require a sig-
(DEPCM =13.0 kcalmol ).
The second step of the reaction is the addition of the cat-
°
ꢀ1
nificant activation energy (DEPCM =29.9 kcalmol
from
the reactants), and turned out to be the rate-determining
ionic complex III-BF to the hydride complex IV to form
4
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Dehydrogenative Silylation of Alcohols
FULL PAPER
Scheme 4. Energy profile in solution (dichloroethane) for the transforma-
ꢀ
1
ꢀ1
4
tion of IV into II-BF (kcalmol ). Gibbs free energies (kcalmol ) in the
gas phase are in parentheses.
the dinuclear gold hydride complex II-BF (Scheme 4). This
4
step is found to be thermodynamically favored (DDE
=
PCM
ꢀ
1
ꢀ17.0 kcalmol ), and despite an extensive search, no transi-
tion state was located for this addition reaction. We note
here that given the exergonicity of this reaction, no equilib-
rium can possibly be considered between the dinuclear spe-
Scheme 5. Energy profile in solution (dichloroethane) for the transforma-
ꢀ
1
ꢀ1
4 4
tion of II-BF into III-BF (kcalmol ). Gibbs free energies (kcalmol )
in the gas phase are in parentheses .
cies II-BF and the two monomeric complexes IV and III-
4
BF4.
At this point the alcohol silylation (Me SiꢀF···BF +
3
3
MeOH!Me SiOMe+HꢀF···BF ) had to be considered.
3
3
This reaction was found to be moderately endothermic
ꢀ
1
(
DEPCM =+8.6 kcalmol ), but no attempts were made to
study its mechanism in detail. As a matter of fact, such alco-
hol silylation reactions are known to occur rather easily,
without the need for the participation of the gold complex,
[23]
but require a proton-trapping molecule (typically a base).
In our case, the proton trap is the binuclear gold hydride
complex II-BF (vide infra).
4
Let us now focus on the last step of the catalytic cycle,
which starts with the approach of HF BF toward II-BF ,
3
4
and leads to the formation of dihydrogen and the regenera-
[24]
tion of the catalytic species III-BF (Scheme 5).
4
Several approaches of the acid HF BF to II-BF were cal-
3
4
culated, and it appeared to be feasible only on the hydride
side of the dinuclear gold complex. In the resulting complex
Figure 3. View of the transition state TSV-BF4-VI-BF4
(except the proton and the hydride) have been omitted for clarity. Select-
. Hydrogen atoms
ed bond lengths [ꢂ] and angles [8]: Au1ꢀP1 2.512, Au1ꢀP2 2.376, AuꢀAu
V-BF a weak interaction between the acid proton and the
4
2
.864, Au2ꢀP3 2.392, Au2ꢀP4 2.366, Au1ꢀH1 1.748, Au2ꢀH1 2.172, H1ꢀ
AuꢀHꢀAu unit is found. The elimination of H goes through
2
H2 1.033, H2ꢀF1 1.185, F1ꢀB 1.580; P1-Au1-P2 109.5, P3-Au2-P4 126.5.
the TS
transition state (Figure 3), and has an acti-
V-BF4-VI-BF4
vation barrier of nearly zero at the PCM level. It is worth
noting that, as was found for TSIII-BF4-IV, the process is con-
certed, with simultaneous formation of the hydrogenꢀhydro-
gen and fluorineꢀboron bonds. This again underlines the im-
by, this last step regenerates the catalytic complex III-BF4
and closes the catalytic cycle.
The entire energy profile for this catalytic cycle involving
the counter anion is given in Scheme 6. The rate-determin-
ing step is the first step, corresponding to the hydride trans-
portance of taking into account the counterion in the overall
mechanism. Finally, although the dinuclear complex VI-BF4
was characterized as a minimum when solvent effects were
considered, the difference in energy compared to the two
fer from the silane to the cationic complex III-BF . This
4
separate mononuclear complexes III-BF is small enough to
allow these structures to be in dynamic equilibrium. There-
mechanism is consistent with the fact that only the dinuclear
gold complex II-BF was observed in P NMR spectroscopy
4
4
3
1
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Y. Jean, N. Mꢀzailles et al.
Table 2. Catalytic results with different complexes for the reaction given
in Equation (1).
Entry
Complex
NMR yield [%]
1
2
no complex
3-OTf
3-OTf
4
100
13
49
34
[
a]
3
4
5
6
7
3-PF
3-NTf
3-BF
2-OTf
6
2
4
25
100
[
a] Room temperature.
(
Table 2, entry 2) was quantitative, whereas only 25% yield
was obtained with complex 3-BF (entry 6). Being very effi-
4
cient, the same reaction was then performed at room tem-
perature with complex 3-OTf, but the yield was only 13%
Scheme 6. Energy profile in solution (dichloroethane) for the complete
ꢀ1
ꢀ
catalytic cycle (kcalmol ) involving the counter anion BF
4
. Gibbs free
ꢀ
1
energies (kcalmol ) in the gas phase are in parentheses.
(
entry 3). Mild heating was therefore very beneficial. Inter-
mediate results were obtained with complexes 3-PF and 3-
6
NTf with yields of 49 and 34%, respectively. Finally, the di-
2
,
during all the catalysis experiments, as this was found to be
nuclear gold hydride complex 2-OTf (featuring the most fa-
vorable counterion effect) appeared as efficient as complex
3-OTf (entry 7). Note that all of these reactions were fol-
lowed using P NMR spectroscopy, and, whichever complex
was used, only the dinuclear gold hydride complex 2-X was
observed (singlet at 23.6 ppm).
ꢀ
1
the structure of lowest energy (DEPCM =ꢀ4.0 kcalmol )
within the catalytic cycle. Note that the two structures on
the right-hand side of the energy profile are lower in energy
3
1
than II-BF , a result which illustrates the fact that the over-
4
all reaction process is exergonic.
A novel question arose from these experimental results:
can they be rationalized by the proposed mechanism? The
energy profile of the first step, that is, the rate-determining
step of the catalytic cycle involving the hydride transfer, was
recalculated with the most efficient system: trifluorometha-
nesulfonate as the counter anion (TfO ). The computed
energy profile for this reaction is shown in Scheme 7.
The activation energy associated with the hydride transfer
Influence of the counter anion on the catalysis: experimen-
tal and theoretical study: The key finding of the involve-
ment of the counterion in the calculated process prompted
us to investigate this involvement experimentally. Indeed,
one could envision that a different counterion would modify
the energies of some intermediates, and therefore alter the
kinetics of the reaction. Therefore, new gold complexes with
different counterions were synthesized in the same way as
[25]
ꢀ
ꢀ
1
is still rather high (DE
=24.7 kcalmol ), but significantly
PCM
ꢀ
1
ꢀ
complex 3-BF using the appropriate silver salts [Eq. (3)].
lower (by 5.2 kcalmol ) than that required with BF as the
4
4
ꢀ
1
counterion (29.9 kcalmol , see Scheme 3). This result is
fully consistent with the experimental kinetic effect of the
counterion, and thus provides a new argument in favor of
the proposed mechanism. The structure of the transition
state TSIII-OTf-IV is given in Figure 4. It is, in fact, quite similar
ꢀ
to the one found for BF4 : the hydride transfer is concerted
with the approach of the OTf toward the silane. It should
also be noted that the addition of IV to III-OTf leads to the
formation of II-OTf, with a smaller gain in energy compared
to the formation of II-BF4 from III-BF4 (DDEPCM
=
ꢀ
1
ꢀ1
ꢀ14.5 kcalmol versus ꢀ17.0 kcalmol ). The last step of
Moreover, a dinuclear hydride complex (II-BF ) was cal-
the mechanism, that is, the formation of dihydrogen (see
Scheme 5), was not calculated with X=OTf, because it was
not expected to affect the kinetics of the catalysis.
4
culated to be a plausible intermediate, and was observed
during the process. We recently reported the synthesis of
such a complex, 2-OTf, which prompted us to test its effi-
ciency in the catalytic process. These complexes were then
engaged in the same reaction [Eq. (1)]. For the sake of com-
parison, the reaction was stopped after two hours (at 508C).
The results are presented in Table 2. We were very pleased
to observe a tremendous influence of the counterion on the
kinetics of the reaction, corroborating the DFT calculations.
Indeed, the yield of the catalysis with the complex 3-OTf
Dinuclear gold hydride as catalyst: The last point of our
study deals with the high catalytic activity of the dinuclear
gold hydride complex 2-OTf (Table 2, entry 7), and the way
in which it could enter the catalytic cycle. Indeed, even
though the formation of this complex during catalysis has
been rationalized above, its own catalytic activity, without
the simultaneous formation of a stoichiometric amount of
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Dehydrogenative Silylation of Alcohols
FULL PAPER
and IV has already been excluded, see Schemes 4 and 7 and
associated discussion).
1
) II-OTf could react with the silane to give dihydrogen,
gold(0), and Me SiOTf; the latter could then react with
3
alcohol to form the acid HOTf. In turn, the acid would
react with a second equivalent of II-OTf to form III-
OTf, similarly to the last step of the catalytic cycle
(
Scheme 8, pathway 1);
Scheme 8. Two computed pathways for the generation of the catalytic
complex III-OTf from the dinuclear species II-OTf.
Scheme 7. Energy profile in solution (dichloroethane) for the transforma-
ꢀ
1
ꢀ1
tion of III-OTf into II-OTf (kcalmol ). Gibbs free energies (kcalmol
)
in the gas phase are in parentheses.
2
) II-OTf could react with the counter anion to give gold(0)
and the acid HOTf; the latter would then react with an-
other equivalent of II-OTf to yield III-OTf (Scheme 8,
pathway 2).
Pathway 1 was studied first. A transition state for the ap-
proach of the silane to II-OTf was located (TSII-OTf-Me3SiH-VII),
which leads to the elimination of H (Scheme 9). However,
2
ꢀ
1
this transition state was found to lie 55.3 kcalmol (DEPCM
)
above the reactants. Therefore, this pathway is too disfa-
Figure 4. View of the transition state TSIII-OTf-IV with OTf as counter
anion. Hydrogen atoms (except the hydride) have been omitted for clari-
ty. Selected bond lengths [ꢂ] and angles [8]: AuꢀP1 2.467, AuꢀP2 2.364,
AuꢀH 1.727, SiꢀH 1.767, SiꢀO1 1.988, SꢀO1 1.528; P1-Au-P2 114.0, Au-
H-Si 172.0, H-Si-O1 73.9.
acid, remained unclear. In the subsequent part, the potential
counterion effect on the catalytic activity of the dinuclear
complex was not studied. To be as close as possible to the
experimental system, the complex II-OTf was used for all
the following calculations. Starting from II-OTf and the
silane, two routes were considered for the formation of the
catalytic gold complex III-OTf (note that a simple equilibri-
um between II-OTf and the two monomeric species III-OTf
Scheme 9. Pathway 1: approach of the silane to II-OTf.
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Y. Jean, N. Mꢀzailles et al.
0
vored on energetic grounds to account for the catalytic ac-
tivity of II-OTf.
Next, we investigated the possibility for the dinuclear
gold hydride complex to react with its counter anion (path-
way 2) under the experimental conditions of the catalysis
process would not stop at this “Au dimer” stage, but rather
would form the more thermodynamically favored Au bulk.
It is worth noting that these two pathways imply the for-
0
mation of Au species for the generation of catalytically
active III-OTf from II-OTf.
(
Scheme 10).
Experimentally, in the early stages of the catalysis with
the dimer 2-OTf, such formation is observed. An experiment
to discriminate the two pathways was carried out based on
the fact that pathway 1 involves the silane, whereas path-
way 2 does not. The thermal stability of complex 2-OTf in
the absence of silane was probed, and was shown to evolve
quantitatively within two hours at 508C in 1,2-dichloro-
ethane [Eq. (4)]. This thermal decomposition formed a mon-
[12]
0
onuclear complex, 8, as well as Au particles (see Support-
ing Information). In parallel, the same complex was synthe-
sized quantitatively by the stoichiometric addition of ligand
1
to 3-OTf (see Supporting Information). The formation of
complex 8 is therefore proof that complex 3-OTf formed
and reacted with the free ligand 1, itself liberated from the
Au species during the formation of nanoparticles. Thus,
ꢀ
Scheme 10. Pathway 2: approach of the counter anion OTf toward the
Au-H-Au unit of II-OTf.
0
complex 3-OTf can be generated efficiently from complex 2-
OTf in the absence of silane.
A transition state corresponding to the formation of
ꢀ
HOTf through the approach of TfO on the AuꢀHꢀAu unit
ꢀ
1
of II-OTf, TS
was located 25.5 kcalmol above com-
II-OTf-VII
plex II-OTf. The structure of TSII-OTf-VII is presented in
Figure 5. The mechanism is concerted between the elonga-
tion of the two AuꢀH bonds, the formation of the HꢀO
bond, and the elongation of the SꢀO bond. A significant
elongation of one of the two AuꢀP bonds is also observed,
in agreement with a rearrangement of the coordination
sphere around the metal: the geometry changes from almost
trigonal planar to linear. This reaction is a formal reduction
0
of the complex into two Au moieties. It is likely that this
Finally, from both experimental and theoretical studies, it
can be concluded that the catalytically active complex III-
OTf is formed through pathway 2 (Scheme 8), by a self-de-
composition process of II-OTf. Overall, the calculated reac-
tivity of the dinuclear gold complex II-X (X=BF , OTf), as
4
well as the experimental observations with 2-OTf, prove
that the bridging hydrogen can react either with an acid (hy-
dride behavior) or with a base (proton behavior), as depict-
ed in Scheme 11.
Conclusion
In conclusion, we have presented a combined experimental
and theoretical study of the dehydrogenative silylation of al-
+
cohols, catalyzed by XDPP–Au complexes. The elucidation
of the mechanism revealed a counterion effect, which was
subsequently proved experimentally. The observation of
+
ꢀ
[
{(XDPP)Au} H] X (2-X) as the only complex observed in
2
Figure 5. View of the transition state TSII-OTf-VII. Hydrogen atoms (except
the hydride) have been omitted for clarity. Selected bond lengths [ꢂ] and
angles [8]: Au1
ꢀP1 2.470, Au1ꢀP2 2.565, Au1ꢀH 1.727, SiꢀH 1.878, Au2ꢀ
H 2.472, HꢀO1 1.241, SꢀO1 1.538; P1-Au1-P2 102.5.
the course of the catalytic process was rationalized by DFT
calculations. It is obtained by the addition of the
[(XDPP)AuH] species, generated in situ,with the starting
2262
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 2256 – 2265

Dehydrogenative Silylation of Alcohols
FULL PAPER
under vacuum. For characterization see ref. [12]. Isolated yields were
above 95%.
General procedure for gold(I)-catalyzed dehydrogenative silylation: The
gold complex 3-OTf (5 mg, 0.01 equiv) was placed in a Schlenk flask
under nitrogen, and dichloroethane (0.6 mL) was added with stirring.
The alcohol (58.4 mL, 0.48 mmol, 1 equiv) and then the silane (78 mL,
0
.48 mmol, 1 equiv) were added to the Schlenk at room temperature. The
mixture was heated at 508C for two hours. The solvent and the silane
were evaporated, and the signals of the remaining alcohol versus the sig-
1
nals of the silyl ether were integrated by H NMR spectroscopy in
CDCl
3
.
Computational details: Calculations were performed with the GAUSSI-
[
27]
[19]
AN 03 series of programs. Density functional theory (DFT) was ap-
[
20]
[28]
plied with the B3PW91 functional. The Def2-QZVP pseudo-poten-
tial was employed for the gold atom, and the 6–31++G(d,p) basis set
AHCTUNGTRENNUNG
was used for atoms in interaction with the metal center (P, all or part of
the counterions, and two hydrogen atoms of the alcohol and silane reac-
tants), and the standard 6–31G(d) basis set was used for all other atoms
[
29]
(
Scheme 12). Geometry optimizations were performed using a model
Scheme 11. The two opposite reactivities of the bridging hydrogen in
complex 2-X.
complex 3-X. In parallel, we have shown here for the first
time the use of an isolated gold hydride complex as an effi-
cient catalyst. It is proved that this Au -H dimer 2-X reacts
2
ꢀ
with the counterion X itself to generate an equivalent of
acid, which then allows 3-X to be formed. Finally, the calcu-
lated reactivity of the dinuclear gold complex II-X, as well
as the experimental observations with 2-X, prove that the
bridging hydrogen can react either with an acid (hydride be-
havior) or with a base (proton behavior), as depicted in
Scheme 11. These two facile decomposition pathways pro-
vide a rationale for the extreme rarity of phosphine/gold hy-
dride species. The very peculiar electronic and steric proper-
Scheme 12. Basis sets combination used for all the calculations.
for the XDPP ligand, in which the two methyl groups of the xantphos
backbone and the phenyl groups of the phosphole rings were replaced by
hydrogen atoms. As far as the reactants were concerned, methanol and
trimethylsilane were used as models. The stationary points (minima, tran-
sition states) were characterized by full vibration frequency calculations,
[
30]
ties of the ligand XDPP is undoubtedly responsible for the
and intrinsic reaction coordinates (IRC) calculations were performed
to ensure which minima were connected by transition states. Finally, the
solvation energies were calculated with dichloroethane on the gas-phase
+
stabilization of the hydride in the form of the Au H dimer.
2
[
21,22]
optimized structures using the polarizable continuum model (PCM),
[
31]
as implemented in Gaussian 03 and using the Bondi radii. No entropic
effect was considered at this point.
Experimental Section
Materials and instrumentation: All reactions were routinely performed
under an inert atmosphere of argon or nitrogen using Schlenk, glove-box
techniques, and dry deoxygenated solvents. Dry hexanes were obtained
by distillation from Na/benzophenone. Dry dichloromethane was distilled
Acknowledgements
The CNRS, the Ecole Polytechnique, and the IDRIS (for computer time,
Project No. 91616) are thanked for supporting this work. S.L. (2009–
2012) and A.E.-L. (2008–2011) thank the Ministꢃre de lꢄEducation et de
la Recherche for Ph.D. grants.
on P
2 5
O . Nuclear magnetic resonance spectra were recorded on a Bruker
1
AC-300 SY spectrometer operating at 300.0 MHz for H, 75.5 MHz for
1
3
31
C, and 121.5 MHz for P NMR spectra. Solvent peaks were used as in-
1
13
ternal references relative to Me
4
Si for H and C chemical shifts (ppm);
P chemical shifts were relative to an 85% H PO external reference.
XDPP (1), 2-OTf, and 3-OTf were prepared as reported previous-
3
1
3
4
[
18]
[12]
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ly. [AuCl ACHTUNGTRENNUNG( tht)] was prepared according to the established procedure.
Dichloroethane was obtained from commercial suppliers. Silver salts
were obtained from commercial suppliers, and were stored and weighed
in a glove-box.
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General synthesis of complex 3-X: [Au
The ligand XDPP (169.4 mg, 0.25 mmol) was added to a solution of
AuCl(tht)]] (80 mg, 0.25 mmol) in dichloromethane (3 mL) at room tem-
4 6 2
ACHTUNGERTUNN(NG XDPP)]X (X=BF , PF , NTf ):
[
ACHTUNGTRENNUNG
perature. The solution was stirred for 5 min. The appropriate silver salt
AgX (0.25 mmol, 1 equiv) was then added to the solution at room tem-
perature. The mixture was stirred for 15 min, and completion of the reac-
3
1
tion was checked by P NMR spectroscopy. The mixture was then fil-
tered and dried. The corresponding solid was washed with hexanes. The
solvent was removed by filtration, and the yellow solid 3-X was dried
Chem. Eur. J. 2011, 17, 2256 – 2265
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ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 2256 – 2265

Dehydrogenative Silylation of Alcohols
FULL PAPER
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2265