Ketone hydrosilylation by Cu(I) diphosphine complexes: A kinetic study

Article abstract of DOI:10.1016/j.jorganchem.2013.07.054

Diphosphine ligands are commonly used for the catalytic hydrosilylation of ketones using Cu(I) complexes. While some DFT studies on the suggested catalytic cycle have been reported, we present herein a complete kinetic study of the copper(I) catalyzed reaction to verify the plausibility of the proposed catalytic cycle from an experimental point of view. The rate constants of the two consecutives steps are determined for BDP (BDP = 1,2-bis(diphenylphosphino) benzene) and rac-BINAP ligands. The two constants are found not to differ significantly, i.e. no rate limiting step is identified. Depending on operation conditions, a switch between rate limiting steps is observed leading to either first order kinetics in silane or ketone.

Full text of DOI:10.1016/j.jorganchem.2013.07.054

Journal of Organometallic Chemistry 745-746 (2013) 133e139
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Ketone hydrosilylation by Cu(I) diphosphine complexes: A kinetic
study
*
Thomas Vergote, Sonia Gharbi, François Billard, Olivier Riant, Tom Leyssens
Institute of Condensed Matter and Nanosciences, Université Catholique de Louvain, Place Louis Pasteur 1, B-1348 Louvain-la-Neuve, Belgium
a r t i c l e i n f o
a b s t r a c t
Article history:
Diphosphine ligands are commonly used for the catalytic hydrosilylation of ketones using Cu(I) com-
plexes. While some DFT studies on the suggested catalytic cycle have been reported, we present herein a
complete kinetic study of the copper(I) catalyzed reaction to verify the plausibility of the proposed
catalytic cycle from an experimental point of view. The rate constants of the two consecutives steps are
determined for BDP (BDP ¼ 1,2-bis(diphenylphosphino)benzene) and rac-BINAP ligands. The two con-
stants are found not to differ significantly, i.e. no rate limiting step is identified. Depending on operation
conditions, a switch between rate limiting steps is observed leading to either first order kinetics in silane
or ketone.
Received 27 May 2013
Received in revised form
27 June 2013
Accepted 18 July 2013
Keywords:
Diphosphine
Copper(I)
Ó 2013 Elsevier B.V. All rights reserved.
Kinetics
Hydrosilylation
Reactivity
Catalysis
1. Introduction
copper-diphosphine catalytic systems were developed [45e52].
Lipshutz et al. postulated the formation of a copper(I) hydride
Carbonyl bond reduction, specifically of aldehydes and ketones,
to the corresponding alcohol functionality via hydride transfer is a
fundamental transformation in organic synthesis [1]. Transition-
metal catalysis has been successfully applied in the reduction of
many carbonyl compounds via hydrogenation or hydrosilylation
[1]. Hydrogenation reactions proceed in good yields but require
high pressure or elevated temperature. Moreover if the reaction is
part of a multi-step synthesis, the resulting free alcohol can require
protection prior to the next synthetic step. In contrast, the softer
reactions conditions of hydrosilylation turned out to be a major
advantage in addition to the fact that both the reduction and the
protection steps are performed in a single atom-efficient step.
The first catalytic systems, based on rhodium, were developed in
the early 1970s [2e4] after which other heavy metals were intro-
duced, ranging from Re, Rh and Ru to Ir [5e14]. As the main
drawback was the cost affiliated with these metals, efforts were
made to find efficient, less expensive alternatives using titanium
[15e22], iron [23e31], manganese [32e34] or zinc [35e43]. In
1984 Brunner and Miehling reported the first asymmetric hydro-
silylation with a copper-diphosphine catalyst [44]. Since, many
species as active catalyst for a system combining a catalytic quan-
tity of CuCl/NaOt-Bu/diphosphine and a stoichiometric amount of
hydrosilylant agent [53e57]. At the same time, Carreira and co-
workers [58] as well as Riant and co-workers [49,59e61] re-
ported CuF2 systems as interesting precursors to copper hydride.
Other air and moisture stable copper(II) salts, as Cu(OAc)2, in
combination with inexpensive and readily available diphosphines
such as BINAP are used efficiently to catalyze the hydrosilylation of
various ketones [47,62,63].
The generally accepted mechanism involves a s-bond metath-
esis between the pre-catalyst and the hydrosilylating reagent to
form a copper(I) hydride species 4. The latter reacts with a ketone
substrate 1 resulting in the formation of a copper alkoxide 5 (step 1)
that subsequently undergoes a s-bond metathesis with the orga-
nosilane 2 to afford the silyl ether 3 and regenerate the CueH
catalyst (Step 2, see Scheme 1) [52,64e67]. Although computa-
tional studies [68e74] confirm this mechanism, little efforts were
made to verify the mechanism experimentally. Considering our
interest in ketone hydrosilylation by diphosphine-copper(I) catal-
ysis [49,59e61], we decided to investigate this mechanism using
chemical kinetics studies. To the best of our knowledge, the only
kinetic studies performed were those by Issenhuth et al. [71] in
which they use initial rate analysis based on NMR spectra to find a
first order in ketone and silane and a 0.5 order in catalyst. However,
* Corresponding author. Tel.: þ32 (0)10 47 28 11.
E-mail address: tom.leyssens@uclouvain.be (T. Leyssens).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.07.054

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T. Vergote et al. / Journal of Organometallic Chemistry 745-746 (2013) 133e139
Scheme 1. Suggested catalytic cycle for the asymmetric hydrosilylation of ketones using diphosphine-copper(I) catalysts.
no efforts were placed in trying to fit these orders with the sug-
gested catalytic scheme. Their observation of a rate limiting second
step, furthermore, seems to contradict the observed first order ki-
netics with respect to the ketone species. In this paper, we want to
apply kinetic studies to verify the plausibility of the suggested
mechanism and investigate the apparent contradictions with
respect to the nature of the rate limiting step [71].
d½4ꢀ
dt
v4
¼
¼
¼ ꢁk1½1ꢀ½4ꢀ þ k2½2ꢀ½5ꢀy0
¼ k1½1ꢀ½4ꢀ ꢁ k2½2ꢀ½5ꢀy0
(2a)
(2b)
d½5ꢀ
dt
v5
Combining Equations (1) and (2) with the knowledge of the total
amount of catalyst used:
2. Kinetic expressions
½4ꢀ þ ½5ꢀ ¼ ½Cuꢀ0
(3)
To verify the plausibility of the catalytic scheme one first needs
to develop a corresponding rate expression. Within the Scheme
shown above (Scheme 1), substrate 1 (ketone) adds to the catalytic
species 4 in step 1. A second step consists of a reaction between
species 5 and a second substrate 2 (the silane) to yield product 3
and species 4 (Scheme 2).
The overall rate equation can be expressed based exclusively on
experimentally readily accessible data ([1], [2] and [Cu]0)
d½1ꢀ
dt
k1k2½1ꢀ½2ꢀ½Cuꢀ0
k1½1ꢀ þ k2½2ꢀ
d½3ꢀ
dt
v1
¼
¼ ꢁ
¼ ꢁ
¼ ꢁv3
(4)
The rate equations for product appearance/reactant disappear-
ance are given by:
The overall rate expression contains a two-term denominator. In
most two-step catalytic cycles, one of the two rate constants is
dominating and one of these terms can be neglected, leading to two
extremes. In a first case, the second step is rate limiting with k1[1]
outweighing k2[2]. This situation is commonly termed “saturation
kinetics” in species 1, with a simplified rate equation being first
order with respect to the silane 2 (Equation (5) in Table 1).
In the opposite case, the formation of intermediate 5 will be
rate-limiting, and the unbound catalyst species 4 is the “resting
state” of the catalyst. The rate equation is now first order with
respect to the ketone 1 (Equation (6) in Table 1).
If none of the terms is dominating, the full rate Equation (4)
needs to be considered. Significant amounts of both 4 and 5 are
present in the system. However, it is possible to force the system to
one of the two limiting situations working with an excess amount
of one of the reactants.
d½1ꢀ
v1
v3
¼
¼
¼ ꢁk1½1ꢀ½4ꢀ
¼ k2½2ꢀ½5ꢀ
(1a)
(1b)
dt
d½3ꢀ
dt
Although concentrations of species 1, 2, and 3 can easily be
measured, those of copper containing species 4 and 5 are less easily
accessible due to the small amount of species present. The overall
rate Equations (1a) and (1b) will therefore be altered to depend
solely on the major species present in solution. This is achieved
assuming the steady state for species 4 and 5 (Equations (2a)
and (2b)).
The catalytic cycle shown in Scheme 1, suggests the copper-
hydride to be monomeric. However, recent studies [71e73,75e
80] postulate the copper hydride to be oligomeric with the
monomer the only active form. This will not influence the overall
expression (4) but will affect the rate order with respect to the
catalyst and possibly explains the 0.5 order observed experimen-
tally [71]. The goal of the current paper is to study the two steps of
Scheme 2. Consecutive elementary reactions corresponding to Scheme 1.

T. Vergote et al. / Journal of Organometallic Chemistry 745-746 (2013) 133e139
135
Table 1
Full and simplified rate equations for the catalytic reaction shown in Scheme 1. Integrated rate expression; a0 and b0 represent the initial concentrations in ketone 1 and silane 2
respectively.
Major Cu species (Scheme 1)
Dominating term in denominator of Equation (4)
Observed rate equation
Integrated rate equation
ꢀ
ꢀ
ꢁ
v ¼ k2½2ꢀ½Cuꢀ0
(5)
5
k1[1]
b0 ꢁ x
ln
¼ ꢁk2½Cuꢀ0t (7)
¼ ꢁk1½Cuꢀ0t (8)
b0
ꢁ
v ¼ k1½1ꢀ½Cuꢀ0
(6)
(4)
a0 ꢁ x
k2[2]
4
ln
a0
k1k2½1ꢀ½2ꢀ½Cuꢀ0
All terms contribute
None
v ¼
k1½1ꢀ þ k2½2ꢀ
Table 2
Experimental plan used working with an excess amount of silane.
both on ketone reactant 1 and alcohol product 3, showing full
conversion.
Using the integrated rate expressions (Table 1), the data is
checked with respect to the two extreme cases.3 Fig. 2 shows the
data following Equation (8). Although there is an excellent linear
behavior indicative of first order kinetics, a change in slope at
around t ¼ 50 min is also observed. A similar observation can be
made when considering the data under Equation (7) (Supporting
information Fig. S.1). This change in slope can suggest a variation
in mechanism or a change in rate limiting step, implying both steps
to be governed by similar rate constants. As standard conditions are
performed using 1 eq. of ketone and 1.2 eq. of silane, the relative
importance of k1[1] vs. k2[2] would in this case depend on the
relative concentrations [1] and [2], which in turn depends on re-
action progress.
Ketone [1] (M)
Silane [2] (M)
[2]/[1]
BDP3
BDP4
BDP5
BDP6
BDP7
BDP8
0.072
0.068
0.145
0.145
0.298
0.303
0.75
0.75
0.75
0.75
0.75
0.75
10
11
5
5
2.5
2.5
the catalytic cycle, without focusing on the hydride equilibrium
involved.
3. Results and discussion
As the hypothesis implying a change in mechanism seems un-
likely, we suspected both elementary steps to have comparable rate
constants. To study both steps, we worked either with an excess
amount of silane (rendering the first step rate limiting; Equations
(6) and (8)) or an excess amount of ketone (rendering the second
step rate limiting; Equations (5) and (7)).
Kinetic studies were conducted using Cu(RCOO)2/BDP (BDP ¼ 1,2-
bis(diphenylphosphino)benzene) and Cu(RCOO)2/(rac)-BINAP cata-
lysts (2 mol%) as both lead to full conversion (>99%) of benzylace-
tone using Me(OEt)2SieH1 as
a hydride source (Scheme 3).
Concentrations of ketone and alcohol were monitored over time
using gas chromatography. Reaction temperature was controlled and
held constant at 298 K. Concentration profiles were constructed from
calibration curves using an internal standard method (see
Experimental section and Supporting material for more details).
Kinetic orders with respect to reactants were obtained from an
exponential fitting analysis of the concentration curves, varying the
amounts of each component (ketone, silane). Rate constants were
obtained through linearization of the obtained data.
3.2. Case 1: excess amount of silane
Under the hypothesis of comparable rate constants, working
with an excess amount of silane 2, the rate expression is first order
with respect to the ketone 1 (Equation (6)). To verify this, an
experimental plan was devised keeping the concentration in silane
constant and in excess with respect to the amount of ketone
(at least 2.5 times more important). The concentration of ketone
was then varied from one experiment to the other (Fig. 3)
All curves show exponential decrease of the ketone concentra-
tion over time (Fig. 3). If the data corresponds to first order kinetics
in ketone 1, representing the conversion over time (Equation (8)
ða0 ꢁ xÞ=a0 ¼ expðꢁk1½Cuꢀ0tÞ) should yield identical curves.4
Fig. 4 confirms this type of behavior.
Taking the natural logarithm of this data (Fig. 5 and Figs S.2 to
S.7), linear curves are obtained, once more highlighting first order
kinetics in ketone. Considering the initial catalyst concentration
[Cu]0 ¼ 0.004 M, a value of k1 ¼ 2.03 ꢂ 0.29 Mꢁ1 sꢁ1 (confidence
interval at 95%) is obtained (Table S.1).
3.1. Repeatability and standard conditions
For kinetic studies the quality of the obtained data is essential.
For this reason, all conditions were repeated twice. Fig. 1 shows the
repeatability of the obtained data for ketone reactant and alcohol
product concentrations over time under standard conditions.2
Fig. 1 shows excellent repeatability between the two trials. This
is also the case for all of the experimental conditions considered
within the scope of this work. Quantitative analysis is performed
1
The authors would like to emphasize that for other silane reactants the results
3
obtained can strongly alter.
As the silane concentration [2] is not measured directly, it is deduced from the
2
Standard conditions, are those used in literature (T ¼ 25 ꢃC, [1] ¼ ꢂ0.2 M;
initial amount of silane added with respect to the ketone 1, and the concentration of
alcohol [3] present.
[2] ¼ ꢂ0.24 M; 2 mol% catalyst, using the BDP ligand); The initial concentration in 1
is taken as the average value of the sum of 1 and 3 throughout the reaction. This
sum is constant, as shown in Supporting information.
4
A same amount of catalyst is used from one experiment to the other, so the
term k1[Cu]0 is constant.

136
T. Vergote et al. / Journal of Organometallic Chemistry 745-746 (2013) 133e139
Scheme 3. Copper-catalyzed hydrosilylation of benzylacetone.
The data is shown not to correspond to first order kinetics with
3.3. Case 2: excess amount of ketone
respect to silane 2 (Equation (7)). Working with an excess amount
of silane, the first step is found to be rate limiting, or in other words
the copper-hydrate species 4 to correspond to the resting state of
the catalyst.
Working with an excess amount of ketone 1, we expect the rate
expression to be given by Equations (5) and (7), being first order
with respect to the silane 2. An experimental plan (Table 3) was
Fig. 1. Concentration of reactant 1 (black) and product 3 (gray) over time under standard conditions (round and triangle points respectively correspond to two independent trials
BDP1 and BDP2).
Fig. 2. Transformation of the data presented in Fig. 1 according to the expression of Equation (8) (data represented for experiment BDP1).

T. Vergote et al. / Journal of Organometallic Chemistry 745-746 (2013) 133e139
137
Fig. 6. Concentration of [2] over time, for experiments from Table 3.
Fig. 3. Concentration of [1] over time, for experiments of Table 2.
Fig. 7. Conversion over time, for experiments from Table 3.
Fig. 4. Conversion over time, for experiments from Table 2.
resting state of the catalyst. A value of k1 ¼ 2.41 ꢂ 0.62 Mꢁ1 sꢁ1 can
be extracted from the data (Table S.1).
A pooled t-test does not show a significant difference between
the values of k1 and k2, validating the hypothesis of comparable rate
constants for both elementary steps of the catalytic cycle.
3.4. Changing the ligand e (rac)-BINAP
To study the effect of different ligands on reaction kinetics,
studies were performed using the (rac)-BINAP ligand. Under stan-
dard conditions, a similar observation is made to that mentioned
above, with the data corresponding to first order kinetics and
showing a change in rate limiting step (Supporting information,
data BIN1 and BIN2). Table 4 shows the experimental plan used,
working either with an excess amount of silane 2 (BIN3 to BIN7) or
ketone 1 (BIN8 to BIN11) (Fig. 8).
Fig. 5. Linearization of the data of experiment BDP8 according to Equation (8).
devised in a similar spirit as above, keeping the concentration in
ketone constant and in excess and varying the concentration of
silane. Exponential decrease of reactant concentration is once more
observed (Fig. 6). Representing the conversion over time (Equation
(7) ðb0 ꢁ xÞ=b0 ¼ expðꢁk2½Cuꢀ0tÞ) yields identical curves, as ex-
pected (Fig. 7).
Plotting the conversion over time, confirms first order behavior
(Fig. S.22). Linearizing the data according to Equations (7) or (8),
Table 4
Experimental plan used for the mechanistic study involving the BINAP ligand.
Linearizing the data, a first order kinetics in silane 2 is confirmed
(Figs S.9 to S.13). This implies the second step to be rate limiting or
in other words the copper-alkoxyde species 5 to correspond to the
Ketone [1] (M)
Silane [2] (M)
[1]/[2]
BIN3
BIN4
BIN5
BIN6
BIN7
0.075
0.133
0.134
0.270
0.266
1.00
1.00
1.00
1.00
1.00
13
7.5
7.5
3.7
3.7
Table 3
Experimental plan used when working with an excess amount of ketone.
Ketone [1] (M)
Silane [2] (M)
[1]/[2]
Ketone [1] (M)
Silane [2] (M)
[2]/[1]
BDP9
0.919
0.922
0.928
0.944
0.847
0.092
0.092
0.223
0.228
0.339
10
10
4.2
4.2
2.5
BDP10
BDP11
BDP12
BDP13
BIN8
BIN9
BIN10
BIN11
0.752
0.772
0.705
0.676
0.180
0.185
0.338
0.324
4.2
4.2
2.1
2.1

138
T. Vergote et al. / Journal of Organometallic Chemistry 745-746 (2013) 133e139
reactant concentrations. The full rate equation should be consid-
ered when working with equal amounts of reactants. Similar results
are obtained using either BDP or BINAP as a ligand.
5. Experimental section
5.1. Instrumentation and chemicals
Unless otherwise noted, all manipulations were performed un-
der an argon atmosphere using standard Schlenk-type glassware.
Toluene was distilled on sodium under an argon atmosphere. Sol-
vents used for work-up were of technical grade. Commercial re-
agents were purchased from Acros, SigmaeAldrich, ABCR, TCI or
Apollo scientific and used as received unless stated otherwise. GC
analyses were recorded on a ThermoFinningan Trace GC apparatus
with a CHIRALSIL-DEX CB (25 m, 0.25 mm, 25
mm) column. Column
chromatography was carried out on silica gel (ROCC 60, 40e63
mm).
TLC analyses were performed on commercial aluminum plates
bearing a 0.25 mm layer of Merck Silica gel 60F254.
5.2. Representative procedure for the hydrosilylation of
benzylacetone
A flame-dried Schlenk was sequentially charged with a mag-
netic stirrer, copper(II) 2-ethylhexanoate (0.02 eq.), and the
diphosphine ligand (0.02 eq.). Freshly distilled toluene (4 ml per
mmol) was added under argon, and the solution was stirred for
15 min at room temperature. The silane (Me(OEt)2SiH, 1.2 eq.) was
added drop wise, followed by the benzylacetone (1.0 eq.) and the
mixture was allowed to stir at room temperature. After total con-
sumption of the starting material as indicated by TLC, a solution of
NaOH in methanol (5% in MeOH) was added (1.5 ml per mmol)
and the solution was stirred for 30 min at room temperature. The
light gray mixture was filtered on a plug silica gel (eluting with
P.E./AcOEt: 7/3). Concentration in vacuo of the filtrate provided the
desired alcohol (95e98 %yield).
Fig. 8. a) Concentration of [1] over time, for experiments BIN3 to BIN7 from Table 4. b)
Concentration of [2] over time, for experiments BIN8 to BIN11 from Table 4.
Table 5
Rate constants obtained within this study.
BDP
BINAP
k1
k2
2.03
2.41
[1.72;2.33]
[1.78;3.03]
3.51
3.04
[2.58;4.44]
[2.00;4.07]
5.3. Representative procedure for the kinetic studies
respectively for the experiments with an excess amount of ketone 1
or silane 2,
a
value of k1
¼
3.51
ꢂ
0.93 Mꢁ1 sꢁ1 and
Hydrosilylation studies were carried out at 298 K. The general
procedure was followed, with the ketone being added at t0. The
k2 ¼ 3.04 ꢂ 1.03 Mꢁ1 sꢁ1 is obtained (Figs S.14 to S.22, Table S.1).
Once more a pooled t-test does not show any significant difference
between both rate constants.
reaction was monitored by withdrawing 100
reaction media at specific time (t1, t2, ., tn > t0). 400
500 L of a solution of NaOH 5% in MeOH were added to each of the
m
L samples of the
mL of Et2O and
Table 5 sums up the rate constants obtained within this study.
The k1 value of the BDP system is significantly different from the k1
value of the (rac)-BINAP system. Nevertheless, the values remain
extremely close and one can conclude that the diphosphine com-
plexes show comparable reactivity. For both systems, no significant
difference is obtained between k1 and k2 values. This implies that
working with an excess amount of silane 2, renders the first step
rate limiting and the copper hydride species 4 the resting state of
the catalyst. Working with an excess amount of ketone, implies the
second step to be rate limiting and the alkoxide species 5 to be the
catalyst resting state. These findings are in agreement with the
earlier results [71], showing a first order with respect to ketone and
silane respectively, as both orders were determined under condi-
tions favoring this outcome.
m
samples, for hydrolysis. After 1 h at room temperature, the sample
mixture is filtered on a capillary plug silica gel mixed with MgSO4
(eluting with 600
a freezer at 255 K for a couple of days without any loss. To each
sample, 400 L of anisole (internal standard, solution of 7500 ppm
mL of Et2O). If necessary, the filtrate can be kept in
m
in Et2O) was then added, bringing the final volume to 2 ml. The
latter were analyzed by GC, giving the ketone and silane concen-
tration through the calibration curves. The initial rates of the
various reactions were determined from an exponential fitting
analysis of the concentration curves.
Acknowledgments
The authors would like to thank the Université Catholique de
Louvain (Thomas Vergote is a UCL research assistant).
4. Conclusions
In this paper, we experimentally confirm the validity of the
suggested catalytic cycle for the hydrosilylation of ketones using
copper(I) diphosphine complexes. The kinetics studies show com-
parable rate constants for the two successive steps of the cycle, with
the rate-limiting step depending on the relative importance of
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.07.054.

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