Unlocking the Catalytic Hydrogenolysis of Chlorosilanes into Hydrosilanes with Superbases

Article abstract of DOI:10.1021/acscatal.1c01515

The efficient synthesis of hydrosilanes by catalytic hydrogenolysis of chlorosilanes is described using an iridium (III) pincer catalyst. A careful selection of a nitrogen base (including sterically hindered guanidines and phosphazenes) can unlock the preparation of Me₃SiH, Et₃SiH, and Me₂SiHCl in high yield (up to 98%) directly from their corresponding chlorosilanes.

Full text of DOI:10.1021/acscatal.1c01515

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Unlocking the Catalytic Hydrogenolysis of Chlorosilanes into
Hydrosilanes with Superbases
Gabriel Durin, Jean-Claude Berthet, Emmanuel Nicolas, and Thibault Cantat*
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ABSTRACT: The efficient synthesis of hydrosilanes by catalytic hydro-
genolysis of chlorosilanes is described using an iridium (III) pincer catalyst. A
careful selection of a nitrogen base (including sterically hindered guanidines and
phosphazenes) can unlock the preparation of Me3SiH, Et3SiH, and Me2SiHCl
in high yield (up to 98%) directly from their corresponding chlorosilanes.
KEYWORDS: hydrosilane synthesis, hydrogenolysis, homogeneous catalysis, superbases, iridium pincer complex
Scheme 1. Different Synthetic Ways from Chlorosilanes to
Hydrosilanes
ydrosilanes are useful molecules in the industry for the
Hproduction of a variety of organosilicon compounds
through hydrosilylation of alkenes or dehydrocoupling
reactions.1,2 In organic synthesis, these mild reducing agents
promote reactions with high selectivity and efficiency such as
reduction of esters into aldehydes3 or ethers4 and amides into
amines or enamines.5,6 Where the use of hydrogen as a
reductive source suffers from thermodynamic limitations,7
hydrosilanes provide convenient alternatives, and recent
studies have highlighted new utilizations to recover catalytically
the valuable organic content of oxygenated feedstocks (lignin,
plastics, and CO2) through C−O bond reduction or to develop
phosphine-catalyzed Wittig reactions.8 However, hydrosilanes
are produced via energy-intensive processes and their
utilization also generates quantities of siloxanes.9 The recycling
of these wastes begins with an acidic treatment (HCl) to
provide chlorosilanes which are key intermediates in the
synthetic route to hydrosilanes.10 At this time, the most used
reagents to reduce [Si]−Cl bonds in chlorosilanes are anionic
metal hydrides, such as LiAlH4 (Scheme 1).11 The catalytic
conversion of [Si]−Cl into [Si]−H derivatives using
dihydrogen as a reductive source remains highly challenging.
Catalytic hydrogenolysis of halosilanes and triflates was
recently introduced by Shimada et al. in 2017 and Schneider et
al. in 2018 (Scheme 1). They reported the efficient
transformation of R3SiX (X = OTf, I, Br) into R3SiH under
dihydrogen with the use of noble transition-metal catalysts in
the presence of a nitrogen base to drive the thermodynamics of
the transformation. This reaction is however particularly
difficult with chlorosilanes because Si−Cl is much stronger
Received: April 2, 2021
Revised: July 30, 2021
https://doi.org/10.1021/acscatal.1c01515
© XXXX American Chemical Society
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than the Si−Br and Si−I bonds, with bond dissociation
energies of 456, 343, and 339 kJ mol−1, respectively.12 As such,
a single example supplied the direct hydrogenolysis of Me3SiCl
into Me3SiH but in a near stoichiometric yield (7%).13 To
tackle these limitations, before initiating catalytic hydro-
genolysis in presence of a base, a chloride abstractor such as
NaI or Na[B(C6H3-3,5-(CF3)2)4] was first added, which
greatly improved the yields in hydrosilane (up to 84%).13,14
We present here an efficient catalytic hydrogenolysis of
chlorosilanes into hydrosilanes using an iridium (III) pincer
catalyst. This novel route based on the careful choice of the
base (guanidine or phosphazene) avoids the use of chloride
abstractors and enables the formation of Me3SiH, Me2SiHCl,
and Et3SiH in yields up to 98%.
1.4 after 7 days,13 1 provided Me3SiH in a promising 6% yield
(TON = 6) after 18 h (eq 1, entry 2). TBDH proved
ineffective and reacted immediately with Me3SiCl to give a
white precipitate of the silylium adduct [TBDHSiMe3]Cl
(Scheme 2, right). The silylium species [TBDHSiMe3]Cl
remained inert under hydrogenolysis conditions, even after 48
h at 90 °C in benzene (Table 1, entry 3). This result was
further supported by calculations (vide infra). Even in CD2Cl2,
where [TBDHSiMe3]Cl is soluble and formed as a major
product, only traces of Me3SiH were detected (Table 1, entry
4). Interestingly, Me3SiH was obtained in 25% yield after 18 h
(Table 1, entry 5) with the use of MeTBD. Using BTMG and
BTPP superbases, yields in Me3SiH were considerably
improved, up to 54% after 18 h (Table 1, entries 6 and 7).
Despite the fact that the catalyst is still active (vide infra, as
compound 2), longer reaction times did not lead to increased
yields, demonstrating that the reaction is in full equilibrium.
These results represent the first efficient generation of a
hydrosilane by hydrogenolysis of a chlorosilane derivative,
without an activator. As the BTPP base is relatively expensive,
its recycling would be appealing. A solution might come from
electrodialysis to recover the BTMG or BTPP bases from their
corresponding hydrogen chloride salt, a process which has
been successfully applied to generate ammonia from
ammonium chloride, albeit in water.19 Such a process must
first be transposed to organic media in the case of phosphazene
or guanidine bases.
In contrast to TBDH, the silylium adducts [BaseSiMe3]Cl
were not formed in benzene with the bases MeTBD, BTMG,
and BTPP, while the hydrogenolysis byproduct [BaseH]Cl
deposited gradually, except for [BTPPH]Cl, which is soluble in
benzene. Decreasing the H2 pressure to 1 bar somewhat
decreased the conversion of Me3SiCl (23%) and the yield in
Me3SiH (18% after 18 h) (Table 1, entry 8).13,14 These
experiments underline the crucial role of the base which must
be as strong as DBU to favor hydrogenolysis. The ability of the
above bases to favor either the hydrogenolysis process or a
nucleophilic substitution on Me3SiCl was correlated with
thermodynamic DFT calculations (Scheme 3). Gibbs free
energies (in kcal mol−1) ΔG1 for hydrogenolysis (in purple)
and ΔG2 for nucleophilic substitution (in orange) were
computed at the PBE0-D3/6-311+G(d,p) level of theory
using the SMD solvation model in benzene (see Supporting
Information page S24 for details).
The hydrogenolysis of silyl halides into hydrosilanes is
thermodynamically unfavoured and, as shown in Scheme 1,
requires a base to form the corresponding ammonium salt as a
byproduct and overcome the thermodynamic limitations.
Schneider et al. have computed that trialkylamines are not
basic enough to perform the hydrogenolysis of chlorosilanes
with their Ru(II) complex and a stoichiometric amount of
additives was needed with NEt3 or iPr2NEt to induce the
reaction (Scheme 1).14 We chose iridium (III) pincer
complexes as potential catalysts because they are competent
in a number of hydrogenation15 and hydrosilylation16 reactions
with the formation of reactive [Ir]−H entities which transfer
hydrides readily.
We thus turned to the precursor [Ir(tBuPOCOP)HCl]17 (1)
(
tBuPOCOP = (C6H3){1,3-OPtBu2}2) as a possible catalyst for
the Si−X to Si−H transformation. Beyond its thermodynamic
role, the base is kinetically determinant to favor the formation
of a metal hydride intermediate from a metal chloride and H2
(Scheme 2). The choice of the base is thus crucial and must be
Scheme 2. Proposed Mechanism for the Catalytic
Hydrogenolysis of Silyl Chlorides and Nucleophilic
Substitution
The highest positive ΔG values are found for the amine
NEt3 in agreement with the absence of reactivity noted
experimentally. Increasing the Brønsted basicity of the base
favors the hydrogenolysis of Me3SiCl as reflected in the drop of
ΔG1 values from +2.6 to −3.9 kcal mol−1 from DBU to BTPP.
While the hydrogenolysis of chlorosilanes (ΔG1) with DBU,
MeTBD, and BTMG is slightly endergonic (+2.6, +1.6, and
+0.9 kcal mol−1, respectively), the reaction proceeds in
benzene as it is driven by the precipitation of the byproduct
salt [BaseH]Cl. Interestingly, with DBU, MeTBD, BTMG, and
BTPP, the formation of the silylium salts [BaseSiMe3]Cl is
disfavored with ΔG2 values ranging from +3.6 to +16.7 kcal
mol−1. The high values obtained for BTMG and BTPP (+14.9
and +16.7 kcal mol−1, respectively) are presumably due to
unfavorable steric interactions. The transition states for the
SN2 pathway range from +10.5 to +22.8 from DBU to BTPP
(see the Supporting Information, Section 4.4). The [Base-
SiMe3]Cl compounds are therefore kinetically accessible. A
distinct behavior is observed for TBDH, where both reactions
rationalized. Organic superbases such as amidines, guanidines,
and phosphazenes are neutral bases, stronger than alkylamines,
and are well-known activators in a variety of base-mediated
organic transformations as well as in catalysis.18 Hydroxide or
alkoxide bases are not compatible with chlorosilanes, and a
series of six neutral bases (NEt3, DBU = 1,8-
diazabicyclo[5.4.0]undec-7-ene, TBDH = 1,5,7-
triazabicyclo[4.4.0]dec-5-ene, MeTBD = 7-methyl-1,5,7-tria-
za-bicyclo[4.4.0]dec-5-ene, BTMG = 2-tBu-1,1,3,3-tetrameth-
yl-guanidine, BTPP = (tBu-imino)tri(pyrrolidino)-
phosphorane) differing by their Brønsted basicity (pKa) and
steric hindrances have thus been considered in this work.
The role of the base was evaluated in the catalytic
hydrogenolysis of Me3SiCl (5 bar of H2, room temperature,
in benzene) with 1 (1 mol %) (eq 1). In the presence of NEt3,
no reaction occurred even after 48 h (Table 1, entry 1). While
Shimada et al. observed by using an Ir(I) complex and DBU
the near stoichiometric formation of Me3SiH with a TON of
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a
Table 1. Screening of Bases for the Hydrogenolysis of Me3SiCl in C6D6 with 1
c
b
b
entry
H2 pressure (bar)
base
NEt3
DBU
TBDH
TBDH
MeTBD
BTMG
BTPP
pKa
conversion (%)
yield (selectivity) (%)
reaction time (h)
1
2
3
5
5
5
5
5
5
5
1
18.8
24.3
26.0
26.0
25.5
26.5
28.4
28.4
0
10
0
0 (0)
6 (60)
0 (0)
48
18
48
18
18
18
18
18
d
4
1
<1
5
6
7
8
30
55
59
23
25 (83)
42 (76)
54 (92)
18 (78)
BTPP
a
General conditions: 0.1 mmol of Me3SiCl, 0.11 mmol of the base, 1 μmol of the catalyst, 0.5 mL of the solvent, at room temperature under 5 bar
b
H2 except entry 8 (1 bar). Conversions, selectivities, and yields were determined by 1H NMR spectroscopy through integration of the R3Si signals
versus an internal standard (1,3,5-trimethylbenzene). pKa in MeCN.20 In CD2Cl2.
c
d
because its solubility is strongly related to the nature of the
Scheme 3. Calculated ΔGC H by DFT (M06-2X-D3/6-
6
6
solvent, we investigated the influence of some polar and
nonpolar solvents in the hydrogenolysis of Me3SiCl (1−5 bar
H2, r.t., 18 h), catalyzed by 1 (1 mol %) in the presence of
BTPP (eq 2, Table 2). Replacing benzene with toluene or
311+G(d,p), SMD: Benzene) of the Hydrogenolysis Process
Me3SiCl + H2 + Base → Me3SiH + [BaseH]Cl (ΔG1) and
the Reaction Me3SiCl + Base → [BaseSiMe3]Cl (ΔG2)
Plotted against the pKa of the Base in MeCN20
Table 2. Influence of the Solvent on the Hydrogenolysis of
a
Me3SiCl
b
b
p(H2)
(bar)
conv.
(%)
yield (select.)
(%)
c
entry
solvent
C6D6
Tol-d8
THF-d8
CD2Cl2
CD2Cl2
CD3CN
DMF-d7
DMSO-d6
ε
1
2
3
4
5
6
7
8
1
1
1
1
5
1
1
1
23
15
17
56
99
12
18
9
18 (78)
13 (89)
17 (99)
56 (99)
98 (99)
4 (30)
0 (0)
2.3
2.4
7.6
8.9
8.9
36.6
38.3
46.7
0 (0)
a
General conditions: 0.1 mmol of Me3SiCl, 0.11 mmol of BTPP, 1
μmol of 1 (1 mol %), 0.5 mL of the solvent, room temperature (r.t.).
b
1
Conversions, selectivities, and yields were determined by H NMR
integration of the Me3Si signals versus an internal standard (1,3,5-
c
trimethylbenzene). Dielectric constant of the solvents.
are thermodynamically favorable. The negative value of ΔG1
compared to other bases may be related to the high stability of
[TBDH2]Cl,21 and the absence of Me3SiH production may
result from either the higher exergonicity for the formation of
silylium compared to the hydrogenolysis (ΔG1 − ΔG2 = +1.0
kcal mol−1) or its low solubility and precipitation in C6D6.
Experimental and computational results show that from a
thermodynamic standpoint, BTPP is the most suitable base for
the production of Me3SiH because it presents a large ΔG1 −
ΔG2 difference with a negative ΔG1 value of −3.9 kcal mol−1,
and it is able to favor the thermodynamics of the hydro-
genolysis while preventing the formation of the silylium side
product [BaseSiMe3]Cl. Since the formation of the hydro-
genolysis byproduct [BaseH]Cl can drive the catalysis and
THF, at 1 bar of H2, somewhat decreased the yields in Me3SiH
to 13 and 17%, respectively (Table 2, entries 2 and 3). In the
more polar solvents DMF, DMSO, and MeCN, the conversion
rates at 1 bar H2 are low, and Me3SiH is observed only in
MeCN and in low quantity (4%) (Table 2, entries 6−8). NMR
analyses actually revealed in the later solvents the formation of
large quantities of the soluble silylium salt [BTPPSiMe3]Cl,
which is detrimental to the catalysis. Finally, at 1 bar H2, the
highest conversion (56%) with excellent yield and selectivity in
Me3SiH (56 and 99%, respectively) was achieved in dichloro-
methane (entry 4). Importantly, under 5 bar H2, Me3SiH was
obtained in near quantitative yield (98%) from Me3SiCl (entry
5). In the case of 1 bar of H2, longer reaction times did not
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a
lead to increased yields similar to the previous observations in
C6D6. The normalized profiles in Me3SiH production in
CD2Cl2 at 1 and 5 bar of H2 have been drawn (see the
Supporting Information, 2.1.5). The two curves almost overlap,
which suggests that the pressure mostly influences the
thermodynamics rather than the kinetics of the reaction.
Capitalizing on these findings, the hydrogenolysis of Et3SiCl
and Me2SiCl2 was attempted (Scheme 4; eqs 3−5). The
Scheme 5. Stoichiometric Reactions
Scheme 4. Catalytic Hydrogenolysis by 1 (1 mol %) of
Et3SiCl and Me2SiCl2 with BTMG or MeTBD in CD2Cl2
a
a
1
Yields were determined by H NMR integration of the RnSi (n = 2
and 3) signals versus an internal standard (1,3,5-trimethylbenzene).
a
Reaction (7) with 2 has been conducted under 1 bar of H2.
conversion of Et3SiCl with BTPP proved more difficult than
Me3SiCl, requiring 40 h to afford Et3SiH selectively in 59%
yield. This is in agreement with the observation of the Shimada
and Schneider groups that an increase in the steric hindrance
of the silyl iodides or triflates led to longer reaction times.13,14
Interestingly, Me2SiCl2, which is more electrophilic22 than
Me3SiCl, proved less reactive under our conditions (Scheme 4,
eq 4), and a mixture of Me2SiHCl (13%) and Me2SiH2 (3%)
was observed after 40 h. This result matches the hydro-
genolysis of Me2Si(OTf)2 by an iridium catalyst reported to be
much slower (7 days vs 8 h for Me3SiOTf).13 These poor
yields in hydrosilanes demonstrated that the hydrogenolysis of
dialkylchlorosilanes must be optimized. Replacing BTPP with
MeTBD (Scheme 4, eq 5) favored a higher conversion rate in
Me2SiCl2 (48%) and the formation of Me2SiHCl as the major
product (37% yield). Increased reaction times led to higher
yields in Me2SiHCl (54% after 7 days). No trace of the silylium
[MeTBDSiClMe2]Cl could be detected by 1H NMR in
dichloromethane. The latter results evidence the formation
of R2SiHCl species from R2SiCl2 under smooth conditions,
while Me2SiH2 is the major product with strong reducing
agents (such as LiAlH4).23
hydride transfer is expected to be much more efficient than
that from the neutral derivatives 2 or 3 (eq 7). Such species are
known, and [Ir(tBuPOCOP)H3]Na24 was previously reported
by Brookhart and co-workers from the treatment of 1 with
NaH. Although never detected in our catalytic experiments,
anionic species may be an intermediate formed by deproto-
nation of 2 with a strong base. Attempts to generate such
species from 3 using only BTPP and H2 have not been
successful. However, the use of a stronger base such as Verkade
superbase iPrVB (iPrVB = 2,8,9-triisopropyl-2,5,8,9-tetraaza-1-
phosphabicyclo[3.3.3]undecane) in the presence of 3 under 10
bar H2 (eq 8) in THF immediately afforded a white deposit.
This solid has been characterized by NMR in acetonitrile as
the ion pair [Ir(tBuPOCOP)H3][iPrVBH] (4), featuring both
the expected anionic hydride and the phosphonium salt.
Complex 4 shows 1H NMR signals similar to those reported in
25
[Ir(tBuPOCOP)H3]Na.24 It is not stable in CD2Cl2 but
reacted in C6D6 with 1 equiv. Me3SiCl to give Me3SiH
quantitatively after 4 h (eq 9). The resulting complexes 3 and
[Ir(tBuPOCOP)H2(HSiMe3)] (5) were identified by NMR (eq
10 and Supporting Information Figure S18/S19). Complex 5 is
analogous to the previously reported [Ir(tBuPOCOP)-
H2(HSiEt3)]26 and displays an Ir−H hydride signal at −8.56
ppm by 1H NMR. This result supports the involvement of the
anionic hydride complex [Ir(tBuPOCOP)H3][BTPPH] (6) as
a key hydride donor in the formation of the hydrosilane.
Generated catalytically and prone to reduce Si−Cl bonds,
complex 6 would be the first anionic trihydride iridium
complex involved in catalytic hydrogenation reactions and
would be responsible for the excellent performances of this
system (Scheme 5).
To gain insights into the mechanism of the catalysis under
the optimized conditions (5 bar H2 in CH2Cl2) (Scheme 5),
we focused on the iridium complexes that might form from 1
either by stoichiometric addition of the reagents (eqs 6, 7, and
1
8) or under catalytic conditions. The H NMR spectrum of 1
is not modified by addition of 1 equiv. of Me3SiCl or BTPP or
when pressurized under H2. 1 is however converted in 1 h into
[Ir(tBuPOCOP)H2(H2)]24 (2) (81%), when treated with
1
BTPP under 5 bar H2. 2 is the only complex detected by H
NMR during the catalysis and at its end. Hydrogenolysis of
Me3SiCl with [Ir(tBuPOCOP)H2] (3) (1 mol %) also led to 2
as the only observable iridium species (see Supporting
Information Figure S7).24 However, 2 and 3 do not react
with stoichiometric quantities of Me3SiCl to give Me3SiH. Our
hypothesis for the active species is based on the formation of
the anionic hydride species [Ir(tBuPOCOP)H3]‑ for which the
Density functional theory calculations (see Supporting
Information page S24 for details) were performed to get a
deeper insight into the mechanism and to confirm the possible
hydride transfer to Me3SiCl from the anionic trihydride species
6. The most favored pathway is depicted in Scheme 6: complex
10858
https://doi.org/10.1021/acscatal.1c01515
ACS Catal. 2021, 11, 10855−10861