Selective Hydrosilylation of Esters to Aldehydes Catalysed by Iridium(III) Metallacycles through Trapping of Transient Silyl Cations

Article abstract of DOI:10.1002/chem.201602867

The combination of an iridium(III) metallacycle and 1,3,5-trimethoxybenzene catalyses rapidly and selectively the reduction of esters to aldehydes at room temperature with high yields through hydrosilylation followed by hydrolysis. The ester reduction involves the trapping of transient silyl cations by the 1,3,5-trimethoxybenzene co-catalyst, supposedly by formation of an arenium intermediate whose role was addressed by DFT calculations.

Full text of DOI:10.1002/chem.201602867

DOI: 10.1002/chem.201602867
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&
Homogeneous Catalysis
Selective Hydrosilylation of Esters to Aldehydes Catalysed by
Iridium(III) Metallacycles through Trapping of Transient Silyl
Cations
Yann Corre,[a, b] Vincent Rysak,[a, b] Frꢀdꢀric Capet,[a] Jean-Pierre Djukic,[c] Francine Agbossou-
Niedercorn,*[a, b] and Christophe Michon*[a, b]
Abstract: The combination of an iridium(III) metallacycle and
1,3,5-trimethoxybenzene catalyses rapidly and selectively the
reduction of esters to aldehydes at room temperature with
high yields through hydrosilylation followed by hydrolysis.
The ester reduction involves the trapping of transient silyl
cations by the 1,3,5-trimethoxybenzene co-catalyst, suppos-
edly by formation of an arenium intermediate whose role
was addressed by DFT calculations.
tion of 2-pyridinyl esters.[8b] The strong organoborane Lewis
acid B(C6F5)3 catalyses efficiently the hydrosilylation of esters
with triphenylsilane to afford silyl acetals along with the over-
reduction products (silyl ethers and alkanes).[8c] More recently,
the synthesis of aldehydes by ester hydrosilylation and hydrol-
ysis of the resulting silyl acetals was performed independently
by Brookhart et al. and Darcel et al.[8d–e] The former used a com-
bination of 0.1–0.5 mol% of [{Ir(cyclooctene)2Cl}2] catalyst and
an excess of diethylsilane to reduce a broad scope of sub-
strates in high yields at temperatures from ambient to 508C.[8d]
The latter developed N-heterocyclic carbene iron carbonyl cat-
alysts for the hydrosilylation of aromatic and aliphatic esters at
1 mol% catalyst loading in the presence of a slight excess of
diethylsilane or diphenylsilane and under UV irradiation at
room temperature.[8e] Besides these significant achievements,
our attention was drawn by the seminal work of Nagashima
et al. who, after a study on ruthenium-catalysed selective re-
duction of esters to alcohols and ethers,[9a] reported on primary
alkylation of electron-enriched aromatic compounds with
esters using a ruthenium catalyst and a silane reagent (Sche-
me 1).[9b,c] During our studies on hydrosilylation reactions of
unsaturated carbon–heteroatom compounds,[10] we observed
a novel and original reactivity by using 1,3,5-trimethoxyben-
zene (TMB) in combination with other catalysts and silane re-
agents. Herein, we report on the selective hydrosilylation of
esters to aldehydes catalysed by an IrIII metallacycle and TMB
at room temperature.
Introduction
Aldehydes are important compounds in organic chemistry.
Three main synthetic pathways can achieve their preparation
starting from widely available chemicals: the hydroformylation
of olefins,[1] the oxidation of primary alcohols[2] and the reduc-
tion of carboxylic acid derivatives.[3] However, because of the
reactivity of aldehydes, problems with chemoselectivity may
be encountered. Although the reduction of esters to aldehydes
is possible by using diisobutylaluminium hydride or lithium tri-
tert-butoxyaluminium hydride, these reagents are not used
regularly on a large scale because of their sensitivity and capri-
cious reputation.[4] Alternatively, the use of hydrosilanes as re-
ductants is an area of growing interest for the mild and selec-
tive reduction of carboxylic acid derivatives by using transition
metal or Lewis acid catalysts.[5] Indeed, this strategy enables
the formation of silyl acetals, which can be converted to alde-
hydes upon hydrolysis. Although the hydrosilylation reaction
has been successful for the selective reduction of esters to al-
cohols or ethers,[6,7] examples of hydrosilylation of esters to al-
dehydes are scarce.[8] A first example using a ruthenium car-
bonyl catalyst at high temperature showed a rather narrow
substrate scope.[8a] A second method applied a catalyst based
on Pd(OAc)2 and triphenylphosphine for the exclusive reduc-
[a] Dr. Y. Corre, V. Rysak, Dr. F. Capet, Dr. F. Agbossou-Niedercorn, Dr. C. Michon
Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181
UCCS - Unitꢀ de Catalyse et Chimie du Solide, 59000 Lille (France)
E-mail: francine.agbossou@ensc-lille.fr
christophe.michon@ensc-lille.fr
Results and Discussion
[b] Dr. Y. Corre, V. Rysak, Dr. F. Agbossou-Niedercorn, Dr. C. Michon
ENSCL, UCCS-CCM-CASECO, (Chimie-C7) CS 90108
59652 Villeneuve d’Ascq Cedex (France)
Following our previous investigations on IrIII metallacycles as
catalysts for the hydrosilylation of imines,[10] we started our
study by focusing on the hydrosilylation of ethyl ester 3a
using triethylsilane without any further hydrolysis (Table 1). In-
terestingly, a catalyst loading of 1 mol% of iridacycle 1 and
2 mol% of sodium tetrakis[(3,5-trifluoromethyl)phenyl]borate
[c] Dr. J.-P. Djukic
Institut de Chimie de Strasbourg, UMR 7177, Universitꢀ de Strasbourg
4 rue Blaise Pascal, 67000 Strasbourg (France)
Supporting information for this article can be found under
http://dx.doi.org/10.1002/chem.201602867.
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Table 1. Catalytic study.
Entry
3
Ir complex
[mol%]
TMB
[mol%]
Yield
[%][a]
Selectivity[a]
4a–c/5a–c/6a–c
Scheme 1. Esters in alkylation and hydrosilylation reactions.
1[b]
2
3[c]
4[c]
5[d]
6[d]
7
8[e]
9[e]
10
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3a
3b
3c
1 (1)
2 (1)
2 (1)
1 (1)
1 (1)
2 (1)
2 (1)
2 (1)
2 (1)
2 (1)
1 (1)
2 (0.1)
–
0
0
0
0
0
0
100
1
58
48
100
100
53
44
100
63
71
100
89
17
0
100
17
14
75
0
0
100
83
0
54
68
19
84
85
0
17
32
0
0
0
–
0
35
23
70
0
0
29
32
6
16
15
0
tr.
tr.
0
(NaBArF24) as additive led selectively to silyl acetal 4a in a 58%
yield, but an additional time lapse did not allow the reaction
to reach completion (Table 1, entry 1). Surprisingly, under simi-
lar reaction conditions, the use of iridacycle 2 in combination
with NaBArF24 led to a mixture of three products, among
which ethyl and silyl ethers 5a and 6a were the major ones
and silyl acetal 4a was the minor one (overall yield: 48%;
Table 1, entry 2). The addition of three equivalents of triethylsi-
lane afforded mainly 5a (68%) along with 6a (32%) and 4a
(14%) in an overall quantitative yield (Table 1, entry 3). When
three equivalents of triethylsilane were used with iridacycle 1,
we obtained 4a as the major product (75%) with some ethers
5a (19%) and 6a (6%), and the overall yield was quantitative
(Table 1, entry 4). With 2 mol% of tritylBArF20, ethyl ether 5a
was formed as the main product in 44–53% overall yield
(Table 1, entries 5 and 6). This trend can be explained by the
propensity of the trityl cation to interact with the silyl ether
group and allow its elimination.
5
68
10
10
10
10
10
10
10
10
10
100
100
100
–
100
60
18
30
100
11
0
0
–
0
12[f]
13
14[g]
15[c]
16[h]
17[e]
18
2(1)
62
100
65
53
100
2 (1)
2 (1)
2 (1)
2 (1)
5
59
tr.
0
[a] Determined by 1H NMR spectroscopy. [b] 68% yield in 24 h. [c] Reac-
tion with 3 equiv triethylsilane. [d] 2 mol% CPh3BArF20. [e] tr.=trace.
[f] 22% in 2 h. [g] 1 equiv Et3SiH. [h] 1.2 equiv of TMDS.
propyl substituent afforded selectively silyl acetal 4c in quanti-
tative yield (Table 1, entry 18).
In an attempt to adjust the silane reactivity, several ether ad-
ditives were tested (see Table S1 in the Supporting Informa-
tion). The use of one equivalent of TMB led to the quantitative
synthesis of silyl acetal 4a (Table 1, entry 7). Whereas 1 or
5 mol% of TMB led to the formation of product 4a with some
ethyl ether 5a in lower overall yields (Table 1, entries 8 and 9),
a TMB loading of 10 mol% again afforded selectively silyl
acetal 4a in quantitative yield (Table 1, entry 10). Under these
reaction conditions, iridacycle 1 led to a less efficient catalyst
(Table 1, entry 11), and a 0.1 mol% loading of complex 2 result-
ed in low yields, independently of the reaction time (Table 1,
entry 12). The presence of an IrIII metallacycle proved to be crit-
ical to allow the hydrosilylation of ester 3a (Table 1, entry 13).
Furthermore, the use of 1.2 equivalents of triethylsilane proved
to be essential to reach full conversion (Table 1, entries 14, 15).
In the presence of TMB, the addition of three equivalents of
triethylsilane allowed the formation of 4a with some ethers 5a
and 6a in a 60:35:5 ratio (Table 1, entry 15). Replacement of
triethylsilane by 1,1,3,3-tetramethyldisiloxane (TMDS) resulted
in lower yield and selectivity (Table 1, entry 16). Finally, we no-
ticed that the hydrosilylation reaction was sensitive to the
nature of the ester group. Indeed, methyl ester 3b, a weaker
electron-donating group, led mainly to methyl ether 5b along
with some silyl acetal 4b (7:3 ratio) in a modest yield (Table 1,
entry 17). By comparison, ester 3c bearing an electron-rich iso-
The scope of the hydrosilylation reaction was studied for
a series of aromatic and aliphatic esters under the optimised
conditions determined above (Scheme 2). Hydrolysis of the re-
sulting silyl acetals to the corresponding aldehydes could be
performed in most cases by simple addition of water to the re-
action mixture. Indeed, hydration of the remaining NaBArF24
resulted in a catalysis producing a Brønsted acid, which could
cleave chemoselectively the generated silyl acetals.[11] Methyl
2-phenylacetate (3b) led to the corresponding aldehyde in
moderate yield, and the hydrosilylation of phenyl acetates 3a
and 3c could be performed readily. Reagent 3d with 2-phenyl-
propanoate skeleton was also reduced selectively without any
effect of the steric hindrance of the substrate. Though the hy-
drosilylation of ester 3e based on a furan heterocycle was
straightforward, the reduction of thiophene-based ester 3 f
proved to be more challenging and led only to the corre-
sponding acetal.[12] Hydrosilylation of benzyl benzoate (3m)
and related ortho- and para-substituted esters 3g–k, 3n pro-
ceeded selectively to the desired aldehydes in high yields inde-
pendently of the substitution pattern of the reagent. However,
ortho substitution by a methoxy group led to unreactive sub-
strate 3l, which probably strongly chelates the catalyst. Finally,
linear and cyclic aliphatic esters 3o–s were reduced selectively
to the corresponding aldehydes in high yields, with the excep-
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Scheme 2. Reaction scope. [a] Yields measured by H NMR spectroscopy, isolated yields in brackets. [b] Isolated as the 2,4-dinitrophenylhydrazone adduct.
[c] 2.2 equiv Et3SiH was used and the product was hydrolysed with 4 equiv H2O. [d] Isolated as the resulting acetal; no hydrolysis observed with HCl (2m) or
NaOH (2m) in THF.
tion of ester 3q, whose resulting acetal could not be hydro-
lysed.[12]
This confirmed the propensity of strong electrophiles such as
triethylsilyl and trityl cations to eliminate ether groups and
form products 5 and 6 (Scheme 3). In such a case, it is likely
that average yields could only be obtained when 1.2 equiva-
lents of triethylsilane were used as two equivalents of iridium
hydride complex 10 are required for that pathway.
Regarding the reaction mechanism, precatalyst 2 is first de-
halogenated by NaBArF24 to give transient cationic complex 8
(Scheme 3).[10b] The iridium catalyst activates triethylsilane
through the formation of silane–iridium adduct 9.[13,14] Such an
activation process would produce iridium hydride complex
10[10a,15] and a triethylsilyl cation.[16] The latter may activate the
carbonyl group of ester substrate 3 and generate a silyloxy car-
bonium species, which is stabilised by the electron-donating
group R’. Reaction with a first equivalent of the iridium hydride
complex 10 affords silyl acetal 4 along with the cationic iridi-
um catalytic species 8. At this stage, the generation of iridium
hydride complex 10 and the triethylsilyl cation seems to be
faster than the formation of silyl acetal 4 and release of species
8. Indeed, we observed that alkyl ether 5 and silyl ether 6
were the major reaction products with respect to 4 when no
TMB was used as co-catalyst (Table 1, entry 2). Hence, TMB
would trap the remaining triethylsilyl cation, probably through
an aromatic electrophilic substitution generating an SiÀC areni-
um (Wheland-type) intermediate.[16] Final hydrolysis of silyl
acetal 4 is catalysed after hydration of the remaining NaBArF24
additive[11] and affords the desired aldehyde product, whereas
the SiÀC Wheland intermediate would probably be involved in
the reduction of another molecule of ester 3. Without TMB,
silyl acetal 4 reacts further through activation of its ether
groups by the triethylsilyl cation. This affords, after reaction
with a second equivalent of iridium hydride complex 10, alkyl
ether 5 and silyl ether 6 as well as the catalytic cationic iridium
species 8. This second pathway was confirmed by some experi-
mental results. First, when a large excess of triethylsilane was
used (3 equiv), the synthesis of ethers 5a and 6a was almost
quantitative (Table 1, entry 3). In addition, the combined use of
iridium precursor, [trityl][B(C6F5)4] additive and triethylsilane led
mainly to ethyl ether 5a in 44–53% yield (Table 1, entries 5, 6).
To further confirm the proposed reaction mechanism, the
role of TMB in the trapping of the triethylsilyl cation was stud-
ied from a broad viewpoint. In the first instance, we did not
consider the actual kinetic aspects of the transfer of the silyli-
um cation Et3Si+, and focused on the thermochemistry of its
interaction, particularly with the arene additive as well as with
the various organic molecules involved in the catalysis. First,
the possible interaction of the silylium-activated ester cation
with TMB was found to be highly unfavourable (Figure 1a and
Figure S2 in the Supporting Information). Conventional
COSMO (CH2Cl2)/DFT-D computation of the Gibbs enthalpy of
the reaction depicted in Figure 1a for a model reaction, with
or without the BArF24 anion, produced an endoergonic value
of around +23 kcalmolÀ1, which cleared doubts about a possi-
ble interaction of intermediate carbocationic species with the
arene additive. Figures 1b–f depict the possible scenarios of
addition of the silylium cation Et3Si+ to the ester
PhCH2C(O)OEt, the acetal PhCH2C(OSiEt3)(OEt), TMB, 1,3-dime-
thoxybenzene and 1,4-dimethoxybenzene. On thermochemical
grounds, the addition of the silylium cation at the carbonyl
oxygen atom is greatly favoured over other competing interac-
tions with the arene additive, which are disfavoured by about
16 kcalmolÀ1 for TMB. It is therefore likely that the first step of
the catalysis, which entails reduction of the ester to the acetal,
does not necessarily involve explicitly the arene additive.The
interaction of the latter with Et3Si+ tends to be more critical in
the second step, in which the acetal undergoes reduction to
an ether. In this case, the silylium cation has an affinity for TMB
of À24 kcalmolÀ1 for a reaction affording a typical SiÀC Whe-
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Scheme 3. Proposed reaction mechanism.
Figure 1. DFT-D (ZORA-PBE-D3(BJ)/all-electron TZP) calculated thermochemistry with COSMO solvation (CH2Cl2, T=298.15 K; see the main text for detail.
land intermediate, and O binding of Et3Si+ is disfavoured by
about 6 kcalmolÀ1 (Figure 1d and Figure 2). The fact that the
affinities of the silylium cation for the acetal and for TMB are
almost equal within 1 kcalmolÀ1 may explain the minimal con-
version of the acetal to ethers in the course of the catalysis. In
this case, we hypothesise that TMB moderates the electrophi-
licity of Et3Si+ by dragging it into an equally probable interac-
tion with the arene, with possible subsequent formation of si-
lylated TMB and release of H+, which would be detrimental to
the hydrido iridium catalytic species and would end the cataly-
sis. The affinity of Et3Si+ for the other two dimethoxybenzene
isomers is characterised by preferential binding to the oxygen
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atoms as formation of the corresponding Wheland intermedi-
ates is disfavoured by about 7–10 kcalmolÀ1. The affinity of the
two dimethoxybenzene isomers for Et3Si+ is overall 3–5 kcal
molÀ1 lower than that of TMB, and the diversion of Et3Si+ by
interaction could be disfavoured.
sis. However, in the absence of TMB, further dealkoxylation of
the silyl acetal occurs and affords alkyl and silyl ether products.
Our results highlight that the reactivity of silanes and related
intermediates is modular and depends on the substituents of
the silicon atoms. We believe that the reactivity of Lewis acids
such as carbocations[17] may be tuned by using such a method-
ology.
Experimental Section
General procedure for catalysis
In a glovebox, iridium(III) catalyst 2 (1 mol%), TMB (10 mol%), and
NaBArF24 (2 mol%) were introduced into a Schlenk tube. Under ni-
trogen, distilled ester (0.31 mmol, 1 equiv) and dry dichlorome-
thane (2 mL) were added followed by triethylsilane (0.34 mmol,
1.1 equiv). The reaction mixture was then heated at 258C with stir-
ring. To monitor the progress of the reaction, aliquots (0.1 mL)
were taken at defined times, filtered through Celite with a CH2Cl2
wash (3 mL), evaporated under vacuum and analysed by 1H NMR
spectroscopy. After 0.5 h, water (2 equiv) was added and the reac-
tion mixture was stirred for an additional 0.5 h. The resulting mix-
ture was filtered through MgSO4 and the solvent evaporated under
vacuum. The crude product was then purified by flash chromatog-
raphy or preparative TLC.
Volatile compounds were isolated as 2,4-dinitrophenylhydrazone
adducts. After reaction, the crude product was filtered through
MgSO4 and diluted with ethanol (1 mL). The resulting solution was
treated with 0.5 mL of Brady’s reagent [solution of 2,4-dinitrophe-
nylhydrazine (0.3 g), H2O (1.5 mL), ethanol (5 mL) and H2SO4 conc.
(1 mL)] for 30 min with vigorous stirring. The resulting orange pre-
cipitate was isolated by filtration through filter paper, washed with
ethanol (10 mL) and dried under vacuum.
Computational details
Computations were performed by DFT with the PBE[18] GGA func-
tional implemented in the Amsterdam Density Functional package
(ADF2013[19]) and augmented with Grimme’s DFT-D3(BJ)[20] imple-
mentation of dispersion with a Becke–Johnson (BJ) damping func-
tion. Scalar relativistic corrections with the Zeroth Order Regular
Approximation[21] were applied with ad hoc all-electron (AE) basis
sets consisting of polarised triple-z (TZP) Slater-type orbitals. Ge-
ometry optimisation by energy gradient minimisation was carried
out in all cases with a numerical grid accuracy between 4.5 and 8,
an energy gradient convergence criterion of 10À3 a.u. and a very
tight SCF convergence criterion. Vibrational modes were computed
to verify that the optimised geometries were related to energy
Figure 2. Gas-phase singlet ground-state geometries of: a) the cationic Whe-
land-type adduct, and b) the O-silylium adduct resulting from the interaction
of Et3Si+ with TMB.
Several attempts were made to crystallise the adduct of TMB
and the triethysilyl cation. Even though such a compound
could not be evidenced, other unrelated arenium species did
crystallise (see Scheme S1–S4, Figure S1 and Table S1 in the
Supporting Information).
minima without considering residual modes between
0 and
90i cmÀ1. Representations of molecular structures and isosurfaces
Conclusions
were produced with ADFview 2013.
The combination of a cyclometallated iridium(III) complex and
TMB catalyses efficiently and selectively the hydrosilylation of
esters with triethylsilane to give aldehydes at room tempera- Acknowledgements
ture in high yields. Hydrolysis of the resulting acetals allows, in
most cases, straightforward formation of the aldehyde. The se-
lective ester reduction is most probably favoured by trapping
of transient silyl cations leading to an arenium intermediate,
which inhibits any further reductive transformation of the
acetal. In the presence of TMB, no further reduction of the key
silyl acetal intermediate ensues, and formation of the aldehyde
product is more often than not straightforward upon hydroly-
The University of Lille 1 and Rꢀgion Nord-Pas de Calais are ac-
knowledged for PhD fellowships (Y.C. and V.R.). The CNRS, the
Chevreul Institute (FR 2638), the Ministꢂre de l’Enseignement
Supꢀrieur et de la Recherche, the LABEX Chimie des Systꢂmes
Complexes (Strasbourg), the Rꢀgion Nord - Pas de Calais and
the FEDER are acknowledged for supporting and partially fund-
ing this work. Mrs. C. Mꢀliet and C. Delabre (UCCS) are thanked
Chem. Eur. J. 2016, 22, 1 – 7
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Keywords: aldehydes
·
homogeneous
catalysis
·
hydrosilylation · iridium · synthetic methods
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Received: June 15, 2016
Published online on && &&, 0000
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Chem. Eur. J. 2016, 22, 1 – 7
www.chemeurj.org
6
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Full Paper
FULL PAPER
Winning combination: A combination
of an IrIII metallacycle and 1,3,5-trime-
thoxybenzene provides efficient cataly-
sis of the hydrosilylation of esters to al-
dehydes (see figure). By using triethylsi-
lane and a further hydrolysis step, the
reactions proceed rapidly with high
yields and selectivities under mild con-
ditions. The trapping of transient silyl
cations was performed for the first time
and studied by DFT calculations.
&
Homogeneous Catalysis
Y. Corre, V. Rysak, F. Capet, J.-P. Djukic,
F. Agbossou-Niedercorn,* C. Michon*
&& – &&
Selective Hydrosilylation of Esters to
Aldehydes Catalysed by Iridium(III)
Metallacycles through Trapping of
Transient Silyl Cations
Chem. Eur. J. 2016, 22, 1 – 7
www.chemeurj.org
7
ꢁ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
&
These are not the final page numbers! ÞÞ