Catalytic hydrosilylation of oxalic acid: Chemoselective formation of functionalized C2-products

Article abstract of DOI:10.1039/c4cy00339j

Oxalic acid is an attractive entry to functionalized C₂-products because it can be formed by C-C coupling of two CO₂ molecules under electrocatalytic reduction. Herein, we describe the first attempts to reduce oxalic acid by catalytic hydrosilylation. Using B(C₆F ₅)₃ as a Lewis acidic catalyst, oxalic acid can be converted to reduced C₂-molecules, with high chemoselectivity, under mild reaction conditions.

Full text of DOI:10.1039/c4cy00339j

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Cantat et al.
Catalytic hydrosilylation of oxalic acid: chemoselective formation of
functionalized C2-products

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Catalytic hydrosilylation of oxalic acid:
chemoselective formation of functionalized
C2-products†
Cite this: Catal. Sci. Technol., 2014,
4, 2230
Received 18th March 2014,
Accepted 12th April 2014
*
Elias Feghali, Olivier Jacquet, Pierre Thuéry and Thibault Cantat
DOI: 10.1039/c4cy00339j
www.rsc.org/catalysis
Oxalic acid is an attractive entry to functionalized C2-products
because it can be formed by C–C coupling of two CO2 molecules
under electrocatalytic reduction. Herein, we describe the first
attempts to reduce oxalic acid by catalytic hydrosilylation. Using
B(C6F5)3 as a Lewis acidic catalyst, oxalic acid can be converted to
reduced C2-molecules, with high chemoselectivity, under mild
reaction conditions.
from the C–O bond coupling of two CO2 molecules, via the
transient formation of formaldehyde.2l So far, oxalic acid,
HO2CCO2H, is the main C2-product accessible by reduction
of CO2. In fact, selective electrocatalysts have been developed
to promote the C–C bond coupling of CO2 with high Faradaic
efficiency.6 In this context, the reduction of oxalic acid could
provide novel routes to the conversion of CO2 to C2-compounds.
While oxalic acid is mostly utilized as a ligand in the extrac-
tion of rare earth elements,7 the reduction chemistry of this
simple dicarboxylic acid remains a largely unexplored area.
In fact, the two C+III atoms of oxalic acid can each undergo
six-electron reduction to yield ethane and the sequential two-
electron reduction of oxalic acid is a potential entry to a vari-
ety of functionalized C2-molecules. As depicted in Scheme 1,
these include glyoxal, glycolic acid, ethylene glycol, acetalde-
hyde and ethanol. Yet, to the best of our knowledge, glycolic
acid and glyoxal are the only products available from the
reduction of oxalic acid. The conversion of oxalic acid to
glycolic acid can be achieved either with powdered magne-
sium or electrochemical reduction at a Hg cathode;8 LiAlH4
favors the formation of glyoxal.9 Importantly, catalytic hydro-
silylation reactions have shown superior chemoselectivity
to classical reduction methods involving metal-hydrides for
the reduction of a wide range of carbonyl groups, including
carboxylic acids.10 Herein, we report the first attempts to
reduce oxalic acid under hydrosilylation conditions and
Because it has a low toxicity and a low cost, CO2 is an
attractive carbon source in organic chemistry.1 Beyond the
industrial applications utilizing CO2 for the production of
urea, carbonates and salicylic acid,1a,c novel methodologies
have been developed over the past few years to use CO2 as a
C1-building block in the formation of formic acid,
formaldehyde, methanol, formamides, formamidines, imines
and methylamines.2 These advances were facilitated on the
one hand by the design of efficient hydrogenation catalysts
which are able to convert CO2–H2 mixtures to reduced C1
compounds.2a,f–h,3 On the other hand, the utilization of
polarized reductants, such as hydrosilanes and hydroboranes,
enabled the development of efficient reduction processes
for the conversion of CO2 at low temperature and low
pressure.2b–e,j–m,4 Key examples of this development include
the efficient hydrogenation of CO2 to methanol2a and the
reductive functionalization of CO2 to methylamines, using
organometallic catalysts.2c,i,5 Nonetheless, these novel
transformations are limited to the formation of C1-products.
In 2012, Sabo-Etienne, Bontemps et al. showed that the
ruthenium catalyzed hydroboration of CO2 to methoxyborane
generated
a
C2-intermediate.2k–m The pinB–OCH2–OCHO
compound (pinB = 4,4,5,5-tetramethyl-1,3,2-dioxaboryl) results
CEA, IRAMIS, NIMBE, CNRS UMR 3299, CEA/Saclay, 91191 Gif-sur-Yvette, France.
E-mail: thibault.cantat@cea.fr; Fax: +33 1 6908 6640; Tel: +33 1 6908 4338
† Electronic supplementary information (ESI) available: General experimental
details, synthetic procedures and data for 1–6, and crystallographic information
in CIF format for 3. CCDC 992213. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4cy00339j
Scheme 1 Product distribution for the sequential 2-electron reduc-
tion of oxalic acid to ethane.
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describe the selective conversion of oxalic acid to ethane and
silylated derivatives of 2-oxoacetic acid, glyoxal, glycolic acid
and glycolaldehyde.
No intermediate could be observed in the reduction of
oxalic acid to ethane with TMDS or PMHS (eqn (1) and (2))
and, using a default amount of the reductant, ethane was
still obtained as the sole organic product, with limited con-
version. Nevertheless, we have observed previously that
Et3SiH presents a lower reactivity, compared to TMDS and
PMHS, in the electrophilic hydrosilylation of lignin model
compounds.13a The sequential reduction of oxalic acid was
therefore carried out with increasing amounts of Et3SiH.
Addition of 3 equiv. Et3SiH to a CH2Cl2 solution of anhydrous
oxalic acid, in the presence of 1.0 mol% B(C6F5)3, resulted in
the rapid formation of 1 in 61% yield (eqn (3)). This reaction
is consistent with previous findings by Brookhart et al., who
demonstrated the efficient conversion of carboxylic acids to
their silylacetals by B(C6F5)3-catalyzed hydrosilylation.15
Reduction of oxalic acid to 1 is accompanied with H2 evolu-
tion (observed by 1H NMR), resulting from the dehydro-
genative silylation of the acidic O–H groups. The silylation of
the carboxylic functionalities likely precedes the reduction of
oxalic acid, although the putative bis(triethylsilyl)oxalate
intermediate could not be detected. In fact, pre-activation of
the substrate can be achieved independently by reacting
oxalic acid with two equivalents of Me3SiCl to prepare oxalate
With the recent development of highly active and selective
molecular catalysts, hydrosilylation reactions have found
compelling success in the reduction of esters, carboxylic
acids, amides and ureas, which are reluctant substrates in
hydrogenation transformations.10d,11 Striking examples
include the design of iron and ruthenium catalysts able to
promote the hydrosilylation of esters to alcohols or aldehydes
selectively.11c,e Notably, the Lewis acid B(C6F5)3 is a potent
hydrosilylation catalyst for the reduction of carbonyl
derivatives, alcohols and ethers12 and it has been recently
utilized for the reduction of lignin models and cellulose.13
In addition, this metal-free catalyst can also convert poly-
methylhydrosiloxane (Me3Si(OSiMeH)nOSiMe3, PMHS) and
tetramethyldisiloxane (Me2SiHOSiHMe2, TMDS), which are
cost-efficient (2–5 € per mole), non-toxic and moisture stable
by-products of the silicone industry.2d,14 Thus, the complete
reduction of oxalic acid was first undertaken in CH2Cl2, using
4.3 equiv. TMDS, which has a slight excess of Si–H function-
alities. Catalyst B(C6F5)3 promotes the reduction of oxalic
acid to ethane within 2 h at 25 °C, with a low catalyst loading
of 1.0 mol% (eqn (1)). The quantitative formation of ethane
and hydrogen was monitored by 1H NMR and GC analyses.
This transformation represents the first example of the
reduction of oxalic acid to ethane. It is noteworthy that the
solvent plays an important role in the reduction of oxalic acid
and the reaction is significantly slower in benzene. Ethane
was also successfully produced in the presence of PMHS
(eqn (2)). Nonetheless, the conversion yield was somewhat
limited (<73%), presumably because of the concomitant for-
mation of siloxane gels which precluded the full conversion
of the Si–H functionalities. Overall, this reaction chemistry
exemplifies the ability of B(C6F5)3 to promote the reduction
of the different functional groups involved in the 12-electron
reduction of oxalic acid to ethane, namely esters, acetals and
ethers. Although ethane is a valuable fuel or chemical for the
production of ethylene, its formation from oxalic acid (or
CO2) is clearly not competitive with its extraction from fossil
feedstocks. The partial reduction of oxalic acid was thus
explored, so as to preserve chemical functional groups in the
products.
1
3 in quantitative yield. The H and 13C NMR spectra of 3 are
in agreement with the reported data and its crystal structure
was determined by X-ray diffraction (Fig. 1).† As depicted in
eqn (3) and (4), with the B(C6F5)3 catalyst, reduction of 3 with
1 equiv. Et3SiH mimics the chemical behaviour of oxalic acid
in the presence of 3 equiv. Et3SiH, thereby confirming the
involvement of a silyloxalate intermediate in the formation of
1. From a mechanistic perspective, previous investigations by
Piers' group have established that the B(C6F5)3-catalyzed
hydrosilylation of carbonyl groups involves the formation of
an ion pair, in which the carbonyl functionality is activated
by coordination to a silylium cation while the active reductant
−
is the HB(C6F5)3 anion.12b It is likely that the formation of
this ion pair is favoured by a polar solvent, therefore account-
ing for the enhanced reaction rates in dichloromethane
(ε = 8.9) compared to benzene (ε = 2.3). It is thus expected that
reduction of 3 to 1 follows this scheme (Scheme 2).
(1)
Fig. 1 X-ray crystal structure of 3, with displacement ellipsoids set at
50% probability. Selected bond lengths [Å] and angles [°]: C2–O4 1.197
(2), C1–O2 1.200 (2), C1–C2 1.524 (2), Si1–O1 1.720 (1), Si2–O3 1.721 (1);
C1–C2–O4 122.12 (13), C2–C1–O2 122.29 (13), C1–C2–O3 111.91 (12),
C2–C1–O1 111.63 (12), Si2–O3–C2 124.38 (10), Si1–O1–C1 123.96 (9).
(2)
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mixture then evolves slowly, until the reductant is fully con-
sumed, to yield a ~1 : 1 mixture of 1 and 4, over 16 h (eqn (6)).
Compound 4 was fully characterized by 1H and 13C NMR spec-
troscopy. 4 is a silylated form of 2-hydroxyacetaldehyde and,
thus, a 6-electron reduction product of oxalic acid. In addi-
tion, using 5.2 equiv. Et3SiH and a greater loading of B(C6F5)3
(5.0 mol%), 4 was obtained in quantitative yield by reduction
of oxalic acid in CH2Cl2, within 1 h at RT (eqn (7)). This reac-
tion chemistry therefore reveals that, in CH2Cl2, the hydrosilyl-
ation of 1 with Et3SiH affords an intermediate with an
increased reactivity towards reduction, leading to the accumu-
lation of 4. In fact, 4 is the end-product in the hydrosilylation
of oxalic acid with Et3SiH and no evolution of ethane (nor
ethoxysilane) was observed when oxalic acid was reacted with
excess Et3SiH (>15 equiv.) and 5.0 mol% B(C6F5)3, even after
48 h at 100 °C.
Scheme 2 Proposed mechanism for the reduction of 3 to 1 based on
mechanistic studies carried out by Piers et al.
(6)
(7)
(3)
(4)
Because the B(C6F5)3-catalyzed hydrosilylation of oxalic
acid is much slower in benzene, a different chemoselectivity
can also be expected when replacing CH2Cl2 with this latter
solvent. Indeed, 5 was formed in 90% yield, after 50 h at
RT, when oxalic acid was reduced with 4.2 equiv. Et3SiH and
7.5 mol% B(C6F5)3 (eqn (8)). Yet, in the reaction mixture, 5
is unstable and it converts slowly to the more stable 6, with
concomitant elimination of Et3SiOSiEt3 siloxane (eqn (8)).
Both 5 and 6 are formally 4-electron reduction products of
oxalic acid and silylated forms of glyoxal and glycolic acid,
respectively. The redox tautomerism at play in the conver-
sion of 5 to 6 is therefore unusual, since both glyoxal and
glycolic acid are stable compounds and they do not inter-
convert at 100 °C. Interestingly, glyoxal conversion to
glycolic acid was reported, for the first time, by Mondelli,
Pérez-Ramirez et al. in 2014.16 The authors showed that,
using zeolites as Lewis acid catalysts, glyoxal could undergo
a Meerwein–Ponndorf–Verley reduction/Oppenauer oxidation
sequence (MPV/O) to produce glycolic acid at 90–100 °C.16
In light of these results, it is likely that the formal 1,2-
hydride shift responsible for the formation of 6 from 5
involves a similar MPV/O mechanism, assisted by catalytic
amounts of B(C6F5)3 or the Et3Si+ silylium cation (Scheme 2).
Importantly, in benzene, 5 and 6 are the end-products of
the hydrosilylation of oxalic acid and 4, ethoxysilane deriva-
tives or ethane is not observed when oxalic acid is reacted
with 10 equiv. Et3SiH.
Importantly, 1 is a reactive intermediate in the reduction
of oxalic acid and it rearranges to triethylsilyl 2-oxoacetate
2 in 31% yield, after 2.5 h at room temperature (eqn (3)
and (4)). 1 and 2 are silylated derivatives of 2-oxoacetic acid
and their formation by reduction of oxalic acid exemplifies
the potential of catalytic hydrosilylation in promoting the
controlled reduction of oxalic acid. Although 2 is unstable
and degrades to unidentified products in CH2Cl2, 1 could be
successfully prepared in quantitative yield from the catalytic
reduction of oxalic acid and 3.5 equiv. Et3SiH, in benzene
(eqn (5)).
(5)
Formally, the 4-electron reduction of oxalic acid can lead
to two isomers, namely glyoxal or glycolic acid (Scheme 1).
Using 4 equiv. Et3SiH, the hydrosilylation of oxalic acid pro-
duces 1 in quantitative yield, within 1 h at 25 °C. The reaction
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Conclusions
Generated by C–C coupling of two CO2 molecules, oxalic acid
affords a desirable platform to access functionalized C2-products.
In this context, we have investigated the partial reduction of
oxalic acid with hydrosilanes. While B(C6F5)3 was successfully
utilized in the hydrosilylation of mono-functional carboxylic
acids, it was shown to be also selective in the reduction of a
geminal di-carboxylic acid. Inexpensive and air-stable PMHS
and TMDS hydrosilanes are able to reduce oxalic acid to eth-
ane at room temperature. In contrast, high chemoselectivity
was achieved in the partial reduction of oxalic acid with
Et3SiH and dissymmetric C2-compounds were successfully
accessed. Depending on the nature of the solvent (CH2Cl2 vs.
benzene), silylated derivatives of 2-oxoacetic acid, glyoxal,
glycolic acid and glycolaldehyde were formed selectively.
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Acknowledgements
For financial support to this work, we acknowledge the CEA,
the CNRS, the CHARMMMAT Laboratory of Excellence and the
European Research Council (ERC starting grant agreement
no. 336467). T.C. thanks the Fondation Louis D. – Institut de
France for its formidable support.
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