Selective Reductive Dimerization of CO2into Glycolaldehyde

Article abstract of DOI:10.1021/acscatal.1c00412

The selective dimerization of CO2 into glycolaldehyde is achieved in a one-pot two-step process via formaldehyde as a key intermediate. The first step concerns the iron-catalyzed selective reduction of CO2 into formaldehyde via formation and controlled hydrolysis of a bis(boryl)acetal compound. The second step concerns the carbene-catalyzed C-C bond formation to afford glycolaldehyde. Both carbon atoms of glycolaldehyde arise from CO2 as proven by the labeling experiment with 13CO2. This hybrid organometallic/organic catalytic system employs mild conditions (1 atm of CO2, 25 to 80 °C in less than 3 h) and low catalytic loadings (1 and 2.5%, respectively). Glycolaldehyde is obtained in 53% overall yield. The appealing reactivity of glycolaldehyde is exemplified (i) in a dimerization process leading to C4 aldose compounds and (ii) in a tri-component Petasis-Borono-Mannich reaction generating C-N and C-C bonds in one process.

Full text of DOI:10.1021/acscatal.1c00412

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Selective Reductive Dimerization of CO2 into Glycolaldehyde
Dan Zhang, Carlos Jarava-Barrera, and Sébastien Bontemps*
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ABSTRACT: The selective dimerization of CO2 into glycolalde-
hyde is achieved in a one-pot two-step process via formaldehyde as
a key intermediate. The first step concerns the iron-catalyzed
selective reduction of CO2 into formaldehyde via formation and
controlled hydrolysis of a bis(boryl)acetal compound. The second
step concerns the carbene-catalyzed C−C bond formation to
afford glycolaldehyde. Both carbon atoms of glycolaldehyde arise
from CO2 as proven by the labeling experiment with 13CO2. This
hybrid organometallic/organic catalytic system employs mild
conditions (1 atm of CO2, 25 to 80 °C in less than 3 h) and
low catalytic loadings (1 and 2.5%, respectively). Glycolaldehyde is
obtained in 53% overall yield. The appealing reactivity of glycolaldehyde is exemplified (i) in a dimerization process leading to C4
aldose compounds and (ii) in a tri-component Petasis−Borono−Mannich reaction generating C−N and C−C bonds in one process.
KEYWORDS: CO2 reduction, organocatalysis, Formose reaction, dimerization, carbohydrates
Scheme 1. (a) Cn Products Obtained from Electroreduction
and Hydrogenation of CO2 and (b) Synthesis and
Utilization of 4e− Reduction Products as the C1 Source
from CO2 Hydrosilylation7 and as the Cn Source from CO2
Hydroboration8
INTRODUCTION
■
CO2 reductions to Cn>1 products are much less developed than
to C1 compounds due to the intrinsic difficulty of combining
two individually challenging steps in a single process: CO2
reduction and C−C bond formation.1 As a consequence,
further developments of CO2 utilization as the Cn source face
two main issues: selectivity and limited scope of products.1,2
Besides oxalate, acetate, and related products,3 most of the
obtained compounds are highly reduced products: aliphatic
hydrocarbons, olefins, and alcohols (Scheme 1a). In these
electroreduction or hydrogenation systems, CO is considered
as the key bifurcation point toward Cn products. Because of
their high energy density, the synthesized products are of
interest as energy carriers. However, if one wants to use CO2 as
a sustainable Cn source, less reduced polyoxygenated
compounds would be highly desirable (Scheme 1a) because
such compounds, featuring multiple alcohol, ketone, and/or
aldehydic functions, exhibit appealing molecular complexity
leading to versatile reactivity. Their isolation from biomass is
currently being explored for their use as a new chemical
feedstock.4 Finding a new access from CO2 would offer a
complementary sustainable alternative to biomass extraction. A
thorough study on Cu-catalyzed electroreduction of CO2
reported the detection of such C2 and C3 oxygenated
compounds as minor species.5 As indicated in the proposed
mechanism and further complemented by an in-depth
theoretical investigation,6 these compounds cannot be
accumulated because they are further reduced in situ (Scheme
1a) under the reaction conditions.
Received: January 28, 2021
Revised: March 18, 2021
Published: March 31, 2021
We believe that (i) new pathways need to be developed for
the generation and accumulation of these reactive intermedi-
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ates, (ii) homogeneous thermal transformations of CO2 into
Cn compounds would have a key role to play in this objective,9
and (iii) HCHO could be an alternative bifurcation point
toward the Cn products. Mild operating conditions should be
pursued to favor in situ characterization and control of the
reaction. In the field of CO2 reduction, hydrosilane and
hydroborane reductants enabled to use particularly mild
conditions and make significant progress in understanding
the mechanisms as well as in the characterization and
functionalization of reactive intermediates.10 More specifically,
it enabled the reduction of CO2 to the formaldehyde level with
the selective generation of bis(silyl)11 and bis(boryl)acetal12
compounds (Scheme 1b). These reactive intermediates were
proven particularly versatile for the synthesis of a large scope of
different C1 products.8,11c,12d,f,13 While these acetal compounds
were used as formaldehyde surrogates, it is only in a recent
example where Parkin et al. reported the actual release of
formaldehyde from a bis(silyl)acetal compound by adding CsF
at 25 °C or heating the solution to 120 °C in DMF.7 The in
situ generated formaldehyde was then involved in C1
transformations (Scheme 1b).
to calculate the yield in formaldehyde (see Table S2 and Figure
S1 in the Supporting Information for details).
Step 2: Carbene-Catalyzed Formose Reaction of
HCHO in the Presence of Water. We then turned our
attention to the formation of short-chain carbohydrates from
HCHO. The so-called formose reaction corresponds to the
oligomerization of formaldehyde into carbohydrate com-
pounds. While this powerful transformation was discovered
in the 19th century,17 it suffers from numerous side reactions,
leading to mixtures of up to 30 different products when
catalyzed by inorganic bases.18 Net improvements in
controlling side reactions and chain growth were achieved
with the use of NHC-type catalysts, notably with thiazolium
and triazolium precursors.18,19 In these studies, it is clear that
formose outcome is very sensitive to the reaction conditions
(solvent, NHC, and reaction time); however, (i) beyond the
accepted general mechanism via the Breslow intermediate,
more defined rationales to explain such variations are
lacking19c−e and (ii) THF and H2O are two solvents that
have been sparingly used.19e,20
In this context, NHC 2 and 3 were selected for initial tests
with commercial para-formaldehyde in DMF, THF, and THF/
H2O media (Table 1). The results reported by Teles et al. in
DMF were first reproduced with catalyst 2, showing that 30
and 60 min were necessary to reach full conversion.
Glycolaldehyde (C2) was the main product after 30 min
while a mixture of C2−4 carbohydrates was observed after 60
min with 0.5 mol % catalyst loading (Table 1, entries 1−3).19e
Two formose reactions were reported in THF with triazolium
precursors, slightly different from 2, leading to the favored
formation of glycolaldehyde in 62 and 46% yields.19e,f We thus
explored the reaction in THF with catalysts 2 and 3. The
reaction was almost complete after 30 min: 88 and 93% total
yield with 2 and 3, respectively (Table 1, entries 5 and 11).
Interestingly, 3 favored the dihydroxyacetone (C3 ketose) with
a measured 67% yield after 30 min, and 2 favored the C4
aldoses (erythrose and threose) with combined yields of 72
and 78% after 30 and 60 min, respectively. C4 ketoses were
never detected. To the best of our knowledge, these latter
results are the best yields and selectivity reported for C4
aldoses. Teles et al. indeed reported two reactions in DMF
in which the C4 carbohydrates were the major products with
yields of 50 and 16% with triazolium and imidazolium
precursors as catalysts, respectively.19e,f We then probed the
impact of the addition of H2O which was reported detrimental
for the reaction yield and the selectivity in DMF/H2O
thiamine-based catalysis.20 In accordance with these data, we
observed a general negative impact of H2O on the yield.
However, while the addition of 10 equiv of water completely
shut down any conversion with catalyst 3 (Table 1, entry 12),
we were pleased to observe a 56% total yield of C2−4 aldose
products with catalyst 2 after 30 min in the same conditions.
The addition of 20 and 40 equiv of water or 1 equiv of
methanol afforded lower total yields (Table 1, entries 8−10).
With these positive results from HCHO in the presence of
water in hands, we then decided to explore this transformation
from CO2.
In the last 10 years, we developed 4e− reductions of CO2
with hydroboranes14 and applied it to C1 chemistry.12f,15 More
recently, we aimed at using this process for the transformation
of CO2 into Cn products. A first example of the synthesis of a
C3 compound was reported from the stoichiometric reaction of
an N-heterocyclic carbene (NHC) with a bis(boryl)acetal
compound.8 However, the coupling step was not catalytic and
the yield (30%) remained modest. Herein, we present the first
catalytic system enabling to generate selectively glycolaldehyde
from the CO2 reductive dimerization in 62% yield from a
bis(boryl)acetal compound.
RESULTS AND DISCUSSION
Step 1: Formaldehyde Release from bis(Boryl)acetal.
As reported earlier, bis(boryl)acetal 1 (Scheme 2) is generated
■
Scheme 2. Step 1: Release of HCHO from the Reactivity of
1 with D2O
from the selective 4e− reduction of CO2 with 9-BBN using
mild conditions (25 °C, 1 atm of CO2, 45 min) and 1 mol % of
the iron complex Fe(H)2(dmpe)2.12f The first objective was to
find a pathway to generate formaldehyde from bis(boryl)acetal
1 under mild and compatible conditions for the subsequent
organocatalyzed coupling step (vide infra). While partial
release of HCHO was mentioned in the literature from a
bis(boryl)acetal featuring pinacolboryl moieties,12d,14b,16 no
system actually reported quantitative generation of HCHO
from the bis(boryl)acetal compound as in Parkin’s example
from a related bis(silyl)acetal.7 In this context, compound 1
was subjected to 1, 2, and 10 equivalents of water (Scheme 2).
While 1 and 2 equiv led to partial release of formaldehyde in
24 and 72% yield, respectively (even after 24 h), the use of 10
equiv afforded formaldehyde in 94% yield within 30 min.
Synthesis of Glycolaldehyde from CO2. We combined
the two reactions developed above in a one-pot system. The
conditions deduced from these independent studies are as
follows: bis(boryl)acetal 1 generated in situ in THF was
hydrolyzed with 10 equiv of water at 25 °C in 1 h. NHC 2
(0.5%) was then added to catalyze the formose reaction at 80
1
Formaldehyde was detected by H NMR analysis both as free
formaldehyde (singlet at 9.58 ppm) and as hydrated or
oligomeric formaldehyde (broad signals at 4.5−5.5 ppm). Both
sets of resonances were measured against an internal standard
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a
Table 1. Step 2: Formose Reaction in DMF, THF, and THF/H2O Media with Catalysts 2 and 3
b
b
b
entry
time min
NHC
solvent
DMF
DMF
DMF
THF
THF
THF
THF + H2O
THF + H2O
THF + H2O
C2 , %
C3 , %
C4 , %
total yield %
1
2
3
4
5
6
7
8
10
30
60
10
30
60
30
30
30
30
30
30
2
2
2
2
2
2
2
2
2
2
3
3
33
62
38
16
16
17
27
31
17
37
12
33
96
100
16
88
100
56
31
17
40
93
0
34
28
34
72
78
11
5
18
c
d
e
9
f
10
11
12
THF + MeOH
THF
THF + H2O
3
67
4
c
a
b
Reaction conditions: 0.5 mmol of paraformaldehyde, 0.0025 mmol of carbene, 5 mL of THF (0.1 M), and 80 °C. GC yields obtained after
c
d
derivatization reactions. 10 equiv of water was added relative to the HCHO amount. 20 equiv of water was added relative to the HCHO amount.
e
f
40 equiv of water was added relative to the HCHO amount. 1 equiv of MeOH was added relative to the HCHO amount.
°C in 30 min (Scheme 3). To our delight, glycolaldehyde was
temperature (temp) of formose reaction. For each parameter,
standard (s) and determined optimal (o) conditions are
indicated among the different conditions tested (in black
color). Details on observed trends for each parameter are
described hereafter in the order indicated in Scheme 4a, while
detailed yields are given in the Supporting Information. A
significant yield improvement of about 10% was observed
when the initial concentration of 1 in THF was divided by half
([1] = 0.05 M). This observed trend is consistent with the
common observation of lower chain length selectivity with
lower concentration of formaldehyde in formose reaction.19c−e
The hydrolysis time of the acetal 1 (t1) had a negligible impact
on the yield when it was varied from 30 min to 24 h. It
indicates that compound 1 is readily hydrolyzed in these
conditions. However, we observed improved yields in
glycolaldehyde when 40 equiv of water were used instead of
2, 5, 10, 20, or 80. Since the hydrolysis time of the acetal 1 (t1)
had no impact on the yield, we believe that the increased yield
with 40 equiv of H2O was not due to a better hydrolysis
process but rather to a dilution factor. Adding 40 equiv of H2O
indeed led to a decrease of the concentration by about half,
which was shown to be beneficial. We confirmed this
assumption by varying the amount of equiv of H2O added
while maintaining [1]. In this case, yield improvement was
attenuated but still in favor of the addition of 40 equiv of H2O.
Although the observed +4% increase is marginal, we kept 40
equiv of H2O as an optimized condition, while maintaining
concentration [1] = 0.05 M. When catalyst loading [2] was
varied from 0.1 to 30 mol %, a narrow variation of 10% was
observed between the highest and the lowest yields. Catalyst
(2.5 mol %) loading was found to be optimal. It first indicates
that the catalyst is very active under these conditions. It also
shows that if H2O has a detrimental impact, its excess
compared to the amount of catalyst (3−4 orders of
magnitude) is too large to be compensated by a 10-fold
increase of the catalyst loading. Finally, the standard conditions
generated in 37% yield under these standard conditions. The
Scheme 3. One-Pot System under Initial Standard
Conditions with Catalysts 2−6
yield is the average yield of four runs with a measured
maximum deviation (Δmax) of 7%. C3 and C4 carbohydrates
were detected in 0.4 and 0.6% yield, respectively. The reaction
was thus particularly selective since no other carbohydrate was
detected, although the yield remained modest. The reaction
took place only in the presence of carbene catalyst 2 or its
triazolium precursor 6. Their absence or the use of carbenes
3−5 did not afford any detectable C2, C3, or C4 products.21
Future work will be dedicated to understand the specificity of
carbene 2 in protic media, keeping in mind the recent
questioning about organocatalyzed Umpolung reactions.22
In order to improve the glycolaldehyde yield, reaction
conditions were optimized using catalyst 2. The optimization
was conducted in two stages: (i) variation of one parameter at
a time (Scheme 4a) and (ii) combination of optimal
conditions and careful comparison of the obtained yields
between standard (s) and optimized (o) conditions (Scheme
4b).
First stage (Scheme 4a): six parameters were optimized: the
initial concentration of compound 1 in THF, the time t1 of the
hydrolysis, the number of equivalents of D2O added for
hydrolysis, the catalyst loading in 2, and the time (t2) and
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Scheme 4. Optimization for the Synthesis of Glycolaldehyde: (a) One-Factor-at-a-Time Study over Six Parameters: (s)
Indicates the Standard Condition and (o) Indicates the Optimal Condition; and (b) Comparison of Yields between Standard
and Optimized Conditions
1
1
Figure 1. Selected areas of H and H{13C} NMR analyses of the crude mixture obtained from the developed 13CO2 reductive coupling process.
for formose reaction time and temperature (t2 = 30 min, temp
= 80 °C) are also the optimal conditions in accordance with
the literature. t2 = 25 or 30 min enabled to record the best
yields, while shorter or longer reaction time had a negative
impact. We confirmed that the reaction took place at 100 °C
but with a slightly lower yield, while expectedly, it did not
proceed at room temperature.
Second stage (Scheme 4b): The presented general
optimization methodology relies on a classical one-factor-at-
a-time method. Although this is a classical method,
combinations of effect are then not considered.23 In order to
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properly assess the impact of the optimized conditions, the
given yields are the average of four runs. In each of them, the
solution containing the in situ generated 1 was split into two
containers which were then subjected in parallel to the
standard conditions (s) for one container and to the optimized
conditions (o) for the other one. With these experiments, a
34% yield (Δmax = 4%) was measured under the standard
conditions in the same range as that initially obtained (i.e.,
37%, Δmax = 7%). Gratifyingly, the yield obtained under the
optimized conditions is calculated to be 62% with a maximum
deviation of 3%. The optimization study thus led to almost
doubling the product yield. Considering the system including
the CO2 reduction step, we developed herein a hybrid one-pot
organometallic/organic system transforming CO2 into glyco-
laldehyde in an overall 53% yield based on hydroborane used
under mild conditions in less than 3 h.
aqueous solution.26 The dimerization of glycolaldehyde was
then conducted with Zn(L-Pro)2 as a catalyst.27 Increasing the
reaction temperature to 50 °C enabled to generate erythrose
(8, 40%) and threose (9, 24%) for a total GC yield of 64% of
C4 aldoses.
CONCLUSIONS
■
In the present publication, the selective dimerization of CO2
into glycolaldehyde is reported for the first time. To achieve
such transformation, particularly mild conditions (1 atm of
CO2, 25 to 80 °C in <3 h) and low catalytic loadings (1 and
2.5%, respectively) were employed with a hybrid organo-
metallic/organic catalytic process. The key aspects of the
presented work rely on the quantitative release of form-
aldehyde from the hydrolysis of a bis(boryl)acetal compound
compatible with the C−C coupling step. Intensive optimiza-
tion led to the generation of glycolaldehyde in 62% yield from
compound 1 and in 53% overall yield from CO2 based on the
amount of hydroborane engaged. The ultimate proof of CO2
being the sole source of carbon was obtained with the labeling
experiment using 13CO2. This homogeneous thermal CO2
process shows that (i) bis(boryl)acetal obtained from mild
CO2 reduction enables a selective and mild access to
formaldehyde, (ii) acetal compounds can be involved in the
generation of Cn compounds, and (iii) carbohydrates such as
glycolaldehyde are valuable reactive feedstocks to be
synthesized from CO2.4e,25 On a broader perspective, the
presented strategy highlights that formaldehyde offers an
appealing new synthetic access to the Cn products of
intermediate reduction stage. Although, in the present case,
the boron-based reductant system is not sustainable and the
formose control only applies to the smallest carbohydrate, we
believe that the strategy presented herein could stimulate
different fields of CO2 reduction.
Synthesis of 13C-Labeled Glycolaldehyde. As a mean to
prove that glycolaldehyde is indeed formed from the reductive
coupling of two molecules of CO2 and to generate valuable
13C-labeled glycolaldehyde, we conducted the reaction using
13CO2. The multiple forms adopted by glycolaldehyde in
solution (monomeric, symmetrical, and unsymmetrical oligo-
meric and hydrated forms) complexify its NMR analysis (see
the Supporting Information).5,24 We present herein the
characterization of the simple monomeric aldehydic form in
the crude THF-d8/D2O mixture. In 13C{1H} NMR analysis,
this species is characterized at δ 203.3 (d, 1JC−C = 41.1 Hz) and
1
69.7 (d, JC−C = 41.1 Hz). The two carbon centers correlate
with the aldehydic and methylenic protons characterized at δ
1
3
9.60 (dd, 1H, JH−C = 173.2 Hz, JH−C = 26.9 Hz) and 4.01
1
3
(dd, 2H, JH−C = 140.9 Hz, JH−1C = 4.0 Hz), respectively.
Figure 1 discloses the stacking of H NMR analysis as well as
the selective and broad-band H{13C} spectra of the selected
1
areas. It enables to visualize that each proton correlates with
1
3
both carbon nuclei of the molecule with observed J and J
scalar couplings, the selective 13C decoupling further high-
lighting these correlations. Importantly, no 12C-labeled
glycolaldehyde was detected in the crude mixture by 1H
NMR, indicating that every molecule of glycolaldehyde
quantified by GC analyses arises from CO2 reductive coupling.
In Situ Transformation of Glycolaldehyde. Carbohy-
drates and glycolaldehyde in this instance are attractive species
because of their reactive nature.4e,25 In order to exemplify this
feature, glycolaldehyde in situ generated from CO2 was
engaged in two reactions depicted in Scheme 5a,b. The tri-
component Petasis−Borono−Mannich reaction was conducted
in water to afford compound 7, isolated in 37% yield in
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acscatal.1c00412.
Experimental details, compounds’ synthesis and charac-
terization, and description of the optimization procedure
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
Sébastien Bontemps − LCC-CNRS, Université de Toulouse,
CNRS, Toulouse 31077, France; orcid.org/0000-0002-
4950-9452; Email: sebastien.bontemps@lcc-toulouse.fr
Scheme 5. Reactivity of In Situ Generated Glycolaldehyde:
(a) Synthesis of 7 by Petasis−Borono−Mannich Reaction
and (b) Synthesis of C4 Carbohydrates
Authors
Dan Zhang − LCC-CNRS, Université de Toulouse, CNRS,
Toulouse 31077, France
Carlos Jarava-Barrera − LCC-CNRS, Université de Toulouse,
CNRS, Toulouse 31077, France
Complete contact information is available at:
https://pubs.acs.org/10.1021/acscatal.1c00412
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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Research Article
of Isotopically Labeled C1 Moieties Derived from Carbon Dioxide
into Organic Molecules. J. Am. Chem. Soc. 2019, 141, 17754−17762.
(8) Béthegnies, A.; Escudié, Y.; Nunez-Dallos, N.; Vendier, L.;
̃
Notes
The authors declare no competing financial interest.
Hurtado, J.; del Rosal, I.; Maron, L.; Bontemps, S. Reductive CO2
Homocoupling: Synthesis of a Borylated C3 Carbohydrate. Chem-
CatChem 2019, 11, 760−765.
(9) (a) Cui, M.; Qian, Q.; He, Z.; Zhang, Z.; Ma, J.; Wu, T.; Yang,
G.; Han, B. Bromide promoted hydrogenation of CO2 to higher
alcohols using Ru-Co homogeneous catalyst. Chem. Sci. 2016, 7,
5200−5205. (b) Tominaga, K.-I.; Sasaki, Y.; Saito, M.; Hagihara, K.;
Watanabe, T. Homogeneous RuCo bimetallic catalysis in CO2
hydrogenation: The formation of ethanol. J. Mol. Catal. 1994, 89, 51−
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ACKNOWLEDGMENTS
■
Chinese Scholarship Council is acknowledged for the PhD
fellowship of D.Z. The ANR (ICC-ANR-17-CE07-0015) and
CNRS-DR14 are acknowledged for the financial and technical
support.
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