Lewis Base Promoted Reduction of CO2 with BH3NH3 into Boryl Formates: CO2 as a Carbon Source in Organic Synthesis Under Mild Conditions

Article abstract of DOI:10.1002/ejoc.201800320

Lewis base promoted selective reduction of CO₂ into boryl formates by using BH₃NH₃ as a reductant under mild conditions has been reported. The boryl formates, generated in situ, were shown to be reactive and versatile sources of formyl compounds to create new C–N, C–O, and C–C bonds. The reactivity of the boryl formates to yield formic acid, formamides, formates, secondary alcohols, and benzoheterocyclic rings was investigated.

Full text of DOI:10.1002/ejoc.201800320

A Journal of
Accepted Article
Title: Lewis Base-Promoted Reduction of CO2 with BH3NH3 into Boryl
Formates: General Approach to Utilize CO2 as a Carbon Source
in Organic Synthesis under Mild Conditions
Authors: Chanjuan Xi, Bo Zhang, Gaixia Du, Wei Hang, and Sheng
Wang
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201800320
Link to VoR: http://dx.doi.org/10.1002/ejoc.201800320
Supported by

10.1002/ejoc.201800320
European Journal of Organic Chemistry
COMMUNICATION
Lewis Base-Promoted Reduction of CO2 with BH3NH3 into Boryl
Formates: General Approach to Utilize CO2 as a Carbon Source in
Organic Synthesis under Mild Conditions**
Bo Zhang,§[a] Gaixia Du,§[a] Wei Hang,[a] Sheng Wang,[a] and Chanjuan Xi*[a][b]
Abstract: Lewis base-promoted selective reduction of CO2 into boryl
formates using BH3NH3 as a reductant under mild conditions has
been reported. The in situ generated boryl formates were shown to
be reactive and versatile sources of formyl compounds to create new
C-N, C-O and C-C bonds. Reactivity of the boryl formates to yield
formic acid, formamides, formates, secondary alcohol, and
benzoheterocyclic ring was investigated.
Figure 1. General Scheme for the Reduction of CO2 Using Hydrosilanes or
Hydroboranes.
Carbon dioxide (CO2) is of great concern as a C1-building
block for the production of chemicals or fuels due to its high
availability, low toxicity, and recyclability.1 Due to its high stability,
well-designed activation of CO2 are required for efficient
transformations. In this respect, in recent years, increasing
To further enlarge the scope of carbon-containing compounds
research efforts have been devoted to promoting the reduction
The formoxy A is the first step reduced product of CO2, which
could be an active and versatile source to platform chemicals.5
accessible from CO2 transformation, we envisioned that the
highly selective formation of the formoxy A using a suitable
reductant to achieve this objective would be prospective,
although the base- or transition metal-catalyzed reduction of
CO2 using hydrosilanes3a,3b,3d or hydrobroanes4b,4c to the
formoxy A have been reported. Recently, several reports on the
catalyst-free reduction of CO2 using hydroboranes have been
reported, some of them have been isolated and characterized
for formation of formyl boranes.3e,4e It is known that ammonia
borane (BH3NH3) is an excellent hydrogen storage material
(hydrogen capacity: 19.6 wt-%) and a easily handed chemical.6
However, the use of BH3NH3 for reduction of CO2 is rarely
reported,7 as an ongoing effort in the capture and utilization of
CO2 in our group,8 herein, we report a base-promoted reduction
of CO2 with BH3NH3 under mild conditions for highly selective
formation of the boryl formate A, and its subsequent use as a
versatile and reactive formyl transfer agent to form not only C−N
and C−O, but also C−C bonds using a one-pot two-step strategy.
The reactivity of CO2 and BH3NH3 was initially performed in
various solvents, such as acetonitrile (CH3CN), tetrahydrofuran
(THF), 1,4-dioxane, toluene, and 1,2-dichloroethane (DCE)
(Table 1, entries 1-5). Trace formic acid was observed after
of CO2 with reductants such as hydrogen,1b-c,2 hydrosilanes,3 or
hydroboranes4 into formates, formaldehydes, methoxides or
methane. However, the reported reactions often required
expensive or complex catalysts such as Ru2b,2c,3c, Rh3b, Ir2h, Pd2a,
Cu4b, Co2d, Fe4g and organo-catalysts such as NHC3g,4l, TBD3a,4m
and FLP4h,4i, whose recycle are difficult, although pyridine-
decorated carbon nanotubes as a metal-free heterogeneous
catalyst for mild CO2 reduction to methanol with hydroboranes
has been reported.4n In addition, most of these processes suffer
from yet high temperature and/or high pressure resulting in low
atomic economy (hydrosilanes or hydroboranes as reductants)
or low selectivity (such as formation of a mixture of formylation
and methylation products with amines). Consequently, the
exploration of catalyst-free, atom economic, high selectivity, and
a general method under mild conditions in functionalization of
CO2 is in accelerating demand. From a mechanistic perspective,
hydrofunctionalization of CO2 using hydrosilanes or
hydroboranes as mild reductants under-goes four consecutively
reductive steps to afford formoxy A, acetal B, methoxy C, and
ultimately methane D, respectively (Figure 1).
,
1
hydrolysis based on the monitored by H NMR, unfortunately. It
may be necessary for addition of a base in this reaction. We
then tried the reaction of CO2 and BH3NH3 in the presence of
o
triethylamine (Et3N) in CH3CN at 80 C (entry 6). To our delight,
the yield of the HCOOD were determined by 1H NMR
integrations in 87% yield in D2O based on the integration of the
[a]
[b]
B. Zhang, G. Du, W. Hang, S. Wang, Prof. C. Xi
Department of Chemistry
MOE Key Laboratory of Bioorganic Phosphorus Chemistry &
Chemical Biology
Tsinghua University, Beijing 100084 (China)
E-mail: cjxi@tsinghua.edu.cn
Prof. C. Xi
State Key Laboratory of Elemento-Organic Chemistry
Nankai University, Tianjin 300071 (China)
Authors contributed equally.
signal for the
formyl group of HCOOD using 1,1,2-
trichloroethene as an internal standard and respecting to
BH3NH3. The reactions were screened in other base such as
KOH, NaOH, t-BuOK, 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU),
and 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) (entries 7-11).
The reactions proceeded well in certain organic bases (entries 6,
10, and 11) and the TBD as a base afforded the the HCOOD in
130 % yield based on the formation of formyl group of HCOOD
using 1,1,2-trichloroethene as an internal standard and
respecting to BH3NH3 (entry 6). The reaction was enlarged to 10
mmol based on BH3NH3, the HCOOD was obtained in 125%
§
[**]
This work was supported by the National Natural Science
Foundation of China (Nos 91645120 and 21472106).
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejoc.201800320
European Journal of Organic Chemistry
COMMUNICATION
Scheme 1. CO2 Hydroboration with BH3NH3 in the Presence of TBD or Et3N..
yield (entry 12). Notably, the borates are formed after hydrolytic
work-up, 11B NMR spectra show one peak around 17-19 ppm.
Table 1. Optimized Reaction Conditionsa
As shown in Table 1 and Scheme 1, CH3CN as solvent and
TBD as a base with a high selectivity formed boryl formates A.
We next tried the reaction in different temperature, reaction time,
and other boranes (such as BH3NMe3, BH3SMe2, and BH3THF)
(see STable 1). The optimized condition for CO2 hydroboration
with BH3NH3 in the presence of TBD were carried out at 80 oC in
CH3CN for 12 h.
Having the first reduced boryl formates A at hand, we then
probed the ability of A to transfer formyl into value-added
chemicals. As described in Scheme 1, the boryl formates A (as
a mixture of A1, A2, and A3) in situ generated by the reaction of
CO2 with BH3NH3 in the presence of TBD in CH3CN at 80 oC for
12 h was treated by acid aqueous solution to afford formic acid 1
in 130% yield based on the BH3NH3 (Scheme 2). The formic
acid is playing a growing role in future non-fossil fuel economy.9
Addition of various amines to the boryl formates A with TBD as a
base and the formamides were obtained in moderate yields
based on the BH3NH3. Both second amines such as 1,2,3,4-
[a] Reaction conditions: BH3NH3 (1 mmol), base (1 mmol), CO2 (1 atm),
solvent (2 mL), [b] The yields were determined by 1H NMR using 1,1,2-
trichloroethene as an internal standard and respecting to BH3NH3. [c] 10 mmol
scale of reaction.
To confirm the structures of the formed boryl formates A. The
tetrahydroisoquinoline,
morpholine,
pyrrolidine,
1-(3-
reaction of CO2 and BH3NH3 were conducted with TBD as a
bromophenyl)-N-methylmethanamine, and primary amines such
as phenylmethanamine, 2-phenylethan-1-amine, and n-
butanamine are tolerated to afford the desired products (4a-4g)
in good yields. Aromatic amines such as N-methylaniline also
afforded formylation product 4h in 60% yield. The reaction of A
with hydrazone afforded formohydrazide 3 in 56% yield. It is
noteworthy that formamide was observed over monitoring the
reaction intermediate, which indicated that ammonia from
BH3NH3 could react with the boryl formates A. Therefore,
addition of ammonia solution to the in situ generated boryl
formates and formamide 2 was obtained in 90% NMR yield. It is
worthy to note that A3 did not participate in the formation of
formamides during the reaction of in situ generated boryl formats
A with amines. Although N-formylation of amines using BH3NH3
and CO2 in high pressure (≥ 10 atm) has been reported,7a this
one-pot two-step procedure for N-formylation of amines
proceeded in an atmospheric pressure.
o
base at 80 C for 12 h. The reaction mixture was monitored by
1
1H NMR, 13C NMR, and 11B NMR spectroscopy. The H NMR
spectrum showed signals located around 8.41-8.24 ppm and
7.99-7.19 ppm, which assigned to –OOCH, B-OOCH, and
HCONH2 (see SFigure 1). The 13C NMR spectrum showed five
peaks located at 167.67, 163.95, 162.87, 152.89, and 151.24
ppm (see SFigure 2), among them, peaks located at 152.89, and
151.24 ppm were identified as quaternary carbons by DEPT-135
(see SFigure 3), which assigned to B-OOCTBD+, respectively.
Other three peaks assigned to the carbon of –OOCH, B-OOCH,
and B-(OOCH)2. The 11B NMR spectrum showed peaks at 2.53
and 1.85 ppm, which indicated that there are -OB-H bonds in the
products (see SFigure 4).4e Furthermore, monitoring of the
1
reaction mixture with H NMR and 11B NMR spectrum at 70 oC
over 3 h, HB-(OOCTBD+)(OOCH)2 grew larger with time (see
SFigure 8 and 9), respectively. Based on the above results,
reaction of CO2 and BH3NH3 in the presence of TBD as a base
afforded three products as shown in Scheme 1. The reaction
was also monitored by NMR in the presence of NEt3 (Scheme 1,
also see SFigure 5-7 and SFigure 10-13). Notably, in the case of
NEt3 as a base, the formyl A1 and A2 were detected, and in this
case, A3 did not observed. The reaction mechanisms were
proposed as shown in SFigure 15-17.
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10.1002/ejoc.201800320
European Journal of Organic Chemistry
COMMUNICATION
In addition, the reaction of the in situ generated boryl formates
Scheme 2. Reaction of in situ generated boryl formates A affording formic
acid and formamidesa
A
with (3-methoxyphenyl)magnesium bromide (4 equiv.)
produced the corresponding secondary alcohol 6 in 40%
isolated (Eq. 1).
Formylation of N-H bond by boryl formates A yielded
formamides 4. The amide function could be reacted in a
cascade reaction with another nucleophile such as an amine,
alcohol. Next, we studied the reactivity of boryl formates A as a
surrogate of formate in representative cascade reactions
(Scheme 4). In situ generated compounds A were reacted at 80
oC with benzene-1,2-diamine to afford 1H-benzo[d]imidazole 7 in
60% isolated yield. The reaction of boryl formates A with 2-
aminophenol
and
2-aminobenzenethiol
afforded
benzo[d]oxazole 8 and benzo[d]thiazole 9 in 53% and 40%
yields, respectively. These results demonstrated that the
complete deoxygenation products (7-9) were available under our
reaction conditions. The catalytically complete deoxygenation of
CO2 into formamidine derivatives has been infrequently
reported.2c,3c
[a] BH3NH3 (1 mmol), TBD (1 mmol), CH3CN (2 mL). [b] D2O (2 mL) and DCl
(2 drops). [c] NH3H2O (3 mL). [d] Hydrazone (1.0 equiv.), isolated yield
respecting to BH3NH3. [e] Amine (1.0 equiv.), NMR yield respecting to BH3NH3.
Scheme 4. Reactions of in situ Generated Boryl Formates
Compounds 7-9a
A affording
Furthermore, direct addition of phenylmethanol to boryl
formates A generated in the presence of TBD and the reaction
did not proceed. To our delight, when TsOH was added and the
formate 5a was obtained in 25% yield based on the BH3NH3.
During this reaction, we found the reaction mixtures became a
slurry. Addition of phenylmethanol to the boryl formates A
generated in situ by the reaction of CO2 with BH3NH3 in the
presence of Et3N and the formate 5a were obtained in 65%
isolated yield. Therefore, esterification reaction was carried out
in the presence of Et3N as a base. A variety of alcohols was
employed in the reaction and formates (5a-5e) were obtained in
good yields (Scheme 3).
Scheme 3. Esterification of in situ generated boryl formates A to afford
formats 5a
[a] BH3NH3 (1 mmol), TBD (1 mmol), CH3CN (2 mL), ben-zene-1,2-diamine, 2-
aminophenol, and 2-aminobenzenethiol was added in 1.0 equiv., respectively,
isolated yield respecting to BH3NH3.
In conclusion, we have demonstrated a base-promated
hydroborylation of CO2 with BH3NH3 in the presence of organic
base under mild conditions to afford boryl formates. The in situ
generated boryl formates could be utilized for synthesis of value-
added products. Direct hydrolysis of the boryl formates afforded
formic acid. The reaction of the boryl formates with amines
(including aniline and hydrazine) and alcohols produced the
corresponding formamides and formates, respectively. Grignard
reagent could also be utilized as nucleophilic reagent to react
with the boryl formates, producing the corresponding secondary
alcohol. Furthermore, the in situ generated boryl formates could
react with benzene-1,2-diamine, 2-aminophenol, and 2-
aminobenzenethiol to form benzoheterocyclic ring. This one-pot
two-step procedure provides a convenient method for the
[a] BH3NH3 (1 mmol), Et3N (1 mmol), CH3CN (2 mL), alcohols (1.0 equiv.),
TsOH (1.0 equiv.).
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10.1002/ejoc.201800320
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oC C for 12 h. The reaction mixture was filtered and evaporated in
vacuum, the residue was purified by column chromatography on silica gel
to give the corresponding products 5.
capture and functionalization of CO2. Further studies of the
reaction mechanism are ongoing in this lab.
Experimental Section
Procedure for the synthesis of bis(3-methoxyphenyl)methanol 6
Procedure for the synthesis of formic acid 1
BH3NH3 (1 mmol, 31 mg), TBD (1 mmol, 140 mg), and 2 mL of CH3CN
were added to a CO2 equipped Schlenk tube. The reaction mixture was
heated at 80 oC for 12 h. Then it was cooled down to room temperature.
Then, CH3CN was removed and followed by addition of THF (3.0 mL)
and (3-methoxyphenyl)magnesium bromide [4.0 mmol, 4 mL (1.0 M in
THF)] under nitrogen. The reaction mixture was kept at room
temperature for 12 h. The mixture was filtered and evaporated in vacuum,
the residue was purified by column chromatography on silica gel to give
bis(3-methoxyphenyl)methanol 6 in 40% yield.
BH3NH3 (1 mmol, 31 mg), TBD (1 mmol, 140 mg), and 2 mL of CH3CN
were added to a CO2 equipped Schlenk tube. The reaction mixture was
heated at 80 oC for 12 h. Then, it was cooled down to room temperature.
The reaction mixture was quenched with diluted DCl solution. The formic
acid 1 was detected in 130% yield by NMR.
Procedure for the synthesis of formamide 2
BH3NH3 (1 mmol, 31 mg), TBD (1 mmol, 140 mg), and 2 mL of CH3CN
were added to a CO2 equipped Schlenk tube. The reaction mixture was
heated at 80 oC for 12 h. Then it was cooled down to room temperature.
The reaction mixture was quenched with saturated NH3·H2O (35% in
aqueous solution, 1 mL). The formamide 2 was detected in 90% yield by
NMR.
General procedure for the synthesis of heterocycles 7-9
BH3NH3 (1 mmol, 31 mg), TBD (1 mmol), and 2 mL of CH3CN were
added to a CO2 equipped Schlenk tube. The reaction mixture was heated
at 80 oC for 12 h. Then it was cooled down to room temperature. Then,
CH3CN was removed and followed by addition of THF (3.0 mL) and 2-
substitued aminobenzene (1.0 mmol) under nitrogen. The mixture was
heated to 80 oC for 12 h. The reaction mixture was evaporated in vacuum,
the residue was purified by column chromatography on silica gel to give
the corresponding heterocycles 7-9.
Procedure
for
the
synthesis
of
N'-
(diphenylmethylene)formohydrazide 3
BH3NH3 (1 mmol, 31 mg), TBD (1 mmol, 140 mg), and 2 mL of CH3CN
were added to a CO2 equipped Schlenk tube. The reaction mixture was
Keywords: boryl formats; CO2; ammoniaborane; formats;
heterocycles
o
heated at 80 C for 12 h. Then it was cooled down to room temperature.
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BH3NH3 (1 mmol, 31 mg), TBD (1 mmol, 140 mg), and 2 mL of CH3CN
were added to a CO2 equipped Schlenk tube. The reaction mixture was
heated at 80 oC for 12 h. Then it was cooled down to room temperature.
Amines (1 mmol) was added to the above the reaction mixture under
nitrogen. The mixture was heated to 80 oC for 12 h. The reaction mixture
was filtered and evaporated in vacuum, the residue was purified by
column chromatography on silica gel to give the corresponding products
4.
General procedure for the synthesis of formates 5
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heated at 80 oC for 12 h. Then it was cooled down to room temperature.
Alcohol (1 mmol) and and p-TsOH (1.0 mmol, 172 mg) were added to the
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European Journal of Organic Chemistry
COMMUNICATION
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This article is protected by copyright. All rights reserved.

10.1002/ejoc.201800320
European Journal of Organic Chemistry
COMMUNICATION
COMMUNICATION
Bo Zhang, Gaixia Du, Wei Hang, Sheng
Wang, and Chanjuan Xi*
Page No. – Page No.
Lewis Base-Promoted Reduction of
CO2 with BH3NH3 into Boryl
Formates: General Approach to
Utilize CO2 as a Carbon Source in
Organic Synthesis under Mild
Conditions
CO2 in Boryl Formates: Lewis base-promoted selective reduction of CO2 into
boryl formates using BH3NH3 under mild conditions has been reported. Boryl
formates can be used to build new C-C, C-O and C-N bonds.
This article is protected by copyright. All rights reserved.