Ammonia borane enabled upgrading of biomass derivatives at room temperature

Article abstract of DOI:10.1039/d0gc02372h

Simplifying biomass conversion to valuable products with high efficiency is pivotal for the sustainable development of society. Herein, an efficient catalyst-free system using ammonia borane (AB) as the hydrogen donor is described, which enables controllable reaction selectivity towards four value-added products in excellent yield (82-100%) under very mild conditions. In particular, the system is uniquely efficient to produce γ-valerolactone (GVL) at room temperature. Combined in situ NMR and computational studies elucidate the hydrogen transfer mechanism of AB in methanol, the novel pathway of GVL formation from levulinate in water, and a competitive mechanism between reduction and reductive amination in the same system. Moreover, carbohydrates are converted directly into GVL in good yield, using a one-pot, two-step strategy. Products of a rather broad scope are prepared within a short reaction time of 30 min by using this catalyst-free strategy in methanol at room temperature. This journal is

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Ammonia borane enabled upgrading of biomass derivatives at
room temperature
Received 00th January 20xx,
Accepted 00th January 20xx
Wenfeng Zhao,a,b Sebastian Meier,*b Song Yang,*a and Anders Riisager*b
DOI: 10.1039/x0xx00000x
Simplifying biomass conversion to valuable products with high in order to achieve a tunable upgrading of biogenic resources to
efficiency is pivotal for the sustainable development of society. a broad range of useful molecules.
Herein, an efficient catalyst-free system using ammonia borane Among the chemicals derived from biomass, levulinic acid (LA)
(AB) as the hydrogen donor is described, which enables controllable and its esters are particularly easily accessible as they can be
reaction selectivity towards four value-added products in excellent formed directly from lignocellulose-based feedstocks.[3]
yield (82-100%) under very mild conditions. In particular, the Levulinic acid and its esters are widely utilized to produce γ-
system is uniquely efficient to produce -valerolactone (GVL) at valerolactone (GVL), a prospective fuel and green solvent.[4] In
room temperature. Combined in situ NMR and computational addition, LA is an intermediate of potential industrial
studies elucidated the hydrogen transfer mechanism of AB in importance in producing various chemicals including 5-
methanol, the novel pathway of GVL formation from levulinate in methyl pyrrolidinone (MPD), which is a good organic solvent,
water, and a competitive mechanism between reduction and dispersant, and surfactant.[5] A reducing agent such as formic
reductive amination in the same system. Moreover, carbohydrates acid, alcohol, hydrogen or hydrosilane is necessary for the
are converted directly into GVL in good yield, using a one-pot, two- conversion of levulinic acid (ester) into GVL and MPD. However,
step strategy. Products of a rather broad scope are prepared within certain shortcomings impede the large-scale application of the
a short reaction time of 30 min by using this catalyst-free strategy above hydrogen sources. For example, the use of formic acid
in methanol at room temperature.
requires materials and catalysts with adequate acid resistance,
while the hydrosilane system suffers from high price and the
need for post-treatment.[6] The use of alcohol as the hydrogen
donor necessitates high reaction temperature, and use of
molecular hydrogen suffers from operational challenges and
from defiance pertaining to storage safety. [7],[8] Moreover, the
use of metal-containing catalysts and the use of excess
hydrogen source are often unavoidable in current conversion
strategies. Finally, it has remained difficult to control the
reaction selectivity towards either reduction or reductive
amination of the carbonyl group in the same reaction system.
Ammonia borane (H3N-BH3, AB) is an attractive solid hydrogen
source with a remarkable fraction of available hydrogen (19.6
wt%) besides being water-soluble, nontoxic, and stable.[9],[10]
The AB molecule is highly polar and is capable of selectively
reducing imines to amines,[11] α, β-unsaturated aldehydes to α,
β-unsaturated alcohols,[12] and aldehydes or ketones to
Bio-refinement of abundant biomass to liquid fuels and valuable
chemicals is promoted as a sustainable strategy to reduce
anthropogenic CO2 emissions.[1] Dedicated efforts have focused
on the transformation of lignocellulose-derived raw materials
into promising platform molecules, including furan-containing
compounds, keto-acids, lactone, lactams, diols, and primary
amines.[2] In order to obtain most of the above target chemicals,
the elaborate design of a suitable catalyst is indispensable,
usually accompanied by the use of relatively harsh reaction
conditions. For the future biorefinery, it will be essential to
develop energy-saving strategies and simple synthesis methods
a. State Key Laboratory Breeding Base of Green Pesticide
& Agricultural
Bioengineering, Key Laboratory of Green Pesticide & Agricultural Bioengineering
Ministry of Education, State-Local Joint Laboratory for Comprehensive Utilization
of Biomass, Center for Research & Development of Fine Chemicals Guizhou alcohols[13] by transfer hydrogenation under very mild reaction
University, Guiyang 550025, P. R. China
conditions. In light of these previous findings, we hypothesized
b. Centre for Catalysis and Sustainable Chemistry, Department of Chemistry,
that AB should be a clean hydrogen source with attractive
selectivity and activity for the reductive conversion of biomass-
derived compounds. Hence, the current study introduces a
versatile catalyst-free reaction system, composed of AB as the
Technical University of Denmark, Kemitorvet Building 207, DK-2800 Kgs. Lyngby,
Denmark
*Correspondence to:: semei@kemi.dtu.dk; jhzx.msm@gmail.com; ar@kemi.dtu.dk
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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hydrogen source in a protic solvent. This system allows methanol can be used as solvent for production of the
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DOI: 10.1039/D0GC02372H
controllable conversion of biomass-derived levulinic acid (ester) corresponding alcohol HPT.
into value-added compounds such as 4-hydroxypentanoic acid
or esters, 4-aminopentanoic acid (APA), GVL, and MPD in
excellent yields (Scheme 1) under unprecedentedly mild
reaction conditions with a tunable selectivity towards the four
individual products by appropriate optimization of the reaction
conditions. As far as we are aware, this is the first time that AB
utilization is described for the reductive conversion of
carbohydrate-derived keto-acids to value-added compounds.
H
O
N
O
O
92%
Water
4 h
95%
O
Heat
H
H
H
H
OR
H
N
B
H
NH3,Methanol
Room temperature
+
Room
temperature
Methanol
15 min
O
NH2
OH
OR
OR
Catalyst-free
O
O
82%
100%
Figure 1. Conversion of ethyl levulinate in water over time.
Scheme 1 Reductive conversion of biomass keto-acid into Reaction conditions: 1 mmol EL, 1 mmol AB, 2 mL water, and
lactam and lactone using a system proposed herein.
room temperature
O
OH
OH
Optionally, a variety of carbohydrates can be directly converted
into GVL in a one-pot, two-step strategy with a first step
employing commercial solid acid catalysts. A broad reaction
O
O
AB
AB
H2O
O
O
OH
rt, H2O
rt, H2O
O
O
O
HPA
EL
GVL
HPT
scope was validated for the AB system by producing various Scheme 2. The conversion pathway of EL in water.
alcohols in neat methanol at room temperature. Moreover,
density functional theory (DFT) calculation and in-situ NMR The AB system was also evaluated with LA as the substrate, but
were used to elucidate the reaction mechanism.
here GVL was not obtained neither in water nor in MeOH at
room temperature opposite to EL. Instead, HPA formed as the
dominating product. This suggested that the carboxylic acid
Results and discussion
In a first set of experiments, the upgrading of ethyl levulinate group of LA altered the reaction pathway and prevented the
(EL) to GVL was investigated using AB as the hydrogen donor in cyclization of HPA, instead of resulting in easy lactonization
water at room temperature (rt). Reaction parameters such as from the HPT ester intermediate.[15] Besides HPA, a non-
reaction time (Figure 1) and AB dosage (Figure S1) allowed the negligible amount of APA product was also found after reaction
GVL yield to improve from 57 to 95% when increasing the in neat MeOH solvent at room temperature (Figure S5).
reaction time from 0.5 to 4 h, while prolonged reaction time Likewise, MPD formed upon heating thus demonstrating that
decreased the yield due to GVL hydrolysis into 4- AB decomposed to ammonia and aminated the keto group in
hydroxypentanoic acid (HPA) (Figure S2 and Scheme 2). The LA.
optimal reaction conditions yielded 95% GVL at room The serendipitous observation of LA amination stimulated
temperature (1 equiv. AB, 4 h). To the best of our knowledge, subsequent attempts to introduce additional ammonia sources
this is the most benign system reported for GVL synthesis and a into the system to enhance the yield of APA and MPD, including
first example demonstrating the catalyst-free synthesis of GVL various ammonium compounds and solvent-dissolved ammonia
in water.
(Figure S6 and Table S3). The results demonstrated that
Additionally, the water solvent allowed simplified isolation of ammonium carbonate and ammonium acetate led to high MPD
the GVL by extraction with ethyl acetate (Figure S3). The use of yields of 50-70% in methanol (Table S3, Entries 1 and 2), while
organic solvent instead of water resulted under else identical methanolic ammonia was an even better ammonia source
reaction conditions in no or low GVL yield, but facilitated affording a yield of 74% of MPD (Table S3, Entry 5). Reaction
quantitative formation of ethyl 4-hydroxypentanoate (HPT) time and temperature were further optimized with methanolic
(Table S1, Entry 1-5). Especially, methanol (MeOH) resulted in ammonia (16 wt.%), and resulted in an MPD yield of 92% after
quantitative HPT formation in only 15 min (Entry 6), while 12 h of reaction at 120 °C (Table S3, Entry 10) and an APA yield
heating of the reaction systems with alcoholic solvent was of 82% after 2 h of reaction at room temperature (Table S3,
needed for high yields of GVL to be obtained (Figure S4 and Entry 13). The influence of ammonia content in methanol on
Table S2). This observation is consistent with the fact that the MPD selectivity was further evaluated (Figure 2). The evaluation
formation of a five-membered lactone ring is entropically illustrated that the selectivity towards MPD improved with
favored.[14] Accordingly, water may be used as a green solvent increasing ammonia content (maximum with 12-16 wt.%
for the selective synthesis of GVL, while the more volatile ammonia), implying that the ammonia addition resulted in
higher yields of MPD at the expense of GVL and HPA. Hence, the
2 | J. Name., 2012, 00, 1-3
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use of AB as hydrogen source overall afforded (1) formation of Scheme 3. Novel plausible reaction mechanism forVfieowrmArtaictleioOnnlionef
HPT with a yield near 100% in methanol at room temperature, GVL in water with AB as the hydrogen soDuOrcI:e1.0.1039/D0GC02372H
(2) formation of GVL with an excellent yield of 95% in water at
room temperature, (3) formation of APA with a yield of 82% at To corroborate the proposed mechanism, in situ NMR
room temperature, and (4) formation of MPD with a yield of spectroscopy was applied to monitor the conversion of EL in
92% at 120 °C.
water by acquiring a series of 1D NMR spectra in a pseudo-2D
manner. The in situ 13C NMR spectra of the GVL formation in
water (Figure 3a) confirmed that EL was almost completely
consumed within 15 min and that GVL was generated from
intermediates, including HPT and derivatized HPT formed in
situ. The 1H-13C HSQC spectra (Figure 3c) of the conversion of EL
in water showed the presence of both borate ester (Int-2) and
HPT intermediates. The in situ 13C NMR spectra with HPT as the
substrate under identical reaction conditions (Figures 3b and
3d) also revealed that HPT was converted slower into GVL than
EL as substrate. Thus, HPT required a longer reaction time (20
h) than EL to reach full conversion, indicating that HPT may not
be the dominating intermediate during the formation of GVL.
Moreover, in situ 11B NMR spectroscopy of EL conversion to GVL
in water detected the formation of boric acid (11.0 ppm)[18]
from AB (Figure S7). These spectroscopic results are consistent
with the proposed reaction pathway shown in Scheme 3 for the
Figure 2. The effect of ammonia content on the production of production of GVL in water at room temperature. To compare
MPD. Reaction conditions: 1 mmol LA, 1 mmol AB, 4 mL MeOH the product distribution in different solvents, in situ 13C NMR
containing ammonia, 120 °C, and 4 h
spectroscopy of EL hydrogenation was performed in methanol
(Figure S8). Here, HPT was observed as the only main product
The remarkably efficient formation of GVL from EL using AB as whereas no GVL was found. Obviously, water and methanol as
hydrogen-donor in water prompted us to study the polar solvents enable EL to form HPT and borate ester in the
corresponding reaction mechanism. Previous investigations presence of AB, while only the water system shows unique
have validated that the AB hydrolysis product in water is boric reactivity toward the production of GVL, while methanol cannot
acid and the alcoholysis product in methanol is trimethyl borate promote cyclization of borate ester into GVL. However, GVL was
(Scheme S1).[16] Thus, one could hypothesize that boric acid accessible when adding water into the methanolic reaction
might be able to promote the cyclization of HPT. To evaluate mixture affording 80% yield at room temperature (Scheme S3),
this hypothesis, HPT was used as reactant to produce GVL with which further illustrated that water played a key role in the
boric acid rather than AB in water and methanol (Scheme S2). hydrolysis of Int-2 and the production of GVL.
The results revealed no GVL formation in methanol nor in water,
demonstrating that boric acid was not promoting the cyclization
of HPT to GVL. Instead, based on the obtained data from the
formation of GVL from EL in water, and consistent with previous
studies,[11,17] we propose a novel, plausible reaction mechanism
for the GVL formation using AB as a hydrogen donor (Scheme
3). In this mechanism, borane (or diborane) generated by
hydrolysis of AB initially forms the intermediate Int-1 by
hydroboration of EL. Subsequently, Int-1 react with LA to
generate Int-2, which in water quickly converts into GVL
through hydrolytic cyclization with co-formation of boric acid
and ethanol.
Figure 3. In situ NMR spectroscopic observations consistent
with the reaction mechanism proposed in Scheme 3. a) In situ
13C NMR spectroscopy of GVL formation from EL via HPT and
borate ester Int-2 in water (0.5 mmol EL, 1 mL water, 0.5 mmol
AB, and room temperature). b) In situ 13C NMR of GVL
production from HPT in water (0.5 mmol HPT, 1 mL water, 0.5
mmol ammonia borane, and room temperature). c) 1H-13C
HSQC spectra of post-reaction mixture from a). d) 13C NMR
spectra of post-reaction mixture from b).
H
H
H
H2B
O
N
O
O
H
+
H
H
O
NH4
H
H
H
H
H
H
O
O
H
H
+
B
H
H
N
B
H
B
H
OH-
O
O
Int-1
H
H
O
O
O
O
OH
H
H
O
HO
B
O
H
O
H
B
OH
O
O
H
H
H
O
O
O
O
O
O
B
O
H
O
H
CH3CH2OH
O
O
O
O
Int-2
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As detailed above, the hydrogenation of LA and its ester in To further support the DFT calculations, deuterium labeling
View Article Online
methanol exhibited high selectivity towards HPA or the experiments were conducted usiDnOgI: 10d.1e0u3t9e/Dra0tGeCd0237A2HB
corresponding ester, while reductive amination could be isotopologues including NH3BD3 (AB(D)), ND3BH3 (A(D)B), and
achieved when conducting the reaction in methanol containing ND3BD3 (A(D)B(D)) to convert LA into GVL and MPD. As shown
16 wt.% ammonia. Consistent with the reactivity of AB in in Figure S9-S14, MPD was generated with high fractions of 100
methanol,[11,18b,19] four possible reaction pathways were amu m/z and 101 amu m/z when employing AB(D) as the
considered for the production of MPD and GVL, as shown in reductant in normal and deuterated methanol, respectively,
Scheme 4. The mechanism contains two ambiguous points, as while only product with 99 amu m/z was afforded with A(D)B or
1
(1) the mode of AB action could be based either on single- AB as the reductant. Additionally, the H NMR spectra of GVL
hydrogen-transfer (SHT) or on double-hydrogen-transfer (DHT), formed by using AB(D) showed the loss of coupling in the methyl
and (2) the competition between reduction of the carbonyl group with chemical shift of 1.4 ppm, and the disappearance of
group and reductive amination remained unclear.
the characteristic signal at 4.5 ppm upon incorporation of a
deuterium during carbonyl reduction (Figure S15). This
observation confirms that the two protons of MPD derived from
borane and alcohol groups, rather than from the nitrogen part
of the AB molecules. Accordingly, these results unambiguously
showed that the predominant conversion route followed the
SHT mechanism.
In the SHT mechanism, the free energy of ts3 (11.4 kcal/mol) in
the reduction of the carbonyl group of LA is higher compared to
the free energy of ts4 (3.8 kcal/mol) in the reduction of the
imine group of IPA, implying that the latter reduction is more
likely. However, the amination of LA to IPA passes transition
states ts1 (37.0 kcal/mol) and ts2 (59.1 kcal/mol) with high-
energy barriers (Scheme S5) in the production of APA. Thus, the
hydrogenation of LA to HPA is more favorable than the
reductive amination of LA to APA, owing to the high free energy
of ts1 and ts2 during the formation of IPA. Nevertheless,
previous reports and our experimental findings show that the
O
H
N
H
MeOBH2NH2H
OH
H
N
H
N
H
H
H
H
NH3
+
+
O
O
O
MeOH
H
N
O
O
H
B H
OH
N
or
H
H2O
MeOH
OH
H
OH
NH
O
H
17.1 kcal/mol
O
H
H
N
IPA
H
H
DHT
H
MeOH
O
H
H
B H
H
OH
O
MeOH
O
OH
MeOH, LA
DHT
NH3-BH3
O
H2O
+
O
MeOBH2NH2HH
15.0 kcal/mol
Me
H
H
O
H
H
H
O
H
B
O
O
H
OH
O
LA
SHT
BH3-MeOH
NH3
OH
BH2OMe
H
O
O
CH3OH
11.4 kcal/mol
NH
SHT
Me
OH
O
H
H
N
H
H
OH
H
H
B
H
O
O
N
O
HN
O
or
H
N
H
IPA
OH
MeOH
H2O
BH2OMe
H
O
O
3.8 kcal/mmol
OH
+
NH3
O
LA
Scheme 4. Possible reaction mechanisms for the conversion of reductive amination was expedited by increasing ammonia
LA into GVL and MPD in methanol based on single-hydrogen- dosage.[15a] Thus, the selectivity of MPD increased with the
transfer (SHT) or double-hydrogen-transfer (DHT).
ammonia content and reached quantitative conversion to MPD
when using approximately 26 equiv. ammonia (see Figure 2). In
Density functional theory (DFT) calculations were implemented situ 13C NMR studies of the conversion of LA into HPA and APA
to determine the variation of free energies of all intermediates were used to probe the theoretically predicted reaction
and products involved in the four pathways. Scheme S4a pathway by monitoring product distributions (Figure 4). It is
displays the resulting energy profiles of formation of HPA upon unambiguous that APA was the dominant product in the
hydrogenation of LA promoted through two conceivable presence of a surplus of ammonia (Figure 4a), and HPA was the
different functions of AB. AB interacts with LA to form ts5 during primary product in the absence of added ammonia (Figure 4b).
the DHT pathway, and the methanol borane complex These results illustrate that the reduction of LA was a dominant
(CH3OHBH3), generated from decomposed AB, approaches LA reaction pathway in the conversion of LA using the AB-methanol
to afford ts3 during the SHT pathway. The calculations system, while reaction selectivity toward reductive amination
established that ts3 has lower free energy (11.4 kcal/mol) than could be realized using the AB-methanol system with added
ts5 (15.0 kcal/mol). Alternatively, the energy profiles of APA ammonia.
formation via the hydrogenation of the imine intermediate 4-
iminopentanoic acid (IPA) through two different modes of AB
action are displayed in Scheme S4b. These energy profiles
indicate that the transition state ts4 (3.8 kcal/mol) produced
from decomposed AB is more accessible than ts6 (17
kcal/mmol) produced from unbroken AB. Theoretically, the SHT
pathway is thus more favorable than DHT both in the reduction
of the carbonyl group and of the imine functionality.
Furthermore, some APA was experimentally detected in the
absence of added ammonia (Figure S5), further illustrating the
tendency of AB to decompose into free ammonia and borane
rather than acting in the intact AB molecular form.
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Figure 4. In situ 13C NMR spectroscopy of (a) LA conversion in comparable, quantitative conversion, likely because the
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DOI: 10.1039/D0GC02372H
the formation of APA (0.5 mmol LA, 0.5 mmol AB, 1 mL reaction system operated too quickly to observe the differences
methanol containing 16 wt.% ammonia, room temperature, and between substrates with electron-donating or electron-
2 h), and in the formation of (b) HPA from LA (0.5 mmol LA, 0.5 withdrawing substituents.
mmol AB, 1 mL methanol, room temperature, and 2 h).
Table 2. Formation of alcohols from biomass-derived aldehydes
Since levulinate esters can be obtained rather swiftly from
carbohydrates by acid catalysis in alcohols,[20] a series of
saccharides was directly employed as feedstock for the
production of GVL using a two-step protocol. In a first reaction
step, methyl levulinate was produced from fructose-based
substrates over Amberlyst-15 in MeOH (Table 1, Entries 1-3)
and from glucose-based substrates over a mixture of Amberlyst-
15 and Sn-beta (Si/Sn ratio of 125) catalyst (Table 1, Entries 4-
6). After the first step, AB and water were directly added into
the resulting reaction mixture without any prior sample workup
and the reaction allowed to proceed for another 8 h at room
temperature. This procedure resulted in the formation of GVL
at a yield of 60% from fructose and of 51% from glucose, while
attractive results were achieved from the other carbohydrate
substrates as well. Thus, the results demonstrated that
implementation of the AB system provides an efficient strategy
for the direct conversion of biomass-derived carbohydrates into
valuable GVL.
and ketones.[a]
OH
R2
O
AB, 0.5 h
RT,MeOH
R1
R1
R2
a
b
OH
OH
OH
OH
O
O
O
OH
OH
O
O
O
O
(2b, >99%)
(3b, >99%)
(4b, 99%）
(5b, 80%)
H3CO
OH
HO
OH
(6b, >99%)
(7b, >99%)
(8b, >99%)
(9b, >99%)
HO
Br
OH
HO
OH
OH
(10b, >99%)
(11b, 75%)
(12b, >99%)
(13b, 91%)
H3CO
OH
O
HO
OH
HO
OH
O
OH
Table 1. Direct production of GVL from biomass saccharides.[a]
(14b, 91%)
(15b, 75%)
(16b, >99%)
(17b, 27%)
Entry
Sugar
Yield GVL
Yield HPA
Conversion
(%)
HO
HO
OH
O
(%)
60
35
22
51
40
45
(%)
9
11
8
10
6
6
OH
OH
O
1
2
3
4[b]
5[b]
6[b]
Fructose
Sucrose
Inulin
Glucose
Cellobiose
Mannose
>99
>99
>99
>99
>99
>99
O
H3CO
(18b, >99%) (19b, >99%)
(20b, 99%)
(21b, >99%)
[a] Reaction conditions: 1 mmol substrate, 1mmol AB, 2 mL
MeOH, 30 min, and room temperature. Yields are indicated in
parenthesis and were quantified with 1H NMR using mesitylene
as internal standard.
[a] Reaction conditions: 180 mg sugar, 100 mg Amberlyst-15,
4.0 g methanol, 30 mg AB and 5.0 g water; First step at 160 ºC
for 5 h and second step after addition of 5 mL water and 30 mg
AB for room temperature and 8 h. [b] Except for adding 50 mg
Sn-beta (125), all other conditions were similar to [a]. Yields
were quantified by 1H NMR spectroscopy using DMF as an
internal standard.
Conclusions
In conclusion, an energy-saving and efficient catalyst-free
strategy to selectively convert biomass-derived aldehydes and
ketones into HPA (or ester), APA, GVL or MPD in excellent yields
has been established using AB as the hydrogen donor under
mild reaction conditions. Interestingly, the solvent directed the
reaction selectivity towards alcohol or GVL, and ammonia
contents in the reaction mixture promoted the selectivity
toward reduction or reductive amination of the carbonyl
groups. Thus, for the first time, a remarkable yield of GVL of up
to 95% was obtained in water at room temperature without any
post-processing operation. In situ NMR illustrated that the
reaction pathway forming GVL from levulinate in water
encompassed hydrolytic cyclization of an intermediate borate
ester. Likewise, in situ NMR and computational studies
elucidated the mechanism of AB action and the competitive
mechanism between the reduction and reductive amination of
the carbonyl groups in the same reaction system. Additionally,
carbohydrates and heterogeneous catalysts were employed to
Finally, the reaction scope of the AB system was evaluated using
other biomass-derived aldehydes and ketones as substrates
(Table 2). Remarkably, the system exhibited good to excellent
activity for conversion of the compounds into the
corresponding alcohols using MeOH as solvent under very mild
conditions of room temperature and 30 min reaction time.
Substrates included levulinate esters with different chain
lengths, aliphatic/aromatic aldehydes and ketones, ,-
unsaturated aldehydes, and various furanics. Notably,
aldehydes showed higher reactivity than ketones (comparison
of 12b and 15b), and some substrates containing electron-
withdrawing groups at the aryl ring displayed faster reactions
than substrates with an electron-donating group, such as 11b,
14b, and 17b. However, the majority of substrates showed
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Green Chemistry
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COMMUNICATION
Journal Name
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directly produce GVL with a two-step strategy, affording a GVL
yield of 51% from glucose. Finally, a wide substrate scope was
demonstrated for the reaction system with the conversion of
various ketones and aldehydes into the corresponding alcohols
in excellent yield at room temperature. The introduced system
with AB as a reducing agent is arguably among the most
efficient, simple, environmentally friendly, and energy-saving
systems developed to date for the selective reductive
conversion of biomass-derived compounds. Accordingly, it can
be anticipated the results will open significant new possibilities
for the reductive conversion of biomass.
View Article Online
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Conflicts of interest
The authors declare no competing interests.
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Acknowledgements
WZ acknowledge The Chinese State Scholarship (No.
201906670009) for supporting a stay at the Technical University
of Denmark to conduct this study. SY acknowledges the
National Natural Science Foundation of China (No. 21576059 16 a) X. Yang, T. Fox, H. Berke, Tetrahedron 2011, 67, 7121-7127.
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and 21666008), while AR and SM acknowledge the Department
of Chemistry, Technical University of Denmark for support. 800
MHz NMR spectra were recorded on the spectrometer of the
DTU NMR Center supported by the Villum Foundation.
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6 | J. Name., 2012, 00, 1-3
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Page 7 of 7
Green Chemistry
View Article Online
DOI: 10.1039/D0GC02372H
Graphic Abstract
catalyst-free
controllable seletivity
mild condition
broad scope
H
O
N
O
O
No catalyst
Methanol
92%
Water
O
Room
temp.
OR(H)
NH3
O
H
Methanol
OH
Methanol
H
H
NH2
OR
B
OR
N
H
O
100%
OH
R2
H
H
O
82%
O
AB, Methanol
Room temperature
More than 20 examples
R1
R2
R1
An efficient catalyst-free system composed of ammonia borane in water or alcohol was developed to selectively
convert biomass-derived feedstock into four value-added products under extremely mild conditions. The
strategy enables the synthesis of a broad range of chemicals and direct upgrading of carbohydrates.