Molybdenum-Catalyzed Deoxygenation Coupling of Lignin-Derived Alcohols for Functionalized Bibenzyl Chemicals

Article abstract of DOI:10.1002/chem.202003776

With the growing demand for sustainability and reducing CO₂ footprint, lignocellulosic biomass has attracted much attention as a renewable, carbon-neutral and low-cost feedstock for the production of chemicals and fuels. To realize efficient utilization of biomass resource, it is essential to selectively alter the high degree of oxygen functionality of biomass-derivates. Herein, we introduced a novel procedure to transform renewable lignin-derived alcohols to various functionalized bibenzyl chemicals. This strategy relied on a short deoxygenation coupling pathway with economical molybdenum catalyst. A well-designed H-donor experiment was performed to investigate the mechanism of this Mo-catalyzed process. It was proven that benzyl carbon-radical was the most possible intermediate to form the bibenzyl products. It was also discovered that the para methoxy and phenolic hydroxyl groups could stabilize the corresponding radical intermediates and then facilitate to selectively obtain bibenzyl products. Our research provides a promising application to produce functionalized aromatics from biomass-derived materials.

Full text of DOI:10.1002/chem.202003776

Accepted Article
Accepted Article
Title: Molybdenum-Catalyzed Deoxygenation Coupling of Lignin-
Derived Alcohols for Functionalized Bibenzyl Chemicals
Authors: Huifang Jiang, Rui Lu, Xiaolin Luo, Xiaoqin Si, Jie Xu, and
Fang Lu
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
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to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: Chem. Eur. J. 10.1002/chem.202003776
Link to VoR: https://doi.org/10.1002/chem.202003776
01/2020

10.1002/chem.202003776
Chemistry - A European Journal
COMMUNICATION
Molybdenum-Catalyzed Deoxygenation Coupling of Lignin-
Derived Alcohols for Functionalized Bibenzyl Chemicals
Huifang Jiang,[a,b] Rui Lu,[a] Xiaolin Luo,[a,b] Xiaoqin Si,[a] Jie Xu[a] and Fang Lu* [a]
[a]
H. Jiang, R. Lu, X. Luo, X. Si, Prof. J. Xu, Prof. F. Lu
State Key Laboratory of Catalysis
Dalian Institute of Chemical Physics
Dalian National Laboratory for Clean Energy
Dalian 116023 (P.R. China)
E-mail: lufang@dicp.ac.cn
[b]
H. Jiang, X. Luo
University of Chinese Academy of Sciences
Beijing 100049 (P.R. China)
Supporting information for this article is given via a link at the end of the document
Abstract: With the growing demand for sustainability and reducing
CO2 footprint, lignocellulosic biomass has attracted much attention as
a renewable, carbon-neutral and low-cost feedstock for the production
of chemicals and fuels. To realize efficient utilization of biomass
resource, it is essential to selectively alter the high degree of oxygen
functionality of biomass-derivates. Herein, we introduced a novel
procedure to transform renewable lignin-derived alcohols to various
functionalized bibenzyl chemicals. This strategy relied on a concise
deoxygenation coupling pathway with economical molybdenum
catalyst. A well-designed H-donor experiment was performed to
investigate the mechanism of this Mo-catalyzed process. It was
proved that benzyl carbon-radical was the most possible intermediate
to form the bibenzyl products. It was also discovered that the para
methoxy and phenolic hydroxyl groups could stabilize the
corresponding radical intermediates and then facilitate to selectively
deoxygenated strategies for the selective conversion of biomass-
derivates. Hydrodeoxygenation (HDO) is one such chemical
procedure performed in high H2 pressures and high temperature
to remove oxygen atoms.[3] For example, in the bio-fuel refining
process, HDO reaction can remove the oxygen atoms of lignin-
derived bio-oils to produce hydrocarbons under noble metals and
metal sulfides catalysts.[4] Deoxydehydration (DODH) reaction, as
a single-step combined with dehydration and deoxygenation
processes, has been used to eliminate adjacent hydroxyls in
polyol and then produce olefins. This type of reaction is carried
out under high-valent rhenium[5], molybdenum[6] and vanadium[7]
catalysts combined with reductants, such as 1-butanol, PPh3, H2,
metal and so on. The ability of oxo-Re, Mo and V complexes to
activate C-O bonds aroused the interest of many researchers to
investigate the deoxygenation of various biomass-derived
alcohols.[8]
obtain bibenzyl products. Our research provides
a promising
application to produce functionalized aromatics from biomass-derived
materials.
Recently, a re-functionalization of mono-alcohol has been
reported via oxo-Re and V complex-catalyzed oxidation/reductive
coupling reactions.[9] Compared with similar C-C bonds
construction from allyl and benzyl alcohols via stoichiometric
titanium-based halide[10] or La/Me3SiCl[11] mediated pathway, this
procedure discovered a new catalytic process to enhance the
selectivity of C-C coupling products. Besides, different from the
above-mentioned HDO and DODH methods, this deoxygenation
coupling process is capable of doubling the carbon number.
Biomass, especially lignocellulose, has been regarded as a
promising renewable energy carrier to satisfy the continuous
necessity of diversified resources for producing valuable
chemicals and fuels.[1] The upgrading transformation of oxygen-
rich biomass feedstocks motivates the extensive interest in
developing chemical processes to re-functionalize the C-O
bonds.[2] These incentives spur to explore innovative and efficient
Scheme 1. The conceptual outline for the production of functionalized bibenzyl chemicals from lignocellulosic feedstock.
1
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shown in Table 1. When the commercial MoO3 and ammonium
heptamolybdrate ((NH4)6Mo7O24, AHM) catalysts were used, the
yields of reduced products were only 22% and 39%, respectively
(Table 1, entry 2 and 3). The related coupling products formed by
reductive homo-coupling of allylic alcohols and benzyl alcohol
have also been observed by Fristrup and coworkers in AHM-
catalyzed DODH of biomass-derived polyols.[15] Based on
previous works[15-16], the weak deoxygenation ability of these Mo
catalysts might be associated with the inherent multinuclear
property of Mo-center.
According to our previous research, nitrogen-donor ligand was
capable of enriching the electron density of Mo and improving its
activity in deoxygenation reaction.[17] Therefore, several
organometallic oxo-molybdenum catalysts with various nitrogen-
donor ligands, including 1,10-phenanthroline (Phen), 2,2’-
bipyridine (Bipy), tetradentate Schiff base (Salen) and 8-
hydroxyquinoline (8-HQ), were prepared (Figure S1-3 in
supporting information) and used in the deoxygenation coupling
reaction. Compared with MoO2(acac)2, a slightly higher yields of
reduced products were obtained with Mo-Bipy and Mo-Salen
(Table 1, entry 4-7). However, it still needed to selectively
enhance the yield of bibenzyl product. Mo-8-HQ, as a more stable
single-site Mo-based catalyst, exhibited the excellent catalytic
performance in this reaction. The total yield of reduced products
was 92% with 48% yield for 4,4’-dimethoxybibenzyl 1a and 44%
yield for 4-methylanisole 1b as shown in Table 1, entry 8 and
Figure S4-10. Methyltrioxorhenium (CH3ReO3) has been reported
to be an efficient catalyst for deoxygenation of biomass-derived
alcohols.[18] In this deoxygenation coupling process, CH3ReO3
could give 87% yield of reduced products, but lower selectivity of
bibenzyl product (Table 1, entry 9) compared with Mo-8-HQ
catalyst. Therefore, Mo-8-HQ catalyst was the best candidate for
the selective production of bibenzyl compounds.
Bibenzyls, as one type of important intermediates in chemical
engineering, have been used as the aromatic monomers for the
production of new type polycarbonate, thermosetting resins and
thermoplastics.[12] Especially equipped with methoxy or phenolic
hydroxyl groups, the functionalized bibenzyls showed a great
potential as the green alternative for ubiquitous bisphenol A in
similar resins.[13] Moreover, natural bibenzyls are widely known as
the key motif of various important natural pharmaceuticals in bio-
medical
area
combined
with
their
cardioprotective,
neuroprotective, anticancer and antiviral bio-activity.[14] In view of
the abundant source of lignin in nature and the structural affinity
between natural bibenzyls and lignin-monomer, we here
described a novel procedure to produce functionalized bibenzyl
chemicals from lignin-derived alcohols (Scheme 1). In this work,
various molybdenum catalysts were employed in the
deoxygenation coupling reactions. And a well-designed H-donor
experiment was conducted to study the mechanism of
molybdenum-catalyzed deoxygenation coupling reaction. The
effect of different functional groups in lignin-derived substrates for
the selective production of bibenzyls was also investigated.
Table 1. Comparison of different catalysts for reduced C-C coupling reaction of
anisic alcohol. [a]
Entry
Catalyst
Conversion
(%)
Reduced
Products
(1a, 1b) (%)
Oxidation
Products
(1c) (%)
1
2
3
4
5
6
7
8
9
blank
MoO3
-
-
-
42
22 (7, 15)
39 (7, 32)
54 (6, 48)
41 (8, 33)
57 (8, 49)
59 (14, 45)
92 (48, 44)
87 (17, 70)
13
23
21
24
25
14
7
(NH4)6Mo7O24
MoO2(acac)2
Mo-Phen
Mo-Bipy
67
>99
65
88
Mo-Salen
Mo-8-HQ
CH3ReO3
>99
>99
>99
5
[a] Reaction conditions: 1 mmol anisic alcohol, 10 mol% catalyst, 1.5 mmol
triphenylphosphine and 20 mL1,4-dioxane were put into 50 mL Parr autoclave,
then reacted at 220 °C for 8 h under Ar. Note: Phen: 1,10-phenanthroline, Bipy:
2,2’-bipyridine, Salen: tetradentate Schiff base, 8-HQ: 8-hydroxyquinoline.
Figure 1. Reaction temperature profile of deoxygenation coupling of anisic
alcohol catalyzed by Mo-8-HQ. Reaction conditions: 1 mmol anisic alcohol, 10
mol% Mo-8-HQ, 1.5 mmol PPh3 and 20 mL1,4-dioxane were put into 50 mL Parr
autoclave, then reacted at a certain temperature for 12 h under Ar.
For selectively obtaining the coupled products, we firstly
investigated the viability of various Mo-based catalysts for
deoxygenation coupling ability of anisic alcohol 1 (all experimental
details were showed in Supporting Information). The reduced
products, including the coupled product, 4,4’-dimethoxybibenzyl
1a, deoxygenation product, 4-methylanisole 1b, and oxidized
byproduct, para-anisaldehyde 1c, were detected by GC-MS as
As shown in Figure 1, a series of reaction temperatures under
Mo-8-HQ catalyst were investigated. Along with the reaction
temperature elevated from 180 to 240 °C, the yield of reduced
products gradually increased with the decrease of the oxidized
byproduct. When the reaction was run at 240 °C, the product
2
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Figure 2. The distribution of reduced products from various benzylic alcohols. [a]
[a] Reaction conditions: 1 mmol alcohol substrate, 10 mol% Mo-8-HQ catalyst, 1.5 mmol triphenylphosphine and 20 mL1,4-dioxane were put into 50 mL Parr
autoclave, then reacted at 220 °C for 8 h under Ar. [b] At 200 °C for 16 h. [c] At 200 °C for 14 h with adding 1 mmol 8-HQ. [d] 1 mmol 8-HQ was added under
reaction condition.
distribution was changed with the increase of 4-methylanisole 1b
and the decrease of 4,4’-dimethoxybibenzyl 1a. It possibly
resulted from the instable intermediate under high reaction
temperature. It showed that suitable reaction temperature was
important for the selectively formation of bibenzyl products.
Depending on the further exploration, it was found that oxygen-
containing reaction atmosphere could not significantly affect the
product distribution (Table S1 in Supporting Information). It
suggested that the Mo-based deoxygenation coupling reaction
possessed strong tolerance and no need for the harsh anaerobic
condition.
Based on the aforementioned condition screening results, the
deoxygenation coupling procedure was also applicable to various
benzyl alcohols, especially lignin-derived alcohols, with Mo-8-HQ
as the catalyst. All of the results were summarized in Figure 2.
The benzhydrol substrate was tested under 200 °C for 16 h. And
nearly quantitatively converted into bibenzyl with four benzene
rings, 1,1,2,2-tetraphenylethane (93%) (Figure 2, column 1).
Interestingly, it was found that 1,1,2,2-tetraphenylethane product
could be recrystallized with high purity from ice-ethanol after the
solvent removed (MS and NMR analysis were shown in Figure
S11-13), and the isolated yield was about 85%. α-Phenethyl
alcohol was also completely converted into reduced products with
a 74% selectivity of 2,3-diphenylbutane (Figure 2, column 2 and
Figure S14-18) at 200 °C for 14 h. Moreover, these results were
even better than the reported results with Re-based catalysts.[9a]
Because the 8-HQ ligand exhibited a special role in promoting
Mo reactive activity, we tried to add 1 mmol 8-HQ into Mo-8-HQ
catalyzed deoxygenation coupling reaction of anisic alcohol 1.
Interestingly, the addition of 8-HQ not only improved the yield of
4,4’-dimethoxybibenzyl 1a (59%, in Figure 2, column 3), but also,
changed the products distribution to a certain extent. Veratryl
alcohol 2 with one para-methoxy (p-OMe) group and one meta-
methoxy (m-OMe) group gave the corresponding reduced
products in good yield (82%), in which 49% yield for 3,3’,4,4’-
tetramethoxybibenzyl 2a (Figure 2, column 4, the GC and mass
spectrum were shown in Figure S19-22) and 33% yield for 3,4-
dimethoxytoluene 2b were obtained. 3,4,5-Trimethoxybenzyl
alcohol 3 with one p-OMe group and two m-OMe groups
underwent the deoxygenation coupling reaction and afforded the
natural bibenzyl product 3a (Figure 2, column 5 and Figure S23-
26), named brittonin A, in one step. 3,5-Dimethoxybenzyl alcohol
4 with two m-OMe groups could also produce the bibenzyl
products 3,3’,5,5’-tetramethoxybibenzyl 4a, although the yield
was low (Figure 2, column 6 and Figure S27-30). Obviously, these
results revealed that the position of methoxy group in benzene
ring affected the product distribution. It was speculated that the
decreased yields of reduced products from 3 and 4 substrates
were likely due to the relatively low reactivity of their
corresponding reaction intermediates; this will be discussed in
more details below.
In view of phenolic hydroxyl (Ar-OH) as a common functional
group in lignin-derivatives, vanillyl alcohol 5 with a p-Ar-OH group
3
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10.1002/chem.202003776
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and a m-OMe group was also used in the deoxygenation coupling
reaction and afforded the reduced products in a good yield (86%,
Figure 2, column 7 and Figure S31-34). Among these products,
53% yield was obtained for 4,4’-dihydroxy-3,3’-dimethoxybibenzyl
5a, which, as a representative, was possibly a promising
alternative for ubiquitous bisphenol A in similar resins.[13]
Compared with veratryl alcohol 2, p-Ar-OH in vanillyl alcohol was
conductive to obtain bibenzyl products selectively.
position of the substituted groups in benzene ring significantly
influenced the stability of conjugation radical intermediate.
Experimental results showed that p-OMe and p-Ar-OH groups
could facilitate the selectively gaining of bibenzyl products.
Based on the previous works on deoxygenation coupling
reactions[9, 23], a rational catalytic cycle for the Mo-catalyzed
deoxygenation coupling reaction was speculated and depicted in
Scheme 3. At first, the MoVI catalyst was reduced by
triphenylphosphine (PPh3) to form MoIV. The PPh3 reductant as an
O-scavenger could avoid excessive consumption of benzyl
alcohol itself as the reductant and suppress the undesired
byproducts. After reaction, this reductant was converted to
triphenylphosphine oxide (OPPh3), which could be reduced and
regenerated to PPh3.[24] Then, MoIV and benzyl alcohol molecule
were combined to form Mo-alkoxide species. Subsequently, the
as-formed reduced Mo-alkoxide species were underwent in-situ
C-O cleavage to benzyl radical intermediate and MoVI. Finally, two
benzyl radical intermediates were coupled with each other to
generate bibenzyl product. It was worthy noted that the benzyl
radical intermediate with stable structure was the key factor for
the selectively formation of bibenzyl products.
Besides, the cross-coupling process with 3 and 5 obtained an
excellent yield of reduced products and gave the mixture of homo-
bibenzyls, 3a (11%) and 5a (11%), the hetero-bibenzyl 6a (4-
hydroxy-3,3’4’5’-tetramethoxybibenzyl, 26%, and the mass
spectrum was shown in Figure S35) and deoxygenation products
3b and 5b (Figure 2, column 8). The hetero-bibenzyl 6a as a type
of natural bibenzyl medicine was named crepidatin. Some of our
bibenzyl products, especially brittonin A and crepidatin, existed in
natural Dendrobium plants and have been widely studied for its
anticancer potential.[19] Certainly, it was considered that various
functionalized bibenzyl chemicals could be synthesized through
this deoxygenation coupling process with corresponding benzyl
alcohols except these above-mentioned products.
H
H
H
H
H
H2C
OH
+
10 mol% Mo-8HQ
1.5 equiv PPh3
H
H
H
H
+
+
20 mL 1,4-dioxane
220 oC, 8 h
MeO
1b
1
OMe
Scheme 2. Trapping Free C-radical Intermediate with 9,10-Dihydroanthracene.
The bibenzyl products were considered to be generated via
homolytic decomposition of benzyl alcohol to form the free benzyl
radicals process.[20] To further investigate the reaction pathway,
we tried to trap the free C-radical intermediate with 9,10-
dihydroanthracene (DHA), which was commonly used as H-atom
donor.[21] As expected, there were no corresponding bibenzyl
product, only 4-methylanisole 1b and the DHA combined with
benzyl radical product (Scheme 2, GC of the reaction mixture and
mass spectrum of trapping free C-radical intermediate with DHA
were shown in supporting information Figure S36 and S37). This
result indicated that the benzyl radical was trapped successfully
under the reaction condition and provided a mechanism evidence
to understand Mo-catalyzed radical deoxygenation coupling
process.
Furthermore, the results of Figure 2 illustrated that benzhydrol
substrate could obtain excellent selectivity of bibenzyl product
under the deoxygenation coupling reaction. The α-phenethyl
alcohol substrate was also transformed into bibenzyl products
with a high yield. These could be explained with the strong
stability of the free benzyl 2˚C-radical intermediates. Besides, p-
Ar-OH and p-OMe groups in benzene ring of benzyl alcohol
substrates were able to promote the yields of bibenzyl products.
It might be due to the factor that these function groups could
generate a stronger conjugated structure and stabilize the
corresponding benzyl radicals even under 220 °C. However, the
substrates with m-OMe group had low activity to produce bibenzyl
in this process. The reason was probably that the meta-
substituent could destabilize the intermediate benzyl radical.[22]
Therefore, the stability of benzyl radical intermediate was the key
point to efficient synthesis of the bibenzyl products. The type and
Scheme 3. Proposed reaction pathway of Mo-Catalyzed Deoxygenation
Coupling Reaction of Benzyl Alcohol.
In summary, we took advantage of lignin-derived alcohols as the
renewable resources to produce functionalized bibenzyl
chemicals. It was implemented via a Mo-catalyzed deoxygenation
coupling procedure. The well-designed H-donor experiment
demonstrated that the benzyl carbon-radical might be the
intermediate for bibenzyl products. It was also found that the
methoxy and phenolic hydroxyl groups in benzene ring of lignin-
derivates obviously affected the products distribution by
influencing the stability of corresponding radical. This research
will provide a new sustainable way to transform abundance
biomass resources into value-added functionalized aromatic
chemicals, especially in medicine and polymer industry.
Acknowledgements
The authors are grateful to the National Key R&D Program of
China (2018YFB1501600 and 2019YFC1905303), the National
Natural Science Foundation of China (No. 21908218 and
21872139) and the Strategic Priority Research Program of the
Chinese Academy of Sciences (XDB17020300) and DICP Grant
(No. I201944) for providing financial support to do this scientific
research.
4
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Conflict of interest
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Keywords: Biomass • Lignin-derived alcohols • Molybdenum
catalyst • Deoxygenation • C-C coupling
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Entry for the Table of Contents
This study opens a new sight to transform abundant biomass resources into important functionalized aromatic chemicals via a
concise Mo-catalyzed deoxygenation coupling procedure.
6
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