Domino lignin depolymerization and reconnection to complex molecules mediated by boryl radicals

Article abstract of DOI:10.1039/d0cy00558d

Chemical degradation of lignin has attracted increasing interest due to its potential for producing chemicals from renewable resources. Herein, we present a new transition metal free degradation procedure utilizing DDQ-oxidation and boryl radical mediated degradation, followed by reconnection of a monomer intermediate to a new dimer in a domino process. Our results include the selective degradation of oxidized β-O-4 model compounds by a boryl radical initiated with a catalytic amount of 4-(4-pyridinyl)benzonitrile and bispinacolborane B₂(pin)₂ as well as its application to organosolv lignin. This sequential procedure expands the toolbox for lignin degradation from simple depolymerization to high-value products by incorporating bond forming transformations within the process, and also provides a new transition-metal free method for the construction of 1,6-diketone fragments.

Full text of DOI:10.1039/d0cy00558d

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PClaetaaslyesidsoScnieontcaed&juTsetcmhnaorlgoignys
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DOI: 10.1039/D0CY00558D
ARTICLE
Domino Lignin Depolymerization and Reconnection to Complex
Molecules Mediated by Boryl Radicals
Longcheng Hong,a Astrid Spielmeyer,b Janin Pfeiffer b and Hermann A. Wegner*a
Received 00th January 20xx,
Accepted 00th January 20xx
Chemicaldegradationofligninhas attractedincreasinginterestdue toits potentialinproducingchemicalsfrom renewable
ressources.Hereinwe presenta newtransitionmetalfree degradationprocedure utilizingDDQ-oxidationandborylradical
mediateddegradation,followedbyreconnectionofthe monomerintermediate toa newdimerina dominoprocess.Our
results include the selective degradationofoxidized -O-4modelcompounds byborylradicalinitiatedbycatalyticamount
of4-(4-pyridinyl)-benzonitrile andbispinacolborane B2(pin)2 as wellas its applicationtoorganosolv-lignin.Thissequential
procedure expands the toolboxforlignindegradationfromsimple depolymerizationtohigh-valueproductsbyincorporating
a bond forming transformations within the process , and also provides a new transition-metal free method for the
construction of 1,6-diketone fragments.
DOI: 10.1039/x0xx00000x
which poses great challenges for developing selective
degradation reactions.
Introduction
The quest for efficient and novel chemical degradation
With the rapid development of the global economy, fossil-
derived energy resources are increasingly consumed posing
severe challenges for the environment as well as for
sustainability.1,2 One way to address this issue is by developing
new catalytic concepts for the utilization of renewable
resources.3,4 Within this context, the valorization of non-food
biomass represents a viable approach avoiding the competition
with food production. Lignin, the third most abundant
methods of lignin initiated extensive fundamental research in
the past decades and has produced variety of strategies
a
including oxidative, reductive, acid-catalyzed as well as base-
promoted depolymerization.18,19,22 Most of the methods target
the cleavage of the -O-4 linkage of the native lignin which is
the most abundant linkage. Recently, some innovative two-step
depolymerization methodologies have been developed aiming
at reducing the BDE of this C–O bond by selective pre-oxidation
component in biomass (∼20%) arising from wood plants, is such
of the alcohol,23-31 which enables
a
subsequent mild
a renewable and abundant bioresource.5 In the past century
lignin, regarded as a waste product in the pulp and paper
industry, has been mainly burned for heating.4,6-8 In the past
decades, exploitation of lignin has been demonstrated to have
potential applications in industries,9,10 such as energy
production from lignin-derived syngas,11,12 application of lignin
as dispersant for particles,12 utilization of lignin as additive to
make polymers more biodegradable during their production13
as well as to improve the UV barrier properties of linen fabrics.14
Besides the applications in material and energy science,15,16 the
chemical degradation and transformation of lignin as chemical
feedstock has attracted increasing attention.17-19 Due to its
highly oxygenated polyphenolic polymer structure (Scheme
depolymerization of the oxidized lignin (Scheme 1b). Protocols
based on the concept of -oxidation include (Scheme 1c): (1)
Formic-acid-induced depolymerization of aspen or birch
lignin;25,29 (2) Zn/NH4Cl promoted depolymerization of birch
lignin;26
(3)
[Ir(ppy)2(dtbbpy)](PF6)
catalyzed
photo-
depolymerization of pine lignin;27 (4) NiMo sulfide catalyzed
reductive depolymerization of birch lignin.28 These two-step
approaches are all efficient for the scission of -O-4 linkage
producing
a variety of aromatic monomers. A subsequent
comparative study by Crocker and co-workers evaluated the
literature methods for oxidized-lignin depolymerization using
Indulin AT kraft lignin and -valerolactone extracted lignin from
maple.30
They
tested
the
Baeyer–Villiger
oxidation,
1a),20 the chemical degradation of lignin offers
a versatile
Cu(OAc)2/1,10-phenanthroline, Dakin oxidation as well as Li-Al
LDH and Au/Li-Al LDH for the subsequent depolymerization on
lignin, concluding that benzylic alcohol oxidation pre-treatment
might decrease the efficiency of lignin depolymerization.
Westwood and Li’s research also showed similar findings and
even employed various determination methods to probe the
possible reasons for the deactivation phenomenon.31 They
proposed that some undesired degradation of linkages,
condensation and modification of lignin occurred during lignin
oxidation pre-treatment.
platform for aromatic chemicals.8,21 In the other hand, the
complex structure of technical lignin provides high physical
stability and thus makes it chemically recalcitrant, though,
a. Institute of Organic Chemistry, Justus Liebig University, Heinrich-Buff-Ring 17,
35392 Gießen, Germany and Center for Materials Research (LaMa), Justus-Liebig-
University, Heinrich-Buff-Ring 16, 35392 Gießen, Germany.
b. Institute of Food Chemistry and Food Biotechnology, Justus Liebig University,
Heinrich-Buff-Ring 17, 35392 Gießen, Germany.
†Electronic Supplementary Information (ESI) available. See DOI: 10.1039/
x0xx00000x
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Journal Name
the intermediate monomers in
well as its application to organosolv-lignin affording
biologically active 1,6-diketone.
a
domino transformation, as
View Article Online
DOI: 10.1039/D0CY00558D
a
Results and discussion
To assess the feasibility of the desired radical degradation
process, the lignin model compound in its oxidized form (1aa)
was treated with 1.5 equiv. of B2(pin)2 (2) and 20 mol% of 4-(4-
pyridinyl)-benzonitrile36 as catalyst in toluene (0.25 M based on
1aa) at 110 °C. The corresponding cleavage products
acetophenone (3a) and phenol (4a) were detected in a yield of
46% and 37% (Table 1, entry 1), respectively, by GC-MS (n-
undecane as standard). Further solvent screening showed the
advantage of polar solvents such as 1,4-dioxane, diglyme and
DMF (entry 3, 4 & 6) in this radical reaction, while DMSO was
not a suitable solvent due to its reduction by the boryl radical.
The best yield was obtained in the reaction used DMF as
solvent. The beneficial effect of DMF might be contributed to
the stabilization of the boryl radical intermediate by
coordination with the solvent.38-41 Raising the reaction
concentration as well as the temperature improved the
outcome of the reaction in case of DMF (see supporting
information, Table S1). A brief screening of different equivalents
of B2(pin)2, resulted in the optimized conditions; 1.5 equiv.
B2(pin)2, 20 mol% of catalyst in DMF (0.5 M) at 140 °C for 24 h
gave the best GC conversion (> 99%) and GC-yield (83% and 91%
of acetophenone (3a) and phenol (4a), respectively). In most of
the screened conditions a significant difference between the
conversion of 1aa and the yield of 3a and 4a has been observed
(entry 3 & 4 in Table 1, entry 4, 6, 7, 9 & 14 in Table S1). Since
the boryl radical shows high reactivity towards carbonyl groups
(substrates as well as the acetophenone product), same
polymerization could have occurred, which could not be
detected by GC-MS or NMR spectroscopy.42,43 Due to the
stabilization of the active radical species in DMF as solvent the
side reactivity could be drastically reduced supporting the
hypothesis.
Scheme 1 (a) A representative lignin substructure displaying typical lignin subunits and
linkages. (b) Selective oxidation of -O-4 linkage to -O-4α-ox or -O-4-ox oxidized form in
lignin and the corresponding calculated C–O BDEs.32,33
R = H or OMe. (c) Reported
depolymerization protocols of -oxidized lignin to aromatic monomers. (d) The herein
developed boryl radical mediated depolymerization of -oxidized lignin.
Lignin valorization through chemo-selective degradation
requires not only efficient methodologies, but also production
of value-added chemicals and the pursuit of diversity. One way
to achieve this valorization, is a further transformation of the
degraded aromatic products into functionalized compounds.
Very recently, Barta and co-worker had developed a synthetic
approach of biologically active seven-membered N-heterocyclic
compounds from lignocellulose.34 This described strategy
consisted of (a) reductive catalytic fractionation (RCF) of
lignocellulose, (b) Ru-catalyzed selective amination of the
produced alcohol and (c) Pictet−Spengler cyclization of the
obtained secondary amines affording the N-heterocyclic
compounds, which could inhibit human hepatoma cell line
(HepG2) with the IC50 values ranged from 30.4 to 53.2 M. On
the other hand, utilization of novel reaction systems beyond the
traditional oxidative or reductive degradation to deliver more
complex molecular entities directly from biomass is also
desirable in the aspect of valorization. Recently, boryl radicals
generated catalytically by pyridine derivatives have been
The scope of the new boryl radical mediated degradation
method has been evaluated on different lignin model
compounds (Scheme 2A). For all substrates high conversion
Table 1 Screening of reaction solvents.a
Yield
Conversion
of 1aa
Entry
Solvent
3a
4a
demonstrated
substrates.35-37 As the same functional entity is present in -
oxidized lignin we propose to use boryl radical for its
depolymerization. The desired radical addition reaction should
generate benzylic ketyl radical which would subsequently
trigger a radical cleavage of the C–O aryl ether bond of -O-4
linkage (Scheme 1d). Herein, we report the successful
development of such a boryl radical degradation of the oxidized
-O-4 model compounds, an unprecedented reconnection of
to
selectively
interact
with
carbonyl
1
2
3
Toluene
n-Octane
1,4-Dioxane
Diglyme
DMSO
52%
67%
87%
99%
10%
99%
 99%
46%
37%
60%
55%
10%
72%
83%
37%
60%
37%
47%
7%
a
4
5
6
7b
a
DMF
DMF
78%
91%
a Reaction conditions: 1aa (0.4 mmol, 1.0 equiv.), 2 (1.5 equiv.) and catalyst (0.08
mmol) were dissolved in the stated solvent under N2 and stirred at 110 C for 24 h.
The yield was determined by GC-MS using n-undecane as internal standard. The
reaction was carried out at 140 C.
b
2 | J. Name., 2012, 00, 1-3
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rates were achieved and the corresponding products were
Since the first report on the homolytic cleavaVgieewoAfrticBle2O(pnilnin)e2
by 4-cyanopyridine in 2016,37 this so generated pyridine-ligated
DOI: 10.1039/D0CY00558D
boryl radical has been applied for
a variety of organic
transformations.56 A number of mechanistic studies have been
conducted using density functional theory (DFT) calculations,35-
37,41,57-60
EPR,36,37,58,61,62 NMR spectroscopy41,63 and single crystal
X-ray diffraction structural analysis.62 According to these
studies, a proposed reaction mechanism of the boryl radical
cleavage of the C–O ether bond in the model compounds is
postulated in Scheme S1 (supporting information), involving a
radical addition of ketone and the subsequent intramolecular
single electron transfer (SET) process.
For the formation of diketone 5c, three possible
mechanisms from model compound 1-cb were proposed as
shown in Scheme 3. Route A begins with a radical addition to
the carbonyl group, while route B and route
C start with
cleavage of the hydroxyl group in -position. In route A, after
the radical cleavage of the C–O-bond in -position, an enolate
pinacolborane intermediate is generated followed by
a
hydrolysis step (A-a) or elimination of HOBpin (A-b).
Nevertheless, in both routes a secondary radical addition takes
place on the carbonyl group and generates a carbon-centered
radical at the -C which subsequently undergoes a dimerization
to the dimer product 5c. In route B, the radical cleavage of the
C–O-bond at the -position forms a carbon radical which under
elimination of guaiacolate pinacolborane provides the vinyl
Scheme 2 (A) Substrates scope of the boryl radical mediated cleavage of -CO bond;
(B) Unexpected formation of dimers 5 observed in the boryl radical degradation of model
compounds 1 bearing -hydroxyl group.
obtained in moderate to good isolated yields. Interestingly, the ketone intermediate 7. In route C, at high temperature the
hydroxyl group in 1db did not affect the degradation providing
p-hydroxyl-acetophenone 3d in 44% isolated yield.
Surprisingly, the model compound 1-cb bearing
a
hydroxymethylene group in -position was transformed to
diketone 5c in 49% isolated yield rather than the expected
simple degradation product, -hydroxyl aryl ketone 3c (Scheme
2B, Eq. 1). In the process not only the C–O-bond of the oxidized
-O-4 model compound has been cleaved, but simultaneously a
new C–C-bond forming event has occurred in a domino process
giving access to a new structural theme not accessible by any
current lignin degradation method. Further raising the amount
of B2(pin)2 to 3.0 equiv. improved the reaction yield to 69%. This
transformation seems general as two other model compounds
(1-eb
&
1-fb) also underwent smoothly the radical
degradation/dimerization process providing the corresponding
dimer 5e and 5f in 52% and 77%, respectively (Eq. 2 & 3). To our
knowledge, the construction of the 1,6-diketone framework is
more challenging than other diketones and has been less
developed. The reported synthetic strategies include: (1)
gold(I)-catalyzed hydration of alkynes,44 palladium-catalyzed
Wacker-type oxyfunctionalization of terminal alkenes,45,46
oxidative
ring-opening
cleavage
of
cyclohexanone,
cyclohexenes and vicinal diols,47-51 Mg/MgBr2 or Yb/TMSBr53
or CoI2/Zn54 promoted reductive homocoupling of enones, and
Mn-catalyzed ring-opening coupling reaction of cyclopropanols
52
with enones.55 As
a transition-metal-free methodology, the
Scheme 3 Possible mechanistic routes from 1-cb to 5c in the presence of the catalytically
generated boryl radical. Route proceeds via radical addition, hydrolysis (a) or
elimination of HOBpin (b) followed by radical dimerization. Route B starts with radical
cleavage of -C–O bond, and route C is initiated by an elimination of H2O. The mechanistic
proposal via route A (grey blue-ground) shows the best coherence with the control
experiments.
A
herein reported boryl radical mediated dimerization of -
hydroxyl ketone would expand the tool box for the construction
of 1,6-diketone.
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