Catalytic Depolymerization of Chitin with Retention of N-Acetyl Group

Article abstract of DOI:10.1002/cssc.201501224

Chitin, a polymer of N-acetylglucosamine units with β-1,4-glycosidic linkages, is the most abundant marine biomass. Chitin monomers containing N-acetyl groups are useful precursors to various fine chemicals and medicines. However, the selective conversion of robust chitin to N-acetylated monomers currently requires a large excess of acid or a long reaction time, which limits its application. We demonstrate a fast catalytic transformation of chitin to monomers with retention of N-acetyl groups by combining mechanochemistry and homogeneous catalysis. Mechanical-force-assisted depolymerization of chitin with a catalytic amount of H₂SO₄ gave soluble short-chain oligomers. Subsequent hydrolysis of the ball-milled sample provided N-acetylglucosamine in 53 % yield, and methanolysis afforded 1-O-methyl-N-acetylglucosamine in yields of up to 70 %. Our process can greatly reduce the use of acid compared to the conventional process.

Full text of DOI:10.1002/cssc.201501224

DOI: 10.1002/cssc.201501224
Communications
Catalytic Depolymerization of Chitin with Retention of
N-Acetyl Group
Mizuho Yabushita,[a] Hirokazu Kobayashi,*[a] Kyoichi Kuroki,[b] Shogo Ito,[b] and
Atsushi Fukuoka*[a]
Chitin, a polymer of N-acetylglucosamine units with b-1,4-gly-
cosidic linkages, is the most abundant marine biomass. Chitin
monomers containing N-acetyl groups are useful precursors to
various fine chemicals and medicines. However, the selective
conversion of robust chitin to N-acetylated monomers current-
ly requires a large excess of acid or a long reaction time, which
limits its application. We demonstrate a fast catalytic transfor-
mation of chitin to monomers with retention of N-acetyl
groups by combining mechanochemistry and homogeneous
catalysis. Mechanical-force-assisted depolymerization of chitin
with a catalytic amount of H2SO4 gave soluble short-chain olig-
omers. Subsequent hydrolysis of the ball-milled sample provid-
ed N-acetylglucosamine in 53% yield, and methanolysis afford-
ed 1-O-methyl-N-acetylglucosamine in yields of up to 70%.
Our process can greatly reduce the use of acid compared to
the conventional process.
hemagglutination, and thus can be used for suppression of in-
fluenza and cancer.[5] MeGlcNAc is also applicable to the syn-
thesis of biodegradable polyesters and polyamides,[6] organo-
catalysts,[7] ligands,[8] and gelators.[9] Additionally, MeGlcNAc is
a useful molecule for further transformations in glycoscience,
as the hemiacetal group is protected by a methyl group.[10]
Despite a great deal of effort in various fields, efficient con-
version of chitin to N-acetylated monomers remains a chal-
lenge. In biochemistry, chitin is depolymerized by enzymes to
form GlcNAc in yields of 64–77% under ambient conditions;
however, it takes 10 days for the reaction to be completed
owing to low catalytic activity.[11] Conventional chemical/
mechanical pretreatments slightly improved the reactivity of
chitin by amorphization, but a harsh pretreatment (supercriti-
cal water with converge milling) was necessary to accelerate
the reaction.[12] In synthetic organic chemistry, HCl provides
GlcNAc in approximately 65% yield, but only when HCl is used
in large excess (i.e., with a molar ratio of HCl to GlcNAc units
in chitin of 100) and at high concentrations of 15–36%.[4a,12a]
Therefore, the serious corrosiveness of HCl becomes an issue,
and a huge amount of acidic waste is produced. Dilution of
HCl in the depolymerization reaction of chitin results in low se-
lectivity for GlcNAc.[13] Moreover, the use of acid results in de-
acetylation of chitin,[13] but preservation of the N-acetyl group
is essential for high physiological activity and wide application
of the resulting chitin monomers.[14] The synthesis of MeGlcNAc
was not studied to the same extent as that of GlcNAc; in fact,
its production from chitin is not yet reported. Fischer glycosi-
dation of GlcNAc was necessary to obtain MeGlcNAc.[15] Herein,
we report catalytic depolymerization of chitin to GlcNAc and
MeGlcNAc (Scheme 1) in a process that reduces the use of
acid by 99.8% and is completed within one day (including all
handling).
The transformation of biomass to value-added chemicals with
particular functional groups is an attractive alternative to multi-
step functionalization of hydrocarbons from fossil fuels.[1] The
most abundant terrestrial and marine biomasses are cellulose
and chitin, respectively. Notably, chitin is a characteristic biopo-
lymer consisting of nitrogen-containing monomer units,
N-acetylglucosamine (GlcNAc), with b-1,4-glycosidic linkages
(Scheme 1). Therefore, it is an attractive renewable feedstock
for organic nitrogen compounds.[2] However, unlike cellulose
conversion, in which thermal and mechanocatalytic depolyme-
rization can both yield monomers,[3] chitin requires selective
depolymerization of glycosidic bonds to prevent the loss of
N-acetyl groups for wide application.
Hydrolysis of the glycosidic bonds of chitin gives GlcNAc,
which is useful as a biologically active agent, an ingredient for
cosmetics, and a raw material for N-containing organic com-
pounds.[4] Meanwhile, methanolysis of chitin can afford
1-O-methyl-N-acetylglucosamine (MeGlcNAc), which inhibits
The first key step in our reaction is mechanocatalytic conver-
sion of chitin to soluble oligomers. It is known that mechanical
force enhances chemical reactions under mild conditions.[16]
Chitin was impregnated with a small amount of H2SO4 [sub-
strate to catalyst ratio (S/C)=8.1], and the resulting mixed
solid was ball-milled at 500 rpm for 6 h to achieve mechano-
catalytic hydrolysis (Scheme 1, product name: Chitin-H2SO4-
BM). The water used for hydrolysis should be physisorbed
water (1.5 wt%) in chitin. The process quantitatively converted
chitin to soluble compounds such as GlcNAc (4.7% yield),
(GlcNAc)2 (7.8% yield), (GlcNAc)3 (11% yield), (GlcNAc)4 (9.7%
yield), and (GlcNAc)5 (8.6% yield), where (GlcNAc)n denotes an
oligomer consisting of n GlcNAc units (Figure 1, Chitin-H2SO4-
BM). Other products were longer and branched oligomers, as
[a] Dr. M. Yabushita, Dr. H. Kobayashi, Prof. Dr. A. Fukuoka
Institute for Catalysis
Hokkaido University
Kita 21 Nishi 10, Kita-ku, Sapporo, Hokkaido 001-0021 (Japan)
E-mail: kobayashi.hi@cat.hokudai.ac.jp
fukuoka@cat.hokudai.ac.jp
[b] K. Kuroki, S. Ito
Graduate School of Chemical Sciences and Engineering
Hokkaido University
Kita 13 Nishi 8, Kita-ku, Sapporo, Hokkaido 060-8628 (Japan)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201501224.
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Scheme 1. Two-step depolymerization of chitin to N-acetylated monomers, catalyzed by H2SO4. The first step is mechanocatalytic depolymerization of chitin
to water-soluble oligomers, and the second is solvolysis of the oligomers to N-acetylated monomers.
reaction (Scheme 2), we propose that amorphization of chitin
first occurs by the ball-milling, which enhances the accessibility
of acid to chitin owing to lower steric hindrance.[18] Then, me-
chanical force assists the cleavage of the connecting points of
polymers, in which polymers are subjected to macro-scale
power as tensile stress.[19] Thus, the use of mechanical force re-
sulted in selective activation of glycosidic bonds connecting
the GlcNAc units rather than amide bonds in the acetamide
Figure 1. Mechanocatalytic depolymerization of chitin. Dissolution condi-
tions: chitin weight 406 mg, distilled water 40 mL, 298 K.
determined by liquid chromatography/mass spectroscopy (LC/
MS) and nuclear magnetic resonance (NMR) spectroscopy (Fig-
ure S1 and S2 in the Supporting Information). It is known that
side chains improve the solubility of oligomers.[16i] Chitin oligo-
mers are useful as antitumor, immunoenhancing, and antimi-
crobial agents,[17] and as precursors for monomers (vide infra).
In controlled experiments (Figure 1), pristine chitin con-
tained no soluble compounds, whereas chitin that was ball-
milled in the absence of H2SO4 (Chitin-BM) afforded soluble
compounds in 5.2% yield, with the chitin becoming amor-
phous (Figure S3). A H2SO4-impregnated chitin that was aged
at 298 K for 6 h (Chitin-H2SO4) had a soluble fraction of only
3.7%. Hence, both ball-milling and H2SO4 are necessary for de-
polymerization of chitin. To examine the synergy between me-
chanical force and H2SO4, chitin was ball-milled, impregnated
with H2SO4, and aged at 298 K for 6 h (Chitin-BM-H2SO4); this
sample had a soluble fraction of only 7.8%. Therefore, mechan-
ical force was only effective for the depolymerization of chitin
in the presence of H2SO4.
Notably, acetyl groups in the GlcNAc units were preserved
after the mechanocatalytic reaction, as Chitin-H2SO4-BM con-
sisted of N-acetylated oligomers and did not contain acetic
acid. This result is specific to the mechanocatalysis, as the con-
ventional hydrolysis using a small amount of acid results in sig-
nificant removal of N-acetyl groups.[13] In the mechanocatalytic
Scheme 2. Proposed schematic of mechanochemical depolymerization. The
mechanical force first amorphizes chitin, which increases the accessibility of
acid to the substrate. The main chain is subjected to strong mechanical
force, which leads to selective dissociation of the connecting points of the
chain (glycosidic bonds), whereas the side chains (acetamide groups) do not
dissociate.
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groups hanging from the units. This is the remarkable merit of
mechanocatalysis in the selective depolymerization of chitin.
The merit is different from that in the conversion of simple
sugar polymers like cellulose that have only one functional
group to be hydrolyzed (glycosidic bond).[16]
and the high stereoselectivity is favorable for further applica-
tions (the formation mechanism of the a-anomer is discussed
in Figure S5). The turnover number (TON) of H2SO4 for
MeGlcNAc production was 5.6, showing that the acid acted as
a catalyst. This result was in sharp contrast to those obtained
from conventional systems, which require a large excess of HCl
(TON<0.01) to obtain N-acetylated monomer.[4a] With regard
to by-products, only a trace amount of methyl acetate (0.90%)
was detected by gas chromatography (flame ionization detec-
tor). Almost no deacetylation occurred during methanolysis
under our conditions. The yield of methanol-insoluble humins
was 4.7 wt% based on the weight of chitin used. The yield of
MeGlcNAc increased to 70% under optimum conditions
(S/C=4.1, entry 8). In contrast, methanolysis of other sam-
ples—pristine chitin, Chitin-H2SO4, Chitin-BM, and Chitin-BM-
H2SO4—gave low yields of MeGlcNAc (ꢀ16%; entries 3–6).
These results clearly show that mechanocatalytic depolymeri-
zation of chitin prior to methanolysis is essential for the high-
yield synthesis of MeGlcNAc. The reactivity of chitin is dramati-
cally different from that of cellulose; the methanolysis of cellu-
lose gives a good yield of methylglucose with no pretreat-
ment.[21]
To produce GlcNAc, Chitin-H2SO4-BM was further hydrolyzed
in water under rapid heating–cooling conditions (Table 1). The
catalyst for hydrolysis was H2SO4 contained in the sample
(S/C=8.1), which gave a 32% yield of GlcNAc at 463 K in 1 h
(entry 1). The yield of GlcNAc was improved to 53% when the
amount of H2SO4 was increased to S/C=2.0 and the reaction
Table 1. Solvolysis of chitin samples to N-acetylated monomers.[a]
Entry Sample
S/C T [K] Solvent
Product[b] [%]
GlcNAc MeGlcNAc Other[c]
1
2
3
4
5
6
7
8
Chitin-H2SO4-BM 8.1 463 Water
32
53
–
–
–
–
–
–
–
–
0.0
4.0
0.0
16
68
70
68
47
0.0
1.7
2.2
4.6
27
28
Chitin-H2SO4-BM 2.0 443 Water
[d]
Chitin
–
463 Methanol
Chitin-H2SO4
Chitin-BM
8.1 463 Methanol
[d]
–
463 Methanol
Chitin-BM-H2SO4 8.1 463 Methanol
Chitin-H2SO4-BM 8.1 463 Methanol
Chitin-H2SO4-BM 4.1 463 Methanol
In summary, we demonstrated the catalytic conversion of
chitin to N-acetylated monomers, notably MeGlcNAc. The pro-
cess consists of mechanocatalytic depolymerization of chitin to
soluble oligomers, followed by solvolysis to monomers. Me-
chanical force was found to be essential to achieving selective
catalytic depolymerization with retention of N-acetyl groups,
demonstrating the remarkable merit of mechanocatalysis. The
second thermocatalytic methanolysis converted oligomers to
the N-acetylated monomer MeGlcNAc in 68–70% yield, owing
to the improved reactivity of the substrate and the good sta-
bility of the product. In the overall process, the amount of acid
used for the reaction was drastically reduced (by 99.8%) com-
pared with the conventional process. Our method overcomes
critical issues in conventional systems for chitin depolymeriza-
tion, and opens the door to various applications of chitin-
based compounds.
[a] Common reaction conditions: chitin weight 406 mg, solvent 40 mL,
rapid heating–cooling conditions (time course is shown in Figure S4).
[b] Based on moles of GlcNAc units. [c] Other soluble products. [d] No
H2SO4 was impregnated on chitin.
temperature was lowered to 443 K (entry 2). However, the for-
mation of by-products was still observed in these reactions be-
cause of the degradation of GlcNAc, which is thermally unsta-
ble in water; in a stability test of GlcNAc, only 17% of GlcNAc
was recovered after being subjected to the same heating con-
ditions (Table S1, entry S1). As the generation of acetic acid
was limited (<1%), the degradation must have resulted from
the conversion of hemiacetal groups in our rapid reaction.
The second key point in our reaction is to overcome the sta-
bility issue by using methanolysis instead of hydrolysis, as the
product MeGlcNAc does not contain a hemiacetal group.
Indeed, 94% of the MeGlcNAc was recovered after heat treat-
ment at 463 K (Table S1, entry S2). It is notable that the advant-
age of MeGlcNAc is not only the good stability but also the
wide applications in various fields as described above.[5–10] Re-
cently, Pierson et al. demonstrated that alcoholysis of chitin
was possible in the presence of H2SO4 (S/C=0.9), although the
deacetylated product [i.e., 1-O-(2-hydroxyethyl)-glucosamine]
was obtained predominantly (24% yield) owing to the
harsh conditions required for direct depolymerization of the
robust chitin.[20] Therefore, we can expect selective synthesis of
MeGlcNAc from the reactive chitin oligomers contained in
Chitin-H2SO4-BM.
Acknowledgements
We would like to thank Dr. Y. Yamakoshi (Hokkaido Research Or-
ganization) for the LC/MS analysis and Dr. A. Shrotri (Hokkaido
University) for useful discussion. This work was supported by
a Grant-in-Aid for Young Scientists (A) (KAKENHI, 26709060) from
the Japan Society for the Promotion of Science (JSPS) and
a Grant-in-Aid for the JSPS Fellows.
Keywords: biomass
·
catalysis
·
chitin
·
hydrolysis
·
mechanochemistry
Chitin samples were subjected to methanolysis at 463 K to
demonstrate selective synthesis of MeGlcNAc. The reaction of
Chitin-H2SO4-BM provided MeGlcNAc in 68% overall yield, even
when only a small amount of H2SO4 was used (S/C=8.1)
(entry 7). The ratio of a- to b-anomers in MeGlcNAc was 5.0,
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Published online on November 5, 2015
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