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Chapter 5
Summary
107
It has been reported in previous researches that some quantitatively investigate the
impacts of various erythro/threo for β-O-4 bond cleavage in alkaline/water system.
Furthermore, the degradation of the phenolic or non-phenolic C6-C2-type β-O-4 lignin
model compounds has been identified in organic solvent systems under mild conditions.
The present study focus on the β-O-4 bond cleavage of non-phenolic C6-C3-type lignin
model compounds, which have more similar chemical structures to natural lignin than
C6-C2-type because of the existence of the erythro (E) and threo (T) diastereomers.
In Chapter 2, the non-phenolic C6-C2-type β-O-4 lignin model compound I, 2-(2methoxyphenoxy)-1-(3,4-dimethoxyphenyl)ethanol, was treated in potassium tertbutoxide (tBuOK)/dimethylsulfoxide (DMSO) system with/without water addition to
verify the consistency of reaction systems by comparing with former researches and to
remove possible impact factors for the degradation process.
According to the results, it can be found that the water addition has a significant
impact on the degradation rate of the C6-C2-type β-O-4 lignin model compound in
tBuOK/DMSO system. When tBuOK/DMSO system is employed as a delignification
system, there is a necessary to avoid the pollution from water for the whole reaction
system with special attention.
In Chapter 3, the influence of the γ-hydroxymethyl group on the β-O-4 bond
cleavage of C6-C3-type erythro (IIe), and C6-C3-type threo (IIt) non-phenolic lignin
model compounds in tBuOK/DMSO system was investigated detailedly by comparing
the degradation rate of C6-C3-type lignin model compounds with that of C6-C2-type
lignin model compound and that of α-hydroxy or γ-hydroxy groups methyl-etherified
C6-C3-type
analogues,
the
erythro
and
threo
isomers
of
3-methoxy-2-(2-
methoxyphenoxy)-3-(3,4-dimethoxyphenyl)propan-1-ol (IIIe and IIIt) and 3-methoxy2-(2-methoxyphenoxy)-1-(3,4-dimethoxyphenyl)propan-1-ol (IVe and IVt).
108
The β-O-4 bond cleavage rates follow the order compound C6-C3-type threo (IIt) >
C6-C3-type erythro (IIe) > C6-C2-type (I) in tBuOK/DMSO system, suggesting that the
γ-hydroxy groups obviously contribute to the cleavage process, which is contrast to their
minor contributions in common alkaline/water system. The possible reason for the faster
cleavage of IIt than that of IIe is that the hydrogen bond has been generated between a
generated alkoxide and the other hydroxyl group at the α- or γ-position of compound IIe
in its most preferential conformer as well another lesser one to interfere with the attack
of the alkoxide on the β-carbon to break the β-O-4 linkage, while compound IIt does not
have it in its most preferential conformer.
In the tBuOK/DMSO system, the rates of the β-O-4 bond cleavage of the methyletherified compounds were following the order compound IIIt > IVe ≥ IVt > IIIe,
suggesting that the γ-hydroxy group has a more observable contribution in the reaction
of compound IIt than in that of compound IIe, though the reaction rate order cannot be
totally explained.
In the degradation process of compound I, compounds V, VI, VIII, and IX were
identified and quantified as products. Other products cannot be identified and quantified
because of the minor generation. The formation of compound VIII, which is further
converted to compounds VI and IX with some other unknown routes, can be explained
by the common neighboring participation mechanism. Compounds V, VI, VII, VIII,
and IX were identified and quantified as reaction products in the reaction of compound
IIe or IIt. All other reaction products must have been minor. It was clarified that
compounds VII and VIII primarily form via the common neighboring group
participation mechanism of the γ-and α-hydroxy groups, respectively. Compound VII
was surprisingly converted to compound VIII in its exclusive major route as well as to
unidentified minor reaction products. Compounds VI and IX must have been generated
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from compound VIII. Because the amounts of unidentified reaction products were
smaller in the reaction of compound IIt than in that of compound IIe, it can be suggested
that the γ-hydroxy group of the former contributes more to the β-O-4 bond cleavage than
that of the latter.
In Chapter 4, the impacts of the different solvent, counter cations, or bases for the
β-O-4 bond cleavage, when C6-C3-type erythro (IIe), C6-C3-type threo (IIt), and/or C6C2-type lignin model compounds were used, were investigated, respectively.
In tBuOK/tBuOH system, the degradation rate of compound I, IIe, or IIt was
slower than that of them in the tBuOK/DMSO system, respectively, which can be
explained by the better dissolvability of tBuO¯ in tBuOH than in DMSO. In both
tBuOK/tBuOH and tBuOK/DMSO systems, the same degradation rates order of
compound IIt, IIe, and I, which was following compound IIt > IIe > I and different from
that in common alkaline/water system, was observed and suggested the significant
participation of the γ-hydroxy groups of C6-C3-type lignin model compounds in the βO-4 bond cleavage process.
When tBuOK, tBuONa, or tBuOLi was employed as a base in DMSO, the β-O-4
bond cleavage of compound IIt was faster than that of compound IIe. Basically, the
degradation rates of both compound IIe and IIt in DMSO systems were following the
order of: tBuONa > tBuOK >> tBuOLi, although that of compound IIe was less great in
the tBuONa system than in the tBuOK system at the initial stage. These phenomena still
cannot be explained clearly but probably be caused by whether each base dissolves in
DMSO as the ion pair or free ions, with resulting basicity changing of the system.
When compound IIe or IIt was treated in DMSO by four bases, their degradation
rate is following the order of: tBuOK >> KH >> iPrOK > EtOKThe degradation rate
order, which suggests that the degradation rates were basically dependent on the
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basicity of these bases except for KH. As for the specific result of KH, one possible
explanation is that the basicity of it is too strong to quantitatively produce the conjugate
base of DMSO in DMSO, with the formation of lots of DMSO originating aggregates
to consequently decrease the basicity of system and the disappearance rate of lignin
model compounds.
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