Cleavage of CC and Co bonds in β-O-4 linkage of lignin model compound by cyclopentadienone group 8 and 9 metal complexes

Article abstract of DOI:10.1246/cl.200037

Degradation of 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphe-noxy)propane-1,3-diol (1), a model compound for lignin β-O-4 linkage was examined with iron, ruthenium, rhodium and iridium complexes bearing cyclopentadienone ligand. Cyclopentadienone iron complex gave only a small amount of degraded product with reduced molecular weight. Cyclopentadienone ruthenium complex, so called Shvo's catalyst, afforded 3,4-dimethoxybenzaldehyde (a3) in 14.3% yield after CαCβ bond cleavage. On the other hand, cyclopentadienone group-9 metal complexes catalyzed CβO bond cleavage to afford guaiacol (b1) as a main product in up to 74.9% yield.

Full text of DOI:10.1246/cl.200037

Advance Publication Cover Page
Cleavage of C–C and C–O bonds in -O-4 Linkage of Lignin Model
Compound by Cyclopentadienone Group 8 and 9 Metals Complexes
Shuhei Kusumoto, Masamichi Kishino, and Kyoko Nozaki*
Advance Publication on the web February 29, 2020
doi:10.1246/cl.200037
© 2020 The Chemical Society of Japan
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1
Cleavage of C–C and C–O bonds in -O-4 Linkage of Lignin Model
Compound by Cyclopentadienone Group 8 and 9 Metals Complexes
Shuhei Kusumoto, 1 Masamichi Kishino, 1 and Kyoko Nozaki *1
1 Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku,
Tokyo 113-8656, Japan.
E-mail: nozaki@chembio.t.u-tokyo.ac.jp
1
2
3
4
5
6
7
8
9
Degradation
of
1-(3,4-dimethoxyphenyl)-2-(2-
methoxyphenoxy)propane-1,3-diol (1), a model compound
for lignin -O-4 linkage was examined with iron, ruthenium,
rhodium and iridium complexes bearing cyclopentadienone
ligand. Cyclopentadienone iron complex gave only a little
amount of degraded product with reduced molecular weight.
Cyclopentadienone ruthenium complex, so called Shvo’s
catalyst, afforded 3,4-dimethoxybenzaldehyde (a3) in
14.3% yield after C–C bond cleavage. On the other hand,
10 cyclopentadienone group-9 metal complexes catalyzed C–
11 O bond cleavage to afford guaiacol (b1) as a main product
12 in up to 74.9% yield.
50
51 Figure 1. Structure and degradation of the -O-4 linkage.
52
53 form dark brown precipitate along with only 0.3% of b1
54 (guaiacol) as the degraded product (entry 1). The formed
55 precipitate was found to have rather larger molecular weight
56 than the starting compound 1, suggesting some undesired
57 dimerization or trimerization took place mediated by iron
58 complex (See supporting information for the detailed
59 analysis). With cyclopentadienone ruthenium complex [Ru],
60 significant amount of a3 (14.3%) and b2 (20.8%) were
61 obtained presumably via dehydrogenation / retro-aldol
62 reaction (entry 2, path (i) in Scheme 1), which is known to
13 Keywords: Lignin, Metal-Ligand Cooperation, Retro-
14 Aldol Reaction
15
Lignin, one of the main components in the woody
16 biomass, has a complex polymeric structure containing
17 many aromatic rings and ether linkages. Its complexity and
18 low solubility make it difficult to efficiently refine lignin to
19 basic chemicals. Great efforts have been made to develop
20 catalytic systems which can cleave either C–O or C–C
21 bonds in lignin or its model compounds aiming at a future
22 valorization of lignin to basic aromatic chemicals.1 With the
23 closer look at the microstructures of lignins, -O-4 structure
24 which consists of 1,3-diol bearing aryl ether at 2- position is
25 the most dominant linkage between lignin monomers.2
26 Therefore, the main focus has been paid upon either the
27 cleavage of the C–C bond or the cleavage of the C–O or
28 C4–O bond (Figure 1).3 Among the enormous number of
29 transition metal catalyzed bond cleavage reactions, we
30 focused our attention onto metal-ligand cooperative
31 functions, which have been rarely applied to degradation of
32 lignin derived materials but well known to be powerful tools
33 for the reduction of polar bonds.4 As we previously
34 reported hydrogenolysis of C–O single bonds by
35 hydroxycyclopentadienyl iridium complexes utilizing their
36 metal-ligand cooperative functions, we envisioned that
37 cyclopentadienone metal complexes can exhibit unique and
38 high activity in degradation of -O-4 linkage via C–O or
39 C–C bond cleavage.5 Here in this paper, we report the
40 catalytic degradation of a-O-4 model compound by
41 cyclopentadienone metal complexes.
63 undergo with
64 tris(diphenylphosphinomethyl)ethane)
a
ruthenium triphos (triphos
=
1,1,1-
There
catlyst.6
65 existed, however, compound c2, the de-hydroxymethylated
66 product, as the major product in 27.7%. Because c2 was
67 formed presumably via the retro-aldol reaction from -
68 hydroxyketone c1, the favorable dehydrogenation of
69 secondary alcohol at C position would have hampered the
70 desired C–C bond cleavage (Scheme 1, path (ii); 1–c1–
71 c2). The yield of a3 or b2 could not be improved by
72 applying higher temperature and longer reaction time (entry
73 5) or optimization of other reaction conditions (see
74 supporting information). The activity of cyclopentadienone
75 metal complexes in retro-aldol reaction can be rationalized
76 because cyclopentadienone metal complexes have both
77 basic carbonyl oxygen and acidic metal center to activate
78 hydroxy group and carbonyl group simultaneously (Scheme
79 2). On the contrary to the observed C–C bond cleavage
80 by ruthenium catalyst [Ru], group-9 metal complexes [Rh]
81 and [Ir] afforded only minimal amount of products a3 and
82 b2 (1.2%, 0.0% with rhodium, 0.0%, 0.0% with iridium,
83 entries 3 and 4). With cyclopentadienone rhodium complex
84 [Rh], c3 after the loss of hydroxymethyl and hydroxy
85 groups was obtained as the major product (20.8%, entry 3).
86 Compound c3 was presumably obtained via hydrogenation
87 of carbonyl group in c2 followed by dehydration (Scheme 1,
88 path (ii); c2–c5–c3).
42
First, cyclopentadienone iron, ruthenium, rhodium and
43 iridium complexes were examined in the reaction of a -O-4
44 model compound 1 at 140 °C for 20 hours, and the results
45 are summarized in Table 1. Compounds a1–a4, b1 and b2
46 are possible products of either C–C or C–O bond cleavage
47 and compounds c1–c4 are the ones without bond cleavage
48 between two aromatic rings A and B. With iron complex
49 [Fe], 68.8% of the starting compound 1 was consumed to

2
1
Table 1. Degradation of 1-(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)propane-1,3-diol (1) by cyclopentadienone metal complexes.(a)
2
3
temp. time
yield (%)(e)
conv.
(%)(c)
entry
cat.
(°C)
140
140
140
140
160
140
140
(h)
20
20
20
20
72
72
20
a1
a2
a3
0.0
14.3 0.0
1.2
0.0
8.0
0.0
0.0
a4
0.0
b1
b2
c1
0.0
c2
c3
c4
1(b)
2
3
4
5
[Fe]
[Ru]
[Rh]
[Ir]
[Ru]
68.8(d) 0.0
0.0
0.2
0.0
1.9
1.9
6.0
0.0
0.3
0.0
0.0
27.7 0.0
0.0
0.0
0.0
96.0
>99
>99
>99
>99
5.1
3.0
1.5
0.9
8.4
1.6
0.0
25.0 20.8 3.8
0.0
5.1
0.0
7.7
0.0
25.7 0.0
43.4 0.0
35.1 5.0
74.9 0.0
0.0
6.8
0.0
2.6
0.0
0.0
6.7
20.8 0.0
9.1 12.7
29.4 0.0
0.0
6
7
[Ir]
9.4
0.0
2.3
0.0
10.9
0.0
Cp*IrCl2(PPh3)
0.2
0.0
1
4
5
6
7
8
(a) Reaction conditions; catalyst (4.5 µmol), substrate (90 µmol). (b) Precipitate (30.9 mg) was obtained after the reaction and analyzed by H NMR
and SEC. (See supporting information for the detailed experimental procedure.) (c) The conversions were determined by 1H NMR analyses with
internal standard (1,1,2,2-tetrachloroethane). (d) The conversion of the starting material was determined by 1H NMR analysis after the separation of the
reaction mixture by HPLC. (e) The yields of a1–a4, b1, b2 and c2 were determined by GC analyses and the yields of c1, c3 and c4 were determined by
1H NMR analyses. (See supporting information for other products.)

3
1
2
Scheme 1. Possible reaction pathways for the degradation of 1.
3
4
5
6
7
8
9
Cyclopentadienone iridium catalyst [Ir] afforded b1, a
product after C–O bond cleavage, as the major product in
43.4% yield along with c4 which is the product after
hydrogenation of c3 (entry 4, path (iv) in Scheme 1).
Although the detailed reaction path to afford b1 by [Ru],
[Rh] and [Ir] is unclear, b1 could be formed via direct C–O
bond hydrogenolysis from the starting material 1 or
27
28 Scheme 2. Possible mechanism for the retro-aldol reaction mediated
10 intermediates
b2–3
or
c1–5
as
similar
29 by cyclopentadienone metal complexes.
11 hydroxycyclopentadienyl iridium complexes have already
12 been reported to catalyze C(sp3)–O bond hydrogenolysis.5
13 Another possible pathway to afford b1 can be a reaction via
14 -aryloxy elimination from the hemiacetal intermediates
15 (c7), which could be formed via hydration of C=C double
16 bond in c3 (Scheme 1, path (iii); c3–c6–c7–(a9+b1)).7 The
17 yield of b1 was further improved to 74.9% with longer
18 reaction time although the counterparts containing aromatic
19 ring A could not be fully characterized, which is most
20 probably because of the undesired polymerization reactions
30
31
The possible reaction path to afford c3, c4 and b1 from
32 benzylic alcohol c5 via dehydration and C–O bond cleavage
33 shown in Scheme 1, path (ii) and (iii) (c5 to a9 and b1), was
34 supported by the additional experiments shown in Scheme 3.
35 When
1-(3,4-dimethoxyphenyl)-2-(2-
36 methoxyphenoxy)ethan-1-ol (c5) was treated with [Ru] or
37 [Ir] under the same conditions as Table 1, [Ru] afforded
38 only 2.7% yield of b1 while [Ir] afforded 25.5% of b1 in
39 addition to 38.1% of the dehydrated product c3.8
21 (entry
6).
The
iridium
complex
bearing
22 pentamethylcyclopentadienyl (Cp*) ligand instead of
23 cyclopentadienone ligand exhibited almost negligible
24 activity (entry 7) suggesting the possible involvement of
25 cyclopentadienone ligand in this C–C and C–O bond
26 cleavage reactions.

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1
2
Scheme
3.
Degradation
of
1-(3,4-dimethoxyphenyl)-2-(2-
3
methoxyphenoxy)ethan-1-ol (c5) by [Ir] and [Ru]. (See supporting
information for other products.)
4
4
5
6
7
8
9
In conclusion, cyclopentadienone iron, ruthenium,
rhodium and iridium complexes are examined in the
degradation
5
6
of
1-(3,4-dimethoxyphenyl)-2-(2-
methoxyphenoxy)propane-1,3-diol (1), a model compound for
7
8
10 -O-4 linkage of lignin. The iron complex afforded only
11 trace amounts of degraded products either after the cleavage
12 of C–C or C–O bonds. Ruthenium complex, so-called
13 Shvo’s catalyst was found to cleave C–Cbonds via
14 dehydrogenation / retro-aldol reaction. Ruthenium, rhodium
15 and iridium complexes all cleaved C–C bonds
16 preferentially than C–Cbonds. Iridium complex showed
17 the highest activity for the formation of guaiacol (b1) by the
18 successive C–O bond cleavage.
19
20 This work was supported by JSPS KAKENHI JP18H05259.
21
22 Supporting
Information
is
available
on
23 http://dx.doi.org/10.1246/cl.******.
24 References and Notes
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