Vinylation of Aryl Ether (Lignin Β-O-4 Linkage) and Epoxides with Calcium Carbide through C?O Bond Cleavage

Article abstract of DOI:10.1002/cssc.201701153

Calcium carbide has been increasingly used as a sustainable, easy-to-handle, and low-cost feedstock in organic synthesis. Currently, methodologies of using calcium carbide as “solid acetylene” in synthesis are strictly limited to activation and reaction w

Full text of DOI:10.1002/cssc.201701153

Accepted Article
Title: Vinylation of Aryl Ether (Lignin β-O-4 Linkage) and Epoxides with
Calcium Carbide via C-O Bond Cleavage
Authors: Yugen Zhang, Siew Ping Teong, and Jenny Lim
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DOI: 10.1002/cssc.200((will be filled in by the editorial staff))
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Vinylation of Aryl Ether (Lignin β-O-4 Linkage) and Epoxides with Calcium Carbide
via C-O Bond Cleavage **
Siew Ping Teong, Jenny Lim and Yugen Zhang*
Abstract: Calcium carbide has been increasingly used as a
sustainable, easy-to-handle, and low-cost feedstock in organic
synthesis. Currently, methodologies of using calcium carbide as
“solid acetylene” in synthesis are strictly limited to activation and
reaction with X-H (X = C, N, O, S) bonds. Herein, a mild and
transition metal-free protocol was developed for the vinylation of
epoxides and aryl ether linkage (β-O-4 lignin model compound)
with calcium carbide via C-O bond cleavage, forming valuable vinyl
ether products. CaC2 plays a vital role in the C-O bond activation
and cleavage, and in providing acetylide source for the formation of
vinylated products. These exciting results may provide new
Scheme 1. Methodologies for using calcium carbide as acetylide
source in organic synthesis.
methodologies for organic synthesis and new insights towards lignin
or biomass-related degradation to useful products.
We previously reported the formation of vinyl ethers from
alcohols with calcium carbide under the Cs2CO3/DMSO system.[15]
While employing this superbasic system on the styrene oxide at
100 °C for 16 h, vinylated product 3a was isolated via ring opening
under this mild condition (Scheme 2). Various conditions were
screened using styrene oxide as the model substrate (Table S1-2).
Additives such as KBr, TBAB and 18-crown-6 were added in an
attempt to improve the efficiency but to no avail (Table S2, entry 3-
5). Similar to alcohol vinylation,[15] small amount of water additive
is important to this ring opening and vinylation reaction (Figure S1).
Optimized product yield and selectivity of 85% was obtained with
calcium carbide (2.5 equiv.) in DMSO in the presence of Cs2CO3
(0.5 equiv.) at 100 °C for 16 h when 6 vol% of water was added into
the reaction system. A control reaction was carried out under
anhydrous conditions where no desired product was observed after
the reaction.
Interesting phenomena were observed from these results, albeit
the limited success of expanding the substrate scope (Scheme 2).
The majority of the substrates (1a – 1e, 1h) gave a similar vinyloxy
product whereas substrate 1f formed a di-vinyloxy product. Initially,
a diol intermediate reaction pathway was proposed (Scheme 3b(I)).
This mechanism is supported by a control reaction where 72% of 3a
was obtained when 1-phenyl-1,2-ethanediol (1i) was used as the
substrate under similar conditions (Scheme 2, Table S3). It was
hypothesized that the small amount of water in the system may have
led to ring opening of epoxide to diol, followed by rapid vinylation
of the in-situ generated diol and -elimination to form the end
product (Scheme 3b(I)). However, efforts to track the diol
intermediate from the reaction of 1a were not successful. When
substrate 1a was carried out at a shorter time, no diol intermediate
was observed. In fact, the control reaction of 1a without calcium
carbide gave only trace amount of diol product (1i), leaving most of
the starting material unreacted. In addition, 1-phenyl-1,2-ethanediol
(1i) is inert under the above conditions (without CaC2) (Scheme 3a).
These results indicate that the superbasic system may be not
favourable for the hydrolysis reaction of 1a and -elimination of 1i.
Calcium carbide plays an important part in facilitating epoxide ring-
opening. With that, a plausible mechanism was proposed as shown
in Scheme 3b(II). Firstly, acetylide interacts with epoxide and
activates the C-O bond.[11c] Ring opening then occurs along with the
new C-O bond formation to give vinyloxyl intermediate (A).
Following with this, transferring of -carbon proton to vinyl anion
and subsequent vinylation gave the final product -vinyloxy styrene.
For cyclohexene oxide 1f, however, diol intermediate was observed
in the absence of CaC2, suggesting that it could undergo the diol
pathway shown in Scheme S1.
Lignin is the world’s second most abundant renewable polymer
besides cellulose. Due to its aromatic-rich units, depolymerization of
the renewable lignin to high value-added aromatic monomers is
particularly attractive in sustainable chemistry.[1] However, the
complexity of the lignin structure also makes selective
depolymerization into small aromatic fragments challenging.[1-4]
Therefore, most of the efforts have been devoted to using model
compounds to mimic the representative linkages in lignin which will
help to further understand the bond cleavage mechanism.[5] -O-4
linkage with aryl ether unit is the most abundant and stable
substructure in lignin. Various methodologies have been developed
for the cleavage of -O-4 linkage which include base[6] or acid[7]
mediated cleavage, oxidative cleavage[8] and reductive cleavage[9]
and others.[10] These methods either require harsh conditions and
expensive reagents or suffer from low selectivity.
In recent years, the attractive calcium carbide has been
increasingly used as a sustainable, easy-to-handle, and low-cost
feedstock in organic synthesis.[11-12] Although many excellent
synthetic methodologies have been developed by using calcium
carbide as the acetylene/acetylide source, the substrate scopes are
still strictly limited to activation and reaction with X-H (X = C, N, O,
S) bonds. As part of our group’s effort[12] in exploring calcium
carbide utilization in organic synthesis, we then questioned if it can
be used as the acetylide source to activate the much stable C-O bond
(Scheme 1). Therefore, CaC2-mediated ring opening of strained
epoxides for further chemical transformation was investigated.
Herein, a mild and transition-metal free system with calcium carbide
was demonstrated to yield highly value-added vinyl ethers[13-14]
from epoxides and lignin -O-4 linkage model compounds via C-O
bond cleavage.
[]
S. P. Teong, J. Lim and Dr. Y.G. Zhang.
Institute of Bioengineering and Nanotechnology, 31
Biopolis Way, The Nanos, Singapore 138669, Singapore.
E-mail: ygzhang@ibn.a-star.edu.sg
[] This work was supported by the Institute of
Bioengineering and Nanotechnology (Biomedical
Research Council, Agency for Science, Technology and
Research, Singapore).
Supporting information for this article is available on the
WWW under http://www.angewandte.org or from the
author.
1
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10.1002/cssc.201701153
ChemSusChem
however, the addition of radical initiator did not help to further
improve the yield (Table S4, entries 9 - 12). For lignin model
compound 5, 82% of 3g was obtained. (1-(vinyloxy)vinyl)benzene
(3a) was characterized to be the second half of the cleavage product
in 57% yield under the optimized condition (Scheme 4b). In
addition to the product 3a and 3g, about 9% of by-product 5b was
also isolated. When different linkages of lignin model compounds 6
(α-O-4) and 7 (-O-5) were tested in this system, starting materials
were quantitatively recovered, even after extending the reaction time
to a week (Scheme 4c). Simple alcohols were used as substrates
under similar conditions that represent the α-O and β-O linkages
(Scheme 4d – e). It was found that generally, α-O alcohols
underwent O-vinylation instead of elimination and vice versa.
Similar to the epoxide reactions, addition of CaC2 is vital. In the
absence of CaC2, no reaction happened and more than 90% of model
compound 5 or 9 was recovered from the reaction system which
exclude the posibility of base catalyzed cleavage of β-O-4 ether
bond via an epoxide intermediate.[17]
Scheme 2. Vinylation of epoxides using calcium carbide
Scheme 4. Reaction of lignin model compounds with/without
calcium carbide.
Delighted by this mild, selective and transition metal-free direct
vinylation method to cleave the aryl-ether linkages and convert to
valuable vinyl ethers in a simple process, we then went on to
conduct kinetic studies with model compound 5. According to
Figure S2, all 3 products increased sharply in first 2 h and then
remained relatively constant as time continued to extend to 24 h.
Maximum yields for 3g and 3a are of 88% and 57%, respectively.
At the beginning of the reaction, trace amount of acetophenone and
phenol was detected. When the reaction was carried out in the
absence of CaC2, approximately 90% of the starting material was
found, alongside with trace amount of 5b and the isomerization
product 5a (Scheme 5). Based on these results, a proposed reaction
pathway is shown in Scheme 5.
It was postulated that isomerization of lignin model
compound 5 could have taken place to form 5a, followed by
elimination which gave product 5b. This side reaction (pathway (I)
in scheme 5) is slow as only small amount of intermediate 5a and
by-product 5b (< 10%) was observed in the systems with or without
calcium carbide. The initial detection of acetophenone and phenol in
Scheme 3. (a) Control reactions in the system without CaC2. (b)
Proposed mechanism for vinylation of epoxides.
Unexpectedly, (vinyloxy)benzene was obtained from substrate
1g, which may be due to cleavage of aryl-ether C-O bond. This led
us to further explore the possibility of this methodology for lignin
which consists of abundant β-O-4 aryl-ether linkages for cleavage.
Promising results were shown in Scheme 4. Initial screenings with
lignin model compound 4 representing the oxidized β-O-4 linkage
showed that C-O bond cleavage occurred with 88% of
(vinyloxy)benzene (3g) after optimization (Scheme 4a). It was noted
that the other half of the cleavage product from 4 could be
acetophenone which is not stable under the given conditions. This
was confirmed by a control reaction using acetophenone as the
starting material where full conversion of acetophenone was
observed, but attempts to characterize these unknowns were
unsuccessful. Addition of metal catalysts was not compatible with
the CaC2 superbase system (Table S4, entries 4 - 7). The cleavage of
β-O-4 linkages in lignin often suggests a radical pathway,[16]
GCMS analysis could have occurred from direct -elimination of 5,
15]
followed by vinylation to form 3g (pathway (II)).[11c,
O-
vinylation of 5 could have taken place as a fast and dominant
reaction in the system to give 5c intermediate which was
experimentally observed in GCMS during the reaction process.
2
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10.1002/cssc.201701153
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Similar to the epoxide reactions, it was assumed that 5c was then
activated by acetylide and went through intermediate C (C-O
cleavage and hydrogen transfer) to give 3a and 3g. It is clear that
pathway (III) is the fast and dominant reaction, while pathway (I)
and (II) coexist in the system as complimentary reaction paths.
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Scheme 5. Proposed pathway for the cleavage and vinylation of
lignin model compound.
In summary, we have developed a mild and transition metal-free
protocol for the vinylation of epoxides and aryl-ether linkage (β-O-4
lignin model compound) via C-O bond cleavage, forming valuable
vinyl ether products. CaC2 plays a vital role in the C-O bond
activation and cleavage, and in providing acetylide source for the
formation of vinylated products. These thrilling results may provide
new methodology for organic synthesis and new insights towards
lignin or biomass-related degradation to useful products.
Experimental Section
General procedure for vinylation of epoxides: In a 8 mL pressure vial,
Cs2CO3 (0.5 mmol) and CaC2 (2.5 mmol) were added. The vial was then
purged and refilled with argon thrice before DMSO + 6 vol% H2O (5 mL)
and epoxide (1 mmol) were added to the vial using a syringe. The reaction
was stirred at 100 °C for 16 h, cooled to room temperature before it was
diluted with H2O (10 mL) and extracted with diethyl ether (3 x 10 mL). The
combined organic layer was dried over anhydrous Na2SO4 and concentrated
in vacuo. The residue was further purified by column chromatography on
silica gel to afford the corresponding vinyl ethers.
General procedure for vinylation of lignin model compounds: In a 8 mL
pressure vial, substrate (1 mmol), Cs2CO3 (0.5 mmol) and CaC2 (4.0 mmol)
were added. The vial was then purged and refilled with argon thrice before
DMSO + 9 vol% H2O (5 mL) was added to the vial using a syringe. The
reaction was stirred at 160 °C for 16 h, cooled to room temperature before it
was diluted with H2O (10 mL) and extracted with diethyl ether (3 x 10 mL).
The combined organic layer was dried over anhydrous Na2SO4 and
concentrated in vacuo. The residue was further purified by column
chromatography on silica gel to afford the corresponding vinyl ethers.
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Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Keywords: C-O bond cleavage · β-O-4 linkage · calcium carbide ·
epoxide vinylation · lignin depolymerisation
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Entry for the Table of Contents (Please choose one layout)
Layout 2:
((Catch Phrase))
Siew Ping Teong, Jenny Lim and Yugen
Zhang*
__________ Page – Page
Vinylation of Aryl Ether (Lignin β-O-4
Linkage) and Epoxides with Calcium
Carbide via C-O Bond Cleavage
A mild and transition metal-free protocol for the vinylation of epoxides and aryl ether
linkage (β-O-4 lignin model compound) with calcium carbide via C-O bond
cleavage, forming valuable vinyl ether products.
.
5
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