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methyltetrahydrofuran together with water to give
2
a in a good yield. Notably, the procedure did not
require the addition of acids or bases as in earlier re-
[5,9]
ports.
Performing the reactions in ethyl acetate
gave higher selectivity with lignin models than in
ethanol. In ethanol, both the dehydrogenation and
the CÀO bond cleavage were accelerated to give
lower selectivity. Another advantage of using ethyl
acetate was that the product was easier to isolate from the re-
action mixture (i.e., recycling). Performing the reaction in the
absence of water gave low reactivity. A plausible explanation is
that water facilitated the dissolution of glucose. Notably, this
mild dehydrogenation methodology was applied to a variety
of primary and secondary alcohols to generate the correspond-
ing aldehydes and ketones in good to excellent yield within
minutes (see the Supporting Information). These reactions
were performed in ethanol and water, with no side reactions
observed.
Scheme 2. Previously reported method yielded low selectivity and no ketone.
Scheme 3. Ketone 4 from lignin as a versatile starting material for the
synthesis of fine chemicals.
Pd/C-catalyzed aerobic oxidation of benzylic alcohols in
lignin model compounds was subsequently investigated. b-O-
4’-Ethanolaryl ethers, which mimic lignin from different sour-
ces, were chosen as model substrates to investigate the scope
[
9]
of palladium. However, metal hydrides are not convenient in
[
10]
large-scale productions. Initial attempts to selectively oxidize
the benzyl alcohol in lignin model compound 1a by Pd/C (see
the Supporting Information) with addition of sodium borohy-
dride led to marginally better results compared to the blank
reaction without additives (Table 1, entries 9–10). Ammonium
formate also gave poor selectivity in the oxidation reaction in
[3c]
of the reaction. Model 5a, with no methoxy substitution of
the aryl, is the major component in for example, switchgrass.
Substitution of the hydrogen atoms in the aryl group by one
3’
methoxy group in the R position (5 f) corresponds to the pre-
dominant lignin substitution pattern in softwood and, by two
[6]
3’
5’
which CÀO bond cleavage was observed (Table 1, entry 8).
methoxy groups in the R and R positions (5g), to the lignin
substitution pattern in hardwood, for example, birch and euca-
lyptus. Gratifyingly, selective oxidation gave the corresponding
products in 83–94% isolated yield (Table 2). No degradation of
the CÀO or CÀC bonds was observed. Interestingly, the substi-
tution pattern of the aryl did not affect the yield or selectivity
to generate the desired ketone. Highly substituted 5g was
transformed in a high yield (Table 2, entry 7). This compound
has previously proved difficult to transform by transition metal
On using alcohols, the conversions increased to above 50% of
the desired product (Table 1, entries 1–7). The best result was
obtained with glucose in which 1a was selectively dehydro-
genated to yield 2a in 68% yield (Table 1, entry 1). An opti-
mum of 40 mol% of glucose was found; increased or de-
creased levels of glucose gave poorer results. Attempts to per-
form the reaction in either an inert or oxygen atmosphere
were unsuccessful (see the Supporting Information). The reac-
tion could be performed in environmentally friendly solvents
such as ethyl acetate, ethanol, cyclopentyl methyl ether, or 2-
[3c]
catalysis. Lignin model compound 5h with a free phenolic
group also underwent selective dehydrogenation to generate
the corresponding ketone 6h in a good yield. Unpro-
tected phenols have been reported to be cumber-
[
a]
Table 1. Influence of additives on oxidation of 1a.
[4a,b,5]
some to transform.
Lignin model compounds with the g-alcohol motif
represent the native lignin structure. These b-O-4’-
glycerolaryl ethers models have previously showed
low or no reactivity in transition-metal-catalyzed reac-
[6,11]
Entry
Additive
2a [%]
tions.
Gratifyingly, the b-O-4’-glycerolaryl ethers
underwent selective benzylic oxidation to generate
ketones 7a–d in moderate to good yields (Table 3).
Compounds containing free phenol (7c) also showed
good reactivity and selectivity.
1
2
3
4
5
6
7
8
9
glucose
sorbitol
2-propanol
sucrose
pinacol
glycerol
ethylene glycol
HCOONH
68
56
53
52
50
48
42
41
36
18
To demonstrate the feasibility of the selective de-
hydrogenation, b-O-4’-glycerolaryl ether 1a was con-
verted to aryl ethyl ketone 4a in a one-pot proce-
dure (Scheme 4). Performing an aerobic oxidation
with Pd/C in an ethyl acetate and water mixture for
4
NaBH
–
4
1
0
[
8
a] Reaction conditions: 0.4 equiv additive, 0.05 equiv Pd/C (5 wt%), EtOAc/H
08C, 12 h.
2
O (4:1),
1
2 h at 808C gave 2a. Addition of base led to dehy-
dration intermediate 3. This intermediate was re-
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Dawange, Monali
