Allylic and benzylic sp3 C-H oxidation in water

Article abstract of DOI:10.1039/c4ob02017k

A copper-catalyzed method for the oxidation of allylic and benzylic sp³ C-H by aqueous tert-butyl hydroperoxide (T-Hydro) in water using a recyclable fluorous ligand has been developed. The reaction procedure is tolerant to additional functional groups and the fluorous ligand could be reused with little loss of catalytic activity. This journal is

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Allylic and benzylic sp3 C-H oxidation in water
Wei Jie Ang and Yulin Lam*
A copper-catalyzed method for the oxidation of allylic and benzylic sp3 C-H by aqueous tert-
butyl hydroperoxide (T-Hydro) in water using a recyclable fluorous ligand has been developed.
The reaction procedure is tolerant to additional functional groups and the fluorous ligand could
be reused with little loss of catalytic activity.
Introduction
Results and discussion
Allylic and benzylic oxidations via the direct sp3 C-H bond In the initial assessment of the allylic oxidation reaction, 1-
phenyl-1-cyclohexene 1a was chosen as the model substrate. We
investigated the optimization reaction conditions by varying the
solvent, catalyst, base and oxidants (Table 1a). When the allylic
oxidation was conducted using 1a, 5 mol% CuI, 5 mol% L1 as
ligand, TBHP (6M in decane), K2CO3 and water as solvent at
room temperature, the desired 3-phenylcyclohex-2-enone 2a was
obtained after 12 h in 29% yield (Table 1a, entry 1). When water
was replaced with acetonitrile, compound 2a was obtained in
similar yield (Table 1a, entry 2). To optimize the reaction, we
varied the metal catalyst and found that the reaction in aqueous
medium was most efficient with CuI (Table 1a, entries 3-10).
Next we varied the base and established that the reaction was
most efficient in the absence of base (Table 1a, entries 11-14).
Subsequently, we screened different oxidants on the reaction
which indicated that TBHP was a more effective oxidant than
H2O2, DTBP or m-CPBA. In addition, both T-Hydro and TBHP
provided compound 2a in similar yields (Table 1a, entries 15-
21). Hence we decided to use T-Hydro as the oxidant since the
absence of decane would increase the environmental friendliness
of the reaction. We also examined the effect of varying the ratio
of CuI and L1 but this provided compound 2a in lower yield
(Table 1b, entries 2-4). The minimum time for the reaction to
reach completion was also determined to be 2h (Table 1b, entry
5). The amount of oxidant employed was also examined.
Lowering the amount of oxidant to 4 equivalents increased the
yield of 2a to 62% but further decreasing the amount of oxidant
was detrimental to the reaction yield (Table 1b, entries 5-8).
It was previously reported that surfactant molecules, could
aid in reaction rates.8a,9 Thus the effect of additives was
investigated and the reaction was most effective when 5 mol%
of sodium dodecyl sulfate (SDS) was added. The addition of 5
mol% SDS not only halved the reaction time but also provided
compound 2a in a higher yield (Table 1b, entries 9-12). Besides
L1, we also tested two other fluorous ligands, L2 and L3 (Fig.
1). but found L1 to be the most effective ligand for facilitating
the allylic oxidation reaction (Table 1b, entries 13-14). Finally,
activation of alkenes and alkylarenes to the corresponding ,-
unsaturated enones and carbonyl compounds are important
transformations with a wide variety of industrial applications1
including the synthesis of drug precursors and building blocks in
many organic syntheses.2 Although several organocatalysts have
been developed for these oxidations,3 the efficiency of metal-
catalyzed allylic and benzylic oxidations remains remarkable.4
To-date several protocols for sp3 C-H oxidation catalysed by
metal complexes in combination with tert-butyl hydroperoxide
(TBHP) have been reported.4a-f,5 However amongst the different
metals, fewer oxidations have been done using Cu as catalyst,
even though copper proteins found in nature are catalysts,
especially for oxidation reactions.6
Previously, we have reported a recyclable, fluorous catalyst
that efficiently promoted carbon-carbon bond formation
reactions in water.7 Since development of aqueous-phase
reaction is an active field in organic synthesis due to demands
for realization of green chemical processes8 and many classic
reactions that are commonly used in organic synthesis are
currently being re-examined in order to develop cleaner chemical
processes and to reduce or eliminate existing drawbacks and
inefficiencies, we herein report a protocol for room temperature,
copper-catalyzed allylic and benzylic oxidation by T-Hydro in
water using a recyclable fluorous ligand L1 (Fig. 1). To the best
of our knowledge, there are no earlier publications on metal-
catalyzed allylic oxidation in water.
Fig. 1 Structure of fluorous ligand L1
.
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13c
4
12
a control experiment in the absence of L1, only trace amount of
2a was obtained (Table 1b, entry 15).
5 mol% L2 instead of L1, 5
35
mol% SDS added
14d
4
4
12
1
5 mol% L3 instead of L1, 5
mol% SDS added
23
Table 1a Optimization of the allylic oxidation reaction of 1-phenyl-1-
cyclohexene 1aa
15
Absence of L1, 5 mol% SDS
Trace
a
Reaction condition: 1a (0.5 mmol), CuI (5 mol%), L1 (5 mol%), T-
b
c
Hydro, H2O (1.0 mL), r.t. Isolated yields. 25% starting material
recovered. d 50% starting material recovered.
Having established the optimal reaction condition, different
alkenes were tested to explore the generality of this oxidation
transformation. As shown in Table 2, the reaction condition was
compatible with both electron-withdrawing and –donating
substituents as well as reactive functional group (entries 3-5).
The oxidation reaction was also regioselective at the least
hindered allylic carbon which is consistent with previously
published results.4e It is worth noting that the allylic oxidation of
substrate 1c did not occur at the methyl substituent (entry 3).
Attempts to extend the substrate scope to linear allylic alkenes
(1-hexene, 1-octene, 1-decene, 3-pentenenitrile) were
unsuccessful under the optimized conditions.
Entry
Catalyst
Oxidant
Base
Yieldb (%)
1
2c
3
CuI
CuI
TBHP
TBHP
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
Ag2CO3
KOAc
KOH
-
29
30
CuBr
CuBr2
Cu(OTf)2
FeCl2
FeCl3
CoCl2
Ni(OAc)2
MnO2
CuI
TBHP
24
4
TBHP
27
5
TBHP
23
6
TBHP
5
7
TBHP
21
8
TBHP
0
9
TBHP
0
10
11
12
13
14
15
16
17
18
19
20
21
TBHP
16
TBHP
6
CuI
TBHP
43
CuI
TBHP
Trace
53
CuI
TBHP
CuI
H2O2
-
Trace
Trace
Trace
Trace
17
Scheme 1 Reaction of 1a’ under optimized reaction condition
FeCl3
CuI
H2O2
-
DTBP
-
Tert-butyl peroxy ether of the allylic substrates was observed
as the by-product in the allylic oxidation reaction (Table 2,
entries 1-5). 3-t-Butylperoxy-1-phenylcyclohexene 1a’ was
isolated from the allylic oxidation of 1a in 10% yield (Table 2,
entry 1). When the isolated 1a’ was subjected to the allylic
oxidation condition, 2a was obtained in 75% yield (Scheme 1).
CuI
Urea, H2O2
NaBO3.H2O
m-CPBA
T-Hydro
-
CuI
-
CuI
-
20
CuI
-
51
a Reaction condition: 1a (0.5 mmol), catalyst (5 mol%), L1 (5 mol%), oxidant,
base (0.5 equiv.), H2O (1.0 mL), r.t., 12 h. b Isolated yields. c Using CH3CN as
solvent instead.
Purification and isolation of tert-butyl peroxy ether of 1b-1e was
1
unsuccessful but H NMR of the by-product fraction shows the
Table 1b Optimization of the allylic oxidation reaction of 1-phenyl-1-
cyclohexene 1aa
t-Bu signal in the 1.2 ppm region.
Table 2 Copper-catalyzed allylic and benzylic oxidationa
T-Hydro
(Equiv.)
Change from standard
condition
Time
(h)
Yieldb
(%)
Entry
1
2
5
5
5
5
5
4
3
2
4
4
4
4
None
10 mol% L1
12
12
12
12
2
51
35
33
30
52
62
41
38
23
60
71
61
Time
(h)
Yieldb
(%)
Entry
1
Substrate
Product
3
10 mol% CuI and 10 mol% L1
1 mol% CuI and 1 mol% L1
None
1
71 (10c)
4
5
6
None
2
2
3
2
58 (10c)
7
None
2
8
None
2
1
62 (5c)
9
5 mol% PhNHNH2 added
5 mol% TBAB added
5 mol% SDS added
1 mol% SDS added
2
10
11
12
1
1
1
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oxidation products in moderate yields (Table 2, entries 9-10).
Alkylarenes bearing an electron-withdrawing group on the alkyl
moiety are difficult substrates for direct oxidation due to the inert
benzylic C-H bond.10 Gratifyingly, we were able to apply our
condition to the oxidation of substrate 1m to obtain compound
2m in 73% yield (Table 2, entry 13). The methyl ester
functionality of substrate 1m was tolerated under the mild
condition with no hydrolyzed product observed. Long chain aryl
alkanes 1n was also successfully oxidized to the corresponding
ketone albeit lower (51%) yield (Table 2, entry 14).
4
5
1
1
58 (8c)
74 (8c)
6
6 (2d)
88 (90d)
7
8
9
6 (2d)
81 (84d)
Table 3 Recycling of L1a
1
70
Cycle
Time (h)
Yieldb (%)
Recovered L1c (wt%)
1
2
3
4
5
1
1
71
69
66
64
60
95
93
93
90
81
3d
51d
1.5
1.5
2
10
11
1
68
a Reaction condition: 1a (2.5 mmol), CuI (5 mol%), L1 (5 mol%), T-Hydro
(4.0 equiv.), SDS (5 mol%), H2O (5.0 mL), r.t. b Isolated yields. c Recovered
via F-SPE.
6 (2d)
85 (86d)
Next we investigated the possibility of recycling and reusing
L1. 1-Phenyl-1-cyclohexene 1a was used for the model study
under the optimized reaction condition. The recycling
experiments were carried out over 5 cycles and the time taken for
the reaction to complete was 1-2 h to provide 2a in 81-95%
yields (Table 3). The recovered L1 showed a slow decline in
catalytic activity after several recoveries. In addition, the
recovery of L1 via F-SPE was good except in the 5th cycle. The
fluorous silica gel used in F-SPE could be reused10 which
minimized waste thus making the recovery of L1 a greener
procedure.
12
13
14
1
1
1
68
73
51
Finally to apply this reaction in gram-scale reaction, 10.8
mmol (1.71 g) of 1a was used for the allylic oxidation reaction
under the optimized reaction condition. The reaction proceeded
smoothly, gave the corresponding product 2a in similar (70%)
yield with 9% of 1a’ and the recovery of L1 was also high (94%).
a Reaction condition: Substrate (0.5 mmol), CuI (5 mol%), L1 (5 mol%),
T-Hydro (4.0 equiv.), SDS (5 mol%), H2O (1.0 mL), r.t. b Isolated yields.
c Yields of tert-butyl peroxy ether by-products. d Oil bath temperature of
50 °C.
Encouraged by the promising results, we extended the
optimized condition to benzylic oxidation (Table 2, entries 6-13).
We found that diphenylmethane type substrates gave the
corresponding products in good yields (Table 2, entries 6, 7 and
11) although the reaction required a longer time to complete.
This could be attributed to these substrates being solids at room
temperature and have poor solubilities in water. When the
temperature was increased to 50 oC, the reaction time was
considerably shortened and the products were obtained in
comparable yields. Substrates bearing a heteroatom were also
compatible with the reaction condition and provided the benzylic
Scheme 2 Gram-scale synthesis of 2a
A proposed catalytic cycle of the L1/CuI catalyzed allylic
oxidation is depicted in Scheme 3. The copper complex
formed in-situ from L1 and CuI. The formation of tert-butyl
peroxy radical IV abstracts a hydrogen atom from the alkene to
I was
give the radical
V which forms the tert-butylperoxy intermediate
VI. This is supported by the reaction of 1a’ in Scheme 1.
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added K2CO3 (1.38 g, 10 mmol) and C8F17(CH2)3I 4 (1.96 g, 3.33
mmol). The reaction mixture was stirred at room temperature for 12
h, quenched with H2O (5 mL) and consecutively washed with EtOAc
(20 mL x 3). The combined organic layer was washed with brine (10
mL x 3), dried over anhydrous MgSO4, filtered and concentrated. The
desired product 5 (2.90 g, 99%) was obtained as a white solid after
purification by F-SPE.
Synthesis
of
N-(4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-
heptadeca fluoroundecyloxy)benzyl)-1-(pyridin-2-yl)-N-(pyridin-
2-ylmethyl)methanamine (L1). To a solution of compound 5 (0.58
g, 1.0 mmol) in dichloroethane (15 mL) was added di-(2-
picolyl)amine (0.22 mL, 1.2 mmol). The reaction mixture was stirred
at room temperature for 1 h. Thereafter NaBH(OAc)3 (305 mg, 1.44
mmol) was added and the reaction mixture was further stirred for 24
h. After which, the reaction mixture was quenched with H2O (5 mL)
and the organic layer was separated. The aqueous layer was further
extracted with dichloromethane (15 mL) and the combined organic
layer was dried over anhydrous MgSO4, filtered and concentrated.
The desired product L1 (377 mg, 62%) was obtained as a yellow oil
after purification by F-SPE.
Scheme 3 Proposed catalytic cycle of L1/CuI catalyzed allylic
oxidation
Synthesis of 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-hepta
decafluoroundecyloxy)-2,6-di(2-pyridyl)pyridine (L2). To a
solution of [2,2':6',2''-terpyridin]-4'-ol
6 (125 mg, 0.5 mmol) in
DMF (5.7 mL) was added K2CO3 (104 mg, 0.75 mmol). The
reaction mixture was stirred at 80 °C for 30 min. Thereafter
Experimental
C8F17(CH2)3I 4 (441 mg, 0.75 mmol) was added and the reaction
General
mixture was further stirred for 24 h, quenched with H2O (2 mL)
and filtered. The desired product L2 (345 mg, 97%) was obtained
as a white solid after washing with hexane (5 mL x 2) to remove
residual C8F17(CH2)3I.
All chemicals purchased were used without further purification.
Compound 611 and 712 were synthesized according to previously
reported procedure. Moisture-sensitive reactions were carried
out under nitrogen with commercially obtained anhydrous
solvents. Analytical thin-layer chromatography (TLC) was
carried out on precoated F254 silica plates and visualized with
UV light. Column chromatography was performed with silica
(Merck, 230 – 400 mesh). F-SPE was performed with
Synthesis
2,2'-(4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-
heptadecafluoroundecyloxy)pyridine-2,6-diyl)bis(1
H-
benzo[
d
]imidazole) (L3). To
a
solution of 2,6-bis(1H-
benzo[d]imidazol-2-yl)pyridin-4-ol
7
(164 mg, 0.5 mmol) in
DMF (5.7 mL) was added K2CO3 (104 mg, 0.75 mmol). The
reaction mixture was stirred at 80 °C for 30 min. After which,
C8F17(CH2)3I 4 (441 mg, 0.75 mmol) was added and the reaction
mixture was further stirred for 24 h, then quenched with H2O (2
mL) and filtered. The desired product L3 (352 mg, 89%) was
obtained as an off-white solid after washing with hexane (5 mL
x 2) to remove residual C8F17(CH2)3I.
1
FluoroFlash® silica gel (40 micron). H and 13C NMR spectra
were recorded at 298K. Chemical shifts are expressed in terms
of δ (ppm) relative to the internal standard tetramethylsilane
(TMS). Mass spectra were performed under EI and ESI mode.
Synthesis of fluorous ligands L1-L3
General procedure for the copper-catalyzed allylic and
benzylic oxidation. A mixture of the substrate (0.5 mmol), L1
(5 mol%), CuI (5 mol%), SDS (5 mol%) and water (1.0 mL) was
stirred at room temperature. T-Hydro (4 equiv.) was added
dropwise to the stirred solution. The reaction was monitored by
TLC. When the reaction has completed, the reaction mixture was
diluted with EtOAc (20 mL) and the organic layer was separated.
The aqueous layer was further extracted with EtOAc (20 mL).
The combined organic layer was dried over anhydrous MgSO4,
filtered and concentrated. The crude product was then purified
by column chromatography.
General procedure for the recycling experiment using F-
SPE. Recycling experiments were carried out on a 2.50 mmol
scale using the general procedure described above. The crude
product was first diluted with THF–H2O = 7: 3 (1 mL) and
loaded into a F-SPE fluorous silica column (column diameter:
2.20 cm; amount of fluorous silica required: 10% of crude
product by weight). The crude product was then eluted using
THF–H2O = 7: 3 (30 mL) as eluent and L1 was subsequently
eluted with THF (20 mL). The fluorous silica was regenerated
and reused (5 times) after washing with acetone (10 mL). To
determine the amount of product formed after each recycling
experiment, the solution of crude product was concentrated,
Scheme 4 Synthesis of fluorous ligands L1 to L3
Synthesis of 4-(4,4,5,5,6,6,7,7,8,8,9,9,10,10,11,11,11-heptadeca
fluoroundecyloxy)benzaldehyde (5). To
a
solution of 4-
hydroxybenzaldehyde 3 (0.61 g, 5.0 mmol) in DMF (10 mL) was
4 | J. Name., 2012, 00, 1-3
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diluted with EtOAc (20 mL) and washed with water (10 mL).
The organic layer was dried over anhydrous MgSO4, filtered,
concentrated and then purified accordingly.
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Conclusion
An environmentally benign procedure for the room temperature,
copper-catalyzed allylic and benzylic oxidation by aqueous
TBHP in water using a recyclable fluorous ligand has been
developed. Pre-generation of the organometallic reagent was not
required. This increases the green aspect of the reaction.
Fluorous ligand L1 was recycled and reused with little loss of
catalytic activity. The oxidation procedure would also be applied
to gram-scale reaction.
5
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