Visible light mediated efficient oxidative benzylic sp3 C-H to ketone derivatives obtained under mild conditions using O2

Article abstract of DOI:10.1039/c5cc06015j

A photooxygenation of benzylic sp³ C-H reaction has been demonstrated using O₂ mediated by visible light. This protocol provides a simple and mild route to obtain ketones from benzylic sp³ C-H bonds. Various benzylic sp³ C-H bonds can be transformed into the desired ketone derivatives in moderate to good yields. The ¹⁸O₂ labelling experiments demonstrated that the oxygen introduced into ketone originated from dioxygen. A plausible mechanism has been proposed accordingly.

Full text of DOI:10.1039/c5cc06015j

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3
Visible Light Mediated Efficient Oxidative Benzylic sp
C−H to Ketone Derivatives under Mild Conditions
using O2
Cite this: DOI: 10.1039/x0xx00000x
a
a
a
a
ab
Hong Yi, Changliang Bian, Xia Hu, Linbin Niu, and Aiwen Lei *
Received 00th January 2014,
Accepted 00th January 2014
DOI: 10.1039/x0xx00000x
www.rsc.org/
3
A photooxygenation of benzylic sp C−H reaction has been induced singleꢀelectron transfer process, visibleꢀlight photocatalysis
demonstrated using O2 mediated by visible light. This provide new protocol to C–H bonds functionalization that enable an
protocol provides a simple and mild route to ketones from environmentally friendly approach to the construction of valuable
3
7
benzylic sp3 C−H bonds. Various benzylic sp C−H bonds can chemicals under mild conditions. The molecular oxygen was also
be transformed into the desired ketone derivatives in used as the oxidant in many visible light mediated oxidative
1
8
moderate to good yields. The O2 labelling experiments reactions. In these photocatalytic reactions, two different roles of O2
demonstrated that the oxygen introduced into ketone are included, oxidases and oxygenases. The oxidases means the
originated from dioxygen. A plausible mechanism has been direct reduction of O2 to water or hydrogen peroxide to couple
proposed accordingly.
diverse oxidation reactions, which exists in most visible light
7h, 7r, 8
mediated oxidative reactions.
The oxygenases means directly
As a representative CꢀH bond functionalization process, direct
benzylic oxidation is one of the most useful and fundamental
transformations due to its wide applications in the synthesis of
incorporate oxygen atoms into the organic molecule and limited
examples have been focused on. In 2012, Xiao and coꢀworkers
realized the visibleꢀlight initiated aerobic oxidative hydroxylation of
1
pharmaceuticals and fine chemicals. Up to now, numerous methods
arylboronic acids and transform the oxygen of O into the product
2
3
9
3
have been developed for the oxidation of benzylic sp C−H bonds
phenol. Herein, we report a photooxygenation of benzylic sp C−H
reaction using O2 mediated by visible light (Scheme 1). This
protocol provides a simple and mild route to ketones from benzylic
into the corresponding ketones. Traditionally, benzylic oxidation
2
reactions involve the use of stoichiometric amounts metal oxidants.
3
During the last decades, great efforts that combining catalytic
amount of transition metal complexes or other additives such as KBr
etc with various oxidants like IBX, PhIO, tertꢀbutylhydroperoxide
sp C−H bonds.
(
TBHP) and oxone under homogeneous and heterogeneous
3
conditions have been made to modify these oxidative processes.
3
Scheme 1. Visible light mediated efficient oxidative benzylic sp C−H to
ketones.
Molecular oxygen (O ) as a cheap, safe and environmentally friendly
2
oxidant has been increasingly used in many common oxidative
4
processes. Recently, direct oxidation of benzylic C–H bond have
+
ꢀ
The 9ꢀmesitylꢀ10ꢀmethylacridinium ion (Acr –Mes ClO ) has
4
been wellꢀdeveloped using molecular oxygen as the oxidant, which
been widely applied in many photocatalytic reactions because of
3
5
provided a green route to ketones from benzylic sp C−H bonds.
+
strong oxidizing and reducing ability of the photoexcitation of Acr –
However, this area always required transition metal catalyst, additive
or high temperature. Thus, the development of simple and mild
conditions for selective benzyl oxidation is highly desirable.
ꢀ 10
Mes ClO4 . The photocatalytic oxygenation of pꢀxylene, durene
and mesitylene has been successfully transformed into the
+
ꢀ 11
corresponding aldehydes using Acr –Mes ClO . Though great
4
Visibleꢀlight, a clean, inexpensive, and ‘almost infinitely’
available source of energy, has received increasing attentions of the
chemistry community. Since 2008, visibleꢀlight photoredox catalysis
has received widespread research interest and various useful radical
success has been achieved, the xenon lamp was used and no ketones
3
from benzylic sp C−H bonds have been reported. Initially, we
started our evaluation of the reaction parameters with
+
ꢀ
diphenylmethane 1a as the model substrate using Acr –Mes ClO as
4
6
chemical transformations have been demonstrated. Based on photoꢀ
the photocatalyst. To obtain optimized conditions for this reaction,
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we tried different photocatalyst and a range of solvents. The
anticipated benzophenone product 2a could be achieved in 82%
+
ꢀ
yield in the presence of 1.0 mol% Acr –Mes ClO4 using CH CN as
3
the solvent in O atmosphere under a 3W blue LEDs (Table 1, entry
2
1
). Other data in Table 1 illustrates the impact of different conditions
on the efficiency of this reaction. Other tested photocatalyst such as
Ru(bpy) Cl or Eosin Y got trace or 22% desired product (Table 1,
3
2
entries 2 and 3). Among the screened solvents, the DMF did not
promote the reaction at all (Table 1, entry 5). The DCE can get
almost the same result as CH CN (Table 1, entry 4). Considering the
3
environmentꢀfriendly solvent effect, we chose the CH CN as our
3
target solvent to perform the oxidative reaction. In the control
experiments, no desired product was observed with neither
photoredox catalyst nor light indicating that the photoredox catalysis
is essential to this process (Table 1, entries 6 and 8). The reaction did
not happen without O (Table 1, entry 7).
2
a
Table 1. Optimization of reaction conditions
a
Reactions were conducted at rt for 12 h using 1 (0.50 mmol), Photo catalyst
+
ꢀ
b
Acr –MesClO
4 3 2
(1.0 mol%) in CH CN (2.0 mL) in O atomosphere under 3W
entry Photo catatlyst
solvent
CH CN
yield(%)
b
+
ꢀ
blue LEDs. Isolated yields are shown. 3.0 mol% Acr ꢀMesClO
4
.
+
1
2
3
4
5
Acr –Mes
3
82(77)
c
Ru(bpy)
3
Cl
2
CH
CH
3
CN
CN
n. d.
To gather insights into this oxidative reaction, radical
scavengers, such as TEMPO, was employed in the reaction
Eosin Y
3
22
80
+
Acr –Mes
DCE
DMF
(
Scheme 2a). The reaction was completely shut down, which
+
c
Acr –Mes
n. d.
d
+
c
could indicate that this transformation involved radical
intermediates. Based on literature, the benzyl alcohol derivative
may be a possible intermediate in the formation of ketone. We
also used the benzhydrol as the substrate under standard
conditions and almost quantitative yield was got (Scheme 2b),
6
7
Acr –Mes
CH
3
CN
n. d.
e
+
c
Acr –Mes
CH3CN
n. d.
c
8
No
CH
CH
3
CN
CN
n. d.
f
+
9
Acr –Mes
3
46
a
Reaction conditions: 1a (0.50 mmol), photocatalyst (1.0 mol%) in solvent
b
(
2
2.0 mL) at rt in O under 3W blue LEDs for 12 h. Determined by GC using which reveals that benzyl alcohol derivative may be an
biphenyl as an internal standard, the yield in the parentheses is isolated yield.
intermediate in this oxidation reaction. The time profile of
photocatalytic reaction shown in Figure 1 revealed that the
reaction was totally inhibited in the absence of light. This result
indicated that continuous irradiation of visible light is essential to
this photoꢀcatalytic transformation.
c
d
e
f
n.d.= no desired product. without light. Under N
2
atmosphere. Under Air
atmosphere, isolated yield.
3
With the optimal conditions established, various benzylic sp
C−H compounds were investigated and the results are summarized
in Table 2. This oxidative reaction was compatible with arylꢀMe, Bu
t
and F groups (2b–2d). The CꢀCl group was well tolerated in this
reaction providing the possibility for further functionalization (2e).
The electronꢀwithdrawing groups such as COOMe (2f) or COMe
(
2g) can also afford the desired ketone products in moderate yields,
while the NO group afforded a poor yield (2h). In addition, this
2
reaction was successfully amenable to the substitutions on the two
arenes (2i, 2j), which demonstrated a diversity of this protocol.
Interestingly, 1,2,3,4ꢀtetrahydronaphthalene could also be employed
to get the target alkenylation product without any difficulties (2k).
Furthermore, the isochromane was also suitable for this reaction and
the corresponding products were obtained in 64% yields (2l).
Scheme 2. Radical ꢀ trapping and benzhydrol oxidation experiments
3
Table 2. Visible light mediated efficient oxidative benzylic sp C−H to
a
ketones
2
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DOI: 10.1039/C5CC060 J
in 43% yields. Thus, these results indicated that 1,2ꢀdiphenylethanꢀ
ꢀone derivatives may be firstly oxidized to benzil derivatives, which
under further transformation into benzoic acids.
0
0
0
0
0
0
0
0
.7
.6
.5
.4
.3
.2
.1
.0
1
0
2
4
6
8
10
Figure 1. Time profile of photocatalytic reaction with and without visible
light.
18
To study where the oxygen comes from, O labeling
experiments were conducted using diphenylmethane 1a and
benzophenone 2a as substrates. The data listed in Table 3
suggests that carbonyl oxygen on benzophenone 2a originates
from O , which can readily exchange with water under the
2
Scheme 3. Visible light mediated oxidative reaction of 1,2ꢀdiphenylethanꢀ1ꢀ
one derivatives and benzil.
1
8
visibleꢀlight conditions. Firstly, we used the labelling O to do
2
1
6
the reaction under the standard conditions and a mixture of of
and O was generated, in which the ratio was 0.5: 1.0. When
O
18
10a
Based on the previous reports
and experimental results, a
1
8
16
using H O to do the labeling experiment, the ratio of O and
2
plausible mechanism is proposed in Scheme 4. Firstly, the
18
18
O was 0.6: 1.0. When the diphenyl ketone product 2a and H2
O
+
ꢀ
photocatalyst Acr –Mes ClO4 is excited by visible light irradiation
1
6
18
was stirring under standard conditions, a mixture O and O was
also generated. This result revealed that carbonyl oxygen on
benzophenone 2a can exchange with water under the visibleꢀlight
+
ꢀ
(
3W blue LEDs) to generate the excited species [Acr –MesClO ]*,
4
which then undergoes single electron transfer (SET) process with
3
+
benzylic sp C−H compound to generate radical cation 6, and [Acr –
12
conditions. In addition, no labeling product was generated when
ꢀ
+
ꢀ
MesClO ] radical anion. The [Acr –MesClO ] radical anion can be
4
4
1
8
just mixing benzophenone 2a and H2 O without visible light and
photocatalyst. This means the visible light and photocatalyst is
essential for the exchange of benzophenone 2a with water.
oxidized by O to recycle the photocatalytic reaction. The generated
2
radical cation 6 loses one proton to generate the radical 7, which can
react with O or O radical anion to generate the hydroperoxidate
2
2
1
8
a
intermediate 8. This intermediate 8 then loses one water and
transformed into the desired ketone product 2. The hydroperoxidate
intermediate 8 may also transformed into the benzyl alcohol
derivative 9, which under further oxidation to ketone 2.
Table 3. O labeling experiment
1
8
a
entry substrate (mmol)
Oꢀreagent
product
b
18
16
18
1
2
3
4
1a (0.2 mmol)
1a (0.2 mmol)
2a (0.2 mmol)
O
2
2a O/ O = 0.5 : 1.0
c
d
e
18
18
18
16
18
H
2
H
2
H
2
O
O
O
2a O/ O = 0.6 : 1.0
16
18
2a O/ O = 0.85 : 1.0
1
6
2a (0.2 mmol)
2
a Only O labeling
a
16
18
b
The ratio of O to O on products was determined by GCꢀMS. Reaction
conditions: 1a (0.20 mmol), Photo catalyst Acr –MesClO
+
ꢀ
4
(1 mol%) in
18
c
CH
conditions: 1a (0.20 mmol), Photo catalyst Acr –MesClO
2.0 mmol) in CH CN (2.0 mL) at rt in O under 3W blue LEDs for 10 h.
Reaction conditions: 2a (0.20 mmol), Photo catalyst Acr –MesClO
3
CN (2.0 mL) at rt in
O under 3W blue LEDs for 10 h. Reaction
2
+ ꢀ 18
4
2
(1 mol%), H O
d
(
3
2
Scheme 4. Proposed mechanism
+
ꢀ
4
(1
1
8
mol%), H
LEDs for 10 h. Reaction conditions: 2a (0.20 mmol), H
CH CN (2.0 mL) at rt for 10 h.
2
O (2.0 mmol) in CH
3
CN (2.0 mL) at rt in O
2
under 3W blue
O (2.0 mmol) in
e
18
In conclusion, we have disclosed a photooxygenation of benzylic
2
3
sp C−H reaction using O
provides a simple and mild route to ketones from benzylic sp C−H
When trying 1,2ꢀdiphenylethanꢀ1ꢀone derivatives as the substrates bonds. Various benzylic sp C−H bonds can be transformed into the
mediated by visible light. This protocol
3
2
3
3
1
8
under standard conditions, no target oxygenation product was desired ketone derivatives in moderate to good yields. The O2
detected. To our surprise, the benzoic acid derivatives were formed labelling experiments demonstrated that the oxygen introduced into
based on GCꢀMS and NMR spectroscopy (Scheme 3a). In this ketone originated from dioxygen. We believe this oxidative reaction
reaction, we proposed that the benzil derivatives 5 may be the key mediated by visible light will help chemists to design more and more
intermediate to the benzoic acids. To verify our hypothesis, we interesting, useful, and sustainable reactions in the near future.
demonstrate the reaction using benzil 5a under our standard
This work was supported by the 973 Program
conditions. The corresponding benzoic acid product 4a was obtained (2012CB725302), the National Natural Science Foundation of
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Program of Introducing Talents of Discipline to Universities of
China (111 Program) is also appreciated.
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