Palladium-catalyzed regio-selective oxidative C-H bond acylation of azoxybenzenes with alcohols

Article abstract of DOI:10.1039/c5ob00089k

A palladium-catalyzed regio-selective acylation of C-H bonds of azoxybenzenes with alcohols was developed using tert-butyl hydroperoxide (TBHP) as an oxidant. Alcohol derivatives can act as effective acyl precursors in situ, which are less toxic, inexpensive, stable, and commercially available. These transformations proceeded smoothly and could tolerate a variety of functional groups. This journal is

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Palladium-Catalyzed Regio-selective Oxidative C-H
Bond Acylation of Azoxybenzenes with Alcohols
Cite this: DOI: 10.1039/x0xx00000x
Lekai Hou,b Xiangxiang Chen,b Shuang Li, Suxian Cai, Yanxia Zhao, Meng Sun*a
and Xiao-Juan Yang
Received 00th January 2012,
Accepted 00th January 2012
DOI: 10.1039/x0xx00000x
www.rsc.org/
followed by further transformations, are rarely reported in the C-H
activation/functionalization process.11
A palladium-catalyzed regio-selective acylation of C-H bond
of azoxybenzenes with alcohols was developed using tert
-
Azoxy compounds are widely used in electronic devices due to
their wonderful liquid crystalline properties,12 and are also key
materials of polymer inhibitors, stabilizers and dyes.13 Although
numerous methods for the synthesis of azoxy compounds have been
developed,14 reports on the ortho-selective functionalization of
butyl hydroperoxide (TBHP) as oxidant. Alcohol
derivatives can act as effective acyl precursors in situ, which
were low toxic, inexpensive, stable, and commercially
available. These transformations proceeded smoothly and
could tolerate a variety of functional groups.
Table 1. Optimization of the reaction conditionsa
General methods for selective transformation of unreactive C-H
bonds have enjoyed tremendous advances owing to their widespread
application to the rapid construction of carbon-carbon or carbon-
heteroatom bonds, particularly in the fields of many useful
polyfunctional compounds.1,2 Directing-group-assisted strategy was
one of the most efficient methods to realize the regioselectivity of C-
H bond cleavage and its further transformation.3 As well as the
oxygen-containing groups,4 a variety of nitrogen-containing groups
have been extensively investigated, such as amides,5 imines,6
amines,7 oximes,8 pyridine9 and other nitrogen-containing
heterocycles.10 However, although significant progress in C-H bonds
functionalization has been achieved with the assistance of diverse
directing groups, there are rare studies on the regioselectivity control
of functionalization of compounds with unique functional groups,
which binds with transition metal to form different rings by making
use of the two or more possible orthogonal co-ordination modes.
Hence, it is very necessary to develop knowledge and associated
strategies for overcoming these hurdles. As far as we know, the
azoxy compounds with an unique directing group, which could be
used as anchors to form two different cyclometalated intermediates
O
N
O
N
N
O
CH2OH
ca a
mo
t l t 10
l
%
s
N
y
+
s
ox an so ven
id
t
l
t
,
H
a
1
a
2
a
3
b
n r
E t
a a
t l
x
an
t
o e
v n
t
S l
e
Yi ld
%
[
t
C
i
d
O
y
y
]
c
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
DTBP
D
C
E
E
E
E
E
E
E
E
E
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
Pd A
O
(
20
3
14
14
1
%
%
%
%
%
0
%
race
race
)
)
2
Pd TFA
D
C
2
(
Pd
2
l
C 2
D
C
D
C
D
C
e
C
Pd
l
M
(
N
)
2
C 2
3
Pd PPh
(
)
3 4
none
D
C
D
C
D
C
D
C
Pd TFA
7
(
(
(
(
)
Pd TFA 2
DD
t
t
t
t
Q
)
Pd TFA 2
K
2S2O8
)
)
)
)
)
)
2
2
2
2
2
Pd TFA
Pd TFA
(
Pd TFA
Pd TFA
(
Pd TFA
Pd TFA
Pd TFA
race
race
c
A
)
2
DCE
PhI
O
(
H2O2
TBHP
TBHP
TBHP
D
C
E
o uene
T l
2
0
%
(
oxane
Di
Ph
n.r.
l
C
23
%
(
(
(
2
2
c
race
t
A
H
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
O
)
)
2
nea
DME
DMF
t
4
2
%
race
t
n.r.
H
O
Pd TFA
( )
2
Pd TFA
(
(
)
Pd TFA 2
N
N
n.r.
%
%
%
%
%
DM
SO
)
Pd TFA 2
56
7
3
58
54
60
c
D
E
C
e
o-se ec ve
ti
(
)
-
on ac va on
d
TFA 2
R
i
l
H
H
g
d
O
N
O
P
d
(
ti ti
D E
C
C H b
)
2
2
2
2
e
Pd TFA
D
C
D
C
D
C
E
E
E
N
O
(
)
)
d f
,
Pd
N
H
O
Pd TFA
(
-ca a .
-ca a .
Pd
Pd
t
l
23
24
t
l
N
N
O
N
d
,
g
Pd TFA
or o-ac
a on
l
ti
y
or o-ac
th
a
on
th
l
y
ti
(
)
N
N
O
CH2OH
or
CH2OH
a All the reactions were carried out in the presence of 0.2 mmol of 1a, 0.6
mmol of 2a and 0.6 mmol of TBHP in 1.0 mL DCE at 100 ºC under air
condition. b Isolated yields. c 6.0 equiv of TBHP was added. d 7.0 equiv of
TBHP was used. e 8.0 equiv of TBHP was used. f At 80 oC. g At 120 oC.
H
Pd
H
n ro uc
O
l
d
t
y p
o
o
serve
b d
N
t
Scheme 1 Pd-catalyzed ortho-acylation of azoxybenzenes with
alcohols.
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the model substrates to optimize the reaction conditions. To our
Table 2. Scope of the ortho-acylation of azoxybenzene with different
alcohols a,b
delight, when the mixture of azoxybenzene (0.20 mmol) and benzoyl
alcohol (0.6 mmol) was treated with 3.0 equiv. of TBHP in DCE
o
(1.0 mL) at 100 C for 20 h in the presence of 10 mol% Pd(OAc)2,
O
N
O
N
the desired product 3a was isolated in 20% yield (Table 1, entry 1).
Further catalyst screening showed that an obvious increasing yield
was achieved when Pd(TFA)2 was used in the reaction, and the
corresponding product was obtained in 32% yield (Table 1, entry 2).
However, other catalysts such as PdCl2, PdCl2(MeCN)2 and
Pd(PPh3)4 were less effective (Table 1, entry 3-5). Subsequently,
different oxidants were investigated in this transformation and TBHP
was the most suitable oxidant compared with other oxidants
including DTBP, DDQ, K2S2O8, PhI(OAc)2, H2O2, etc. (Table 1,
entry 7-11). Notably, no desired product was detected in the absence
of catalyst even in high temperature (Table 1, entry 6). Additionally,
the effect of solvents on this transformation was also tested, and
DCE proved to be crucial, while a slight lower yield was also
isolated in PhCl and comparative yield was achieved under neat
condition (Table 1, entry 12-19). It should be noted that increasing
the stoichiometry of TBHP significantly increased the yield of the
corresponding azoxy ketones. The best result was observed in 73%
with 7 equiv of TBHP, while increasing or lowering the amount of
oxidant suppressed the efficiency (Table 1, entry 20-24). Finally, the
optimized reaction conditions for the ortho-C-H acylation of azoxy
compound were obtained as follows: 10 mol% Pd(TFA)2 as catalyst,
7.0 equiv of TBHP as oxidant, and DCE as solvent, at 100 oC under
air for 20 h.
With the optimized reaction conditions in hand, the applicability
of this protocol was subsequently investigated with regard to
different alcohols as the in situ generated acyl source. It was found
that a variety of alcohols with both electron-donating and electron-
withdrawing groups substituted on aromatic ring were tolerated and
moderate to good yields were obtained for most case. For example,
the substrates with electron donating methyl group on the aromatic
ring gave the desired products in good yields (Table 2, 3ab-3ad),
while para electron-withdrawing group-substituted benzyl alcohols
afforded comparative results (Table 2, 3ah-3aj). It is worth noting
that ortho-hindered benzyl alcohol was feasible coupling partner in
this catalytic system. Moreover, lower yields were obtained when 4-
mo
Pd TFA 2 10
l
%
N
N
O
(
)
+
R
CH2OH
TBHP D E 100 o
C
C
,
,
H
R
a
1
2
3
O
N
O
N
N
O
N
aa
=
=
=
=
3
R
H 7
3
,
,
,
,
,
%
65
68
a
-
e
e
3 b R 4 M
%
%
0
%
O
3
ac
a
-
M
3
R
3
2
,
ae
,
4
6
%
-
e
M
7
,
3 d R
,
R
O
N
O
N
N
O
N
O
a
3 f
e
e
M
%
38
%
M
O
O
29
,
g
,
a
3
e
M
O
O
N
O
N
N
O
N
O
a
=
-
,
3 h R 4 F
66
%
,
a
=
=
-
l 75%
C
,
3 i R
4
,
a
3
- r
4 B 2
7
,
%
R
,
j
a
3 k 4
6
%
,
R
N
C
O
N
O
N
N
O
N
O
a
3 l 4
0
%
,
am
3
39
%
,
Table 3. Scope of azoxy compounds in ortho-acylation reactions a,b
N
O2
a All the reactions were carried out in the presence of 0.2 mmol of 1a, 0.6
mmol of 2a and 1.4 mmol of TBHP in 1.0 mL DCE at 100 ºC under air. b
O
R
O
N
N
R
+
N
O
CH2OH
R
mo
Pd TFA 2 10
l
%
N
(
)
Isolated yields.
R
R
TBHP DCE 100 o
C
,
,
azoxybenzenes are scarce.11 Besides, it is well known that aryl
ketones are very important structural motifs of natural products and
pharmaceuticals.15 During the development of preparation of aryl
ketones, the emergence of direct functionalization of C-H bonds
provides an atom-economic and highly efficient strategy16 compared
with the traditional methods of Friedel-Crafts acylation relying on
Lewis acids or Bronsted acid17 and various oxidants.18 To the best of
our knowledge, there are only two reports on the acylation of
azoxybenzenes,11a,11c and this restriction has greatly limited the
method to preparation of azoxy ketones. Consideration of the
limitations and extremely challenge in the control of two different
kinds of C-H bonds, it is necessary to develop a new method to
realize regio-selective acylation of C-H bond of azoxy compounds.
Herein, we describe a facile synthesis of azoxy ketones by Pd-
catalyzed ortho-C-H bond acylation employing alcohol derivatives
as the reliable coupling partners,19 which show obvious advantage of
H
a
1
3
2
e
OM
O
N
O
N
R
N
N
O
a
,
=
=
=
-
e
e
3b
R
R
R
4 M
1
6 %
,
O
e
M O
ea
,
3
75
%
ca
,
-
M
-
e
2 M 70
,
3
3
68%
,
c
a
,
3d
%
t
CO2E
O
N
O
N
R
N
O
N
O
R
a
g
=
=
-
3f
3
R
R
4 F 4
,
0%
,
EtO2C
a
3h
, 0%
a
,
-
4
C
l 54
%
,
high stability, easy to handle and low cost compared with α- a All the reactions were carried out in the presence of 0.2 mmol of
1, 0.6
mmol of 2a and 1.4 mmol of TBHP in 1.0 mL DCE at 100 ºC under air. b
oxocarboxylic acid and aldehydes.
Isolated yields. c At 120 oC, 36 h.
We initially used azoxybenzenes (1a) and benzoyl alcohol (2a) as
2 | J. Name., 2012, 00, 1-3
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conditions. The results of the reaction with the azoxy derivatives
bearing electron-donating groups were better than with those bearing
electron-withdrawing groups (Table 3). For example, 4,4’-dimethyl
azoxybenzene and 4,4’-azoxydianisole gave the corresponding
azoxy ketones in good yields, while sharp reducing yields were
observed with electron-deficiency substituents at the para-positions
of azoxybenzenes, especially for the CO2Et, no C-H cross-coupling
reaction occurred at all.
To further investigate the possible mechanism, some control
experiments were performed to obtain mechanistic insight (Scheme
2). It is observed that benzaldehyde was formed under our reaction
conditions in the absence of azoxy compound. Very recently, we
H
C
O
mo
Pd TFA 2 10
l
%
H
O
(
)
TBHP D E 100 o 20 h
C
C
,
,
,
a
2
a
e
2 '
61 GC i ld
,
%
y
O
N
O
N
H
C
O
mo
Pd TFA 2 10
l
%
N
N
O
(
)
+
TBHP D E 100 o
C
C
,
,
H
R
a
1
a
2 '
aa
3
66
,
%
O
N
O
mo
e u v.
i
q
)
Pd TFA 2 10
l
%
(
)
N
have developed
a method to realize the ortho-acylation of
.
TEMP 1 0
N
H
O
N
O
(
+
TBHP D E 100 o
azoxybenzene with benzaldehyde under similar reaction
conditions.11c Moreover, treatment of azoxybenzene (1a) and benzyl
alcohol (2a) in the presence of the radical scavenger TEMPO
suppressed the reaction completely, which indicated that a radical
process was possibly involved in this procedure.20
C
C
O
,
,
H
R
a
1
a
2 '
aa
3
no e ec e
t d t t d
Based on the previous literatures19,21 and above-mentioned control
experiments, a plausible mechanism pathway for this transformation
is depicted in Scheme 3. First, the active palladium catalyst reacts
with azoxybenzene to generate intermediate I22 by chelation-directed
C-H activation with N atom, which is generally considered to be a
better coordinating atom than O. At the same time, aldehyde was
formed via the oxidation of the alcohol by TBHP, followed by
transferred to an acyl radical according to the literature. Second, the
Pd(II) intermediate I would react with the acyl radicals to afford the
Pd(IV) intermediate II.23,24 Finally, the intermediate II underwent
reductive elimination to afford the ortho-acylated product and the
active Pd(II) was regenerated.
In summary, we have developed a novel catalytic system for the
synthesis of ortho-acetyl functionalization of azoxybenzenes via
regio-selective directing-group-assisted strategy. Alcohol derivatives
can be employed as effective acyl precursors in the oxidative
coupling between two C-H bonds. Further exploration of the
synthetic utility of this chemistry and other functionalization of
azoxy compounds are currently underway in our lab.
Scheme 2 Control reactions.
methoxybenzyl alcohol and 3,5-dimethoxybenzyl alcohol were
involved, and the corresponding azoxy ketones were achieved in 38%
and 29% yields, respectively, presumably due to the difficult C-H
bond cleavage of aldehydes generated in situ. In particular, the
bromo group remained intact during the course of the reaction,
which could be further transformed into other important structures.
To our delight, a representative structure of 3ai was confirmed by X-
ray single-crystal analysis (see Supporting Information (SI) for more
details), which proved the single isomer product. Encouraged by
these positive results, we also extended the substrate scope to benzyl
alcohol derivatives with strong electron-withdrawing groups and
aliphatic alcohols, and obtained the desired acylated azoxy
compounds in moderate yields (Table 2, 3ak-3am). Unfortunately,
heterocyclic-substituted alcohols could not participate in this
procedure, which often showed positive results in acylation reactions.
Next, the scope of this transformation with regard to a range of
azoxybenzenes was then explored with benzyl alcohol under our best
We are grateful to the Northwest University (PR13089,
NF14020) and National Found for Fostering Talents of Basic
Science (NFFTBS-J1103311, J1210057) for financial support.
O
N
O
N
Pd TFA
(
)
2
N
N
O
H
Notes and references
a
1
aa
3
c c o a a a on
ll d ti
aKey Laboratory of Synthetic and Natural Functional Molecule Chemistry
of Ministry of Education, Department of Chemistry & Materials Science,
Northwest University, Xi’an 710069, P. R. China
bThese two authors contributed equally to this work.
*E-mail: sunmeng@nwu.edu.cn
l
y
p
CF3COOH
re uc ve e m na on
li
ti ti
d
i
† Electronic supplementary information (ESI) available: CCDC 1037307.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c000000x/
O
O
N
N
N
]
N
PdII
Pd
[
[
]
R
I
1
For selected reviews, see: (a) G. Dyker, Handbook of C-H
Transformations: Applications in Organic Synthesis, Wiley-VCH,
Weinheim, 2005; (b) K. I. Goldberg and A. S. Goldman, Eds.
Activation and Functionalization of C-H Bond, ACS Symposium
Series 885, American Chemical Society, Washington, DC, 2004. (c)
R. Giri, B.-F. Shi, K. M. Engle, N. Maugel and J.-Q. Yu, Chem. Soc.
Rev., 2009, 38, 3242; (d) J.-Q. Yu and Z.-J. Shi, C-H Activation,
Springer, Berlin, Germany, 2010.
O
II
o
e ca
th ti l Pd IV i t
n erme a e
di t
)
h
yp
(
-
u
-
u
t B OOH
t B O
HO
H
O
O
-
u
-
u
t B OH
t B OH
a
2
2
(a) S. Murai, F. Kakiuchi, S. Sekine, Y. Tanaka, A. Kamatani, M.
Sonoda and N. Chatani, Nature, 1993, 366, 529; (b) D. A. Colby, R.
Scheme 3 Proposed reaction mechanism.
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10 For selected examples, see: (a) R. Giri, X. Chen and J.-Q. Yu, Angew.
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24 However, a Pd(II) intermediate can not be ruled out at this stage,
since the formation of cyclopalladation, insertion of aldehyde into the
C-Pd bond and β-hydride elimination are precedented
transformations for Pd(II) species.
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