Palladium(II)-Catalyzed Acetoxime Directed ortho-Arylation of Aromatic Alcohols

Article abstract of DOI:10.1002/chem.201503284

Biaryls, which contained a benzyloxy motif, were directly constructed through a ligand-promoted Pd^II-catalyzed ortho-arylation of masked aromatic alcohols. A variety of acetoxime ethers could be coupled with a diverse range of arylboronic acid

Full text of DOI:10.1002/chem.201503284

DOI: 10.1002/chem.201503284
Full Paper
&
CÀH Activation
Palladium(II)-Catalyzed Acetoxime Directed ortho-Arylation of
Aromatic Alcohols
[a]
[a]
[b]
[a]
Kun Guo, Xiaolan Chen, Jingyu Zhang, and Yingsheng Zhao*
Abstract: Biaryls, which contained a benzyloxy motif, were
directly constructed through a ligand-promoted Pd -cata-
bioactive biaryls in modest to good yield. Not only could
acetoxime be subsequently removed without a separation,
functionalized hydroxylamine derivatives could also be ob-
tained.
II
lyzed ortho-arylation of masked aromatic alcohols. A variety
of acetoxime ethers could be coupled with a diverse range
of arylboronic acid pinocol esters, giving direct access to
Introduction
a demand to more efficiently and directly construct biaryls
containing a benzyloxy motif, which are the omnipresent com-
ponent of many drug candidates, for example, in known HNF-
Development of controlling site-specific functionalization of
CÀH bonds offers an innovative approach to construct the
core structures of pharmaceutical agents, natural products,
[
11]
[12]
[13]
4a modulators, JTT-305,
and VEGF receptor inhibitors
(Figure 1; HNF=hepatocyte nuclear factor, VEGF=vascular en-
[1]
and organic materials. Harnessing a directing group to
enable site-selective CÀH functionalization is one of the most
[2]
successful strategies; however, such transformations are
sometimes in conflict with the directing groups available in
a particular starting material. Despite great efforts that have
been made in the last few decades to exploit varieties of auxil-
[
3]
[4]
[5]
iaries for amines, acids, and ketones in transition-metal-
catalyzed direct CÀH functionalization, it remains a significant
challenge to develop alcohol or masked alcohol-directed CÀH
Figure 1. Biologically active compounds containing a benzyloxy motif.
[6]
oxidation and CÀC formation reactions, due to the possible
oxidation of the hydroxyl group, weak coordination of alcohols
II
with Pd , and the constraints of introducing suitable surrogate
dothelial growth factor). Recently, our group reported acetox-
groups for alcohols.
ime directed ortho-olefination of masked aromatic alcohols
In 2010, breakthroughs were achieved by Yu and co-workers,
through
a
six- or seven-membered exo-palladacycle
II
[14]
who made a series of reports on Pd -catalyzed hydroxy-direct-
(Scheme 1A). Though the crystallization of exo-palladacycle
intermediate demonstrated the ability of exo-directing mode,
we have not tapped its potential. Herein, we report the first
[
7]
[8]
ed ortho-olefination, intramolecular cyclization, and carbon-
[
9]
ylation of phenethyl alcohols. However, the use of secondary-
or primary alcohols afforded lower yields. Meanwhile, Hartwig
and co-workers developed a practical hydroxyl-directed ortho-
silylation of benzyl alcohols by employing an Ir catalyst, fol-
lowed by a separate Hiyama-type coupling of the benzoxasi-
II
Pd -catalyzed cross-coupling of O-benzylacetoximes with aryl-
boron reagents, in which oxime serves as an exo-type directing
[10]
loles with aryl halides. Despite these advances, there is still
[
a] K. Guo, X. Chen, Prof. Dr. Y. Zhao
Key Laboratory of Organic Synthesis, Jiangsu Province College of Chemistry
Chemical Engineering and Materials Science, Soochow University
Suzhou 215123 (P. R. China)
E-mail: yszhao@suda.edu.cn
[
b] Dr. J. Zhang
College of Physics
Optoelectronics and Energy & Collaborative Innovation Center
Suzhou Nano Science and Technology, Soochow University
Suzhou 215006 (P. R. China)
II
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201503284.
Scheme 1. Pd -catalyzed acetoxime-assisted CÀH functionalization. NHPI=N-
hydroxyphthalimide, DEAD=diethyl azodicarboxylate.
Chem. Eur. J. 2015, 21, 17474 – 17478
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group, and can be subsequently removed without a separation
[a]
Table 1. Optimization of reaction conditions.
(
Scheme 1B).
While oxime is well-known for transition-metal-catalyzed
[
5,15]
CÀH activation of masked ketones,
Dong and co-workers
developed the first exo-oxime-directed acetoxylation of
II
IV
[16]
masked alcohols under Pd /Pd catalysis.
In this seminal
3
report, they developed a catalytic alcohol b-C(sp )ÀH activation
and CÀO formation reaction, where using a 2,6-dimethoxylben-
zaldoxime as novel bidentate directing group was found to be
[b]
Entry
Catalyst
Oxidant
Base
–
Yield
%]
[
1
2
3
4
5
6
7
8
9
Pd(TFA)
Pd(TFA)
2
Ag
Ag
Ag CO
Ag
Ag
Ag
Ag
2
2
CO
CO
3
3
3
3
3
3
3
3
85
5
[
16a]
crucial.
Later, they expanded its application in Pd-catalyzed
2
K
3
PO
K2HPO4
KH PO
NaHCO
Na CO
Li CO
NaBF4
KBF
4
[16b]
ortho-acetoxylation of masked benzyl alcohol.
However, we
Pd(TFA)2
31
83
49
38
87
84
93
51
77
80
77
84
18
39
12
3
2
envisioned that using concise acetoxime as monodentate di-
recting group should also be beneficial to the arene CÀH acti-
vation, because 1) O-benzylacetoximes are simple and readily
Pd(TFA)
Pd(TFA)
Pd(TFA)
Pd(TFA)
2
2
2
2
2
2
2
2
CO
CO
CO
CO
2
4
3
2
3
2
3
available, which can be easily synthesized from an S 2 reaction
Pd(TFA)2
Pd(TFA)
Pd(OAc)
2
Ag CO
N
[
17]
of benzyl halides with acetone oxime,
widely available alcohols though
Scheme 1); 2) the acidic a-hydrogens in acetoxime are rather
or prepared from
2
Ag
2
CO
3
4
10
2
Ag
Ag
Ag
Ag CO
Ag
Ag
2
2
2
CO
CO
CO
3
3
3
3
3
KBF
KBF
KBF
4
a
one-pot procedure
1
1
Pd(PPh
3
)Cl
2
4
(
12
Pd(CH
PdCl2
3
CN)
2
Cl
2
4
[
14]
stable under mild oxidative conditions; 3) tuning the reactiv-
ity and selectivity of catalysts can be achieved through the use
13
14
15
KBF4
2
[c]
Pd(TFA)
Pd(TFA)
Pd(TFA)
Pd(TFA)
2
2
2
2
2
2
CO
O
KBF
KBF
KBF
KBF
4
4
4
4
[
18]
of external ligands; 4) not only is acetoxime used as a trace-
[
d]
1
1
6
7
AgOAc
[
13,19]
less directing group, but hydroxylamine
active molecules.
also exists in bio-
Cu(OAc)
BQ
2
18
Pd(TFA)2
KBF4
[e]
1
2
9
0
Pd(TFA)
-
2
Ag
Ag
2
2
CO
CO
3
3
KBF
KBF
4
4
8
0
Results and Discussion
[
(
1
a] Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), Pd catalyst
10 mol%), oxidant (2.0 equiv), base (2.0 equiv) in t-amylOH (0.5 mL),
208C, 10 h. [b] GC yield determined using tridecane as internal standard.
Initially, we treated acetoxime ether 1a with phenylboronic
acid pinacol ester 2a at 1208C for 10 h with a combination of
different oxidants, bases, and ligands (Table 1). Preliminary re-
[
c] Pd(TFA) (5 mol%), Ac-Gly-OH (5 mol%). [d] AgOAc (4 equiv). [e] With-
2
out Ac-Gly-OH.
sults revealed that the use of Pd(TFA) (10 mol%), Ac-Gly-OH
2
(
10 mol%), and Ag CO (2 equiv) in t-amylOH (0.5 mL) gave the
2 3
ortho-arylation product 3a in 85% yield (Table 1, entry 1). In
a further investigation into the performance of different bases,
no positive effect was observed in the presence of common
ined to test the generality of this methodology (Scheme 2). To
our great delight, the electron-rich arenes provided the corre-
sponding arylated products in moderate to good yield (3c–
g,i,j). In particular, both meta- and para-substituted benzyl al-
cohol derivatives afforded the corresponding mono-arylated
products in good yield (64%–83%). Trace diarylated products
were also observed in the reaction system (3e–h). It is proba-
ble that the steric hindrance and coordinated effect inhibited
the further arylation reaction. Furthermore, the a-substituted
benzyl alcohols were all well-tolerated, affording arylated prod-
ucts in good to excellent yield (3k–m,q). In particular, the ace-
toxime-protected 1,4-benzenedimethanol gave the mono-ary-
lated product 3o in a synthetically acceptable yield. Rather dis-
appointingly, O-phenylacetoxime would be completely decom-
weak bases (entries 2–8). Interestingly, the addition of KBF fa-
4
cilitated this transformation, giving 3a in 93% yield (entry 9).
Although the reason needs to be further explored, we specu-
lated that the KBF would play the role of a pH buffer in the
4
catalytic cycle, which would stabilize the palladium intermedi-
ate. Among the palladium catalysts screened, though
Pd(OAc) , Pd(PPh )Cl , Pd(CH CN) Cl , and PdCl resulted in
2
3
2
3
2
2
2
moderate to good yield (entries 10–13), Pd(TFA) was found to
2
II
be superior. Because of competitive Pd -mediated protonation
and homocoupling of the arylboron reagents, the use of
1
0 mol% of catalyst was deemed necessary (entries 9 and 14).
II
0
Having identified the best catalyst, we proceeded to evaluate
posed into undetectable smaller molecules under Pd /Pd cat-
alysis (3p). In contrast, the acetoxime-protected phenethyl
alcohol afforded the arylated product in good yield with
2.5 equiv of 2a, which may go through a rare seven-mem-
the optimal oxidant. The oxidants such as AgOAc, Ag O,
2
Cu(OAc) , and 1,4-benzoquinone (BQ) were all tested (en-
2
tries 15–18); however, Ag CO still turned out to be the best.
2
3
[
14]
From previous studies, the ligands can adjust the steric and
bered exo-palladacycle intermediate.
II
[18d,e,g,20]
electronic properties of coordinated Pd centre.
Thus,
The efficiency of this acetoxime-assisted CÀH transformation
encouraged us to evaluate the scope of arylboronic acid pina-
col esters (Ar-BPins). In general, various para- and meta-substi-
tuted Ar-BPins were tolerated in this transformation, affording
the corresponding products in good to excellent yield
(Scheme 3). Gratifyingly, this synthetic protocol is compatible
with various functional groups, including F, Cl, Ac, CF , SO Me,
the ligand of Ac-Gly-OH, which could significantly promote
2
C(sp )ÀH activation and subsequent coupling reactions, was in-
dispensable (entries 9 and 19). In the absence of a palladium
catalyst, the control experiment gave no product (entry 20).
With optimized conditions for the cross-coupling in hand,
various masked aromatic alcohols were prepared and exam-
3
2
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2
Scheme 2. C(sp )ÀH cross-coupling of Ar-BPin with various functionalized
acetoxime ethers. [a] Reaction was carried out with 2a (0.5 mmol).
2
Scheme 3. C(sp )ÀH cross-coupling of acetoxime ethers with different Ar-
BPins. [a] Reaction was carried out with 2 (0.5 mmol).
NO , Me, and OMe. Moreover, the 2-naphthylboronic acid pina-
2
col ester gave the corresponding coupling product in good
yield (4g). Unfortunately, poor conversions were obtained for
ortho-substituents on account of steric hindrance (4n and 4o).
Similarly, heteroarylboronates that contained thianaphthene
(
4p) and furan (4q) were unreactive under this condition.
To further demonstrate the synthetic simplicity and versatili-
ty of the acetoxime group, we developed a sequential proce-
dure, which could accomplish arylation and removal of the di-
recting group with one separation, to access arylated benzyl
alcohols (Scheme 4). The key to make this protocol successful
[
14]
depends on the reduction of acetoxime by Raney Ni catalyst.
Alternatively, hydrolysis of acetoxime could obtain hydroxyla-
Scheme 4. Sequential arylation and removal of directing group.
[
21]
mine derivatives.
Based on the above results and pioneering stud-
releases the target molecule 3, with concomitant generation of
[3f,g,h,5a,16,18d,22]
0
I
ies,
the mechanism of acetoxime-directed ortho-
Pd species D. Reoxidation with 2 equiv of Ag regenerates the
II
arylation reaction is proposed as depicted in Scheme 5.To
active Pd species A for another catalytic cycle.
II
begin with, the Pd species A, which is bound by amino acid
ligand, coordinates with the substrate 1 and subsequently acti-
vates the ortho-CÀH bond, forming a six-membered exo-palla-
Conclusion
[14]
dacycle intermediate B. Transmetalation with Ar-BPin 2 pro-
We have developed the first example of acetoxime-directed
[18d,3h]
duces intermediate C,
followed by reductive elimination,
ortho-arylation of masked aromatic alcohol derivatives via the
Chem. Eur. J. 2015, 21, 17474 – 17478
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Natural Science Foundation of China (No. 21402133), and PAPD
for financial support.
Keywords: alcohols · arylation · CÀH activation · cross-
coupling · synthetic methods
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Received: August 19, 2015
1
8570–18572; g) G.-J. Cheng, Y.-F. Yang, P. Liu, P. Chen, T.-Y. Sun, G. Li,
Published online on October 9, 2015
Chem. Eur. J. 2015, 21, 17474 – 17478
www.chemeurj.org
17478
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