Nickel-catalyzed one-pot deoxygenation and reductive homocoupling of phenols via C-O activation using TCT reagent

Article abstract of DOI:10.1021/ol503560e

A new method for C-O bond activation of phenolic compounds has been achieved using 2,4,6-trichloro-1,3,5-triazine to utilize in one-pot Ni-catalyzed deoxygenation and reductive homocoupling reactions. With this simple method, phenolic compounds were converted to their corresponding arenes or biaryl compounds under mild conditions. The introduced methodology has a broad scope and demonstrates good functional group compatibility.

Full text of DOI:10.1021/ol503560e

Letter
pubs.acs.org/OrgLett
Nickel-Catalyzed One-Pot Deoxygenation and Reductive
Homocoupling of Phenols via C−O Activation Using TCT Reagent
Nasser Iranpoor* and Farhad Panahi
Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran
S
* Supporting Information
ABSTRACT: A new method for C−O bond activation of
phenolic compounds has been achieved using 2,4,6-trichloro-
1,3,5-triazine to utilize in one-pot Ni-catalyzed deoxygenation
and reductive homocoupling reactions. With this simple
method, phenolic compounds were converted to their
corresponding arenes or biaryl compounds under mild
conditions. The introduced methodology has a broad scope
and demonstrates good functional group compatibility.
arbon−oxygen bond activation is one of the most
application in one-pot conversion of phenolic compounds to
other products. First, 3 equiv of phenols immediately reacts with
1 equiv of TCT to generate the corresponding 2,4,6-triaryloxy-
1,3,5-triazine (TAT),24 providing an atom-economical approach.
In fact, TAT was the introduced aryl C−O electrophile in our
strategy. To the best of our knowledge, there is no report in the
literature on the use of TAT as an aryl C−O electrophile in
metal-catalyzed coupling reactions. These materials possess a
range of physical and chemical properties.25 Second, the C−O
bond in TAT (1.42 Å) is longer than that in phenol (1.36 Å),
making its strength weaker and demonstrating that TCT acts as a
C−O bond activator (Figure 1).26 Finally, the ortho-nitrogen
atom in the structure of TAT can coordinate with metals to
facilitate the oxidative addition step.27
C
important strategies in organic chemistry applied in organic
transformations.1 Phenolic compounds (widely found in nature)
can be converted to aryl C−O electrophiles using C−O bond-
activating agents. This strategy makes the C−O electrophiles
more suitable alternatives than aryl halides in carbon−carbon
and carbon−heteroatom bond formation reactions due to their
higher diversity, abundance, and availability.2 Along this line, a
range of aryl C−O electrophiles such as ethers,3,4 esters,4
pivalates,5,6 acetates,6,7 phosphonates,7 triflates,7 carba-
mates,4,8−10 carbonates,4,10,11 phosphoramides,12 phosphates,13
tosylates,7,14 mesylates,4,14−16 sulfamates,4,9,10,16−18 and hetero-
aryl ethers19 have been introduced for application in metal-
catalyzed cross-coupling reactions as coupling partners. To use
phenolic compounds in coupling reactions, there are two
strategies. In the most widely used method (stepwise), phenols
are first converted to the corresponding aryl C−O electrophile
and then used for the next reaction.3−19 One-pot activation of
phenols is another approach for the generation of an active
coupling partner in metal-catalyzed cross-coupling reac-
tions.20−22 Bromo-trispyrrolidino phosphoniumhexafluoro-
phosphate is the most widely used reagent for one-pot activation
of phenolic compounds.20 In some cases, phenolic salts were
used in the one-pot activation process.21 Recently, Chen et al.
reported an efficient nickel-catalyzed one-pot Suzuki−Miyaura
cross-coupling of phenols and arylboronic acids mediated by
N,N-ditosylaniline.22 The one-pot activation strategy possesses
important advantages in comparison to a stepwise method. First,
the preactivation step limits the efficiency of a two-step protocol
in view of overall yields and step economy. On the other hand,
conversion of some of the phenolic compounds to aryl C−O
electrophiles is difficult and has restrictions.1k For these reasons,
the one-pot phenol activation strategy has received more
attention recently.20−22
Figure 1. Phenol activation using TCT.
As part of our program aimed at activating phenolic
compounds for participation in metal-catalyzed coupling
reactions,23 we demonstrate a user-friendly and operationally
simple Ni-catalyzed deoxygenation and homocoupling of
phenols using TCT (Scheme 1). To the best of our knowledge,
this strategy represents the first one-pot Ni-catalyzed deoxyge-
nation28,29 and homocoupling30 of phenols.
Deoxygenation of phenolic compounds into the arene
counterparts plays a key role in organic synthesis. In most
cases, the function of the hydroxyl group is to increase the
reactivity of the substrate toward functionalization as well as
In our previous study, we reported that 2,4,6-trichloro-1,3,5-
triazine (TCT) is an efficient reagent for Ni-catalyzed amination
of phenols.23 There are some important features in the structure
of TCT which encouraged us in the development of its
Received: November 12, 2014
© XXXX American Chemical Society
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Letter
Scheme 1. Stepwise and One-Pot Deoxygenation/
Homocoupling of Phenols
acting as a directing group. Afterward, efficient catalytic
deoxygenation methods are needed to remove the hydroxyl
group to obtain the target product.31 Note that the
deoxygenation method should be applicable to a range of
substrates with various functional groups. The frequently used
protocol for deoxygenation of phenols is their conversion into
aryl C−O electrophiles followed by metal-catalyzed reductive
cleavage. However, the selective, simple, and efficient deoxyge-
nation of phenolic compounds is still a challenge.28
We first set out to obtain proper conditions for one-pot Ni-
catalyzed deoxygenation of phenolic compounds using 2-
naphthol as a simple model substrate (Table 1S, Supporting
Information). Conditions for the TAT preparation step were
selected based on our previous work.23 Next, different reduction
systems were checked to find the most suitable conditions.
According to the work of Sasaki et al., the MeOH/Zn/KI
system29 was selected as a reducing agent at the beginning, and in
the presence of NiCl2(PCy3)2 catalyst, 85% of arene was
obtained. Also, other hydrogen donor systems such as Et3SiH
[NiCl2(PCy3)2], Ph3SiH [NiCl2(PCy3)2],28e Mg/MeOH/
NH4 OAc (Pd/C),2 8 c N-(n-Pr)3 /HCOOH [Pd-
(OAc)2(PPh3)2],28h K2CO3/i-PrOH [PdCl2(dppf)2],28d and
NaBH4 [NiCl2(PCy3)2]28a were investigated, but no superiority
was observed. Then, diverse Ni catalysts was tested, among
which NiCl2(PCy3)2 was recognized as the best one. The
optimum amount of catalyst was found to be 10 mol %. As a
result, the optimized conditions for the efficient deoxygenation
of phenols using TCT is shown in Scheme 2.
Figure 2. Ni-catalyzed deoxygenation of phenols via C−O activation
using TCT under optimized conditions. All yields are isolated.
excellent chemoselectivity profile. Methoxy, amino, nitro,
cyano, and keto, and aldehyde substituents tolerated the reaction
conditions well. For substrates containing nitro and aldehyde
functionalities, no reduced products were observed. As shown for
2h−j, the method is also applicable for heterocyclic substrates.32
The methodology could be extended to biphenyls and
phenanthrene phenolic substrates,33 and their corresponding
products (2k−m) were obtained in moderate yields. Interest-
ingly, coumarin (2n) was obtained in 68% yield from 4-
hydroxycoumarin. The structural diversity of this reaction was
further increased using estrone, leading to the formation of 3-
deoxyestrone (2o) in 72% yield.
The potential utility of this method in organic synthesis was
demonstrated in Scheme 3. As shown in Scheme 3, 2p could be
Scheme 3. Hydroxy Group as a Removal Handle for
Regioselective Arene Functionalization and Synthesis of 2p
Using Our Method
Scheme 2. Optimized Reaction Conditions for
Deoxygenation of Phenolic Compounds Using TCT
selectively prepared in high yield from p-cresol (1p) in a four-
step process. First, 1p′ was synthesized using the ability of the
hydroxyl group to direct functionalization in the ortho position of
the aromatic ring.34 Then, 1p′ was converted to its
corresponding benzyl chloride substrate (1p″).35 Nucleophilic
addition of 1-methyl-1H-indole to 1p″ resulted in the production
of compound 1p‴ in 65% isolated yield.36 In the final step, 1p‴
was deoxygenated using our procedure, and 2p was obtained as a
novel bisindole derivative.37
Subsequently, we carried out a deuterium-labeling experiment
to assemble evidence regarding the reaction pathway (Scheme
4). The successful synthesis of 2a′ indicates that our procedure
can be used to introduce deuterium in the structure of aromatic
backbones from readily available precursors.38
Under the optimized conditions, various phenolic compounds
were successfully deoxygenated in good to excellent yields
(Figure 2).
As shown in Figure 2, 2-naphthol gave naphthalene in 85%
isolated yield. The lower yield obtained for 1-naphthol may be
due to the presence of some steric hindrance.28g,h The effect of
steric hindrance on the progress of the reaction is more
observable in the synthesis of 2c. Next, we studied the electronic
effects of substituents on the aryl ring. Interestingly, phenolic
substrates with electron-donating and -deficient substituents
gave corresponding products in moderate to good yields. This
one-pot Ni-catalyzed deoxygenation of phenols shows an
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Scheme 4. Deuterium Labeling of 2-Naphthol Using Ni-
Catalyzed Reductive Deoxygenation via C−O Bond
Activation Using TCT
Here, we report another important application of TCT as an
efficient reagent for the one-pot homocoupling reaction of
phenolic compounds. The C−C bond formation using an aryl
C−O electrophile is an important strategy in terms of
accessibility of phenolic compounds and environmental
impact.39 One of the significant applications of this strategy is
the synthesis of biaryls. Biaryl compounds have an imperative
structural motif, which exists extensively in pharmaceuticals,
advanced materials, and natural compounds.40 Transition-metal-
catalyzed homocoupling of aryl C−O electrophiles such as aryl
triflates, aryl tosylates, and aryl mesylates, has been known as a
potent and essential method for the synthesis of symmetrical
biaryls.30 However, in the reported methods for the homocou-
pling of aryl C−O electrophiles, phenolic compounds were first
converted to their corresponding electrophiles and then used in
the metal-catalyzed reactions. Clearly, a one-pot transformation
from phenol would be the best option to resolve complication as
it avoids the additional steps of group transfer and the production
of organic waste.
Figure 3. Products of the one-pot homocoupling reaction of phenolic
compounds using TCT. All yields are isolated.
TAT during the reaction process can act as an aryl C−O
electrophile (Scheme 6).
Scheme 6. Conversion of TAT to Deoxygenated and/or
Homocoupling Product
The efficiency of this method for homocoupling of p-cresol as
a simple model substrate was first evaluated. According to the
data obtained from the deoxygenation reaction and pioneering
work of Percec et al.,30c the following conditions were first tested
and the homocoupling product was obtained in 84% isolated
yield (Scheme 5).
When 1,3,5-tris(p-tolyloxy)benzene (5)41 was used instead of
4, no product was observed. This experiment clarifies that the
ortho-nitrogen atom in the structure of TAT plays an important
role in the activation of the C−O bond (Scheme 7).
Scheme 5. One-Pot Homocoupling Reaction of 4-
Methylphenol Using TCT as the Model Reaction
Scheme 7. Effect of Nitrogen on TAT in C−O Activation
Further changes to the catalyst, solvent, and reaction
temperature did not result in any noticeable improvement in
the performance of the reaction, thus the above condition was
selected as the optimum. To determine the scope of the reaction
for the preparation of symmetrical biaryls, a number of
commercially available phenols were coupled under our
optimized reaction conditions, and the results are shown in
Figure 3.
As shown in Figure 3, phenols with electron-donating groups
furnished the coupling product in good yield (3a and 3c).
Functional groups such as −NO2, −CN, and −CO2Me were
compatible under these reaction conditions (3d−f). Sterically
hindered substrates also coupled smoothly to give the desired
product in moderate yields (3g and 3h). Heterocyclic substrates
also provided moderate to good yields of the corresponding
coupled products (3i−l).
In conclusion, TCT was found to be an efficient reagent for
C−O activation of phenolic compounds to convert them into the
corresponding aryl and biaryl compounds. These methods are
characterized by their simplicity, wide preparative scope, and the
availability of the substrates employed. Remarkably, this study
does not require the use of toxic reagents and preactivation of
phenolic compounds, thus representing an additional benefit
when compared to current methods.
ASSOCIATED CONTENT
■
S
* Supporting Information
Some experiments were also performed to achieve a deeper
insight into the reaction mechanism. In an experiment, under
optimized conditions, 2,4,6-tris(p-tolyloxy)-1,3,5-triazine (4)24
was converted to 3a and 2q, demonstrating that the generated
Table of optimization study, experimental procedures, and
copies of 1H and 13C NMR for all synthesized compounds. This
material is available free of charge via the Internet at http://pubs.
acs.org.
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Letter
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AUTHOR INFORMATION
Corresponding Author
■
*E-mail: iranpoor@susc.ac.ir.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support from the research councils of Shiraz University
and a grant from the Iran National Elite Foundation are gratefully
acknowledged.
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