Directed meta-Selective Bromination of Arenes with Ruthenium Catalysts

Article abstract of DOI:10.1002/anie.201507100

A Ru-catalyzed direct C?H activation/meta-bromination of arenes bearing pyridyl, pyrimidyl, and pyrazolyl directing groups has been developed. A series of bromo aryl pyridines and pyrimidines have been synthesized, and further coupling reactions have also been demonstrated for a number of representative functionalized arenes. Preliminary mechanistic studies have revealed that this reaction may proceed through radical-mediated bromination when NBS is utilized as the bromine source. This type of transformation has opened up a new direction for the radical non-ipso functionalization of metal with regard to future C?H activation development that would allow the remote functionalization of aromatic systems.

Full text of DOI:10.1002/anie.201507100

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Angewandte
Communications
International Edition: DOI: 10.1002/anie.201507100
German Edition: DOI: 10.1002/ange.201507100
À
C H Activation
Directed meta-Selective Bromination of Arenes with Ruthenium
Catalysts
Qingzhen Yu, Le’an Hu, Yue Wang, Shasha Zheng, and Jianhui Huang*
À
Abstract: A Ru-catalyzed direct C H activation/meta-bromi-
nation of arenes bearing pyridyl, pyrimidyl, and pyrazolyl
directing groups has been developed. A series of bromo aryl
pyridines and pyrimidines have been synthesized, and further
coupling reactions have also been demonstrated for a number
of representative functionalized arenes. Preliminary mecha-
nistic studies have revealed that this reaction may proceed
through radical-mediated bromination when NBS is utilized as
the bromine source. This type of transformation has opened up
a new direction for the radical non-ipso functionalization of
a variety of representative meta functional group introduc-
tions possible by further functional group interconversion on
the corresponding aryl bromides (Scheme 1).
À
metal with regard to future C H activation development that
would allow the remote functionalization of aromatic systems.
À
D
irect functionalization of non-activated aromatic C H
bond has been considered as one of the most powerful
methods in modern molecular design and synthesis. Transi-
À
tion metal catalyzed ligand-assisted ortho-C H activation and
direct functionalization has been very well studied in the last
decade.[1] However, the methods for meta-selective C H
À
activation are less established.[2] The Smith/Malezcka and
À
Hartwig/Miyaura/Ishihara groups have disclosed meta-C H
direct functionalizations under Ir-catalyzed conditions,[3]
while Gaunt utilized Cu for the meta-arylation.[25] More
recently, Ackermann and Frost have reported Ru-catalyzed
meta-alkylation and sulfonation of aryl pyridines.[4] Later, Yu
À
Scheme 1. meta-C H bromination and the introduction of a range of
representative functional groups.
and Dong discovered Pd-catalyzed remote-directed and
[5]
À
norbornene-mediated meta-C H arylation and alkylation.
Aryl bromide is one of the most utilized organic
intermediates in modern organic chemistry owing to its
versatile characteristics. For example, this type of building
block is well recognized for the introduction of functional
groups by the halogen/main group metal exchange, and
transition metal-catalyzed coupling reactions for the func-
tionalization of aromatic systems.[7] The direct bromination of
arenes is generally accomplished by: i) The conventional
electrophilic aromatic bromination,[8] or more recently devel-
oped aromatic bromination under activated conditions;[9] or
ii) Directed ortho-bromination under Rh-,[10] Pd-,[11] Ru-,[12] or
Cu-catalyzed[13] conditions. Inspired by Roper and Wrightꢀs
early discoveries on electrophilic aromatic substitution reac-
tion (SEAr) of the aryl organoruthenium or organoosmium
reagents (Scheme 1, Eq. 1), the bromination of organometal-
lic reagent 1 is directly occurring selectively at the para-
position to the metal substituent to give the meta-brominated
product 2.[14]
However, these methods so far have only provided
approaches for the introduction of a limited set of functional
groups. Herein, we report the directed meta-bromination for
the synthesis of a series of meta-substituted aryl pyridines and
pyrimidines. During the submission of this manuscript,
a similar Ru-catalyzed meta-functionalization process for
aryl pyridines was also reported.[6] Unlike the Frost and
Ackermann procedures, this transformation has also made
[*] Dr. Q. Yu,[+] L. Hu,[+] Y. Wang, S. Zheng, Prof. Dr. J. Huang
School of Pharmaceutical Science and Technology
Tianjin University
92 Weijin Road, Nankai District, Tianjin 300072 (China)
and
Collaborative Innovation Center of Chemical Science and
Engineering (Tianjin)
Tianjin 300072 (China)
and
Based on their discoveries, we have proposed an approach
to access brominated arenes at the meta-position to a directing
group in similar regioselective fashion. As demonstrated in
Scheme 1, Eq. 2, starting from a directed arene 3, under
Tianjin Key Laboratory for Modern Drug Delivery and
High-Efficiency (China)
E-mail: jhuang@tju.edu.cn
[+] These authors contributed equally to this work.
À
plausible directed C H metallation processes, an ortho-
metallic intermediate 2 could be formed and analogous to
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507100.
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Table 1: Reaction scope on benzamides.[a]
Roper and Wrightꢀs organometallic bromination, meta-bro-
minated arene 4 could be formed after the further reductive
elimination of the transition metal or protonation of the
organometallic compound to eliminate the transition metal
species.
With this hypothesis in hand, we have set up experiments
to test its validity. Further inspired by Frostꢀs[4b] sulfonation
and Ackermannꢀs[4a,c] alkylation reactions, we chose Ru
catalysts for our initial studies. During our preliminary
solvent screening, we found that solvents such as toluene,
MeCN, dioxane, and DMSO did not give any desired product;
in those cases, we have only recovered starting material.
Fortunately, when a mixture of 2-phenylpyridine 3a, 5 mol%
of [RuCl2(p-cymene)]2, and NBS in DMAwas heated at 708C,
we found that our desired product 4a was obtained in good
65% yield with 32% of recovered starting material (Support-
ing Information, Table S1, entry 1).
The screening of various Ru catalysts was carried out, and
we were pleased to find that [RuCl2(p-cymene)]2 and
RuCl2(cod) both worked very well, as our desired meta-
brominated product 4a was isolated in 67% and 63% yields,
respectively. However, the commonly used Ru photocatalyst,
Ru(bpy)3Cl2, was less efficient for this type of reaction, while
only ortho-bromination was observed and the corresponding
aryl bromide 4a was only obtained in 4% yield. Grubbsꢀ
catalyst as well as [RuCp*Cl2]n both gave our desired product
in good yields. Interestingly, we found that when the RuIII
catalyst RuCl3 was utilized,[15] the results were comparable
with those with [RuCl2(p-cymene)]2 (Table S1, entry 6).
When the temperature was increased to 808C, our desired
product 4a was isolated in an excellent 91% yield with an
excellent regioselectivity (Table S1, entry 7). Elevated tem-
perature did not provide better selectivity, instead more
bisbrominated product 6a was observed. No product was
formed in the absence of Ru catalyst under similar conditions.
Other transition metal catalysts, such as Pd and Rh, failed to
deliver the desired product 4a, reactions are instead following
[a] Reaction conditions: Substrate 3 (0.2 mmol), NBS (0.4 mmol),
[RuCl2(p-cymene)]2 (5 mol%) in DMA (1.0 mL) at 808C for 24 h. Isolated
yields are in the parentheses, yields based on the recovered starting
material are listed in the square brackets.
be achieved in good yields (Table 1). Under the standard
reactions conditions, unsubstituted pyrazole underwent back-
ground SEAr reaction to give 4-bromo pyrazole 3v exclusively
in quantitative yield.[16] When halo-pyrazoles were utilized as
the directing groups, the syntheses of aryl bromides also
became possible to give the corresponding products 4u and
4v, albeit with low isolated yields. These experimental
observations have opened up a new direction for the future
synthesis development. Unfortunately, under similar condi-
tions, the attempts on other arenes bearing commonly used
directing groups (the failed examples 4w–4ae, see the
Supporting Information, Table S2) and the introduction of
other halogens using Selectfluor, NBS, and NIS all failed to
give the corresponding aryl halides.
To demonstrate the application of the aryl bromides, Pd-
catalyzed coupling reactions of bromobenzene 4a were
carried out.[17,18] As expected, when alkene or aryl boronic
acids were used as the coupling partners, the coupled products
7 and 8 were synthesized in over 80% yields (Scheme 2).
Coupling reactions of bromobenzene 4a with acetylene and
alkyl Grignard reagent were also fruitful, with the desired
alkynyl and alkyl substituted arenes 10 and 11 obtained in
86% and 75% yields, respectively.[19,20] Aryl bromide 4a
could also be converted smoothly into the corresponding
boronic ester 11 in excellent yield under standard borylation
À
the ortho-C H activation/bromination pathways to give the
ortho-brominated arene 5a in 73% and 36% yields (Table S1,
entries 11 and 12).
With the optimal conditions in hand, we have evaluated
the scope and limitations of this reaction. As illustrated in
Table 1, a number of substituted 2-pyridyl arenes have been
successfully obtained. The meta-bromo arenes 4b–4e were
successfully obtained in good to excellent yields when the
para-substituents were electron-rich or electron-deficient
groups. Similar reactivity was also observed once the sub-
stituents were on the ortho-position to the pyridine directing
group; the corresponding aryl bromides 4 f–4h were isolated
in 54%–69% yields respectively.
The synthesis of aryl bromide 4i was less successful, owing
to the strong electron donation of the methoxy group to the
arene, and aryl bromide 4i’ was isolated in 90% yield. Once
the electron-rich methoxy group was introduced to the para-
position to the pyridine nitrogen, the reaction yield dropped
to 44%. Methyl groups at the meta-position gave rise to
slightly reduced yield, and methyl substituents at the ortho-
position leave the reaction intact. We found that bromo-
benzoquinoline 4p and bromo-phenylpyrimidine 4q can also
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Scheme 2. Pd-catalyzed coupling reactions with various coupling part-
ners.
conditions.[21] These reactions allow us to make a variety of
meta-functionalized arenes in a practical manner (for exper-
imental details see the Supporting Information).
Scheme 4. H/D exchange experiments.
The meta-bromination method has also been applied in
the concise synthesis of anti-cancer drug Vismodegib 16. As
demonstrated in Scheme 3, the sequential one-pot reaction by
molecule. Furthermore, it is possible that the aryl pyridine
was acting as a crucial ligand; a plausible intermediate 17 may
form during the transformation, which could explain the H/D
cross-over results.
Surprisingly, we found that reaction using prepared aryl
ruthenium compound 18 as the starting material under our
standard conditions only gave our desired product 4a in
a trace amount, whereas reaction using a catalytic amount of
catalyst 18 gave the brominated product in 87% yield.
(Scheme 5, Eqs. 3 and 4) These results suggest that catalyst
18 is not an active catalyst with regard to the bromination
processes; the aryl pyridine may help this transformation by
acting as an external ligand.
Scheme 3. Synthesis of Vismodegib 16.
À
a meta-C H bromination followed by an ortho-directed
chlorination gave aryl pyridine 12 in 76% yield. After CuI-
catalyzed amidation and substituent replacement, our desired
product Vismodegib 16 was successfully isolated in an overall
47% yield from cheap and readily available starting material
aryl pyridine 3a (Scheme 3).
To explore the reaction pathway, we have also carried out
H/D exchange experiments. Interestingly, when 3a-D2 was
treated with NBS under our standard conditions, unlike
Ackermann and Frostꢀs results,[4] we did not observe any
deuterium loss during the reaction, which is similar to the
observations we had during our Pd-catalyzed reactions.[22]
(Scheme 4) Surprisingly, when the cross-over H/D exchange
experiment was carried out, we found that the D has shifted
from 3a-D5 to the structurally similar aryl pyridine 3d, which
showed that deuterium was scrambling within the two
Scheme 5. Experiments using stoichiometric amount of Ru catalyst.
In terms of better understanding of the bromination step,
control experiments in the presence of a radical scavenger
were carried out (Scheme 6). Interestingly, in the presence of
3.0 equivalents of radical scavenger 19, the reaction was
totally shut down under the standard conditions, similar to
Ackermannꢀs observation under their conditions.[4c] No
product was observed, and only starting material was
recovered. These results indicate that our reaction may
undergo radical processes during the bromination step.
À
molecules. These results indicate that during the C H
activation processes, the H is very likely to be appearing as
a possible ligand on the Ru metal center during the process up
until the reductive elimination to give the H back to the
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give brominated aryl-Ru E with the loss of succinamide,
followed by reductive elimination, forming a Ru intermediate
B’, which after ligand exchange gives the active RuIIL2 species
for the next catalytic cycle.
In conclusion, we have developed an approach for the
À
meta-selective bromination of arenes by directed C H metal-
lation. We have demonstrated the usefulness of our method-
ology by the preparation of a range of meta-brominated
arenes, as well as the introduction of a number of represen-
tative functional groups. Preliminary mechanistic studies have
revealed that this reaction may undergo a RuII/RuIV catalytic
cycle, and the functional group introduction may involve
radical processes. Further detailed investigations on the
mechanistic studies are still in progress.
Scheme 6. Experiments in the presence of radical scavenger.
With regard to the reaction pathway, kinetic isotope effect
experiments were carried out. We found that during the
competition experiment between phenyl pyridines 3a and
3a-D3, the KIE was calculated as 1.43:1 (Supporting Infor-
mation, Section 2.15, Scheme S1, Eq. S1). These results
showed a possible secondary KIE for the re-aromatization
during the SEAr processes, which may leave the initial
À
directed ortho-C H activation as the primary KIE as the Experimental Section
A mixture of phenylpyridine (0.3 mmol), NBS (0.6 mmol), and
rate-determining step. Comparable KIEs have recently been
[RuCl2(p-cymene)]2 (5 mol%) in DMA (1.5 mL) was heated at
808C or 1008C under air atmosphere for 24–48 h. After the reaction
was completed, the reaction mixture was allowed to cool down to
room temperature, and H2O (20 mL) was added. The resulting
mixture was extracted with EtOAc (10 mL ” 5). The combined
organic layer was dried over anhydrous Na2SO4, filtered, and the
solvent was removed under reduced pressure to provide the crude
product. The purification was performed by flash column chroma-
tography on silica gel.
À
observed for related ruthenium-catalyzed meta-C H functio-
nalizations.[6b] However, when a mixture of phenylpyridines
3a and 3a-D5 were introduced in the reaction, a similar KIE
value of 1.56:1 was observed after the reaction was heated
under the standard conditions for 3 h (Supporting Informa-
tion, Section 2.15, Scheme S1, Eq. S2). These results revealed
À
that neither the ortho-C H metallation nor deprotonation/re-
aromatization is the rate-determining step.
Based on these preliminary experimental results, a plau-
sible reaction mechanism is proposed (Scheme 7). The first
Acknowledgements
À
C H activation of aryl pyridine 3a gives the Ru complex B
Financial support from the National Basic Research Program
of China (2015CB856500), the National Natural Science
Foundation of China (21302136), and the Tianjin Natural
Science Foundation (13JCQNJC04800) is gratefully acknowl-
edged.
À
Keywords: bromination · C H activation · meta-selective ·
radical processes
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15284–15288
Angew. Chem. 2015, 127, 15499–15503
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Received: July 31, 2015
Revised: October 13, 2015
Published online: October 30, 2015
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