Fast and Selective Dehydrogenative C-H/C-H Arylation Using Mechanochemistry

Article abstract of DOI:10.1021/acscatal.6b00861

A fast and site-selective biaryl synthesis via dehydrogenative C-H/C-H arylation in a ball mill was developed. In this paper, both electron-deficient oximes and electron-rich anilides quickly and under mild conditions provided arylation with various arenes to give the biaryl products in high-level selectivity. Given the solventless mechanochemical conditions, the transformation obviated the use of large amounts of arene coupling partners as solvents and allowed utilization of more-complicated and more-expensive arenes as coupling partners, which may dramatically enhance the application prospect of the C-H/C-H arylation strategy.

Full text of DOI:10.1021/acscatal.6b00861

Letter
pubs.acs.org/acscatalysis
Fast and Selective Dehydrogenative C−H/C−H Arylation Using
Mechanochemistry
Shao-Jie Lou,‡ Yang-Jie Mao,‡ Dan-Qian Xu,* Jiang-Qi He, Qi Chen, and Zhen-Yuan Xu
Catalytic Hydrogenation Research Center, State Key Laboratory Breeding Base of Green Chemistry−Synthesis Technology, Zhejiang
University of Technology, Hangzhou 310014, People’s Republic of China
S
* Supporting Information
ABSTRACT: A fast and site-selective biaryl synthesis via
dehydrogenative C−H/C−H arylation in a ball mill was
developed. In this paper, both electron-deficient oximes and
electron-rich anilides quickly and under mild conditions
provided arylation with various arenes to give the biaryl
products in high-level selectivity. Given the solventless
mechanochemical conditions, the transformation obviated the use of large amounts of arene coupling partners as solvents
and allowed utilization of more-complicated and more-expensive arenes as coupling partners, which may dramatically enhance
the application prospect of the C−H/C−H arylation strategy.
KEYWORDS: mechanochemistry, C−H bond activation, selective arylation, biaryls, ball milling
Scheme 1. Biaryl Synthesis via Mechanochemical C−H/C−
reparation of biaryl compounds has consistently been of
Pgreat interest to the chemical community, because of their
wide applications in pharmaceuticals and agrochemicals.1
Despite the classic coupling reactions, transition-metal-
catalyzed dehydrogenative C−H/C−H activation arylation
has emerged as the most efficient way to access the biaryl
scaffold.2 Great effort has been exerted and various systems
have been developed in the past decade.3 However, several
challenges still must be overcome. First, let us examine
regioselective cross-coupling between substituted arenes and
substrates. Most examples in C−H/C−H bond arylation were
less selective, especially for the monosubstituted aromatic
coupling partners. The Yu group reported a paraselective C−H
bond arylation of monosubstituted arenes, in which the specific
directing group as well as the external oxidants were crucial for
the unique selectivity.3d Very recently, a ligand-controlled
strategy was developed to achieve high-level paraselectivity.3a
Second, a large excess amount of coupling partner must be
considered (usually, simple arenes were used as solvents, which
were difficult to handle and were not eco-friendly, especially for
the high boiling point and expensive arenes). Third, the reactions
required a long period of time and were less efficient for the
electron-deficient arenes. With these considerations in mind,
we envisioned developing a novel and efficient C−H/C−H
arylation, using a ball mill to overcome the aforementioned
problems (see Scheme 1).
H Arylation in This Work Versus in Batch
metal-catalyzed C−H bond activation was detected by Curic
́
and co-workers using solid-state Raman spectroscopy.6 Very
recently, practical examples of mechanochemically Rh-catalyzed
C−H bond olefination and Ir-catalyzed C−H bond amidation
were developed by Bolm and co-workers.7 This work has
revealed that C−H bond activation in a ball mill will be
applicable for the reduction of solvent amount and reaction
time. Given our continued interest in the C−H bond
functionalization and green synthesis in a ball mill, we herein
reported the first Pd-catalyzed mechanochemical biaryl syn-
thesis via dehydrogenative C−H/C−H arylation under a liquid-
assisted grinding (LAG)8 process. In our previous nitrate-
promoted C−H bond fluorination of ketone oximes, a small
amount of biaryl product was detected when using toluene as a
solvent.9 Given the synthetic and economic potential of
ketoxime ethers,10 arylation of oximes with simple arenes is
of great importance for the synthesis of useful biaryl
skeletons.11 With slight modification of the reaction condition,
we found that arylated product 3a was formed in 33% GC-MS
Ball milling, as a mechanochemical technique in synthetic
chemistry, has been receiving great attention, because of their
environmentally friendly and solventless synthesis approaches,
which remarkably enhance the efficiency of the reactions.4
Several metal-catalyzed relevant reactions have been realized
using ball-milling techniques in the past few years.5 However,
application of mechanochemical technique in C−H bond
functionalization is less studied. First, ball-milling transition-
Received: March 24, 2016
Revised: May 10, 2016
© XXXX American Chemical Society
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ACS Catalysis
Letter
yield along with good paraselectivity at 40 °C for 24 h (see
were inefficient oxidants for this transformation (Table 1,
entries 9−13). Furthermore, dimethylformamide (DMF), as an
additive, might play the crucial role of ligand during the
process, according to the previous report.3 Omission of Pd
catalyst, oxidant, or DMF led to negative results (Table 1,
entries 14−16). To the best of our knowledge, this is the first
time to use a ball-milling technique to promote the C−H/C−H
bond arylation, which represented a highly efficient and
selective cross-coupling protocol for the biaryl skeleton
construction.
Scheme 2). Given the unique advance in mechanochemistry,
Scheme 2. C−H/C−H Bond Arylation of Oxime in Batch
With the optimized condition in hand, we then investigated
the scope and limitation of the present mechanochemical
arylation. Various arenes including monosubstituted and
disubstituted arenes were subjected to the system and excellent
regioselectivity was obtained (Table 2). Monosubstituted
arenes bearing electron-donating functional groups, e.g., alkyl,
alkyloxy, and aryloxy underwent C−H/C−H cleavage cross-
coupling smoothly to give the biaryl products in moderate to
we expected that such an arylation in the ball mill would be
more efficient and require much less arene coupling partners in
comparison with the traditional batch process.
To our delight, cross-coupling product 3a was obtained in
61% yield with 6.0 equiv toluene after 1 h of milling [6 × (10
min + 1 min break)]. Notably, high paraselectivity was
observed under this condition [p/(o + m) = 25/1] (Table 1,
a
Table 1. Screening of Reaction Conditions
Table 2. Mechanochemical C−H/C−H Arylation with
a
Various Arenes
variation from “standard
condition”
conversion of 1a
yield of 3a
b
entry
(%)
(%)
1
2
without TfOH
68
96
93
94
80
74
44
74
62
6
61
85
80
84
76
66
36
63
54
0
none
3
TsOH·H2O instead of TfOH
Pd(TFA)2 instead of Pd(OAc)2
Pd2(dba)3 instead of Pd(OAc)2
Pd(PPh3)4 instead of Pd(OAc)2
K2S2O8 instead of Na2S2O8
(NH4)2S2O8 instead of Na2S2O8
oxone instead of Na2S2O8
BQ instead of Na2S2O8
4
5
6
7
8
9
10
11
12
13
Cu(OAc)2 instead of Na2S2O8
PhI(OAc)2 instead of Na2S2O8
2
0
62
88
0
SELECTFLUOR instead of
Na2S2O8
80
14
15
16
17
NFSI instead of Na2S2O8
without Pd(OAc)2
without Na2S2O8
52
3
35
0
2
0
without DMF
5
0
a
1a (0.3 mmol), 2a (6.0 equiv), Pd(OAc)2 (10 mol %), Na2S2O8 (2.0
equiv), DMF (2.0 equiv), TfOH (1.0 equiv), 600 rpm in ball mill
[6(10 min +1 min break)], 60 stainless steel grinding balls (2 mm
diameter); DMF = N,N-dimethylformamide, Selectfluor = F-TEDA-
b
BF4, NFSI = N-fluorobenzenesulfonimide. Determined by GC-MS
using dodecane as an internal standard.
entry 1). Pleasingly, the yield was significantly enhanced to 85%
when 1.0 equiv of trifluoromethanesulfonic acid (TfOH)3,12
was added (Table 1, entries 2 and 3), whereas the selectivity
remained unchanged.13 Various combinations of Pd catalysts
and oxidants were then surveyed. Pd(OAc)2 was selected as the
best catalyst. Other peroxides, such as K2S2O8, (NH4)S2O8, and
oxone could also deliver the desired product, albeit in lower
yields (Table 1, entries 4−8). Electrophilic fluorinating agents
such as SELECTFLUOR and N-fluorobenzenesulfonimide
(NFSI) were good external oxidants to give comparable
results.3,14 PhI(OAc)2, benzoquinone (BQ), or Cu(OAc)2
a
1a (0.3 mmol), 2 (1.80 mmol, 6.0 equiv), Pd(OAc)2 (10 mol %),
Na2S2O8 (0.6 mmol, 2.0 equiv), DMF (2.0 equiv), TfOH (0.3 mmol,
1.0 equiv), 600 rpm in ball mill [6(10 min + 1 min break)], 60
stainless steel grinding balls (2 mm diameter); isolated yields and
regioselectivity (p/others) determined by gas chromatography (GC),
unless otherwise noted. 1a (0.3 mmol), 2 (1.5 mL), Pd(OAc)2 (10
mol %), Na2S2O8 (0.6 mmol, 2.0 equiv), DMF (2.0 equiv), TfOH (0.3
b
c
d
mmol, 1.0 equiv), 40 °C, 24 h. 2 (0.9 mmol, 3.0 equiv). Isolated
yields and regioselectivity (m/others) determined by GC.
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Letter
good yields, along with excellent paraselectivity (3a−3e).
Aromatic halides are also good coupling partners and gave
excellent paraselectivity as well, which might provide
opportunities for further decorations of the C−X (X = Cl or
Br) bonds (3f−3h). Electron-deficient arenes as C−H/C−H
arylation coupling partners were less studied in the previous
reports, because of their lower reactivity. Notably, in this case,
the electron-deficient aromatics such as acetophenone and
methyl benzoate were also found reactive under the present
mechanochemical condition, and a high meta-selectivity was
observed (3i, 3j), presumably because of the meta-C−H site of
acetophenone and methyl benzoate are more reactive for the
electrophilic substitution (SEAr).15 Furthermore, several
disubstituted arenes were investigated. Highly site-selective
C−H/C−H arylation were delivered at the electronic-favored
and less sterically hindered C−H bonds of the arenes
respectively (3k−3o). Note that only 3.0−6.0 equiv of arene
coupling partners (6.0 equiv for volatile arenes and 3.0 equiv
for less-volatile arenes) were required in all cases and arenes
that were hardly used in the previous solvent-based C−H/C-H
arylation were now possible, to serve as successful coupling
partners, which would dramatically enhance the application
prospect of the C−H/C−H arylation strategy.
Table 3. Mechanochemical C−H/C−H Arylation of
a
Oximes
We then explored the reactivity of oximes with different
functional groups. Electronic-rich acetophenone O-methyl
oximes were more reactive, whereas electron-deficient oximes
were less effective and required longer reaction time to obtain
compromised yields. No obstacles were occurred when other
carbonyl O-methyl oximes were subjected to the present
condition (Table 3).
Moreover, olefinic sp2 C−H bond could also be arylated with
different arenes (e.g., toluene, anisole, or aromatic fluoride) to
afford the excellent paraselective products in good yields under
the mechanochemical conditions (Scheme 3).16
Apart from the electron-deficient oxime substrates, electron-
rich anilide derivatives, which were widely used in this type of
C−H/C−H arylation,3 were also carried out to evaluate the
universality of this method. Anilides including urea and
acetanilide could couple with different arenes to form various
useful biarylaniline products in good yields, along with
consistent high-level selectivity (Table 4).
Finally, both biaryl oximes and anilides could undergo
removal of directing groups using simple workup, which
enhanced the application prospect of this method.14
a
1 (0.3 mmol), 2a (1.80 mmol, 6.0 equiv), Pd(OAc)2 (10 mol %),
Na2S2O8 (0.6 mmol, 2.0 equiv), DMF (2.0 equiv), TfOH (0.3 mmol,
1.0 equiv), 600 rpm in ball mill [6(10 min +1 min break)], 60 stainless
steel grinding balls (2 mm diameter); isolated yields and
b
regioselectivity (p/others) determined by GC. 600 rpm in ball mill
[9(10 min + 1 min break)].
Scheme 3. Mechanochemical Olefinic C−H Arylation with
Simple Arenes
To gain mechanistic insight of this mechanochemical
process, kinetic isotope effects (KIE) experiments were
performed using deuterated substrate [D5]1a and coupling
partner [D8]2a, respectively. Intermolecular H/D competition
between [D5]1a and 1a with 2a had a significant KIE value of
3.0, which indicated that the initial step of oximyl-directing-
group-assisted C−H bond activation might be involved in the
rate-determined step (Scheme 4a). However, a competition
between 2a and [D8]2a gave a H/D isotope effect of 1.0, which
suggested that the C−H bond activation of arene coupling
partner is not the slow step in the catalytic cycle (Scheme 4b).
The KIE values under the present mechanochemical condition
are consistent with the data reported in the similar trans-
formation using arenes as solvents.3 Based on the preliminary
mechanism study and the previous report, two sequential C−H
bond activations, including the initial cyclopalladation of
oximes or amides and the fast C−H bond activation of arene
substrates at the PdIV center, might be responsible for the high-
level selectivity.17 In addition, the solventless condition
significantly promoted the transformation by reducing the
coupling partner content and reaction time.
In conclusion, we have developed an efficient mechano-
chemical dehydrogenative C−H/C−H arylation of both
electron-deficient oximes and electron-rich anilides. Notably,
the solventless conditions allowed us to use various simple
arenes including monosubstituted and disubstituted arenes with
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Letter
Author Contributions
Table 4. Mechanochemical C−H/C−H Arylation of
a
‡These authors contributed equally.
Anilides
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors acknowledge the National Nature Science
Foundation of China (No. 21361130021), China Postdoctoral
Science Foundation (No. 2014M560494) and the Postdoctoral
Science Foundation of Zhejiang Province for financial support.
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