Pd-Catalyzed C(sp3)-H Biarylation via Transient Directing Group Strategy

Article abstract of DOI:10.1021/acs.orglett.0c02353

Here, we describe a highly selective Pd-catalyzed C(sp3)-H biarylation of 2-methylbenzaldehydes using cyclic diaryliodonium salts as arylation reagents. The key strategy is the employment of tert-leucine as a bidentate transient directing group for the proximity-driven metalation to achieve reactivity and selectivity in C-H activation. Various functionalized biaryls bearing both aldehyde and iodine functional groups were prepared successfully, which could be further transformed into a wide range of compounds with potential applications in pharmaceutical chemistry and materials science.

Full text of DOI:10.1021/acs.orglett.0c02353

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Letter
3
Pd-Catalyzed C(sp )−H Biarylation via Transient Directing Group
Strategy
Mingruo Ding, Wenkai Hua, Min Liu, and Fengzhi Zhang*
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sı Supporting Information
ABSTRACT: Here, we describe a highly selective Pd-catalyzed
3
C(sp )−H biarylation of 2-methylbenzaldehydes using cyclic diary-
liodonium salts as arylation reagents. The key strategy is the
employment of tert-leucine as a bidentate transient directing group
for the proximity-driven metalation to achieve reactivity and
selectivity in C−H activation. Various functionalized biaryls bearing
both aldehyde and iodine functional groups were prepared successfully, which could be further transformed into a wide range of
compounds with potential applications in pharmaceutical chemistry and materials science.
arbon−hydrogen bond activation and functionalization
substituted aryl iodides as arylation reagents led to either poor
yields or no reaction probably due to the neighboring steric
C
have proven to be a powerful synthetic strategy to rapidly
construct complex compounds from relatively simple starting
materials and hold great promise as one of the most effective
methodologies for the late stage structural modification of
3
effect (Scheme 1a). Therefore, more novel benzylic C(sp )−H
arylation methods are highly demanded in order to address this
issue and deliver more diverse and valuable 2-arylated
benzaldehydes.
1
functional molecules. To achieve the high selectivity and
reactivity, a directing group normally is required in transition-
metal catalyzed C−H activation. However, the prior covalent
installation and final removal of the stoichiometric directing
groups are obviously not atom- and step-economic and, thus,
limit their synthetic applications. In order to address this issue,
3
Scheme 1. Pd-Catalyzed C(sp )−H Arylation of 2-Methyl-
benzaldehydes via Transient Directing Group Strategy
2
transient directing group strategies were developed recently.
2
The remarkable progress has been applied in C(sp )−H
functionalization of aromatic aldehydes and ketones by
employing monodentate or bidentate transient directing
3
3d,4
5
groups, such as arylation, halogenation,
hydroxylation,8
4
b
4a,6
7
methoxylation, alkylation,
amidation, and selenylation.
3
Meanwhile, more challenging C(sp )−H bond functionaliza-
tions of aliphatic aldehydes and ketones with various transient
9
ligands have been significantly developed by the groups of Yu,
1
0
11
12
13
14
15
Ge, Bull, Wei, Zhou, Zhang,3 and Li.
Unsurprisingly, the benzylic C(sp )−H functionalization of
-alkylbenzaldehyde, a critical building block, has been
16
2
accomplished with this novel synthetic strategy. In 2016, Yu
and co-workers described the seminal example of Pd-catalyzed
3
benzylic enantioselective C(sp )−H bond arylation of aromatic
aldehydes using L-tert-leucine as a transient ligand and aryl
9c
iodides as arylation reagents. The same group also reported a
3
Pd(II)-catalyzed enantioselective C(sp )−H fluorination of 2-
alkylbenzaldehyde substrates using amino acid derived diethyl
17
Received: July 15, 2020
amide as a transient chiral ligand. Subsequently, the benzylic
3
C(sp )−H arylation of 2-methylbenzaldehyde was also
18
19
20
21
achieved by Hu, Kim, Li, and Zhang, which used
acetohydrazide, an amino amide bearing a hydroxyl group,
semicarbazide, and glycine as the transient ligand, respectively.
However, in these previous works, the reactions with ortho-
©
XXXX American Chemical Society
https://dx.doi.org/10.1021/acs.orglett.0c02353
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Inspired by these elegant works, and based on our previous
With optimum conditions in hand, the substrate scopes were
investigated by testing various structurally diverse 2-methyl-
benzaldehydes initially (Scheme 2). First, 2-methylbenzalde-
2
2
studies about cyclic diaryliodonium salt chemistry, we
envisioned that cyclic diaryliodonium salts could be employed
3
as arylation reagents for the Pd-catalyzed C(sp )−H
a
biarylation through a transient directing group strategy
Scheme 2. Scope of 2-Methylbenzaldehydes
(
Scheme 1b), and anticipated that the corresponding novel
biphenylated 2-methylbenzaldehydes bearing an iodine func-
tional group could be obtained successfully, which would
provide a complementary method for the selective benzylic
3
C(sp )−H functionalization.
To test our hypothesis, 2-methylbenzaldehyde 1a was used
as the initial substrate and cyclic diaryliodonium salt 2a was
employed as an arylation reagent (Table 1). We initially chose
a
Table 1. Reaction Conditions Optimization
b
entry
TDG
MS
solvents
yield (%)
1
TDG1
TDG2
TDG3
TDG4
TDG5
TDG4
TDG4
TDG4
TDG4
TDG4
TDG4
TDG4
TDG4
TDG4
−
−
−
−
−
−
−
−
−
−
−
AcOH
AcOH
AcOH
AcOH
AcOH
HFIP
HFIP/AcOH (9/1)
HFIP/AcOH (19/1)
HFIP/AcOH (9/1)
HFIP/AcOH (9/1)
HFIP/AcOH (9/1)
HFIP/AcOH (9/1)
HFIP/AcOH (9/1)
HFIP/AcOH (9/1)
HFIP/AcOH (9/1)
21
nr
2
3
4
5
6
7
8
9
1
1
1
1
1
1
18
31
27
trace
42
25
49
47
56
60
65
nr
c
d
c
0
1
2
3
4
5
3Å MS
3Å MS
3Å MS
3Å MS
3Å MS
c
c
c
,
,
,
e
f
,
f g
,
fh
nr
a
Reaction conditions: 1a (0.2 mmol), 2a (0.4 mmol), Pd(OAc) (10
2
a
1
(0.2 mmol), 2a (0.8 mmol), Pd(OAc) (10 mol %), AgTFA (1.5
2
mol %), AgTFA (1.5 equiv), TDGs (20 mol %), solvent (1.0 mL),
b
c
d
equiv), tert-leucine (40 mol %), 3Å MS MS, HFIP/AcOH = 9:1 (0.2
M), 120 °C, 12 h. 36 h.
1
20 °C, 12 h. Isolated yield. TDGs (40 mol %). TDGs (60 mol
b
e
f
%
). 1a (0.4 mmol), 2a (0.2 mmol). 1a (0.8 mmol), 2a (0.2 mmol).
g
h
Without palladium catalyst. Without TDGs. nr = no reaction, 3Å
hydes bearing diverse groups at the para-position of aldehyde,
such as the methoxy group, methyl, F, Cl, and Br, reacted
smoothly to provide the corresponding desired products in
good to excellent yields (3b−3f), respectively. The substrate
MS = 3Å molecular sieve, HFIP = 1,1,1,3,3,3- hexafluoro-2-propanol.
Pd(OAc) (10 mol %) as a catalyst, glycine (TDG1) (20 mol
with strong electron-withdrawing group NO afforded the
2
2
%
) as a transient directing group, and AgTFA (1.5 equiv) as a
product 3g in good yield for 36 h. The para-phenylthiolated
substrate gave the product 3h in 53% yield. The derivatives
from adamantane acid and xanthoic acid were successfully
coupled to generate the corresponding arylated adducts 3i and
3j. The substrates with a substituent on the meta-position of
aldehyde also gave the desired products (3k−3r) in good
yields except 3o. Interestingly, the substrates with a
neighboring substituent next to methyl group reacted well
(3q and 3r). Although 2-fluoro-6-methylbenzaldehyde was
successfully transformed into the desired product 3s in 70%
yield, the substrate with a methyl substituent next to the
aldehyde group did not react probably due to the neighboring
steric effect (3t). It is worth mentioning that various functional
groups including halides (F, Cl, Br, and I), nitro, aldehyde,
ester, and ether were compatible with the reaction conditions
which provide functional handles for further transformations.
Next, the scope of cyclic diaryliodonium salts was
investigated (Scheme 3). First, we examined the scope of
symmetrical cyclic diaryliodonium salts with 2-methylbenzal-
dehyde as a model substrate, which afforded the corresponding
products in good yields (4a−4d). A series of unsymmetrical
cyclic diaryliodonium salts were then tested (4e−4m). It was
halide scavenger, and the reaction was performed in AcOH at
3
1
20 °C for 12 h. To our delight, the desired benzylic C(sp )−
H arylated product 3a was obtained in 23% yield without the
2
detection of a C(sp )−H arylated byproduct (Table 1, entry
1
). To improve the yield of this reaction, a screening of
different ligands (TDG1−TDG5) was then carried out (Table
, entries 1−5). It was found that the reaction with tert-leucine
TDG4) as the directing group gave a better yield (Table 1,
entry 4); further screening of the solvents (Table 1, entries 6−
) showed that the mixed solvent system HFIP/AcOH (9/1)
proved to be more effective leading to a 42% yield of 3a (Table
, entry 7). Next, the yield was increased to 49% by increasing
the amount of transient directing ligand (TDG4) to 40 mol %
Table 1, entry 9). The yield was further improved to 56% by
1
(
8
1
(
adding 3Å MS (120 mg) into the reaction system (Table 1,
entry 11). Finally, the ratio of 1a to 2a was investigated, and
the optimum result was obtained in 65% yield when 1a (0.8
mmol) and 2a (0.2 mmol) were employed (Table 1, entry 13).
The control experiments in the absence of either palladium
catalyst or the transient directing group were negative (Table
1
, entries 14 and 15).
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Scheme 3. Scope of Cyclic Diaryliodonium Salts
was employed as a functional handle for the construction of
C−C and C−X bonds via Pd-catalyzed coupling reactions.
Various functional groups were introduced such as diphenyl-
phosphine 5, trimethylsilyl 6, alkynyl 9, and alkenyl 10.
Moreover, the cyclized products 7, 8 were obtained
successfully after treatment of 3a with iodobenzene and
diphenylacetylene. Next, the aldehyde group was employed as
a functional handle for further transformations. By treatment
with TfOH as a catalyst in hexafluoroisopropanol (HFIP), 3a
was readily converted into the polycyclic aromatic hydro-
carbons (PAHs) product anthracene 11 in good yield, which
has potential applications in organic light-emitting diodes
21
(
OLEDs). Benzyl alcohol, hydrazine, and imidazole deriva-
tives 12, 13, and 14 also were prepared in excellent yields by
treatment of 3a with ethyl magnesium bromide, 1-amino-4-
methylpiperazine, and o-phenylenediamine, respectively.
To gain more detailed information for a better under-
standing of this reaction, several control experiments were
performed (Scheme 5). First, the reaction employing the
Scheme 5. Control Experiments
found that the reaction gave a single product with the methyl
attached to the less hindered and electron-deficient side of the
cyclic diaryliodonium salts. Both electron-withdrawing and
-
donating groups, including various functional groups such as
fluorine (4f), chlorine (4g), ester (4h), trifluoromethyl group
4j), and nitro group (4k) were compatible with the optimum
(
conditions.
Because of the importance of iodine and aldehyde groups, a
gram scale preparation of 3a was conducted in 66% yield, and
further derivatization studies were carried out to explore the
synthetic transformations (Scheme 4). First, the iodine group
Scheme 4. Further Transformations of 3a
biphenyl iodide 2b or linear diaryliodonium salt 2c as an
arylation reagent respectively provided only trace amount of
product 15 (Scheme 5-I,II), which demonstrates the unique
importance of cyclic diaryliodonium salts as biarylation
reagents. With 2-ethyl-benzaldehyde 1b as the substrate, the
reaction did not produce any arylated product under the
optimal conditions (Scheme 5-III). Interestingly, with 2′-
methylacetophenone 1c as substrate and alanine as a transient
directing group at 130 °C for 24 h, the desired adduct 17 was
obtained successfully, albeit in 20% yield (Scheme 5-IV).
Although the yield cannot be improved by further optimizing
3
reaction conditions, it did prove that the benzylic C(sp )−H
C
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Letter
bond of aromatic ketones could be functionalized via this
strategy. Next, a deuteration experiment was performed
Experimental procedures and spectroscopic character-
ization data (PDF)
(Scheme 5-V) and the intermolecular kinetic isotope effects
of C−H biarylation were determined to be KIE ≈ 4.0,
AUTHOR INFORMATION
Corresponding Author
3
■
indicating that the C(sp )−H bond cleavage might be the rate-
limiting step.
23
On the basis of these results and the related literature,
a
Fengzhi Zhang − College of Pharmaceutical Science, Zhejiang
University of Technology, Hangzhou 310014, P. R. China;
orcid.org/0000-0001-9542-6634; Email: zhangfengzhi@
zjut.edu.cn
putative mechanism for this method is outlined in Scheme 6.
Scheme 6. Proposed Reaction Mechanism
Authors
Mingruo Ding − College of Pharmaceutical Science, Zhejiang
University of Technology, Hangzhou 310014, P. R. China
Wenkai Hua − College of Pharmaceutical Science, Zhejiang
University of Technology, Hangzhou 310014, P. R. China
Min Liu − College of Pharmaceutical Science, Zhejiang University
of Technology, Hangzhou 310014, P. R. China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c02353
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the National Natural Science
Foundation of China (Grant No. 21871234), and Zhejiang
Provincial NSFC for Distinguished Young Scholars under
Grant No. LR15H300001.
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