Readily Removable Directing Group Assisted Chemo- and Regioselective C(sp3)-H Activation by Palladium Catalysis

Article abstract of DOI:10.1002/anie.201505932

Currently used directing groups for selective aliphatic β-functionalization of carbonyl compounds show excellent reactivity and selectivity with an amide as a linker. Described herein is 2-piconimide, used for the first time with commercially available 2-picolinamide/2-picolic acid as precursors, to direct C-H arylation/alkenylation by palladium catalysis. The directing group is essential for promoting the sequnetial primary and secondary C(sp³)-H arylation with different aryl iodides in one substrate. The directing group was easily removed under simple reaction conditions at room temperature.

Full text of DOI:10.1002/anie.201505932

.
Angewandte
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International Edition: DOI: 10.1002/anie.201505932
German Edition: DOI: 10.1002/ange.201505932
CÀH Activation
Readily Removable Directing Group Assisted Chemo- and
3
À
Regioselective C(sp ) H Activation by Palladium Catalysis
Yun-Fei Zhang, Hong-Wei Zhao, Hui Wang, Jiang-Bo Wei, and Zhang-Jie Shi*
Dedicated to Professor Xuelong Hou on the occasion of his 60th birthday
Abstract: Currently used directing groups for selective ali-
phatic b-functionalization of carbonyl compounds show
excellent reactivity and selectivity with an amide as a linker.
Described herein is 2-piconimide, used for the first time with
commercially available 2-picolinamide/2-picolic acid as pre-
cursors, to direct CÀH arylation/alkenylation by palladium
catalysis. The directing group is essential for promoting the
3
sequnetial primary and secondary C(sp )ÀH arylation with
different aryl iodides in one substrate. The directing group was
easily removed under simple reaction conditions at room
temperature.
T
ransition-metal-catalyzed direct CÀH functionalization has
[1]
attracted much interest in the past decades. Compared to
Scheme 1. Directing groups for b-functionalization of carboxylic acid
derivatives by palladium catalysis. FG=functional group.
2
[2]
3
C(sp )ÀH functionalization,
C(sp )ÀH activation faces
[3]
many more challenges and the progress is far behind. To
conquer the challenges of both efficiency and selectivity in
3
C(sp )ÀH activation, the use of directing groups (DGs) has
of new directing groups, which are commercially available
and readily removable under mild reaction conditions, is
highly desirable.
proven to be the most powerful, common, and practical
[4]
strategy. Because of the importance of carboxylic acid
derivatives, site-specific activation of their CÀH bonds is
To solve this problem, one can modify the directing
groups to facilitate their removal. Chen and co-workers
[5]
[9a]
highly appealing. To approach the b-functionalization of
carboxylic acid derivatives, well-established directing groups
have been developed since the first example was demon-
and others made significant contributions to successfully
[
9]
modifying the 8-aminoquinyl scaffold. Shi and co-workers
[
6]
[10]
strated by Daugulis and co-workers in 2005 (Scheme 1). Of
the reported directing groups, bidentate ones showed much
developed the PIP group,
which is now commercially
available, based on the previous reports by Chatani and co-
[
7]
[11]
better performance.
Another beautiful example was
workers.
However, the structural complexity requires
reported by Yu and co-workers on the use of 2,3,5,6-
tetrafluoro-4-(trifluoromethyl)aniline as a successful mono-
tedious synthetic procedures and thus increases of cost. Our
goal is to develop commercially available and easily remov-
able directing groups to facilitate site-selective aliphatic CÀH
[8]
dentate directing group. Undoubtedly, current directing
groups have an excellent ability to promote the reactivity and
selectivity. However, the stable amides linkers are difficult to
remove, and some of them are relatively expensive and
require several steps to prepare. Therefore, the development
activation. Therefore, we turned our attention to the much
more labile imide linker between the directing group and the
substrate.
We considered 2-picolinamide since it is commercially
available and inexpensive. Potentially, it can serve as a DG
because it contains coordinating N atoms. However, some
challenges exist: 1) the high acidity of NH of imides,
compared to those of amides, might intrinsically affect the
reactivity; 2) the high reactivity of the imide seems beneficial
for its removal while the stability of such a labile imide linker
is unpredictable under the relatively harsh reaction conditions
required for CÀH functionalization in the presence of base or
[
*] Y.-F. Zhang, H. Wang, J.-B. Wei, Prof. Dr. Z.-J. Shi
Beijing National Laboratory of Molecular Sciences (BNLMS) and Key
Laboratory of Bioorganic Chemistry and Molecular Engineering of
Ministry of Education, College of Chemistry and Molecular Engi-
neering and Green Chemistry Center, Peking University, Beijing,
1
00871 (China)
Prof. Dr. H.-W. Zhao
College of Chemistry and Chemical Engineering,
Shanxi University, Shanxi, 030006 (China)
acid; 3) last but not the least, the introduction of such a DG by
the formation of the imide might not be as easy as the
corresponding amide.
To explore the possibility based on our design, we first
developed an efficient protocol to prepare the substrate by
formation of the imide. Indeed, it was an easy and reliable
Prof. Dr. Z.-J. Shi
State Key Laboratory of Organometallic Chemistry, Chinese Academy
of Sciences, Shanghai 200032 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201505932.
1
3686
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protocol through a one-pot sequence involving the deproto-
nation of picolinamide and acylation with acyl chloride
product. For example, cyclohexane gave a better result
(entry 4) and with toluene 3aa was observed in 40% yield
(entry 5). Other aromatic solvents were tested, and chlor-
obenzene and tert-butylbenzene gave 3aa in 42 and 60%
yield, respectively (entries 6 and 7). Silver salts were crucial.
Other silver salts were tested, such as Ag CO and AgBF , but
[
Eq. (1); THF = tetrahydrofuran].
2
3
4
better results were not obtained (entries 8 and 9). Fortunately,
1
Ag PO gave much better results and the yield ( H NMR) was
3
4
improved to 83% (entry 10). When Pd(OTFA) was used
2
instead of Pd(OAc) as the catalyst, 3aa was isolated in 83%
2
(
entry 11). The amount of Ag PO4 could be reduced to
3
0
.9 equivalents and the efficacy was not obviously affected
With the starting material in hand, we set out to screen the
(entry 12). Further reaction optimization indicated that this
arylation took place at 1008C for 3 hours to deliver the
product in 82% yield (entries 13–16). A control experiment
confirmed an essential role of the palladium catalyst
(entry 17).
reaction conditions for the desired CÀH arylation with 1a as
a model substrate and PhI (2a; Table 1). At the beginning,
Pd(OAc) was used as a catalyst in DCE at 1208C in the
2
presence of AgOAc (2.0 equiv) as a base. The product 3aa
was obtained but the yield was very low after 3 hours (less
than 10% by H NMR analysis). As predicted, the rest of 1a
was decomposed (entry 1). However, this result encouraged
us to search for other reaction conditions to promote the
arylation because the observation of 3aa indicated that the
piconimide is a potential DG, albeit with a much lower
The substrate scope was further investigated and the
representative data with different aryl iodides are shown in
Table 2. The results demonstrate the broad substrate scope
1
Table 2: Palladium-catalyzed direct arylation of 1a with different aryl
[a]
iodides (2).
pK value. Considering the solubility and stability of imides,
a
the solvent effect should be very important. Polar solvents,
such as tAmylOH and THF, severely accelerated the decom-
position of 1a (entries 2 and 3). The nonpolar solvents were
beneficial for stabilizing both the starting material and
3
Table 1: Paladium-catalyzed Direct C(sp )ÀH arylation of 1a with 2a
[
a]
under different reaction conditions.
+
[b]
Entry
Catalyst
Ag salt
Solvent
Yield [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OAc)2
Pd(OTFA)2
Pd(OTFA)2
Pd(OTFA)2
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
Ag CO
DCE
tAmylOH
THF
c-hexane
<10
decomposed
decomposed
30
40
42
60
52
[a] Reaction conditions: 1a (0.10 mmol), 2 (0.20 mmol), Pd(OTFA)2
(10 mol%), Ag PO (0.090 mmol) in 1.0 mL of TBB (tert-butylbenzene)
at 1008C for 3 h and open to air. All the yields reported are those for the
3
4
isolated products. [b] Diarylated products were isolated.
toluene
chlorobezene
t-butylbenzene
t-butylbenzene
t-butylbenzene
t-butylbenzene
t-butylbenzene
t-butylbenzene
t-butylbenzene
t-butylbenzene
t-butylbenzene
t-butylbenzene
t-butylbenzene
with 1a as a partner, and the corresponding arylated products
2
3
(3) were obtained in good to excellent yields. Both electron-
AgBF4
n.r.
83
87 (83)
rich (2b,c, 2e–h) and electron-deficient (2i–n) aryl iodides
performed well. Aryl iodides bearing various functional
groups, such as alkyl (2b–f), acetyl (2l), ester (2m), trifluoro-
methyl (2n) etc., proceeded smoothly. Steric effects were
critical, and o-tolyl iodide (2d) failed to react. Methoxy (2g),
halo (2i–2k), and pivaloyl (2h) groups survived, thus provid-
ing the potential for orthogonal functionalization through
0
1
2
3
4
5
6
7
Ag PO
3
4
4
4
4
4
4
4
4
Ag PO
3
[
c]
Ag PO
86 (83)
3
[
c,d]
Ag PO
30
3
[
[
c,e]
Pd(OTFA)2
Ag PO
3
84 (82)
85 (82)
50%
n.r.
c,e,f]
Pd(OTFA)2
Pd(OTFA)2
–
Ag PO
3
[
c,e,g]
Ag PO
3
Ag PO
3
[12]
well-established cross-couplings.
Heteroaryl iodides, for
[
a] Reaction conditions: 1a (0.1 mmol), 2a (0.2 mmol), catalyst
example, N-tosyl-5-iodo-1H-indole (2o), also gave the ary-
lated product in good yield. Notably, aryl iodides, bearing
strong electron-withdrawing substituents on the phenyl ring
(10 mol%), Ag salt (0.2 mmol) in 1.0 mL of solvent at 1208C for 12 h in
a 50 mL sealed tube if without further note. [b] 1H NMR yields using
benzo[d][1,3]dioxole as an internal standard. Yield of isolated product
shown within parentheses. [c] 0.9 equiv Ag PO . [d] 808C. [e] 1008C.
(
2i, 2j, 2l–n) reacted well at the primary CÀH bonds, but
3
4
[
f] 3 h. [g] 1 h. TFA=trifluoroacetate.
a second CÀH activation, at the secondary carbon atom of the
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cyclohexyl ring, did occur, probably because of their better
oxidative ability toward in situ generated palladium(II)
species. Both mono- and diarylated products were isolable
by flash chromatography. It is worth noting that transition-
propionylamide, and the cyclic substrate bearing an a-CÀH
[4e]
bond were problematic substrates. However, in our reac-
tion, all of them (4l–o) reacted. This chemistry showcases the
potential of developing new directing groups.
3
metal-catalyzed C(sp )ÀH arylation with much less reactive
With the successful access to the direct arylation of
3
3
aryl bromides still remain challenging. However, in our
present system, a good yield was obtained by using 4’-
bromoacetophenone (2p).
C(sp )ÀH bonds, we started exploring C(sp )ÀH alkenyla-
[13]
tion. Although alkenyl bromides were commercially avail-
able and inexpensive, they had never been used as an
3
A variety of aliphatic the carboxylic acid derivatives were
examined. The structure of carboxylic acids had intrinsic
effects on the reactivity. As show in Table 3, a tertiary a-
carbon atom was important for this transformation. In the
absence of a CÀH bond, acyclic aliphatic imides reacted very
alkenylating reagent in C(sp )ÀH alkenylation. For the first
time we tested (2-bromovinyl)benzene (5a) as an alkenylat-
ing reagent, and to our delight, the desired product 6a was
obtained in 54% yield, upon isolation, with very good
distereoselectivity (Scheme 2). It is important to note that
smoothly. The monomethyl substrates were arylated in good
to excellent yields (4a–g). It was very important to note that,
the arylation only took place at the primary carbon center,
thus leaving secondary and activated benzyl CÀH, and even
2
sp -hybridized aryl CÀH bonds untouched. Similar to pre-
vious reports with the amide linker, imides bearing two
methyl groups at the a-position also performed well, and
mono- and diarylated products at both methyl groups were
obtained (4h, 4i, 4l). Similar to 1a, cyclic substrates showed
good reactivity (4j,k). In previous reported systems, isobu-
tyramide, 2-methylbutanamide, 2-phenylpropionylamide,
Table 3: Palladium-catalyzed direct arylation of different imides (1) with
Scheme 2. Palladium-catalyzed direct alkenylation with alkenyl bro-
mides.
[
a,b]
2.
this unexpected alkenylation took place at the secondary CÀ
H bond on the backbone instead of at the primary carbon
center, which was completely different from previous
[13]
reports. Different alkenyl bromides were tested and both
electron-rich (6b–e) and electron-deficient (6 f,g) alkenyl
bromides gave acceptable yields. Alkyl (6b–e), halo (6 f,g),
and naphthyl (6h) groups were suitable substrates. The CÀH
bond of a seven-membered ring was alkenylated but in
a lower yield and with lower distereoselectivity (6i).
To further explore the potential application of this
arylation, the reaction of 1a and 2b was scaled up to
5
.0 mmol in a 50 mL one-necked flask, open to the air, and
the same efficiency was maintained [Eq. (2)].
We then explored the sequential aliphatic CÀH function-
alization of both primary and secondary CÀH bonds in the
same molecule (Scheme 3). With the use of 2b as the first
arylating reagent, 1a was successfully and selectively arylated
at the primary CÀH bond of the methyl group in an excellent
[
(
1
a] Reaction conditions: 1 (0.10 mmol), 2 (0.20 mmol), Pd(OTFA)2
10 mol%), Ag PO (0.090 mmol) in 1.0 mL of tert-butylbenzene at
3 4
008C for 12 h and open to air. All the yields reported are those for the
1
2
yield. After the transformation finished, another arylating
isolated products. [b] Ar =4-acetylphenyl, Ar =4-methylphenyl. [c] At
308C.
1
reagent, 2l, was submitted with an additional palladium
1
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at room temperature. Studies on the application of 2-
picolinamide as a directing group in other transformations
and a clear understanding of mechanism are underway.
Experimental Section
Scheme 3. Sequential double CÀH arylation.
General procedure for gram-scale reaction: Pd(OTFA) (166.2 mg,
2
0
5
.5 mmol), Ag PO (1.8836 g, 4.5 mmol), imide (1a, 1.2315 g,
.0 mmol), and aryl halide (2b, 4.3606 g, 10 mmol) were added to
3 4
catalyst and silver salt. Thus the second arylation took place
solely at the methylene group on the cyclohexyl ring with high
chemo- and regioselectivity, and the formed, more active
benzyl CÀH was untouched. This sequential double CÀH
a 50 mL one-neck round-bottom flask open to the air. Then the
solvent (TBB, 20.0 mL) was added. The mixture was stirred at 1008C
in oil bath for 3 h. After the reaction was complete, the system was
cooled to room temperature and purified directly (without removal of
TBB) by flash chromatography on silica gel with petroleum ether/
EtOAc (10:1!3:1) to give the product. 3ab was obtained as
a colorless liquid in 85% yield (1.4288 g).
arylation showed good efficiency and ideal selectivity, thus
providing a new protocol to produce the complex molecules
through sequential CÀH activation with the same directing
group.
To further test the ability to remove this directing group,
we chose different types of arylated product, including 4o, 4n,
Acknowledgements
3
ab, and 4i to explore the reaction conditions. To our delight,
Support of this work by the “973” Project from the MOST of
China (2013CB228102, 2015CB856600), NSFC (No.
all these products were hydrolyzed under mild reaction
conditions in a short time (please refer to the Supporting
Information). For 4o and 4n, the carboxylic acid and 2-
picolinamide were obtained in quantitative yields, thus
showing the great recovery of the directing group [Eq. (3)].
However, both 3ab and 4i gave the picolic acid/ester and the
corresponding amide as products with very high efficiency
2
1332001), and NSF of Shanxi Province (No. 2014011014-4)
is gratefully acknowledged.
Keywords: arenes · CÀH activation · palladium · silver ·
synthetic methods
[
6]
[
Eq. (4)].
How to cite: Angew. Chem. Int. Ed. 2015, 54, 13686–13690
Angew. Chem. 2015, 127, 13890–13894
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Received: June 28, 2015
Revised: July 30, 2015
Published online: September 1, 2015
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3690 www.angewandte.org
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