Heteroarene-Directed Oxidative sp2 C-H Bond Allylation with Aliphatic Alkenes Catalyzed by an (Electron-Deficient η5-Cyclopentadienyl)rhodium(III) Complex

Article abstract of DOI:10.1021/acs.orglett.6b01288

It has been established that the oxidative sp² C-H bond allylation with aliphatic alkenes proceeds under mild conditions by using heteroarenes as directing groups and an (electron-deficient η⁵-cyclopentadienyl)rhodium(III) complex, [

Full text of DOI:10.1021/acs.orglett.6b01288

Letter
pubs.acs.org/OrgLett
Heteroarene-Directed Oxidative sp2 C−H Bond Allylation with
Aliphatic Alkenes Catalyzed by an (Electron-Deficient
η5‑Cyclopentadienyl)rhodium(III) Complex
Yuji Takahama,† Yu Shibata,*,‡ and Ken Tanaka*,†,‡
†Department of Applied Chemistry, Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei,
Tokyo 184-8588, Japan
‡Department of Chemical Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8550, Japan
S
* Supporting Information
ABSTRACT: It has been established that the oxidative sp2 C−H bond
allylation with aliphatic alkenes proceeds under mild conditions by using
heteroarenes as directing groups and an (electron-deficient η5-cyclopenta-
dienyl)rhodium(III) complex, [CpERhCl2]2, as a precatalyst. In sharp
contrast, the use of [Cp*RhCl2]2 instead of [CpERhCl2]2 led to a complex
mixture of products under the same reaction conditions.
he transition-metal-catalyzed allylation of arenes is widely
employed as an important tool for the synthesis of
required in the same reactions using a widely used commercially
available rhodium(III) complex, [Cp*RhCl2]2, instead of 1.10,11a
In addition to the annulation with alkynes, the oxidative sp2 C−H
bond olefination of acetanilides with aliphatic alkenes also
proceeded under ambient conditions by using 1 as the precatalyst
to give styrene derivatives in good yields (Scheme 1, left). In
T
allylarenes.1 Specifically, the direct allylation of sp2 C−H bonds
of arenes is a highly attractive protocol from the viewpoint of
atom- and step-economy.2 A number of successful examples of
the sp2 C−H bond allylation with allylic compounds, possessing
heteroatom-containing leaving groups (e.g., allyl carboxylates,
allyl carbonates, and allyl halides), have been reported3−7) by
using various late transition-metal complexes (e.g., Pd(II),3
Rh(III),4 Fe(III),5 Co(III),6 and other metal complexes7) as
catalysts. Yet, the direct oxidative (dehydrogenative) sp2 C−H
bond allylation with aliphatic alkenes is an extremely atom- and
step-economical method, since this process produces only water
as a byproduct and requires no preactivation or prefunctionaliza-
tion of both coupling partners. Despite the high synthetic utility
of this transformation, the oxidative sp2 C−H bond allylation
with aliphatic alkenes has been succeeded in a limited number of
examples8,9 and it is awaiting the expanded substrate scope. For
example, although several Pd(II)-catalyzed reactions have been
reported,8 the desired allylation products were obtained in a few
examples8a−c as minor products8d or by using uncommon
substrates (polyfluorobenzenes).8e,f The Rh(III)-catalyzed re-
actions have also been reported,9 while examples have been
limited to the use of a N-alkyl (or phenyl)-N-nitrosoamino
directing group9a or strained unimolecular reactants.9b
Scheme 1
sharp contrast, when using [Cp*RhCl2]2 instead of 1 under the
same reaction conditions, no reaction was observed.11c In this
paper, we have established that not the olefination but the
allylation of the aromatic sp2 C−H bonds with aliphatic alkenes
proceeds by using heteroarenes13 in place of N−H amides as
directing groups and 1 as the precatalyst (Scheme 1, right).
We first examined the reaction of 1-phenylpyrazole (2a, 2
equiv) and 1-octene (3a, 1 equiv) in the presence of a cationic
rhodium(III)/CpE catalyst, generated in situ from 1, AgSbF6, and
Cu(OAc)2·H2O. We were pleased to find that not the olefination
but the allylation of 2a with 3a proceeds at room temperature
under air to give monoallylated product 4aa as a mixture of E/Z
On the other hand, our research group reported that several
oxidative sp2 C−H bond functionalization reactions of arenes can
be catalyzed by a bis(ethoxycarbonyl)-substituted cyclopenta-
dienyl−rhodium(III) complex, [CpERhCl2]2 (1),10 under mild
conditions.11,12 For example, the oxidative [3 + 2] annulation of
anilides with internal alkynes using 1 as a precatalyst proceeded
under ambient conditions (room temperature, under air) to give
multisubstituted indoles in high yields, while elevated temper-
ature and a stoichiometric oxidant (or oxygen atmosphere) were
Received: May 3, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b01288
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
a
isomers in 56% yield along with diallylated product 5aa in 26%
yield at the 85% conversion of 3a (Table 1, entry 1). In order to
Scheme 2
a
Table 1. Optimization of Reaction Conditions
c
yield (%)
b
entry
n
AgX
temp
rt
2a/3a conv (%) of 3a
4aa 5aa
1
2
3
4
5
5
AgSbF6
AgPF6
2:1
2:1
2:1
2:1
2:1
2:1
2:1
3:1
1.1:1
3:1
3:1
3:1
85
88
86
96
84
88
92
89
66
60
90
>99
56
51
48
39
53
49
51
62
28
50
73
3
26
26
30
34
30
15
24
20
19
8
5
rt
5
AgBF4
rt
5
AgNTf2
AgOTf
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
AgSbF6
rt
5
rt
d
6
e
5
rt
7
5
rt
8
5
rt
9
5
rt
10
11
2.5
2.5
2.5
rt
40 °C
40 °C
16
0
f
12
a
1 (0.0063 mmol), AgX (0.025 mmol), Cu(OAc)2·H2O (0.050
mmol), 2 (0.75 mmol), 3 (0.25 mmol), and acetone (1.3 mL) were
b
used. As 3a could not be recovered after evaporation, conversions of
1
3a were estimated from recovered 2a, which was determined by H
NMR using 1,3,5-trimethylbenzene as an internal standard.
a
1 (0.0063 mmol), AgSbF6 (0.025 mmol), Cu(OAc)2·H2O (0.050
mmol), 2 (0.75 mmol), 3 (0.25 mmol), and acetone (1.3 mL) were
used. The cited yields are of the isolated products.
c
1
Determined by H NMR using 1,3,5-trimethylbenzene as an internal
d
e
f
standard. Solvent: (CH2Cl)2. Solvent: 2-butanone. [Cp*RhCl2]2
was used instead of 1.
yields than 4aa and 4ba presumably due to steric repulsion
between the methyl group (red) on the heteroarene ring and the
ortho hydrogen (blue) on the benzene ring (Figure 1), which
increase the conversion of 3a and the selectivity for 4aa,
optimization of reaction conditions was conducted. Screening of
silver salts (entries 1−5) revealed that AgSbF6 is the most
effective one (entry 1). With respect to solvents, the use of
dichloroethane and 2-butanone slightly lowered the product
yields (entries 6 and 7). In order to inhibit the formation of
diallylated product 5aa, the ratio of 2a to 3a was investigated. As
expected, increasing the amount of 2a increased the yield of 4aa
(entry 8); in contrast, decreasing the amount of 2a decreased the
conversion of 3a (entry 9). Gratifyingly, when the amount of 1
was reduced to 2.5 mol %, the selectivity for 4aa was improved,
although the conversion of 3a was decreased (entry 10). Finally,
when the reaction was conducted at slightly elevated temperature
(40 °C), 4aa was obtained in the highest yield (entry 11).
Importantly, the use of [Cp*RhCl2]2 instead of 1 afforded only a
trace amount of 4aa (entry 12). Other than this product, an
extremely complex mixture of unidentified products was
generated (entry 12).
With the optimized reaction conditions in hand, we next
focused our attention into the substrate scope (Scheme 2). A
wide variety of heteroarylbenzenes could be allylated with 1-
octene (3a), although the product yields were varied.14 A 2-
pyridyl group worked as a good directing group to give the
desired monoallylated product 4ba in good yield, which is
comparable to that of 4aa. However, sterically demanding
substituted heteroarylbenzenes, 1-phenyl-3,5-dimethylpyrazole
(2c) and 3-methyl-2-phenylpyridine (2d), reacted with 3a to
give monoallylated products 4ca and 4da, respectively, in lower
Figure 1. Steric repulsion between the methyl group (red) and the ortho
hydrogen (blue).
deters the rhodacycle formation through the C−H bond
activation. The reactions of heteroarylbenzenes 2e and 2f,
possessing more than one nitrogen atom, were also investigated,
which revealed that the yields of the desired monoallylated
products 4ea and 4fa were lower than those of 4aa and 4ba, as a
result of the formation of significant amounts of diallylated
products 5ea and 5fa.15,16 With respect to the substituents on the
benzene ring, para-substituted and meta-disubstituted hetero-
arylbenzenes 2g−i reacted with 3a to give monoallylated
products 4ga−ia in good yields. The reaction of meta-substituted
heteroarylbenzene 2j with 3a proceeded with perfect regiose-
lectivity in the same yield as 4aa. The scope of aliphatic alkenes
was also examined. Not only 1-octene (3a) but also sterically
demanding branched alkenes 3b and 3c and allylbenzene 3d
could be employed for this process. Furthermore, functionalized
alkenes 3e and 3f, bearing chloro and ethoxycarbonyl groups,
B
DOI: 10.1021/acs.orglett.6b01288
Org. Lett. XXXX, XXX, XXX−XXX

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Letter
were also capable of reacting with 2a to give monoallylated
products 4ae and 4af in good yields without affecting the
functional groups. With respect to the stereochemistry of the
allylation products, E isomers were obtained as major products.
It was anticipated that the use of 1-(2-methylphenyl)pyrazole
(2k) would afford the corresponding monoallylated products in
good yields without using excess 2k. However, steric repulsion
between the ortho methyl group (red) on the benzene ring and
hydrogen (blue) on the heteroarene ring (Figure 1) would deter
the rhodacycle formation through the C−H bond activation. As
shown in Scheme 3, the reaction of 2k and 3a in the ratio of 3 to 1
A plausible explanation for the effect of the directing groups on
the reaction pathways (oxidative allylation vs olefination) is
shown in Scheme 5. The cationic rhodium(III) complex A reacts
Scheme 5
a
Scheme 3
with 1-phenylpyrazole (2a) and 1-octene (3a) to give five-
membered rhodacycle B through sp2 C−H bond cleavage. Next,
regioselective insertion of 3a into B gives seven-membered
rhodacycle C. β-Hydride elimination away from the benzene
ring8,9,17 gives heteroarene- and olefin-coordinated intermediate
D. Subsequent oxidative decomplexation affords allylated
product 4aa and regenerates the Rh(III) catalyst. Yet, when
using acetanilide (7) in place of 2a, the sp2 C−H bond cleavage
followed by insertion of 3a gives eight-membered rhodacycle E.
In this intermediate, β-hydride elimination adjacent to the
benzene ring proceeds to give olefinated product 8 through
carbonyl- and olefin-coordinated intermediate F. The selectivity
of the β-hydride elimination might be determined by flexibility of
the rhodacycles C and E induced by both the ring size (seven- or
eight-membered ring) and the coordinative functional group
(heteroarene or carbonyl).18
a
1 (0.0063 mmol), AgSbF6 (0.025 mmol), Cu(OAc)2·H2O (0.050
mmol), 2 (0.25 mmol), 3 (0.275 mmol), and acetone (1.3 mL) were
used. The cited yields are of the isolated products. 2 (0.75 mmol) and
3 (0.25 mmol) were used.
b
proceeded sluggishly to give the corresponding monoallylated
product 4ka in lower yield than 4aa as a result of the insufficient
conversion of 3a. Pleasingly, when 1.1 equiv of 3a with respect to
2k was used, the yield of 4ka was increased to 63%. This yield is
markedly higher than that of 4aa under the same conditions
(28%: Table 1, entry 9). Unfortunately, the reaction of 1-(2-
chlorophenyl)pyrazole (2l) with 3a was sluggish even using 1.1
equiv of 3a with respect to 2l. In contrast to the poor yields of 4ea
and 4fa shown in Scheme 2, the use of 2-methylphenyl-
substituted heteroarenes 2m and 2n, possessing more than one
nitrogen atom, dramatically increased the yields of monoallylated
products 4ma and 4na. Sterically demanding branched alkene 3c
also reacted with 1n to give monoallylated product 4nc in high
yield. The formation of E isomers as major products is the same
as the case shown in Scheme 2.
In summary, we have established that the oxidative sp2 C−H
bond allylation with aliphatic alkenes proceeds under mild
conditions by using heteroarenes as directing groups and an
(electron-deficient η5-cyclopentadienyl)rhodium(III) complex,
[CpERhCl2]2, as a precatalyst. In sharp contrast, the use of
[Cp*RhCl2]2 instead of [CpERhCl2]2 led to a complex mixture of
products under the same reaction conditions. The selective
formation of the allylated products might arise from the selective
β-hydride elimination away from the benzene ring in the
heteroarene-coordinated seven-membered rhodacycle.
Under the optimized reaction conditions employed in Scheme
2, the oxidative coupling of 2a with “activated” alkenes, styrene
(3g) and methyl acrylate (3h), also proceeded to give the
corresponding olefinated products 6ag and 6ah in good yields
(Scheme 4).
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b01288.
■
S
Experimental procedures and compound characterization
data (PDF)
Scheme 4
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: yshibata@apc.titech.ac.jp.
*E-mail: ktanaka@apc.titech.ac.jp.
C
DOI: 10.1021/acs.orglett.6b01288
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
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This work was supported partly by ACT-C from the Japan
Science and Technology Agency (JST, Japan), Grants-in-Aid for
Scientific Research (Nos. 26102004 and 25105714) from the
Ministry of Education, Culture, Sports, Science and Technology
(MEXT, Japan), and a Grant-in-Aid for Research Activity Start-
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Science (JSPS, Japan). We are grateful to Umicore for generous
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D
DOI: 10.1021/acs.orglett.6b01288
Org. Lett. XXXX, XXX, XXX−XXX