Direct Alkenylation of Allylbenzenes via Chelation-Assisted C-C Bond Cleavage

Article abstract of DOI:10.1021/jacs.8b03718

A novel method for direct transformation of allyl groups in allylbenzene derivatives to alkenyl groups via rhodium-catalyzed C-C bond cleavage is reported. The alkenylation with styrenes of allylbenzenes containing pyridyl and pyrazolyl groups as a directing group proceeded efficiently to give alkenylation products. We also developed a new protocol for transformation of an ortho-prenylated phenol to an aniline derivative.

Full text of DOI:10.1021/jacs.8b03718

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Communication
Direct Alkenylation of Allylbenzenes via
Chelation-Assisted C–C Bond Cleavage
Shunsuke Onodera, Soya Ishikawa, Takuya Kochi, and Fumitoshi Kakiuchi
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 22 May 2018
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Direct Alkenylation of Allylbenzenes via Chelation-Assisted C–C Bond
Cleavage
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Shunsuke Onodera, Soya Ishikawa, Takuya Kochi, and Fumitoshi Kakiuchi*
Department of Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa
223-8522, Japan.
Supporting Information Placeholder
a trace amount of 4aa was obtained in the absence of AgSbF6 (entry 3).
The use of other rhodium catalysts such as Cp*Rh(OAc)2·H2O,
RhH(CO)(PPh3)3, and RhH(PPh3)4 resulted in the formation of less
than a trace amount of 4aa (entries 4-6). Finally, extension of the reac-
tion time to 48 h was found
ABSTRACT: A novel method for direct transformation of allyl groups
in allylbenzene derivatives to alkenyl groups via rhodium-catalyzed C–
C bond cleavage is reported. The alkenylation with styrenes of
allylbenzenes containing pyridyl and pyrazolyl groups as a directing
group proceeded efficiently to give alkenylation products. We also
developed a new protocol for transformation of an ortho-prenylated
phenol to an aniline derivative.
Scheme 1. Metal-Catalyzed Cleavage of Unstrained C–C Bonds
Catalytic bond formation via cleavage of unreactive bonds has at-
tracted considerable attention and has provided unconventional, effi-
cient routes to synthesize complex organic molecules. Particularly,
metal-catalyzed conversion of inert, unstrained C–C bonds to new C–
C bonds has been recognized as one of the most challenging transfor-
mation in this field, because of the limited availability of effective
methods to cleave such thermodynamically stable bonds.1 Formation
of a stable chelate has been frequently used as a driving force for cata-
lytic cleavage of the unstrained C–C bonds, including pioneering
works of Suggs,2 Jun,3 and Murai4 as well as recent works of Douglas,5
Shi,6 Wang,7 Dong,8 Morandi,9 Johnson,10 Xu and Wei,11 but the cleav-
able bonds have been limited to those right next to a heteroatom such
as ketones, imines, and alcohols.
Here we report a chelation-assisted catalytic alkenylation of al-
lylbenzene derivatives via C–C bond cleavage. A simple Cp*Rh cata-
lyst/alcohol system was found to be effective for the conversion of
allylbenzenes possessing a directing group to stilbene derivatives. Many
reactions involving cleavage of allylic C–C bonds have been reported
but mostly proceed via either formation of ketones from homoallyl
alcohols12 or generation of stabilized carbanions.13 This deallylative
alkenylation reported here is the first catalytic C–C bond formation
involving chelation-assisted regioselective cleavage of bonds between
an aromatic ring and an allyl group.14
to improve the NMR yield to 91%, and product 4aa was isolated in
87% yield (entry 7).
Substrate scope was then examined for the pyridyl-directed deallyla-
tive alkenylation (Table 2). First, the reactions of 1a with styrenes
having various substituents were investigated. High yields (86-92%) of
the alkenylation products were obtained in the reactions with styrenes
having a variety of para substituents including electron-donating (Me,
tBu) and -withdrawing (CF3) groups and halogeno groups (F, Cl, Br).
The reaction with 2-methylstyrene also proceeded to give the corre-
sponding alkenylation product 4ah in 78% yield. In the case of 2-
vinylnaphthalene, alkenylation product 4ai was obtained in 70% yield
by increasing the catalyst loading to 8 mol %. Next, the deallylative
alkenylation was examined for arylpyridine substrates possessing vari-
ous substituents. While the reaction of a substrate bearing a methoxy
group at the position ortho to the pyridyl group afforded 92% yield of
When a reaction of a prenylbenzene derivative bearing a pyridyl di-
recting group (1a) with styrene (2a) was performed in the presence of
4 mol % of [Cp*Rh(CH3CN)3][SbF6]2 (3) under EtOH refluxing
conditions for 24 h, substitution of the prenyl group with a β-styryl
group proceeded to give alkenylation product 4aa in 84% NMR yield
(Table 1, entry 1). Several rhodium catalysts were then examined for
this alkenylation. While the reaction using
a combination of
[Cp*RhCl2]2 and AgSbF6 provided 4aa in a lower yield (entry 2), only
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product 4ba, installation of an electron-withdrawing trifluoromethyl
group instead of the methoxy group reduced the product yield to 35%.
The alkenylation of substrates having meta-substituents such as NMe2,
Me, Ph, and CF3 proceeded efficiently to give the corresponding
alkenylation products 4da-ga in 75-97% yields.15 The reaction of a 2-
(2-pyridyl)benezene derivative possessing prenyl groups at both 2- and
5-positions (ortho and meta positions) only convert the ortho prenyl
group into an alkenyl group to provide 4ha in 68% yield, and no by-
product formation by cleaving off the
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Table 1. Deallylative Alkenylation of 1a with Styrene (2a)a
entry
1
catalyst
[Cp*Rh(CH3CN)3][SbF6]2 (3)
[Cp*RhCl2]2/AgSbF6
[Cp*RhCl2]2
yieldb
84%
2c
3d
4
72%
aReaction conditions: 1 (0.3 mmol), 2 (0.9 mmol), 3 (0.012 mmol),
EtOH (0.6 mL), reflux (90 °C), 48 h. Isolated yields are shown. Per-
formed with 8 mol % (0.024 mmol) of 3 in 3.0 mL of EtOH.
b
trace
Cp*Rh(OAc)2·H2O
RhH(CO)(PPh3)3
RhH(PPh3)4
trace
nde
meta prenyl group was observed. The reaction of
a 2-(3-
5
methylpyridyl)benzene derivative also gave the corresponding alkenyl-
ation product 4ia in 72% yield.
6
nde
In the reactions of substrates possessing two ortho prenyl groups,
both prenyl groups can be converted to alkenyl groups (Scheme 2).
While the reaction of a substrate having a methoxy group at the meta
position (1j) provided mono- and dialkenylation products, 4ja and 5ja,
in 10 and 66% yields, respectively, installation of a triisopropylsilyl
group at the meta position (1k) seemed to suppress the alkenylation at
the more hindered site to provide monoalkenylation product 4ka as a
major product.
7f
3
91% (87%)g
aReaction conditions: 1a (0.3 mmol), 2a (0.9 mmol), catalyst (0.012
b
1
mmol), EtOH (0.6 mL), reflux. Yields were determined by H NMR
c
using allylbenzene as an internal standard. Performed with 0.06 mmol
(2 mol %) of [Cp*RhCl2]2 and 0.24 mmol (8 mol %) of AgSbF6. dPer-
formed with 0.06 mmol (2 mol %) of [Cp*RhCl2]2. eNot detected.
f
Performed for 48 h. g The isolated yield is shown in parentheses.
Scheme 2. Deallylative Alkenylation of Arylpyridines Possessing Two
Ortho Prenyl Groups
Table 2. Deallylative Alkenylation of Arylpyridines 1 with Styrene
Derivatives 2a
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The deallylative alkenylation was also examined with substrates hav-
A possible catalytic cycle of the deallylative alkenylation is shown in
Figure 1. Coordination of substrate 1 or 6 to rhodium-hydride species
A, generated in situ from the Cp*Rh complex and EtOH,17,18 gives
complex B, and subsequent hydrometalation provides alkyl rhodium
complex C. β-Carbon elimination then proceeds to give D with the
assistance of stable five-membered chelate formation.6c-f,9 Alkene ex-
change with 2 occurs to form rhodacycle E, and carbometalation, fol-
lowed by β-hydride elimination and alkene dissociation provides the
alkenylation product 4 or 7 with the regenerated complex A. Many
examples of catalytic reactions via C–C bond cleavage by β-carbon
elimination has been reported,19 but the β-carbon elimination mostly
occurs from an alkoxide complex to generate carbonyl compounds.20
ing an allyl group other than a prenyl group (eq 1). The alkenylation of
an arylpyridine possessing a methallyl group (1l) gave the correspond-
ing product 4aa in 76% yield. The reaction of a substrate with a simple
allyl group (1m) also proceeded, but the product yield was decreased
to 57%, because the C–C bond formation with the double bond in
another molecule of 1m competed with the reaction with 2a.
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Pyrazolyl group was also found to function as an effective directing
group16 for this deallylative alkenylation (Table 3). When arylpyrazole
6a was reacted with 3 equiv of styrene 2a in 2.4 mL of EtOH solvent,
alkenylation product 7aa was obtained in 80% isolated yield. Electronic
nature of styrenes did not seem to have significant influence on the
reaction efficiency, and styrenes having a variety of substituents can be
applied for this reaction to form the corresponding alkenylation prod-
ucts 7aa-7ah in 64-86% yields. Substrates possessing a substituent oth-
er than a methyl group such as isopropyl and methoxy groups can also
be converted to alkenylation products 7ba and 7ca in high yields.
Figure 1. A Proposed Mechanism of the Deallylative Alkenylation
In order to investigate the fate of the cleaved allyl group in the
alkenylation reaction, the reaction of allylbenzene derivative 1n with 2a
was performed (Scheme 3). Although 47% of substrate 1n still remains
unreacted after 12 h, the reaction gave 43% NMR yield of the corre-
sponding alkenylation product 4ea along with 3-(4-bromophenyl)-1-
propene in 43% NMR yield. No formation of 1-(4-bromophenyl)-1-
propene was observed by GC-MS measurement. The result of this
experiment supports the proposed catalytic cycle shown in Figure 1, in
which the C–C bond was cleaved via hydrometalation/-carbon elimi-
nation rather than direct oxidative addition to give an allylrhodium
intermediate.
Table 3. Pyrazolyl-Directed Deallylative Alkenylation of Allylben-
zene Derivatives 6 with Styrene Derivatives 2
Scheme 3. Detection of the Cleaved Allyl Group
aReaction conditions: 6 (0.3 mmol), 2 (0.9 mmol), 3 (0.012 mmol),
EtOH (2.4 mL), reflux (90 °C), 48 h.
ortho-Prenylated phenols are a class of easily-accessible compounds
and can be found in many molecules of biological interest.21 Therefore,
application of the deallylative alkenylation to transformation of an
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ortho-prenylated phenol was examined (Figure 2). First, ortho-
This work was supported in part by JSPS KAKENHI Grant Numbers
17K19126, 16H04150 and 15H05839 (Middle Molecular Strategy).
T.K. is also grateful for support by JSPS KAKENHI Grant Number
16H01040 and 18H04271 (Precisely Designed Catalysts with Cus-
tomized Scaffolding). S.O. is also grateful for support by the Research
Grant of Keio Leading-edge Laboratory of Science & Technology.
prenylated phenol 8 was converted to an aryl triflate, and palladium-
catalyzed coupling with a pyrazole derivative,22 followed by deprotec-
tion, gave arylpyrazole 6d. The deallylative alkenylation of 6d with
styrene (2a) provided the corresponding stilbene derivative 7da. Final-
ly, the reaction of 7da with TMSCl in the presence of
TMPMgCl·LiCl,23 followed by treatment with HCl in MeOH provided
ortho-alkenylated aniline 9.
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In conclusion, a novel method for direct conversion of allyl groups in
allylbenzene derivatives to alkenyl groups via C–C bond cleavage is
described. The reaction proceeded without additives only in the pres-
ence of rhodium catalyst in EtOH solvent. We also developed a new
protocol for transformation of an ortho-prenylated phenol to an ortho-
alkenylated aniline derivative. We believe the catalytic cycle involving
the chelation-assisted C–C bond cleavage via a hydrometalation/β-
carbon elimination pathway can be applicable to other reactions, and
development of new reactions based on this strategy is currently in
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website.
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Full experimental details and characterization data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*kakiuchi@chem.keio.ac.jp
ORCID
Takuya Kochi: 0000-0002-5491-0566
Fumitoshi Kakiuchi: 0000-0003-2605-4675
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
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J. Am. Chem. Soc. 2007, 129, 14158-14159. (e) Bour, J. R.; Green, J. C.; Winton,
V. J.; Johnson, J. B. J. Org. Chem. 2013, 78, 1665-1669. (f) Mahendar,
L.; Satyanarayana, G. J. Org. Chem. 2014, 79, 2059-2074. (g) Iwasaki, M.;
Araki, Y.; Nishihara, Y. J. Org. Chem. 2017, 82, 6242-6258. (h) Smits, G.; Audic,
B.; Wodrich, M. D.; Corminboeuf, C.; Cramer, N. Chem. Sci. 2017, 8, 7174-
7179.
(20) Only a few reports on catalytic unstrained C–C bond cleavage by β-
carbon elimination not through an alkoxide complex have been reported: (a)
Youn, S. W.; Kim, B. S.; Jagdale, A. R. J. Am. Chem. Soc. 2012, 134, 11308-
11311. (b) Ye, J.; Limouni, A.; Zaichuk, S.; Lautens, M. Angew. Chem., Int. Ed.
2015, 54, 3116-3120.
(21) (a) Chen, X.; Mukwaya, E.; Wong, M.-S.; Zhang, Y. Pharm. Biol. 2014,
52, 655-660. (b) Šmejkal, K. Phytochem. Rev. 2014, 13, 245-275.
(22) A copper-catalyzed C–N bond formation using aryl triflates derived
from a calix[4]arene derivative with a simple pyrazole has been reported, but
was not applicable to our substrates: Rawat, V.; Press, K.; Goldberg, I.; Vigalok,
A. Org. Biomol. Chem. 2015, 13, 11189-11193. We found that using 3(5)-
trimethylsilylpyrazole as a coupling partner in the palladium-catalyzed reaction
provided efficient access to the amination product. To the best of our
knowledge, no palladium-catalyzed coupling of aryl triflates with pyrazoles to
form arylpyrazoles was reported previsouly.
(23) In this reaction, we believe that deprotonation and silylation at the C-5
position of the pyrazole ring occurred first, and further deprotonation at the C-3
position led to the formation of a magnesiated species, which underwent ring-
opening formation of an enamine derivative. A base-mediated ring-opening
reaction of pyrazole derivatives has been reported. Schlosser, M.; Volle, J.-N.;
Leroux, F.; Schenk, K. Eur. J. Org. Chem. 2002, 2913-2920.
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