Cp?CoIII-Catalyzed Dehydrative C-H Allylation of 6-Arylpurines and Aromatic Amides Using Allyl Alcohols in Fluorinated Alcohols

Article abstract of DOI:10.1021/acs.orglett.6b00846

Cp?Co^III-catalyzed C-H allylation of various aromatic C-H bonds using allyl alcohols as allylating reagents is described. Improved reaction conditions using fluorinated alcohol solvents afforded efficient directed C-H allylation of 6-arylpurine

Full text of DOI:10.1021/acs.orglett.6b00846

Letter
pubs.acs.org/OrgLett
Cp*CoIII-Catalyzed Dehydrative C−H Allylation of 6‑Arylpurines and
Aromatic Amides Using Allyl Alcohols in Fluorinated Alcohols
Youka Bunno,† Nanami Murakami,† Yudai Suzuki,‡ Motomu Kanai,‡ Tatsuhiko Yoshino,*,†,§
and Shigeki Matsunaga*,†,§
†Faculty of Pharmaceutical Sciences, Hokkaido University, Sapporo 060-0812, Japan
‡Graduate School of Pharmaceutical Sciences, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan
§ACT-C, Japan Science and Technology Agency, Sapporo 060-0812, Japan
S
* Supporting Information
ABSTRACT: Cp*CoIII-catalyzed C−H allylation of various
aromatic C−H bonds using allyl alcohols as allylating reagents is
described. Improved reaction conditions using fluorinated alcohol
solvents afforded efficient directed C−H allylation of 6-arylpurines,
benzamides, and a synthetically useful Weinreb amide with good
functional group compatibility.
llylation of aromatic compounds is very important in
organic synthesis because the allyl moiety can be further
Although it was reported that N-methylbenzamide can be
allylated using an allyl alcohol, the yield was low, and its gener-
ality was not demonstrated.19 During the preparation of our
manuscript, Kapur reported RuII-catalyzed C−H allylation using
allyl alcohols, but the scope was still limited to indoles, an
indoline, and 2-phenylpyridine.22 Here we report an improved
protocol for Cp*CoIII-catalyzed dehydrative C−H allylation
of aromatic compounds using allyl alcohols. The use of fluo-
rinated alcohol solvent significantly enhanced the reactivity,
enabling allylation of 6-arylpurines, benzamides, and an aromatic
Weinreb amide in moderate to excellent yield.
We first selected 6-phenylpurine 1a as a model substrate
(Table 1). Transition-metal-catalyzed C−H bond functionaliza-
tion of 6-arylpurines has recently attracted much attention23 due
to their biological activities.24 Previously reported conditions for
allylation of indoles20 using [Cp*Co(CO)I2] (5 mol %), AgOTf
(10 mol %), and AgOAc (10 mol %) in DCE at 60 °C selectively
afforded monoallylated product 3a, but in only 13% yield (entry 1).
Screening of the solvent led to a significantly improved yield of 3a
(entries 2−6). Although most of the solvents were ineffective,
2,2,2-trifluoroethanol (TFE) and 1,1,1,3,3,3-hexafluoro-2-propanol
(HFIP) exhibited high reactivity,25 and 3a was obtained in good
yields (entries 5, 6). Changing AgOTf to other Ag salts did
not affect the reactivity (entries 7−10), whereas the yield was
decreased when no cationic Ag salts were used (entry 11). Other
acetate bases were less effective than AgOAc (entries 12−14).
Almost no product was detected in the absence of acetate bases
(entry 15), indicating that a carboxylate-assisted concerted
A
converted into a wide variety of functional groups. In terms of
atom economy,1 step economy,2 and functional group com-
patibility, direct allylation of inert C−H bonds under transition
metal catalysis3 is more attractive than classical allylation
methods, such as cross-coupling reactions of functionalized
arenes.4,5 Therefore, aromatic and olefinic C−H allylation
reactions using allyl acetates,6 carbonates,7 phosphates,8 halides,9
and ethers10 as allylating reagents were developed over the past
decade using various transition metals. Among them, Cp*RhIII-
catalyzed11 C−H allylation reactions using preactivated allyl
alcohols6c,7b−d,8d,9d,10b are especially noteworthy due to their
mild reaction conditions and broad substrate scope.12 Those
reactions are thought to proceed via C−H metalation, olefin
insertion, and subsequent β-elimination of oxygen-containing
functional groups.
We13 and other groups14,15 have investigated Cp*CoIII
catalysts for C−H bond functionalization as an inexpensive
alternative16 to Cp*RhIII catalysts after our first report in 2013.13a
Glorius17 and Ackermann18 independently reported Cp*CoIII-
catalyzed C−H allylation of indoles, pyrroles, and 2-phenyl-
pyridines using preactivated allyl alcohol derivatives under mild
conditions. The scope of this reaction was further expanded
to benzamide derivatives.19 On the other hand, we recently
reported that unactivated allyl alcohols can be used directly
for aromatic C−H allylation under Cp*CoIII catalysis, while
Cp*RhIII exhibited inferior catalytic activity.20 Key to this
reaction is likely the more favored β-hydroxy elimination over the
potentially competing β-hydride elimination after insertion of a
C−C double bond.21 The substrate scope of our previous
protocol, however, was limited to indoles and 1-phenylpyrazole.
Received: March 23, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b00846
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
a
Table 1. Optimization of Reaction Conditions
Scheme 1. Scope of Dehydrative Allylation of 6-Arylpurines
b
entry
solvent
Ag salt
base
yield (%)
1
DCE
THF
toluene
MeOH
TFE
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgSbF6
AgNTf2
AgBF4
AgPF6
none
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
AgOAc
NaOAc
KOAc
13
trace
6
2
3
4
3
c
5
81 (78 )
6
HFIP
TFE
72
78
79
80
73
42
8
7
8
TFE
9
TFE
10
11
12
13
14
15
TFE
TFE
TFE
AgOTf
AgOTf
AgOTf
AgOTf
TFE
29
12
trace
TFE
CsOAc
none
TFE
a
The reactions were run using 1a (0.10 mmol) and 2a (0.15 mmol) in
b
indicated solvent (0.1 M) at 60 °C. Determined by 1H NMR analysis
a
The reactions were run using 1 (0.30 mmol), 2, [Cp*Co(CO)I2]
(5 mol %), AgOTf (10 mol %), AgOAc (10 mol %) in TFE (0.1 M) at
60 °C for 8 h unless otherwise noted. The indicated yields are isolated
of the crude mixture using dibenzyl ether as an internal standard.
c
Isolated yield after silica gel column chromatography at 0.30 mmol
b
scale.
yields after silica gel column chromatography. 1.0 g (3.5 mmol) of 1b
c
was used, and 1.08 g of 3b was isolated. The reaction time was 24 h.
d
The reaction was run at 70 °C.
metalation−deprotonation mechanism works in this reaction.26
Under the best reaction conditions in entry 5, the allylated
product 3a was isolated in 78% yield after chromatographic
purification.
a
Scheme 2. Dehydrative Allylation of Benzamides
After optimizing the conditions, the scope of 6-arylpurines was
investigated (Scheme 1). In addition to N9-alkylated purine
derivatives 1a, 1b, and N9-tosylated purine 1c, fully acetylated
6-phenylpurine riboside 1d was allylated without any difficulty to
give 3d in 88% yield. Both electron-donating and -withdrawing
groups were compatible on the phenyl moiety, and products
3e−g were obtained in 70%−93% yield. Heteroarylpurine 1h
also successfully afforded allylated product 3h in 59% yield. The
reaction using tertiary allyl alcohol 2b proceeded γ-selectively to
afford prenylated product 3i. The scalability of the new protocol
was also confirmed by a gram-scale experiment in which 3b was
obtained in 95% yield.
Next we turned our attention to the allylation of other
aromatic compounds. After minor modification of the reaction
conditions,27 benzamide derivatives 4 were also allylated using
20 mol % of AgNTf2 and HFIP solvent, as shown in Scheme 2.
Unsubstituted benzamide 4a and p-Me-benzamide 4b afforded
the corresponding allylated products in moderate yields along
with the bis-allylated byproducts.28 Allylation of o-F-benzamide
4c proceeded in 71% yield. Various m-substituted benzamide
derivatives, including I-substituted substrate 4h, afforded the
monoallylated products in acceptable yields with >10/1 regio-
selectivity (5d−5h). N-Methylbenzamide 4i and N,N-dimethyl-
benzamide 4j also afforded the allylated products 5i and 5j albeit
in diminished yields. To confirm the importance of the fluo-
rinated alcohol, a negative control experiment using previously
a
The reactions were run using 4 (0.30 mmol), 2 (0.90 mmol),
[Cp*Co(CO)I2] (5 mol %), AgNTf2 (20 mol %), AgOAc (10 mol %)
in HFIP (1.0 M) at 80 °C for 24 h unless otherwise noted.
The indicated yields are isolated yields after silica gel column
b
chromatography. Using DCE instead of HFIP as solvent.
used DCE as a solvent was performed for 4d. As expected, a
lower yield was obtained compared with that in HFIP (34% in
DCE vs 67% in HFIP).
B
DOI: 10.1021/acs.orglett.6b00846
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
Synthetically useful Weinreb amide29 6 was also allylated
in moderate yield (eq 1). It is noteworthy that the Weinreb
In summary, efficient and general reaction conditions for
Cp*CoIII-catalyzed aromatic C−H allylation of 6-arylpurines,
and benzamide derivatives using allyl alcohols as allylating
reagents were developed. Use of fluorinated alcohol solvents
was crucial for improving the scope of the dehydrative C−H
allylation. We also describe our preliminary result of allylation of
an aromatic Weinreb amide to further demonstrate the possi-
bility of Cp*CoIII-catalyzed C−H functionalization reactions for
the production of synthetically useful compounds.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00846.
■
amide-directed C−H bond functionalization reaction is rather
limited possibly due to its weak coordinating ability,30 and this is
the first example of Weinreb amide-directed C−H bond
functionalization under high-valent cobalt catalysis.
S
Figure 1 shows a plausible catalytic cycle for the allyla-
Experimental procedures, characterization data, and copy
of NMR spectrum (PDF)
tion of 6-arylpurine 1 based on previous Cp*RhIII-7b and
AUTHOR INFORMATION
Corresponding Authors
■
*E-mail: tyoshino@pharm.hokudai.ac.jp.
*E-mail: smatsuna@pharm.hokudai.ac.jp.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported in part by ACT-C from JST, Scientific
Research on Innovative Area from MEXT, Grant-in-Aid for
Research Activity Start-up from JSPS, and Asahi Glass
Foundation. Y.S. thanks JSPS for the fellowship.
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