Sterically demanding unsymmetrical diaryl-λ3-iodanes for electrophilic pentafluorophenylation and an approach to α-pentafluorophenyl carbonyl compounds with an all-carbon stereocenter

Article abstract of DOI:10.1002/open.201402045

A sterically demanding unsymmetrical pentafluorophenyl-triisopropylphenyl-λ³-iodane was developed as an effective reagent for the electrophilic pentafluorophenylation of various β-keto esters and a β-keto amide. 17 examples of α-pentafluorophen

Full text of DOI:10.1002/open.201402045

DOI: 10.1002/open.201402045
3
Sterically Demanding Unsymmetrical Diaryl-l -iodanes for
Electrophilic Pentafluorophenylation and an Approach to
a-Pentafluorophenyl Carbonyl Compounds with an All-
Carbon Stereocenter
[
a]
[a]
[a]
[b]
[a]
Kohei Matsuzaki, Kenta Okuyama, Etsuko Tokunaga, Motoo Shiro, and Norio Shibata*
[5a,b]
A sterically demanding unsymmetrical pentafluorophenyl-tri-
a small protein enhance the stability of its tertiary structure.
Nucleoside analogues containing a C F instead of natural nu-
3
isopropylphenyl-l -iodane was developed as an effective re-
6
5
agent for the electrophilic pentafluorophenylation of various
b-keto esters and a b-keto amide. 17 examples of a-pentafluor-
ophenylated 1,3-dicarbonyl compounds 3 having a quaternary
carbon center are provided. The resulting compounds were
nicely transformed into chiral a-pentafluorophenyl ketones
with an all-carbon stereogenic center in high yields and high
enantioselectivities using asymmetric organocatalysis (up to
cleobases have been used as tools for studying base pairing
[5c,d]
and DNA replication factors.
sal base in peptide nucleic acids (PNAs). Although introduc-
tion of the C F group into a C center has been actively in-
The C F group is also a univer-
6 5
[5e]
sp2
6
5
vestigated by using radical and transition metal-catalyzed
[6]
cross-coupling reactions, these are not applicable for the con-
struction of a stereogenic Csp center. Instead, silylated or
metal-C F reagents are a suitable choice for this purpose, and
3
98% ee) or asymmetric metal catalysis (up to 82% ee).
6
5
their utility is limited to the nucleophilic pentafluorophenyla-
[7]
tion of carbonyls, imines and related compounds. Pentafluor-
ophenylboronate serves as an aryl donor for a rhodium-cata-
lyzed cross-coupling reaction with a-aryldiazoacetate, produc-
Trifluoromethylation and fluorination are now key technologies
in innovations in pharmaceutical, agrochemical and advanced
[
1]
[8]
material industries. On the other hand, direct pentafluoro-
phenylation is still undeveloped, despite the potential use of
the pentafluorophenyl (C F ) group in liquid crystal materials,
ing an a-pentafluorophenyl carboxylic acid ester. On the
other hand, methods for the electrophilic pentafluorophenyla-
tion of a stereocenter are rare, except for a nucleophilic attack
6
5
[
2]
[9]
organic semiconductors and bioactive compounds. Looking
on hexafluorobenzene. In this method, however, the scope of
[
3]
at the success of chiral drug industries as well as fluorinated
the substrate is limited due to the need for drastic conditions
[
1]
[9]
pharmaceuticals, we are interested in molecules containing
the C F group attached to a C center as drug candidates in
(high temperature and a strong base).
sp3
As part of an on-going synthetic program focused on orga-
6
5
[10]
future markets.
nofluorine reagents, we herein disclose our studies describ-
3
Due to the strong electron-accepting property of the C6F5
group with a planar p system, characteristic stacking effects by
polar p interactions have been observed between the C F
ing the synthesis and use of aryl-pentafluorophenyl-l -iodanes
1 (aryl pentafluorophenyl iodonium salts) as effective electro-
philic pentafluorophenylating reagents. A variety of b-keto
esters 2 is nicely a-pentafluorophenylated by 1 providing
a-pentafluorophenyl-1,3-dicarbonyl compounds 3 with a qua-
ternary carbon center in good yields under mild conditions. In
particular, a structurally new and sterically demanding unsym-
6
5
[
4]
group and non-fluorinated counterparts. Therefore, bioactive
compounds with a C F moiety are expected to interact with
6
5
ubiquitous aromatic moieties of biomolecules such as amino
[
5]
acids and nucleic acids. Indeed, Gellman and co-workers dis-
closed that the pentafluorophenyl variants of phenylalanine in
3
metrical diaryl-l -iodane 1d was found to be most effective.
The resulting a-pentafluorophenyl b-keto esters 3 were trans-
formed into chiral a-pentafluorophenyl ketones with an all-
carbon stereogenic center by organocatalysis or metal cataly-
sis, that is, by decarboxylation followed by enantioselective al-
kylation under cinchona alkaloids catalysis, or asymmetric de-
carboxylative allylation under the Tsuji–Trost condition using
palladium(II) catalysis (Scheme 1).
[
a] K. Matsuzaki, K. Okuyama, E. Tokunaga, Prof. Dr. N. Shibata
Department of Nanopharmaceutical Science &
Department of Frontier Materials, Nagoya Institute of Technology
Gokiso, Showa-ku, Nagoya 466-8555 (Japan)
Fax: (+81)527357543
E-mail: nozshiba@nitech.ac.jp
[b] Dr. M. Shiro
3
It is a well-known fact that diaryl-l -iodanes react with eno-
Rigaku Corporation
[11]
3
-9-12 Mastubara-cho, Akishima, Tokyo 196-8666 (Japan)
lates providing a-arylated carbonyl compounds. While one
3
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/open.201402045.
of the two aryl groups of diaryl-l -iodanes is transferred into
the substrates, a more electron-deficient aryl group plays a fun-
damental role in the arylation of enolates with unsymmetrical
ꢀ
2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited and is not used for commercial purposes.
3
[12]
diaryl-l -iodanes. Despite of a large number of papers on
the arylation of enolates using this concept, pentafluoropheny-
3
lation using diaryl-l -iodanes has surprisingly never been re-
ꢀ
2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemistryOpen 2014, 3, 233 – 237 233

www.chemistryopen.org
[17]
amined.
The yield improved
slightly to 17% in the presence
of K CO and tetrabutylammoni-
2
3
um iodide in N,N-dimethylforma-
mide (DMF; Entry 7). In an at-
tempt to further improve the
yield, the effect of the base and
solvent on the reaction was ex-
amined. The combination of or-
ganic or inorganic bases with
solvents had an impact on yield
Scheme 1. Electrophilic pentafluorophenylation by 1 and transformations to chiral a-pentafluorophenyl ketones
having an all-carbon stereocenter.
(
Entries 8–17), which was finally
improved to 72% when the re-
[
11,12]
ported.
This fact encouraged us to design a series of un-
3
[a]
symmetrical C F -containing diaryl-l -iodanes as electrophilic
Table 1. Initial studies on pentafluorophenylation of b-keto ester 2a.
6
5
pentafluorophenylation reagents for enolates. The reasons why
3
unsymmetrical diaryl-l -iodanes are targeted are indicated
next. Due to the electron-deficient nature of the C F group,
6
5
another set of aryl ligands were easily designed to stay at-
tached to the parent iodanes. Furthermore, electron-poor sym-
3
metrical salts of bispentafluorophenyl-l -iodanes are difficult
[
b]
[
13]
3
Entry
1
Base
Additive
Solvent
Time [h]
Yield [%]
to prepare.
signed in this study were prepared in three steps from iodo-
pentafluorobenzene via Koser-type reagent, hydroxy-
tosyloxy)iodopentafluorobenzene 6 according to a modified
version of the published procedure, including oxidation and
The five unsymmetrical diaryl-l -iodanes de-
1
2
3
1a
1a
1a
1a
1b
1c
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1d
1a
1b
1c
1e
DBU
DABCO
tBuOK
–
–
–
CH
2
CH
2
CH
2
Cl
Cl
Cl
2
2
2
2
18
23
8
4
42
2
2
4
20
4
11
6
2
6
4
8
2
2
2
2
6
6
7
9
6
a
[
14]
(
[c]
4
[
K
2
CO
3
Bu
4
NI
DMF
DMF
DMF
DMF
DMF
c]
5
6
7
8
9
K2CO3
4
Bu NI
[
15]
[c]
c]
the Friedel–Crafts reaction (Scheme 2).
K
K
2
CO
CO
3
3
Bu
Bu
–
4
NI
NI
trace
17
48
28
8
58
21
58
53
44
50
49
72
20
10
5
[
2
4
We first examined the pentafluorophenylation of methyl in-
NaH
NaH
NaH
NaH
3
danone carboxylate 2a using 1 (Table 1). The use of diaryl-l -
–
–
–
–
–
–
–
–
–
–
–
–
–
–
toluene
THF
iodane 1a with a mesityl (2,4,6-Me C H ) group as a dummy
10
3
6
2
1
1
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
CH
2
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
2
2
2
2
2
2
2
2
2
2
2
2
12
13
14
15
16
17
18
19
20
21
22
iPr
2
NEt
K
2
CO
3
Cs
2
CO
3
NaOH
tBuOK
KF
K
3
K
3
K
3
K
3
K
3
PO
PO
PO
PO
PO
4
4
4
4
4
2
11
Scheme 2. Reagents and conditions: a) Oxone, TFA/CHCl
b) TsOH·H O, CH CN, RT, 3 h (87% over 2 steps); c) ArH, CF
overnight.
3
, RT, 2 h;
[a] Reagents and conditions: a) 1a–e (1.1 equiv), base (1.2 equiv), additive
(10 mol%), solvent, RT, time. All reactions were carried out on a 19.0 mg
2
3
3
CH OH or TFA, RT,
2
2 3
(0.100 mmol) scale. [b] Isolated yield. [c] K CO (3.0 equiv) was used.
[
16]
ligand was expected to be highly suitable in chemoselective
pentafluorophenylation, but the outcome was disappointing
with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU)/CH Cl , yielding
action of 2a with 1d was carried out in the presence of K PO
3
4
in CH Cl (Entry 18). Re-optimization of reagents 1 under the
2
2
2
2
6
% of 3a and a detectable amount of an a-mesitylation side-
best reaction conditions of K PO in CH Cl could not improve
3
4
2
2
product (Entry 1, Table 1). Attempts to improve the disappoint-
ing yield by changing the base, solvent, and additive were all
unsuccessful (Entries 2–4). We next fine-tuned both the elec-
tronic and steric effects of the dummy ligand on 1. However,
the yield (Entries 19–22, Table 1).
With optimized reaction conditions in hand, the substrate
scope was explored (Table 2). Reagent 1d was identified as the
most practical reagent for the electrophilic pentafluoropheny-
lation of various substrates 2 in the presence of K PO in
3
diaryl-l -iodanes 1b–c with electron-donating methoxy
3
4
group(s) on the benzene ring were also poor reagents for this
transformation (Entries 5 and 6). Hence, sterically highly de-
CH Cl at room temperature. When the reaction was carried
2 2
out with b-keto esters 2a–m derived from 1-indanones, it pro-
ceeded efficiently affording desired products 3a–m in good
3
manding triisopropyl-substituted diaryl-l -iodane 1d was ex-
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2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemistryOpen 2014, 3, 233 – 237 234

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phase-transfer catalyst of choice in aqueous toluene
in the presence of KOH at À408C. The scope of the
asymmetric alkylation of 4 is shown in Table 3. In all
cases, high yields with excellent enantioselectivities
up to 98% ee were accomplished almost independ-
ent of both the electronic and steric natures of elec-
trophiles RÀBr, and substrates 4 (indanone 4a and
tetralone 4b). The absolute configuration of 5e was
determined as R by X-ray crystallography (Figure 1),
and the stereochemistry of 5a–d, 5 f–i was tentative-
ly assigned by analogy.
[a]
Table 2. Substrate scope on pentafluorophenylation of b-keto esters 2.
In another approach to chiral pentafluorophenyl
compounds, direct asymmetric transformation of
allyl-a-pentafluorophenyl-b-keto esters 3d and 3o to
a-allyl-a-pentafluorophenyl-ketones 5a and 5i was
achieved by palladium(II)-catalyzed asymmetric de-
[20]
carboxylative allylation in the presence of the Trost
ligand with high enantioselectivities (82% and 80%
ee, respectively; Scheme 4). The absolute stereochem-
istry of 5 obtained by this procedure is opposite to
that obtained by direct alkylation mentioned in
Table 3.
In conclusion, this study discloses aryl-pentafluoro-
3
phenyl-l -iodanes 1 as effective reagents for the elec-
trophilic pentafluorophenylation of b-keto esters and
amide 2. Among several reagents designed, an un-
symmetrical diaryl iodonium salt 1d with a highly
sterically demanding 2,4,6-triisopropyl phenyl group
as a dummy ligand was most effective for the elec-
trophilic pentafluorophenylation reaction. In the pres-
ence of K PO in CH Cl at room temperature, the
[
(
(
a] The reaction of 2 with 1d (1.1 equiv) was carried out in the presence of K
1.2 equiv) in CH Cl at RT, unless noted otherwise. [b] 1d (1.6 equiv) and K
1.7 equiv) were used. [c] Reaction time was 22 h.
3
PO
PO
4
4
2
2
3
3
4
2
2
yields. Substrates with sterically demanding tert-butyl ester 2c
and transformable allyl ester 2d were reacted to give desired
products 3c and 3d. The electronic or steric nature of the ben-
zene ring and ester moiety of the substrates might not have
affected the yields of 3 that much. The yields derived from tet-
ralone 2n and 2o or cyclopentanone 2p were slightly lower
due to the less enolizable nature of the substrates. Notably,
b-keto amido 2q was also reactive giving 3q in 41% yield (see
Table 2).
pentafluorophenylation of nucleophiles proceeded smoothly
to yield the desired products. The resulting products were
The allyl a-pentafluorophenyl-b-keto esters 3d and 3o that
were obtained were easily transformed into a-pentafluoro-
phenyl ketones 4d and 4o by palladium(II)-catalyzed decar-
Scheme 3. Palladium-catalyzed decarboxylation of b-keto esters 3d and 3o
to a-pentafluorophenyl ketones 4a and 4b. Reagents and conditions:
a) Pd (dba) (2.5 mol%), dppe (6.25 mol%), Meldrum’s acid (2.5 equiv), THF,
2 3
RT or 508C, 1–2 h.
boxylation
using
tris(dibenzylideneacetone)dipalladium(0)
(
Pd (dba) ), 1,2-bis(diphenylphosphino)ethane (dppe) and Mel-
2
3
[
18]
drum’s acid as a proton source in tetrahydrofuran (THF) with
96% and 93% yield, respectively (Scheme 3).
With a-pentafluorophenyl ketones 4 in hand, we were ready
to investigate the enantioselective functionalization of 4 to
create a quaternary chiral carbon center bearing a C F group
6
5
by asymmetric organocatalysis. Asymmetric alkylation of a-
[19]
phenyl ketones has been actively researched in recent years.
However, there are no examples of the use of a-pentafluoro-
phenyl ketones as substrates. Fortunately, a brief survey of re-
action conditions consisting of chiral catalysts, bases and sol-
vents for allylation (see table S1 in the Supporting Information)
led to the rapid selection of cinchoninium bromide as the
Figure 1. X-ray crystallographic structure of (R)-5e (CCDC 1009021).
ChemistryOpen 2014, 3, 233 – 237 235
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2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

www.chemistryopen.org
[
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[
b]
[c]
Entry
4, n
R
Time [h]
5
Yield [%]
ee [%]
[
d]
1
2
3
4
5
6
7
8
4a, n=1
4a, n=1
4a, n=1
4a, n=1
4a, n=1
4a, n=1
4a, n=1
4b, n=2
allyl
benzyl
96
24
48
72
48
96
24
48
5a
5b
5c
5d
5e
5 f
95
95
91
84
87
83
89
93
87 (À)
96 (+)
96 (À)
95 (À)
98 (À)
92 (+)
94 (À)
96 (À)
CH
2
CH
2
CH
2
CH
2
CH
2
-p-Tol
-p-MeOPh
-p-BrPh
-1-Naphthyl
-2-Naphthyl
[
e]
[
[
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[
21]
the potential of these reagents are now in progress.
Acknowledgements
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[
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This study was financially supported in part by the Platform for
Drug Discovery, Informatics, and Structural Life Science, Scientific
Research (B) from the Ministry of Education, Culture, Sports, Sci-
ence and Technology (MEXT), Japan (25288045) and the Ad-
vanced Catalytic Transformation (ACT-C) from the Japan Science
and Technology (JST) Agency. We thank Mr. Tatsuya Ochiai, Pro-
fessor Tomohiro Ozawa and Professor Hideki Masuda (Nagoya In-
stitute of Technology) for their help in X-ray crystallographic
analysis.
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Received: August 25, 2014
Published online on September 17, 2014
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2014 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemistryOpen 2014, 3, 233 – 237 237