Photoredox-Catalyzed Benzylic Esterification via Radical-Polar Crossover

Article abstract of DOI:10.1021/acs.orglett.1c01645

Photoredox-catalyzed C-O bond formation reactions are reported. The decarboxylative esterification reaction allows the conversion of a variety of arylacetic acids into the corresponding benzyl carboxylates. Furthermore, the use of (diacetoxyiodo)benzene allows the conversion of the benzylic C-H bond through hydrogen atom transfer. The reactions were applied to the divergent transformation of pharmaceuticals via decarboxylative or C-H esterification reactions.

Full text of DOI:10.1021/acs.orglett.1c01645

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Letter
Photoredox-Catalyzed Benzylic Esterification via Radical-Polar
Crossover
⊥
⊥
Bumpei Maeda, Yota Sakakibara, Kei Murakami,* and Kenichiro Itami*
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sı Supporting Information
ABSTRACT: Photoredox-catalyzed C−O bond formation reac-
tions are reported. The decarboxylative esterification reaction
allows the conversion of a variety of arylacetic acids into the
corresponding benzyl carboxylates. Furthermore, the use of
(
diacetoxyiodo)benzene allows the conversion of the benzylic
C−H bond through hydrogen atom transfer. The reactions were
applied to the divergent transformation of pharmaceuticals via
decarboxylative or C−H esterification reactions.
enzyl carboxylates have been regarded as an important
fundamental organic chemistry (Figure 1B). From a synthetic
point of view, it is important to develop a new efficient
transformation that allows the conversion of different types of
feedstock molecules. Recently, photoredox-catalyzed reactions
have been emerging because a photoexcited catalyst can
generate various reactive intermediates under mild reaction
B
scaffold that is ubiquitously found in various bioactive
1
molecules (Figure 1A). For the synthesis of benzyl
carboxylates, the S 2-type reaction of benzylic halides and
N
the esterification of benzyl alcohols have been well-known in
2
conditions. In general, a radical intermediate is generated
through the reaction of a substrate with a photoredox catalyst.
When alkylcarboxylic acid is reacted with a photoexcited
catalyst, the corresponding alkyl radical is generated through
3
decarboxylation. The resulting radical is generally nucleo-
philic, which reacts with electrophilic reagents. Radical-polar
crossover (RPC) has been known as an efficient umpolung
method that converts an alkyl radical into an alkyl cation. The
4
−7
generated cation smoothly reacts with anionic nucleophiles.
A pioneering example was reported by Murphy et al. where
4
tetrathiafulvalene was utilized as a catalyst for a RPC reaction.
Recently, Nagao and Ohmiya reported an efficient decarbox-
ylative transformation that allows the formation of a C−O
5
bond with hindered alkyl cations. Doyle et al. revealed a
decarboxylative fluorination over RPC strategy that enabled
6
the synthesis of various tertiary alkyl fluorides. During the
course of our study on a decarboxylative C−heteroatom bond
8
formation reaction, we found that benzyl carboxylates can be
synthesized via RPC through the reaction of benzyl carboxylic
acids and hypervalent iodine reagents (Figure 1C). Moreover,
the use of (diacetoxyiodo)benzene (PIDA) allows the
Received: May 15, 2021
Published: June 16, 2021
Figure 1. (A) Examples of pharmaceuticals containing benzyl ester
moieties. (B) Synthetic methods of benzyl esters. (C) This work.
©
2021 American Chemical Society
https://doi.org/10.1021/acs.orglett.1c01645
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Letter
abstraction of a benzylic hydrogen atom that directly
functionalizes the benzylic C−H bond.
3
The treatment of arylacetic acid 1a with 1-(benzyloxy)-1λ -
benzo[d][1,2]iodaoxol-3(1H)-one (IBB) in the presence of
Ru(bpy) ·2PF as the photoredox catalyst (PC) under blue
3
6
light irradiation afforded the resulting product 3a in a 78%
isolated yield (Table 1, entry 1). The yield significantly
a
Table 1. Screening of the Reaction Conditions
entry
photoredox catalyst (PC)
solvent
NMR yield (%)
b
1
2
3
4
5
6
7
8
9
Ru(bpy) ·2PF
CH Cl
78
3
6
2
2
2
2
2
2
2
CH Cl
21
31
15
23
28
60
36
34
13
10
2
PC1
PC2
PC3
PC4
CH Cl
2
CH Cl
2
CH Cl
2
CH Cl
2
Ru(bpy) ·2PF
ClCH CH Cl
3
6
2
2
Ru(bpy) ·2PF
CHCl3
3
6
6
6
6
Ru(bpy) ·2PF
CH CN
3
3
10
1
Ru(bpy) ·2PF
DMF
3
c
1
Ru(bpy) ·2PF
CH Cl
2
3
2
a
Reaction conditions are as follows: 1a (0.4 mmol), 2a (1 mmol), and
a
b
c
Figure 2. Scope of the reaction. The structure of hypervalent iodine
PC (0.01 mmol) in solvent (2 mL) for 24 h. Isolated yield. Without
blue light irradiation.
b
2
is described in the Supporting Information. Conducted on a 0.2
mmol scale.
decreased when the reaction was performed in the absence of
the catalyst (Table 1, entry 2). Notably, the addition of other
catalysts such as PC1−4 did not increase the yield compared
with that of the noncatalytic conditions (Table 1, entry 2 vs
entries 3−6). Then, the solvent effect was investigated.
Employing less polar 1,2-dichloroethane provided the product
in a 60% yield, whereas much more polar solvents such as
chloroform, acetonitrile, and DMF suppressed the reaction
tron-donating group-substituted benzoates reacted to give 3e−
3g in moderate yields. We then investigated the scope of the
arylacetic acid derivatives. Electron-donating methoxy- and
methyl-substituted phenylacetic acid were efficiently converted
to the corresponding products 4a and 4b in 63% and 72%
yields, respectively. Thienylacetic acid or naphthylacetic acid
was transformed into product 4c or 4d, respectively, in a
moderate yield. Diphenylacetic acid and adamantanecarboxylic
acid, which provides a secondary or tertiary radical, gave 4e
and 4f in a 43% and 35% yields, respectively. To show the
utility of the reaction, we converted a variety of pharmaceut-
(
Table 1, entries 7−10, respectively). It is notable that the
reaction slightly proceeded without the irradiation of blue light
Table 1, entry 11). Overall, the combination of the Ru(bpy)3·
PF catalyst and CH Cl as the solvent resulted in the best
(
9
2
icals that had an arylacetic acid moiety. Fortunately,
6
2
2
yield.
flurbiprofen, ketoprofen, and isoxepac were smoothly con-
verted to the corresponding products in moderate to good
yields (4g−4i, respectively).
With the optimized conditions in hand, we then investigated
the scope of the reaction (Figure 2). The reaction was
applicable to other hypervalent iodine compounds. The
reaction with chlorinated hypervalent iodine compound 2b
provided 3b in a 47% yield. Furthermore, not only the cyclic
reagent but also other hypervalent iodine reagents were utilized
in the reaction. The reaction of 1a with benzoate- or
naphthoate-substituted hypervalent iodine (2c or 2d, respec-
tively) gave the product in a 50% and 43% yield, respectively.
Gratifyingly, electron-withdrawing group-substituted or elec-
During the course of our study, we disclosed that the
10−12
benzylic C−H bond could be converted to a C−O bond
under similar reaction conditions via hydrogen atom transfer.
The treatment of 4-phenyltoluene (5a) with PIDA in CH Cl
1
3
2
2
in the presence of PC4 (for the structure, see Table 1) as the
catalyst under blue light irradiation afforded 6a in a 51% yield.
The reaction proceeded with a variety of substrates (Figure 3).
3
4-Propylbiphenyl (5b), which has secondary benzylic C(sp )−
5
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Letter
a
Figure 3. Scope of the C−H esterification. PIDA (4.0 equiv) was
used.
H bonds, was converted into the corresponding product 6b in
an 81% yield. Functional groups such as chloride (6c), acetoxy
(6d), trimethylsilyl (6e), and methoxy (6f) were tolerated
under the reaction conditions. The reaction of 1,2-diphenyl-
ethane gave 6g in a 54% yield when 4 equiv of PIDA was used.
Although an excess amount of PIDA was used, no
difunctionalization product was observed. Diarylmethane 5h
was smoothly converted to 6h in a 74% yield. Furthermore, the
reaction allowed the late-stage functionalization of Celestolide
to the acetoxylated product 6i in a 41% yield. Unfortunately,
the reaction did not afford 1-acetoxyadamantane (6j) from the
tertiary C−H bond. Additionally, hypervalent iodine 2h was
not applicable to the reaction.
Figure 4. Proposed mechanisms for (A) decarboxylative esterification
and (B) C−H bond esterification. (C) Radical trapping experiments.
For the decarboxylative esterification, we propose a reaction
conducted (Figure 4C). When the esterification of 4-
propylbiphenyl (5b) was conducted under standard conditions
in the presence of 2.0 equiv of 1,1-diphenylethylene, the
esterified compound 6b was not obtained. Instead, we
observed methylated 1,1-diphenylethylene 7 in a 72% yield.
This result nicely supports the generation of a methyl radical,
which reacts with 1,1-diphenylethylene.
These reactions were applicable for the divergent trans-
formation of ibuprofen (8a) and its methyl ester 8b (Figure 5).
The treatment of 8a under the decarboxylative esterification
conditions provided 9a in a 55% yield. On the other hand, the
reaction of 8b under the C−H esterification conditions
furnished 9b in a 60% yield. The two developed trans-
formations enable the diversification of bioactive arylacetic acid
derivatives, which will accelerate the development of novel
bioactive molecules.
8
mechanism based on our previous study, as shown in Figure
4
A. Arylacetic acid 1 reacts with hypervalent iodine 2 to afford
intermediate A. Single-electron transfer takes place between
intermediate A and the photoactivated Ru catalyst. Carboxylate
B and benzyl radical C are formed by the decarboxylation of
intermediate A. Ru(III) then oxidizes benzyl radical C to give
the benzyl cation D, which reacts with carboxylate B to give
product 3. For the C−H esterification reaction, a proposed
mechanism is shown in Figure 4B. Photoactivated Ru reacts
with PIDA to give the methyl radical and acetate through
decarboxylation. As the resulting methyl radical has a higher
reactivity, benzylic C−H abstraction provides benzyl radical C,
which then is oxidized by the catalyst to give intermediate D.
Intermediate D reacts with acetate to give product 6. To probe
this proposed mechanism, a radical trapping experiment was
5
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Letter
Yota Sakakibara − Institute of Transformative Bio-Molecules
WPI-ITbM) and Graduate School of Science, Nagoya
(
University, Chikusa, Nagoya 464-8602, Japan
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.1c01645
Author Contributions
⊥
These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
This work was supported by JSPS KAKENHI Grants
9H05463 (K.I.) and JP19H02700 (K.M.) and Japan Science
■
1
and Technology Agency (JST) PRESTO Grant JPMJPR20D8.
The Institute of Transformative Bio-Molecules (ITbM) is
supported by the World Premier International Research
Center Initiative (WPI), Japan.
Figure 5. Divergent transformation of ibuprofen and its derivative.
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Corresponding Authors
■
Kenichiro Itami − Institute of Transformative Bio-Molecules
(
WPI-ITbM) and Graduate School of Science, Nagoya
University, Chikusa, Nagoya 464-8602, Japan; orcid.org/
000-0001-5227-7894; Email: itami@chem.nagoya-u.ac.jp
Kei Murakami − Institute of Transformative Bio-Molecules
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