Aerobic Direct Dioxygenation of Terminal/Internal Alkynes to α-Hydroxyketones by an Fe Porphyrin Catalyst

Article abstract of DOI:10.1002/asia.202101019

We herein report a new synthetic method for the preparation of α-hydroxyketones by the dioxygenation of alkynes. The reaction proceeds at room temperature under the action of Fe porphyrin and pinacolborane under air as a green oxidant to produce α-hydroxyketones. The mild reaction conditions allow chemoselective oxidation with functional group tolerance. Terminal alkynes in addition to internal alkynes are applicable, affording unsymmetrical α-hydroxyketones that are difficult to obtain by any reported dioxygenation of unsaturated C?C bonds.

Full text of DOI:10.1002/asia.202101019

CHEMISTRY
AN ASIAN JOURNAL
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Accepted Article
Title: Aerobic Direct Dioxygenation of Terminal/Internal Alkynes to form
α-Hydroxyketones by Fe Porphyrin Catalyst
Authors: Kento Kimura, Takuya Kurahashi, and Seijiro Matsubara
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10.1002/asia.202101019
Chemistry - An Asian Journal
COMMUNICATION
Aerobic Direct Dioxygenation of Terminal/Internal Alkynes to
form α-Hydroxyketones by Fe Porphyrin Catalyst
Kento Kimura, Takuya Kurahashi,* and Seijiro Matsubara*
Abstract: We herein report
a new synthetic method for the
Scheme 1. Direct synthesis of α-hydroxyketone moiety.
preparation of α-hydroxyketones by the dioxygenation of alkynes.
The reaction proceeds at room temperature under the action of Fe
porphyrin and pinacolborane under air as a green oxidant to produce
(a) Oxidation of ketone / aldehyde
O
O
α-hydroxyketones.
The
mild
reaction
conditions
allow
R2
OH
R2
R1
R1
chemoselective oxidation with functional group tolerance. Terminal
alkynes in addition to internal alkynes are applicable, affording
unsymmetrical α-hydroxyketones that are difficult to obtain by any
reported dioxygenation of unsaturated C–C bonds.
(b) Acyloin condensation
O
O
O
R2
OH
+
R1
R1
H
R2
H
α-Hydroxyketones are important structural motifs in natural
products[1] and drug molecules,[2] as well as useful intermediates
in organic chemistry.[3] Various methods have been reported for
the synthesis of the α-hydroxyketone moiety.[4-6] Among these,
methodologies using carbonyl compounds as starting materials
for transformations such as α-hydroxylation[4] and the acyloin
condensation[5] are widely known, although unsymmetrical α-
hydroxyketone synthesis with full regiocontrol remains
challenging (Scheme 1a, 1b). On the other hand, the direct
dioxygenation of a carbon–carbon unsaturated bond could be
considered one of the most reasonable strategies from the
perspective of step economy. Indeed, the dioxygenation of
olefins to α-hydroxyketones has been developed through the
use of noble metal catalysts such as osmium,[6a,6b]
(c) Dioxygenation of olefins
R
O
R
OH
Scheme 2. Catalytic aerobic oxidation by ferric porphyrin catalyst.
(a) Generation of ferric boroperoxo porphyrin
Catalytic oxidation
δ–
Bpin
MsO
hydrogen
bonding
H
O
Fe porphyrin
O
ruthenium,[6c,6d]
tungsten,[6e]
and
palladium.[6f]
For
+
FeIII
Reduction of O2 in air
HBpin
environmentally friendly oxidation reactions, metal-free
conditions employing iodine[7] and N-bromosuccinimide[8] were
recently reported (Scheme 1c). However, the oxidation of
internal olefins to unsymmetrical internal α-hydroxyketones is
very limited due to the difficulty of regioselective oxidation.
Alkynes are another class of compounds containing unsaturated
C–C bonds. Although α-acetoxyketone synthesis by the
oxidation of alkynes has been reported,[9] the oxidation of
alkynes to α-hydroxyketones has not been described, to the best
of our knowledge.
Dioxygen in ambient air is an ideal oxidant for green
chemistry by virtue of its nominal cost and ready availability; it
has been used for various oxidation reactions.[10,11] However, the
aerobic oxidation of alkynes is generally difficult owing to the
generation of unstable radical/ionic intermediates during the
reaction compared to alkenes.[12] Iron-catalyzed oxidations are
well known in enzymatic reactions in vivo and are also attractive
in terms of eco-friendliness because of their low toxicity and
(air = O2 21 vol% / N2 78% vol%)
Ferric boroperoxo
porphyrin
(b) Working hypothesis
regioselective oxidation
O
R2
R1
R2
R1
OH
Ferric boroperoxo
porphyrin
Bpin
O
Bpin
O
O
R2
O
O
R1
R2
R2
OBpin
R1
R1
cheapness compared with other rare metals.[13] In our previous
work, we reported the generation of a ferric boroperoxoporphyrin
species by use of a ferric porphyrin complex and pinacolborane
in air atmosphere (Scheme 2a).[14,15] Following upon our
previous study, we envisioned the oxidation of alkynes by the
ferric porphyrin complex to obtain α-hydroxyketones. As shown
in Scheme 2b, the ferric boroperoxoporphyrin that is formed
from ferric porphyrin, pinacolborane, and oxygen was expected
to add to an alkyne regioselectively to form a boroperoxo alkynyl
complex. This species can undergo homolytic scission of the O–
[a]
K. Kimura, Prof. T. Kurahashi, and Prof. S. Matsubara
Department of Material Chemistry, Graduate School of Engineering,
Kyoto University, Kyoto 615-8510, Japan
Email: kurahashi.takuya.2c@kyoto-u.ac.jp
Email: matsubara.seijiro.2e@kyoto-u.ac.jp
O
bond to yield the corresponding alkoxy radicals that
Supporting information for this article is given via a link at the end of
the document.
recombine at the α-position of the carbonyl moiety to form the α-
hydroxyketone.
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To prove our hypothesis, we initially examined the
oxidation of phenylacetylene (1a) using [Fe(TPP)]Cl (iron(III)
tetraphenylporphyrin chloride) as the catalyst and pinacolborane
(HBpin) as the reductant in toluene at room temperature in air
(Table 1). Unfortunately, the reaction did not proceed, although
a trace amount of 2-hydroxy-1-phenylethan-1-one (2a) was
obtained (entry 1). However, upon changing the counter anion of
the catalyst from Cl to mesylate (OMs), the yield of 2a was
dramatically increased to 78% (entry 2). Other iron porphyrin
catalysts were not suitable for the reaction (entries 3–5). Using
more reactive boron and silane reducing agents gave poor
yields of 2a due to over reduction of the resulting product
(entries 6–9). FeCl3, the efficient catalyst used in the aerobic
olefin oxidation reaction,[16] did not promote this reaction (entry
10). Finally, control experiments in the absence of an iron
catalyst, reductant, or air atmosphere resulted in no product
formation (entries 11–13).
alkyl moieties (2c, 2d), and electron-withdrawing groups such as
halides (2f–2h) or carbonyl groups (2i–2k), were oxidized to
furnish the corresponding α-hydroxyketones. These products
would be difficult to obtain by the previously reported reaction
that oxidized the α-position of a carbonyl compound.[4] In
addition, a sterically hindered aromatic terminal alkyne (2e) was
suitable for the reaction. Moreover, polycyclic aromatic alkynes
with an acetal (2l) or estrone (2m) moiety were compatible and
provided the desired products.
Figure 1. Substrate scope for the Fe porphyrin-catalyzed oxidation of aromatic
terminal alkynes.[a]
[Fe(TPP)]OMs (10 mol%)
HBpin (1.5 eq)
O
Ar
H
Ar
toluene, air, r.t.,
(air = O2 21 vol% / N2 78% vol%)
OH
1
2
Table 1. Optimization of the reaction conditions for Fe porphyrin-catalyzed
oxidation of alkynes.[a]
Product (%)[b]
catalyst (10 mol%)
reductant (1.5 eq)
O
toluene, air, r.t., 16 h
(air = O2 21 vol% / N2 78% vol%)
O
O
O
OH
1a
2a
OH
OH
OH
OH
OH
OH
OH
OH
OH
EtO
Bu
2b
78%, 8 h
2c
2d
64%, 20 h
Entry
1
Catalyst
Reductant
HBpin
Yield (%)[b]
49%, 23 h
[Fe(TPP)Cl]
<5
78
31
51
<5
21
<5
<5
<5
<5
<5
<5
<5
O
O
O
2
[Fe(TPP)OMs]
[Fe(TMP)OMs]
HBpin
3
HBpin
OH
Br
Cl
2e[c]
4
[Fe(TPFPP)OMs] HBpin
2f
2g
56%, 23 h
87%, 10 h
50%, 19 h
5
[Fe(OEP)OMs]
[Fe(TPP)OMs]
[Fe(TPP)OMs]
[Fe(TPP)OMs]
[Fe(TPP)OMs]
FeCl3
HBpin
9-BBN
NaBH4
TESH
PMHS
HBpin
HBpin
—
6
O
O
O
7
OH
F
OHC
8
O
2h
75%, 17 h
2i
2j
51%, 16 h
79%, 10 h
9
10
11
12
13[c]
O
O
O
—
O
O
H
H
OH
[Fe(TPP)OMs]
[Fe(TPP)OMs]
O
N
H
H
2k
2l
64%, 23 h
2m
70%, 22 h
HBpin
66%, 23 h
O
[a] Reaction conditions: 1 (0.1 mmol), catalyst (10 mol%), reductant (1.5 equiv),
solvent (2 mL, 0.05 M), room temperature, air (1 atm; O2, 21 vol%), 16 h. [b]
1H NMR yield using 1,1,2,2-tetrachloroethane as internal standard. [c]
Reaction under N2. TPP: 5,10,15,20-tetraphenylporphyrin. TMP: 5,10,15,20-
[a] Reaction conditions: 1 (0.1 mmol), catalyst (10 mol%), reductant (1.5 equiv),
solvent (2 mL, 0.05 M), room temperature, air (1 atm; O2, 21 vol%), indicated
time. [b] 1H NMR yield using 1,1,2,2-tetrachloroethane as internal standard. [c]
3.0 equivalents of HBpin were used.
tetramesitylporphyrin.
TPFPP: 5,10,15,20-tetrapentaflurophenylporphyrin.
OEP: 2,3,7,8,12,13,17,18-octaethyl-21H,23H-porphine. TESH: triethylsilane.
PMHS: polymethylhydrosiloxane.
With optimized reaction conditions in hand, we first tested
various types of terminal alkynes (Figure 1). Substituted
aromatic terminal alkynes with electron-donating ether (2b) or
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Figure 2. Substrate scope for the Fe porphyrin-catalyzed oxidation of aromatic
internal alkynes.[a]
acetoxy (4j), and hydroxyl groups (4k), also afforded the
corresponding
α-hydroxyketones.
In
particular,
the
chemoselective oxidation of an aromatic internal alkyne bearing
a terminal aliphatic alkyne moiety gave 4l. This result highlights
the good chemoselectivity of the oxidative transformation.
[Fe(TPP)]OMs (10 mol%)
HBpin (1.5 eq)
O
R
Ar
R
Ar
toluene, air, r.t.,
(air = O2 21 vol% / N2 78% vol%)
OH
3
4
A plausible mechanism for the oxidation is postulated in
Figure 3. Initially, Fe porphyrin (CP1) is activated by oxygen and
pinacolborane to afford an Fe peroxoporphyrin complex (CP2).
We previously reported that Fe boroperoxoporphyrin CP2 is
formed via the reduction of oxygen by CP1 and
pinacolborane.[14] The hydrogen boroperoxoate moiety on CP2
adds to alkyne 1 in accord with the Markovnikov rule to give CP3.
The addition likely proceeds in a stepwise process, with initial
C–H bond formation via a vinyl cation-like transition state and
subsequent C–O bond formation, which would lead to the
selective addition of the boroperoxo moiety to the alkyne. The
boroperoxyalkenyl intermediate then dissociates from the Fe
porphyrin to provide A and regenerate CP1. Homolytic O–O
bond scission in A generates corresponding radical species B,
and radical recombination proceeds to give C. Finally, C
undergoes hydrolysis to afford 2 and the pinacol ester of boric
acid.
Product (%)[b]
O
O
O
OH
OH
OH
4c
MeO
OHC
4a
4b
65%, 23 h
78%, 9 h
70%, 11 h
O
O
O
OH
OH
OH
NC
O2N
pinB
4d
4e
4f
75%, 14 h
55%, 22 h
66%, 19 h
O
O
O
Figure 3. Plausible mechanism.
OMe
Cl
OH
OH
OH
4g
54%, 19 h
4h
4i
O
72%, 22 h
93%, 17 h
+
O2 (air)
Ferric boroperoxo
porphyrin
B
O
H
hydrogen
bonding
δ–
O
O
O
O
O
B
OAc
OH
MsO
H
O
OH
OH
OH
OMs
O
4j
4k[c]
4l
Ph
Ph
84%, 20 h
50%, 13 h
72%, 11 h
Ph
Ph
N
Fe
N
N
N
Fe
N
N
N
N
Ph
Ph
O
Ph
Ph
CP1
CP2
S
OH
4m
70%, 22 h
H
Ar
O
R
O
B
O
Ar
R
MsO–
Ph
O
[a] Reaction conditions: 1 (0.1 mmol), catalyst (10 mol%), reductant (1.5 equiv),
solvent (2 mL, 0.05 M), room temperature, air (1 atm; O2, 21 vol%), indicated
time. [b] 1H NMR yield using 1,1,2,2-tetrachloroethane as internal standard. [c]
3.0 equivalents of HBpin were used.
1
Ph
O
B
N
Fe+
N
N
O
O
N
O
Ph
Ph
R
Ar
A
CP3
We next expanded the scope of the oxidation reaction with
aromatic internal alkynes; the products from these reactions
would be challenging to access from the dioxygenation of
olefins[6] (Figure 2). Gratifyingly, each alkyne underwent
oxidation with full regioselectivity. 1-Phenylhexyne with aromatic
ring substituents such as an electron-donating methoxy group
(4b), electron-withdrawing cyano (4d) and nitro substituents (4e),
and easily oxidized formyl (4c) and boronic acid groups (4f)
underwent oxidation. Heteroaromatic alkyne (4m) was
applicable in the reaction. Functional groups on the alkyl chain
portion of the alkyne, including methoxy (4h), chloro (4i),
O
O
O
hydrolysis
R
O
R
R
Ar
Ar
Ar
O
O
O
OH
B
B
O
B
C
2
O
observed by ESI-MS
observed by ESI-MS
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[2]
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The proposed mechanism is supported by the following
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Scheme 3. Detection of reaction intermediate by ESI–MS.
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[Fe(TPP)]OMs (10 mol%)
HBpin (1.5 eq)
Galvinoxyl (2 eq)
O
(a)
toluene, air, r.t., 3 h
(air = O2 21 vol% / N2 78% vol%)
Galvinoxyl
3a
5
found 591.3811
calcd for C39H52O3Na
[M+Na]+ 591.3809.
[Fe(TPP)]OMs (10 mol%)
HBpin (1.5 eq)
O
[5]
[6]
a) R. S. Menon, A. T. Biju, V. Nair, Beilstein J. Org. Chem. 2016, 12,
444–461; b) N. Kuhl, F. Glorius, Chem.Commun., 2011, 47, 573–575;
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(b)
toluene, air, r.t., 3 h
(air = O2 21 vol% / N2 78% vol%)
O
O
B
O
3a
6
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calcd for C16H23BO4Na
[M+Na]+ 313.1582.
In summary, we report the first example of the aerobic direct
dioxygenation of alkynes to yield α-hydroxyketones. The
reaction proceeds in the presence of oxygen in air as a green
oxidant and an iron catalyst at room temperature. The protocol,
with its high chemoselectivity and functional group tolerance.
Efforts to expand the scope of the alkyne substrates and further
application of this reaction are now in progress.
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Acknowledgements
This work was supported by Grants-in-Aid for Scientific
Research (Nos. 20H02737, 18H04253, and 17KT0006) from
MEXT (Japan). Part of this study was performed at the BL14B2
beamline of the SPring-8 synchrotron radiation facility (Proposal
Nos. 2015B1770, 2019A1712, 2019B1842, 020A1624,
2020A1766, and 2021B1720).
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Keywords: oxidation • alkyne • α-hydroxyketone • ferric
porphyrin complex • compound 0
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COMMUNICATION
Entry for the Table of Contents
Complete Regioselectity & Chemoselective Oxidation
Kento Kimura, Takuya Kurahashi,* and
Seijiro Matsubara*
via
MsO
Ph
H
O
Example
O
O
Bpin
[Fe(TPP)]OMs catalyst
HBpin
O
Ph
N
FeIII
H
H
R'
OH
R
R'
OH
N
R
Page No. – Page No.
N
room temperature
ambient air
H
N
Ph
Ph
Aerobic Direct Dioxygenation of
Terminal/Internal Alkynes to α-
Hydroxyketones by Fe Porphyrin
Catalyst
O
ferric boroperoxoporphyrin
"a compound 0 analogue"
26 examples
up to 93% yield
We report the first synthesis of α-hydroxyketones through the dioxygenation of
alkynes. The reaction is mediated by an Fe porphyrin complex and pinacolborane
at room temperature under air as a green oxidant, furnishing α-hydroxyketones in
up to 93% yields. The mild reaction conditions allow chemoselective oxidation with
good functional group tolerance.
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