Biomimetic aerobic oxidative hydroxylation of arylboronic acids to phenols catalysed by a flavin derivative

Article abstract of DOI:10.1039/c3ob42081g

Flavin-catalysed oxidative hydroxylation of substituted arylboronic acids by molecular oxygen with the assistance of hydrazine or ascorbic acid resulted in phenols in high yields. This mild organocatalytic protocol is compatible with a variety of functional groups and it is alternatively usable for transformation of alkylboronic acids to alcohols. Reaction takes place also in water and fulfils criteria for a green procedure.

Full text of DOI:10.1039/c3ob42081g

Organic &
Biomolecular Chemistry
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Biomimetic aerobic oxidative hydroxylation of
arylboronic acids to phenols catalysed by a flavin
derivative†
Cite this: Org. Biomol. Chem., 2014,
2, 2137
1
a,b
b
a
c
Hana Kotoučová, Iveta Strnadová, Martina Kovandová, Josef Chudoba,
c
a
Hana Dvořáková and Radek Cibulka*
Received 18th October 2013,
Accepted 23rd January 2014
Flavin-catalysed oxidative hydroxylation of substituted arylboronic acids by molecular oxygen with the
assistance of hydrazine or ascorbic acid resulted in phenols in high yields. This mild organocatalytic pro-
tocol is compatible with a variety of functional groups and it is alternatively usable for transformation of
alkylboronic acids to alcohols. Reaction takes place also in water and fulfils criteria for a green procedure.
DOI: 10.1039/c3ob42081g
www.rsc.org/obc
Introduction
Design of biomimetic systems inspired by enzymatic catalytic
processes usually yields new efficient green methodologies in
1
organic synthesis. Accordingly, oxygenation reactions occur-
ring in flavin-dependent monooxygenases provided inspiration
for artificial systems based on flavinium salts which are now
used as powerful catalysts for chemoselective and stereo-
2
–4
selective oxidations with oxygen or hydrogen peroxide.
In monooxygenases, flavin cofactor Fl (FMN or FAD) reduced
by NADPH reacts with oxygen to give flavin-4a-hydroperoxide
FlOOH (Scheme 1), which oxidizes a substrate. After oxygen
2
,5
transfer, the cofactor is regenerated by water elimination.
Artificial aerial oxidations catalysed by flavinium salts (e.g. by
compound 1 in Scheme 1) in the presence of a sacrificial redu-
2
,4
cing agent proceed via the same mechanism. The use of 5-
alkylflavinium salts instead of neutral flavin is necessary
because the 5-alkyl group stabilizes hydroperoxide 1-OOH so
2
b,6
that it can be utilized for oxidations.
Despite their consider-
able potential, flavinium catalysts have still been tested only in Scheme 1 Mechanism of aerobic oxidations of substrate S catalysed by
4
a,f
4f
neutral flavin Fl (in enzymes) or flavinium salt 1 (in artificial systems) with
the assistance of reductant AH2.
2
O oxidations of sulfides to sulfoxides, amines to N-oxides,
4
e
4c
and in Baeyer–Villiger (B.V.) and Dakin oxidations. Here, we
report flavinium-catalysed oxidative hydroxylation
of arylboronic acids, thus extending the portfolio of flavin-
mediated oxidations and bringing an organocatalytic pro-
cedure for the transformation of arylboronic acids to phenols.
Analogous biocatalytic oxidation of C–B bonds with Baeyer–
Villiger oxygenases employing the flavin cofactor has been
a
Department of Organic Chemistry, Institute of Chemical Technology, Prague,
Technická 5, 16628 Prague 6, Czech Republic. E-mail: cibulkar@vscht.cz;
Fax: +420 220 444 288; Tel: +420 220 443 688
7
7b
reported, but with a substrate scope focused on alkylboronates.
b
Charles University of Prague, Faculty of Education, M. D. Rettigové 4, 116 39 Prague 1,
Currently, the hydroxylation of arylboronic acid represents
a useful alternative to classical phenol synthesis methods, i.e.
nucleophilic aromatic substitution of aryl halides or hydrolysis
of arene diazonium salts, which often suffer from low func-
tional group compatibility and poor accessibility of the starting
compounds. Arylboronic acids can be hydroxylated by strong
Czech Republic
c
Central Laboratories, Institute of Chemical Technology, Prague, Technická 5,
16628 Prague 6, Czech Republic
†
Electronic supplementary information (ESI) available: Synthesis and character-
ization of catalyst 2, characterization of products of oxidative hydroxylations and
NMR studies. See DOI: 10.1039/c3ob42081g
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oxidizing agents such as hydrogen peroxide, oxone, or MCPBA range of reducing agents and solvents from efficient protocols
8
4a,d–f
which are usually used in stoichiometric amounts. Because designed for aerobic sulfoxidations and B.V. oxidations.
there is a need for mild and environmentally friendly oxidation We used simple flavinium catalyst 2 (Scheme 2) which is
methods tolerating other functionalities, catalytic oxidative readily available by a two-step procedure from commercial
4
d
hydroxylations of boronic acids became a subject of intensive material. We initially employed the oxidation of phenylboro-
9
–12
research in the last decade.
As a result, oxidations with nic acid (3a) with oxygen (1 atm., balloon) with 5 mol% of the
9
molecular oxygen catalysed by copper(II) or palladium(II) salts,
copper-promoted electrochemical hydroxylation,
with electrochemically generated superoxide anions,
catalyst as a model reaction (Table 1).
1
0a
reaction
When hydrazine is used as a sacrificial reducing agent, the
1
0b
and choice of solvent is essential (entries 1–4). In methanol, which
photocatalytic aerobic oxidative hydroxylation mediated by a dissolves phenylboronic acid well, only 5% conversion to
ruthenium or methylene blue sensitizer and visible light have phenol was observed after 10 min. Reaction in acetonitrile and
1
1
been developed. Recently, the metal-free mild oxidation with in an acetonitrile–ethyl acetate–water mixture led to 35% and
N-oxides has been reported, but it requires a stoichiometric 12% conversions, respectively. Therefore we turned our atten-
1
2
amount of organic oxidant. Unexpected phenol production tion to trifluoroethanol which is considered to be a suitable
from arylboronic acid in the presence of oxygen and naphtho- solvent for aerobic oxidations due to high solubility of
4
f
quinone is still the only example of an organocatalytic aerobic oxygen. Addition of the methanol co-solvent was necessary to
1
3
process mentioned in the literature.
homogenize the reaction mixture. In the resulting trifluoro-
ethanol–methanol (2 : 1) solvent system we observed the best
result by far with a conversion of 95% after 10 min of oxidation
(entry 4). As expected, lower catalyst loading as well as the use
of air instead of oxygen led to deceleration of the process
Results and discussion
When searching for suitable conditions for the flavin-based
oxidative hydroxylation of arylboronic acids, we evaluated a
(
entries 5 and 6). It is important to note that oxidation in the
absence of the flavin catalyst does not take place (entry 7).
Hydroxylation also proceeds with zinc or ascorbic acid as sacri-
ficial reductants; however, the rates were lower than that with
hydrazine under optimized conditions (entries 9–14). Reac-
tions with zinc or ascorbic acid failed to accelerate either by
changing the solvent or by adding sodium acetate to generate
4
a
ascorbate with a higher reduction power and/or by generat-
ing the flavin hydroperoxide anion which is more powerful for
4
e,14
nucleophilic oxidations
(entries 11 and 13). Interestingly,
Scheme 2 Flavinium catalyst 2 and the corresponding hydroperoxide.
flavin-based hydroxylations take place also in aqueous
a
Table 1 Oxidation of phenylboronic acid with oxygen catalyzed by alloxazinium salt 2
b
Conv. 10 min.
[%]
Entry
Catalyst
Reducing agent
Solvent + additives
1
2
3
4
5
2
2
2
2
N
N
N
N
N
N
N
N
2
2
2
2
2
2
2
2
H
H
H
H
H
H
H
H
4
4
4
4
4
4
4
4
·H
·H
·H
·H
·H
·H
·H
·H
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
CH
CH
CH
3
3
3
OH
CN
5
35
12
95
10
15
0(0)
7(95)
39
12
13
6
d
e
e
e
e
2
CN–EtOAc–H O
CF
CF
CF
CF
3
CH
2
2
2
2
OH–CH
OH–CH
OH–CH
OH–CH
3
3
3
3
OH
OH
OH
OH
2 (1 mol%)
3
3
3
CH
CH
CH
c
6
2
—
2
2
2
2
2
2
2
7
8
9
2
H O (pH = 7.8)
Zn
Zn
Zn
CH
CH
CH
3
3
3
OH
CN–EtOAc–H
CN–EtOAc–H
d
d
10
11
12
13
14
2
O
2
O + CH
3
COONa (1 equiv.)
f
Ascorbic acid
Ascorbic acid
Ascorbic acid
CF
CF
3
CH
CH
2
OH–CH
OH–CH
3
OH–H
OH–H
2
O
2
f
3
2
3
3
O + CH COONa (1 equiv.)
8
27
2
H O (pH = 7.8)
a
Conditions: phenylboronic acid (0.079 mmol), 2 (5 mol% unless otherwise indicated), reducing agent (0.106 mmol), oxygen (1 atm., balloon),
b
solvent 0.6 mL, R.T. (for details see procedure A in the Experimental section). Conversion after 10 minutes (conversion after 3 hours in
brackets) determined by H NMR. Air (1 atm., balloon) used instead of oxygen. 8 : 1 : 1. 2 : 1. 7 : 3 : 2.
1
c
d
e
f
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solutions (entries 8 and 14). The reaction is slower compared Table 2 Isolated yields and reaction times for oxidative hydroxylations
a
of arylboronic (and alkylboronic) acids on a preparative scale
to that in trifluoroethanol. Most probably, the reaction rate in
water is negatively influenced by low solubility of oxygen
which is approx. 10 times lower as compared with organic and
1
5
fluorinated solvents. Even so, quantitative conversion was
almost observed in aqueous medium with hydrazine as the
reductant after extended time (3 h).
Series of boronic acids with an electron-withdrawing or
electron-donating group were screened in preparative experi-
ments to investigate the substrate scope of the reaction
(Table 2; see the Experimental section and ESI† for details).
Most substituted phenols 4 were obtained in quantitative con-
version and with good to excellent yields from arylboronic
acids 3 using our optimized protocol, i.e. the 2/O2 (1 atm.)/
hydrazine system in trifluoroethanol–methanol. It should be
mentioned that the procedure is suitable also for ortho-substi-
tuted and even for ortho,ortho′-disubstituted derivatives with
only longer reaction time being required. The reaction is
slower also for ortho- and para nitro derivatives 3g and 3i. For
oxidation of o-nitrophenylboronic acid (3i), 51% conversion
was only observed even after extended reaction time. This is
probably caused (in addition to a steric effect in the case of 3i)
1
6
by relatively high acidity of the resulting nitrophenols which
are able to protonate hydrazine. Hydrazine acts as a flavin
reducing agent and a base generating flavinhydroperoxide
anion. Both these processes could be decelerated by hydrazi-
1
8
nium/hydrazine equilibrium. Addition of a weak base, e.g.
sodium bicarbonate or sodium acetate, speeds up the reaction
significantly: quantitative and 85% conversion was detected
for 3g and 3i, respectively, in the presence of 5 equivalents of
sodium acetate after 2 h.
Special attention has been paid to arylboronic acids posses-
sing moieties sensitive to oxidation: aldehyde group, pyridine
nitrogen, double bond and (hydroxymethyl)phenyl group. As
expected, hydrazine is not compatible with aldehyde function
and hydrazone 5 is formed by the original protocol from p-for-
mylphenylboronic acid (3p). On the other hand, the procedure
with ascorbic acid gave p-hydroxybenzaldehyde (4p) in an
2
almost quantitative yield. This indicates that the 2/O /ascorbic
acid system is also useful on the preparative scale and, more-
over, the procedure is chemoselective leaving the aldehyde
1
9
function non-oxidized. The protocol with ascorbic acid was
efficiently applied also to hydroxylation of o-nitrophenylboro-
nic acid (3i) and 4-vinylphenylboronic acid (3m). The hydra-
zine based procedure is excluded for 3m to avoid double bond
reduction by diimide which can be formed from hydrazine by
2
0
the action of flavinium salts. On the other hand, the pro-
cedure with hydrazine succeeded in producing corresponding
hydroxy derivatives from pyridin-3-ylboronic acid (3n) and
a
4-(hydroxymethyl)phenylboronic acid (3o) in quantitative conver-
Conditions: boronic acid (0.79 mmol), catalyst 2 (5 mol%), hydrazine
1.06 mmol), oxygen (1 atm., balloon), CF CH OH–CH OH (2 : 1,
mL), R.T. (see procedure B in the Experimental section). Conversion
(
6
>
3
2
3
sion and with good yields. No side-oxidation of the double
bond, pyridine nitrogen, as well as (hydroxymethyl)phenyl
b
c
d
95%. Conversion 51%. 5 equiv. of sodium acetate added (see
e
f
group was ever observed. Finally, cyclohexyl- (6a) and dodecyl- procedure D in the Experimental section). Conversion 85%. Ascorbic
acid + sodium acetate instead of hydrazine (see procedure C in the
boronic acid (6b) were oxidized to the corresponding alcohols
a and 7b by the unchanged protocol with hydrazine showing
g
Experimental section). Conversion 76%.
7
its applicability to alkylboronic acids.
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3 3 2 3 2
Fig. 1 Course of aerial oxidation of 3d to 4d mediated by the 2/ascorbic acid/CH COONa system in CF CH OH–CD CN–D O (7 : 4 : 3) monitored
1
11
by H (a) and B (b) NMR; for full spectra, see ESI.†
In a preliminary mechanistic study on the course of oxi- oxidation. It is also applicable for the transformation of alkyl-
dative hydroxylation, we tried to vary the amount of hydrazine boronic acids to alcohols. The reaction is performed under
relative to the substrate. We observed the quantitative pro- mild conditions with potential use of water as a solvent. The
duction of phenol with only 0.5 equivalent of hydrazine use of oxygen as a stoichiometric oxidant and hydrazine as a
4
d,f
showing that, similar to sulfoxidations,
one equivalent of sacrificial reducing agent producing water and nitrogen as the
hydrazine generates two equivalents of dihydroflavine (the pre- only by-products ranks this method among the green pro-
cursor of flavin hydroperoxide) in the catalytic cycle. During cedures. The efficiency of the described catalytic system
the first reduction step, hydrazine is oxidized to diimide which should be further increased by optimizing the structure of the
is still strong enough to reduce the second molecule of flavi- flavin catalyst, and this is now underway in our laboratory.
4
d 1
nium salt while it itself is oxidized to molecular nitrogen.
H
1
1
and B NMR monitoring of the p-methylphenylboronic acid
3d) hydroxylation mediated by the 2/ascorbic acid/CH COONa
system in CF CH OH–CD CN–D O revealed a simple reaction
course with no by-products (Fig. 1; see also ESI†). It also shows
(
3
Experimental
Materials and methods
2
1
3 2 3 2
that the likely intermediates, e.g. adduct of 2-OOH on arylboro- NMR spectra were recorded on a Varian Mercury Plus 300
1
13
nic acid and the corresponding arylborate, are readily con- (299.97 MHz for H, and 75.44 MHz for C) and a Bruker
1
13
verted to the reaction products (phenol and boric acid). Avance DRX 500 (500.13 MHz for H, 125.77 MHz for C and
1
1
Similarly, a simple course of hydroxylation was observed in 160.4 MHz for B) at 298 K unless otherwise indicated.
aqueous solution (see ESI† for details).
Chemical shifts δ are given in ppm using residual solvent or
1
13
tetramethylsilane as an internal standard for H and C NMR
11
and BF
C, H, N) were performed on a Perkin-Elmer 240 analyser. High-
resolution mass spectra were obtained on an LTQ Orbitrap Velos
3 2
·Et O as an external standard for B. Elemental analyses
(
Conclusions
In conclusion, flavinium salt 2, in the presence of a sacrificial (Thermo Fisher Scientific), equipped with an orbitrap mass ana-
reducing agent, catalysed the oxidative hydroxylation of aryl- lyzer. The mass spectrometer was operated in ESI mode (ESI
boronic acids to phenols with molecular oxygen which is a source temperature 250 °C, potential 3000 V) with a mass range
new synthetic application for the flavin-based biomimetic from 200 to 2000 a.m.u. TLC analyses were carried out on a
systems. The flavinium-based method is organocatalytic, thus DC Alufolien Kieselgel 60 F254 (Merck). Preparative column chro-
distinguishing it from other catalytic procedures for the matography separations were performed on a silica gel Kieselgel 60
hydroxylation of arylboronic acids. The method is character- (0.040–0.063 mm) (Merck). Melting points were measured on
ized by a broad substrate scope, including ortho-substituted a Boetius melting point apparatus and are uncorrected. Start-
arylboronic acids and derivatives with groups sensitive to ing materials, reagents and substrates were obtained from
2140 | Org. Biomol. Chem., 2014, 12, 2137–2142
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commercial suppliers and used without further purification.
The solvents were purified and dried using standard pro-
Acknowledgements
cedures. Catalyst 2 was prepared according to the modified This work was financially supported by the Czech Science
4
d
protocol from the literature
characterization).
(see ESI† for details and Foundation (grant no. P207/12/0447).
Notes and references
General procedures for oxidative hydroxylations
Oxidations carried out on an analytical scale – general pro-
1 L. Marchetti and M. Levine, ACS Catal., 2011, 1, 1090–1118.
2 Applications of flavinium salts in catalysis are reviewed in:
(a) G. de Gonzalo and M. W. Fraaije, ChemCatChem, 2013, 5,
403–415; (b) F. G. Gelalcha, Chem. Rev., 2007, 107, 3338–3361;
(c) Y. Imada and T. Naota, Chem. Rec., 2007, 7, 354–361.
−5
cedure A. Boronic acid (7.9 × 10 mol) and a reducing agent
5
(
10.6 × 10− mol) were dissolved or suspended in 0.6 mL of
−
5
solvent. Then catalyst 2 (0.4 × 10 mol) was added and the
reaction mixture was shaken for 10 min in a small flask under
oxygen (balloon, 1 atm.). The solvents were evaporated and the
3 Very recent papers on H O oxidations catalyzed by flavi-
2
2
residue was dissolved in CD
Preparative oxidations – general procedure B (hydrazine
hydrate used as a reducing agent). Boronic acid (7.9 × 10
3
OD for NMR measurement.
nium salts: (a) Y. Imada, T. Kitagawa, S. Iwata, N. Komiya
and T. Naota, Tetrahedron, 2014, 70, 495–501;
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R. Cibulka, Adv. Synth. Catal., 2013, 355, 3451–3462;
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1571–1583.
−
4
−
4
mol) and hydrazine hydrate (7.92 mL, 10.6 × 10 mol) were
dissolved in trifluoroethanol (4.0 mL) and methanol (2.0 mL).
−
4
Then catalyst 2 (0.4 × 10 mol) was added and the reaction
mixture was shaken in a flask under oxygen (balloon, 1 atm.).
The solvents were evaporated and the crude product was puri-
fied by column chromatography.
Preparative oxidations – general procedure C (ascorbic acid
used as a reducing agent). Boronic acid (7.9 × 10 mol),
−
4
−
4
natrium acetate (131.2 mg, 16.0 × 10 mol) and ascorbic acid
−
4
(
218.8 mg, 16.0 × 10 mol) were dissolved in trifluoroethanol
(
2
3.5 mL), water (1.0 mL) and methanol (1.5 mL). Then catalyst
(14.8 mg, 0.4 × 10 mol) was added and the reaction
−
4
mixture was shaken in a flask under oxygen (balloon, 1 atm.).
The solvents were evaporated and the residue was dissolved/
suspended in water (20 mL). The resulting mixture was
extracted with dichloromethane (3 × 15 mL) and dried over
magnesium sulfate. After evaporation of solvents, the crude
product was purified by column chromatography.
4 Papers on aerial oxidations catalyzed by flavinium salts:
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T. Naota, Tetrahedron Lett., 2013, 54, 621–624; (b) S. Chen,
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Preparative oxidations – general procedure D (procedure B
−4
with the application of a base). Boronic acid (7.9 × 10 mol),
−4
natrium acetate (162 mg, 3.95 × 10 mol) and hydrazine
−
4
hydrate (81.4 mg, 16.0 × 10 mol) were dissolved in trifluoro-
ethanol (4.0 mL) and methanol (2.0 mL). Then catalyst 2
4
(
14.8 mg, 0.4 × 10− mol) was added and the reaction mixture
was shaken in a flask under oxygen (1 atm.) for 2 hours. The
solvents were evaporated, the residue was dissolved/suspended
in water (20 mL) and the mixture was acidified with hydro-
chloric acid (pH = 1). The resulting mixture was extracted with
dichloromethane (3 × 15 mL) and dried over magnesium
sulfate. After evaporation of solvents, the crude product was
purified by column chromatography.
8
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(
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Phenols 4a–4p and 5 and alcohols 7a and 7b resulting from
oxidative hydroxylations of boronic acids were characterized by
1
13
H and C NMR and the HR-MS technique. Spectral data cor-
respond to those published in the literature (see ESI† for
details).
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