Nanoflow microreactor for dramatic increase not only in reactivity but also in selectivity: Baeyer-Villiger oxidation by aqueous hydrogen peroxide using lowest concentration of a fluorous lanthanide catalyst

Article abstract of DOI:10.1016/j.jfluchem.2006.03.012

Not only the reaction rate but also the regioselectivity in the scandium bis(perfluorooctanesulfonyl)amide-catalyzed Baeyer-Villiger reaction is remarkably increased by the development of the fluorous nanoflow microreactor system continuously controlled by the nanofeeder DiNaS (Direct Nanoflow System) even in the lowest concentration of the catalyst (?0.1 mol%). The Baeyer-Villiger reaction completes within few seconds as a contact time in the nanoflow microreactor to give the lactone products with high regioselectivity without hydrolysis.

Full text of DOI:10.1016/j.jfluchem.2006.03.012

Journal of Fluorine Chemistry 127 (2006) 592–596
www.elsevier.com/locate/fluor
Nanoflow microreactor for dramatic increase not only in reactivity
but also in selectivity: Baeyer–Villiger oxidation by aqueous hydrogen
peroxide using lowest concentration of a fluorous lanthanide catalyst
a
a
Koichi Mikami a, , Masahiro Yamanaka , Md. Nazrul Islam ,
*
Takayuki Tonoi a, Yoshimitsu Itoh a, Masaki Shinoda b, Kenichi Kudo c
a Department of Applied Chemistry, Graduate School of Science and
Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan
b Fuji Electric Co. Ltd., 1 Fuji-machi, Hino-city, Tokyo 191-8502, Japan
c KYA Technologies Corporation, 16-4 Kawa-machi, Hachioji-city, Tokyo 191-0154, Japan
Received 17 March 2006; accepted 18 March 2006
Available online 28 March 2006
Abstract
Not only the reaction rate but also the regioselectivity in the scandium bis(perfluorooctanesulfonyl)amide-catalyzed Baeyer–Villiger reaction is
remarkably increased by the development of the fluorous nanoflow microreactor system continuously controlled by the nanofeeder DiNaS (Direct
Nanoflow System) even in the lowest concentration of the catalyst (ꢀ0.1 mol%). The Baeyer–Villiger reaction completes within few seconds as a
contact time in the nanoflow microreactor to give the lactone products with high regioselectivity without hydrolysis.
# 2006 Elsevier B.V. All rights reserved.
Keywords: Nanoflow; Microreactor; Fluorous lanthanide catalyst; Baeyer–Villiger reaction
1. Introduction
highly hydrophobic and its miscibility to organic solvent is
critically temperature dependent. When fluorous ponytails are
present in sufficient length and/or quantity, fluorous phase
affinities (fluorophilicities) of fluorous lanthanide catalysts can
be extremely high. We have thus developed the lanthanide(III)
tris(perfluorooctanesulfonyl)amide complexes without any
hydrocarbon spacer, which are super Lewis acidic and soluble
in both aliphatic fluorocarbons and fluoroaromatic solvents
such as benzotrifluoride (BTF) as fluorous/organic hybrid
solvents [2,3].
Fluorous phase is the third phase orthogonal to organic and
aqueous phases. Molecules can be rendered fluorous by the
attachment of perfluoroalkyl groups, which are referred to as
ponytails [1]. In the fluorous biphasic catalysis, a fluorous
catalyst is immobilized in the fluorous phase by one to several
fluorous ponytails. The strongly electron-withdrawing properties
of perfluoroalkyl ponytails could have a dramatic effect on the
catalytic activity of the fluorous catalyst. This is the reason why
hydrocarbon spacers have to be set on ligands to minimize the
electron-withdrawing effect on the late transition metal catalysts.
The weaker van der Waals interactions of perfluoroalkyl
groups result in lower boiling points of the perfluoroalkanes and
their low miscibility with organic solvents. Fluorous biphasic
catalysis is based on this low miscibility with conventional
organic solvents at room temperature. Fluorous solvents are
The miniaturization of analytic device using micro-total
analysis systems (m-TAS) has shown a new paradigm especially
in biochemical field [4]. Recently, a miniaturized chemical
reactor, that is, microreactor, attracts much attention of
synthetic organic chemists [5]. The inherent benefits of
microreactors are efficient heat transfer, rapid generation of
small but detectable quantities of reaction products, fluidic
control and short molecular diffusion distance can be applied in
organic synthesis [6].
Strict control of the flow rates of the reaction media is the
key to develop the fully integrated microreactor system. Usual
* Corresponding author. Tel.: +81 3 5734 2142; fax: +81 3 5734 2776.
E-mail address: kmikami@o.cc.titech.ac.jp (K. Mikami).
0022-1139/$ – see front matter # 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jfluchem.2006.03.012

K. Mikami et al. / Journal of Fluorine Chemistry 127 (2006) 592–596
593
flow rate in the recent micro-pumping systems is in the range of
10–500 mL/min [5]. The development of high-performance
nanoscale flow pumping system (‘‘nanoflow system’’) has been
required for further downsizing of the microreactors by three
orders from micro to nano. Electro osmotic flow (EOF) and
hydrodynamic pumping technique are employed in micro-
reactors [7]. However, the flow rate is simply proportional to the
polarity of the solvent in EOF, and hence, non-polar solvents
such as fluorous media cannot be used. By contrast, our
nanoflow microreactor system can be employed through strict
fluidic control in nano-ordered level even by using non-polar
fluorous solvents.
borosilicate microreactors are delivered from Fuji Electric Co.
and fabricated by using a standard fabrication procedure [15].
The reaction path dimension is 3 cm length ꢁ 30 mm
depth ꢁ 30 mm width. With decreasing cell dimensions the
surface area to volume ratio increases, thereby arising the most
striking property of the micro-cell. Due to small dimensions
and internal volumes, the reaction rate remarkably increases.
Fick’s second law of diffusion indicates that the time of
transportation T is reduced by shortening the width as shown in
the diffusion equation: T ꢂ L2/D (L: width, D: diffusion
coefficient) [16].
The nature of fluid flow is of great influence on the mixing in
the cell. Under laminar flow conditions, molecular diffusion is
the only mechanism for exchange of molecules. In turbulent
flow, a fluid portion does not stay within a certain layer but
moves in a random manner across the flow path. As compared
to the length of molecular diffusion for mixing of fluid in
laminar flow, the distance traveled by small volume elements in
the turbulent flow is much higher, leading to fast and
continuous mixing [17]. Due to low flow rates (25–200 nL/
min) and small characteristic dimensions, laminar flow
conditions are sometimes encountered. Scheme 1 shows the
introduction of two solutions (aq. H2O2 and BTF) into a
microreactor at constant flow rates for different runs [14].
Diffusion between the adjacent domains leads to a homo-
geneous mixing at molecular level and larger interfacial areas
lead to enhance mass and heat transfer as well as promotion of
chemical reactions [17].
The Baeyer–Villiger (B–V) reaction has been widely
employed in organic synthesis by virtue of the unique
transformation. An oxygen atom can be inserted into a
carbon–carbon single bond by an oxidant [8]. Particularly,
hydrogen peroxide is an environmentally friendly oxidant to
waste only water. This environmental advantage can make
hydrogen peroxide attractive for industrial use. Hydrogen
peroxide has, in turn, disadvantages: less reactivity, formation
of the regioisomeric mixtures from a-substituted cyclopente-
none substrates in particular, and hydrolysis of the ester or
lactone products particularly in the presence of strong acids [9].
To overcome these disadvantages, a wide variety of homo-
geneous and heterogeneous metal catalysts have been devel-
oped for the B–V oxidation [8].
We report the remarkable increase not only in the reaction
rate but also in the regioselectivity of the B–V oxidation with
30% H2O2 even in the low concentration of a fluorous
lanthanide complex [10,11] with bis(perfluorooctanesulfony-
l)amide ponytail [2] (e.g., Sc[N(SO2C8F17)2]3) in our nanoflow
microreactor system. This fluorous lanthanide catalyst can be
fully employed through the microfabricated flow device strictly
controlled by nanofeeder Direct Nanoflow System [12–14]
(DiNaS) (Scheme 1).
A batch system of fluorous Sc-amide complex was first
examined for the B–Voxidation of 2-methylcyclopentanone in
CF3C6F11 biphasic system with 30% H2O2 but no product was
observed even after 3 h stirring. By using BTF as a fluorous-
organic hybrid solvent, the lactone product (regioselectivity
ratio 67:33) was formed. In the nanoflow microreactor, a
substrate and Sc[N(SO2C8F17)2]3 (5 ꢁ 10ꢃ5 M) in BTF [18]
and 30% H2O2 were separately introduced via two micro-inlets
(Scheme 1); significantly the B–Voxidation reactions of cyclic
ketones were completed within few seconds at r.t. even in very
low concentration (5 ꢁ 10ꢃ5 M) of Sc[N(SO2C8F17)2]3. The
flow rate was continuously controlled by 100 nL/min for both
phases and thus the reactions were examined for 8.1 s as
biphasic contact time through 30 mm ꢁ 30 mm ꢁ 3 cm micro-
channel. Virtually complete regioselectivity of the product as
well as high chemical yield were obtained (Table 1). By
contrast, lower yield and regioselectivity were obtained in the
batch system. It should be noted here that the virtually complete
regioselectivity could be achieved along with quantitative yield
by the nanoflow microreactor even with cyclopentanone
substrates.
2. Results and discussion
The nanoflow microreactor system is illustrated in Scheme
1. DiNaS can now be supplied from KYATECH Corp. as a high
pressure (up to 20.0 MPa) syringe delivery system controlling
the tunable flow of solution from 1 to 200,000 nL/min [12]. The
The success in the nanoflow system shows that the faster
dispersion of H2O2 into BTF including the substrate and the Sc
catalyst can be achieved. The perfect regioselectivity and
significant increased chemical yield by the nanoflow micro-
reactor can be rationalized by the formation of the metal
peroxide [19,20] by the faster dispersion of H2O2 into BTF
including the fluorous Sc catalyst.ꢃThe metal peroxide can
Scheme 1. The Baeyer–Villiger reaction by nanoflow microreactor.
increase the nucleophilicity of HO2 .

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K. Mikami et al. / Journal of Fluorine Chemistry 127 (2006) 592–596
Table 1
The B–V reaction in the nanoflow microreactor and batch systemsa
Substrate
Product
Time (s)
8.1 (5 h)
% Yield
63 (22)b
Regioisomer ratio
8.1 (5 h)
8.1 (5 h)
91 (28)
74 (17)
100:0 (69:31)b
100:0 (100:0)b
8.1 (5 h)
8.1 (5 h)
99 (53)
92 (55)
97:3 (67:33)b
99:1 (70:30)b
a
Substrates (0.1 M), 30% H2O2 aqueous solution and a catalyst (0.05 mol%) in BTF were used in the nanoflow system and flow rates for both layer were maintained
by 100 nL/min at r.t.
b
Catalyst (1 mol%) in BTF was used in batch system.
In order to support for the metal peroxo formation in the
nanoflow system [20], two experimental procedures were
compared in the batch system (round bottomed flasks) for 2-
methylcyclopentanone (Table 2). In the standard batch
experiment (Method a) under almost the same condition as
the nanoflow system except for the high Sc catalyst
concentration (otherwise, oxidation did not proceed within
reasonable reaction times). To the Sc catalyst in BTF
(1 ꢁ 10ꢃ3 M), the substrate was added (0.1 M), followed by
30% H2O2. The low regioselectivity was observed in the first
batch experiment (Scheme 2, Method a). In the premixing batch
experiment (Method b), the Sc catalyst was premixed with aq.
H2O2 for 30 min prior to substrate addition in the same
concentrations of the substrate (0.1 M) and the Sc catalyst
(1 ꢁ 10ꢃ3 M) as the Method a. In contrast to the standard
Method a, the higher regioselectivity was achieved in the
premixing Method b.
illustrated in terms of efficient metal peroxide formation in the
nanoflow system as shown in Scheme 2. It is crucial either the
complexation of the Sc catalyst with H2O2 to give more
nucleophilic Sc peroxide species (path b) or the coordination of
the Sc catalyst to the ketone (path a).
On the basis of these experimental observations in the batch
system, the significant increase in regioselectivity can be
Table 2
The pre-complexation of the Sc peroxo speciesa
Time (h)
Method a
Method b
Regioselectivity
% yield
Regioselectivity
% yield
1
2
4
60:40
65:35
70:30
17
22
39
95:5
94:6
91:9
17
22
43
a
Substrates (0.1 M) and catalyst (1 mol%) in BTF were used in batch system;
yield and regioselectivity were determined by GC analysis.
Scheme 2. The mechanism of the metal peroxo formation followed by the
regiospecific oxygen atom insertion.

K. Mikami et al. / Journal of Fluorine Chemistry 127 (2006) 592–596
595
3.2. The B–V reaction of cyclic ketones for the batch and
nanoflow systems
3.2.1. For batch system
In the BTF (1 ml) solution was added 1 mol% of
Sc[N(SO2C8F17)2]3 (3 mg, 1 ꢁ 10ꢃ3 mmol) and solubilize it
by stirring at room temperature. Cyclic ketones (0.1 mmol)
were added to the fluorous catalyst solution. The reaction
mixture was stirred at room temperature for 30 min followed by
the addition of 30% aq. hydrogen peroxide solution. The
mixture including n-decane (0.1 mmol) as an internal standard
was allowed to stir at that temperature for 5 h monitoring by
TLC. The reaction mixture was quenched with 10% aqueous
Na2SO3 solution, extracted with Et2O, washed with brine and
dried over anhydrous magnesium sulfate. After evaporation
under reduced pressure, the resultant residue was purified by
flash column chromatography to give the lactone products. The
yield and regioselectivity were determined by GC analysis.
Scheme 3. Asymmetric Baeyer–Villiger reaction in nanoflow microreactor.
In path b, the intramolecular nucleophilic attack of the
hydroperoxide moiety regioselectively proceeds from the Sc
complex to the coordinated ketone. The direction of O–O bond
anti to the migratory carbon bearing R group leads to high
regioselectivity with the formation of the Sc hydroxide
complex. In the nanoflow microreactor, the faster dispersion
of H2O2 into BTF phase can efficiently lead to the re-form of
the Sc peroxide from the Sc hydroxide complex with H2O2. The
highly regioselective path b is in contrast to path a, where the
intermolecular H2O2 attack should proceed from both sides.
We have also examined the asymmetric B–V reaction by our
nanoflow system in moderate enantioselectivity. The reaction
rate is significantly increased with 60% H2O2 even by use of a
fluorous lanthanide complex with (S,S)-1,2-N,N0-bis-(trifluor-
omethanesulfonylamino)-1,2-diphenylethane (DPENTf) (5 ꢁ
10ꢃ2 mol%) (Scheme 3). The fluorous Sc complex and
DPENTf ligand were premixed to form the Sc–DPENTf
complex in BTF before introducing into the nanoflow
microreactor. The reaction proceeded within several seconds
(ꢂ50% conversion) in the microreactor to give moderate level
of kinetic resolution (26% ee).
3.2.2. For nanoflow system
The B–V reactions were performed by introducing a BTF
solution containing cyclic ketones (0.1 mmol), and
Sc[N(SO2C8F17)2]3 [5 ꢁ 10ꢃ5 M (5 ꢁ 10ꢃ2 mol%)] and n-
decane (0.1 mmol) as an internal standard through DiNaS-A
nanofeeder and 30% aq. hydrogen peroxide solution through
DiNaS-B nanofeeder, respectively, at room temperature. The
product was collected in a vial containing ethyl acetate as an
eluating solvent. Contact time of the starting materials in the
nanoflow device were determined by total volume of the flow
path and volume flow rates of the two phases. The yield and
regioselectivity were determined by GC analysis.
In summary, we have developed that the Sc-amide-catalyzed
B–V reaction is significantly increased not only in the
regioselectivity but also in the reaction rate along with
moderate level of kinetic resolution by nanoflow microreactor
even in the low concentration of the catalyst (ꢀ0.1 mol%).
3.2.3. For the asymmetric B–V reaction
The asymmetric B–V reaction as performed by introducing a
BTF solution containing 2-phenylcyclohexanone (0.1 M), (S,S)-
DPENTf and Sc[N(SO2C8F17)2]3 [5 ꢁ 10ꢃ5 M (0.05 mol%)]
and n-decane (0.1 M) as an internal standard through DiNaS-A
and 60% aqueous hydrogen peroxide solution through DiNaS-B
at 0–5 8C temperature. The product was collected in a vial
containing ethyl acetate as an eluating solvent. Reaction time of
the reacting materials in the microreactor device were
determined by total volume of the flow path and volume flow
rates of the two phases. The flow rate was maintained by
100–200 nL/min. The product conversion and enantioselectivity
were determined by chiral GC analysis (SHIMADZU GC14B,
column: CP-cyclodextrin 0.25 mm ꢁ 25 m).
3. Experimental
3.1. General
1H and 13C NMR spectra were measured on a Varian
GEMINI 300 (300 MHz), a JEOL GSX-500 (500 MHz) and a
JEOL EX-270 (270 MHz) spectrometers. Chemical shifts of 1H
NMR were expressed in parts per million relative to chloroform
(d 7.26) or tetramethylsilane (d 0.00) as an internal standard in
chloroform-d. Chemical shifts of 13C NMR were expressed in
parts per million relative to chloroform-d (d 77.0) as an internal
standard. IR spectra were measured on a JASCO FT/IR-5000
spectrometer. GC analysis was carried out in SHIMADZU GC-
1700. DiNaS was supplied from KYA TECH Corp. as a high
pressure (up to 20.0 MPa) syringe delivery system controlling
the continuous and tunable flow of solution from 1 to
200,000 nL/min. The borosilicate microreactors used in this
nanoflow reaction system were prepared by Fuji Electric Co.
using a standard fabrication procedure.
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