Pauson-Khand reactions in a photochemical flow microreactor

Article abstract of DOI:10.1021/ol4008519

Pauson-Khand reactions were achieved at ambient temperature without any additive using a photochemical flow microreactor. The efficiency of the reaction was better than that in a conventional batch reactor, and the reaction could be operated continuously for 1 h.

Full text of DOI:10.1021/ol4008519

ORGANIC
LETTERS
2013
Vol. 15, No. 10
2398–2401
PausonꢀKhand Reactions in a
Photochemical Flow Microreactor
Keisuke Asano, Yuki Uesugi, and Jun-ichi Yoshida*
Department of Synthetic Chemistry and Biological Chemistry, Graduate School of
Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
yoshida@sbchem.kyoto-u.ac.jp
Received March 28, 2013
ABSTRACT
PausonꢀKhand reactions were achieved at ambient temperature without any additive using a photochemical flow microreactor. The efficiency of
the reaction was better than that in a conventional batch reactor, and the reaction could be operated continuously for 1 h.
The PausonꢀKhand reaction is a powerful tool for the
construction of cyclopentenone frameworks by [2 þ 2 þ 1]
cocyclization between alkynes, alkenes, and CO.1,2 The
utility has been verified by many examples employing the
reaction as a key step in natural product syntheses.2
A plausible mechanism of this reaction was proposed
by Magnus and Schore,3 and it is widely recognized that
the initial step is decarbonylation from an alkyneꢀcobalt
complex to provide a vacant coordination site for an
alkene.4 Various protocols to promote this event have
been studied. For example, the use of additives, such as
(1) (a) Khand, I. U.; Knox, G. R.; Pauson, P. L.; Watts, W. E.
J. Chem. Soc. D 1971, 36a. (b) Khand, I. U.; Knox, G. R.; Pauson, P. L.;
Watts, W. E. J. Chem. Soc., Perkin Trans. 1 1973, 975. (c) Khand, I. U.;
Knox, G. R.; Pauson, P. L.; Watts, W. E.; Foreman, M. I. J. Chem. Soc.,
Perkin Trans. 1 1973, 977. (d) Pauson, P. L.; Khand, I. U. Ann. N.Y.
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r
10.1021/ol4008519
Published on Web 05/03/2013
2013 American Chemical Society

amine oxides,5aꢀg amines,5h,i phosphine oxides,5j phosphines,5k
sulfoxides,5c,l sulfides,5l,m and other compounds,5n,o proved
to have considerable advantages over the traditional thermal
conditions. However, the use of an additive inherently leads
to production of wastes derived from it. To circumvent the
use of an additive, other methods for activation have also
been investigated. However, such methods also suffer from
some drawbacks. For example, the irradiation of micro-
waves should be classified as a harsh condition which is not
suitable for the synthesis of complex molecules sensitive to
heat.6 Ultrasound needs to be used with additives to obtain
a large accelerating effect.5g,j The dry state adsorption
techniques were also shown to be efficient, but the synthesis
on a solid surface is challenged in the process of scale-up.7
Meanwhile, light irradiation was found to be effective,
and the photochemical method serves as a milder, greener
approach for activation.4a,8,9 However, photochemical
reactions generally suffer from problems in efficiency and
scale-up. Thus, we attempted to use a flow microreactor
system10,11 for the light-mediated PausonꢀKhand reac-
tions, because the short light path length and high surface-
to-volume ratios of flow microreactors are generally
beneficial for photochemical reactions from the viewpoints
Table 1. PausonꢀKhand Reaction of Phenylacetyleneꢀ
Dicobalt Complex 1a and Norbornene (2a) in a Photochemical
Flow Microreactora
entry
t
R (s)
conversion (%)b
yield (%)b
1
22
55
55
110
55
83
94
71
99
<1
62
88
71
90
<1
2
3c
4
5d
a For the microchannel: depth, 200 μm; width, 1000 μm; length,
916.04 mm; area, 916.83 mm2. Flow rate: 0.1ꢀ0.5 mL/min. The light
source: medium pressure mercury lamp. b Determined by 1H NMR
analysis of the crude products obtained by passing through a short silica
gel pad to remove highly polar materials. c Run using the microreactor
with 500 μm depth. d Run without light.
(6) (a) Iqbal, M.; Vyse, N.; Dauvergne, J.; Evans, P. Tetrahedron
Lett. 2002, 43, 7859. (b) Fischer, S.; Groth, U.; Jung, M.; Schneider, A.
€
Synlett 2002, 2023. (c) Fager-Jokela, E.; Kaasalainen, E.; Leppanen, K.;
Tois, J.; Helaja, J. Tetrahedron 2008, 64, 10381.
of efficiency and scalability.12ꢀ14 Herein, we demonstrate
that PausonꢀKhand reactions could be effectively carried
out in a photochemical flow microreactor in highefficiency
and good productivity.
(7) (a) Smit, W. A.; Gybin, A. S.; Shashkov, A. S.; Strychkov, Y. T.;
Kyzmina, L. G.; Mikaelian, G. S.; Caple, R.; Swanson, E. D. Tetra-
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A. S.; Shashkov, A. S.; Mikaelian, G. S.; Tarasov, V. A.; Ibragimov, I. I.;
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see: (a) Oelgemoller, M. Chem. Eng. Technol. 2012, 35, 1144. (b) Shvydkiv,
€
O.; Oelgemoller, M. In CRC Handbook of Organic Photochemistry and
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Photobiology, 3rd ed.; Griesbeck, A. G., Oelgemoller, M., Ghetti, F., Eds.; CRC
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Press: Boca Raton, FL, 2012; Chapter 3, Vol. 1, p 49. (c) Oelgemoller, M.;
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Shvydkiv, O. Molecules 2011, 16, 7522. (d) Coyle, E. E.; Oelgemoller, M.
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Org. Lett., Vol. 15, No. 10, 2013
2399

We initiated our investigations with the reaction of
phenylacetyleneꢀdicobalt complex 1a and norbornene
(2a) in a photochemical flow microreactor (YMC,
KeyChem-Lumino 2) (Table 1). A solution of 1a and 2a
in toluene was passed through the microchannel (depth,
200 μm; width, 1000 μm; length, 916 mm) covered with
fused quartz glass, which was irradiated by a medium
pressure Hg lamp (80 W) at a flow rate of 0.1ꢀ0.5 mL/min.
The temperature (25 °C) was controlled by a Peltier cool-
ing system. The collected solution was additionally stirred
at ambient temperature for 5 min in the batch reactor.15
The solution was concentrated under reduced pressure
to obtain the crude product, which was purified by flash
chromatography. Although the conversion of 1a and
the yield of cyclopentenone product 3aa were not high
with the residence times shorter than 55 s (entry 1), high
conversions and yields were obtained with the residence
time being longer than 55 s (entries 2ꢀ4). In addition, the
use of a microreactor of a deeper channel (depth: 500 μm)
was less effective (entry 3), indicating the importance
of short light path length. It was also confirmed that the
irradiation was essential for the process (entry 5).
Table 2. Intermolecular PausonꢀKhand Reactions in a
Photochemical Flow Microreactora
To demonstrate the advantage of the flow microreactor
for the photochemical PausonꢀKhand reactions, the reac-
tionof 1awith2awas examinedusinga conventional batch
Scheme 1. Photochemical PausonꢀKhand Reaction of 1a with
2a in a Conventional Batch Reactora
a Reaction was run in 30 mL flask, 10 cm distant from the light source.
Scheme 2. Scalability of PausonꢀKhand Reaction of 1a with 2a
in a Photochemical Flow Microreactora
a For the microchannel: depth, 200 μm; width, 1000 μm; length,
916.04 mm; area, 916.83 mm2. The light source: medium pressure
mercury lamp. b Determined by 1H NMR analysis of the crude products
obtained by passing through a short silica gel pad to remove highly polar
materials. Values in parentheses are the conversion of 1. c Concentration
of 1: 0.05 M. d Concentration of 1: 0.1 M. e Run using the microreactor
with 500 μm depth. f The products were obtained as a single diaster-
eomer. See Supporting Information (SI) for details. g The regioisomer
ratio was 1.4:1.3:1. See SI for details.
macroscale reactor as a control experiment (Scheme 1).
The reaction was carried out in a 30 mL flask, which was
placed 10 cm distant from the light source. The light
source was the same one used for the reaction in a flow
a For the microchannel: depth, 200 μm; width, 1000 μm; length,
916.04 mm; area, 916.83 mm2. The light source: medium pressure
mercury lamp.
2400
Org. Lett., Vol. 15, No. 10, 2013

microreactor, and the irradiation time was 5 min. The
product was obtained only in 32% yield in batch, while the
flow microreactor reaction afforded 3aa in 88% yield
(Table 1, entry 2; 0.05 mmol scale; flow rate, 0.2 mL/
min; operation time, 5 min), indicating that the flow
microreactor method is superior to the batch method.
In addition, the scalability, another important feature
of a photochemical flow microreactor system, was also
verified (Scheme 2). The reaction of 1a with 2a in the flow
microreactor system could be continuously operated for
1 h with high efficiency.
Table 3. Intramolecular PausonꢀKhand Reactions in a
Photochemical Flow Microreactora
Under the optimized conditions (tR = 55 s), the reac-
tions of other alkyneꢀdicobalt complexes 1bꢀ1e with 2a
were investigated (Table 2, entries 2ꢀ5). Good yields were
obtained with both electron-poor and -rich arylacetylene
complexes (entries 2 and 3). A complex of an aliphatic
alkyne also yielded the corresponding cyclopentenone
product (entry 4). A disubstituted alkyne complex was
unfortunately not applicable to the reaction (entry 5).
Cyclopentene (2b) could also be used, although 10 equiv
of 2b and a much longer residence time (tR = 548 s) were
needed (entries 6ꢀ8). The reaction of 1d and 2b, however,
gave rise to a modest yield of the product (entry 9). In
addition, methylenecyclopropane 2c could also be used to
give the products as a mixture of regioisomers (1.4:1.3:1) in
good yield (entry 10).16
Next, we examined intramolecular PausonꢀKhand re-
actions using the photochemical flow microreactor
(Table 3). The reaction of 4a in toluene afforded the cor-
responding product only in 32% yield, although the start-
ing material was consumed almost completely (entry 1).
In this case, 1,2-dimethoxyethane (1,2-DME) was found to
be a more efficient solvent, and the product was obtained
in 76% yield (entry 3).5o Under the conditions (tR = 55 s in
1,2-DME), nitrogen-bridged substrates could also be used
(entries 4ꢀ6). It is noteworthy that intramolecular reac-
tions of the complexes of disubstituted alkynes could be
accomplishedtoobtain thecorresponding productsinhigh
yield (entries 5 and 6).
In summary, we have demonstrated that a photochemi-
cal protocol using a flow microreactor system is effective
for PausonꢀKhand reactions. The method allowed for an
effective activation of alkyneꢀcobalt complexes, thereby
leading to the high efficiency of PausonꢀKhand reactions
a For the microchannel: depth, 200 μm; width, 1000 μm; length,
916.04 mm; area, 916.83 mm2. The light source: medium pressure
mercury lamp. b Determined by 1H NMR analysis of the crude products
obtained by passing through a short silica gel pad to remove highly polar
materials. Values in parentheses are the conversion of 4. c 1,2-DME =
1,2-dimethoxyethane.
at ambient temperature without any additive. Easy scale-
up by elongating the operation time is beneficial for
preparative purposes.
Acknowledgment. This work was financially supported
by the Grant-in-Aid for Scientific Research on Innovative
Areas (No. 2015).
Supporting Information Available. Experimental pro-
cedures including spectroscopic and analytical data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
(15) The collected solution was additionally stirred in order to ensure
that the reaction was complete.
(16) Unstrained alkenes failed to give the corresponding products in
practical yields (<20%).
The authors declare no competing financial interest.
Org. Lett., Vol. 15, No. 10, 2013
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