DOI: 10.1002/chem.201406653
Communication
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Flow Chemistry
Immobilization and Continuous Recycling of Photoredox Catalysts
in Ionic Liquids for Applications in Batch Reactions and Flow
Systems: Catalytic Alkene Isomerization by Using Visible Light
David C. Fabry, Meria A. Ronge, and Magnus Rueping*[a]
numerous side reactions. Additionally, reversibility of the reac-
tions can occur.[14] Mechanistic studies by Caldwell revealed[15]
Abstract: A catalytic (E)- to (Z)-isomerization of olefins
using a photoredox catalyst under mild reaction condi-
tions is presented. A variety of (Z)-alkenes can be pre-
pared in the presence of visible light. A new reaction
system allows an easy and efficient scale-up, as well as
a continuous flow process in which the photocatalyst is
immobilized in an ionic liquid and continuously recycled
by simple phase separation.
that isomerizations of the double bond in (E)-b-cyclopropylstyr-
ene proceed via triplet–biradical species. Due to the ring strain
radical ring-opening occurred in the case of a cyclopropyl sub-
stituent and the formation of 3-phenylcyclopentene provided
proof for the biradical transition state.
In our previous efforts in the area of photoredox catalysis
we concentrated on the formation of radical intermediates and
applications were mainly restricted to single-electron trans-
fers.[16] The application of sensitizers for energy transfer pro-
cesses onto the corresponding substrate was not investigat-
ed.[17] Thus, the excitation of stilbene systems with use of the
photoredox catalyst as sensitizer was the next step.
Stilbene and derivatives represent broadly applicable mole-
cules which show characteristic absorption and fluorescence
due to their conjugated p-system and can therefore be photo-
chemically excited.[1] Owing to these properties such olefins
are important additives in the production of industrial dyes,[2]
chromatic lasers[3] and scintillators.[4]
Prior to this, Osawa and co-workers realized the isomeriza-
tion of 4-cyanostilbene to the corresponding (Z)-isomer.[18,19] In
this work a ruthenium complex was attached to the nitrile
group of the substrate and then photochemically excited.
Weaver and co-workers published an isomerization of (E)-
amino styrenes to the corresponding (Z)-products postulating
amino radicals and biradicals as intermediates, respectively.[20]
In addition they reported the isomerization of styrene deriva-
tives and allylic alcohols.
Furthermore they continuously gain importance in the field
of photophysics and photochemistry,[5] as well as biochemistry,
in which they found applications as anticancer drugs.[6] Espe-
cially in the latter case, precise control of the stereochemistry
regarding (E)/(Z) isomers plays a crucial role. Whereas the (E)-
alkene is readily accessible, the synthesis of the (Z)-isomer still
poses a major challenge. Current procedures for the synthesis
are Wittig reactions,[7] the Still–Gennari modification of the
Horner–Wadsworth–Emmons reaction,[8] Pd-[9] and Ni-cata-
lyzed[10] cross-couplings of (Z)-alkenyl halides, stereoselective
reductions of alkynes by Lindlar catalysis[11] or hydrometala-
tions, (Z)-selective metathesis,[11] the Shapiro reaction[12] and
Peterson olefinations.[13] The highlighted reactions allow no
subsequent modifications of the double bond geometry. Also,
specific functional groups and electronic environments are re-
quired that are more energy-rich than the thermodynamically
harder accessible (Z)-product.
Herein, we describe a catalytic system which allows in the
presence of visible light under mild reactions conditions the
(E)- to (Z)-isomerization of alkenes. The reaction tolerates differ-
ent solvents and can be readily scaled-up. Considering that
these products are needed in larger amounts, a convenient
procedure and recycling of the catalyst is highly desirable
(Scheme 1).
The subsequent isomerization of double bonds via UV light
has been known for many years, but is still limited in its appli-
cation due to the high energy of UV light which can lead to
Scheme 1. Realization of a photoredox catalyzed isomerization for the syn-
thesis of (Z)-olefins.
We here report on the E/Z isomerization of stilbenes using
readily available photoredox catalysts. First reactions in aceto-
nitrile with 3 mol% of the photosensitizer [Ir(ppy)2(bpy)]PF6
showed quantitative conversions already after 18 hours
(Table 1, entry 4). A broad solvent screening (see Supporting
Information) revealed that high conversions can be achieved if
all components are soluble in the corresponding solvent. Ace-
[a] M. Sc. D. C. Fabry, B. Sc. M. A. Ronge, Prof. Dr. M. Rueping
Institut fꢀr Organische Chemie
RWTH Aachen
Landoltweg 1, 52074 Aachen (Germany)
Fax: (+49) 241-809-2127
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201406653.
Chem. Eur. J. 2015, 21, 1 – 6
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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