DOI: 10.1002/chem.201600776
Communication
&
Homogeneous Catalysis
A Thermo- and Photo-Switchable Ruthenium Initiator For Olefin
Metathesis
[
a]
Volodymyr Sashuk* and Oksana Danylyuk
cis- to trans-dichloro isomerization induced by light gives rise
Abstract: A ruthenium carbene complex bearing azoben-
zene functionality is reported. The complex exists in the
form of two isomers differing by the size of the chelate
ring. Both isomers were isolated by applying kinetic or
thermodynamic control during the synthesis and charac-
terized by X-ray diffraction analysis. The isomerization of
the complex was studied by UV/Vis spectroscopy. The
stable isomer was tested as a catalyst in olefin metathesis.
The complex was activated at about 1008C to promote
ring-closing and ring-opening polymerization metathesis
reactions. The activation took place also at room tempera-
ture under middle ultraviolet radiation.
to the metathesis activity of a thermally switchable ruthenium
[
13]
catalyst containing S-chelating moiety. Surprisingly, this was
the only report on the dual-responsiveness so far. Therefore,
we decided to expand the portfolio of orthogonally activated
metathesis initiators.
Here we present a new olefin metathesis catalyst that can
be switched either by temperature or light. The catalyst con-
tains a N-chelating azo benzene moiety, which coordinates to
the ruthenium center through either of the two nitrogen
atoms leading to ring-size isomerism. A five-membered chelate
ring complex (CAST1) is a kinetic product that converts quickly
in solution into a six-membered thermodynamically favored
isomer (CAST2). The latter is switched on at elevated tempera-
tures to promote ring-closing (RCM) and ring-opening poly-
merization (ROMP) metathesis reactions. Also, it initiates the
ROMP at room temperature under UV irradiation.
Olefin metathesis is a convenient chemical tool for creating
[1]
carbon–carbon double bonds. Though known for a long
time, the reaction became popular only after the discovery of
The CAST1 and CAST2 catalysts were prepared by carbene
exchange reaction using Grubbs-II ruthenium complex (GII)
and (E)-1-phenyl-2-(2-vinylphenyl)diazene (AST). The latter was
readily accessible in one step from commercially available ni-
trosobenzene and 2-vinylaniline. The reaction schemes for
ligand and catalyst preparation are presented in Scheme 1. The
carbene exchange reaction was complete at room temperature
within 15 min, yielding both isomers as indicated by thin layer
chromatography (TLC). To obtain pure thermodynamic product
the reaction mixture was aged for an additional few hours. The
product was isolated by chromatography and then crystallized
by vapor diffusion technique from dichloromethane-pentane
[2]
the well-defined ruthenium Grubbs-type catalysts. Since their
inception, the ruthenium catalysts were developed primarily
[3]
towards increasing their efficacy. With time, however, the
focus on catalyst design was shifted. Nowadays, an increasing
interest is observed for the catalysts capable of switching the
[4]
reactions on demand. It is highly desirable to have catalytic
systems that are dormant at ambient conditions but able to
be activated under external stimuli. The most promising are
physical stimuli that barely change the chemical composition
of catalytic system and can be applied with good spatial and
[
5]
temporal resolution. Temperature and light are arguably the
simplest and the most used physical triggers for olefin meta-
thesis. The general strategy to achieve the thermal switching is
based on the use of strongly chelating ligands containing
[
6]
[7]
[8]
[9]
atoms such as nitrogen, phosphorus, sulfur, oxygen, or
[
10]
even iodine. On the other hand, when the labile ligands are
employed, the catalysts are readily activated by light irradia-
[11]
[12]
tion or ultrasound. At first glance, the requirements for
thermal- and light-responsiveness seem to be incompatible.
Fortunately, there are plenty of other light-activation mecha-
nisms that can be utilized to make a strongly chelated catalyst
[
5c]
photoswitchable. Lemcoff et al. have recently shown that
[
a] Dr. V. Sashuk, Dr. O. Danylyuk
Institute of Physical Chemistry
Polish Academy of Sciences
Kasprzaka 44/52, 01-224 Warsaw (Poland)
E-mail: vsashuk@ichf.edu.pl
Scheme 1. Synthesis of: a) AST ligand, yield: 20%; b) CAST1 complex, yield:
not determined due to instability, and CAST2 complex, yield: 67%.
Chem. Eur. J. 2016, 22, 6528 – 6531
6528
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim