Angewandte
Chemie
Photochemistry
Regioselective Chromatic Orthogonality with Light-Activated
Metathesis Catalysts**
Efrat Levin, Sudheendran Mavila, Or Eivgi, Eyal Tzur, and N. Gabriel Lemcoff*
Abstract: The ability to selectively guide consecutive chemical
processes towards a preferred pathway by using light of
different frequencies is an appealing concept. Herein we
describe the coupling of two photochemical reactions, one the
photoisomerization and consequent activation of a sulfur-
chelated latent olefin-metathesis catalyst at 350 nm, and the
other the photocleavage of a silyl protecting group at 254 nm.
Depending on the steric stress exerted by a photoremovable
neighboring chemical substituent, we demonstrate the selective
formation of either five- or six-membered-ring frameworks by
light-triggered ring-closing metathesis. The orthogonality of
these light-induced reactions allows the initiation of these
processes independently and in interchangeable order, accord-
ing to the wavelength of light used to promote them.
tion of ruthenium olefin-metathesis initiators has clear uses in
polymer synthesis,[9] the development of a photocatalytic
orthogonal process could expand the utility of these catalysts
for other synthetic applications. Herein, we show how the
order of light irradiation of a latent catalyst and a protected
substrate influences the selectivity for the generation of either
a five- or six-membered cycloalkene isomer as the preferred
final product.
The activation of latent precatalysts 1 and 2 (Scheme 1) by
light encouraged us to search for applications of this process
in novel light-guided synthetic pathways. Thus, 1 and 2 were
irradiated at two different wavelengths in the presence of the
benchmark ring-closing-metathesis (RCM) substrate diethyl
A
rguably one of the most impressive examples of efficient
use of energy is the way nature has exploited photons to
advance living systems. The wavelength of light determines
the exact energy delivered by a photon, and chemists, like
nature, have harnessed this energy in processes called photo-
chemical reactions.[1] A novel concept developed by Bochet
and co-workers[2] is the use of chromatically orthogonal
procedures as sequential chemical reactions that are carried
out selectively in the presence of light of different energies.[3]
These processes have been developed mostly by the use of
complex functional-group-deprotection schemes, in which the
photoinduced removal of a specific protecting group is
necessary in the presence of another protecting group that
remains intact.[4]
Scheme 1. Photoisomerization of sulfur-chelated ruthenium benzyli-
dene complexes.
diallylmalonate (DEDAM). As previously reported, photo-
isomerization of the precatalysts by irradiation at 350 nm
afforded the active trans isomers and led to concurrent ring
closing of DEDAM.[8a,10] However, when solutions of 1 and 2
in dichloromethane were irradiated with ultraviolet light at
254 nm, the cis isomer remained intact, and, as expected, no
RCM reaction occurred (see the Supporting Information).
Having shown that olefin metathesis could be induced with
UV chromatic selectivity, we decided to combine the photo-
activation of RCM with an orthogonal photodeprotection
reaction. We chose to pursue this approach by using the
tris(trimethylsilyl)silyl protecting group (supersilyl),[11,12]
a photochemically removable protecting group that may be
photocleaved at the selected wavelength, 254 nm, and is not
deprotected at 350 nm.
Photocatalytic processes that enable efficient chemical
reactions to occur have been widely used and studied.[5] At
times, light promotes these catalytic processes by activation of
a dormant catalyst.[6] For example, the lower activity of cis-
dichloro ruthenium olefin-metathesis catalysts with respect to
the corresponding trans-dichloro isomers[7] has been exploited
for the activation of sulfur-chelated ruthenium benzylidenes
by a photoisomerization process.[8] Whereas the photoactiva-
[*] E. Levin, Dr. S. Mavila, O. Eivgi, Prof. N. G. Lemcoff
Department of Chemistry, Ben-Gurion University of the Negev
Beer-Sheva 84105 (Israel)
E-mail: lemcoff@bgu.ac.il
To study the orthogonality of the system, we prepared 4-
[tris(trimethylsilyl)silyloxy]hepta-1,6-diene (4) from 1,6-hep-
tadien-4-ol (3; Scheme 2a). First, the supersilyl protecting
group in 4 was fully cleaved after irradiation for 20 min at
254 nm in CH2Cl2 with methanol to regenerate 3 (Sche-
me 2b). Moreover, irradiation of 4 at 350 nm for 1 h did not
cleave the protecting group: an essential requisite for the
orthogonal system. In a parallel experiment, the olefin-
Dr. E. Tzur
Department of Chemical Engineering
Shamoon College of Engineering
Ashdod 77245 (Israel)
[**] The Israel Science Foundation is gratefully acknowledged for
financial support.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
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