Angewandte
Research Articles
Chemie
LED Photochemistry
Blue LED Irradiation of Iodonium Ylides Gives Diradical
Intermediates for Efficient Metal-free Cyclopropanation with Alkenes
Abstract: A facile and highly chemoselective synthesis of
doubly activated cyclopropanes is reported where mixtures of
alkenes and b-dicarbonyl-derived iodonium ylides are irradi-
ated with light from blue LEDs. This metal-free synthesis gives
cyclopropanes in yields up to 96%, is operative with cyclic and
acyclic ylides, and proceeds with a variety of electronically-
diverse alkenes. Computational analysis explains the high
selectivity observed, which derives from exclusive HOMO to
LUMO excitation, instead of free carbene generation. The
procedure is operationally simple, uses no photocatalyst, and
provides access in one step to important building blocks for
complex molecule synthesis.
Iodonium ylides are synthetically versatile,[6] readily under-
going ionic reactions,[7] and serving as precursors in C H
À
insertion,[8] cycloaddition,[9] cyclopropanation,[10] or tandem
reactions thereof,[11] as both racemic and asymmetric proc-
esses.[10f,12] The reliance on the use of b-dicarbonyl or similarly
b-distabilized species to generate iodonium ylides render
these unique carbene precursors ideally functionalized for the
synthesis of important synthetic motifs, such as doubly
activated cyclopropanes.[13,14]
Iodonium ylides can be converted to singlet carbenes
under irradiation using a mercury (Hg) lamp, and the groups
of Hadjiarapoglou,[9c] Matveeva,[7c,15] and Spyroudis[9f,16] have
reported examples of cyclopropanation and related cyclo-
addition events. These reactions thus proceed via free
carbenes, which are formed upon cleavage of the ylide
Introduction
À
Recently, the use of hypervalent iodine(III) (HVI)
reagents in organic chemistry has undergone tremendous
growth, owing to the inherent advantages of mild reaction
conditions and broad reactivity, and of recyclable, environ-
mentally-benign by-products.[1] Cutting edge strategies to-
wards reactions that are catalytic in iodoarene have been
developed, including catalytic asymmetric variants.[2] In
addition, recent efforts to merge visible light-mediated
photochemistry with HVI-based reactions has revealed new
and exciting potential for organic synthesis.[3] However,
compared with the major achievements realized for photo-
redox catalysis with transition metal complexes,[4] the inter-
section of HVI-mediated reactions and visible light photo-
redox catalysis or photoactivation is still in its infancy.
C IPh bond, but the heat generated by the lamp is often an
unavoidable source of technical complications and chemical
selectivity issues. For example, Hadjiarapoglou showed that
irradiating a iodonium ylide 1a with a Hg lamp leads
exclusively to a dihydrofuran in 96% yield, in part due to
the heat generated during the reaction, which is sufficient to
isomerize any of cyclopropane 3a formed (Scheme 1a). In
Our interest in HVI-mediated reactions stems from the
movement towards safer and more sustainable organic syn-
thesis practices. By developing new processes in which
iodonium ylides serve as diazonium ylide surrogates,[5] we
exploit the increased and often complementary reactivity that
they display, which originates from their hypervalent bonding.
Scheme 1. Light-mediated reactions of iodonium ylides.
[*] T. Chidley, I. Jameel, S. Rizwan, Prof. Dr. W. S. Hopkins,
Prof. Dr. G. K. Murphy
this regard, light-emitting diodes (LEDs) constitute new tools
for testing the reactivity of iodonium ylides, as light irradi-
ation in the lower-energy 400–500 nm (violet-blue) range can
be easily achieved without significant heating, hence enabling
the practical, convenient and selective generation of reactive
intermediates under much milder conditions. Recently, Da-
vies and Jurberg reported the photochemical decomposition
of aryldiazoacetates with blue LEDs in the presence of
styrene.[17,18] However, these cyclopropanation reactions were
only successful with donor/acceptor diazo derivatives, as both
monostabilized ethyl diazoacetate and distabilized dimethyl
Department of Chemistry, University of Waterloo
Waterloo, Ontario, N2L3G1 (Canada)
E-mail: shopkins@uwaterloo.ca
Dr. P. A. Peixoto, Prof. Dr. L. Pouysꢀgu, Prof. Dr. S. Quideau
Univ. Bordeaux, ISM (CNRS-UMR 5255)
351 cours de la Libꢀration, 33405 Talence Cedex (France)
E-mail: stephane.quideau@u-bordeaux.fr
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
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ꢀ 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2019, 58, 2 – 9
These are not the final page numbers!