Angewandte
Chemie
Photoredox Catalysis
Photooxidizing Chromium Catalysts for Promoting Radical Cation
Cycloadditions**
Susan M. Stevenson, Matthew P. Shores,* and Eric M. Ferreira*
III
Abstract: The photooxidizing capabilities of selected Cr
complexes for promoting radical cation cycloadditions are
described. These complexes have sufficiently long-lived excited
states to oxidize electron-rich alkenes, thereby initiating [4+2]
processes. These metal species augment the spectrum of
catalysts explored in photoredox systems, as they feature
unique properties that can result in differential reactivity from
the more commonly employed ruthenium or iridium catalysts.
development of transformations optimal for these catalyst
systems. Herein, we report the realization of this goal in the
discovery of photooxidizing chromium-catalyzed Diels–Alder
cycloadditions that proceed via radical cation intermediates
(Figure 1).
O
ver the past several years, photoredox catalysis has arisen
as a powerful tool in organic synthesis for its ability to form
[
1]
CꢀC bonds. A light-generated excited-state metal complex
can act as an oxidant or reductant, yielding unconventional
radical intermediates from organic substrates through single-
electron transfer. An advantage of photocatalysis is its
utilization of a simple energy source (i.e., light) to initiate
these valuable transformations. Despite this benefit, the
photocatalysts most commonly employed in these processes
are complexes of ruthenium and iridium, two of the most rare
and expensive transition metals. In contrast, examples of the
use of first-row metal photocatalysts for synthetic applications
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Figure 1. Photooxidizing Cr -catalyzed cycloadditions.
The judicious use of Ru and Ir photoredox catalysts for
promoting organic transformations, based on their inherent
oxidative or reductive demands, is well appreciated. Select
examples of prevailing Ru and Ir catalysts are shown in
Figure 2. With regard to first-row metals, the photooxidizing
properties of dipyridyl and phenanthrolinyl chromium(III)
complexes were reported by Serpone, Hoffman, and co-
workers over 30 years ago. One of us with Damrauer and co-
workers had further investigated the oxidizing properties of
these homoleptic and related homo- and heteroleptic com-
are sparse; in 1987, Kern and Sauvage reported the applica-
+
tion of [Cu(dap) ] in the reductive couplings of benzylic
2
[
2,3]
I
halides. More recent investigations of Cu photocatalysis—
[
4,5]
[6]
primarily in reductive processes—have been described, yet
reports are relatively few, and to our knowledge no analogous
disclosures of metal-catalyzed oxidative systems exist. The
discovery and employment of photoredox catalysts based on
alternative, earth-abundant, first-row transition metals for
synthetic organic transformations would be a major advance.
Moreover, the potential for photocatalyst development with
different reactive properties may enable the downstream
[7]
plexes, with specific examples also shown in Figure 2.
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Ground-state Cr complexes are generally substitution
inert, and these specific species absorb strongly in the
ultraviolet region (p!p*), with weaker absorptions in the
[
8]
near-UV (NUV) and visible regions (charge-transfer). The
III
excited state lifetimes (tobs*) of the Cr species are substan-
tially longer than those of the common Ru/Ir photocatalysts,
with values up to 304 and 425 ms for complexes 3 and 4,
respectively. The oxidizing capabilities of these species are
also unique and intriguing. Excited-state reduction potentials
for the conventional metal photocatalysts range from
[
*] S. M. Stevenson, Prof. Dr. E. M. Ferreira
Department of Chemistry, University of Georgia
Athens, GA 30602 (USA)
E-mail: emferr@uga.edu
Prof. Dr. M. P. Shores
Department of Chemistry, Colorado State University
Fort Collins, CO 80523 (USA)
[9]
+
0.31 V (vs. SCE) for fac-Ir(ppy) to + 1.45 V for the
3
2
+ [1]
III
photooxidizing [Ru(bpz) ] . The Cr complexes feature
3
E-mail: matthew.shores@colostate.edu
comparatively higher excited state reduction potentials,
ranging from + 1.40 to + 1.84 V, nearing the potentials of
select strong organic photosensitizers (e.g., pyrilium/acridi-
[
**] Funding for this research was provided by the Colorado Collabo-
ratory for Light-Activated Earth Abundant Reagents (C-CLEAR),
a Network for Sustainable Molecular Design and Synthesis (the
National Science Foundation and the Environmental Protection
Agency, CHE-1359020). We thank the Popik group (UGA) and the
Williams group (CSU) for sharing UV and GC equipment. We
acknowledge collaborators Anthony Rappꢀ, Niels Damrauer, and
Tomislav Rovis for enriching discussions.
[10]
nium salts; E1/2 generally >+ 2.0 V). Overall, these data
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indicate that the Cr complexes may serve as reactive
oxidizing alternatives to the more commonly utilized photo-
catalysts of rare metals.
To start, we directed our attention toward radical-cation-
mediated Diels–Alder cycloadditions, which are initiated by
single-electron oxidation of the dienophile. Bauld and co-
Angew. Chem. Int. Ed. 2015, 54, 1 – 6
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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