Angewandte
Chemie
DOI: 10.1002/anie.201405359
Photochemistry
Hot Paper
[2+2] Cycloaddition of 1,3-Dienes by Visible Light Photocatalysis**
Anna E. Hurtley, Zhan Lu, and Tehshik P. Yoon*
Abstract: [2+2] Photocycloadditions of 1,3-dienes represent
a powerful yet synthetically underutilized class of reactions. We
report that visible light absorbing transition metal complexes
enable the [2+2] cycloaddition of a diverse range of 1,3-dienes.
The ability to use long-wavelength visible light is attractive
because these reaction conditions tolerate the presence of
sensitive functional groups that might be readily decomposed
by the high-energy UVC radiation required for direct photo-
excitation of 1,3-dienes. The resulting vinylcyclobutane prod-
ucts are poised for a variety of further diversification reactions,
and this method is consequently expected to be powerfully
enabling in the synthesis of complex organic targets.
Figure 1. [2+2] Cycloadditions of 1,3-dienes provide access to syntheti-
cally versatile vinylcyclobutanes.
P
hotochemical reactions provide uniquely direct access to
(< 75 kcalmolꢀ1) would thus greatly increase the versatility of
this vastly underexploited transformation and facilitate its use
in synthesis.
molecular structures that are often difficult to synthesize
using other reaction types.[1] The utility of photochemical
[2+2] cycloaddition reactions, in particular, has become
widely appreciated both because of the prevalence of cyclo-
butanes in a diverse family of bioactive natural products[2] and
because of the utility of strain-releasing fragmentation
reactions in the assembly of larger-ring systems.[3] Over the
past several years, our research group has taken advantage of
the unique properties of transition metal chromophores to
develop a variety of photocycloaddition reactions that can be
accomplished with visible light.[4,5] In examining synthetic
targets to demonstrate the utility of visible light induced
photocycloadditions, we realized that [2+2] cycloadditions of
1,3-dienes would afford versatile vinylcyclobutanes that are
ideally poised for diverse synthetic elaborations by manipu-
lation of their alkenyl substituents (Figure 1). Notably, only
a few examples of [2+2] diene–olefin photocycloadditions
have been reported in the context of total synthesis.[6,7] This
stands in sharp contrast to the multitude of syntheses that
feature analogous [2+2] enone–olefin photocycloadditions.
One important reason for this discrepancy is that the direct
photoexcitation of dienes requires irradiation with very high-
energy UVC light (ca. l = 240–265 nm).[8] These high-energy
photons (108–120 kcalmolꢀ1) are incompatible with the
highly functionalized organic substrates generally required
for late-stage synthetic applications. The ability to promote
the [2+2] cycloaddition of dienes with low-energy visible light
Most of the recent interest in visible light mediated
photochemical reactions has focused on photoinduced redox
activation of organic functional groups using transition metal
photocatalysts such as [Ru(bpy)3]2+ (1, Table 1).[5] We have
reported a complementary approach that uses iridium com-
plex 2 to perform the [2+2] cycloaddition of a variety of
electronically diverse styrenes via an energy-transfer mech-
anism.[9] We speculated that this strategy might also be
capable of activating dienes towards [2+2] cycloaddition
reactions using visible light. Although dienes are more
resistant to one-electron oxidation than styrenes,[10] their
lowest-lying triplet states are quite similar in energy (ca. 55–
60 kcalmolꢀ1).[11] We reasoned, therefore, that the same
visible light activated photocatalysts that proved to be
effective for sensitization of styrenes might also activate
simple 1,3-dienes and could thus provide access to a wide
range of synthetically valuable vinylcyclobutane products.
Triene 3 was selected as a model substrate for our
preliminary investigations (Table 1). The direct photoexcita-
tion of this compound requires short-wavelength UVC light,
and as expected, irradiation at l = 254 nm resulted in rapid
and complete decomposition after 30 min (entry 1), consis-
tent with the destructive nature of these high-energy photons.
No trace of the desired [2+2] cycloadduct could be observed
even at partial conversion (entry 2). In contrast, we were
pleased to find that vinylcyclobutane 4 was formed in high
yield upon irradiation of 3 with a household CFL bulb in the
presence of 1 mol% of iridium complex 2 (entry 3).[12]
Importantly, we did not observe any products arising from
competitive electrocyclization or [4+2] cycloaddition events
under these conditions. The observation that 1, which
[*] A. E. Hurtley, Dr. Z. Lu, Prof. T. P. Yoon
Department of Chemistry, University of Wisconsin-Madison
1101 University Avenue, Madison, WI 53706 (USA)
E-mail: tyoon@chem.wisc.edu
possesses
a substantially lower triplet energy (47 kcal
[**] Financial support was provided by the National Institutes of Health
(GM095666). The NMR spectroscopy facility at UW-Madison is
funded by the NSF (CHE-1048642).
molꢀ1),[13] fails to promote this reaction (entry 4) is consistent
with the proposed role of 2 as a triplet sensitizer rather than
a photoredox catalyst. Finally, no reaction occurred in the
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2014, 53, 1 – 5
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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