Divergent Photocatalytic Reactions of α-Ketoesters under Triplet Sensitization and Photoredox Conditions

Article abstract of DOI:10.1021/acs.orglett.0c02314

The long-lived triplet excited states of transition metal photocatalysts can activate organic substrates via either energy- or electron-transfer pathways, and the rates of these processes can be influenced by rational tuning of the reaction conditions. The characteristic reactive intermediates generated, however, are distinct and can exhibit very different reactivity patterns. This mechanistic diversity available to photocatalytic reactions might thus offer an opportunity to engineer divergent reactions that give markedly different chemical outcomes under superficially similar conditions. Herein, we show that the photocatalytic reactions of benzoylformate esters with alkenes can be directed toward either Paternò-Büchi cycloadditions or allylic functionalization reactions under conditions favoring energy transfer or electron transfer, respectively. These studies provide a framework for designing other divergent photocatalytic methods that produce different sets of reaction outcomes under photoredox and triplet sensitization conditions.

Full text of DOI:10.1021/acs.orglett.0c02314

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Letter
Divergent Photocatalytic Reactions of α‑Ketoesters under Triplet
Sensitization and Photoredox Conditions
Jian Zheng, Xiao Dong, and Tehshik P. Yoon*
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ABSTRACT: The long-lived triplet excited states of transition metal photocatalysts can activate organic substrates via either energy-
or electron-transfer pathways, and the rates of these processes can be influenced by rational tuning of the reaction conditions. The
characteristic reactive intermediates generated, however, are distinct and can exhibit very different reactivity patterns. This
mechanistic diversity available to photocatalytic reactions might thus offer an opportunity to engineer divergent reactions that give
markedly different chemical outcomes under superficially similar conditions. Herein, we show that the photocatalytic reactions of
̀
benzoylformate esters with alkenes can be directed toward either Paterno−Buchi cycloadditions or allylic functionalization reactions
̈
under conditions favoring energy transfer or electron transfer, respectively. These studies provide a framework for designing other
divergent photocatalytic methods that produce different sets of reaction outcomes under photoredox and triplet sensitization
conditions.
discovery of compounds with new chemical and biological
he development of new photocatalytic reactions has
Trecently reemerged as an important theme of research in properties.
synthetic chemistry.1 The most common photocatalysts
utilized for these methods belong to a family of Ru and Ir
polypyridyl complexes. Several features make these inorganic
complexes ideal for synthetic applications, including strong
absorption in the visible range, excellent chemical stability, and
predictable tunability across a wide range of excited-state redox
potentials and triplet-state energies.2 Most importantly, these
complexes undergo efficient intersystem crossing to produce
long-lived triplet excited states that can initiate useful organic
transformations via either energy-transfer (triplet sensitization)
or electron-transfer (photoredox) mechanisms. Though the
same photocatalytic triplet excited state is responsible for both
pathways, the reactive intermediates that are generated by each
are different and exhibit distinctive reactivity patterns. The
optimization of photocatalytic reactions, therefore, often
requires careful tuning of reaction conditions to ensure that
one pathway is favored to the exclusion of the other.3 The
mechanistic diversity available to photocatalytic reactions,
however, might also offer an opportunity to engineer divergent
reactions that give markedly different chemical outcomes
under superficially similar conditions. This goal would be
valuable because reactions that produce significant structural
diversity using a small set of simple reactants can facilitate the
Photoreactions of carbonyl compounds have been an
important topic of study throughout the history of organic
photochemistry, and the mechanisms of their reactions have
been extensively investigated.4 Among the oldest and best-
̀
known photocycloadditions is the Paterno−Bu
̈
chi reaction,5
which classically involves the combination of excited-state aryl
ketones with alkenes to afford structurally complex oxetane
products.6,7 As a starting point for our investigations, we
examined the photocatalytic reaction of benzoylformate esters.
Neckers has explored the photochemistry of these compounds
and demonstrated that they are excellent reactants for
̀
Paterno−Bu
̈
chi cycloadditions.8 The triplet energies of
benzoylformate esters have been reported to be ∼62
kcal/mol;9 we proposed, therefore, that irradiation of methyl
benzoylformate (1) and tetramethylethylene (2) in the
presence of a fluorinated Ir(III) complex with a high triplet-
Received: July 11, 2020
https://dx.doi.org/10.1021/acs.orglett.0c02314
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
Scheme 1. Divergent Outcomes of Triplet Sensitization and Photoredox Reactions of Benzoylformate Esters
state energy would generate long-lived triplet 1* via Dexter
energy transfer (Scheme 1, left). Subsequent [2 + 2]
cycloaddition via the standard stepwise biradical mechanism
̀
Table 1. Optimization Studies for Paterno−Bu
̈
chi
a
Cycloaddition
̀
of the Paterno−Buchi reaction would then afford oxetane 3a.
̈
Indeed, upon irradiation of 1 and 2 in acetonitrile with 465 nm
LEDs in the presence of 1 mol % [Ir(dF(CF3)ppy)2(dtbbpy)]-
10
BArF, Paterno−Bu
̀
̈
chi cycloadduct 3a was obtained in 74%
b
entry
photocatalyst
solvent
yield
yield.
Our laboratory has a long-standing interest in the use of
Lewis acid cocatalysts to increase the driving force for
photoinduced electron transfer to carbonyl-containing sub-
strates.11 We imagined that an appropriate Lewis acid could
accelerate electron transfer to ketoester substrate 1, out-
competing the rate of energy transfer (Scheme 1, right). The
resulting Ir(IV) complex could then oxidize electron-rich
alkene 2 to the corresponding radical cation (2•+).
Deprotonation would afford an allylic radical (5) that could
generate product either by combination with the persistent
Lewis acid stabilized ketyl radical ([Y]-1•−)12 or via a Lewis
acid catalyzed addition to another equivalent of ketoester.13
Under otherwise identical conditions used for the
1
2
3
4
5
6
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF (61)
[Ir(Fppy)2(dtbbpy)]PF6 (53)
[Ir(ppy)2(dMeObpy)]PF6 (51)
[Ir(ppy)2(dtbbpy)]PF6 (51)
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF
none
MeCN
MeCN
MeCN
MeCN
CH2Cl2
toluene
toluene
toluene
toluene
74%
40%
5%
6%
81%
92%
97%
0%
c
7
8
d
9
15%
a
Reactions conducted using 0.1 mmol of 1, 0.5 mmol of 2, 1 mol %
photocatalyst, and 5 mL of solvent and irradiated with a 16 W LED
lamp (465 nm) for 16 h unless otherwise noted. Yields were
determined by H NMR analysis. Values in parentheses represent
the photocatalyst triplet energies in kcal/mol. Reaction conducted in
1.5 mL of toluene. Reaction conducted in the dark.
b
1
c
d
̀
Paterno−Buchi reaction, addition of 15 mol % Y(OTf)3 and
̈
30 mol % phenanthroline led to the formation of 58% allylic
functionalization products as a 4:1 mixture of regioisomers (4a
and 4a′), along with 18% of the oxetane cycloadduct.
The observation that the partitioning of the reaction
(entries 5−7), with the highest yields observed in toluene. The
yield is increased at higher concentrations (entry 7). A control
experiment shows that no product is observed in the absence
̀
between Paterno−Buchi and allylic functionalization pathways
̈
̀
of light (entry 8); however, a slow Paterno−Buchi reaction was
̈
could be influenced by reaction conditions was intriguing, and
we elected to optimize each of these transformations
independently. First, Table 1 summarizes the results of studies
observed in the absence of photocatalyst, which results from a
tailing of the absorption of 1 into the visible region (entry 9).
Nevertheless, product formation in this reaction is dominated
by the photosensitized reaction pathway.
̀
to improve the yield of the Paterno−Buchi reaction. As
̈
expected for a triplet sensitization process, the yield of oxetane
is highest using high triplet energy photocatalysts14 (entries 1−
4). Also consistent with a triplet mechanism is the relative
insensitivity of the reaction toward the polarity of the solvent
Figure 1 summarizes an investigation of the scope of this
̀
photosensitized Paterno−Buchi cycloaddition. We first exam-
̈
ined the scope with respect to the ketoester reaction
component. Ester moieties bearing potentially reactive func-
B
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̀
Figure 1. Paterno−Bu
̈
chi scope studies. Reactions conducted using 0.5 mmol of 1, 2.5 mmol of 2, 1 mol % [Ir(dF(CF3)ppy)2(dtbbpy)]BArF, and
a
7.5 mL of toluene and irradiated with a 16 W LED lamp (465 nm) for 16 h unless otherwise noted. Reaction conducted using 0.5 mol %
b
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF. Reaction conducted for 24 h.
tional groups including alkenes (3d), alkynes (3e), halides
(3f), and azides (3g) are well-tolerated. The arene could also
be easily modified with various electron-donating and
-withdrawing groups (3h−3o), although ortho substituents
retard the reaction somewhat (3p). Heteroaroylformate esters
that participate are good substrates (3q and 3r). The scope
with respect to the alkene was examined next. The reaction was
fastest with relatively electron-rich alkenes, consistent with the
oxyl radical character of the (n,π*) triplet ketone intermediate,
although disubstituted aliphatic alkenes afforded reasonable
yields of the oxetane cycloadduct (3s and 3t). In all cases, the
regioselectivity of the reaction could be predicted by
considering the stability of the intermediate 1,4-diradical
expected from the conventional stepwise mechanism of the
̀
Paterno−Buchi cycloaddition (3u−3x).
̈
As a further validation of the triplet mechanism of this
photoreaction, we wondered if other canonical photoreactions
of (n,π*) triplet ketones could be accessed using the
Figure 2. Norrish type II reactions under energy-transfer conditions.
Reactions conducted using 0.5 mmol of ketoester, 1 mol %
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF, and 7.5 mL of toluene and
irradiated using a 16 W LED lamp (465 nm) for 16 h.
̀
conditions optimized for the Paterno−Buchi cycloaddition.
̈
We targeted the Norrish−Yang photocyclization15,16 of ortho-
methyl benzoylformates17 to access benzofused cyclobutanes
that are important structural elements of bioactive drugs and
natural products.18 Indeed, a variety of ortho-methyl
benzoylformates undergo smooth transformation to the
benzocyclobutenol products 6a−6h (Figure 2).
improve the yield of this reaction (entries 4−7). Indeed,
[Ir(ppy)2(dMeObpy)]PF6 gave exclusively the allylic function-
alization product in 89% yield (entry 7). Control experiments
indicated that the photocatalyst, Lewis acid cocatalyst, and
light were all required for the formation of the allylic
functionalization product (entries 8−10).
We next undertook studies to optimize the photoredox
allylic functionalization process (Table 2). We speculated that
the product distribution might be diverted completely toward
allylic functionalization by appropriately tuning the identity of
the photocatalyst and Lewis acid cocatalyst to maximize the
rate of photoredox activation and minimize the rate of energy
transfer. First, a screen of alternate ligands for Y(OTf)3
revealed that a slightly more electron-deficient 5-chlorophe-
nanthroline ligand afforded somewhat improved yields of the
allylic functionalization product (entries 1−3). We next
examined photocatalysts with lower triplet energy and more
negative excited-state reduction potentials that might further
Scope studies exploring the generality of this allylic
functionalization are summarized in Figure 3. All of the
̀
benzoylformates examined in the Paterno−Buchi reaction
̈
performed well in this transformation but afforded the allylic
functionalization products (4a−4p) to the exclusion of the
previously observed oxetanes. In general, we obtained relatively
moderate regioselectivity for the formation of the tetrasub-
stituted olefin product over the 1,1-disubstituted alkene
isomer. However, those benzoylformates bearing bulky ortho
substituents gave better selectivity (4p−4v). Importantly, in
C
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a
addition of Y(OTf)3 and phen results in a positive shift to
−0.70 V, which is easily within the range accessible by the
excited states of both [Ir*(dF(CF3)ppy)2(dtbbpy)]+ (−0.89
V)10 and the more strongly reducing [Ir*(ppy)2(dMeObpy)]+
(−1.26 V). Second, this mechanism predicts that the oxidation
of tetramethylethylene (+1.45 V) by the oxidized [Ir(ppy)2-
(dMeObpy)]2+ catalyst (+1.21 V) would be slightly ender-
gonic by 5.5 kcal/mol. Consistent with this expectation, less-
electron-rich trisubstituted alkenes such as 7 (+1.98 V)19 fail to
give any C−H functionalization products (Scheme 2A).20 On
the other hand, electron-rich methallylsilane 8 (+1.24 V)21
participates readily in this reaction to afford Sakurai addition
product 9 in 90% yield (Scheme 2B).
Table 2. Optimization Studies for Allylic Functionalization
entry
photocatalyst
ligand
phen
bpy
5-Cl-phen
5-Cl-phen
5-Cl-phen
5-Cl-phen
5-Cl-phen
5-Cl-phen
none
yield
r.r.
Scheme 2. Support for a Photoredox Mechanism
1
2
3
4
5
6
7
8
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF
[Ir(dF(CF3)ppy)2(dtbbpy)]BArF
[Ru(bpy)3](PF6)2
58%
32%
62%
19%
78%
85%
89%
1%
4:1
4:1
4:1
4:1
4:1
4:1
4:1
−
Ir(ppy)3
[Ir(Fppy)2(dtbbpy)]PF6
[Ir(ppy)2(dMeObpy)]PF6
none
[Ir(ppy)2(dMeObpy)]PF6
[Ir(ppy)2(dMeObpy)]PF6
b
9
2%
0%
−
−
c
10
5-Cl-phen
a
Reactions conducted using 0.1 mmol of 1, 0.5 mmol of 2, 1 mol %
photocatalyst, 15 mol % Y(OTf)3, 30 mol % ligand, and 1.5 mL of
MeCN and irradiated with a 16 W LED lamp (465 nm) for 16 h
unless otherwise noted. Yields were determined by 1H NMR analysis.
b
c
Reaction conducted without Y(OTf)3. Reaction conducted in the
dark.
In summary, we have shown that divergent reactivity is
accessible in the photocatalytic reaction of benzoylformates
with alkenes. Under conditions that favor energy transfer, the
these cases, we observed no trace of the Norrish−Yang
cyclization products.
̀
triplet-state benzoylformate undergoes smooth Paterno−Buchi
̈
Several lines of evidence support our proposal that the allylic
functionalization reaction involves a photoredox process. First,
cyclic voltammetry performed with benzoylformate 1 showed a
quasireversible reduction wave at −1.28 V vs SCE. The
cycloaddition to afford complex oxetane scaffolds. Under
photoredox conditions, on the other hand, the same starting
materials undergo selective transformation to allylic function-
alization products. These results provide a valuable demon-
Figure 3. Allylic functionalization scope studies. Reactions conducted using 0.5 mmol of 1, 2.5 mmol of 2, 1 mol % [Ir(ppy)2(dMeObpy)]PF6, 15
mol % Y(OTf)3, 30 mol % 5-Cl-phen, and 7.5 mL of toluene and irradiated with a 16 W LED lamp (465 nm) for 16 h unless otherwise noted.
Values in parentheses represent the ratio of regioisomers. aReaction conducted for 32 h. bReaction conducted for 24 h. cReaction conducted using 1
mol % [Ir(Fppy)2(dtbbpy)]PF6.
D
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ASSOCIATED CONTENT
* Supporting Information
■
sı
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c02314.
Detailed experimental procedures, full spectroscopic
data for all new copounds (PDF)
(4) (a) Dinda, B. Photochemistry of Carbonyl Compounds. In
Essentials of Pericycli cand Photochemical Reactions, Vol. 93; Springer:
Switzerland, 2017; pp 241−275. (b) Dantas, J. A.; Correia, J. T. M.;
FAIR data, including the primary NMR FID files, for
compounds 3a−x, 4a−v, 6a−h, 9, and S1−S10 (ZIP)
̂
Paixao, M. W.; Correa, A. G. Photochemistry of Carbonyl
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Compounds: Application in Metal-Free Reactions. ChemPhotoChem.
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AUTHOR INFORMATION
Corresponding Author
■
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Tehshik P. Yoon − Department of Chemistry, University of
WisconsinMadison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0002-3934-4973; Email: tyoon@
chem.wisc.edu
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Jian Zheng − Department of Chemistry, University of
WisconsinMadison, Madison, Wisconsin 53706, United
States; orcid.org/0000-0002-4698-3407
Xiao Dong − Department of Chemistry, University of
WisconsinMadison, Madison, Wisconsin 53706, United
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Soc. 2019, 141, 8741−8745. (c) Mateos, J.; Vega-Penaloza, A.;
̃
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The authors declare no competing financial interest.
An early preprint of this research appeared on ChemRxiv.22
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ACKNOWLEDGMENTS
■
We dedicate this paper to Prof. Ilhyong Ryu (Osaka Prefecture
University) on the occasion of his 70th birthday. We are
grateful to Katie Rykaczewski and Prof. Corinna Schindler
(University of Michigan) for sharing data. Funding for this
project was provided by the NIH (GM095666). NMR and MS
facilities at UWMadison are funded by the NIH (1S10
OD020022-1) and a generous gift from the Paul J. and
Margaret M. Bender Fund.
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