Visible-Light-Promoted Generation of α-Ketoradicals from Vinyl-bromides and Molecular Oxygen: Synthesis of Indenones and Dihydroindeno[1,2-c]chromenes

Article abstract of DOI:10.1002/anie.201702953

Ortho-alkynylated α-bromocinnamates can be converted by a visible-light-mediated photocascade reaction with molecular oxygen into either indenones or dihydroindeno[1,2-c]chromenes. The one-step process features key photochemical steps, that is, the initial activation of vinyl bromides through energy transfer to give α-ketoradicals in a reaction with molecular oxygen, followed by α-oxidation of an arene moiety by 6-π electrocyclization, and subsequent hydroxylation by an electron-transfer process from the same photocatalyst leads to the dihydroindeno[1,2-c]chromenes.

Full text of DOI:10.1002/anie.201702953

A Journal of the Gesellschaft Deutscher Chemiker
Angewandte
Chemie
International Edition
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Accepted Article
Title: Visible-light Promoted Generation of alpha-Ketoradicals from
Vinyl-bromides and Molecular Oxygen: Synthesis of Indenones
and Dihydroindeno[1,2-c]chromenes
Authors: Santosh K. Pagire, Peter Kreitmeuer, and Oliver Reiser
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201702953
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Angewandte Chemie International Edition
DOI: 10.1002/anie.200((will be filled in by the editorial staff))
Photoredox Catalysis
Visible-Light-Promoted Generation of α-Ketoradicals from Vinyl-
bromides and Molecular Oxygen: An Unprecedented Synthesis of
Indenones and Dihydroindeno[1,2-c]chromenes**
Santosh K. Pagire, Peter Kreitmeier, and Oliver Reiser*
Dedicated to Emeritus Prof. M. S. Wadia on the occasion of his 80th birthday
Abstract: Ortho-alkynylated α-bromocinnamates can be converted
by a visible light mediated photocascade reaction with molecular
oxygen either to indenones or to dihydroindeno[1,2-c]chromenes.
The one-step process features key photochemical steps, i.e. the
initial activation of vinyl bromides by energy transfer giving rise to
α-ketoradicals in the reaction with molecular oxygen followed by α-
oxidation of an arene moiety by 6-π electrocyclization, and
subsequent hydroxylation by electron transfer process from the
same photocatalyst leads to the dihydroindeno[1,2-c]chromenes.
guously supported by mechanistic studies and control experiments
(vide infra).
Scheme 1: Photocatalyzed synthesis of indenones
dihydroindeno[1,2-c]chromenes 3
2
and
OH
O
EtO2C
EtO2C
O
O2
CO2Et O2
R1
R
Br
R
R
Z
R1
[Ir]
visible light
Z
[Ir]
visible light
Z
O
O
1
2: 49 - 68%
3: 38 - 63%
R1
R/R1 = EDG
R/R1 = EWG
Visible-light photoredox catalysis not only offers an
alternate pathway to established transformations triggered by radical
initiators such as peroxides and n-Bu3SnH under thermal conditions
but holds the promise to arrive at new bond forming reactions owing
to the mild reaction conditions and functional group compatibility
under which photocatalysts generally operate.[1] Recently, we found
that α-bromochalcones can be coupled with a broad variety of
electron rich heteroarenes[2] or alkenes[3] giving rise to polycyclic
aromatic scaffolds. However, photochemical activation of vinyl
halides is rare, requiring either an extended π-system or weak bonds
as found in vinyl iodides.[4]
Exposing 1a (Ered = –1.38 V vs. SCE, see SI) in an oxygen atmos-
phere to irradiation with 5 mol % [Ir{dF(CF3)ppy}2(dtb-bpy)]PF6
[9]
(EIr(IV)/Ir(III)* = –0.89 V vs. SCE; excited state life-time (τ) = 2300
ns),[7a] [dF(CF3)ppy = 2-(2,4-difluorophenyl)-5-trifluoromethylpyri-
dine, dtb-bpy = 4,4’-di-tert-butyl-2,2’-dipyridyl]) with blue light
(LED455, Table 1, entry 1), indeed resulted in a productive trans-
formation, leading to 2a and 3a. In contrast, only E/Z-isomerization
was observed in the absence of oxygen (Table 1, entry 2), which is
in agreement with the proposed activation (vide infra) of vinyl bro-
mides 1 by the photocatalyst via a diradical intermediate generated
by a sensitization process.[10]
In general, vinyl bromides have much higher reduction
potentials than typical photocatalysts in their excited states[2,5,6] (see
also SI). Thus, ruthenium or iridium-based photocatalysts generally
do not possess a sufficient excited-state reduction potential to
transfer an electron and ultimately form a vinyl radical after bromide
extrusion by an oxidative quenching cycle. In contrast, utilizing the
reductive quenching cycle for the activation of vinyl bromides by
having an amine present as a sacrificial electron donor results in
simple dehalogenations.[2,6,7]
Table 1: Synthesis of 2a and 3a: Catalyst screening and reaction
optimization[a]
As an alternative to a direct photo-induced electron
Yield
Temp.
(%)[b]
transfer, substrates can also be activated by energy transfer from the
excited photocatalysts.[8] In line with such a mechanism, we report
here the photochemical activation of vinyl bromides that is utilized
in the coupling of ortho-alkynylated α−bromocinnamates 1 and mo-
lecular oxygen to give rise to indenones 2 or to dihydroindeno[1,2-
c]chromenes 3, depending on the electronic prerequisites of the
substrates. As the key step in this process, the transformation of the
vinyl bromides into α-ketoradicals is proposed, which is unambi-
Entry
Photocatalyst (mol %)
(oC)
rt
2a
3a
01
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (5)
16
41
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (5)
no oxygen
02[c,d]
rt
-
-
03
04
05
06
[Ir(ppy)2(dtb-bpy)]PF6 (5)
fac[Ir(ppy)3] (5)
rt
rt
10
14
17
20
22
33
38
61
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (1)
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (2)
rt
[∗]
M.Sc. S. K. Pagire, Dr. P. Kreitmeier, and Prof. Dr. O. Reiser
Institut für Organische Chemie, Universität Regensburg,
Universitätsstraße 31, 93053 Regensburg (Germany).
E-mail: oliver.reiser@chemie.uni-regensburg.de
60
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (1)
no light
07
rt
-
-
08
09
10
11
12
13
no catalyst
rt
rt
rt
rt
rt
rt
-
5
3
-
-
26
8
-
[∗∗] This work was supported by DAAD (fellowship for SKP) and
DFG (GRK-1626 Photocatalysis). We are grateful to Dr. I.
Ghosh, Mr. R. Weinzierl, and Mr. P. Bayer for carrying out Stern-
Volmer quenching, ESR, and micro(flow)reactor experiments,
respectively. We also thank Dr. M. Bodensteiner and Dr. U.
Chakraborty for single crystal X-ray analysis. Mr. A. Hossain and
Mrs. R. Hoheisel are further acknowledged for helpful
discussions on the reaction mechanism and cyclic-voltammetry
experiments, respectively. Helpful comments of the reviewers are
also gratefully acknowledged.
Ru(bpy)3Cl2 (5)
Eosin Y (5), 530 nm
[Cu(dap)2Cl] (5), 530 nm
Rose Bengal (5), 530 nm
ZnCl2/porphyrin (5), 530 nm
-
-
-
-
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (5),
DIPEA (3 equiv)
14[e]
rt
-
-
Table 1: Reaction Conditions: [a] Substrate 1a (0.5 mmol), photocatalyst (1-5 mol %),
O2, DMF, 72 h, rt, 60 oC, LED455. [b] Isolated yields. [c] Under nitrogen. [d] E/Z-
isomerization of 1a, Z/E = 81:19 (before irradiation) to Z:E = 50:50 (after irradiation).
[e] Dehalogenation product was identified by crude 1H-NMR analysis.
Supporting information for this article is available on the WWW
under http://www.angewandte.org from the author
1
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10.1002/anie.201702953
Angewandte Chemie International Edition
[11]
Other iridium photocatalysts such as [Ir(ppy)2(dtb-bpy)]PF6
Table 3: Synthesis of dihydroindeno[1,2-c]chromenes 3[a]
(EIr(IV)/Ir(III)* = –0.96 V vs. SCE, τ = 557 ns) [ppy = 2-phenyl-
pyridine, dtb-bpy = 4,4’-di-tert-butyl-2,2’-dipyridyl] and neutral
fac[Ir(ppy)3][12] (EIr(IV)/Ir(III)* = –1.73 V vs. SCE, τ = 1900 ns) [ppy =
2-phenylpyridine], (entries 3, 4) gave a similar product distribution,
however, the product yields were lower.
In all cases we noted a deep red coloring of the solution in the
course of the reaction, probably blocking the photo process, which
could explain the incomplete conversion and long reaction time of
the overall transformation. Lowering the catalyst concentration to 1
mol % (entry 5) gave almost identical results, indicating that catalyst
deactivation was not the limiting factor of the reaction. A significant
increase in yield was observed upon carrying out the reaction at 60
°C, which was found to be optimal in combination with 2 mol %
catalyst loading (entry 6). In the absence of the iridium catalyst or
light, no conversion of 1a was observed (entries 7, 8).
Other photocatalysts such as [Ru(bpy)3Cl2][13] (ERu(III)/Ru(II)*
=
–0.81 V vs. SCE, τ = 1100 ns) (bpy = 2,2’-bipyridine), Eosin Y,[14]
and [Cu(dap)2Cl][15] (ECu(II)/Cu(I)* = –1.43 V vs. SCE, τ = 270 ns) [dap
= 2,9-bis(p-anisyl)-1,10-phenanthroline], gave rise to low yields of
2a/3a (Table 1, entries 9-11). Furthermore, no reaction was
observed in the presence of rose bengal or ZnCl2 / porphyrin, ruling
out that the process is driven by singlet oxygen (Table 1, entries 12-
13). A reductive quenching experiment by adding a tertiary amine
(e.g. diisopropylethylamine, DIPEA) as a sacrificial electron donor
(Table 1, entry 14), thus utilizing ground state reduction potentials
of [Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (EIr(III)/Ir(II) = –1.37 V vs. SCE),
only resulted in direct dehalogenation (99%).
Table 4: Substrates 1, leading to a mixture of 2 and 3[a]
Under the optimized reaction conditions (Table 1, entry 6), a
variety of ortho-alkynylated α-bromocinnamates 1 (employed as
E/Z mixtures) was subsequently tested (Table 2-4). A pronounced
electronic effect was observed, which determined the overall course
of the reaction: While α-bromocinnamates bearing +I (inductive
donor) or +M (mesomeric / resonance donor) substituents yielded
exclusively indenones 2b-k (Table 2), derivatives with only electron
withdrawing substituents gave rise to dihydroindeno[1,2-c]chro-
menes 3b-h (Table 3). An ortho substituent R1 overrides the
electronic preference (cf. 2l, Table 2 and 3i, Table 3). However, a
mixture of both products 2 and 3 was observed for electronically
unbiased substrates 1 (Table 4). Notably, carrying out the reaction in
a rotating film reactor enhanced the rate of the reaction (from 72
h/60 °C to 24 h/25 °C, see SI for more experimental details) due to
the larger surface area of the resulting reaction mixture that was
exposed to the oxygen atmosphere and visible-light irradiation (cf
Table 2-4). Furthermore, performing the reaction in the advanced
micro(flow)reactor systems[16] provides comparative yields but in
very short reaction times of 15-40 minutes (Tables 2 and 3; also see
the SI for detailed experimental set-up).
Tables 2-4: Reaction Conditions: [a] Substrate 1 (0.5 mmol, 1.0 equiv), photocatalyst
(2 mol %), O2 at 60 oC, in DMF, 72 h, LED455, isolated yields of separated 2 and 3 are
stated. [b] Reaction performed on rotating film reactor with 1 mol % photocatalyst, rt,
24 h. [c] 2.0 mmol scale reaction. [d] Reaction performed on micro(flow)reactor with 1
mol % photocatalyst, 40 min at rt (or 15 min at 60 oC). Compound color is mentioned in
parentheses.
The mechanistic picture of this transformation is complex (Figure 1).
The fluorescence quenching studies (see SI) revealed that the
starting material 1a strongly interacts with the photocatalysts. A
direct photoelectron transfer from the iridium(III)-catalyst to 1 is
thermodynamically unfavorable due to the large difference in
reduction potentials of substrates 1 and the employed photocatalyst
(vide supra). We, therefore, propose the generation of the activated
vinyl bromide species I* (a more stable diradical species / a triplet
excited state)[10,17] by energy transfer from the excited *Ir(III)-
catalyst to 1. Noteworthy, the [2+2]-cycloaddition of 1 was not
observed in contrast to the non-brominated analog 6 (Scheme 2, B).
Also, no oxidation of the triple bond in 6 to the diketone occurs
under the reaction conditions,[18] thus, the reaction of I* via attack of
oxygen on the triple bond appears to be unlikely. Instead, I* loses
bromine Br• to form the critical vinyl radical II. Evidence for II is
found by a trapping experiment with TEMPO giving rise to 9 as
well as the 5-exo-trig cyclization reaction of 4 to 5 (Scheme 2, A).
In addition, in the ESR spectra also a single radical species (ge =
1.984) was detected under inert atmospheric conditions at low
temperature (See SI). In contrast, a direct 5-exo-dig or 6-endo-dig
cyclization of II was not feasible, probably due to the unfavorable
orientation of the reacting groups (See SI for the crystal structure of
1c). Nevertheless, the vinyl radical intermediate II is capable of
trapping molecular oxygen to give rise to intermediate III, which
subsequently breaks down to the α-ketoradical IV. The independent
Table 2: Synthesis of indenones 2[a]
2
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10.1002/anie.201702953
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Scheme 2: Control experiments
generation of IV from 8 by photoinduced electron transfer in the
presence of oxygen indeed gives rise to 2a and 3a (Scheme 2, C). It
should be noted, however, that the conversion of 8 to 2a and 3a
requires stoichiometric amounts of photocatalyst since the
photoredox cycle cannot be closed in the absence of a sacrificial
electron donor. The α-ketoradical IV then undergoes 5-endo-dig
rather than 4-exo-dig cyclization[19] followed by trapping by a
second molecule of oxygen to V. Elimination of water then leads to
the observed indenones 2. An isotope labeling experiment with pure
18O2 and a mixture of 18/16O2 proved that the oxygen atoms
incorporated into the indenones 2 indeed stem from two different
molecules of molecular oxygen (Scheme 3).
Figure 1: Proposed Reaction Mechanism
Reaction conditions: [a] Substrates 4,
6
or
8
(0.5 mmol, 1.0 equiv),
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (2 mol %), O2 (or under N2), at 60 oC (or at rt), in
anhydrous DMF, 24-72 h, LED455
. [b] Substrate 8 (0.5 mmol, 1.0 equiv),
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (2 mol %), DIPEA (2.2 equiv), O2, at 60 oC, in
anhydrous DMF, 72 h. [c] Substrate 8 (0.05 mmol, 1.0 equiv), [Ir{dF(CF3)ppy}2(dtb-
bpy)]PF6 (100 mol %), O2, at 60 oC, in anhydrous DMF, 12 h.
Scheme 3: Isotope labeling experiments
Indenones 2 having electron withdrawing substituents in either
aromatic ring undergo facile 6π-electrocyclization, leading to
dihydroindeno[1,2-c]chromenes 3. The 18O2 labeling experiment
(Scheme 3) revealed that the termination of the electrocyclization by
the introduction of a hydroxyl group also was ultimately derived
from molecular oxygen.[14,20]
Scheme 4: 6π-electrocyclization
Independent
experiments
under
visible
light
irradiation/photocatalyst or IV irradiation in the presence of oxygen
(Scheme 2) with indenone 2 showed the feasibility of the 6π-
electrocyclization reaction.[21] However, the yields obtained by this
process were low compared to starting from the corresponding
compounds 1.
However, the electronic effect favouring electron poor starting
materials 1 to preferentially undergo the cyclization to 3 rather than
to 2 remains unclear at this point.
Conditions A: Substrate 2 (0.25 mmol, 1.0 equiv), [Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 (2
mol %), O2, at 60 oC, in DMF, 72 h, LED455. Conditions B: Substrate 2 (0.25 mmol, 1.0
equiv), UV-Light (125 W Hg-lamp, λ > 300 nm), O2, at 60 oC, in anhydrous DMF, 72 h.
3
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10.1002/anie.201702953
Angewandte Chemie International Edition
[16] For advanced micro(flow) reaction set-up, see: J. Schachtner, P.
Bayer, A. Jacobi von Wangelin, Beilstein J. Org. Chem. 2016, 12,
1798.
[17] For diradical stabilization on alkyne system, see: R. K. Mohamed,
S. Mondal, K. Jorner, T. F. Delgado, V. V. Lobodin, H. Ottosson,
I. V. Alabugin, J. Am. Chem. Soc. 2015, 137, 15441.
[18] For the photochemical oxidation of alkynes by molecular oxygen
see: H-T- Qin, X. Xu, F. Liu, ChemCatChem 2017, 9, 1409.
[19] For a discussion on 4-exo-dig vs 5-endo-dig radical cyclization,
see: a) J. E. Baldwin, J. C. S. Chem. Comm. 1976, 734; b) I. V.
Alabugin, K. Gilmore, M. Manoharan, J. Am. Chem. Soc. 2011,
133, 12608; c) K. Gilmore, I. V. Alabugin, Chem. Rev. 2011, 111,
6513.
[20] For oxygen anion radical generation by photocatalyst, see: a) S.
Zhu, A. Das, L. Bui, H. Zhou, D. P. Curran, M. Rueping, J. Am.
Chem. Soc. 2013, 135, 1823; b) S. K. Han, T.-M. Hwang, Y.
Yoon, J.-W. Kang, Chemosphere 2011, 84, 1095; c) D. C. Fabry,
M. Rueping, Acc. Chem. Res. 2016, 49, 1969; d) Y. Cheng, J.
Yang, Y. Qu, P. Li, Org. Lett. 2012, 14, 98.
In summary, we have shown for the first time the
generation of α-ketoradicals from vinyl bromides and molecular
oxygen by energy transfer process operated by an Ir-F photocatalyst.
The resulting vinyl radical II, being formed from a diradical
intermediate upon bromine extrusion, could be engaged in a
productive reaction cascade with up to three molecules of molecular
oxygen, leading to complex molecule architectures in a single
photochemical step under the mild reaction conditions. The newly
obtained tetracyclic compounds 3 could serve as valuable building
blocks in medicinal chemistry. Such compounds having a hetero-
atom impregnated into the [6-5-6]-ABC skeleton were previously
advertised as Taiwaniaquinoids related scaffolds by Panda et al,[22]
which are closely related to the naturally occurring family of Tai-
waniaquinoids possessing a common [6-5-6]-abeo-abietane carbon
architecture with broad biological activities.[23]
Received: ((will be filled in by the editorial staff))
Published online on ((will be filled in by the editorial staff))
Keywords: Photocascade, dioxygen activation, α-ketoradicals,
indenones, Taiwaniaquinoids
[21] For
photochemical
6π-electrocyclization,
see:
a)
O.
Anamimoghadam, M. D. Symes, C. Busche, D.-L. Long, S. T.
Caldwell, C. Flors, S. Nonell, L. Cronin, G. Bucher, Org. Lett.
2013, 15, 2970; b) A. C. Hernandez-Perez, S. K. Collins, Acc.
Chem. Res. 2016, 49, 1557; c) S. Parisien-Collette, A. C.
Hernandez-Perez, S. K. Collins, Org. Lett. 2016, 18, 4994; d) O.
Kikuchi, Tet. Lett. 1981, 22, 859.
References:
[1] For recent leading reviews and perspectives on photocatalysis, see:
a) D. Ravelli, S. Protti, M. Fagnoni, Chem. Rev. 2016, 116, 9850;
b) O. Reiser, Acc. Chem. Res. 2016, 49, 1990; c) N. A. Romero, D.
A. Nicewicz, Chem. Rev. 2016, 116, 10075; d) M. H. Shaw, J.
Twilton, D. W. C. MacMillan, J. Org. Chem. 2016, 81, 6898; e) T.
P. Yoon, M. A. Ischay, J. Du, Nat. Chem. 2010, 2, 527.
[22] For Taiwaniaquinoid related scaffolds, see: a) R. Singh, G. Panda,
Org. Biomol. Chem. 2011, 9, 4782; b) R. Singh, M. K. Parai, G.
Panda, Org. Biomol. Chem. 2009, 7, 1858.
[23] For Taiwaniaquinoids, see: a) G. Majetich, J. M. Shimkus, J. Nat.
Prod. 2010, 73, 284; b) G. Liang, Y. Xu, I. B. Seiple, D. Trauner,
J. Am. Chem. Soc. 2006, 128, 11022.
[2] S. Paria, O. Reiser, Adv. Synth. Catal. 2014, 356, 557.
[3] S. Paria, V. Kais, O. Reiser, Adv. Synth. Catal. 2014, 356, 2853.
[4] J. D. Nguyen, E. M. D'Amato, J. M. R. Narayanam, C. R. J.
Stephenson, Nat. Chem. 2012, 4, 854.
[5] For redox potentials and excited state lifetimes of the
photocatalysts, see: a) H. G. Roth, N. A. Romero, D. A. Nicewicz,
Synlett 2016, 27, 714; b) C. K. Prier, D. A. Rankic, D. W. C.
MacMillan, Chem. Rev. 2013, 113, 5322.
[6] S. K. Pagire, O. Reiser, Green Chem. 2017, 19, 1721.
[7] T. Maji, A. Karmakar, O. Reiser, J. Org. Chem. 2011, 76, 736.
[8] For
the
significance
of
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6
photocatalyst in energy transfer process, see: a) Z. Lu, T. P. Yoon,
Angew. Chem. Int. Ed. 2012, 51, 10329; For selected energy
transfer examples, see: b) N. J. Turro, J. Chem. Educ. 1966, 43,
13; c) W. L. Dilling, Chem. Rev. 1969, 69, 845; d) M. Wrighton, J.
Markham, J. Phys. Chem. 1973, 77, 3042; e) E. Brachet, T.
Ghosh, I. Ghosh, B. König, Chem. Sci. 2015, 6, 987; f) S. K. Han,
T.-M. Hwang, Y. Yoon, J.-W. Kang, Chemosphere 2011, 84,
1095.
[9] For the synthesis and photophysical properties of
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 photocatalyst, see: M. S. Lowry, J.
I. Goldsmith, J. D. Slinker, R. Rohl, R. A. Pascal, G. G. Malliaras,
S. Bernhard, Chem. Mater. 2005, 17, 5712.
[10] For E/Z-isomerization of olefins via diradical formation, see: J. B.
Metternich, R. Gilmour, J. Am. Chem. Soc. 2015, 137, 11254.
[11] J. D. Slinker, A. A. Gorodetsky, M. S. Lowry, J. Wang, S. Parker,
R. Rohl, S. Bernhard, G. G. Malliaras, J. Am. Chem. Soc. 2004,
126, 2763.
[12] J. Sun, W. Wu, J. Zhao, Chem. Eur. J. 2012, 18, 8100.
[13] F. Teplý, Collect. Czech. Chem. Commun. 2011, 76, 859.
[14] D. P. Hari, B. Konig, Chem. Comm. 2014, 50, 6688.
[15] For Cu-photocatalysis, see: a) J. M. Kern, J. P. Sauvage, J. Chem.
Soc. Chem. Commun. 1987, 546; b) S. Paria, O. Reiser,
ChemCatChem 2014, 6, 2477; c) S. K. Pagire, S. Paria, O. Reiser,
Org. Lett. 2016, 18, 2106; d) M. Pirtsch, S. Paria, T. Matsuno, H.
Isobe, O. Reiser, Chem. Eur. J. 2012, 18, 7336.
4
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10.1002/anie.201702953
Angewandte Chemie International Edition
Photoredox Catalysis
Santosh K. Pagire, Peter Kreitmeier,
and Oliver Reiser*
“Visible-Light-Promoted Generation of α-
Ketoradicals from Vinyl-bromides and
Molecular Oxygen: An Unprecedented
Synthesis of Indenones and Dihydro-
indeno[1,2-c]chromenes”
The activation of vinyl bromides by visible light photoredox catalysis and molecular oxygen
is developed utilizing dual energy and electron transfer mechanisms using
[Ir{dF(CF3)ppy}2(dtb-bpy)]PF6 as a photocatalyst. Following this photocascade reaction
mode, the synthesis of indenones or dihydroindeno[1,2-c]chromenes is possible from readily
available ortho-alkynylated α-bromocinnamates.
5
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