Photoinduced Hydroarylation and Cyclization of Alkenes with Luminescent Platinum(II) Complexes

Article abstract of DOI:10.1002/anie.202011841

Photoinduced hydroarylation of alkenes is an appealing synthetic strategy for arene functionalization. Herein, we demonstrated that aryl radicals generated from electron-deficient aryl chlorides/bromides could be trapped by an array of terminal/internal aryl alkenes in the presence of [Pt(O^N^C^N)] under visible-light (410 nm) irradiation, affording anti-Markovnikov hydroarylated compounds in up to 95 % yield. Besides, a protocol for [Pt(O^N^C^N)]-catalyzed intramolecular photocyclization of acrylanilides to give structurally diverse 3,4-dihydroquinolinones has been developed.

Full text of DOI:10.1002/anie.202011841

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Accepted Article
Title: Photoinduced Hydroarylation and Cyclization of Alkenes with
Luminescent Platinum(II) Complexes
Authors: Chi-Ming Che, Hanchao Cheng, Tsz-Lung Lam, Yungen Liu,
and Zhou Tang
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10.1002/anie.202011841
Angewandte Chemie International Edition
RESEARCH ARTICLE
Photoinduced Hydroarylation and Cyclization of Alkenes with
Luminescent Platinum(II) Complexes
Hanchao Cheng,[a,b] Tsz-Lung Lam,[c] Yungen Liu,[a] Zhou Tang[c] and Chi-Ming Che*[a,c,d]
[a]
Dr. H. Cheng, Prof. Dr. Y. Liu, Prof. Dr. C.-M. Che
Department of Chemistry
Southern University of Science and Technology
Shenzhen 518055, Guangdong P. R. China
E-mail: cmche@hku.hk
[b]
Dr. H. Cheng
Hefei National Laboratory for Physical Sciences at Microscale
Department of Chemistry
University of Science and Technology of China
Hefei 230026, P. R. China
[c]
[d]
Dr. T.-L. Lam, Dr. Z. Tang, Prof. Dr. C.-M. Che
State Key Laboratory of Synthetic Chemistry, Department of Chemistry
University of Hong Kong
Pokfulam Road, Hong Kong, P. R. China
Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation
Shenzhen, Guangdong 518057, P. R. China
Supporting information for this article is given via a link at the end of the document.
Abstract: Photoinduced hydroarylation of alkenes is an appealing
synthetic strategy for arene functionalization. Herein, we
demonstrated that aryl radicals generated from electron-deficient aryl
chlorides/bromides could be trapped by an array of terminal/internal
aryl alkenes in the presence of [Pt(O^N^C^N)] under visible-light (410
nm) irradiation, affording anti-Markonikov hydroarylated compounds
in up to 95% yield. Besides, a protocol for [Pt(O^N^C^N)]-catalyzed
intramolecular photocyclization of acrylanilides to give structurally
diverse 3,4-dihydroquinolinones has been developed.
diisopropylphenyl)perylene-3,4,9,10-bis(dicarboximide) (PDI)[7] or
by harnessing a photosensitization-initiated reductive activation
strategy using [Ru(bpy)3]2+ photosensitizer with pyrene as
photoreductant[8] for C–H arylation reactions (Scheme 1a). Jui[3d,e]
Introduction
Photoinduced hydroarylation of alkenes constitutes an appealing
method of C(sp2)–C(sp3) bond formation reactions for
arene/alkene functionalization,[1] which complements the
conventional reductive Heck coupling reactions.[2] Because of the
distinct mechanistic pathway, for intermolecular reactions, the
former approach, typically operating through radical addition
coupling, is highly anti-Markonikov selective while the latter
usually shows less satisfactory regioselectivity for non-directing
group bearing or sterically/electronically unbiased alkenes and
could be complicated by the competing Heck reactions. The key
step of photoredox hydroarylation reactions involves the
generation of reactive aryl radical via single electron reduction in
the presence of
a photocatalyst, which is subsequently
intercepted by an alkene coupling partner.[3] While aryl radicals
are often derived from pre-functionalized aryl substrates with
redox active groups such as aryl diazonium salts and
diaryliodonium salts,[4,5] examples of direct use of aryl
chlorides/bromides as radical precursor, despite their broad
commercial availability, are scarce, presumably due to their highly
negative reduction potentials and high C(sp2)–Cl/C(sp2)–Br bond
dissociation energies.[6] In this regard, König and co-workers
demonstrated the reductive activation of Ar–Br and Ar–Cl bonds
Scheme 1. Thermal and photoinduced inter- and intramolecular hyrdoarylation
reactions and chemical structures of Pt-1 and Pt-2.
by
photoexcited
radical
anion
of
N,N-bis(2,6-
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RESEARCH ARTICLE
recently reported that highly anti-Markonikov selective
intermolecular hydropyridylation of alkenes could be achieved
with bromopyridines using an iridium(III) photocatalyst and
Hantzsch ester as sacrificial reductant (Scheme 1b).
phosphorescent OLED emitters.[17] Their synthesis and
characterizations are detailed in Supporting Information. These
complexes show strong absorption band at 400–450 nm in
CH3CN with absorptivities of ca. 1×104 M–1 cm–1, attributed to an
admixture of 1MLCT (1[5dπ (Pt) →π* (O^N^C^N)]) and 1ILCT (1[π
(phenolate) → π* (N^C^N)]) transitions (Fig. 1). They exhibit
strong phosphorescence in deaerated CH3CN solution at room
temperature with emission quantum yield of 48% and 50%,
respectively. Pt-1 displays a broad emission band at 532 nm with
an emission lifetime of 3.2 s coming from a mixed 3MLCT/3ILCT
excited state while the vibronic-structured emission of Pt-2 with
maxima at 494, 526 and 564 nm with an emission lifetime of 15.0
s is attributed to 3LC excited state (Fig. 1).[17] The triplet energies
of Pt-1 and Pt-2 estimated from the emission onset are 2.57 and
2.63 eV, respectively. In cyclic voltammogram (CV), Pt-1 exhibits
a pair of quasi-reversible oxidation and reduction couples with E1/2
= +0.43 and –1.78 V vs SCE, respectively, in MeCN (Fig. S2).
These redox processes become irreversible in MeCN/H2O (6:1)
(Fig. S2). For Pt-2, both the first oxidation and reduction in
MeCN/CH2Cl2 (9:1, v/v) solution are irreversible, occurring at Epa
= +0.83 and Epc = –1.92 V vs SCE (Fig. S3). On the basis of
spectroscopic and electrochemical data, the excited state redox
potentials, E(Pt*/Pt–)/E(Pt+/Pt*), for Pt-1 and Pt-2 are estimated
to be +0.79/–2.14 and +0.71/–1.80 V vs SCE, respectively. Thus,
both Pt* and Pt– states of Pt-1 and Pt-2 are strong one-electron
reductant.
Apart from single electron transfer radical pathway, energy
transfer is also
photoinduced hydroarylation of alkenes. As examples, 3,4-
dihydroquinolinones, group of medicinally bioactive
a viable mechanism for intramolecular
a
compounds,[9] could be synthesized via 6π-photocyclization of
acrylanilides,[10] though a high-energy UV irradiation of 300–350
nm is necessary.[11] A recent work by Song and co-workers[12]
showed that this intramolecular cyclization could be accomplished
by a thermal Cu/Pd cooperative catalysis (Scheme 1c).
Luminescent platinum(II) complexes represent an emerging new
class of photosensitizers and photocatalysts for light-induced
organic transformations.[13] Because of substantial intraligand and
relatively small metal-to-ligand charge transfer character, the
emissive triplet excited states of tetradentate Pt(II) complexes,
3
commonly assigned as MLCT/3IL in nature, generally display a
longer lifetime than that of conventional luminescent Ru(II) and
Ir(III) complexes,[14] which could facilitate bimolecular
photochemical processes. Besides, the planar coordination
geometry of Pt(II) complexes with vacant axial coordination sites
allows for inner sphere substrate binding and trapping of radical
intermediate(s) via Pt-C/Pt-X (X = halogen)/Pt-H bond formation,
[13d,15]
thereby giving rise to opportunities for new photo-catalysis.
We previously reported that long-lived powerful photo-reductants
could be generated from phosphorescent tetradentate
platinum(II)-NHC complexes which could be used for photo-
induced reductive debromination of aryl bromides.[16] Herein, we
describe the use of two tetradentate [Pt(O^N^C^N)] complexes,
Pt-1 and Pt-2, as photocatalyst to achieve (i) aryl halides C–X (X
= Cl, Br) bond reductive activation to produce aryl radicals, which
could be trapped by terminal/internal aryl alkenes to afford various
We started our investigation of hydroarylation reaction by LED
irradiation (410 nm) of a mixture of ethyl 4-chlorobenzoate (a1)
anti-Markonikov
hydroarylated
compounds,
and
(ii)
photocyclization of triplet-sensitized acrylanilides to give 3,4-
dihydroquinolinones at room temperature under visible-light
irradiation of 410 nm (Scheme 1). Control experiments revealed
the superior efficiency of these Pt-catalyzed reactions over those
of [Ru(bpy)3]2+ and [Ir(ppy)3] photocatalysts (Table S1 and S2).
Results and Discussion
Complexes Pt-1 and Pt-2 were previously reported as efficient
Scheme 2. Condition A: Pt-1 (1 mol%), (hetero)aryl halide (0.2 mmol), alkene
i
(0.6 mmol), Cs2CO3 (0.4 mmol), Pr2NEt (0.24 mmol) in 6 mL of CH3CN/H2O
(6:1, v/v), under argon, irradiated by 410 nm LEDs (3 W x 4) for 10 h, isolated
[a]
yields.
Using Pt-1 (1 mol%), a1 (1 mmol) and b1 (1.2 mmol) in 10 mL of
CH3CN/H2O. [b] Using Pt-1 (0.5 mol%) a1 (2 mmol) and b1 (2.4 mmol) in 10 mL
Figure 1. Electronic absorption and emission spectra of Pt-1 (black) and Pt-2
of CH3CN/H2O.
(red) in CH3CN.
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10.1002/anie.202011841
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and 1,1-diphenylethylene (b1), complex Pt-1, a base additive,and
a sacrificial electron donor (Table S1). In the presence of Pt-1 (1
mol%), cesium carbonate (2 equiv) and a slight excess of
diisopropylethylamine (iPr2NEt) in acetonitrile at room
temperature, the reaction afforded product c1 in 35% yield based
on 37% conversion of a1 (Table S1, entry 1). Several inorganic
bases, including K2CO3, Na2CO3 and K3PO4, were further tested
and none of them showed better effects than that of Cs2CO3.
(Table S1, entries 2–4). Product c1 was not obtained in the
absence of a base. As for solvent, acetonitrile (CH3CN) was found
to be superior to dimethyl formamide (DMF), methanol (MeOH),
tetrahydrofuran (THF) and 1, 2-dichloroethane (DCE) (Table S1,
entries 6–9). Simliar to Zeitler’s work,[18] the presence of water
showed a dramatically positive impact and the best ratio of
CH3CN/H2O was 6:1, in which c1 was obtained in 87% yield
(Table S1, entry 11). Control experiments confirmed the necessity
of Pt complex and light irradiation. Finally, the optimal result was
obtained with Pt-1 (1 mol%), 0.2 mmol of aryl chloride, 2.0 equiv
substrate, 2-chloropyrimidine, and afforded product c10 in 95%
yield. Besides aryl chlorides, some common aryl bromides were
tested under the standard conditions. The reaction afforded c11
in 68% yield as major alkylation product when methyl 3-bromo-4-
chlorobenzoate was used, suggesting that the cleavage of C–Br
bond preferentially occurred. Sulfonyl substituted alkane c12 was
readily accessible with 65% yield. The reactions of pyridyl-
containing substrates with b1 proceeded smoothly, affording the
coupling products c13 and c14 in 79% and 55% yields,
respectively. Furthermore, 2-bromobenzothiazole and 5-
bromopyrimidine gave c15 and c16 in moderate yields.
Then we turned our attention to the investigation of alkene
substrates. 1,1-Diarylethylenes bearing various substituents
such as Me, OMe, F, Cl, CF3 and Bpin were subjected to the
standard conditions in the presence of a1 to give 1,1-
diarylalkanes c17–28 in 25-87% yields (Scheme 3). The
diarylalkenes with both electron-donating and -withdrawing
groups could undergo the reaction smoothly with similar product
yield. Except for c24, where the reaction afforded the
corresponding product in 54% yield, all other products were
obtained in yields up to 87%. In addition, diversely unsymmetric
ortho-substituted alkenes were efficiently converted to the desired
i
of Cs2CO3, and 1.2 equiv of Pr2NEt in 6 mL mixed solvent of
CH3CN/H2O (6:1, v/v) (Condition A). With the optimized conditions
in hand, a variety of aryl chlorides (a1–10) was investigated with
1,1-diphenylethylene (b1) as the coupling partner (Scheme
2).Generally, the reaction is compatible with various aryl chlorides, products. Notably, alkenes containing heterocyclic moieties
including cyano, ketone, ester, alkyl, halo, and trifluoromethyl
substitution, affording the corresponding products in moderate to
good yields (c1–9). Methyl/ethyl 4-chlorobenzoates with electron-
withdrawing -F or -Cl groups gave better yields than that with
electron-donating -Me group (c6, c7 vs c5). Interestingly, methyl
3,4-dichloro benzoate and methyl 2,4-dichlorobenzoate
underwent regioselective alkylation at the para position to
methoxycarbonyl group and gave products c7 and c8,
respectively. This protocol also worked well for heterocyclic
including 3-thiophene and 3-pyridine were tolerated in this
Scheme 3. Condition A: Pt-1 (1 mol%), aryl halide (0.2 mmol), alkene (0.6
i
mmol), Cs2CO3 (0.4 mmol), Pr2NEt (0.24 mmol) in 6 mL of CH3CN/H2O (6:1,
Scheme 4. Condition B: Pt-2 (1.0 mol%), alkene (0.1 mmol), Cs2CO3 (0.2 mmol),
in 2.0 mL of CH3CN, under argon, irradiated by 410 nm LEDs (3 W x 4) for 10
h, isolated yields. [a] Using d1 (5 mmol), Cs2CO3 (1 mmol) and Pt-2 (0.2 mol%)
in 10 mL of CH3CN.
v/v), under argon, irradiated by 410 nm LEDs (3 W x 4) for 10 h, isolated yields.
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10.1002/anie.202011841
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reaction, affording the desired alkylated products c29 and c30 in
moderate yields. This protocol was also applicable to 1,2-di- and
tri-substituted alkenes (c31 & c32), alkylaryl alkenes (c33–37),
styrene (c38) and isopropenyl acetate (c39), albeit with lower
product yields (20–60%). In general, the hydroarylation reaction
exhibited good anti-Markonikov regioselectivity in all cases that
C–C bond formation occurred at the terminal carbon atom of the
terminal alkenes.
With respect to photoinduced intramolecular cyclization of
alkenes, the initial optimal conditions for the cyclization utilized Pt-
2 as photocatalyst and N-methyl-N-phenylmethacrylamide (d1)
as model substrate at room temperature in MeCN (Table S2). The
reaction gave the cyclization product e1 in 10% yield under light
irradiation for 10 h. It is found that the addition of Cs2CO3 as
additive could dramatically improve the product yield and afford
e1 almost quantitatively (Table S2). Control experiments revealed
that both catalyst Pt-2 and LEDs irradiation were necessary for
the cyclization. Consequently, the optimal conditions were found
to be 1 mol% catalyst Pt-2, 0.1 mmol of alkene, 2.0 equiv of
Cs2CO3 in CH3CN (2 mL) under LEDs irradiation (410 nm) for 10
h (Condition B). Next we focused on the cyclization allowing the
conversion of alkenes d into 3,4-dihydroquinolinones e (Scheme
4). A wide range of acrylamides proceeded very well, regardless
of the substitution pattern. Acrylamides with functional groups on
the aromatic ring, for instance, alkyl, methoxy, cyano, halo,
trifluoromethyl, naphthyl, alkynyl, ester, sulfonyl and ketone, were
compatible to the standard conditions, leading to the
corresponding products in 30-96% yields. The alkenes bearing
meta substituents exhibited good reactivity as well and delivered
the mixture of e17–19 and e17'–19', albeit with low
regioselectivity. Replacement of methyl group on nitrogen atom of
acrylamide by other alkyl and aromatic substituents resulted in an
insignificant impact on the reaction efficiency (e20–22). Substrate
with a phenyl group at the  and β position of alkene d23–24 also
afforded products e23–24 in 86% and 80% yields, respectively.
Heteroaromatic substrates, such as quinoline, benzothiazole,
1,2,3,4-tetrahydroquinoline, and indoline, could be readily to the
desired products e25–28 in 75-95% yields.
To demonstrate the synthetic utility of the as-developed Pt-
catalyzed protocols, scaled-up reactions for the photo-induced
hydroarylation of alkene and cyclization of acrylanilide have been
performed. Light (410 nm) irradiation of the reaction mixtures
containing 1-2 mmol of a1 as limiting substrate and 1.2–2.4 mmol
of b1 in the presence of 0.5-1 mol% of Pt-1 for 24 h afforded c1
in 50-77% yields (Scheme 2). Similarly, photo-cyclization of d1
catalyzed by Pt-2 (0.2 mol%) could proceed smoothly on a 5
mmol scale in 24 h and e1 was obtained in 75% yield (Scheme 4).
These protocols were also applied in the modification of three
complex organic compounds (Scheme 5). Aryl chlorides bearing
(+)-menthol (f1) and pregnenolone motifs (g1) reacted with diaryl
alkene b1 to afford the corresponding products f2 and g2 in 60
and 66% yields, respectively. In addition, with optimized
cyclization condition (Condition B), alkene d18 was directly
transferred to e18 in one step, which could be further transformed
into atypical schizophrenia drug (±)-SIPI 6360 according to a
reported protocol.[19]
Scheme 5. Synthesis of (a) f2, (b) g2 and (c) (±)-SIPI 6360 (f3).
diphenylethene with quenching rate constant of 7.1 × 107 and 8.1
× 108 M–1 s–1, respectively, but not by ethyl 4-chlorobenzoate. In
the presence of DIPEA (12 mM), the ns-TA signals of Pt-1 at 335,
390 and 440–800 nm decayed with a shortened time constant
from 2.4 to 0.8 s and a long-lived species showing absorption
features at 330, 430 and 690 nm was developed, which persisted
over 50 s (Fig. 2). As Pt-1* is capable of oxidizing DIPEA
(E(Pt*/Pt–) = +0.79 V; E(DIPEA+/0) = +0.64 V vs SCE in CH3CN)
(Fig. S6), the observed new species in ns-TA is assigned as the
reduced Pt-1 (Pt-1•–). Conversely, the emission quenching of Pt-
1* by 1,1-diphenylethene is due to energy-transfer because (i)
1,1-diphenylethene is redox inactive within the solvent window
(Fig. S5) and (ii) no new species was generated in the ns-TA
experiment of Pt-1 and1,1-diphenylethene (Fig. S8). Low
temperature X-band EPR experiment (100K) of
a frozen
CH3CN/H2O (6:1, v/v) solution of Pt-1 and DIPEA obtained by light
(410 nm) irradiation of the solution for 1.5 h at room temperature
revealed a sharp signal at g ~2.0 comprising of three lines
spanning a field range of ~100 G (Fig. S16), which is reminiscent
of the nearly isotropic signals of the reported [Pt(II)(N^N)]•– and
[Pt(II)(O^O)]•– species,[20] and could be approximately fitted using
the parameters: g1 = 2.0042, g2 = 2.0035 and g3 = 2.0021 (giso
2.0033), A1,Pt = 30 G, A2,Pt = 40 G, A3,Pt = 80 G (Aiso,Pt = 54 G) and
A1,N = 4 G, A2,N = 6 G, A3,N = 8 G (Aiso,N = 6 G). The g and A values
=
To probe the mechanism of these two types of photocatalytic
reactions, emission quenching, nano-second time-resolved
absorption spectroscopy (ns-TA), electron paramagnetic
resonance (EPR) and CV measurements were conducted. The
emission of Pt-1 could be quenched by both DIPEA and 1,1-
Figure 2. (a) Nanosecond time-resolved absorption difference spectra of Pt-1
(40 μM) in the presence of DIPEA (12 mM) in degassed CH3CN/H2O (v/v = 6:1).
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relevant tertiary benzylic radicals were reported to have Ered of ca.
−1.35 V vs SCE in MeCN from previous studies,[22] it is expected
that the third SET that generates the respective benzylic
carbanion should derive from Pt-1•– (E(Pt/Pt-) = −1.78 V vs SCE).
Although SET from Pt-1* (E(Pt+/Pt*) = −2.14 V vs SCE) is more
thermodynamically feasible, the insufficient reducing power of
DIPEA to reduce the oxidized Pt-1 (Pt-1+; E(Pt+/Pt) = +0.43 V vs
SCE) back to Pt-1 does not favour this pathway. Finally, alkylation
product c is formed after protonation of the benzylic carbanion.
As for the intramolecular cyclization reaction, the emission
quenching rate constant of Pt-2 by acrylanilide substrate d1 was
found to be 1.4 × 106 M–1 s–1. Because d1 is electrochemically
inactive from –2.2 to +1.5 V vs SCE in MeCN, neither reductive
nor oxidative electron transfer quenching of Pt-2* by d1 is
deemed feasible. Previous mechanistic studies established that
photocyclization of related acrylanilides in the presence of a triplet
sensitizer could be triggered by triplet-triplet energy-transfer
(TTEnT) followed by a cascade of radical ring closure of the
diradicaloids and hydrogen migration.[11] The triplet energy of d1,
determined from 77 K phosphorescence, is about 2.82 eV (Figure
S12)[12b] and the presumed TTEnT process from Pt-2* to d1 is,
therefore, endergonic by ca. 0.2 eV (4.61 kcal mol–1). Such an
uphill EnT event could be made possible by thermal population of
the higher vibrational/rotational levels of the triplet state of the
photosensitizer. A recent work by Guldi and Glorius demonstrated
an efficient [2+2]-photocycloaddition of benzothiophene involving
an endergonic TTEnT step of 5.77 kcal mol–1 from an Ir(III)-
photosensitizer to benzothiophene.[23] It is conceived that similar
photo-coupled-thermal activated TTEnT mechanism between Pt-
2* and d1 could also be operative. Addition of TEMPO as triplet
quencher inhibited the reaction and the starting materials were
recovered, lending support to an EnT mechanism. Despite the
small kq, the extended triplet excited state lifetime of Pt-2 (15.0
s) results in a practical quenching fraction (ηq) of Pt-2* by d1 of
Scheme 6. Mechanistic studies of Pt-1-catalyzed photoinduced hydroarylation
reactions.
are well comparable to those of the [Pt(II)(N^N)]•– species.[20,21]
This EPR signal could be quenched upon addition of ethyl 4-
chlorobenzoate (a1) following an extended period of light
irradiation (Fig. S19). The Pt-1•– species generated
electrochemically in CV at –1.78 V vs SCE could react with a1
(Ered = –2.01 V vs SCE) in CH3CN/H2O as revealed by the
catalytic current with an Eonset of –1.69 V in the presence of the
latter in excess (Fig. S7). The presence of water facilitates this
reduction by stabilizing the LUMO of ethyl 4-chlorobenzoate by
0.11 V (Fig. S4). The involvement of aryl radical in the reaction
under Condition A is substantiated by (i) the radical trap
experiment that the aryl-TEMPO adduct was observed in HRMS
(m/z = 276.1953 [M+H]+) from the reaction mixture of TEMPO (5
equiv) and 4’-bromoacetophenone (a17) (Scheme 6a) and (ii) the
radical clock experiment in which a ring-open E-alkene product
(c38) was obtained as a major product in 56% yield (Scheme 6b).
The isotope labelling experiment of a1 and b1 using MeCN/D2O
51.2% (ηq
= kqτ0[quencher]/(1+kqτ0[quencher])) under the
(6:1) as solvent resulted in
hydroarlyation product (c1:c1-d
a
high ratio of deuterated
11:89) (Scheme 6c),
experimental conditions that effectively generates 3d1* per
=
photoexcitation. From energy consideration, an exergonic reverse
suggesting the formation of a benzylic anion intermediate in the
catalytic cycle. Thus, the photoinduced hydroarylation reaction
begins with single electron transfer (SET) from DIPEA to Pt-1* to
give Pt-1•– (Scheme 7). The Pt-1•– species then reacts with aryl
halide to restore to Pt-1 and afford a reactive aryl radical after a
reductive Ar–Cl/Br bond scission, which is subsequently trapped
by the alkene (b) furnishing a tertiary benzylic radical intermediate
with greatest electronic stabilization. This radical addition step is
conceived to be the key mechanistic step that confers high
regioselectivity for anti-Markonikov addition products. Since
3
TTEnT event from d1* to Pt-2 with a considerably higher rate
constant than the forward TTEnT would be more favorable. Thus,
the possibility of occurrence of a reversible EnT dynamics leading
3
to a diminished d1* population cannot be excluded.[23b] In this
scenario, the ensuing radical ring closure step has to be much
faster than the reverse TTEnT process and/or irreversible to drive
the productive reaction forward. The results from deuterium-
labeling and kinetic isotopic effect studies confirmed an
intramolecular [1,5]-H shift, which is non-rate limiting, to take
place and afford photoproduct e1. Based on the above
findings/considerations and established mechanistic studies,[11]
a
plausible mechanism for photocyclization of acrylanilides
mediated by Pt-2 is proposed and depicted in Scheme 8.
Scheme 8. Proposed reaction pathway for photocyclization of acrylanilides
catalyzed by Pt-2.
Scheme 7. Proposed reaction pathway for hydroarylation catalyzed by Pt-1.
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Conclusion
In summary, we described efficient visible-light (410 nm)
[Pt(O^N^C^N)] photocatalysis that realized (i) reductive coupling
of aryl chlorides/bromides with terminal/internal aryl alkenes to
give anti-Markonikov hydroarylated compounds via aryl radical
intermediacy and (ii) cyclization of acrylanilidines to access a
variety of 3,4-dihydroquinolinones.
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Acknowledgements
This work was supported by Guangdong Major Project of Basic
and Applied Basic Research (2019B030302009), Shenzhen
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Science
and
Technology
Innovation
Commission
(JCYJ20180508162429786), China Postdoctoral Science
Foundation (2019M662156) and Hong Kong Research Grant
Council (HKU 17330416). We thank the Southern University of
Science and Technology for financial support. Dr Wai-Pong To
and Mr Yu-Kan Tang were thanked for the help in photophysical
measurements.
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Keywords: aryl halides • hydroarylation • intramolecular
cyclization • photocatalysis • platinum
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This article is protected by copyright. All rights reserved.

10.1002/anie.202011841
Angewandte Chemie International Edition
RESEARCH ARTICLE
Entry for the Table of Contents
In the presence of luminescent [Pt(O^N^C^N)] complexes under visible light (410 nm) irradiation, aryl radicals derived from reductive
aryl–chloride/bromide bond cleavage were trapped by aryl alkenes and afforded a series of anti-Markonikov hydroarylated compounds.
Besides, a protocol for [Pt(O^N^C^N)]-catalyzed intramolecular photocyclization of acrylanilides to give structurally diverse 3,4-
dihydroquinolinones has been developed.
This article is protected by copyright. All rights reserved.