Photocatalytic Giese-Type Reaction with Alkylsilicates Bearing C,O-Bidentate Ligands

Article abstract of DOI:10.1002/chem.202005300

Herein, a photocatalytic Giese-type reaction with alkylsilicates bearing C,O-bidentate ligands as stable alkyl radical precursors has been reported. The alkylsilicates were prepared in one step from organometallic reagents. Not only primary, secondary, and tertiary alkyl radicals, but also elusive methyl radicals, could be generated by using the present reaction system. The generated radicals were trapped by electron-deficient olefins bearing various functional groups to give the desired alkyl adducts. The silicon byproduct can be recovered after the photoreaction. The radical generation process was investigated by theoretical calculations, which provided an insight into the facile generation of methyl radicals from methylsilicate bearing C,O-bidentate ligands.

Full text of DOI:10.1002/chem.202005300

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&
Synthesis Design
Photocatalytic Giese-Type Reaction with Alkylsilicates Bearing
C,O-Bidentate Ligands
Tatsuya Morofuji,*[a] Yu Matsui,[a] Misa Ohno,[a] Gun Ikarashi,[a, b] and Naokazu Kano*[a]
Abstract: Herein, a photocatalytic Giese-type reaction with
alkylsilicates bearing C,O-bidentate ligands as stable alkyl
radical precursors has been reported. The alkylsilicates were
prepared in one step from organometallic reagents. Not
only primary, secondary, and tertiary alkyl radicals, but also
elusive methyl radicals, could be generated by using the
present reaction system. The generated radicals were
trapped by electron-deficient olefins bearing various func-
tional groups to give the desired alkyl adducts. The silicon
byproduct can be recovered after the photoreaction. The
radical generation process was investigated by theoretical
calculations, which provided an insight into the facile gener-
ation of methyl radicals from methylsilicate bearing C,O-bi-
dentate ligands.
Introduction
drawbacks: 1) bis(catecholato)silicates require a two-step prep-
aration that includes the prior synthesis of neutral organosi-
lanes and ligand exchange to form ate complexes,[9a,11] and
2) silicon byproducts are unstable to hydrolysis and become
waste after the reaction.[12]
The alkyl radical is one of the most fundamental active species
in organic chemistry.[1] The addition of alkyl radicals to elec-
tron-deficient olefins is a powerful method for creating CÀC
bonds through a reaction called the Giese reaction.[2] The
Giese reaction is indispensable for synthesizing complex mole-
cules, such as natural products and medicinal compounds.
However, the original Giese reaction has the drawback of in-
volving toxic tin reagents to generate alkyl radicals. Therefore,
a mild and convenient method for generating alkyl radicals is
still highly desired. A remarkable achievement in the genera-
tion of alkyl radicals in the last decade is the use of visible-
light photoredox catalysts (PCs),[3] leading to the development
of various photocatalytic Giese-type reactions. Carboxylic acid
derivatives,[4] oxalates,[5] pyridinium salts,[6] alkyl halides,[7] orga-
noborates,[8] organosilicates,[9] and sulfinamides[10] are used as
radical precursors in photocatalytic Giese-type reactions.
Herein, we focused on alkylsilicates 1 bearing the C,O-biden-
tate ligands developed by the group of 3Martin
(Scheme 1b).[13] Alkylsilicates 1 can be directly synthesized by
reacting alkyllithium or alkylmagnesium compounds with Mar-
tin’s spirosilane 2. Notably, alkylsilicates 1 are highly stable to
heat, water, and air because the electron-withdrawing C,O-bi-
Photocatalytic Giese-type reactions with bis(catecholato)sili-
cates are highly attractive for the following reasons:[9a] 1) the
process is redox neutral, that is, no extra redox reagents are re-
quired; 2) a wide range of alkyl radicals can be efficiently gen-
erated to produce various products in high yields; and 3) bis-
(catecholato)silicates are bench-stable and easy to handle
(Scheme 1a). However, these reactions also have the following
[a] Dr. T. Morofuji, Y. Matsui, M. Ohno, G. Ikarashi, Prof. N. Kano
Department of Chemistry, Faculty of Science, Gakushuin University
1-5-1 Mejiro, Toshima-ku, Tokyo 1718588 (Japan)
E-mail: tatsuya.morofuji@gakushuin.ac.jp
naokazu.kano@gakushuin.ac.jp
[b] G. Ikarashi
Department of Chemistry, Graduate School of Science
The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku
Tokyo 1130033 (Japan)
Scheme 1. The photocatalytic generation of alkyl radicals by using silicate
complexes as radical precursors: a) previous studies, and b) this work. Alkyl-
silicates bearing C,O-bidentate ligands were used as radical precursors. EWG:
electron-withdrawing group.
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/chem.202005300.
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dentate ligands can stabilize the pentacoordinated structure
both electronically and geometrically. In addition, we recently
reported that the photocatalytic oxidation of 1 could generate
alkyl radicals.[14]
Table 1. Optimization of the photocatalytic Giese-type reaction with al-
kylsilicates.
In this context, we report herein on the photocatalytic
Giese-type reaction with
1 as alkyl radical precursors
(Scheme 1b). As described above, compound 1 can be pre-
pared in one step from organometallic reagents without any
ligand exchange of neutral organosilicon compounds. The
alkyl radical generated by the photocatalytic single-electron
oxidation of 1 was trapped by electron-deficient olefin 3 to
give desired radical adduct 4. Byproduct spirosilane 2 was
stable and could be recovered after the photoreaction. Nota-
bly, an elusive methyl radical could also be generated by using
this system. This radical generation process was investigated
by theoretical calculations to explain the facile generation of
methyl radicals from the methylsilicate bearing C,O-bidentate
ligands.
Entry
1
3a
[equiv]
PC
Yield[b] ([%])
4
([equiv])
5
1
1a (1)
1a (1)
1a (1)
1a (1)
1a (1)
1b (1)
1b (1)
1b (1.2)
1b (1.2)
4
4
4
4
4
4
4
1
1
PC1
PC1
none
PC2
PC3
PC1
PC2
PC1
PC2
4a (85)
4a (0)
4a (0)
4a (86)
4a (0)
4b (60)
4b (72)
4b (10)
4b (58)
(94)
(0)
(0)
(100)
(0)
(87)
(100)
(40)
(100)
2[c]
3
4
5
6
7
8
9
Results and Discussion
Optimization of the reaction conditions
The reaction conditions were optimized for the photocatalytic
Giese-type reaction by using cyclohexylsilicate 1a (Eox
=
+1.47 V)[14] as a radical precursor and diethyl benzylidenemalo-
nate (3a) as a radical acceptor. Irradiation of 1a and 3a with
blue light in the presence of acridinium photocatalyst PC1
(Ered*= +2.06 V)[15] in acetone/MeOH (10:1) gave alkyl adduct
4a and hydroxysilicate 5 in good yields (Table 1, entry 1). Both
irradiation and the photocatalyst were essential for the reac-
tion to progress (Table 1, entries 2 and 3). Photocatalyst PC2
(Ered*= +2.15 V)[16] also worked as well as PC1 (Table 1,
entry 4). In contrast, the use of PC3 did not yield the desired
product because of the low reduction potential of the excited
state of Ru* (Ered*= +0.82 V; Table 1, entry 5).[3a] If 1b was
used as a radical precursor, catalyst PC2 was found to be more
suitable than PC1 (Table 1, entries 6 and 7). This tendency was
more remarkable if the equivalents of radical precursor 1b and
radical acceptor 3a were changed (Table 1, entries 8 and 9).
[a] Silicate 1 (0.10 or 0.12 mmol), radical acceptor 3a (0.40 or 0.10 mmol),
and photocatalyst PC (5 mol%) were stirred at 238C in acetone/MeOH
(10/1, 1.1 mL) for 24 h under blue-light irradiation (LED: light-emitting
diode). [b] Determined by means of 1H NMR spectroscopic analysis.
[c] The reaction was carried out in the dark.
The scope of the generated alkyl radicals was also examined.
Primary alkyl radicals could be efficiently generated and used
for the Giese-type reaction (4b, 4l–4n). Selective CÀSi bond
cleavage occurred if (trimethylsilyl)methylsilicate was used as a
radical precursor (4n). Secondary and tertiary alkyl groups can
also be introduced into electron-deficient olefins to produce
the corresponding radical adducts (4o–4r).
Although the photocatalytic Minisci-type methylation of het-
eroaromatics was unsuccessful under previously reported reac-
tion conditions,[14] methyl radicals could be generated from
methylsilicate 1c and trapped with electron-deficient olefins
by using the present photocatalytic system (Scheme 3; 4s–4x).
Notably, methylation via methyl radicals is a highly challenging
transformation because of its extreme instability.[17] In fact, a
mild method for generating methyl radicals is quite rare, and
their generation by photocatalytic single-electron oxidation of
methylsilicate has never been reported. This successful genera-
tion of methyl radicals from 1c contrasts sharply with the radi-
cal generation method with bis(catecholato)silicates.[9a,18] Un-
fortunately, the reaction with phenylsilicate gave the desired
product in low yield (see the Supporting Information for de-
tails).
Scope of the photocatalytic Giese-type reaction
Next, the scope of the present Giese-type reaction was exam-
ined, as shown in Scheme 2. The reaction of 3a with 1a or 1b
gave the corresponding radical adducts 4a or 4b, respectively.
Various electron-deficient olefins as radical acceptors were in-
vestigated with 1a as a radical precursor (4c–4k). Reactions
with benzyl methacrylate and acrylate, diethyl ethylidenemalo-
nate, and benzylidenemalononitrile also produced the desired
products (4c–4 f). The reaction worked well on a 1.0 mmol
scale (4c). 1,2-Disubstituted olefins, such as N-methylmalei-
mide and diethyl fumarate, are also suitable radical acceptors
(4g, 4h). Additionally, cyclic a,b-unsaturated ketones, such as
cyclohexenone and cyclopentenone, gave the corresponding
products (4i, 4j). The coumarin derivative was converted into
alkyl adduct 4k in excellent yield.
We also developed a method for recovering Martin’s spirosi-
lane 2 after the photocatalytic reaction of 1a with benzyl
methacrylate. Hexane was added to the crude material to pro-
duce a slurry. After precipitation, the supernatant was deca-
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Scheme 3. Photocatalytic generation and trapping of a methyl radical. Meth-
ylsilicate 1c (0.20–0.50 mmol), radical acceptor 3 (0.16–0.20 mmol), and pho-
tocatalyst PC2 (5–7 mol%) were stirred at 238C in acetone/MeOH (1/1,
3 mL) for 24–42 h under blue-light irradiation. Yields of products isolated are
shown. See the Supporting Information for more details.
Mechanistic study and reaction mechanism
To obtain a mechanistic insight, two further experiments were
performed. First, Stern–Volmer quenching experiments were
carried out. A solution of PC2 in acetone/MeOH (0.05 mm, 1/1)
was irradiated at 417 nm, and the fluorescence intensity was
measured at 506 nm in both the presence and absence of ad-
ditives. The obtained results indicated that 1a efficiently
quenched the excited state of the acridinium photocatalyst,
whereas benzyl methacrylate did not (Figure 1a). The oxidation
potential of 1a (Eox = +1.47 V) and the reduction potential of
the excited state of PC2 (PC2*; Ered(PC2*)= +2.15 V)[16] strong-
ly support single-electron transfer from 1a to PC2*.
Second, a radical clock experiment was performed by using
alkylsilicate 1d bearing an olefin moiety (Figure 1b). The reac-
tion gave cyclized product 4y in 49% yield; however, no linear
product was observed. This result confirmed the intermediacy
of alkyl radicals.
Scheme 2. Scope of the photocatalytic Giese-type reaction with alkylsilicates
bearing C,O-bidentate ligands. Silicate 1 (0.20–0.45 mmol), radical acceptor 3
(0.20–0.80 mmol), and photocatalyst PC2 (5 mol%) were stirred at 238C in
acetone/MeOH (2.2–3.3 mL) for 3–24 h under blue-light irradiation. Yields of
products isolated are shown. See the Supporting Information for more de-
tails. [a] The reaction was run on a 1.0 mmol scale. [b] Acetone/EtOH was
used as the solvent. Bn: benzyl.
Based on the experimental results discussed above, a plausi-
ble reaction mechanism is proposed (Figure 1c). Catalyst PC2
is excited by photoirradiation to generate PC2*, which oxidizes
alkylsilicate 1 to give the corresponding alkyl radical, spirosi-
lane 2, and reduced photocatalyst PC2(À1). The use of PC2,
which has a high reduction potential in the excited state, is im-
portant for the successful oxidation of 1. The alkyl radical
reacts with radical acceptor 3 to give electron-deficient radical
6, which is reduced by PC2(À1) and protonated by MeOH to
produce the desired product 4 and MeOÀ. The resulting MeOÀ
is trapped by spirosilane 2 and the resulting methoxysilicate is
nted to separate it from the precipitate. The supernatant con-
tained 4c and the precipitate contained hydroxysilicate 5. The
solid was acidified with a 0.5m aqueous solution of H2SO4 and
extracted with CH2Cl2 to give spirosilane 2 in 96% yield. This
method would reduce the amount of waste produced.
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For comparison, radical generation processes from 7 were
also examined. In contrast to 1c, the HOMO of 7a was delocal-
ized over the p orbitals in the two catecholato ligands, but not
the CÀSi s orbital (Figure 2b). Then, we computationally opti-
mized the structure of the single-electron-oxidized silicate by
using the geometry of 7a as an initial structure (IN II). We
found a local minimum structure, 8a, which was more stable
than the initial structure by À4.8 kcalmolÀ1. The spin density
of radical 8a was delocalized on a catecholato ligand. Methyl
radical generation from radical 8a was found to have a high
activation energy (DG° = +31.0 kcalmolÀ1) and was thermody-
namically disfavored (DG= +9.6 kcalmolÀ1). The disfavor of
the generation of methyl radicals from 7a is much higher than
that of the ethyl radical from 7b (DG° = +23.7 kcalmolÀ1,
DG= +2.1 kcalmolÀ1, see the Supporting Information). These
calculation results are in good agreement with the reported
experimental results, according to which the generation of
methyl radicals from 7a is difficult, in contrast to other alkyl
radicals from 7.[9a,18]
A plausible mechanism for the generation of alkyl radicals
from alkylsilicate bearing C,O-bidentate ligand 1 is shown in
Figure 2c. As mentioned above, the CÀSi s orbital of the alkyl
group contributes the HOMO of 1. Therefore, if 1 meets a
single-electron oxidant, such as the excited state of PC2, one
electron transfers from the CÀSi s bond to the oxidant, result-
ing in a rapid CÀSi s bond cleavage to generate an alkyl radi-
cal without forming a stable intermediate. This process con-
trasts sharply with the generation of alkyl radical with 7 (Fig-
ure 2d). One p electron is transferred from a catecholato
ligand to the oxidant to form radical 8. Radical 8 would be in a
resting state, the CÀSi s bond of which would thermally cleave
to slowly generate an alkyl radical. Because of such a mecha-
nistic difference, alkylsilicate 1c would be more suitable for
generating methyl radicals than the use of 7a.
Figure 1. Mechanistic studies and plausible reaction mechanism. a) Stern–
Volmer plots for the emission of PC2 in the presence of a quencher (Q=1a
or benzyl methacrylate). b) A radical clock reaction. c) Plausible reaction
mechanism.
Conclusion
converted into hydroxysilicate 5 with a small amount of water
during workup.
We developed a photocatalytic Giese-type reaction with alkylsi-
licates bearing C,O-bidentate ligands 1. This redox-neutral CÀC
bond-forming process utilized stable alkyl radical precursors,
which were prepared through a one-step reaction by using or-
ganometallic reagents. Primary, secondary, and tertiary alkyl
radicals, as well as elusive methyl radicals, could be generated
under the present reaction system. The generated radicals
were trapped by electron-deficient olefins bearing various
functional groups to produce the desired alkyl adducts. Addi-
tionally, we developed a concise method for the recovery of
Martin’s spirosilane 2, which would reduce the amount of
waste produced. DFT calculations suggested a mechanistic dif-
ference in the alkyl radical generation process from bis(cate-
cholato)silicates 7 and that from alkylsilicates bearing C,O-bi-
dentate ligand 1, which provided insights into the generation
of methyl radicals from methylsilicate 1c. These mechanistic in-
sights can be useful guidelines for future work on silicate-
based radical chemistry.
Theoretical calculations
DFT calculations were conducted to investigate the alkyl radical
generation process from alkylsilicates bearing C,O-bidentate li-
gands 1. The CÀSi s orbital of the methyl group contributes
the HOMO of the anion of 1c (Figure 2a). We attempted to
computationally optimize the structure of the single-electron-
oxidized silicate by using the geometry of the anion of 1c as
an input file (IN I). However, a local minimum structure was
not found, and the calculation diverged to provide a free
methyl radical and Martin’s spirosilane 2. The sum of the Gibbs
energies of the methyl radical and 2 is much lower than that
of IN I (À37.6 kcalmolÀ1). These calculation results are almost
the same as those obtained if ethylsilicate bearing the C,O-bi-
dentate ligands was used (see the Supporting Information).
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Figure 2. DFT calculations for the generation of alkyl radicals from silicates. a) An optimized structure of methylsilicate bearing C,O-bidentate ligand 1c and a
comparison of the Gibbs energy of oxidized 1c with the sum of Gibbs energies of Martin’s spirosilane 2 and the methyl radical. b) Free energy profile of the
process for the generation of the methyl radical with bis(catecholato)methylsilicate 7a. c) A plausible mechanism for radical generation by using silicates
bearing C,O-bidentate ligands 1. d) A plausible mechanism for radical generation with bis(catecholato)silicate 7.
Computational details
Experimental Section
All DFT calculations were conducted by using the Gaussian 09 pro-
gram.[19] The structures of all intermediates and transition states
General procedure for the photocatalytic Giese reactions
A 30 mL Schlenk tube equipped with a magnetic stirring bar and a
septum was dried under vacuum with heating. After cooling the
tube to 238C, it was purged with argon gas. Alkylsilicate 1 (0.16–
0.45 mmol), olefin (0.20–0.80 mmol), photocatalyst PC2 (5–
7 mmol%), and acetone/MeOH (2.2–3.3 mL) were added to the
tube. After the mixture was bubbled with argon gas for 30 min,
photoirradiation was carried out at room temperature with stirring
for 3–42 h. The solvents were evaporated to give a crude material,
which was purified by flash chromatography and gel permeation
chromatography to obtain desired product 4.
were calculated at the (U)B3LYP/6-311+G(d,p) level with the polar-
ized continuum model (PCM; acetone). Frequency calculations
were carried out to characterize the stationary points and estimate
the Gibbs free energies at 298.15 K and 1 atm.
Acknowledgements
This research was partially supported by a MEXT-supported
program for Strategic Research Foundations at Private Univer-
sities, JSPS KAKENHI (grant no. JP19K15570), the Tokyo Ohka
Foundation for The Promotion of Science and Technology,
Tokyo Chemical Industry Foundation, Merck & Co., Inc., and
the Fukuoka Naohiko Foundation. This research was also sup-
ported by crowdfunding (academist). We thank Yumi Hayashi,
Subaru Kawasaki, Yuki Miyabayashi, Kazufumi Nakano, Nobu,
Kenta Tanaka, Yoshiyuki Yagi, and Jun Yamaoka for funding. We
also thank Prof. Koichi Iwata and Dr. Akira Takakado for their
kind assistance with the Stern–Volmer quenching experiments.
DFT calculations were performed by using the Research Center
for Computational Science, Okazaki, Japan.
Recovery of Martin’s spirosilane 2 after the photoreaction
Similar to the above procedure, alkylsilicate 1a (0.17 g, 0.24 mmol),
benzyl methacrylate (36 mg, 0.21 mmol), PC2 (0.01 mmol), and ace-
tone/MeOH (1/1, 3 mL) were stirred under photoirradiation at
room temperature for 4 h. Evaporation of the solvents gave a
crude material containing 4c and hydroxysilicate 5. Hexane was
added to the crude to obtain a slurry. After precipitation, the su-
pernatant was decanted to separate it from the precipitate. The su-
pernatant contained desired product 4c in over 95% yield, which
1
was determined by H NMR spectroscopic analysis with 1,1,2,2-tet-
1
rachloroethane as an internal standard. H NMR spectroscopic anal-
Conflict of interest
ysis also revealed that the main component of the precipitate was
hydroxysilicate 5. The solid was acidified with a 0.5m aqueous so-
lution of H2SO4 and extracted with CH2Cl2 (3”30 mL). The organic
layer was dried over Na2SO4 followed by evaporation and vacuum
drying to produce spirosilane 2 (0.12 g, 96%). 1H, 13C, 19F, and
29Si NMR spectra of recovered spirosilane 2 are attached in the
Supporting Information.
The authors declare no conflict of interest.
Keywords: Giese reaction · ligand effects · photocatalysts ·
radicals · silicates
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[1] Radicals in Organic Synthesis (Eds.: P. Renaud, M. P. Sibi), Wiley-VCH,
Weinheim, 2001.
[2] a) B. Giese, Angew. Chem. Int. Ed. Engl. 1983, 22, 753–764; Angew.
Chem. 1983, 95, 771–782; b) C. P. Jasperse, D. P. Curran, T. L. Fevig,
Chem. Rev. 1991, 91, 1237–1286; c) G. S. C. Srikanth, S. L. Castle, Tetrahe-
dron 2005, 61, 10377–10441.
[10] F. Xue, F. Wang, J. Liu, J. Di, Q. Liao, H. Lu, M. Zhu, L. He, H. He, D.
Zhang, H. Song, X.-Y. Liu, Y. Qin, Angew. Chem. Int. Ed. 2018, 57, 6667–
6671; Angew. Chem. 2018, 130, 6777–6781.
[11] G. A. Molander, D. L. Sandrock, Curr. Opin. Drug Discov. Devel. 2009, 12,
811–823.
[12] H. Meyer, G. Nagorsen, Angew. Chem. Int. Ed. Engl. 1979, 18, 551–553;
Angew. Chem. 1979, 91, 587–588.
[13] a) E. F. Perozzi, J. C. Martin, J. Am. Chem. Soc. 1979, 101, 1591–1593;
b) W. H. Stevenson III, S. Wilson, J. C. Martin, W. B. Farnham, J. Am.
Chem. Soc. 1985, 107, 6340–6352.
[14] G. Ikarashi, T. Morofuji, N. Kano, Chem. Commun. 2020, 56, 10006–
10009.
[15] S. Fukuzumi, H. Kotani, K. Ohkubo, S. Ogo, N. V. Tkachenko, H. Lemme-
tyinen, J. Am. Chem. Soc. 2004, 126, 1600–1601.
[16] N. A. Romero, K. A. Margrey, N. E. Tay, D. A. Nicewicz, Science 2015, 349,
1326–1330.
[17] a) D. A. DiRocco, K. Dykstra, S. Krska, P. Vachal, D. V. Conway, M. Tudge,
Angew. Chem. Int. Ed. 2014, 53, 4802–4806; Angew. Chem. 2014, 126,
4902–4906; b) A. Hu, J.-J. Guo, H. Pan, Z. Zuo, Science 2018, 361, 668–
672; c) K. Zhang, L. Chang, Q. An, X. Wang, Z. Zuo, J. Am. Chem. Soc.
2019, 141, 10556–10564.
[18] a) M. Jouffroy, D. N. Primer, G. A. Molander, J. Am. Chem. Soc. 2016, 138,
475–478; b) N. R. Patel, C. B. Kelly, A. P. Siegenfeld, G. A. Molander, ACS
Catal. 2017, 7, 1766–1770.
[19] Gaussian 09, Revision A.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, G. A. Pe-
tersson, H. Nakatsuji, X. Li, M. Caricato, A. Marenich, J. Bloino, B. G. Jane-
sko, R. Gomperts, B. Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov,
J. L. Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F. Egidi, J.
Goings, B. Peng, A. Petrone, T. Henderson, D. Ranasinghe, V. G. Zakrzew-
ski, J. Gao, N. Rega, G. Zheng, W. Liang, M. Hada, M. Ehara, K. Toyota, R.
Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E. Peralta, F.
Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov,
T. Keith, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C.
Burant, S. S. Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C.
Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K. Morokuma, O. Farkas,
J. B. Foresman, D. J. Fox, Gaussian, Inc., Wallingford CT, 2016.
[3] a) C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Chem. Rev. 2013, 113,
5322–5363; b) N. A. Romero, D. A. Nicewicz, Chem. Rev. 2016, 116,
10075–10166; c) D. M. Schultz, T. P. Yoon, Science 2014, 343, 1239176.
[4] a) L. Chu, C. Ohta, Z. Zuo, D. W. C. Macmillan, J. Am. Chem. Soc. 2014,
136, 10886–10889; b) G. Pratsch, G. L. Lackner, L. E. Overman, J. Org.
Chem. 2015, 80, 6025–6036; c) A. Millet, Q. Lefebvre, M. Rueping,
Chem. Eur. J. 2016, 22, 13464–13468; d) A. Gualandi, E. Matteucci, F.
Monti, A. Baschieri, N. Armaroli, L. Sambri, P. G. Cozzi, Chem. Sci. 2017,
8, 1613–1620; e) N. P. Ramirez, J. C. Gonzalez-Gomez, Eur. J. Org. Chem.
2017, 2154–2163; f) J.-Q. Chen, R. Chang, Y.-L. Wei, J.-N. Mo, Z.-Y. Wang,
P.-F. Xu, J. Org. Chem. 2018, 83, 253–259.
[5] a) G. L. Lackner, K. W. Quasdorf, L. E. Overman, J. Am. Chem. Soc. 2013,
135, 15342–15345; b) C. C. Nawrat, C. R. Jamison, Y. Slutskyy, D. W. C.
Macmillan, L. E. Overman, J. Am. Chem. Soc. 2015, 137, 11270–11273.
[6] a) J. T. M. Correia, V. A. Fernandes, B. T. Matsuo, J. A. C. Delgado, W. C.
de Souza, M. W. Paix¼o, Chem. Commun. 2020, 56, 503–514; b) M.-M.
Zhang, F. Liu, Org. Chem. Front. 2018, 5, 3443–3446; c) J. Wu, P. S.
Grant, X. Li, A. Noble, V. K. Aggarwal, Angew. Chem. Int. Ed. 2019, 58,
5697–5701; Angew. Chem. 2019, 131, 5753–5757.
[7] a) A. Elmarrouni, C. B. Ritts, J. Balsells, Chem. Sci. 2018, 9, 6639–6646;
b) M. Giedyk, R. Narobe, S. Weiß, D. Touraud, W. Kunz, B. Kçnig, Nat.
Catal. 2020, 3, 40–47.
[8] a) Y. Yasu, T. Koike, M. Akita, Adv. Synth. Catal. 2012, 354, 3414–3420;
b) T. Chinzei, K. Miyazawa, Y. Yasu, T. Koike, M. Akita, RSC Adv. 2015, 5,
21297–21300; c) D. R. Heitz, K. Rizwan, G. A. Molander, J. Org. Chem.
2016, 81, 7308–7313; d) H. Huo, K. Harms, E. Meggers, J. Am. Chem.
Soc. 2016, 138, 6936–6939; e) F. Lima, U. K. Sharma, L. Grunenberg, D.
Saha, S. Johannsen, J. Sedelmeier, E. V. Van der Eycken, S. V. Ley, Angew.
Chem. Int. Ed. 2017, 56, 15136–15140; Angew. Chem. 2017, 129, 15332–
15336; f) Y. Sato, K. Nakamura, Y. Sumida, D. Hashizume, T. Hosoya, H.
Ohmiya, J. Am. Chem. Soc. 2020, 142, 9938–9943.
[9] a) V. CorcØ, L.-M. Chamoreau, E. Derat, J.-P. Goddard, C. Ollivier, L. Fen-
sterbank, Angew. Chem. Int. Ed. 2015, 54, 11414–11418; Angew. Chem.
2015, 127, 11576–11580; b) Y. Deng, Q. Liu, A. B. Smith, J. Am. Chem.
Soc. 2017, 139, 9487–9490; c) M. Uygur, T. Danelzik, O. García Manche-
Ço, Chem. Commun. 2019, 55, 2980–2983.
Manuscript received: December 11, 2020
Accepted manuscript online: December 31, 2020
Version of record online: February 11, 2021
Chem. Eur. J. 2021, 27, 6713 –6718
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