Aryl Ketone Catalyzed Radical Allylation of C(sp3)-H Bonds under Photoirradiation

Article abstract of DOI:10.1021/acs.orglett.6b03586

The catalytic introduction of an allyl group at nonacidic C(sp³)-H bonds was achieved under photoirradiation, in which 1,2-bis(phenylsulfonyl)-2-propene acts as an allyl source and 5,7,12,14-pentacenetetrone (PT) works as a C-H bond-cleaving catalyst. A variety of substances, including alkanes, carbamates, ethers, sulfides, and alcohols, were chemoselectively allylated in a single step under neutral conditions. The present transformation is catalyzed solely by an organic molecule, PT, and proceeds smoothly even under visible light irradiation (425 nm) in the case of alkanes as a starting substance.

Full text of DOI:10.1021/acs.orglett.6b03586

Letter
pubs.acs.org/OrgLett
Aryl Ketone Catalyzed Radical Allylation of C(sp3)−H Bonds under
Photoirradiation
Shin Kamijo,*,† Kaori Kamijo,† Kiyotaka Maruoka,‡ and Toshihiro Murafuji†
‡
†Graduate School of Sciences and Technology for Innovation and Department of Biology and Chemistry, Yamaguchi University,
Yamaguchi 753-8512, Japan
S
* Supporting Information
ABSTRACT: The catalytic introduction of an allyl group at
nonacidic C(sp3)−H bonds was achieved under photoirradiation,
in which 1,2-bis(phenylsulfonyl)-2-propene acts as an allyl source
and 5,7,12,14-pentacenetetrone (PT) works as a C−H bond-
cleaving catalyst. A variety of substances, including alkanes,
carbamates, ethers, sulfides, and alcohols, were chemoselectively
allylated in a single step under neutral conditions. The present
transformation is catalyzed solely by an organic molecule, PT, and
proceeds smoothly even under visible light irradiation (425 nm) in the case of alkanes as a starting substance.
he allyl group is one of the most important carbon units in
organic chemistry because of its synthetic versatility.1 For
Scheme 1. Strategies for Substitutive Allylation of C(sp3)−H
T
Bonds
instance, the allyl group provides multiple opportunities for
further bond-forming transformations since the olefinic moiety
can be easily converted to electrophilic functional groups, such as
aldehydes and epoxides, by oxidative manipulations. The
synthetic utility of the allyl group has been further accentuated
by the emergence of metathesis, the formation of a new olefinic
product by the recombination of two olefins.2 For this reason,
intensive investigations have been carried out on allylation
reactions from a wide array of aspects, such as the development of
new allylating agents, stererochemical control of allylation, and
catalytic allylation as well as their mechanistic investigations.
Among the substitutive introductions of the allyl group at
C(sp3)−H bonds, the majority of methods involve anionic
allylation after generation of a carbanion by making use of the
acidic nature of C−H bonds adjacent to electron-withdrawing
groups (E, Scheme 1-1). LDA and LHMDS are representative
bases for deprotonation to generate the carbanion (Scheme 1-
1a),3 and an allyl carbonate−Pd(0) catalyst is a standard
combination for allylation of active methine/methylene moieties
(Scheme 1-1b).4 Despite numerous examples for allylation of
acidic C(sp3)−H bonds, the methods for substitutive introduc-
tion of an allyl group at nonacidic C(sp3)−H bonds are still
underdeveloped.5,6
catalyzed solely by the organic molecule, PT, and the allylation of
alkanes smoothly proceeds even under irradiation of visible light
at 425 nm wavelength.
At the outset of our investigation, we screened aryl ketones
that are potentially effective for C−H bond cleavage of
cyclooctane with employment of 1,2-bis(phenylsulfonyl)-2-
propene as the allyl source9,10 (Table 1). The reaction was
catalyzed by benzophenone (Ph2CO, 10 mol %), and the
allylation product 2a was obtained in 52% yield (entry 1). This is
a rare example of a photoinduced substitutive C−H function-
We therefore investigated allylation of unreactive C(sp3)−H
bonds by applying a photochemical C−H functionalization
strategy (Scheme 1-2).7,8 The keys to realizing such a
transformation are to find a suitable aryl ketone as a precursor
for an oxyl radical species to cleave the C−H bonds, and the
design of an allyl source to trap the in situ generated carbon
radical A. We herein report the photoinduced catalytic allylation
of nonacidic C(sp3)−H bonds, in which 1,2-bis(phenylsulfonyl)-
2-propene acts as the allyl source and 5,7,12,14-pentacenetetrone
(PT) works as the C−H bond-cleaving catalyst. The character-
istic features of the present transformation are that the reaction is
Received: December 1, 2016
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.6b03586
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
ab
,
Table 1. Optimization of Reaction Conditions
Scheme 2. Allylation of Alkanes
bc
,
entry
aryl ketone
mol %
hν, nm
solvent
yield, %
d
1
2
3
4
5
6
7
8
Ph2CO
4-BzPy
2-ClAQ
PT
10
10
10
10
10
10
10
5
365
365
365
365
425
455
425
425
425
365
425
MeCN
MeCN
MeCN
MeCN
MeCN
MeCN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
52
d
30
61
d
58
PT
55
d
PT
34
PT
59
55
60
63
38
PT
e
9
PT
5
e
10
PT
5
e
11
PT
1
a
a
Conditions: alkane 1 (5 equiv), allyl source (1 equiv), PT (5 mol %),
Conditions: cyclooctane 1a (10 equiv), allyl source (1 equiv), aryl
K2CO3 (1.1 equiv), CH2Cl2 (0.05 M), photoirradiation using a 425
ketone (1−10 mol %), solvent (0.05 M), photoirradiation using an
b
b
nm LED lamp at rt for 24 h unless otherwise noted. Isolated yield.
LED lamp at rt for 24 h. Isolated yields unless otherwise noted.
c
d
c
d
The alkanes 1b and 1d (10 equiv) were used. Isolated yield of
Recovery of the allyl source was observed in general. Yield based on
e
e
1H NMR analysis of the crude mixture. The reaction was conducted
with addition of K2CO3 (1.1 equiv).
reaction using a 365 nm LED lamp. Minute amounts of the
methylene adducts were observed (<4%). The reaction was
f
g
conducted without addition of K2CO3. The reaction was conducted
h
using 1f (1 equiv) and the allyl source (1.2 equiv). Yield based on 1H
alization that was efficiently promoted even with a catalytic
amount of aryl ketone.7i,8c,d,g The use of 4-benzoylpyridine (4-
BzPy)7h,i,11 reduced the product yield (30%, entry 2), and that of
quinone-based ketones, 2-chloroanthraquinone (2-ClAQ)7j and
PT, improved the yield (61% and 58% yield, entries 3 and 4). The
application of PT enabled the use of visible light at 425 nm
wavelength, and 2a was formed in practically the same yield as
observed by the reaction under 365 nm light irradiation (entry
5).12 The use of 455 nm light, however, appeared to retard the
reaction progress (entry 6). A halogenated solvent, CH2Cl2,
slightly improved the solubility of PT (entry 7), and its amount
could be reduced (5 mol %, entry 8).13 Further investigations
revealed that the addition of K2CO3 slightly increased the yield of
2a (60%, entry 9).14 The irradiation of 365 nm light again gave
almost the same yield of 2a under the optimized conditions
(63%, entry 10), whereas the reaction with 1 mol % PT decreased
the product yield (38%, entry 11).
NMR analysis of the crude mixture.
the product 2j in 31% yield. Although the yield of 2j remained
moderate, an alternative method for one-step conversion of 1j to
2j is not readily available at the moment.
We next examined the catalytic allylation of heteroatom-
containing substances 1k−s (Scheme 3). The reaction of
Scheme 3. Allylation of Heteroatom-Containing
a b
,
Substances
We then explored the applicability of the catalytic allylation of
nonacidic C−H bonds by treating various alkanes 1b−j with the
allyl source (1 equiv) in the presence of 5 mol % PT and 1.1 equiv
of K2CO3 under photoirradiation in CH2Cl2 (Scheme 2). The
reactions of cyclohexane 1b and cyclododecane 1c using 425 nm
light afforded the corresponding products 2b and 2c in 69% and
58% yield, respectively. Irradiation of 365 nm light promoted the
allylation as well for these cycloalkanes (63% and 42% yield,
respectively). The reaction of adamantane 1d predominantly
provided the 1-allyladamantane 2d along with a minute amount
of 2-allyladamantane 2d′.15 This result clarified that the reactivity
of the methine C−H bond is higher than that of the methylene
one. The reaction of functionalized adamantanes 1e and 1f, with
tosylamide and hydroxy groups, proceeded selectively at the
methine carbon to furnish the respective adducts 2e and 2f. It is
noteworthy that allylation at the amide and hydroxy functional
groups was not detected at all under the present conditions. The
benzoylated adamantanol 1g and the adamantanemethanol
derivative 1h furnished the expected allylation products 2g and
2h. The allylation of adamantaneethanol 1i took place at the
adamantane core as well. The acyclic compound 1j was
chemoselectively allylated at the methine C−H bond to furnish
a
Conditions: starting substance 1 (5 equiv), allyl source (1 equiv), PT
(5 mol %), K2CO3 (1.1 equiv), CH2Cl2 (0.05 M), photoirradiation
using a 365 nm LED lamp at rt for 24 h unless otherwise noted.
b
c
Isolated yield. The heterocycle 1k or 1p (10 equiv) was used.
d
e
Irradiated using a 425 nm LED lamp. The reaction using a 425 nm
1
LED lamp gave <26% yield based on H NMR analysis of the crude
mixture.
tetrahydrofuran (THF) 1k revealed that the allyl group was
chemoselectively introduced to the C-center adjacent to the O-
atom to form the product 2k in 64% yield with 365 nm light and
73% yield with 425 nm light. The allylation of ambroxide 1l
proceeded with the same chemoselectivity without touching
potentially reactive methine C−H bonds. In the case of allylation
B
DOI: 10.1021/acs.orglett.6b03586
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
of N-Boc pyrrolidine 1m, the irradiation of 365 nm light (76%)
was more effective than that of 425 nm light (<26% yield
determined by NMR analysis). The allylation of the morpholine
derivative 1n clarified that the C−H bond proximal to the N-Boc
group has higher reactivity than that next to the O-atom (2n).
The acyclic starting material 1o derived from butylamine
furnished the expected adduct 2o as well. The cyclic sulfides,
such as tetrahydrothiophene 1p and thioxane 1q, were allylated
at the C−H bond adjacent to the S-atom to produce the
corresponding adducts 2p (81%) and 2q (57%). The allylation of
cyclohexanol 1r took place at the hydroxylated C-center to give
the tertiary homoallyl alcohol 2r. The construction of a
tetrasubstituted C-center via selective allylation at the most
sterically congested methine C−H bond is of note. The allyl
group was similarly introduced to the primary alcohol 1s to
provide the secondary alcohol 2s. This result illustrates that the
reacting C−H bond can be controlled by the proper choice of
alcohol protecting group (see Scheme 2, 1j → 2j).
determining step based on the KIE experiment. The derived
radical A then adds to the electron-deficient vinyl sulfone moiety
of the allyl source to furnish the radical C.7e,j Subsequent
elimination of the sulfonyl radical furnishes the allylated product
2, and the released sulfonyl radical most probably accepts the
hydrogen from B to regenerate PT,18 completing the catalytic
cycle for the present substitutive allylation of nonacidic C−H
bonds.
To highlight the superiority of the present one-step allylation
of nonacidic C(sp3)−H bonds for rapid assembly of several
molecules and for construction of a cyclic molecular framework,
we performed the following two transformations (Scheme 5).
Scheme 5. Transformations of Allylated Products
Having succeeded in the photoinduced catalytic substitutive
introduction of an allyl group to a variety of substances, we
turned our attention to obtaining mechanistic information on the
present transformation. We conducted the allylation of cyclo-
octane 1a in the presence of TEMPO (see Supporting
Information for details). The analysis of the crude mixture
revealed the formation of the TEMPO adduct 3a along with a
trace amount of the allylated product 2a and recovery of the allyl
source. This result strongly indicated the generation of carbon
radical A during the course of the reaction (Schemes 1 and 4).
The first one is the reaction of the allylated adduct 2e with
isopropyl alcohol by the photoinduced Michael-type radical
addition (3e).7e,j,8h,19 Thus, two different molecules could be
successfully linked with 1,2-bis(phenylsulfonyl)-2-propene as a
tethering precursor utilizing the two different types of photo-
induced C−H functionalization reactions. The second one is the
preparation of azacycles 3o and 4o20 from acyclic homoallyl-
amine 2o. The pyrrolidine core could be constructed in merely
two steps from N-Boc butylamide 1o as a starting substance.
In conclusion, we have developed the photoinduced catalytic
allylation of nonacidic C(sp3)−H bonds of a variety of
substances, including alkanes, carbamates, ethers, sulfides, and
alcohols, in a chemoselective manner. The reaction is proposed
to proceed through a radical mechanism, in which the homolytic
cleavage of nonacidic C(sp3)−H bonds is effected solely by
photoexcited 5,7,12,14-pentacenetetrone (PT), and the allyl
group is delivered from 1,2-bis(phenylsulfonyl)-2-propene. In
this study, we showed for the first time that (1) the organic
molecule, PT, acts as an excellent C−H bond cleaving agent; (2)
the direct allylation of nonacidic C(sp3)−H bonds can be
efficiently catalyzed by PT; and (3) PT allows the use of visible
light (425 nm) for the functionalization of alkanes. The newly
developed radical allylation of nonacidic C(sp3)−H bonds
complements conventional methods for anionic introduction of
an allyl group at acidic C(sp3)−H bonds under basic conditions.
In addition, the present C−H allylation provides rapid access to
structurally complex substances by taking advantage of the
electrophilic nature of the installed vinyl sulfone moiety for
further transformations. The protocol developed in this study,
therefore, should serve as a unique tool for carbon chain
extension stemming from nonacidic C(sp3)−H bonds.
Scheme 4. Proposed Mechanism for the Photoinduced
Catalytic Radical Allylation of Nonacidic C(sp3)−H Bonds
We then measured the kinetic isotope effect by treating a
mixture of cyclohexane 1b and its deuterated analogue 1b-d with
the allyl source under the standard conditions (see Supporting
Information for details). The value of the kinetic isotope effect
(KIE) was determined to be 3.6, indicating that C(sp3)−H bond
cleavage is involved in the rate-determining step.
Taking all this information into account, we propose a reaction
mechanism as illustrated in Scheme 4. The photoexcited aryl
ketone, PT, having biradical reactivity, abstracts a hydrogen from
the starting substance 1 to generate the carbon radical
intermediate A and the semiquinone-type radical B.16,17 The
generation of the carbon radical A was strongly supported by the
TEMPO trap (3a), and this step was suggested to be the rate-
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b03586.
Additional experimental details (PDF)
C
DOI: 10.1021/acs.orglett.6b03586
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
135, 17494. (d) Xia, J.-B.; Zhu, C.; Chen, C. Chem. Commun. 2014, 50,
11701. (e) Cantillo, D.; de Frutos, O.; Rincon, J. A.; Mateos, C.; Kappe,
́
C. O. J. Org. Chem. 2014, 79, 8486. (f) Ota, E.; Mikame, Y.; Hirai, G.;
Nishiyama, S.; Sodeoka, M. Synlett 2016, 27, 1128. (g) Lipp, A.; Lahm,
G.; Opatz, T. J. Org. Chem. 2016, 81, 4890. (h) For a recent review, see:
Chen, C. Org. Biomol. Chem. 2016, 14, 8641.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kamijo@yamaguchi-u.ac.jp.
ORCID
Shin Kamijo: 0000-0002-5662-5371
Notes
(9) For reports using allyl sulfones as an allyl source in radical
chemistry, see: (a) Baldwin, J. E.; Adlington, R. M.; Birch, D. J.;
Crawford, J. A.; Sweeney, J. B. J. Chem. Soc., Chem. Commun. 1986, 1339.
(b) Padwa, A.; Murphree, S. S.; Yeske, P. E. Tetrahedron Lett. 1990, 31,
2983. (c) Kim, S.; Lim, C. J. Angew. Chem., Int. Ed. 2002, 41, 3265.
(d) Lee, S.; Lim, C. J.; Kim, S. Bull. Korean Chem. Soc. 2004, 25, 1611.
(e) Lee, J. Y.; Kim, S. Bull. Korean Chem. Soc. 2006, 27, 189.
(f) Schaffner, A.-P.; Renaud, P. Angew. Chem., Int. Ed. 2003, 42, 2658.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research was supported by the Program to Disseminate
Tenure Tracking System (MEXT, Japan) and the research grant
of Astellas Foundation for Research on Metabolic Disorders. We
thank Prof. Masayuki Inoue and Dr. Yuuki Amaoka at The
University of Tokyo for valuable suggestions on design of the
allyl source. We also acknowledge Prof. Masahiro Terada at
Tohoku University for continuous encouragement.
(g) Xu, G.; Luthy, M.; Habegger, J.; Renaud, P. J. Org. Chem. 2016, 81,
̈
1506. (h) De Dobbeleer, C.; Pospisi
F.; Marko, I. E. Chem. Commun. 2009, 2142. (i) Quinn, R. K.; Schmidt,
V. A.; Alexanian, E. J. Chem. Sci. 2013, 4, 4030. (j) Corce, V.;
̌
l, J.; De Vleeschouwer, F.; De Proft,
́
́
Chamoreau, L.-M.; Derat, E.; Goddard, J.-P.; Ollivier, C.; Fensterbank,
L. Angew. Chem., Int. Ed. 2015, 54, 11414. (k) Cui, L.; Chen, H.; Liu, C.;
Li, C. Org. Lett. 2016, 18, 2188. (l) Heitz, D. R.; Rizwan, K.; Molander,
G. A. J. Org. Chem. 2016, 81, 7308. (m) Qi, L.; Chen, Y. Angew. Chem.,
Int. Ed. 2016, 55, 3312.
(10) For protocols of the allyl source preparation, see: (a) Padwa, A.;
Kline, D. N.; Norman, B. H. Tetrahedron Lett. 1988, 29, 265. (b) Padwa,
A.; Kline, D. N.; Murphree, S. S.; Yeske, P. E. J. Org. Chem. 1992, 57, 298.
(c) Chang, M.-Y.; Wu, M.-H. Synlett 2014, 25, 411.
(11) Kamijo, S.; Tao, K.; Takao, G.; Tonoda, H.; Murafuji, T. Org. Lett.
2015, 17, 3326.
(12) Baxter, I.; Cameron, D. W.; Titman, R. B. J. Chem. Soc. C 1971,
1253.
(13) The reactions in acetone, benzene, and t-BuOH gave lower yields.
(14) Inorganic salts, such as Li2CO3, Na2CO3, Cs2CO3, and CaCO3, as
well as 2,6-di(tert-butyl)pyridine, gave similar results. Accordingly, it is
presumed that these bases neutralize in situ generated PhSO2H to
suppress undesired side reactions. A similar base effect was observed in
ref 8g.
(15) 2-Allyladamantane 2d′ was prepared for unambiguous structure
confirmation; see: (a) Wang, G.-Z.; Jiang, J.; Bu, X.-S.; Dai, J. J.; Xu, J.;
Fu, Y.; Xu, H.-J. Org. Lett. 2015, 17, 3682. (b) Inomata, K.; Sasaoka, S.-i.;
Kobayashi, T.; Tanaka, Y.; Igarashi, S.; Ohtani, T.; Kinoshita, H.;
Kotake, H. Bull. Chem. Soc. Jpn. 1987, 60, 1767.
REFERENCES
■
(1) For selected reviews on allylation, see: (a) Yamamoto, Y.; Asao, N.
Chem. Rev. 1993, 93, 2207. (b) Yus, M.; Gonzal
́
ez-Gom
́
ez, J. C.;
Foubelo, F. Chem. Rev. 2013, 113, 5595.
(2) For selected reviews on olefin metathesis, see: (a) Handbook of
Metathesis; Grubbs, R. H., Wenzel, A. G., O’Leary, D. J., Khosravi, E.,
Eds.; Wiley-VCH: Weinheim, 2005. (b) Hoveyda, A. H.; Malcolmson, S.
J.; Meek, S. J.; Zhugralin, A. R. Angew. Chem., Int. Ed. 2010, 49, 34.
(c) Nicolaou, K. C.; Bulger, P. G.; Sarlah, D. Angew. Chem., Int. Ed. 2005,
44, 4490.
(3) For a book Chapter on alkylation of enols and enolates, see: Stoltz,
B. M.; Bennett, N. B.; Duquette, D. C.; Goldberg, A. F. G.; Liu, Y.;
Loewinger, M. B.; Reeves, C. M. In Comprehensive Organic Synthesis, 2nd
ed.; Knochel, P., Molander, G. A., Eds.; Elsevier: Amsterdam, 2014; Vol.
3, pp 1−55.
(4) For selected reviews on π-allylpalladium chemistry, see: (a) Tsuji, J.
Palladium Reagents and Catalysts; Wiley: Chichester, 2004; Chapter 4,
pp 431−517. (b) Trost, B. M.; Crawley, M. L. Chem. Rev. 2003, 103,
2921.
(5) For pioneering works on the allylation of nonacidic C(sp3)−H
bonds, see: (a) Xiang, J.; Evarts, J.; Rivkin, A.; Curran, D. P.; Fuchs, P. L.
Tetrahedron Lett. 1998, 39, 4163. (b) Tanko, J. M.; Sadeghipour, M.
Angew. Chem., Int. Ed. 1999, 38, 159. (c) Zhang, J.; Li, Y.; Zhang, F.; Hu,
C.; Chen, Y. Angew. Chem., Int. Ed. 2016, 55, 1872.
(16) For a UVspectrum of PT, see: Baxter, I.; Cameron, D. W.; Titman,
R. B. J. Chem. Soc. C 1971, 1253.
(17) For the structure of the anion radical of PT, see: (a) Forkner, M.
W.; Miller, L. L.; Rak, S. F. Synth. Met. 1990, 36, 65. (b) Almlof, J. E.;
̈
(6) For recent representative reports on the C(sp3)−H functionaliza-
tion using a tungsten photocatalyst, see: (a) Ravelli, D.; Protti, S.;
Fagnoni, M. Acc. Chem. Res. 2016, 49, 2232. (b) Murphy, J. J.; Bastida,
D.; Paria, S.; Fagnoni, M.; Melchiorre, P. Nature 2016, 532, 218.
(c) Yamada, K.; Okada, M.; Fukuyama, T.; Ravelli, D.; Fagnoni, M.; Ryu,
I. Org. Lett. 2015, 17, 1292. (d) Okada, M.; Fukuyama, T.; Yamada, K.;
Ryu, I.; Ravelli, D.; Fagnoni, M. Chem. Sci. 2014, 5, 2893.
Feyereisen, M. W.; Jozefiak, T. H.; Miller, L. L. J. Am. Chem. Soc. 1990,
112, 1206.
(18) Although the possibility of a radical chain mechanism cannot be
exclusively ruled out at present, we confirmed that the allylation took
place only when the light was irradiated (see Supporting Information for
details) and that the recovery of a majority of PT was observed after the
reaction. More examinations are required to account for the precise
reaction mechanism, see: Cismesia, M. A.; Yoon, T. P. Chem. Sci. 2015,
6, 5426.
(19) Pioneering work on the photoinduced alkylation of isopropanol:
Schenck, G. O.; Koltzenburg, G.; Grossmann, H. Angew. Chem. 1957,
69, 177.
(7) (a) Kamijo, S.; Hoshikawa, T.; Inoue, M. Tetrahedron Lett. 2011,
52, 2885. (b) Kamijo, S.; Hoshikawa, T.; Inoue, M. Org. Lett. 2011, 13,
5928. (c) Hoshikawa, T.; Yoshioka, S.; Kamijo, S.; Inoue, M. Synthesis
2013, 45, 874. (d) Hoshikawa, T.; Kamijo, S.; Inoue, M. Org. Biomol.
Chem. 2013, 11, 164. (e) Amaoka, Y.; Nagatomo, M.; Watanabe, M.;
Tao, K.; Kamijo, S.; Inoue, M. Chem. Sci. 2014, 5, 4339. (f) Kamijo, S.;
Hirota, M.; Tao, K.; Watanabe, M.; Murafuji, T. Tetrahedron Lett. 2014,
55, 5551. (g) Kamijo, S.; Tao, K.; Takao, G.; Murooka, H.; Murafuji, T.
Tetrahedron Lett. 2015, 56, 1904. (h) Kamijo, S.; Watanabe, M.; Kamijo,
K.; Tao, K.; Murafuji, T. Synthesis 2016, 48, 115. (i) Kamijo, S.; Takao,
G.; Kamijo, K.; Hirota, M.; Tao, K.; Murafuji, T. Angew. Chem., Int. Ed.
2016, 55, 9695. (j) Kamijo, S.; Takao, G.; Kamijo, K.; Tsuno, T.;
Ishiguro, K.; Murafuji, T. Org. Lett. 2016, 18, 4912.
(20) Coldham, I.; Leonori, D. J. Org. Chem. 2010, 75, 4069.
(8) For related examples of C(sp3)−H functionalization using a
photoexcited ketone, see: (a) Hoshikawa, T.; Inoue, M. Chem. Sci. 2013,
4, 3118. (b) Nagatomo, M.; Yoshioka, S.; Inoue, M. Chem. - Asian J.
2015, 10, 120. (c) Xia, J.-B.; Zhu, C.; Chen, C. J. Am. Chem. Soc. 2013,
D
DOI: 10.1021/acs.orglett.6b03586
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