Visible-Light-Promoted Photocatalytic B-C Coupling via a Boron-Centered Carboranyl Radical: Facile Synthesis of B(3)-Arylated o-Carboranes

Article abstract of DOI:10.1002/anie.201511251

A visible-light-mediated in situ generation of a boron-centered carboranyl radical (o-C₂B₁₀H₁₁^.) has been described. With eosin Y as a photoredox catalyst, 3-diazonium-o-carborane tetrafluoroborate [3-N₂-o-C₂B₁₀H₁₁][BF₄] was converted into the corresponding boron-centered carboranyl radical intermediate, which can undergo efficient electrophilic substitution reaction with a wide range of (hetero)arenes. This general and simple procedure provides a metal-free alternative for the synthesis of 3-(hetero)arylated-o-carboranes.

Full text of DOI:10.1002/anie.201511251

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201511251
German Edition: DOI: 10.1002/ange.201511251
Photocatalysis
À
Visible-Light-Promoted Photocatalytic B C Coupling via a Boron-
Centered Carboranyl Radical: Facile Synthesis of B(3)-Arylated
o-Carboranes
Da Zhao and Zuowei Xie*
Abstract:
A visible-light-mediated in situ generation of
a boron-centered carboranyl radical (o-C2B10H11C) has been
described. With eosin Yas a photoredox catalyst, 3-diazonium-
o-carborane tetrafluoroborate [3-N2-o-C2B10H11][BF4] was
converted into the corresponding boron-centered carboranyl
radical intermediate, which can undergo efficient electrophilic
substitution reaction with a wide range of (hetero)arenes. This
general and simple procedure provides a metal-free alternative
for the synthesis of 3-(hetero)arylated-o-carboranes.
B
oron-centered radical species (boryl radicals) have
recently attracted considerable attention.[1] The major work
in this area focuses on the isolation and characterization of
such reactive species by overcoming daunting synthetic
difficulties.[2,3] In contrast, a growing interest has been
directed towards the reactivities and applications of in situ
generated boryl radicals.[2f, 4] For instance, N-heterocyclic
carbene (NHC) stabilized boryl radicals have been used in
various organic radical reactions, such as radical deoxygena-
tion of xanthates,[2f, 4c,5] radical reductions of alkyl halides,[6]
radical chain homolytic substitution reactions,[7] and reductive
decyanation of organic nitriles.[8] However, their applications
are far less developed compared with other main group
element based radicals, such as carbon-, tin-, silicon-, nitro-
gen-, or oxygen-centered radical species. This gap may be
attributed to limited number of available methods for
generating transient boryl radicals. The established methods
rely on hydrogen abstraction from the parent boranes or the
Scheme 1. In situ generation of boryl radicals.
and aromatic units.[14] Despite the remarkable progress in
carborane chemistry, straightforward and general syntheses of
3-aryl-o-carboranes, in particular 3-heteroaryl-o-carboranes,
still represents a very challenging task.[15] They are generally
prepared either by the reaction of [nido-7,8-C2B9H11]2À with
ArBX2 (X = Cl, Br, I),[16] or the palladium-catalyzed cross-
coupling reaction of 3-iodo-o-carboranes with either aryl
Grignard/organozinc reagents or boronic acids.[17] Recently,
we reported another approach to 3-aryl-o-carboranes by
using an aromatic ene reaction of 1,3-dehydro-o-carborane.[18]
However, the substrate scope is limited to arenes bearing
À
homolysis of B S bonds in NHC–boryl sulfides (Sche-
me 1).[2f, 4,9] Meanwhile, it has been documented that the
À
photolysis
of
bis(m-carboran-9-yl)mercury
[(m-
benzylic C H bonds. Relatively lower yields coupled with
C2B10H11)2Hg] leads to the formation of a boron-centered
carboranyl radical,[10] a nonclassical boron-centered radical.
However, its synthetic utility has been limited because of the
harsh reaction conditions required and the use of toxic
reagents.
competitive side reactions (Diels–Alder reactions) further
restrict its synthetic application. In addition, such an aromatic
ene reaction is not compatible with heteroarenes. To the best
of our knowledge, there has been no example on the synthesis
of 3-heteroaryl-o-carboranes in the literature.[19]
Owing to their unique steric/electronic properties, carbor-
anes are finding many applications in drug design,[11] as well as
in materials[12] and organometallic/coordination chemistry.[13]
A growing interest has been in the design and synthesis of
compounds which combine o-carborane (o-C2B10H12) clusters
Inspired by the photocatalytic transformations of aryl
diazonium salts into the corresponding aryl radicals,[20] we
wondered whether a boron-centered carboranyl radical (o-
C2B10H11C) could be generated by visible-light-induced photo-
redox catalysis from 3-diazonium-o-carborane tetrafluorobo-
rate ([3-N2-o-C2B10H11][BF4], 1;[18] Scheme 1). Herein, we
describe the generation of such a carboranyl radical by
photoredox catalysis and its reactions with (hetero)arenes for
high-yielding syntheses of 3-(hetero)aryl-o-carboranes.
To test our hypothesis, direct carboranylation of thio-
phene (2a) with the diazonium salt 1 in the presence of the
commercially available organic dye eosin Y (2 mol %), as the
photoredox catalyst,[21] was conducted to establish the optimal
[*] Dr. D. Zhao, Prof. Dr. Z. Xie
Department of Chemistry and State Key Laboratory of Synthetic
Chemistry, The Chinese University of Hong Kong
Shatin, N.T., Hong Kong (China)
E-mail: zxie@cuhk.edu.hk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201511251.
3166
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 3166 –3170

Angewandte
Communications
Chemie
Table 1: Optimization of reaction conditions.[a]
Table 2: Synthesis of 3-heteroaryl-o-carboranes.[a]
Entry
Reaction conditions
Yield [%][b]
1
2
2a (10 equiv)
2a (5 equiv)
83
89
3
2a (2 equiv)
86
4
2a (5 equiv), no eosin Y, no light
2a (5 equiv), no eosin Y, 12 h
2a (5 equiv), no light, 12 h
n.r.
35
trace
5[c]
6[d]
[a] Reaction conditions: 1 (0.1 mmol), 2a (0.2, 0.5 or 1.0 mmol), eosin Y
(2 mol%), CH3CN (2 mL), 12 W LED (530 nm), room temperature for
2 h. n.r.=no reaction. [b] Yield of isolated product. [c] Main products
were 3-fluoro-o-carborane and 3-NHCOCH3-o-carborane. [d] The starting
material 1 was recovered.
reaction conditions (Table 1). The desired 3-thienyl-o-carbor-
ane 3a was obtained in very good yields in the presence of 2–
10 equivalents of 2a (entries 1–3). Control experiments con-
firmed that both light and eosin Y were required for
significant conversion into the product (entries 4–6).[22] It
was noted that this reaction proceeded with excellent
regioselectivity as only substitution at the C2-position of the
thiophene was observed.
Under the optimal reaction conditions (Table 1, entry 2),
À
various thiophene substrates underwent C H carboranyla-
tion with the precursor 1 (Table 2, entries 1–5). In addition to
thiophene derivatives, this method was also compatible with
a variety of heterocycles (entries 6–16). Different substituents
such as alkyl, halogen, alkoxy, carbonyl, phenyl, and ester
groups were well-tolerated and the desired B(3)-(hetero)-
arylated products 3 were produced in very good to excellent
yields. The Boc group in 3j was easily removed to afford 3-
pyrrolyl-o-carborane.[22] It was noted that this reaction
proceeded with excellent regioselectivity as the carboranyla-
tion occurred only at the C2-position of the heteroarenes. If
the C2-position was unavailable, substitution at the C3-
position would take place instead (entries 4, 8, and 15). It is
noteworthy that for both 2b and 2o, which feature a phenyl
substituent, substitution at the phenyl ring was not observed
(entries 2 and 15). These results may indicate that electron-
rich arenes are more favored compared to the less-electron-
rich ones, thus suggesting the electrophilic nature of the
boron-centered carboranyl radical. For benzothiazole (2p),
a mixture of C2- and C7-carboranylated products was formed
with a molar ratio of 5:2 (entry 16).
[a] Reaction conditions: 1 (0.1 mmol), heteroarene 2a (0.5 mmol for
thiophene derivatives, 0.2 mmol for other heteroarenes), eosin Y
(2 mol%), CH3CN (2 mL), 12 W LED (530 nm), room temperature for
2 h. Yield is that of the isolated product. Boc=tert-butyloxycarbonyl.
(2v) and anthracene (2w) worked well to give the corre-
sponding carboranylated products 3v and 3w in 75 and 62%
yields, respectively, upon isolation (entries 6 and 7). More-
over, for ferrocene (2x), which can also serve as an one-
electron reductant,[23] the desired product was obtained
quantitatively after 5 minutes with only 1 equivalent of
ferrocene (entry 8).
À
To further explore the efficiency of this light-mediated
coupling reaction, simple benzene derivatives were also
examined and the results are compiled in Table 3. In general,
compared to those electron-rich heterocycles depicted in
Table 2, the reaction efficiency of these simple arenes was
lower as 10 equivalents of the arene were needed to achieve
a high conversion (Table 3, entries 1–5). Notably, for N,N-
The C H carboranylation of heteroarenes with 1 using
eosin Y is expected to proceed through a radical mecha-
nism,[24] and preliminary mechanistic investigations support
this assumption. Addition of 1.2 equivalents of 2,2,6,6-tetra-
methylpiperdinoxyl (TEMPO) to the reaction mixture com-
pletely suppressed this carboranylation process and the
TEMPO-trapped adduct 4 was isolated in 89% yield,[22]
thus suggesting that a radical pathway is involved in this
photoreaction [Eq. (1)].
À
dimethyl aniline (2u) the C H carboranylation occurred only
at the para-position (entry 5). The reactions of naphthalene
Angew. Chem. Int. Ed. 2016, 55, 3166 –3170
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3167

Angewandte
Communications
Chemie
Table 3: Scope with respect to the simple arenes.[a]
radical A. Addition of A to the arene 2 gives the intermediate
B. Oxidation of B by either the eosin Y radical cation (path a)
or the diazonium salt 1 through a radical-chain transfer
(path b) generates the carbocation intermediate C. Deproto-
nation of C leads to the final rearomatized product 3.
The possibility that 3 is produced from the Friedel–Crafts
reaction via the carboranyl boronium cation[25] D can be ruled
out by the following observations. First, for the indole 2m,
which can easily undergo Friedel–Crafts reaction at the C3-
position, carboranylation occurred only at the C2-position
(Table 2, entry 13), which is consistent with the fact that
radicals preferentially tend to add at C2 of the indole
nucleus.[26] Secondly, if D was formed in the reaction, it
would be trapped by the solvent CH3CN to generate the
corresponding solvated adduct.[25] However, no such species
was detected.
In summary, this work shows that boron-centered carbor-
anyl radical can be generated in situ by photoredox catalysis
and it can undergo electrophilic substitution with a broad
range of (hetero)arenes to efficiently produce 3-(hetero)aryl-
o-carboranes. More importantly, this general and convenient
procedure provides a metal-free approach to 3-heteroaryl-o-
carboranes, and may find applications in medicine and
materials science.[27] Efforts to utilize such carborane boryl
radicals for other synthetically useful transformations are
underway in our laboratory.
[a] Reaction conditions: 1 (0.1 mmol), arene 2a (1.0 mmol), eosin Y
(2 mol%), CH3CN (2 mL), 12 W LED (530 nm), room temperature for
2 h. Yield is that of the isolated product. [b] 5 equiv (0.5 mmol) of 2 was
used. [c] 1 equiv (0.1 mmol) of 2x was used, 5 min.
Acknowledgments
This work was supported by grants from the Research Grants
Council of the Hong Kong Special Administration Region
(Project No. CUHK7/CRF/12G), the National Basic
Research Program of China (973 Program, Grant No.
2012CB821600), and NSFC-RGC Joint Research Scheme
(Project No. N_CUHK442/14).
On the basis of the above experimental results, a plausible
reaction mechanism is proposed in Scheme 2. Irradiation of
eosin Y by visible light generates a photoexcited species,
eosin Y*, which can undergo single-electron transfer (SET)
with 1 (E1/2red = À0.23 V vs. SCE in CH3CN; see the Support-
ing Information) to afford the boron-centered carboranyl
Keywords: arylation · carborane · heterocycles · photocatalysis ·
radicals
How to cite: Angew. Chem. Int. Ed. 2016, 55, 3166–3170
Angew. Chem. 2016, 128, 3218–3222
[1] For selected reviews, see: a) C. D. Martin, M. Soleilhavoup, G.
Bertrand, Chem. Sci. 2013, 4, 3020; b) D. P. Curran, A. Solovyev,
M. M. Brahmi, L. Fensterbank, M. Malacria, E. Lacôte, Angew.
Chem. Int. Ed. 2011, 50, 10294; Angew. Chem. 2011, 123, 10476;
c) J. C. Walton, Angew. Chem. Int. Ed. 2009, 48, 1726; Angew.
Chem. 2009, 121, 1754; d) H. Braunschweig, R. D. Dewhurst,
Organometallics 2014, 33, 6271.
[2] For recent examples of neutral boryl radicals, see: a) A. J.
Rosenthal, M. Devillard, K. Miqueu, G. Bouhadir, D. Bourissou,
Angew. Chem. Int. Ed. 2015, 54, 9198; Angew. Chem. 2015, 127,
9330; b) P. Bissinger, H. Braunschweig, A. Damme, I. Krumme-
nacher, A. K. Phukan, K. Radacki, S. Sugawara, Angew. Chem.
Int. Ed. 2014, 53, 7360; Angew. Chem. 2014, 126, 7488; c) F.
Dahcheh, D. Martin, D. W. Stephan, G. Bertrand, Angew. Chem.
Int. Ed. 2014, 53, 13159; Angew. Chem. 2014, 126, 13375; d) R.
Bertermann, H. Braunschweig, R. D. Dewhurst, C. Hçrl, T.
Kramer, I. Krummenacher, Angew. Chem. Int. Ed. 2014, 53,
5453; Angew. Chem. 2014, 126, 5557; e) Y. Aramaki, H. Orniya,
Scheme 2. Proposed reaction pathways.
3168
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 3166 –3170

Angewandte
Communications
Chemie
M. Yamashita, K. Nakabayashi, S.-i. Ohkoshi, K. Nozaki, J. Am.
Prior, Q. He, C. J. Kiely, M. Brust, C. ViÇas, J. Am. Chem. Soc.
Chem. Soc. 2012, 134, 19989; f) S. H. Ueng, A. Solovyev, X.
Yuan, S. J. Geib, L. Fensterbank, E. Lacôte, M. Malacria, M.
Newcomb, J. C. Walton, D. P. Curran, J. Am. Chem. Soc. 2009,
131, 11256.
2012, 134, 212; h) M. Koshino, T. Tanaka, N. Solin, K. Suenaga,
H. Isobe, E. Nakamura, Science 2007, 316, 853.
[13] For selected reviews, see: a) N. S. Hosmane, J. A. Maguire in
Comprehensive Organometallic Chemistry III, Vol. 3 (Eds.:
R. H. Crabtree, D. M. P. Mingos), Elsevie, Oxford, 2007,
p. 175; b) Z. Xie, Coord. Chem. Rev. 2002, 231, 23; c) Z. Xie,
Acc. Chem. Res. 2003, 36, 1; d) L. Deng, Z. Xie, Coord. Chem.
Rev. 2007, 251, 2452; e) Z.-J. Yao, G.-X. Jin, Coord. Chem. Rev.
2013, 257, 2522.
[14] For recent examples, see: a) D. Zhao, J. Zhang, Z. Xie, J. Am.
Chem. Soc. 2015, 137, 9423; b) D. Zhao, J. Zhang, Z. Xie, J. Am.
Chem. Soc. 2015, 137, 13938; c) A. El-Hellani, V. Lavallo,
Angew. Chem. Int. Ed. 2014, 53, 4489; Angew. Chem. 2014, 126,
4578; d) K. R. Wee, Y. J. Cho, J. K. Song, S. O. Kang, Angew.
Chem. Int. Ed. 2013, 52, 9682; Angew. Chem. 2013, 125, 9864;
e) J. H. Wright II, C. E. Kefalidis, F. S. Tham, L. Maron, V.
Lavallo, Inorg. Chem. 2013, 52, 6223; f) K. R. Wee, W. S. Han,
S. O. Kang, Angew. Chem. Int. Ed. 2012, 51, 2677; Angew. Chem.
2012, 124, 2731; g) K.-R. Wee, Y.-J. Cho, S. Jeong, S. Kwon, J.-D.
Lee, I.-H. Suh, S. O. Kang, J. Am. Chem. Soc. 2012, 134, 17982;
h) C. Shi, H. Sun, X. Tang, H. Lv, H. Yan, Q. Zhao, J. Wang, W.
Huang, Angew. Chem. Int. Ed. 2013, 52, 13434; Angew. Chem.
2013, 125, 13676.
[15] a) R. N. Grimes, Carboranes, 2nd ed., Academic Press, New
York, 2011; b) N. S. Hosmane, Boron Science: New Technologies
and Applications, CRC Press, Boca Raton, 2012; c) V. I.
Bregadze, Chem. Rev. 1992, 92, 209; d) J. Zhang, Z. Xie, Acc.
Chem. Res. 2014, 47, 1623.
[16] For selected examples, see: a) C. ViÇas, G. Barberà, J. M. Oliva,
F. Teixidor, A. J. Welch, G. M. Rosair, Inorg. Chem. 2001, 40,
6555; b) G. Barberà, A. Vaca, F. Teixidor, R. Sillanpää, R.
Kivekäs, C. ViÇas, Inorg. Chem. 2008, 47, 7309; c) M. F.
Hawthorne, P. A. Wegner, J. Am. Chem. Soc. 1968, 90, 896;
d) A. V. Safronov, N. I. Shlyakhtina, M. F. Hawthorne, Organo-
metallics 2012, 31, 2764.
[17] a) K. Aizawa, K. Ohta, Y. Endo, Heterocycles 2010, 80, 369;
b) K. Ohta, H. Yamazaki, Y. Endo, J. Organomet. Chem. 2009,
694, 1646; c) L. Eriksson, I. P. Beletskaya, V. I. Bregadze, I. B.
Sivaev, S. Sjçberg, J. Organomet. Chem. 2002, 657, 267; d) M. J.
Bayer, A. Herzog, M. Diaz, G. A. Harakas, H. Lee, C. B.
Knobler, M. F. Hawthorne, Chem. Eur. J. 2003, 9, 2732.
[18] D. Zhao, J. Zhang, Z. Xie, Angew. Chem. Int. Ed. 2014, 53, 8488;
Angew. Chem. 2014, 126, 8628.
[19] For examples of B-heteroaryl-substituted m- and p-carboranes,
see: a) I. P. Beletskaya, V. I. Bregadze, V. A. Ivushkin, P. V.
Petrovskii, I. B. Sivaev, S. Sjçberg, G. G. Zhigareva, J. Organo-
met. Chem. 2004, 689, 2920; b) I. P. Beletskaya, V. I. Bregadze,
K. Z. Kabytaev, G. G. Zhigareva, P. V. Petrovskii, I. V. Glukhov,
Z. A. Starikova, Organometallics 2007, 26, 2340.
[3] For recent examples of anionic boryl radicals, see: a) A. Hübner,
A. M. Diehl, M. Diefenbach, B. Endeward, M. Bolte, H.-W.
Lerner, M. C. Holthausen, M. Wagner, Angew. Chem. Int. Ed.
2014, 53, 4832; Angew. Chem. 2014, 126, 4932; b) H. Braunsch-
weig, V. Dyakonov, J. O. C. Jimenez-Halla, K. Kraft, I. Krum-
menacher, K. Radacki, A. Sperlich, J. Wahler, Angew. Chem. Int.
Ed. 2012, 51, 2977; Angew. Chem. 2012, 124, 3031; c) H.
Braunschweig, F. Breher, C. W. Chiu, D. Gamon, D. Nied, K.
Radacki, Angew. Chem. Int. Ed. 2010, 49, 8975; Angew. Chem.
2010, 122, 9159.
[4] For selected examples, see: a) J. A. Baban, B. P. Roberts, J.
Chem. Soc. Chem. Commun. 1983, 1224; b) J. A. Baban, V. P. J.
Marti, B. P. Roberts, J. Chem. Soc. Perkin Trans. 2 1985, 1723;
c) M. Lucarini, G. F. Pedulli, L. Valgimigli, J. Org. Chem. 1996,
61, 4309; d) S.-H. Ueng, M. M. Brahmi, É. Derat, L. Fenster-
bank, E. Lacôte, M. Malacria, D. P. Curran, J. Am. Chem. Soc.
2008, 130, 10082; e) J. LalevØe, N. Blanchard, M.-A. Tehfe, A.-C.
Chany, J.-P. Fouassier, Chem. Eur. J. 2010, 16, 12920; f) J. C.
Walton, M. M. Brahmi, J. Monot, L. Fensterbank, M. Malacria,
D. P. Curran, E. Lacôte, J. Am. Chem. Soc. 2011, 133, 10312.
[5] S.-H. Ueng, L. Fensterbank, E. Lacôte, M. Malacria, D. P.
Curran, Org. Lett. 2010, 12, 3002.
[6] X. Pan, E. Lacôte, J. LalevØe, D. P. Curran, J. Am. Chem. Soc.
2012, 134, 5669.
[7] X. Pan, A.-L. Vallet, S. Schweizer, K. Dahbi, B. Delpech, N.
Blanchard, B. Graff, S. J. Geib, D. P. Curran, J. LalevØe, E.
Lacôte, J. Am. Chem. Soc. 2013, 135, 10484.
[8] T. Kawamoto, S. J. Geib, D. P. Curran, J. Am. Chem. Soc. 2015,
137, 8617.
[9] S. Telitel, A.-L. Vallet, S. Schweizer, B. Delpech, N. Blanchard, F.
Morlet-Savary, B. Graff, D. P. Curran, M. Robert, E. Lacôte, J.
LalevØe, J. Am. Chem. Soc. 2013, 135, 16938.
[10] a) B. L. Tumanskii, V. T. Kampel, V. I. Bregadze, N. N. Bubnov,
S. P. Solodovnikov, A. I. Prokof’ev, E. S. Kozlov, N. N. God-
ovikov, M. I. Kabachnik, Russ. Chem. Bull. 1986, 35, 425; b) V. I.
Bregadze, V. T. Kampel, A. Y. Usiatinsky, N. N. Godovikov,
Pure Appl. Chem. 1991, 63, 357; c) V. I. Bregadze, V. T. Kampel,
A. Y. Usiatinsky, N. N. Godovikov, Pure Appl. Chem. 1991, 63,
835.
[11] For selected examples, see: a) G. R. Kracke, M. R. VanGordon,
Y. V. Sevryugina, P. J. Kueffer, K. Kabytaev, S. S. Jalisatgi, M. F.
Hawthorne, ChemMedChem 2015, 10, 62; b) S. M. Wilkinson, H.
Gunosewoyo, M. L. Barron, A. Boucher, M. McDonnell, P.
Turner, D. E. Morrison, M. R. Bennett, I. S. McGregor, L. M.
Rendina, M. Kassiou, ACS Chem. Neurosci. 2014, 5, 335; c) M.
´
Scholz, G. N. Kaluderovic, H. Kommera, R. Paschke, J. Will,
[20] For recent reviews, see: a) A. Studer, D. P. Curran, Angew.
Chem. Int. Ed. 2016, 55, 58; Angew. Chem. 2016, 128, 58; b) D. P.
Hari, B. Kçnig, Angew. Chem. Int. Ed. 2013, 52, 4734; Angew.
Chem. 2013, 125, 4832; c) C. K. Prier, D. A. Rankic, D. W. C.
MacMillan, Chem. Rev. 2013, 113, 5322; d) F. Mo, G. Dong, Y.
Zhang, J. Wang, Org. Biomol. Chem. 2013, 11, 1582; e) J. Xuan,
W.-J. Xiao, Angew. Chem. Int. Ed. 2012, 51, 6828; Angew. Chem.
2012, 124, 6934; f) J. M. R. Narayanam, C. R. J. Stephenson,
Chem. Soc. Rev. 2011, 40, 102; g) M. R. Heinrich, Chem. Eur. J.
2009, 15, 821.
[21] For recent reviews on the applications of organic dyes as
photoredox catalysts, see: a) D. A. Nicewicz, T. M. Nguyen, ACS
Catal. 2014, 4, 355; b) D. P. Hari, B. Kçnig, Chem. Commun.
2014, 50, 6688.
[22] For experimental details, cyclic voltammetry, and complete
characterization data, see the Supporting Information.
W. S. Sheldrick, E. Hey-Hawkins, Eur. J. Med. Chem. 2011, 46,
1131; d) M. L. Beer, J. Lemon, J. F. Valliant, J. Med. Chem. 2010,
53, 8012; e) A.-I. Stoica, C. ViÇas, F. Teixidor, Chem. Commun.
2009, 4988.
[12] For selected examples, see: a) H. Naito, Y. Morisaki, Y. Chujo,
Angew. Chem. Int. Ed. 2015, 54, 5084 – 5087; Angew. Chem. 2015,
127, 5173 – 5176; b) J. Guo, D. Liu, J. Zhang, Q. Maio, Z. Xie,
Chem. Commun. 2015, 51, 12004; c) S. G. McArthur, L. Geng, J.
Guo, V. Lavallo, Inorg. Chem. Front. 2015, 2, 1101; d) C. A.
Lugo, C. Moore, A. Rheingold, V. Lavallo, Inorg. Chem. 2015,
54, 2094; e) A. Ferrer-Ugalde, A. Gonzµlez-Campo, C. ViÇas, J.
Rodríguez-Romero, R. Santillan, N. Farfµn, R. Sillanpää, A.
Sousa-Pedrares, R. NfflÇez, F. Teixidor, Chem. Eur. J. 2014, 20,
9940; f) R. Visbal, I. Ospino, J. M. López-de-Luzuriaga, A.
Laguna, M. C. Gimeno, J. Am. Chem. Soc. 2013, 135, 4712;
ˇ
´
g) A. M. Cioran, A. D. Musteti, F. Teixidor, Z. Krpetic, I. A.
Angew. Chem. Int. Ed. 2016, 55, 3166 –3170
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3169

Angewandte
Communications
Chemie
[23] a) N. G. Connelly, W. E. Geiger, Chem. Rev. 1996, 96, 877;
Narayanam, J. W. Tucker, C. R. J. Stephenson, Org. Lett. 2010,
12, 3104; c) Y. M. Osornio, R. Cruz-Almanza, V. Jimenez-
Montano, L. D. Miranda, Chem. Commun. 2003, 2316.
[27] For a recent review on the applications of carboranes, see R. N.
Grimes, Dalton Trans. 2015, 44, 5939.
b) R. R. Gagne, C. A. Koval, G. C. Lisensky, Inorg. Chem. 1980,
19, 2854.
[24] D. P. Hari, P. Schroll, B. Kçnig, J. Am. Chem. Soc. 2012, 134,
2958.
´
[25] B. Ringstrand, P. Kaszynski, Acc. Chem. Res. 2013, 46, 214, and
references therein.
[26] For selected examples of the addition of radicals to the indole
nucleus, see: a) T. W. Greulich, C. G. Daniliuc, A. Studer, Org.
Lett. 2015, 17, 254; b) L. Furst, B. S. Matsuura, J. M. R.
Received: December 4, 2015
Revised: December 28, 2015
Published online: January 28, 2016
3170
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 3166 –3170