Visible-Light-Mediated Aromatic Substitution Reactions of Cyanoarenes with 4-Alkyl-1,4-dihydropyridines through Double Carbon-Carbon Bond Cleavage

Article abstract of DOI:10.1002/cctc.201600037

Novel aromatic substitution reactions of cyanoarenes with 4-alkyl-1,4-dihydropyridines as alkylating reagents in the presence of a catalytic amount of a photoredox catalyst proceed smoothly to give the corresponding alkyl-substituted arenes in good to high yields. The present reaction system realizes a novel C-C bond-forming reaction between two fragments generated from the C-C bond-cleavage reactions of two independent substrates.

Full text of DOI:10.1002/cctc.201600037

DOI: 10.1002/cctc.201600037
Communications
Visible-Light-Mediated Aromatic Substitution Reactions of
Cyanoarenes with 4-Alkyl-1,4-dihydropyridines through
Double Carbon–Carbon Bond Cleavage
Kazunari Nakajima,[a] Sunao Nojima,[a] Ken Sakata,*[b] and Yoshiaki Nishibayashi*[a]
Novel aromatic substitution reactions of cyanoarenes with 4-
alkyl-1,4-dihydropyridines as alkylating reagents in the pres-
ence of a catalytic amount of a photoredox catalyst proceed
smoothly to give the corresponding alkyl-substituted arenes in
good to high yields. The present reaction system realizes
a novel CÀC bond-forming reaction between two fragments
generated from the CÀC bond-cleavage reactions of two inde-
pendent substrates.
succeeded in performing oxidative CÀC bond-cleavage reac-
tions of amines by using photoredox catalysts.[4]
As other CÀC bond-cleavage reactions under photochemical
conditions, aromatic substitution reactions of cyanoarenes
with various alkylating reagents under UV[5–7] and visible light[8]
are also quite attractive. Aromatic substitution reactions pro-
ceed by radical–radical coupling reactions between a cyanoar-
ene radical anion and an alkyl radical and the subsequent CÀC
bond-cleavage reaction of the CÀCN moiety in the resulting al-
kylated cyanoarene. Recently, MacMillan and co-workers re-
ported photoredox-catalyzed aromatic substitution reactions
of cyanoarenes with alkylating reagents, for which alkyl radicals
were generated from CÀH bond-cleavage reactions of alkylat-
ing reagents (Scheme 2a).[8]
Molecular transformation of unstrained carbon–carbon (CÀC)
bonds is one of the most challenging subjects in organic
chemistry.[1] Specifically, the development of novel CÀC bond-
forming reactions between two fragments generated from
CÀC bond-cleavage reactions of two independent substrates
(Scheme 1) is one of the most difficult issues. To achieve the
target reaction, we paid attention to the use of photoredox
catalysis for the activation and transformation of CÀC bonds,
because radical cations and radical anions formed by single-
electron-transfer processes in photoredox catalysis are consid-
ered to be suitable intermediates.[2]
Scheme 2. Aromatic substitution reactions of cyanoarenes by alkyl radicals
generated from CÀH and CÀC bond cleavage.
Scheme 1. Concept of new CÀC bond formation by double CÀC bond cleav-
age.
On the basis of this research background, our group[9] de-
signed novel photoredox-catalyzed aromatic substitution reac-
tions of cyanoarenes with alkylating reagents, for which alkyl
radicals are generated from CÀC bond-cleavage reactions of al-
kylating reagents (Scheme 2b). Novel CÀC bond-forming reac-
tions between the two fragments generated from the CÀC
bond-cleavage reactions of two independent substrates
(Scheme 1) can be realized in this reaction system. To the best
of our knowledge, there is no successful example of such
a unique reaction based on visible-light-photoredox catalysis.[7]
To achieve this type of reaction, we envisaged the use of cyclic
amines such as 4-alkyl-1,4-dihydropyridines as alkylating re-
agents, because alkyl radicals are generated from CÀC bond
cleavage at the 4-position of the dihydropyridine ring under
photochemical[10] and thermal[11] reaction conditions. In fact,
we disclose that novel aromatic substitution reactions of cya-
noarenes with 4-alkyl-1,4-dihydropyridines as alkylating re-
agents in the presence of a catalytic amount of a photoredox
catalyst proceed smoothly to give the corresponding alkyl-sub-
The generation of radical cations through the single-electron
oxidation of amines is a promising way to achieve CÀC bond-
cleavage reactions, because dissociation of inert bonds such as
CÀC bonds proceeds rapidly in the course of the fragmenta-
tion of radical cations.[3] Several research groups have already
[a] Dr. K. Nakajima, S. Nojima, Prof. Dr. Y. Nishibayashi
Institute of Engineering Innovation
School of Engineering
The University of Tokyo
Yayoi, Bunkyo-ku, Tokyo, 113-8656 (Japan)
E-mail: ynishiba@sogo.t.u-tokyo.ac.jp
[b] Prof. Dr. K. Sakata
Faculty of Pharmaceutical Sciences
Hoshi University
Ebara, Shinagawa-ku, Tokyo, 142-8501 (Japan)
E-mail: sakata@hoshi.ac.jp
Supporting Information for this article can be found under http://
dx.doi.org/10.1002/cctc.201600037.
ChemCatChem 2016, 8, 1028 – 1032
1028
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications
stituted arenes in good to high yields. Typical results are de-
scribed in this paper.
KOAc gave the products in lower yields with similar isomeric
ratios (Table 1, entries 8 and 9). We also examined the effect of
other iridium complexes (e.g., 4b and 4c) and ruthenium com-
plex 5 as photocatalysts. The use of 4b and 4c gave the prod-
ucts in slightly lower yields (Table 1, entries 10 and 11), but the
use of 5 did not afford the corresponding product (Table 1,
entry 12). Separately, we confirmed that the reaction did not
proceed at all in the absence of a photocatalyst or in the dark
(Table 1, entries 13 and 14). These results clearly indicate that
both a photoredox catalyst and visible-light irradiation are es-
sential to promote the reaction.
Initial studies on reaction conditions were performed with
1,4-dicyanobenzene (1a) and 4-benzyl-3,5-bis(ethoxycarbonly)-
2,6-dimethyl-1,4-dihydropyridine (2a) as substrates, from
which 1-benzyl-4-cyanobenzene (3a) was obtained as the
main product (Table 1). In the presence of 2 mol% of photore-
dox catalyst 4a, the reaction of 1a with 2a (2 equiv.) in 1,3-di-
methyl-2-imidazolidinone (DMI) at 258C for 18 h under visible-
light illumination with a 14 W white light-emitting diode (LED)
gave 3a in 45% yield as a single isomer (Table 1, entry 1). The
use of N,N-dimethylformamide (DMF) and dimethyl sulfoxide
(DMSO) as the solvents gave 3a in slightly lower yields
(Table 1, entries 2 and 3). However, other solvents such as tet-
rahydrofuran (THF), dichloromethane, and acetonitrile were
not effective at all (Table 1, entries 4–6).
With the optimized reaction conditions in hand, we per-
formed reactions of 1a with various 4-alkylated-1,4-dihydropyr-
idines 2 (Table 2). All reactions performed with 2 bearing 2-, 3-,
and 4-methyl groups and other electron-donating or electron-
withdrawing groups such as tert-butyl, methoxy, fluoro, and
chloro moieties on the benzene ring of the benzyl group pro-
ceeded smoothly to give corresponding products 3 in excel-
lent yields with similar isomeric ratios (Table 2, entries 1–7).
The use of dihydropyridine bearing a secondary alkyl group
(i.e., compound 2i) also afforded 3i/3i’ in 91% yield (Table 2,
entry 8).
Table 1. Reactions of 1,4-dicyanobenzene (1a) with 4-benzyl-3,5-bis-
(ethoxycarbonly)-2,6-dimethyl-1,4-dihydropyridine (2a).[a]
Table 2. Reactions of 1a with dihydropyridines 2.[a]
Entry
Catalyst
Solvent
Additive
Yield[b] [%]
3a/3a’
1
2
3
4
5
6
7[c]
8
4a
4a
4a
4a
4a
4a
4a
4a
4a
4b
4c
5
DMI
–
–
–
–
–
–
45
35
27
0
0
0
88
51
69
76
78
0
100:0
100:0
100:0
–
DMF
DMSO
THF
CH2Cl2
MeCN
DMI
DMI
DMI
DMI
DMI
–
–
NaOAc
PhCO2Na
KOAc
NaOAc
NaOAc
NaOAc
NaOAc
NaOAc
86:14
86:14
81:19
84:16
84:16
–
Entry
R (2)
Yield[b] [%] (3/3’)
Ratio of 3/3’
1[c]
2[c]
3[c]
4[c]
5[c]
6[c]
7[c]
8[c]
9[c]
10[c]
11
CH2(2-MeC6H4) (2b)
CH2(3-MeC6H4) (2c)
CH2(4-MeC6H4) (2d)
CH2(4-tBuC6H4) (2e)
CH2(4-MeOC6H4) (2 f)
CH2(4-FC6H4) (2g)
CH2(4-ClC6H4) (2h)
CH(Me)Ph (2i)
iPr (2j)
2-pentyl (2k)
CH2OMe (2l)
CH2N(CH2Ph)2 (2m)
91 (3b/3b’)
86 (3c/3c’)
90 (3d/3d’)
84 (3e/3e’)
76 (3 f/3 f’)
81 (3g/3g’)
72 (3h/3h’)
91 (3i/3i’)
88:12
83:17
82:18
80:20
78:22
79:21
88:12
81:19
79:21
89:11
>99:1
89:11
9
10
11
12
13
14[d]
DMI
DMI
DMI
–
4a
0
0
–
–
63 (3j/3j’)
73 (3k/3k’)
60 (3l/3l’)[d]
59 (3m/3m’)
12[e]
[a] All reactions of 1a (0.25 mmol) with 2 (0.50 or 0.75 mmol) were per-
formed in the presence of 4a (0.005 mmol) and NaOAc (0.50 mmol) in
DMI (1.25 mL) with 14 W white LED illumination at 258C. [b] Yield of iso-
[a] All reactions of 1a (0.25 mmol) with 2a (0.50 mmol) were performed
in the presence of the photocatalyst (0.005 mmol) and additive
(0.50 mmol) in solvent (1.25 mL) with 14 W white LED illumination at
lated isomeric mixture. [c] Compound
2 (0.75 mmol, 3 equiv.), 40 h.
[d] Yield was determined by NMR spectroscopy. [e] Catalyst 4b was used
instead of 4a.
258C for 18 h. [b] Yield of isolated product. [c] 66% yield of
(0.33 mmol). [d] In the dark.
6
Next, we examined the use of base as an additive. Upon
using NaOAc (2 equiv.), the corresponding product was ob-
tained as a mixture of 1,4- and 1,2-substituted isomers 3a and
3a’ in 88% yield with an isomeric ratio of 86:14 (Table 1,
entry 7). In this case, the formation of corresponding dealkylat-
Instead of benzylic groups in 2, some alkyl groups such as
isopropyl and 2-pentyl groups were also viable alkylating re-
agents, and they gave the corresponding products (i.e., com-
pounds 3j/3j’ and 3k/3k’) in high yields (Table 2, entries 9 and
10), whereas methyl and n-hexyl groups did not undergo the
reaction. The use of 4-alkoxymethyl- and aminomethyl-substi-
tuted 3,5-bis(ethoxycarbonyl)-2,6-dimethyl-1,4-dihydropyridines
ed pyridine derivative
6 was confirmed in 66% yield
(0.33 mmol). The use of other bases such as PhCO2Na and
ChemCatChem 2016, 8, 1028 – 1032
www.chemcatchem.org
1029
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications
(2l and 2m) gave the corresponding products in good yields
(Table 2, entries 11 and 12). It is also noteworthy that com-
pounds 2 containing heteroatom-substituted alkyl groups
were also used as alkylating reagents.
Next, we investigated reactions of a variety of dicyanoben-
zenes with 2a (Table 3). Upon using 1,4-dicyanobenzene deriv-
atives bearing fluoro, chloro, methyl, and ethyl groups, corre-
sponding products 3n–q/3n’–q’ were obtained in excellent
yields as a mixture of two isomers. Reactions of 2,3,5,6-tetra-
chloro-1,4-dicyanobenzene and 9,10-dicyanoanthracene were
also successful and gave corresponding products 3r and 3s as
single isomers.
Scheme 3. Reactions of cyanoheteroarenes with 2a.
Table 3. Reactions of dicyanobenzenes 1 with 2a.[a]
To obtain some information on the reaction pathway, we
performed the reaction of 1a with 2a in the presence of
2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO). The formation of
3a and 3a’ was not observed at all, whereas a benzyl radical
was captured by TEMPO as shown in [Eq. (2)]. This result indi-
cates that the benzyl radical is generated as a key reactive in-
termediate in the present reaction system.
[a] All reactions of 1 (0.25 mmol) with 2a (0.50 or 0.75 mmol) were per-
formed in the presence of 4a (0.005 mmol) and NaOAc (0.50 mmol) in
DMI (1.25 mL) with 14 W white LED illumination at 258C. [b] Compound
2a (0.75 mmol, 3 equiv.), 40 h.
We also performed the reactions of cyano-substituted pyri-
dine derivatives, instead of dicyanobenzenes (Scheme 3). From
4-cyanopyridine and 4-cyano-7-azaindole, corresponding prod-
ucts 3t and 3u were obtained in good yields as single isomers,
whereas 2-cyanopyridine resulted in the formation of product
3v in a lower yield. These results indicate that the reaction
system is applicable not only to dicyanobenzenes but also to
electron-deficient heterocycles such as cyano-substituted pyri-
dines.
Next, we investigated an electron-transfer process in this re-
action. At first, we determined the quantum yield of the reac-
tion of 1a with 2a to be 0.19. A value below unity indicates
that the reaction proceeds through a sequential redox process
rather than through a radical chain process.[9,13] Then, we per-
formed Stern–Volmer analyses of 4a with substrates 1a and
2a.[12] The rate constants for reduction of 1a and oxidation of
2a were determined to be k=8.3”107 mÀ1 sÀ1 and k=6.5”
108 mÀ1 À1, respectively, which indicates that both oxidation of
s
Reactions of 1,2-dicyanobenzene and 1,3-dicyanobenzene
were also performed under the same reaction conditions by
using 4a as the catalyst. From 1,2-dicyanobenzene, substituted
product 3a’ was obtained as the major isomer along with 3a
as the minor isomer [Eq. (1)]. Separately, we confirmed that iso-
merization of 1a to 1,2-dicyanobenzene and that of 3a to 3a’
did not occur under the reaction conditions.[12] These results in-
dicate that the ipso-substitution reaction of cyanoarenes leads
to the formation of alkylated arenes as the major products. No
reaction of 1,3-dicyanobenzene proceeded at all under the
same reaction conditions.
2a and reduction of 1a are possible[14] but that the oxidation
of 2a is slightly faster than the reduction of 1a.
On the basis of the experimental results, we propose a plau-
sible reaction pathway, as shown in Scheme 4. First, single-
electron oxidation of dihydropyridine 2 by the excited catalyst
(*cat) and subsequent reduction of cyanoarene 1 by the re-
duced catalyst (catÀ) occur to give radical cation A and radical
anion B. Then, CÀC bond cleavage at the 4-position of the di-
hydropyridine ring in A proceeds to give alkyl radical inter-
mediate C along with the formation of a pyridine derivative.
Radical–radical coupling between B and C gives D or D’. De-
ChemCatChem 2016, 8, 1028 – 1032
www.chemcatchem.org
1030
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications
Scheme 6. Relative Gibbs free energies [kcalmolÀ1] at 298.15 K.
Scheme 4. A plausible reaction pathway.
VII via transition state TSII. Finally, dissociation of VII into the
cyanide ion CNÀ and product III occurs through TSIII. The DG
of the final product state (III+IV+1[Ir(ppy)3]) is À53.8 kcal
molÀ1, which is largely exergonic. On the other hand, BnÀ may
also attack the ortho carbon atom of I to form VII’. However,
subsequent paths make a huge difference between ipso and
ortho attacks. Either hydride elimination (TSIII’) or hydride
transfer (TSIV’) has a higher activation barrier (DG=11.8 or
35.0 kcalmolÀ1, respectively). Thus, the formation of para-prod-
uct III is preferred. Experimentally, only para-product III was
observed in the case without an additive such as an acetate
ion. The present result indicates that the additive acetate ion
plays a role in the formation of the ortho product.
cyanation from D affords 4-substituted arene 3 as the major
product. However, decyanation from D’ after 1,3-shift may
occur to give 2-substituted arene 3’ as a minor product. As to
the formation of the minor product, a similar reaction pathway
proceeding through a 1,3-shift has been proposed by Schuster
and Lan[6e] in a UV-mediated reaction of cyanoarenes with al-
kylborates.
The reaction pathway for the model reaction (Scheme 5)
was examined by wB97X level DFT calculations.[12] The lowest
triplet (T1) state of [Ir(ppy)3] (ppy=2-phenylpyridinato),
3[Ir(ppy)3], which is generated by visible-light absorption of the
In summary, we herein disclosed photoredox-cata-
lyzed aromatic substitution reactions of cyanoarenes
with 4-alkyl-1,4-dihydropyridines as alkylating re-
agents to give the corresponding alkylated arenes in
good to high yields. The reaction system realizes
novel CÀC bond-forming reactions between two frag-
ments generated from CÀC bond-cleavage reactions
of two independent substrates. Further work on the
synthetic utilization of 4-alkyl-1,4-dihydropyridines as
alkylating reagents in the presence of photoredox
catalysts is now underway.
Scheme 5. Model reaction system for DFT calculations.
1
electronic ground singlet (S0) state, [Ir(ppy)3], and subsequent
intersystem crossing, is able to give both reductive quenching
and oxidative quenching. Experimental Stern–Volmer analyses
indicate that the former quenching cycle is preferred. Our cal-
culations show that the single-electron transfer (SET) process
from dihydropyridine II to 3[Ir(ppy)3] is 22.1 kcalmolÀ1 ender-
gonic, whereas the other SET process from [Ir(ppy)3]À to dicya-
nobenzene I is 19.6 kcalmolÀ1 exergonic. As a result, the
change in the Gibbs energy, DG, through the reductive
quenching pathway from the initial state (I+II+3[Ir(ppy)3]) to
the state (IÀ +II+ +1[Ir(ppy)3]) is +2.5 kcalmolÀ1.
Acknowledgements
This work was supported by Core Research for Evolutionary Sci-
ence and Technology (CREST), Japan Science and Technology
Agency (JST). We thank the Japan Society for the Promotion of
Science (JSPS) and the Ministry of Education, Culture, Sports, Sci-
ence and Technology of Japan (MEXT) for Grants-in-Aid for Scien-
tific Research (Nos. 26288044, 26620075, 26105708, 26105756,
and 26870120).
Then, we examined the reaction between IÀ and II+, as
shown in Scheme 6. Dihydropyridine cation II+ easily separates
C
into the benzyl radical Bn and IV via transition state TSI (DG=
9.4 kcalmolÀ1) and product complex V (DG=À3.8 kcalmolÀ1).
Keywords: arenes
reactions · photoredox catalysts · radicals
·
aromatic substitution
·
cleavage
After electron transfer from dicyanobenzene anion IÀ to Bn ,
C
the benzyl anion BnÀ attacks the ipso carbon atom of I to form
ChemCatChem 2016, 8, 1028 – 1032
www.chemcatchem.org
1031
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communications
[1] a) Y. Nakao, Bull. Chem. Soc. Jpn. 2012, 85, 731; b) K. Ruhland, Eur. J. Org.
Chem. 2012, 2683; c) H. Liu, M. Feng, X. Jiang, Chem. Asian J. 2014, 9,
3360; d) F. Chen, T. Wang, N. Jiao, Chem. Rev. 2014, 114, 8613.
[2] a) J. M. R. Narayanam, C. R. J. Stephenson, Chem. Soc. Rev. 2011, 40, 102;
b) S. Maity, N. Zheng, Synlett 2012, 1851; c) M. A. Ischay, T. P. Yoon, Eur.
J. Org. Chem. 2012, 3359; d) J. Xuan, W.-J. Xiao, Angew. Chem. Int. Ed.
2012, 51, 6828; Angew. Chem. 2012, 124, 6934; e) T. Koike, M. Akita, Syn-
lett 2013, 2492; f) C. K. Prier, D. A. Rankic, D. W. C. MacMillan, Chem. Rev.
2013, 113, 5322.
[3] a) E. R. Gaillard, D. G. Whitten, Acc. Chem. Res. 1996, 29, 292; b) M.
Schmittel, A. Burghart, Angew. Chem. Int. Ed. Engl. 1997, 36, 2550;
Angew. Chem. 1997, 109, 2658; c) M. Mella, M. Fagnoni, M. Freccero, E.
Fasani, A. Albini, Chem. Soc. Rev. 1998, 27, 81; d) E. Baciocchi, M. Bietti,
O. Lanzalunga, J. Phys. Org. Chem. 2006, 19, 467; e) A. Houmam, Chem.
Rev. 2008, 108, 2180.
[4] a) S. Cai, X. Zhao, X. Wang, Q. Liu, Z. Li, D. Z. Wang, Angew. Chem. Int.
Ed. 2012, 51, 8050; Angew. Chem. 2012, 124, 8174; b) Y. Zhao, S. Cai, J.
Li, D. Z. Wang, Tetrahedron 2013, 69, 8129; c) J. K. Laha, K. S. S. Tumma-
lapalli, A. Gupta, Org. Lett. 2014, 16, 4392.
[5] a) M. Kako, Y. Nakadaira, Bull. Chem. Soc. Jpn. 2000, 73, 2403; b) D. Man-
gion, D. R. Arnold, Acc. Chem. Res. 2002, 35, 297; c) M. Fagnoni, D.
Dondi, D. Ravelli, A. Albini, Chem. Rev. 2007, 107, 2725; d) N. Hoffmann,
Chem. Rev. 2008, 108, 1052.
[7] UV-mediated aromatic substitution reactions based on CÀC bond cleav-
age of alkylating reagents have been known in limited substrates: a) A.
Sulpizio, A. Albini, N. d’Alessandro, E. Fasani, S. Pietra, J. Am. Chem. Soc.
1989, 111, 5773; b) R. Popielarz, D. R. Arnold, J. Am. Chem. Soc. 1990,
112, 3068; c) M. Mella, E. Fasani, A. Albini, J. Org. Chem. 1992, 57, 3051;
d) M. Mella, M. Fagnoni, A. Albini, J. Org. Chem. 1994, 59, 5614; e) L.
Bardi, E. Fasani, A. Albini, J. Chem. Soc. Perkin Trans. 1 1994, 545.
[8] a) A. McNally, C. K. Prier, D. W. C. MacMillan, Science 2011, 334, 1114;
b) K. Qvortrup, D. A. Rankic, D. W. C. MacMillan, J. Am. Chem. Soc. 2014,
136, 626; c) Z. Zuo, D. W. C. MacMillan, J. Am. Chem. Soc. 2014, 136,
5257; d) J. D. Cuthbertson, D. W. C. MacMillan, Nature 2015, 519, 74.
[9] Our previous work on photoredox catalyses: a) Y. Miyake, K. Nakajima,
Y. Nishibayashi, J. Am. Chem. Soc. 2012, 134, 3338; b) K. Nakajima, M. Ki-
tagawa, Y. Ashida, Y. Miyake, Y. Nishibayashi, Chem. Commun. 2014, 50,
8900; c) K. Nakajima, Y. Ashida, S. Nojima, Y. Nishibayashi, Chem. Lett.
2015, 44, 545.
[10] S. Fukuzumi, T. Suenobu, M. Patz, T. Hirasaka, S. Itoh, M. Fujitsuka, O.
Ito, J. Am. Chem. Soc. 1998, 120, 8060.
[11] a) G. Li, R. Chen, L. Wu, Q. Fu, X. Zhang, Z. Tang, Angew. Chem. Int. Ed.
2013, 52, 8432; Angew. Chem. 2013, 125, 8590; b) G. Li, L. Wu, G. Lv, H.
Liu, Q. Fu, X. Zhang, Z. Tang, Chem. Commun. 2014, 50, 6246; c) L. Cao,
L. Zheng, Q. Huang, J. Organomet. Chem. 2014, 768, 56.
[12] See the supporting information for details.
[6] Selected examples: a) M. Ohashi, K. Miyake, Chem. Lett. 1977, 615; b) C.
Pac, A. Nakasone, H. Sakurai, J. Am. Chem. Soc. 1977, 99, 5806; c) R. M.
Borg, D. R. Arnold, T. S. Cameron, Can. J. Chem. 1984, 62, 1785; d) K.
Mizuno, M. Ikeda, Y. Otsuji, Tetrahedron Lett. 1985, 26, 461; e) J. Y. Lan,
G. B. Schuster, J. Am. Chem. Soc. 1985, 107, 6710; f) D. R. Arnold, X. Du,
J. Am. Chem. Soc. 1989, 111, 7666; g) K. Nakanishi, K. Mizuno, Y. Otsuji,
Bull. Chem. Soc. Jpn. 1993, 66, 2371; h) R. Torriani, M. Mella, E. Fasani, A.
Albini, Tetrahedron 1997, 53, 2573; i) A. N. Frolov, Russ. J. Org. Chem.
1998, 34, 139; j) M. Zhang, Z.-F. Lu, Y. Liu, G. Grampp, H.-W. Hu, J.-H. Xu,
Tetrahedron 2006, 62, 5663; k) Y. Yoshimi, T. Itou M. Hatanaka, Chem.
Commun. 2007, 5244; l) T. Itou, Y. Yoshimi, T. Morita, Y. Tokunaga, M. Ha-
tanaka, Tetrahedron 2009, 65, 263; m) J. Tang, J.-J. Yue, F.-F. Tao, G.
Grampp, B.-X. Wang, F. Li, X.-Z. Liang, Y.-M. Shen, J.-H. Xu, J. Org. Chem.
2014, 79, 7572.
[13] a) U. Megerle, R. Lechner, B. Kçnig, E. Riedle, Photochem. Photobiol. Sci.
2010, 9, 1400; b) M. A. Cismesia, T. P. Yoon, Chem. Sci. 2015, 6, 5426;
c) M. D. Kärkäs, B. S. Matsuura, C. R. J. Stephenson, Science 2015, 349,
1285; d) H. Huo, C. Wang, K. Harms, E. Meggers, J. Am. Chem. Soc. 2015,
137, 9551.
[14] For redox potentials of 1a, 2a, and 4a, see: a) I. M. Dixon, J.-P. Collin, J.-
P. Sauvage, L. Flamigni, S. Encinas, F. Barigelletti, Chem. Soc. Rev. 2000,
29, 385; b) J.-P. Cheng, Y. Lu, X.-Q. Zhu, Y. Sun, F. Bi, J. He, J. Org. Chem.
2000, 65, 3853; c) A. A. Isse, A. Gennaro, J. Phys. Chem. A 2004, 108,
4180.
Received: January 12, 2016
Published online on February 19, 2016
ChemCatChem 2016, 8, 1028 – 1032
www.chemcatchem.org
1032
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim