Peripherally Silylated Porphyrins

Article abstract of DOI:10.1002/chem.201502563

Silylation of peripherally lithiated porphyrins with silyl electrophiles has realized the first synthesis of a series of directly silyl-substituted porphyrins. The meso-silyl group underwent facile protodesilylation, whereas the β-silyl group was entirely

Full text of DOI:10.1002/chem.201502563

DOI: 10.1002/chem.201502563
Communication
&
Substituent Effects |Hot Paper|
Peripherally Silylated Porphyrins
Kenichi Kato,[a] Keisuke Fujimoto,[a] Hideki Yorimitsu,*[a, b] and Atsuhiro Osuka*[a]
ble synthetic protocols to introduce a silyl group at porphyrin.
We recently developed methods to generate highly reactive
porphyrinyl Grignard[9] and lithium[10] reagents through halo-
gen–metal exchange reactions. We disclose herein the first syn-
thesis of peripherally silylated porphyrins through the reactions
of these porphyrinyl metal species with electrophilic silylating
reagents.
Our initial attempts to obtain NiII meso-trimethylsilylporph-
yrin 2Ni by the reaction of meso-magnesiated porphyrin with
trimethylsilyl cyanide[11] resulted in failure. Protonation instead
occurred after hydrolysis, as well as oxidative dimerization of
the Grignard species, because of the low nucleophilicity of the
sterically demanding magnesium reagent.[9] We then turned
our attention to more nucleophilic porphyrinyllithium
reagents.[10] Treatment of meso-iodoporphyrin with nBuLi at
À988C followed by an addition of trimethylsilyl cyanide yield-
ed desired 2Ni (Scheme 1, condition a). Although the reaction
Abstract: Silylation of peripherally lithiated porphyrins
with silyl electrophiles has realized the first synthesis of
a series of directly silyl-substituted porphyrins. The meso-
silyl group underwent facile protodesilylation, whereas the
b-silyl group was entirely compatible with standard work-
up and purification on silica gel. The meso-silyl group
caused larger substituent effects to the porphyrin
compared with the b-silyl group. Silylation of b-lithiated
porphyrins with 1,2-dichlorodisilane furnished b-to-b
disilane-bridged porphyrin dimers.
A doubly b-to-b
disilane-bridged NiII-porphyrin dimer was also synthesized
from a b,b-dilithiated NiII-porphyrin and characterized by
X-ray crystallographic analysis to take a steplike structure
favorable for interporphyrinic interaction. Denickelation of
b-silylporphyrins was achieved upon treatment with a 4-
tolylmagnesium bromide to yield the corresponding free-
base porphyrins.
Porphyrins are an important class of compounds playing
a wide range of vital roles in living systems. Their attractive
optical, electrochemical, and biological properties have led to
applications in various fields, such as materials science, bio-
chemistry, and catalysis.[1] Peripheral introductions of functional
groups onto porphyrins can cause electronic perturbations of
porphyrins and, thus, represent an effective means to modify
the electronic properties and steric environments of
porphyrins.[2]
Silyl groups attached peripherally on a p-conjugated system
have been demonstrated to interact with the p-system
through s–p and s*–p* conjugation.[3–5] Peripheral silylation of
arenes generally gives rise to modifications of the optical and
electronic properties of the parent arenes such as improved
fluorescent quantum yields, and has thereby been extensively
investigated.[6,7] Despite their potential, there are no reports on
peripherally silylated porphyrins,[8] because of the lack of suita-
[a] K. Kato, K. Fujimoto, Prof. Dr. H. Yorimitsu, Prof. Dr. A. Osuka
Department of Chemistry, Graduate School of Science
Kyoto University, Sakyo-ku Kyoto, 606-8502 (Japan)
Fax: (+81)75-753-3970
Scheme 1. Synthesis and denickelation of silylated porphyrins. a) i) 1.5 equiv
nBuLi, THF, À988C, 30 min, ii) 2 equiv TMSCN, THF, À988C to RT, 2 h;
b) i) 1.5 equiv iPrMgCl·LiCl, THF, À408C, 2 h, ii) 2 equiv TMSCN, THF, À408C to
RT, 2 h; c) i) 1.00 equiv nBuLi, THF, À988C, 30 min, ii) 0.45 equiv
E-mail: yori@kuchem.kyoto-u.ac.jp
osuka@kuchem.kyoto-u.ac.jp
[b] Prof. Dr. H. Yorimitsu
Me2(Cl)SiSi(Cl)Me2, THF, À988C to RT, 1 h; d) i) 2.00 equiv nBuLi, THF, À988C,
30 min, ii) 0.47 equiv Me2(Cl)SiSi(Cl)Me2, THF, À988C, 20 min, iii) 0.47 equiv
Me2(Cl)SiSi(Cl)Me2, THF, À408C, 20 min; e) i) 10 equiv 4-TolMgBr, toluene, RT,
4 h, ii) 3 m HCl (aq), CH2Cl2, RT, 10 min; f) i) 40 equiv 4-TolMgBr, 10 equiv
CH2CHSPh, toluene, 508C, 4 h, ii) 3 m HCl (aq), CH2Cl2, RT, 10 min.
ACT-C (Japan) Science and Technology Agency,
Sakyo-ku, Kyoto 606-8502 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201502563.
Chem. Eur. J. 2015, 21, 13522 – 13525
13522
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
was efficient (ca. 70% NMR yield), facile protodesilylation of
2Ni proceeded during purification on silica gel to isolate 2Ni
in only 18% yield, along with the formation of the protonated
product.[12,13] The instability of 2Ni may be ascribed to the
electron-rich nature of the silylated meso-position.
In a similar fashion, we synthesized b-trimethylsilylporphyrin
3Ni through b-lithioporphyrin (Scheme 1, condition a). In con-
trast to 2Ni, 3Ni is stable under acidic conditions,[12] and is
compatible with silica gel column chromatography to isolate
3Ni in 79% yield. It is worth noting that 3Ni was also access-
ible from the corresponding Grignard reagent in 67% yield
(Scheme 1, condition b), reminding us of the smaller steric
hindrance around the b-position.
These successes encouraged us to prepare b-to-b disilane-
bridged porphyrin dimers with an expectation of interporphyr-
inic interaction through s(Si–Si)-p(porphyrin) conjugation.[4c,7]
After more careful and precise preparation of b-lithiated
porphyrin with 1.00 equiv of nBuLi, slow addition of
0.45 equiv of 1,2-dichloro-1,1,2,2-tetramethyldisilane provided
singly b-to-b disilane-bridged porphyrin dimer 4Ni in 57%
yield (Scheme 1, condition c).
Doubly b-to-b disilane-bridged porphyrin dimer 5Ni would
be a promising target, which would take a more fixed confor-
mation for more efficient interporphyrinic interaction, judging
from Isobe’s reports on disilane-bridged anthracene dimers.[7]
To prepare 5Ni, b,b-dilithiated porphyrin was utilized; this was
successfully generated from b,b-diiodoporphyrin under our
conditions for iodine-lithium exchange.[10,14] Stepwise construc-
tion of two disilane bridges was effective to suppress forma-
tion of unbridged and acyclic products, and doubly bridged
dimer 5Ni was obtained in 4% yield by slowly adding
the dichlorodisilane at À988C and subsequently at À408C
(Scheme 1, condition d).
Figure 1. X-Ray crystal structures. a) Top view and b) side view of 2Ni. c) Top
view and d) side view of 4Ni. e) Top view and f) side view of 5Ni. The ther-
mal ellipsoids are drawn at 30% probability. Solvent molecules, tert-butyl
groups and aryl hydrogen atoms are omitted for clarity. Schematic represen-
tation of dihedral angle g) w1 and h) w1’.
In addition to NMR analysis (see the Supporting Informa-
tion), the structures of meso-silylporphyrin 2Ni and b-to-b
disilane-bridged dimers 4Ni and 5Ni were unambiguously re-
vealed by X-ray crystallographic analysis (Figure 1). meso-Silyl-
porphyrin 2Ni has a distorted porphyrin plane to avoid steric
repulsion between the porphyrin plane and the trimethylsilyl
group.[15] In the solid state, singly bridged dimer 4Ni has
a non-symmetric helical steplike structure in which one meso-
aryl group in one porphyrin is located just above the other
Figure 2. UV/Vis absorption spectra of 1Ni–5Ni in CH2Cl2.
1
porphyrin. In contrast, its H and 13C NMR spectra display a sin-
glet peak for the methyl groups on the two silicon atoms, indi-
cating free rotation of the disilane bridge in solution. Doubly
bridged dimer 5Ni has a steplike structure with trans geome-
try, in which the four dihedral angles w1 and w1’ between the
Si-Si-Cb planes and the Cb-Cb’-Ca-Si mean planes are ca. 758
(see the Supporting Information). This conformation looks
favorable for s–p interaction between the two porphyrin units
through the two SiÀSi bonds.
540 nm, respectively) than b-silylporphyrin 3Ni (lmax =414 and
525 nm, respectively), due to the more efficient interaction at
the meso position. Characteristically, dimers 4Ni and 5Ni show
broadened Soret bands[16] as a consequence of small yet
distinct exciton coupling between the two porphyrin units.
To obtain further understanding of s–p conjugation and in-
terporphyrinic interaction in the dimers, DFT calculations were
performed at the B3LYP/6-31G(d) (C,H,N,Si)+LANL2DZ (Ni)
level using Gaussian 09 package.[17] meso-Silylporphyrin 2Ni
has a little narrower HOMO–LUMO gap (2.87 eV) than the
parent porphyrin 1Ni (2.99 eV), whereas the frontier orbital
energy gaps of b-silylporphyrins remain almost unchanged
(see the Supporting Information). Small yet obvious orbital
The UV/Vis absorption spectra of 2Ni–5Ni as well as the
unsubstituted parent porphyrin 1Ni in CH2Cl2 are shown in
Figure 2. Each silylated porphyrin exhibits a typical absorption
spectrum comprising a sharp Soret band in 400–450 nm and
weak Q bands in 500–550 nm. meso-Silylporphyrin 2Ni shows
the larger red shift of the Soret and Q bands (lmax =421 and
Chem. Eur. J. 2015, 21, 13522 – 13525
www.chemeurj.org
13523
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
Figure 4. UV/Vis absorption (black lines) and fluorescence (gray lines) spectra
of 1FB, 3FB, and 4FB in CH2Cl2.
Figure 3. Optimized structure and Kohn–Sham orbital representations ob-
tained by DFT calculations at the B3LYP/6-31G(d) (C,H,N,Si)+LANL2DZ (Ni)
level. a) HOMO, b) LUMO, and c) optimized structure of 2Ni. d) HOMO of
5Ni. tert-Butyl groups were replaced with hydrogen atoms to simplify the
calculations.
In summary, a series of peripherally silylated porphyrins
were synthesized for the first time. meso-Silylporphyrin shows
the larger red shift in the UV/Vis absorption spectrum due to
the more efficient s–p conjugation between the meso-silyl
group and the porphyrin p system, but is unstable under
acidic conditions. In contrast, b-silylporphyrins are stable, and
the electronic perturbation by the b-silyl group is small.
b-to-b Disilane-bridged porphyrin dimers were prepared and
characterized to exhibit weak yet distinct interporphyrinic
interaction. Further study is underway to explore the applica-
tion of peripherally metalated porphyrins in our laboratory.
densities are located at the SiÀMe bonds in the HOMO and
LUMO of meso-silylporphyrin 2Ni (Figure 3a and b), whereas
such densities are negligible in those of b-silylporphyrin 3Ni,
which is consistent with their UV/Vis absorption spectra. The
high HOMO density at the meso position as well as the strain
around the meso-trimethylsilyl group[15] would explain the ob-
served facile protodesilylation reaction. In dimers 4Ni and 5Ni,
contributions of s–p conjugations are mostly trivial in their
frontier orbitals (see the Supporting Information), reflecting
the b-to-b connection. This is consistent with their absorption
spectra, which are not significantly perturbed. Characteristical-
ly, the HOMO of 5Ni has been calculated to exhibit small but
distinct anti-bonding interaction between p-orbitals on the
b-positions and s-orbitals of the SiÀSi bonds (Figure 3d).
Considering the instability of silylated porphyrins under
typical denickelation conditions (H2SO4 in trifluoroacetic acid),
denickelation of silylated porphyrins with a 4-tolylmagnesium
bromide[18] was conducted to obtain freebase porphyrins for
fluorescence study. Although denickelation of 3Ni proceeded
successfully (Scheme 1, condition e), attempted denickelation
of 4Ni resulted in failure, probably due to the oxidative inser-
tion of Ni0, generated by the action of the Grignard reagent,
into the SiÀSi bond. To deactivate the Ni0 species, we found
that an addition of phenyl vinyl sulfide was effective to
furnish denickelated product 4FB in 35% yield (Scheme 1,
condition f).
Acknowledgements
This work was supported by Grants-in-Aid from MEXT
(Nos.: 25107002 “Science of Atomic Layers”) and from JSPS
(Nos.: 25220802 (Scientific Research (S)), 24685007 (Young Sci-
entists (A)), 26620081 (Exploratory Research). H.Y. thanks Kansai
Research Foundation for Technology Promotion and The Asahi
Glass Foundation for financial support. K.F. acknowledges
a JSPS Fellowship for Young Scientists.
Keywords: lithiation · porphyrin · porphyrin dimer · silylation ·
substituent effects
[1] a) Handbook of Porphyrin Science Vol. 1–10 (Eds.: K. M. Kadish, K. M.
Smith, R. Guilard), World Scientific Publishing, Singapore, 2010;
b) Handbook of Porphyrin Science Vol. 11 –15 (Eds.: K. M. Kadish, K. M.
Smith, R. Guilard), World Scientific Publishing, Singapore, 2011; c) Hand-
book of Porphyrin Science Vol. 16–25 (Eds.: K. M. Kadish, K. M. Smith, R.
Guilard), World Scientific Publishing, Singapore, 2012; d) The Porphyrin
Handbook, Vol. 1–10 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Aca-
demic Press, San Diego, 2000; e) Handbook of Porphyrin Science Vol. 11 –
20 (Eds.: K. M. Kadish, K. M. Smith, R. Guilard), Academic Press, San
Diego, 2003; f) D. Dolphin, The Porphyrins, Vol. 1 and 2, Academic Press,
New York, 1979.
[2] For selected recent reviews, see: a) M. O. Senge, Chem. Commun. 2011,
47, 1943–1960; b) S. Fox, R. W. Boyle, Tetrahedron 2006, 62, 10039–
10054; c) H. Shinokubo, A. Osuka, Chem. Commun. 2009, 1011–1021;
d) H. Yorimitsu, A. Osuka, Asian J. Org. Chem. 2013, 2, 356–373. For se-
lected recent articles, see: e) H. Hata, H. Shinokubo, A. Osuka, J. Am.
Chem. Soc. 2005, 127, 8264–8265; f) Y. Kawamata, S. Tokuji, H. Yorimit-
Figure 4 represents the UV/Vis absorption and fluorescence
spectra of b-trimethylsilylporphyrin 3FB, singly bridged dimer
4FB, and the parent porphyrin 1FB. b-Silylporphyrins 3FB and
4FB display spectra similar to that of 1FB with small red shifts.
The Soret band of 4FB is broadened owing to exciton
coupling.[16] The b-silyl groups on the periphery have limited
effects on fluorescence quantum yields: 3FB (f =0.037), 4FB
f
(f =0.048), and 1FB (f =0.043).
f
f
Chem. Eur. J. 2015, 21, 13522 – 13525
www.chemeurj.org
13524
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
su, A. Osuka, Angew. Chem. Int. Ed. 2011, 50, 8867–8870; Angew. Chem.
2011, 123, 9029–9032; g) K. Fujimoto, H. Yorimitsu, A. Osuka, Org. Lett.
2014, 16, 972–975; h) K. Oda, M. Akita, S. Hiroto, H. Shinokubo, Org.
Lett. 2014, 16, 1818–1821; i) Q. Chen, Y.-Z. Zhu, Q.-J. Fan, S.-C. Zhang, J.-
Y. Zheng, Org. Lett. 2014, 16, 1590–1593.
Chem. Lett. 2007, 36, 1112–1113; k) Y. Kobori, Y. Shibano, T. Endo, H.
Tsuji, H. Murai, K. Tamao, J. Am. Chem. Soc. 2009, 131, 1624–1625.
[9] K. Fujimoto, H. Yorimitsu, A. Osuka, Eur. J. Org. Chem. 2014, 4327–4334.
[10] K. Fujimoto, H. Yorimitsu, A. Osuka, Chem. Eur. J. DOI: 10.1002/
chem.201502215.
[3] Selected articles about s–p conjugation: a) H. Sakurai, M. Kumada, Bull.
Chem. Soc. Jpn. 1964, 37, 1894–1895; b) H. Bock, H. Alt, J. Am. Chem.
Soc. 1970, 92, 1569–1576; c) C. G. Pitt, H. Bock, J. Chem. Soc. Chem.
Commun. 1972, 28–29; d) C. G. Pitt, R. N. Corey, E. C. Toren Jr., J. Am.
Chem. Soc. 1972, 94, 3806–3811; e) H. Sakurai, J. Organomet. Chem.
1980, 200, 261–286; f) S. Yamaguchi, K. Tamao, Chem. Lett. 2005, 34, 2–
7.
[4] For conformational studies of s–p conjugation, see: a) H. Sakurai, S.
Tasaka, M. Kira, J. Am. Chem. Soc. 1972, 94, 9285–9286; b) M. Kira, T.
Miyazawa, N. Mikami, H. Sakurai, Organometallics 1991, 10, 3793–3795;
c) H. Tsuji, Y. Shibano, T. Takahashi, M. Kumada, K. Tamao, Bull. Chem.
Soc. Jpn. 2005, 78, 1334–1344.
[11] Screening of silylating reagents is given in Supporting Information
(Table S1). For discussion about the favorable effect of cyanide in silyla-
tion of Grignard reagents, see: P. J. Lennon, D. P. Mack, Q. E. Thompson,
Organometallics 1989, 8, 1121–1122.
[12] Stirring meso-silylporphyrin 2Ni (3 mmol) in CH2Cl2 (5 mL) with 1m HCl
(aq, 5 mL) for 30 min at RT quantitatively gave the protodesilylated
product. In contrast, b-silylporphyrin 3Ni was compatible even with 3m
HCl (aq).
[13] Protodesilylation of typical arylsilanes needs harsher conditions, see:
a) C. Eaborn, J. Chem. Soc. 1953, 3148–3153; b) C. Eaborn, J. Chem. Soc.
1956, 4858–4864; c) C. Eaborn, J. Organomet. Chem. 1975, 100, 43–57.
[14] Addition of D2O to the dilithiated porphyrin prepared with 2.5 equiv
nBuLi at À988C for 30 min gave NiII 5,10,15-tris(3,5-di-tert-butylphenyl)-
2,18-dideuteroporphyrin (H/D=5/95) in 98% yield.
[5] For a representative review on the influence of conformations of oligo-
silanes on s conjugation, see: H. Tsuji, J. Michl, K. Tamao, J. Organomet.
Chem. 2003, 685, 9–14.
[15] The mean plane deviation of 2Ni consisting of the porphyrinic four ni-
trogen atoms and 20 carbon atoms was calculated to be 0.326 Š
(0.292 Š for the structure optimized by DFT calculations). This is larger
than that of tetraphenylporphyrin nickel complex in a crystal. (0.223 Š):
A. L. Maclean, G. J. Foran, B. J. Kennedy, P. Tumer, T. W. Hambley, Aust. J.
Chem. 1996, 49, 1273–1278.
[6] a) S. Kyushin, M. Ikarugi, M. Goto, H. Hiratsuka, H. Matsumoto, Organo-
metallics 1996, 15, 1067–1070; b) S. Kyushin, T. Kitahara, H. Matsumoto,
Chem. Lett. 1998, 27, 471–472; c) S. Kyushin, Y. Ishikita, H. Matsumoto,
H. Horiuchi, H. Hiratsuka, Chem. Lett. 2006, 35, 64–65; d) M. Shimizu, K.
Oda, T. Bando, T. Hiyama, Chem. Lett. 2006, 35, 1022–1023; e) S. Kyush-
in, N. Takemasa, H. Matsumoto, H. Horiuchi, H. Hiratsuka, Chem. Lett.
2003, 32, 1048–1049; f) H. Maeda, Y. Inoue, H. Ishida, K. Mizuno, Chem.
Lett. 2001, 30, 1224–1225; g) S. Kyushin, T. Matsuura, H. Matsumoto, Or-
ganometallics 2006, 25, 2761–2765.
[7] a) W. Nakanishi, S. Hitosugi, A. Piskareva, Y. Shimada, H. Taka, H. Kita, H.
Isobe, Angew. Chem. Int. Ed. 2010, 49, 7239–7242; Angew. Chem. 2010,
122, 7397–7400; b) W. Nakanishi, S. Hitosugi, Y. Shimada, H. Isobe,
Chem. Asian J. 2011, 6, 554–559.
[8] For reports on porphyrins silylated indirectly on the meso-aryl groups,
see: a) H. Horiuchi, T. Tanaka, K. Yoshimura, K. Sato, S. Kyushin, H. Matsu-
moto, H. Hiratsuka, Chem. Lett. 2006, 35, 662–663; b) S. Kyushin, K.
Yoshimura, K. Sato, H. Matsumoto, Chem. Lett. 2009, 38, 324–325; c) K.
Kakiage, E. Fujimura, H. Mine, T. Izumizawa, T. Katoh, K. Yoshimura, S.
Kyushin, T. Kyomen, M. Hanaya, Chem. Lett. 2010, 39, 1063–1065; d) H.
Horiuchi, M. Hosaka, M. Terata, S. Ishida, S. Kyushin, T. Okutsu, T. Takeu-
chi, H. Hiratsuka, Chem. Eur. J. 2014, 20, 6054–6060; e) H. Horiuchi, T.
Kano, H. Uehara, T. Okutsu, Chem. Lett. 2014, 43, 1423–1425; f) S.
Kyushin, Y. Saito, K. Yoshimura, H. Horiuchi, H. Hiratsuka, Heteroat.
Chem. 2014, 25, 514–517; g) H. Tsuji, M. Sasaki, Y. Shibano, M. Toganoh,
T. Kataoka, Y. Araki, K. Tamao, O. Ito, Bull. Chem. Soc. Jpn. 2006, 79,
1338–1346; h) Y. Shibano, M. Sasaki, H. Tsuji, Y. Araki, O. Ito, K. Tamao, J.
Organomet. Chem. 2007, 692, 356–367; i) M. Sasaki, Y. Shibano, H. Tsuji,
Y. Araki, K. Tamao, O. Ito, J. Phys. Chem. A 2007, 111, 2973–2979; j) Y.
Shibano, M. Sasaki, Y. Kawanishi, Y. Araki, H. Tsuji, O. Ito, K. Tamao,
[16] The full widths at half maxima of Soret bands are 1Ni 1200 cmÀ1, 2Ni
1200 cmÀ1
, , , , 1FB
3Ni 1200 cmÀ1 4Ni 1900 cmÀ1 5Ni 1800 cmÀ1
600 cmÀ1, 3FB 700 cmÀ1, and 4FB 1700 cmÀ1
.
[17] Gaussian 09, Revision C.02, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Men-
nucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian,
A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara,
K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O.
Kitao, H. Nakai, T. Vreven, J. A. Montgomery, Jr. , J. E. Peralta, F. Ogliaro,
M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Ko-
bayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyen-
gar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B.
Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann,
O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin,
K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg,
S. Dapprich, A. D. Daniels, Ö. Farkas, J. B. Foresman, J. V. Ortiz, J. Cio-
slowski, D. J. Fox, Gaussian, Inc. Wallingford CT, 2009.
[18] K. Murakami, Y. Yamamoto, H. Yorimitsu, A. Osuka, Chem. Eur. J. 2013,
19, 9123–9126.
Received: June 30, 2015
Published online on August 19, 2015
Chem. Eur. J. 2015, 21, 13522 – 13525
www.chemeurj.org
13525
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim