Copper-catalyzed dehydrogenative cross-coupling of benzothiazoles with thiazoles and polyfluoroarene

Article abstract of DOI:10.1021/ol3023165

A copper-catalyzed dehydrogenative cross-coupling of benzothiazoles with thiazoles and polyfluoroarene under mild reaction conditions is described. This protocol provides a straightforward and operationally simple method for the synthesis of the 2,27prime;-linkage of thiazoles and 2-polyfluoroarylthiazoles of interest in life and material sciences.

Full text of DOI:10.1021/ol3023165

ORGANIC
LETTERS
2012
Vol. 14, No. 18
4950–4953
Copper-Catalyzed Dehydrogenative
Cross-Coupling of Benzothiazoles with
Thiazoles and Polyfluoroarene
Shilu Fan,† Zhao Chen,‡ and Xingang Zhang*,†
Key Laboratory of Organofluorine Chemistry, Shanghai Institute of Organic Chemistry,
Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China, and School
of Chemistry and Chemical Engineering, Southwest Petroleum University, 8 Xindu
Avenue, Chengdu, Sichuan 610500, China
xgzhang@mail.sioc.ac.cn
Received August 19, 2012
ABSTRACT
A copper-catalyzed dehydrogenative cross-coupling of benzothiazoles with thiazoles and polyfluoroarene under mild reaction conditions is
described. This protocol provides a straightforward and operationally simple method for the synthesis of the 2,20-linkage of thiazoles and
2-polyfluoroarylthiazoles of interest in life and material sciences.
As an attractive alternative to traditional cross-coupling
in the construction of carbonꢀcarbon (CꢀC) bonds,
transition-metal-catalyzed direct functionalization of
CꢀH bonds has gained considerable attention.1 In partic-
ular, dehydrogenative cross-coupling from two simple
CꢀH bonds represents the most efficient and straightfor-
ward access to the target molecules, since this tandem
oxidation of CꢀH bonds avoids the prefunctionalization
steps of both cross-coupling partners.2 In recent years,
impressive progress has been made in the formation of
arylꢀaryl bonds via this strategy.3 However, in most of the
cases, palladium catalysts have been used in conjunction
with stoichiometric amounts of copper and silver reoxi-
dants. Although these achievements are promising, new
catalytic systems using less expensive and abundant metal
catalysts, such as copper, to execute such CꢀH bond
(3) (a) Li, R.; Jiang, L.; Lu, W. Organometallics 2006, 25, 5973. (b)
Stuart, D. R.; Fagnou, K. Science 2007, 316, 1172. (c) Dwight, T. A.;
Rue, N. R.; Charyk, D.; Josselyn, R.; DeBoef, B. Org. Lett. 2007, 9,
3137. (d) Stuart, D. R.; Villemure, E.; Fagnou, K. J. Am. Chem. Soc.
2007, 129, 12072. (e) Hull, K. L.; Sanford, M. S. J. Am. Chem. Soc. 2007,
129, 11904. (f) Li, B.-J.; Tian, S.-L.; Fang, Z.; Shi, Z.-J. Angew. Chem.,
† Shanghai Institute of Organic Chemistry.
‡ Southwest Petroleum University.
(1) For selected recent reviews related to CꢀH bonds activation, see:
(a) Campeau, L.-C.; Fagnou, K. Chem. Commun. 2006, 1253. (b)
Daugulis, O.; Zaitzev, V. G.; Shabashov, D.; Pham, Q.-N.; Lazareva,
A. Synlett 2006, 3382. (c) Alberico, D.; Scott, M. E.; Lautens, M. Chem.
Rev. 2007, 107, 174. (d) Giri, R.; Shi, B. F.; Engle, K. M.; Maugel, N.;
Yu, J.-Q. Chem. Soc. Rev. 2009, 38, 3242. (e) Ackermann, L.; Vicente,
R.; Kapdi, A. R. Angew. Chem., Int. Ed. 2009, 48, 9792. (f) Colby, D. A.;
Bergman, R. G.; Ellman, J. A. Chem. Rev. 2010, 110, 624. (g) Lyons,
T. W.; Sanford, M. S. Chem. Rev. 2010, 110, 1147. (h) Sun, C.-L.; Li, B.-J.;
Shi, Z.-J. Chem. Commun. 2010, 46, 677.
(2) For selected recent reviews related to transition-metal catalyzed
dehydrogenative cross-coupling, see: (a) Yeung, C. S.; Dong, V. M.
Chem. Rev. 2011, 111, 1215. (b) Bras, J. L.; Muzart, J. Chem. Rev. 2011,
111, 1170. (c) Liu, C.; Zhang, H.; Shi, W.; Lei, A. Chem. Rev. 2011, 111,
1780. (d) Han, W.; Ofial, A. R. Synlett 2011, 133, 13864. (e) Cho, S. H.;
Kim, J. Y.; Kwak, J.; Chang, S. Chem. Soc. Rev. 2011, 40, 5068. (f)
Bugaut, X.; Glorius, F. Angew. Chem., Int. Ed. 2011, 50, 7479.
Int. Ed. 2008, 47, 1115. (g) Brasche, G.; Garcıa-Fortanet, J.; Buchwald,
´
S. L. Org. Lett. 2008, 10, 2207. (h) Hull, K. L.; Sanford, M. S. J. Am.
Chem. Soc. 2009, 131, 9651. (i) Xi, P.; Yang, F.; Qin, S.; Zhao, D.; Lan,
J.; Gao, G.; Hu, C.; You, J. J. Am. Chem. Soc. 2010, 132, 1822. (j) Wei,
Y.; Su, W. J. Am. Chem. Soc. 2010, 132, 16377. (k) Li, H.; Liu, J.; Sun,
C.-L.; Li, B.-J.; Shi, Z.-J. Org. Lett. 2011, 13, 276. (l) Wang, X.; Leow,
D.; Yu, J.-Q. J. Am. Chem. Soc. 2011, 133, 13864. (m) Han, W.; Mayer,
P.; Ofial, A. R. Angew. Chem., Int. Ed. 2011, 50, 2178. (n) Wang, Z.; Li,
K.; Zhao, D.; Lan, J.; You, J. Angew. Chem., Int. Ed. 2011, 50, 5365. (o)
Mandal, D.; Yamaguchi, A. D.; Yamaguchi, J.; Itami, K. J. Am. Chem.
Soc. 2011, 133, 19660. (p) Kuhl, N.; Hopkinson, M. N.; Glorius, F.
Angew. Chem., Int. Ed. 2012, 51, 8230. (q) Yamaguchi, A. D.; Mandal,
D.; Yamaguchi, J.; Itami, K. Chem. Lett. 2011, 6, 555. (r) Dong, J.;
Huang, Y.; Qin, X.; Cheng, Y.; Hao, J.; Wan, D.; Li, W.; Liu, X.; You, J.
Chem.;Eur. J. 2012, 18, 6158. (s) A metal-free reaction: Brasse, M.;
Ellman, J. A.; Bergman, R. G. Chem. Commun. 2011, 47, 5019.
r
10.1021/ol3023165
Published on Web 09/13/2012
2012 American Chemical Society

functionalization reactions are appealing. During the past
few years, considerable effort has been devoted to using
copper catalysis in the construction of Cꢀheteroatom and
CꢀC bonds through selective catalytic functionalization
of aryl CꢀH bonds.4 However, the copper-catalyzed de-
hydrogenative diaryl cross-coupling reaction remains a
great challenge and largely unexplored. The reason is that
this strategy is often difficult to realize, as both coupling
partners, in particular coupling partners with similar
structure and reactivity, are susceptible to oxidative homo-
coupling in the presence of copper salts.5 To the best of our
knowledge, rare examples have been reported in this chal-
lenging area so far.6 Consequently, new copper-catalytic
systems to overcome these challenges are highly desirable.
In continuing our efforts in transition-metal-catalyzed de-
hydrogenative cross-couplings,7 herein, we describe a copper-
catalyzed dehydrogenative cross-coupling of benzothiazoles
with thiazoles through dual CꢀH bond functionalization
under mild reactions in a highly efficient manner and with
excellent regioselectivity. Furthermore, the methodology
could also be extended to polyfluoroarene.
In view of the importance of azoles,8 in particular,
bithiazaole derivatives, that are found in many biologically
active compounds, natural products,9 and materials (e.g.,
solar cells),10 as well as the fact that no copper-catalyzed
cross-coupling of both similar thiozoles has been re-
ported due to difficulties in suppressing undesired homo-
couplings,5 benzothiazole 1a and 4,5-dimethylthiazole 2a
were chosen as model substrates. Initially, no desired
product 3a was observed, when 1a (1.0 equiv) and 2a
(2.0 equiv) were treated with tBuOLi in DMF at 80 °C by
using Ag2CO3 as an oxidant, Cu(OAc)2 (20 mol %) as a
catalyst, and 1,10-phenanthroline (phen) as a ligand (Table 1,
entry 1). Further investigating the solvent effect, we found
that nonpolar solvent toluene was the best reaction medium,
providing 3a in 35% yield (Table 1, entry 4). Encouraged
by these preliminary results, we subsequently tried to
optimize the reaction conditions by using different bases,
copper salts, ligands, and oxidants. tBuONa, tBuOK, and
(tBuO)2Mg all failed to give 3a; only tBuOLi is a suitable
base. The choice of copper salts is also critical to the
reaction efficiency; CuI proved to be the optimum catalyst
with 60% isolated yield of 3a obtained when 2,20-bipydyl
(bpy) was used as a ligand (Table 1, entry 6). Other oxi-
dants such as AgOAc, AgNO3, air, and O2 showed less or
no activity (Table 1, entries 8ꢀ11). Further improvement
of the reaction efficiency by increasing the amount of 2a to
3.0 equiv led to a higher yield (Table 1, entry 12). Interest-
ingly, the absence of bpy also furnished 3a in a comparable
yield (Table 1, entry 13), whereas the absence of CuI or
Ag2CO3 failed to give any desired product (Table 1, entries
14ꢀ15), thus implying that a copper redox catalytic cycle
was involved in the reaction. Finally, the best yield of 3a
(71%) was afforded by decreasing the loading of CuI to
10 mol % (Table 1, entry 16).
(4) For selected examples, see: (a) Chen, X.; Hao, X.-S.; Goodhue,
C. E.; Yu, J.-Q. J. Am. Chem. Soc. 2006, 128, 6790. (b) Li, Z.; Li, C.-J.
J. Am. Chem. Soc. 2006, 128, 56. (c) Do, H.-Q.; Daugulis, O. J. Am.
Chem. Soc. 2007, 129, 12404. (d) Do, H.-Q.; Khan, R. M. K.; Daugulis,
O. J. Am. Soc. Chem. 2008, 130, 15185. (e) Brasche, G.; Buchwald, S. L.
Angew. Chem., Int. Ed. 2008, 47, 1932. (f) Ueda, S.; Nagasawa, H.
Angew. Chem., Int. Ed. 2008, 47, 6411. (g) Phipps, R. J.; Grimster, N. P.;
Gaunt, M. J. J. Am. Chem. Soc. 2008, 130, 8172. (h) Ban, I.; Sudo, T.;
Taguchi, T.; Itami, K. Org. Lett. 2008, 10, 3607. (i) Kawano, T.;
Yoshizumi, T.; Hirano, K.; Satoh, T.; Miura, M. Org. Lett. 2009, 11,
3072. (j) Bernini, R.; Fabrizi, G.; Sferrazza, A.; Cacchi, S. Angew. Chem.,
Int. Ed. 2009, 48, 8078. (k) Jia, Y.-X.; Kundig, E. P. Angew. Chem., Int.
Ed. 2009, 48, 1636. (l) Phipps, R. J.; Gaunt, M. J. Science 2009, 323, 1593.
(m) Zhao, D.; Wang, W.; Yang, F.; Lan, J.; Yang, L.; Gao, G.; You, J.
Angew. Chem., Int. Ed. 2009, 48, 3296. (n) Do, H.-Q.; Daugulis, O.
J. Am. Chem. Soc. 2011, 133, 13577. (o) Hachiya, H.; Hirano, K.; Satoh,
T.; Miura, M. Org. Lett. 2011, 13, 3076.
With the optimum reaction conditions determined
(Table 1, entry 16), various benzothiazoles and thiazoles
were then investigated (Scheme 1). The mild reaction con-
ditions allow for preparation of a variety of 2,20-linkages
of thiazoles in moderate to good yields. Different substi-
tuted benzothiazoles furnished the corresponding products
smoothly (3aꢀf). Importantly, 5-(2-(benzyloxy)ethyl)-4-
methylthiazole also tolerated reaction conditions, thus pro-
viding opportunities for further transformations (3g). It
should be mentioned that excellent regioselectivity at the
C-2 position of thiazoles was also observed when thiazole
and 4-methylthiazole were investigated (3hꢀi),11 which is in
contrast to a previous reported method by which a diary-
lated product is normally obtained.4c Consequently, these
resulting products 3hꢀi could be further transformed by
sequential CꢀH functionalization at the C4 position of the
thiazole part,12 thus featuring the utility of this protocol.
(5) (a) Do, H.-Q.; Daugulis, O. J. Am. Chem. Soc. 2009, 131, 17052.
(b) Monguchi, D.; Yamamura, A.; Fujiwara, T.; Somete, T.; Mori, A.
Tetrahedron Lett. 2010, 51, 850. (c) Li, Y.; Jin, J.; Qian, W.; Bao, W. Org.
Biomol. Chem. 2010, 8, 326. (d) Zhu, M.; Fujita, K.-i.; Yamaguchi, R.
Chem. Commun. 2011, 47, 12876. For nickel-, manganese-, cobalt-, and
iron-catalyzed dehydrogenative arene homocoupling, see: (e) Truong,
T.; Alvarado, J.; Tran, L. D.; Daugulis, O. Org. Lett. 2010, 12, 1200.
(6) For copper-mediated dehydrogenative arene cross-coupling with
the chelation assistance of a 2-pyridyl group, see: (a) Kitahara, M.;
Umeda, N.; Hirano, K.; Satoh, T.; Miura, M. J. Am. Chem. Soc. 2011,
133, 2160. During our manuscript preparation, a copper-mediated and -
catalyzed dehydrogenative cross-coupling of indoles and oxazoles with
the chelation assistance of 2-pyrimidyl group has been reported; see: (b)
Nishino, M.; Hirano, K.; Satoh, T.; Miura, M. Angew. Chem., Int. Ed.
2012, 51, 6993. A copper-mediated dehydrogenative diarylation has also
been reported during our manuscript preparation; see: (c) Mao, Z.;
Wang, Z.; Xu, Z.; Huang, F.; Yu, Z.; Wang, R. Org. Lett. 2012, 14, 3854.
(7) (a) Zhang, X.; Fan, S.; He, C.-Y.; Wan, X.; Min, Q.-Q.; Yang, J.;
Jiang, Z.-X. J. Am. Chem. Soc. 2010, 132, 4506. (b) He, C.-Y.; Fan, S.-L.;
Zhang, X. J. Am. Chem. Soc. 2010, 132, 12850. (c) He, C.-Y.; Min, Q.-Q.;
Zhang, X. Organometallics 2012, 31, 1335. (d) Chen, F.; Feng, Z.; He, C.-Y.;
Wang, H.-Y.; Guo, Y.-l.; Zhang, X. Org. Lett. 2012, 14, 1176.
(8) For selected papers, see: (a) Ichiba, T.; Yoshida, W. Y.; Scheuer, P. J.;
Higa, T.; Gravalos, D. G. J. Am. Chem. Soc. 1991, 113, 3173. (b) Razavi, H.;
Palaninathan, S. K.; Powers, E. T.; Wiseman, R. L.; Purkey, H. E.;
Mohamedmohaideen, N. N.; Deechongkit, S.; Chiang, K. P.; Dendle,
M. T. A.; Sacchettini, J. C.; Kelly, J. W. Angew. Chem., Int. Ed. 2003, 42,
2758. (c) Coqueron, P. Y.; Didier, C.; Ciufolini, M. A. Angew. Chem., Int. Ed.
2003, 42, 1411. (d) Wiglenda, T.; Gust, R. J. Med. Chem. 2007, 50, 1475.
(9) (a) Bagley, M. C.; Dale, J. W.; Merritt, E. A.; Xiong, X. Chem.
Rev. 2005, 105, 685. (b) Nicolaou, K. C.; Zou, B.; Dethe, D. H.; Li, D. B.;
Chen, D. Y. Angew. Chem., Int. Ed. 2006, 45, 7786. (c) Hughes, R. A.;
Moody, C. J. Angew. Chem., Int. Ed. 2007, 46, 7930. (d) Naumov, P.;
Kochunnoony, M. J. Am. Chem. Soc. 2010, 132, 1156.
(10) (a) Zhu, W.; Wu, Y.; Wang, Z. S.; Li, W.; Li, X.; Chen, J.; Wang,
Z.; Tian, H. Adv. Funct. Mater. 2011, 21, 756. (b) He, J.; Wu, W.; Hua, J.;
Jiang, Y.; Qu, S.; Li, J.; Long, Y.; Tian, H. J. Mater. Chem. 2011, 21,
6054. (c) He, J.; Guo, F.; Li, X.; Wu, W.; Yang, J.; Hua, J. Chem.;Eur.
J. 2012, 18, 7903.
(11) For pKa values of representative heteroarenes, see: Shen, K.; Fu,
Y.; Li, J.-N.; Liu, L.; Guo, Q.-X. Tetrahedron 2007, 63, 1568.
(12) Lapointe, D.; Markiewicz, T.; Whipp, C. J.; Toderian, A.;
Fagnou, K. J. Org. Chem. 2011, 76, 749.
Org. Lett., Vol. 14, No. 18, 2012
4951

Table 1. Representative Results for Optimization of
Cu-Catalyzed Dehydrogenative Cross-Coupling of
Benzothiazole 1a with 4,5-Dimethylthiazole 2aa
Scheme 1. Cu-Catalyzed Dehydrogenative Cross-Coupling of
Benzothiazoles 1 with Thiazoles 2a
yield
[%]b
entry
[Cu]
L
oxidant
solvent
1
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
Cu(OAc)2
CuI
phen
phen
phen
phen
phen
bpy
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
Ag2CO3
AgOAc
AgNO3
air
DMF
trace
trace
21
2
DMSO
DME
3
4
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
35
5
44
6
CuI
70(60)
72
7
CuI
bpy
8
CuI
bpy
65
9
CuI
bpy
45
10
11
12c
13c
14c
15c
16c,d
CuI
bpy
trace
trace
(66)
(68)
NR
CuI
bpy
O2
CuI
bpy
Ag2CO3
Ag2CO3
Ag2CO3
none
CuI
none
none
none
none
none
CuI
NR
CuI
Ag2CO3
(71)
a Reaction conditions (unless otherwise specified): 1(0.3mmol,1.0equiv),
2 (2.0ꢀ3.0 equiv), tBuOLi (3.0ꢀ4.0 equiv), toluene (2.5 mL), 10 h, 80 °C.
All reported reaction yields are isolated yields. The number in parentheses
is the homocoupling yield of compound 1.
a Reaction conditions (unless otherwise specified): 1a (0.3 mmol,
1.0 equiv), 2a (2.0 equiv), solvent (2.5 mL), 10 h. b GC yield; the yield of
the isolated product is in parentheses. c 3.0 equiv of 2a and 4.0 equiv of
tBuOLi were used. d 10 mol % of CuI was used.
improved by using CuCl (0.2 equiv) as the catalyst, 4,40-
dimethoxy-2,20-bipyridine L (0.2 equiv) as the ligand,
tBuOOtBu as the oxidant, and 1,2-dichloroethane (DCE)
as the solvent (Scheme 2). Different thiazoles were then in-
vestigated. Generally, the electronic effect plays an impor-
tant role in the reaction efficiency. As shown in Table 3,
5-Arylthiazoles are also suitable substrates, providing
3kꢀn in good yields (3kꢀn). However, oxazoles and benz-
imidazoles are not suitable substrates under standard
reaction conditions.
Encouraged by these results, we proceeded to employ
this copper-catalyzed direct diaryl cross-coupling protocol to
the synthesis of 2-polyfluoroarylthiazoles. Because not only
this kind of fluorinated compounds can be used as a fluo-
rescent probe,13 but also polyfluoroarylated compounds
play an important role in life and materials science.14 In
addition, to the best of our knowledge, methods for con-
struction of this structural motif through transition-metal
catalyzed dehydrogenative cross-coupling have not been
reported so far.15 However, under the same reaction con-
ditions, no desired product 5a was observed, when ben-
zothiazole 1a and pentafluoroarene 4 were tested. After a
survey of reaction parameters (for details see Supporting
Information), the reaction efficiency was significantly
Scheme 2. Cu-Catalyzed Dehydrogenative Cross-Coupling of
Benzothiazoles 1 with Pentafluorobenzene 4a
(13) Tanaka, K.; Kumagai, T.; Aoki, H.; Deguchi, M.; Iwata, S.
J. Org. Chem. 2001, 66, 7328.
(14) (a) Amii, H.; Uneyama, K. Chem. Rev. 2009, 109, 2119. (b)
Meyer, E. A.; Castellano, R. K.; Diederich, F. Angew. Chem., Int. Ed.
2003, 42, 1210. (c) Murphy, A. R.; Frechet, J. M. J. Chem. Rev. 2007, 107,
1066. (d) Babudri, F.; Farinola, G. M.; Naso, F.; Ragni, R. Chem.
Commun. 2007, 1003.
(15) For transition-metal-catalyzed direct arylation of polyfluoro-
arenes with aryl (pseudo)halides, see: (a) Lafrance, M.; Rowley, C. N.;
Woo, T. K.; Fagnou, K. J. Am. Chem. Soc. 2006, 128, 8754. (b) Do,
H.-Q.; Daugulis, O. J. Am. Chem. Soc. 2008, 130, 1128. (c) Fan, S.;
Yang, J.; Zhang, X. Org. Lett. 2011, 13, 4374.
a Reaction conditions (unless otherwise specified): 1 (2.0 equiv), 4
(0.3 mmol, 1.0 equiv), tBuOLi (0.5 equiv), DCE (2.5 mL), 6 h, 80 °C. All
reported reaction yields are isolated yields. The number in parentheses is
the homocoupling yield of compound 4. L, 4,40-dimethoxy-2,20-bipyr-
idine. b2.5 equiv of tBuOLi were used. c1.5 equiv of tBuOLi were used.
4952
Org. Lett., Vol. 14, No. 18, 2012

benzothiazole 1a furnished the corresponding product 5a in
good yield, but both electron-rich and -deficient benzothia-
zoles led to slightly lower yields (5bꢀc). 5-Arylthiazoles
are also suitable substrates with reasonable yields ob-
tained (5dꢀe).
In conclusion, a copper-catalyzed dehydrogenative cross-
coupling of benzothiazoles with thiazoles and polyfluoro-
arene has been demonstrated. High levels of selectivity for the
cross-coupled products over homocoupling of either cou-
pling partner were generally observed. The notable features
of this reaction are its mild reaction conditions, synthetic
simplicity, and usage of less expensive copper as the catalyst.
We believe that these intriguing results should prompt a
much wider examination of this copper-catalyzed direct
diaryl cross-coupling chemistry. Further studies to expand
the substrate scope and uncover the reaction mechanism are
now in progress.
While a detailed reaction mechanism of this transforma-
tion remains to be elucidated, on the basis of previous
research on the copper-catalyzed homocoupling of arenes
orheteroarenes,5 weproposed thatthe present direct diaryl
cross-coupling may begin with the base promoted cupra-
tion of one of the coupling partner (heteroarene or poly-
fluoroarene), which then reacts with the other partner
copper species (heteroaryl or polyfluoroarylcoper species)
or lithium species (heteroaryl or polyfluoroaryl lithium)
generated by the reaction of heteroarene or polyfluoro-
arene with tBuOLi to form key intermediate biheteroaryl
or heteroarylpolyfluoroaryl copper complex I. I undergoes
reductive elimination in the presence of oxidant, silver salt
or tBuOOtBu,16 to deliver the desired product and release
the copper to complete the catalytic cycle.
Acknowledgment. This work was financially supported
by the NSFC (Nos. 21172242, 20902100, 20832008), the
National Basic Research Program of China (973 Program)
(No. 2012CB821600), and SIOC.
Supporting Information Available. Detailed experimen-
tal procedures and characterization data for new com-
pounds. This material is available free of charge via the
Internet at http://pubs.acs.org
(16) A Cu(III) complex is supposed to be formed in the presence of
oxidant before reductive elimination. However, the detailed mechanism
at this stage unclear and will be addressed in the future.
The authors declare no competing financial interest.
Org. Lett., Vol. 14, No. 18, 2012
4953