Barium manganate in microwave-assisted oxidation reactions: synthesis of solvatochromic 2,4,6-triarylpyrimidines

Article abstract of DOI:10.1016/j.tetlet.2009.09.116

Synthesis of 2,4,6-trisubstituted pyrimidines by tandem oxidation/heterocyclocondensation of propargylic alcohols and amidines is effected rapidly and efficiently under microwave dielectric heating using barium manganate as oxidant. Irradiation at 150 °C

Full text of DOI:10.1016/j.tetlet.2009.09.116

Tetrahedron Letters 50 (2009) 6818–6822
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Barium manganate in microwave-assisted oxidation reactions: synthesis
of solvatochromic 2,4,6-triarylpyrimidines
*
*
Mark C. Bagley , Zhifan Lin, Simon J. A. Pope
School of Chemistry, Main Building, Cardiff University, Park Place, Cardiff CF10 3AT, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 July 2009
Revised 4 September 2009
Accepted 21 September 2009
Available online 24 September 2009
Synthesis of 2,4,6-trisubstituted pyrimidines by tandem oxidation/heterocyclocondensation of propargy-
lic alcohols and amidines is effected rapidly and efficiently under microwave dielectric heating using bar-
ium manganate as oxidant. Irradiation at 150 °C in ethanol–acetic acid for 45 min results in dramatic
improvements in yield over the corresponding manganese dioxide-mediated method and establishes a
rapid route to triarylpyrimidines in order to investigate their photophysical properties.
Ó 2009 Elsevier Ltd. All rights reserved.
Efficient synthetic routes to functionalised chromophoric com-
pounds possessing tunable absorption and emission properties are
essential in the search for materials for optoelectronic devices,
light-energy harvesting and fluorescence imaging microscopy.1 Syn-
thetic2 and theoretical3 studies have shown that unusual electrochro-
mic, photochromic, luminescent or nonlinear optical properties can
be developed by incorporating aromatic and heteroaromatic groups
with different electronic properties onto a chromophoric interlink
within a donor–acceptor (D–A) molecular framework.4–6 The use of
a pyrimidine scaffold in this regard has received very little attention,
tron-rich 1,3-diarylpropynone precursors,11 which threatened the
scope of any photophysical review. Furthermore, when considering
a tandem oxidation–heterocyclocondensation route, only a very
limited range of pyrimidines had been prepared before by this
methodology.12
These concerns were indeed found to be well-justified upon
investigation. Synthesis of 2,4-diphenylpyrimidine (4a) by cyclo-
condensation of benzamidine 1 and propynone 2a, either used di-
rectly or produced in situ by tandem oxidation of propargylic
alcohol 3a with MnO2, was highly efficient under microwave irradi-
ation, whereas the corresponding process for the synthesis of 2-phe-
nyl-4,6-bis(4-methoxyphenyl)pyrimidine (4b) was less efficient
(Scheme 1, Table 1). The tandem oxidation–heterocyclocondensa-
tion of amidine 1 and an even more electron-richpropargylic alcohol
3c with MnO2 under microwave irradiation barely gave any yield of
pyrimidine 4c at all. It was concluded that a new or refined method
was required in order to access a library of 2,4,6-triarylpyrimidines
with different D–A properties for photophysical study.
until recentlywhenthe fluorescence properties of p-extendedpyrim-
idine systems was recognised.7 As part of our interest in the synthesis
of heterocycles with readily-modulated photophysical properties,8
we rationalised that the tandem oxidation/heterocyclocondensation
of propargylic alcohols and amidines under microwave irradiation
wouldenablearapidrouteto alibraryofp-extendedpyrimidinesthat
could incorporate a range of donor and acceptor groups in order to
tune their photophysical behaviour.
Microwave dielectric heating has received increasing attention in
recent years as a valuable alternative to the use of conductive heating
for accelerating transformations, both in synthetic chemistry and the
biosciences.9 Our own studies have demonstrated that microwave
irradiation provides a rapid and convenient alternative to conductive
heating methods for the cyclocondensation of amidines and ethynyl
ketones to give disubstituted pyrimidinesingood yield.10 Despite this
goodprecedent, therewereanumberofconcernspriortotheoutsetof
a microwave-mediated approach to this pyrimidine library: our pre-
viously established method had been found to be less efficient both
for the synthesis of trisubstituted pyrimidines10 and when using elec-
It has been shown that barium manganate is a useful reagent for
the oxidation of benzylic, allylic and propargylic alcohols13,14 and,
when used in a tandem oxidation/Wittig reaction,15 was efficient
O
method
A
2a-c
3a-c
Ph
R1
R2
NH·HCl
NH2
1·HCl
N
N
or
OH
Ph
R1
R2
4a-c
R1
method
B
R2
* Corresponding authors. Tel./fax: +44 29 2087 4030 (M.C.B.); tel.: +44 29 2087
9316; fax: +44 29 2087 4030 (S.J.A.P.).
Scheme 1. Method A10,11 (cyclocondensation): Na2CO3, MeCN,
method B12 (tandem oxidation): MnO2, Na2CO3, MeCN,
, 150 °C, 45–60 min. –
= not determined; denotes microwaves.
lx, 120 °C, 40 min;
lx
E-mail addresses: bagleymc@cf.ac.uk (M.C. Bagley), popesj@cf.ac.uk (S.J.A. Pope).
lx
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.09.116

M. C. Bagley et al. / Tetrahedron Letters 50 (2009) 6818–6822
6819
Table 1
Table 2
Isolated yield (%) of 4a–c using method A or B
Optimising the microwave-assisted synthesis of pyrimidine 4a
R1
R2
A
B
Entry
Reagents and conditionsa
MnO2,
, 140–150 °C,c MeCN, 45 min
BaMnO4, , 150 °C, MeCN, 45 min
MnO2, MeCN, reflux, 10 h
BaMnO4, MeCN, reflux, 10 h
Yieldb
1
2
l
x
lx
85
88
72
76
50
62
82
92
86
64
90
a
b
c
H
98
84
3d
4d
5
BaMnO4,
BaMnO4,
BaMnO4,
BaMnO4,
BaMnO4,
BaMnO4,
BaMnO4,
l
l
l
x
x
x
x
x
, 150 °C, MeOH, 45 min
, 150 °C, EtOH, 45 min
, 120 °C, EtOH–AcOH, 45 min
, 150 °C, EtOH–AcOH, 45 min
, 170 °C, EtOH–AcOH, 45 min
, 150 °C, EtOH–AcOH, 30 min
, 150 °C, EtOH–AcOH, 60 min
6
7
8
9
10
11
41
—
52
19
l
MeO
MeO
OMe
l
l
l
x
x
a
NMe2
l
x denotes experiments were conducted under microwave irradiation in a
10 mL Pyrex vessel using a single mode CEM DiscoverÒ microwave synthesiser at
the given temperature through moderation of the initial microwave power (150 W),
unless noted otherwise. The optimum conditions are indicated in bold.
and gave higher yields than manganese dioxide.16 Following our
previous observation that a related tandem oxidation/heteroannu-
lation process was more facile under microwave irradiation with
BaMnO4 than with MnO2, due to its more efficient coupling with
the rapidly oscillating electric field,17 the use of this oxidant
(3 equiv) was investigated in the microwave-assisted tandem oxi-
dation/heterocyclocondensation of propargylic alcohol 3a (1 equiv)
and benzamidine 1 (1 equiv). It was thought that this would be a
good substrate to study, as the heterocyclisation was highly effi-
cient (Table 1). Under the conditions investigated (Table 2), the
use of BaMnO4 in this tandem process gave pyrimidine 4a in good
yield, with microwave irradiation at 150 °C in a mixture of EtOH–
acetic acid for 45 min appearing to be optimum (entry 8).
b
Yield refers to the percentage isolated yield after purification by column
chromatography on silica gel.
Although the temperature was set to 150 °C, the actual temperature of reaction
was considerably lower due to inefficient coupling with the microwave irradiation.
c
d
Carried out by traditional conductive heating using an oil bath.
These refined conditions (Method 1)18 were tested using a
range of propargylic alcohols 3 (Table 3), prepared according to
the method of Tykwinski.14 The isolated yield of the pyrimidine
product was compared with two other procedures: microwave
irradiation in MeCN using MnO2 as oxidant (Method 2) and an
existing traditional method of pyrimidine formation under con-
ductive heating by cyclocondensation of amidine 1 and the corre-
Table 3
Scope of the microwave-assisted tandem oxidation/heterocyclocondensation using BaMnO4
Entry Alcohol 3
Pyrimidine 4
Yield%a Method 1b Yield%a Method 2c Yield%a Method 3d
Ph
OH
N
N
1
2
92
72
85
42
0e
3a
4a
Ph
Ph
Ph
OH
OH
N
N
N
N
30
3b
3c
4b
4c
MeO
MeO
OMe
MeO
MeO
OMe
N
N
3
4
5
72
79
82
19f
10f
32
24
11
NMe2
NMe2
OH
4d
3d
S
S
MeO
MeO
Ph
OH
N
N
29
3e
4e
S
S
OMe
OMe
(continued on next page)

6820
M. C. Bagley et al. / Tetrahedron Letters 50 (2009) 6818–6822
Table 3 (continued)
Entry Alcohol 3
Pyrimidine 4
Yield%a Method 1b Yield%a Method 2c Yield%a Method 3d
Ph
OH
N
N
6
86
44
40
4f
3f
S
S
S
S
Ph
Ph
OH
OH
N
N
7
73g
48
31
3g
4g
OMe
OMe
N
N
8
68
68
74
62
80
76
36
—
—
—
—
—
24
3h
4h
S
S
Ph
OH
N
N
9
0e
3i
4i
S
S
NMe2
NMe2
Ph
OH
OH
OH
OH
N
N
10
11
12
13
36
3j
4j
NMe2
NMe2
NMe2
Br
Ph
N
N
0e
3k
4k
NC
NC
NMe2
Ph
N
N
40
3l
4l
Br
Ph
N
N
22
3m
4m
NC
Br
NC
Br
a
Isolated yield of pyrimidine 4 after purification by column chromatography on silica gel.
Method 1 (method of choice): microwave irradiation of benzamidine 1 (1 equiv) and propargylic alcohol 3 (1 equiv) at 150 °C using BaMnO4 (3 equiv) in EtOH–AcOH in a
b
sealed tube for 45 min using a CEM DiscoverÒ microwave synthesiser by moderating the initial microwave power (150 W).
c
Method 2 (for comparison): microwave irradiation of benzamidine 1 (1 equiv) and propargylic alcohol 3 (1 equiv) at 150 °C (set temperature; actual temperature may be
17
135–150 °C; see Ref.
for details) using MnO2 (3 equiv) in MeCN in a sealed tube for 45 min using a CEM DiscoverÒ microwave synthesiser by moderating the initial
microwave power (150 W). — = not determined.
d
Method 3 (for comparison): conductive heating of benzamidine 1 (1 equiv) and the corresponding chalcone (1 equiv) at reflux in EtOH in the presence of NaOH, moisture
and air for 10 h, according to the method of Dodson and Seyler.19
e
The reaction failed to give any of the desired pyrimidine product 4.
Reaction time was extended to 1 h.
No purification by column chromatography was required.
f
g

M. C. Bagley et al. / Tetrahedron Letters 50 (2009) 6818–6822
6821
HN
Ph
Ph
N
N
Cu(neocup)(PPh3)Br21
N
N
R
R
KOtBu, PhMe
microwaves
120 °C, 1 h
N
Br
4l R = 2-naphthyl
4m R = 4-cyanophenyl
4n R = 2-naphthyl (68%)
4o R = 4-cyanophenyl (74%)
Scheme 2. Further library diversification by copper-mediated N-arylation.
sponding chalcone (Method 3), according to the method of Dodson
and Seyler,19 for select cases. It was gratifying to observe that a
tandem oxidation/heterocyclocondensation reaction offered a via-
425
525
625
wavelength / nm
ble route to a range of p-extended pyrimidines. Furthermore, in all
cases, the refined conditions using BaMnO4 gave significant
improvements in yield over the other methods making it by far
the more efficient procedure.
Figure 1. Normalised emission (kex = 370 nm) spectra showing the solvent depen-
dence of compound 4n (black: DMSO; grey: CHCl3; light grey: cyclohexane).
In order to broaden the electronic profile of the
p-extended
for 4n upon increasing solvent polarity. Taken together these re-
sults suggest that the room temperature visible fluorescence from
the donor–acceptor appended pyrimidine species is due to intra-
molecular CT, and is in agreement with a recent report into oligo-
meric pyrimidine species25 and other related species.7 These
results are also consistent with our previous findings on related
donor–acceptor frameworks based upon cyanopyridines,8 which
also show pronounced solvatochromic behaviour from an emitting
CT-excited state.
In conclusion, tandem oxidation/heterocyclocondensation of a
propargylic alcohol and benzamidine using BaMnO4 under micro-
wave irradiation provides a rapid route to pyrimidines, which
can be further derivatised by microwave-assisted copper-mediated
pyrimidines accesible by this methodology, bromides 4l,m were
transformed by copper-mediated N-arylation20 using the pre-
formed Cu(I) catalyst Cu(neocup)(PPh3)Br.21 The original conduc-
tive heating procedure was modified22 and carried out in the
presence of pyrrolidine and potassium tert-butoxide at 120 °C for
one hour in toluene under microwave irradiation23 to give pyrroli-
dino-derivatives 4n,o, respectively, in good yield (Scheme 2).
The photophysical properties of a selection of the functionalised
pyrimidines were assessed.24 The pyrimidine species displayed
solution-state (CHCl3) fluorescence at room temperature: each
possessed a single broad emission in the visible region following
excitation at 360 nm. The short (<10 ns) emission lifetimes
(mono-exponential, consistent with a single decaying excited
state) were also characteristic of a fluorescence in each case. With
the exception of 4a, notable Stokes shifts were attributed to the
presence of the donor and acceptor groups, which are likely to in-
duce significant charge transfer character to the excited state. Con-
sequently, the origin of the fluorescence in 4a is more likely to be a
N-arylation. The
p-extended pyrimidines so-formed by this ap-
proach are highly fluorescent in the visible region, displaying sol-
vent-dependent emission wavelengths suggestive of charge
transfer-dominated excited states.
Acknowledgement
locally excited singlet
p
p* transition. The most significant red-
shifts in both kabs and kem were achieved with a naphthyl unit as
the acceptor component. Therefore the charge transfer character
was confirmed by assessing the solvent dependence of the emis-
sion from 4j in cyclohexane, chloroform and DMSO (Table 4). The
steady state emission spectra showed that the fluorescence band
red-shifts significantly with a concomitant broadening as solvent
polarity increases, together with a corresponding reduction in
quantum yield. The corresponding excitation spectra also revealed
a low-energy broadening of the lowest energy band (360–400 nm),
which correlate with the observations from the absorption spectra.
An additional example is shown graphically in Figure 1 again
showing the significant low-energy shift in emission maximum
We thank the EPSRC for funding together with Cardiff
University.
References and notes
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8. Bagley, M. C.; Lin, Z.; Pope, S. J. A. Chem. Commun. 2009, 5165.
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Chem. Soc. Rev. 2008, 37, 1127; (c) Collins, J. M.; Leadbeater, N. E. Org. Biomol.
Chem. 2007, 5, 1141; (d) Adam, D. Nature 2003, 421, 571; (e)Microwaves in
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Angew. Chem., Int. Ed. 2004, 43, 6250; (g) Hayes, B. L. Aldrichim. Acta 2004, 37,
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Tierney, J.; Wathey, B.; Westman, J. Tetrahedron 2001, 57, 9225; (j) Loupy, A.;
Petit, A.; Hamelin, J.; Texier-Boullet, F.; Jacquault, P.; Mathé, D. Synthesis 1998,
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Chem. Soc. Rev. 1998, 27, 213; (l) Galema, S. A. Chem. Soc. Rev. 1997, 26, 233; (m)
Caddick, S. Tetrahedron 1995, 51, 10403; (n) Strauss, C. R.; Trainor, R. W. Aust. J.
Chem. 1995, 48, 1665.
Table 4
Solution state photophysical properties of some selected pyrimidines
Compound
kabs (log
e
)/nm
kem/nm
s
/ns
/
fl
4aa
4ca
4ia
278 (4.26)
365 (4.54)
371 (4.64)
379 (4.84)
368 (4.68)
382 (4.92)
376 (4.82)
430 (4.40)
385 (4.38)
311
453
473
510
422
599
495
498
518
0.6
7.1
7.0
2.4
2.9
1.6
3.5
2.4
3.3
0.51
0.49
0.46
0.40
0.48
0.22
0.44
0.19
0.23
4ja
4jb
4jc
4ka
4na
4oa
a
b
c
In chloroform.
In cyclohexane.
In dimethylsulfoxide.
10. Bagley, M. C.; Hughes, D. D.; Taylor, P. H. Synlett 2003, 259.

6822
M. C. Bagley et al. / Tetrahedron Letters 50 (2009) 6818–6822
11. Bagley, M. C.; Hughes, D. D.; Lubinu, M. C.; Merritt, E. A.; Taylor, P. H.;
Tomkinson, N. C. O. QSAR Comb. Sci. 2004, 23, 859.
12. Bagley, M. C.; Hughes, D. D.; Sabo, H. M.; Taylor, P. H.; Xiong, X. Synlett 2003,
1443.
13. (a) Firouzabadi, H.; Karimi, B.; Abbassi, M. J. Chem. Res. 1999, 236; (b) Kim, K. S.;
Chung, S.; Cho, I. H.; Hahn, C. S. Tetrahedron Lett. 1989, 30, 2559; (c)
Firouzabadi, H.; Ghaderi, E. Tetrahedron Lett. 1978, 839; (d) Fatiadi, A. J.
Synthesis 1987, 85.
petroleum ether (1:6 v/v), gave 4-[6-(4-bromophenyl)-2-phenylpyrimidin-4-
yl]benzonitrile (4m) (0.18 g, 76%) as colourless crystals, mp 231–232 °C
(found: MÅ+, 411.0356. C23H1479BrN3 [M] requires 411.0371); IR (KBr) vmax
2919, 2849, 2224, 1592, 1572, 1525, 1363; 1H NMR (400 MHz, CDCl3) d 8.71
(2H, m, 2,6-PhH), 8.41 (2H, d, J 8.8, 30,50-H), 8.19 (2H, d, J 8.8, 300,500-H), 8.01 (1H,
s, 5-H), 7.88 (2H, d, J 8.8, 20,60-H), 7.73 (2H, d, J 8.8, 200,600-H), 7.56 (3H, m, PhH);
13C NMR (100 MHz; CDCl3) d 165.1 (C), 163.2 (C), 162.8 (C), 142.4 (C), 138.2 (C),
137.4 (C), 136.4 (C), 133.1 (CH), 132.0 (CH), 131.6 (CH), 131.4 (CH), 129.6 (CH),
128.9 (CH), 127.8 (CH), 126.0 (C), 114.0 (CN), 110.6 (CH); MS (EI) m/z (rel.
intensity) 413 (M[81Br]Å+, 97%), 411 (M[79Br]Å+, 100).
14. Shi Shun, A.; Chernick, E.; Eisler, S.; Tykwinski, R. J. Org. Chem. 2003, 68, 1339.
15. For reports on MnO2-mediated tandem oxidation/Wittig reactions and related
methodology, see: (a) Lang, S.; Taylor, R. J. K. Tetrahedron Lett. 2006, 47, 5489;
(b) Phillips, D. J.; Pillinger, K. S.; Li, W.; Taylor, A. E.; Graham, A. E. Chem.
Commun. 2006, 2280; (c) Wilfred, C.; Taylor, R. J. K.; Wilfred, C.; Taylor, R. J. K.
Synth. Commun. 2005, 35, 2859; (d) Raw, S. A.; Reid, M.; Roman, E.; Taylor, R. J.
K. Synlett 2004, 819; (e) Aitken, D. J.; Faure, S.; Roche, S. Tetrahedron Lett. 2003,
44, 8827; (f) Foot, J. S.; Kanno, H.; Giblin, G. M. P.; Taylor, R. J. K. Synthesis 2003,
1055; (g) Blackburn, L.; Kanno, H.; Taylor, R. J. K. Tetrahedron Lett. 2003, 44,
115; (h) Runcie, K. A.; Taylor, R. J. K. Chem. Commun. 2002, 974; (i) Blackburn,
L.; Pei, C.; Taylor, R. J. K. Synlett 2002, 215; (j) Blackburn, L.; Taylor, R. J. K. Org.
Lett. 2001, 3, 1637; (k) Wei, X.; Taylor, R. J. K. J. Org. Chem. 2000, 65, 617; (l)
Blackburn, L.; Wei, X.; Taylor, R. J. K. Chem. Commun. 1999, 1337; (m) Wei, X.;
Taylor, R. J. K. Tetrahedron Lett. 1998, 39, 3815.
16. Shuto, S.; Niizuma, S.; Matsuda, A. J. Org. Chem. 1998, 63, 4489.
17. See preceding manuscript. Barium manganate in microwave-assisted oxidation
reactions: synthesis of lactones by oxidative cyclization of diols. Bagley, M. C.;
Lin, Z.; Phillips, D. J.; Graham, A. E. Tetrahedron Lett. 2009, 50, 6823.
18. In a typical procedure for the microwave-assisted in situ tandem oxidation-
heteroannulation reaction mediated by BaMnO4, a mixture of benzamidine (1)
(0.57 mmol, 1 equiv), 1-(4-cyanophenyl)-3-[4-bromophenyl]prop-2-yn-1-ol
(3m) (0.57 mmol, 1 equiv) and barium manganate (1.70 mmol, 3 equiv) in
EtOH–AcOH (5:1) (5 mL) was irradiated at 150 °C in a sealed pressure-rated
reaction tube (10 mL), at an initial power of 150 W, for 45 min in a self-turned
single mode CEM DiscoverÒ Focused Synthesiser. The mixture was cooled
rapidly to room temperature, by passing compressed air through the
microwave cavity for 5 min, and then filtered through Celite. The filtrate was
poured into water (15 mL) and extracted with EtOAc (8 mL). The aqueous layer
was further extracted with EtOAc (8 mL) and the organic extracts were
combined, washed sequentially with saturated aqueous sodium hydrogen
carbonate (10 mL) and brine (10 mL), dried (Na2SO4) and evaporated in vacuo.
Purification by column chromatography on silica gel, eluting with EtOAc–
19. Dodson, R.; Seyler, J. J. Org. Chem. 1951, 16, 461.
20. Venkataraman, D.; Gujadhur, R. K.; Kintigh, J. T. Tetrahedron Lett. 2001, 42,
4791.
21. Venkataraman, D.; Gujadhur, R. K.; Bates, C. G. Org. Lett. 2001, 3, 4315.
22. In a typical procedure for the microwave-assisted Cu-mediated N-arylation, a
solution of benzonitrile 4m (0.57 mmol, 1 equiv) in toluene (3 mL) was added
to a stirred solution of pyrrolidine (1.13 mmol, 2 equiv), Cu(neocup)(PPh3)Br21
(10 mol %) and potassium tert-butoxide (0.85 mmol, 1.5 equiv) in toluene
(3 mL) in a pressure-rated Pyrex reaction tube (10 mL). The vessel was sealed
and irradiated at 120 °C, at an initial power of 150 W, in a self-turned single
mode CEM DiscoverÒ Focused Synthesiser for 1 h. The mixture was then cooled
rapidly to room temperature, by passing compressed air through the
microwave cavity for 5 min, and then filtered through Celite. The filtrate was
poured into water (15 mL) and extracted with Et2O (15 mL). The aqueous layer
was further extracted with Et2O (15 mL) and the organic extracts were
combined, dried (Na2SO4) and evaporated in vacuo. Purification by column
chromatography on silica gel, eluting with EtOAc–petroleum ether (1:6 v/v),
gave
4-{2-phenyl-6-[4-(pyrrolidin-1-yl)phenyl]pyrimidin-4-yl}benzonitrile
(4o) (0.17 g, 74%) as orange crystals, mp 227–228 °C, with satisfactory
spectroscopic and spectrometric characterisation data.
23. For related CuI-mediated microwave-assisted procedures for Ullmann-
condensation, see: Bagley, M. C.; Dix, M. C.; Fusillo, V. Tetrahedron Lett. 2009,
50, 3661 and references cited therein.
24. Steady state spectra were recorded using a Perkin–Elmer LS55. Luminescence
lifetimes were obtained on a JobinYvon-Horiba Fluorolog spectrometer fitted
with a JY TBX picosecond photodetection module with a pulsed source 372 nm
NanoLED (operating at 1 MHz). All lifetimes were obtained using the JY-Horiba
FluorHub single photon-counting module.
25. Achelle, S.; Nouira, I.; Pfaffinger, B.; Ramondenc, Y.; Ple, N.; Rodriguez-Lopez, J.
J. Org. Chem. 2009, 74, 3711.