Synthesis of mono-(p-dimethylamino)styryl-containing BOPHY dye for a turn-on pH sensor

Article abstract of DOI:10.1039/c4ra15914d

Mono-substitutional bis(difluoroboron)1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine (BOPHY) 3a with a (p-dimethylamino)styryl group in the α-position was confirmed to be synthesized by the Knoevenagel-type condensation. Dimethylamino-containing BOPHY dye 3a is almost non-fluorescent by the ICT effect. Upon the protonation of the tertiary amine function of 3a, the strong fluorescence (Φ_f = 0.98) was released and the fluorescence intensity was dramatically increased by one thousand fold. BOPHY 3a can be used as a pH probe. This journal is

Full text of DOI:10.1039/c4ra15914d

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Synthesis of mono-(p-dimethylamino)styryl-
containing BOPHY dye for a turn-on pH sensor†
Cite this: RSC Adv., 2015, 5, 16735
a
Xin-Dong Jiang, Yajun Su,a Shuai Yue,a Chen Li,a Haifeng Yu,a Han Zhang,b
*
Chang-Liang Sunc and Lin-Jiu Xiaoa
Mono-substitutional bis(difluoroboron)1,2-bis((1H-pyrrol-2-yl)methylene)hydrazine (BOPHY) 3a with a
(p-dimethylamino)styryl group in the a-position was confirmed to be synthesized by the Knoevenagel-
type condensation. Dimethylamino-containing BOPHY dye 3a is almost non-fluorescent by the ICT
effect. Upon the protonation of the tertiary amine function of 3a, the strong fluorescence (Ff ¼ 0.98)
was released and the fluorescence intensity was dramatically increased by one thousand fold. BOPHY 3a
can be used as a pH probe.
Received 7th December 2014
Accepted 27th January 2015
DOI: 10.1039/c4ra15914d
www.rsc.org/advances
urge us to investigate the BOPHY structural motif for a func-
tionalization. Our recent research interest lies in the novel
1. Introduction
BODIPY/aza-BODIPY family of uorescent dyes and their
application.5 During preparation of this manuscript, modica-
tions on BOPHY dyes, including BOPHYs 3a and 3b, were
reported by Jiao and Hao group (Fig. 2).6 However, the synthesis
of 3b was not shown, and no study of BOPHY as a pH uores-
cent probe was explored either. Therefore, we herein commu-
nicate our studies on a (p-dimethylamino)styryl-containing
BOPHY 3a as a turn-on uorescent probe for pH (Fig. 2).
In recent decades, 4,40-diuoro-4-bora-3a,4a-diaza-sindacene
(abbreviated as BODIPY) dyes have received increased attention
because of their high uorescence quantum yields, high absorp-
tion coefficients, good solubility in organic solvents, and excellent
thermal and photochemical stabilities.1 Since the discovery of the
excellent photochemical properties of BODIPY, various modica-
tions on the BODIPY dye 1 (Fig. 1a) including the modication on
the core or aza-BODIPY,2 have been widely explored for sensors,
laser dyes, light harvesters, organic light emitting diodes, and
sensitizers for solar cells and so forth.3 Therefore, the BODIPY dyes
have been attracting increasing interest in the design and
synthesis of these novel uorescent dyes to date.
Ziegler et al. recently presented another type of unique
pyrrole-BF2-based uorophore bis(diuoroboron)1,2-bis((1H-
pyrrol-2-yl)methylene)hydrazine (BOPHY) 2 (Fig. 1a).4 The new
uorescent BOPHY dye 2 can be successfully obtained by the
reaction of pyrrole-2-carboxaldehyde with hydrazine, and fol-
lowed by complexation with Et3N–BF3$Et2O, based on the
traditional classical method (Fig. 1b).4 The aesthetic symmetric
structure is composed of four rings at the same plane, including
two BF2 units in six-membered chelate rings in the center and
two pyrrole units on the periphery (Fig. 1).4 The uorescence
quantum yield for the unmodied BOPHY 2 is so high to near to
be 100%.4 Since the new BOPHY dye 2 has a grid structure,
excellent optical properties and is similar to BODIPY 1, these
2. Experimental section
2.1 General
1H NMR spectra were recorded on a Bruker AVANCE III
500 MHz spectrometer. 1H NMR chemical shis (d) are given in
aCollege of Applied Chemistry, Shenyang University of Chemical Technology,
Shenyang, 110142, China. E-mail: xdjiang@syuct.edu.cn; Tel: +86 024 89387219
bSchool of Materials Science and Chemical Engineering, Tianjin University of Science &
Technology, Tianjin, 300457, China
cCenter of Physical and Chemistry Test, Shenyang University of Chemical Technology,
Shenyang 110142, China
† Electronic supplementary information (ESI) available: Experimental details and
1H NMR spectra. See DOI: 10.1039/c4ra15914d
Fig. 1 The core structure of BODIPY and BOPHY.
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(500 MHz, CDCl3): d (ppm) 7.96 (s, 1H), 7.85 (s, 1H), 7.46 (d,
J ¼ 8.5 Hz, 2H), 7.22 (d, J ¼ 12.5 Hz, 2H), 6.71 (d, J ¼ 6.0 Hz, 2H),
6.68 (s, 1H), 6.17 (s, 1H), 3.03 (s, 6H), 2.49 (s, 3H), 2.35 (s, 3H),
2.33 (s, 3H). 13C NMR (125 MHz, CDCl3): d (ppm) 151.6, 151.0,
150.2, 140.7, 140.1, 137.7, 133.7, 132.4, 128.9, 124.4, 124.0,
123.3, 118.2, 114.5, 112.8, 111.9, 110.9, 40.2, 14.1, 11.2, 11.1.
HRMS-MODAI (ESI) calcd for C23H25N5B2F4 [M + H]+: 470.2305,
found 470.2305.
Fig. 2 Structure of BOPHY 3a and 3b.
3. Results and discussion
3.1 Synthesis and characterization of BOPHY 3a
ppm downeld from Me4Si, determined by chloroform (d ¼ 7.26
ppm). 13C NMR spectra were recorded on a Bruker AVANCE III
125 MHz spectrometer. 13C NMR chemical shis (d) are repor-
ted in ppm with the internal CDCl3 at d 77.0 ppm as standard.
Toluene solvents were distilled over CaH2. Merck silica gel 60
was used for the column chromatography. All pH measure-
ments were performed with a PHS-3E pH meter.
Fluorescence spectra were recorded on FluoroSENS spec-
trophotometer. UV/Vis spectra were recorded on UV-2550
spectrophotometer at room temperature. The refractive index
of the medium was measured by 2 W Abbe's refractometer at
20 ꢀC. The uorescence quantum yields (Ff) of the BOPHY
system was calculated using the following relationship (eqn (1)):
BOPHY dyes 3a and 3b with a (p-dimethylamino)styryl group in
a/b-position of the pyrrole (Fig. 2), were found to occur in the
text and the ESI in Jiao and Hao's paper, respectively.6 It is
illusive to which one was synthesized in the Knoevenagel-type
condensation. Therefore, we curiously repeated this reaction
from BOPHY 4 (ref. 4 and 6) via the Knoevenagel-type conden-
sation (Scheme 1).7 We successfully synthesized the mono-
dimethylaminostyryl BOPHY 3a (Scheme 1). Though the use
of 10 eq. 4-dimethylaminobenzaldehyde, no bis-dimethylamino
styryl BOPHY was observed.
The structure of dye 3a was conrmed by 1H NMR spectrum.
Due to the shield from the full effect of the applied eld by their
surrounding electrons,8 the chemical shi of Ha is well-known
(1) to be lower than that of Hb in 1H NMR spectrum in dye 3a
and 4 (Scheme 1). By the measure of the 1H NMR spectrum of 3a
using a Bruker AVANCE III 500 MHz spectrometer, three sets of
distinct hydrogen signals of the methyl group (d ¼ 2.49 (s, 3Hc),
2.35 (s, 3Ha), 2.33 (s, 3Hb) ppm in CDCl3) were showed (Fig. 3
and Scheme 1). Therefore, the logical structure was thought to
be the dye 3a not 3b which should have two sets of hydrogen
signals in the low eld and a set in the high eld (Scheme 1 and
Fig. 2).
2
Ff ¼ FrefFsamplArefnsampl2/FrefAsamplnref
here, F denotes the integral of the corrected uorescence
spectrum, A is the absorbance at the excitation wavelength, ref
and sampl denote parameters from the reference and unknown
experimental samples, respectively. The reference systems used
was rhodamine 6G in methanol as standard (Ff ¼ 0.78, in air
equilibrated water and deaerated solutions) for 3a–H+.
The MO calculations were performed at the DFT level, and
the frontier molecular orbitals of BOPHY 3a and 3a–H+ at the
MP2/6-31G* level with Gaussian 03.
3.2 A turn-on uorescent probe for pH
The most popular strategy for pH-responsive uorescent
sensors takes advantage of intramolecular charge transfer (ICT)
or the photoinduced electron transfer (PET).9 Dimethylamino
2.2 Synthesis of BOPHY 3a
4-N,N-Dimethylbenzaldehyde (33 mg, 0.22 mmol), compound 4
(40 mg, 0.12 mmol) (see following Scheme 1), AcOH (0.5 mL),
and piperidine (0.5 mL) were stirred for 24 h at 95 ꢀC in dry
toluene (6 mL) in the presence of a small amount of activated
˚
4 A molecular sieves. The mixture was cooled to room temper-
ature, quenched with water, extracted with CH2Cl2, washed with
brine, dried over Na2SO4, evaporated and puried by TLC to
afford BODIHY 3a (15 mg, 28%) as red solids. 1H NMR
Scheme 1 Synthesis of the BOPHY dye 3a.
16736 | RSC Adv., 2015, 5, 16735–16739
Fig. 3 Partial 1NMR spectra of the BOPHY dye 3a.
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is “switched off” and in turn the emission of uorescence is
“switched on”. To the best of our knowledge, a functionaliza-
tion of BOPHY dye for a pH probes has not been documented.
Therefore, we continue to explore the response of the
dimethylamino-containing BOPHY dye 3a to pH value.
Photoimage of 3a were taken under normal room illumina-
tion and UV light, and notable changes of relatively vivid bright
colors of 3a with the pH can be easily observed with naked eye
(Fig. 4). The mechanism of BOPHY 3a to pH value is same to
that of the above described BODIPY (Fig. 5). Upon addition of
hydrochloric acid to BOPHY 3a with a –NMe2 group as a
pH-sensitive functionality, 3a was protonated at relatively low
pH value.11 A stepwise decrease of the absorption intensity was
observed in the 543 nm band of 3a, and this peak disappeared
completely at 1 M (Fig. 6). The formation of a new band at
504 nm was rst observed in pH 3, and another new peak
subsequently arose at 481 nm (Fig. 6). The absorption intensity
of 3a–H+ reached the maximum when 4 M HCl was used, and
the absorption intensity was subsequently decreased at 6 and
8 M HCl. The absorption band of 3a–H+ is blue-shied by about
50 nm compared to that of 3a. The uorescence quantum yield
of 3a in CH3CN–H2O (1 : 1, v/v) is very low (Ff ¼ 0.01 at pH 7)
due to the ICT effect; however, with decreasing pH the twin
emission maxima were shied to 532, 573 nm (Fig. 7a).
A dramatic increase in uorescence intensity at 532 nm by 1200
folds (Ff ¼ 0.98 when treated with HCl to 4 M) (Fig. 7b).
Fig. 4 Photograph of solutions of 5 mM BOPHY dye 3a at pH 0, 1, 1.5, 2,
2.5, 3, 3.5, 4 and 7 in CH3CN–H2O (1 : 1, v/v) under normal room
illumination (upper row) and under UV irradiation (bottom row),
respectively.
Fig. 5 Protonation of BOPHY 3a to generate 3a–H+ and release the
strong fluorescence by switching off the ICT effect.
group is one of the fragments frequently used for the purpose of
ICT. The BODIPY is a family of very widely used uorescent dye
for potential applications and oen designed for pH probes,
wherein the excited state of the uorophore can be quenched by
the electron transfer from electron donating amine to the u-
orophore.10 Upon recognition of a proton, the electron transfer
Fig. 7 (a) Corresponding fluorescence spectra (pH 7, 5, 4, 3.5, 3, 2.8,
Fig. 6 Absorption spectra (pH 7, 5, 4, 3.5, 3, 2.8, 2.5, 2.2, 2.0, 1.5, 1 and 2.5, 2.2, 2.0, 1.5, 1 and 1 M, 2 M, 4 M, 6 M, 8 M of HCl, lex ¼ 490 nm) of
1 M, 2 M, 4 M, 6 M, 8 M of HCl) of 5 mM dye 3a in CH3CN–H2O (1 : 1, v/v) 5 mM dye 3a in CH3CN–H2O (1 : 1, v/v) as a function of pH. (b) Fluo-
as a function of pH.
rescence titration responses of 3a towards HCl at 532 nm.
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3a–H+ relative to that (2.53 eV) of 3a, and the energy level of
frontier orbitals in 3a–H+ was lower than that of 3a. BOPHY 3a
as a pH probe can be used. Further efforts for development of
probes for thiol5b,g based on BOPHY dyes in biotechnology are
ongoing in our lab.
Acknowledgements
This work was supported by National Training Programs of
Innovation and Entrepreneurship for Undergraduates
(201410149052, 20141049000052), the Public Research Foun-
dation of Liaoning Province for the Cause of Science
(2014003009), the Scientic Research Foundation for the
Returned Overseas Chinese Scholars, State Education Ministry,
and the start-up funds from Shenyang University of Chemical
Technology.
Notes and references
1 (a) A. Loudet and K. Burgess, Chem. Rev., 2007, 107, 4891; (b)
L. Yuan, W. Lin, K. Zheng, L. He and W. Huang, Chem. Soc.
Rev., 2013, 42, 622; (c) R. Weissleder and M. J. Pittet, Nature,
2008, 452, 580; (d) L. Yuan, W. Lin, S. Zhao, W. Gao, B. Chen,
L. He and S. Zhu, J. Am. Chem. Soc., 2012, 134, 13510; (e)
Z. Zhang, B. Xu, J. Su, L. Shen, Y. Xie and H. Tian, Angew.
Chem., Int. Ed., 2011, 50, 11654; (f) X. Chen, Y. Zhou,
X. Peng and J. Yoon, Chem. Soc. Rev., 2010, 39, 2120; (g)
Y. Yang, Q. Zhao, W. Feng and F. Li, Chem. Rev., 2013, 113,
192; (h) N. Boens, V. Leen and W. Dehaen, Chem. Soc. Rev.,
2012, 41, 1130; (i) H. Peng, W. Chen, Y. Cheng, L. Hakuna,
R. M. Strongin and B. Wang, Sensors, 2012, 12, 15907; (j)
L. Yuan, W. Lin, K. Zheng, L. He and W. Huang, Chem.
Soc. Rev., 2013, 42, 622; (k) H. Jung, X. Chen, J. Kim and
J. Yoon, Chem. Soc. Rev., 2013, 42, 6019; (l) G. Ulrich,
A. Harriman and R. Ziessel, Angew. Chem., Int. Ed., 2008,
47, 1184; (m) A. Bessette and G. S. Hanan, Chem. Soc. Rev.,
2014, 43, 3342; (n) H. Lu, J. Mack, Y. Yang and Z. Shen,
Chem. Soc. Rev., 2014, 43, 4778.
Fig. 8 The frontier molecular orbitals (MOs) of BOPHY 3a and 3a–H+.
The energy levels of the MOs are shown (eV). Calculations are based
on ground state geometry by DFT at the MP2/6-31G* level with
Gaussian 03. HOMO/LUMO (eV) ¼ ꢁ4.81/ꢁ2.28 for 3a; HOMO/LUMO
(eV) ¼ ꢁ7.42/ꢁ4.77 for 3a–H+.
Moreover, the molecular geometries of BOPHY 3a and 3a–H+
were optimized using density functional theory (DFT) at the
MP2/6-31G* level.12 The calculated HOMO and LUMO orbital
energy levels were summarized in Fig. 8. Upon excitation of the
BOPHY uorophore, an electron of the highest occupied
molecular orbital (HOMO) is promoted to the lowest unoccu-
pied molecular orbital (LUMO), which enables ICT from the
donor (amino nitrogen atom) to that of the BOPHY uorophore,
causing uorescence quenching of the latter. Upon the
protonation of 3a, ICT is no longer possible, and the uores-
cence intensity of the probes was enhanced. The protonation of
BOPHY 3a (3a–H+: labs ¼ 481, 504 nm) resulted in a remarkable
hypsochromic shi compared to that (labs ¼ 540 nm) of the
BOPHY 3a. It is due to the increase in the HOMO–LUMO band
gap (2.65 eV) for the lowest energy absorption bands observed
for 3a–H+ relative to that (2.53 eV) of 3a by MO calculations
(Fig. 8). However, the energy level of frontier orbitals in 3a–H+
was lower than that of 3a (Fig. 8).
2 (a) M. A. T. Rogers, J. Chem. Soc., 1943, 596; (b) E. B. Knott, J.
Chem. Soc., 1947, 1196.
3 (a) M. A. T. Rogers, Nature, 1943, 151, 504; (b) M. A. T. Rogers,
J. Chem. Soc., 1943, 590; (c) J. Killoran, L. Allen, J. Gallagher,
W. Gallagher and D. F. O'Shea, Chem. Commun., 2002, 1862;
(d) A. Gorman, J. Killoran, C. O'Shea, T. Kenna,
W. M. Gallagher and D. F. O'Shea, J. Am. Chem. Soc., 2004,
126, 10619; (e) L. Jiao, Y. Wu, Y. Ding, S. Wang, P. Zhang,
C. Yu, Y. Wei, X. Mu and E. Hao, Chem.–Asian J., 2014, 9,
805; (f) S. O. McDonnell, M. J. Hall, L. T. Allen, A. Byrne,
W. M. Gallagher and D. F. O'Shea, J. Am. Chem. Soc., 2005,
127, 16360; (g) M. J. Hall, L. T. Allen and D. F. O'Shea, Org.
Biomol. Chem., 2006, 4, 776; (h) J. Killoran and
D. F. O'Shea, Chem. Commun., 2006, 1503; (i) H. Lu,
S. Shimizu, J. Mack, Z. Shen and N. Kobayashi, Chem.–
Asian J., 2011, 6, 1026; (j) V. F. Donyagina, S. Shimizu,
N. Kobayashi and E. A. Lukyanets, Tetrahedron Lett., 2008,
49, 6152; (k) Y. Wu, C. Cheng, L. Jiao, C. Yu, S. Wang,
Y. Wei, X. Mu and E. Hao, Org. Lett., 2014, 16, 748.
4. Conclusions
Mono-substitutional BOPHY 3a with a (p-dimethylamino)styryl
group in a-position was conrmed to be synthesized by a
Knoevenagel-type condensation, and no dis substitutional
BOPHY with a (p-dimethylamino)styryl group was observed.
BOPHY 3a is almost non-uorescent by ICT effect. Upon the
protonation of the tertiary amine function of 3a, the strong
uorescence (Ff ¼ 0.98) was released and the uorescence
intensity was dramatically increased by 1200 folds. Based on
MO calculations the increase in the HOMO–LUMO band gap
(2.65 eV) for the lowest energy absorption bands observed for
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4 I.-S. Tamgho, A. Hasheminasab, J. T. Engle, V. N. Nemykin
O. van den Berg and F. C. Grozema, J. Org. Chem., 2010, 75,
and C. J. Ziegler, J. Am. Chem. Soc., 2014, 136, 5623.
2169.
5 (a) X.-D. Jiang, J. Zhang, T. Furuyama and W. Zhao, Org. Lett., 10 (a) S. Hoogendoorn, K. L. Habets, S. Passemard, J. Kuiper,
2012, 14, 248; (b) X.-D. Jiang, J. Zhang, X. Shao and W. Zhao,
Org. Biomol. Chem., 2012, 10, 1966; (c) X.-D. Jiang, H. Zhang,
Y. Zhang and W. Zhao, Tetrahedron, 2012, 68, 9795; (d)
X.-D. Jiang, Y. Fu, T. Zhang and W. Zhao, Tetrahedron Lett.,
2012, 53, 5703; (e) X.-D. Jiang, R. Gao, Y. Yue, G.-T. Sun
and W. Zhao, Org. Biomol. Chem., 2012, 10, 6861; (f)
R. Kang, X. Shao, F. Peng, Y. Zhang, G.-T. Sun, W. Zhao
and X.-D. Jiang, RSC Adv., 2013, 3, 21033; (g) J. Zhang,
X.-D. Jiang, X. Shao, J. Zhao, Y. Su, D. Xi, H. Yu, S. Yue,
L.-J. Xiao and W. Zhao, RSC Adv., 2014, 4, 54080; (h)
X.-D. Jiang, D. Xi, J. Zhao, H. Yu, G.-T. Sun and L.-J. Xiao,
RSC Adv., 2014, 4, 60970; (i) P. Shi, X.-D. Jiang, R. Gao,
G. A. van der Marel, B. I. Florea and H. S. Overklee,
Chem. Commun., 2011, 47, 9363; (b) P. Song, X. Chen,
Y. Xiang, L. Huang, Z. Zhou, R. Wei and A. Tong, J. Mater.
Chem., 2011, 21, 13470; (c) X. Gao, Y. Zhang and B. Wang,
Org. Lett., 2003, 5, 4615; (d) B. Tang, F. Yu, P. Li, L. Tong,
X. Duan, T. Xie and X. Wang, J. Am. Chem. Soc., 2009, 131,
3016; (e) X. Ma, Y. Wang, T. Zhao, Y. Li, L.-C. Su, Z. Wang,
G. Huang, B. D. Sumer and J. Gao, J. Am. Chem. Soc., 2014,
136, 11085; (f) S. Grunder, P. L. McGrier, A. C. Whalley,
M. M. Boyle, C. Stern and J. F. Stoddart, J. Am. Chem. Soc.,
2013, 135, 17691; (g) A. Thottiparambil, P. R. Anil Kumar
and L. Chakkumkumarath, RSC Adv., 2014, 4, 56063.
Y. Dou and W. Zhao, Chin. Chem. Lett., 2015, DOI: 10.1016/ 11 (a) S. O. McDonnell and D. F. O'Shea, Org. Lett., 2006, 8,
j.cclet.2014.05.035.
6 C. Yu, L. Jiao, P. Zhang, Z. Feng, C. Cheng, Y. Wei, X. Mu and
E. Hao, Org. Lett., 2014, 16, 1952.
3493; (b) K. Rurack, M. Kollmannsberger and J. Daub,
Angew. Chem., Int. Ed., 2001, 40, 385.
12 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, V. G. Zakrzewski,
J. A. Montgomery Jr, R. E. Stratmann, J. C. Burant,
S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,
R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S. Clifford,
J. Ochterski, G. A. Petersson, P. Y. Ayala, Q. Cui,
K. Morokuma, D. K. Malick, A. D. Rabuck,
K. Raghavachari, J. B. Foresman, J. Cioslowski, J. V. Ortiz,
A. D. Baboul, B. B. Stefanov, G. Liu, A. Liashenko,
P. Piskorz, I. Komaromi, R. Gomperts, R. L. Martin,
D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, C. Gonzalez, M. Challacombe,
P. M. W. Gill, B. Johnson, W. Chem, M. W. Wong,
J. L. Andres, C. Gonzalez, M. Head-Gordon, E. S. Replogle
and J. A. Pople, Gaussian 03, Gaussian Inc., Pittsburgh, PA,
2003.
7 (a) Q. Zheng, G. Xu and P. N. Prasad, Chem.–Eur. J., 2008, 14,
5812; (b) D. P. Wang, Y. Shiraishi and T. Hirai, Chem.
Commun., 2011, 47, 2673; (c) T. Bura, R. Pascal, G. Ulrich
and R. Ziessel, J. Org. Chem., 2011, 76, 1109; (d) S. Zhu,
J. Zhang, G. Vegesna, F. Luo, S. A. Green and H. Liu, Org.
Lett., 2011, 13, 438.
8 (a) H. R. Westover, J. Chem. Educ., 1967, 44, 486; (b) D. Parker,
Chem. Rev., 1991, 91, 1441; (c) K. C. Wong, J. Chem. Educ.,
2014, 91, 1103.
9 (a) Y. Chen, D. Qi, L. Zhao, W. Cao, C. Huang and J. Jiang,
Chem.–Eur. J., 2013, 23, 7342; (b) C. Thivierge, J. Han,
R. M. Jenkins and K. Burgess, J. Org. Chem., 2011, 76, 5219;
(c) A. W. Czarnik, Acc. Chem. Res., 1994, 27, 302; (d)
W. Rettig, Angew. Chem., Int. Ed., 1986, 25, 971; (e) B. Tang,
X. Liu, K. Xu, H. Huang, G. Yang and L. An, Chem.
Commun., 2007, 3726; (f) W. F. Jager, T. S. Hammink,
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