Fluorophores for Excited-State Intramolecular Proton Transfer by an Yttrium Triflate Catalyzed Reaction of Isocyanides with Thiocarboxylic Acids

Article abstract of DOI:10.1002/anie.201702488

Discovery of new chemical reactivity of a given functional group can often result in innovative synthesis of important chemical entities that possess unprecedented properties. We designed and developed a one-step synthesis of 5-amino-4-carboxamidothiazoles 1 by an yttrium-triflate-catalyzed reaction of thiocarboxylic acids 2 with isocyanides 3. In this reaction, both reactants 2 and 3 deviated from their normal reactivities because of metal coordination. The resulting heterocycles are novel prototypical structures for the double ESIPT process. Some of them were excited by visible light irradiation and emitted fluorescence at the NIR region with large Stokes shift, high quantum yield, and strong solvatochromism.

Full text of DOI:10.1002/anie.201702488

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Communications
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International Edition: DOI: 10.1002/anie.201702488
German Edition: DOI: 10.1002/ange.201702488
Heterocylces
Fluorophores for Excited-State Intramolecular Proton Transfer by an
Yttrium Triflate-Catalyzed Reaction of Isocyanides with
Thiocarboxylic Acids
Shuo Tong, Shun Zhao, Qing He, Qian Wang, Mei-Xiang Wang, and Jieping Zhu*
Abstract: Discovery of new chemical reactivity of a given
functional group can often result in innovative synthesis of
important chemical entities that possess unprecedented proper-
ties. We designed and developed a one-step synthesis of 5-
amino-4-carboxamidothiazoles 1 by an yttrium-triflate-cata-
lyzed reaction of thiocarboxylic acids 2 with isocyanides 3. In
this reaction, both reactants 2 and 3 deviated from their normal
reactivities because of metal coordination. The resulting
heterocycles are novel prototypical structures for the double
ESIPT process. Some of them were excited by visible light
irradiation and emitted fluorescence at the NIR region with
large Stokes shift, high quantum yield, and strong solvato-
chromism.
O
rganic compounds containing an intramolecular hydro-
gen-bond can undergo excited-state intramolecular proton
transfer (ESIPT) reaction to emit fluorescence at long
[1,2]
wavelength.
attention due to its fundamental importance in chemistry, in
biology, and in developing novel functional molecules.
ESIPT process has attracted multidisciplinary
[3]
Scheme 1. Double ESIPT molecules: Design and synthesis.
Although being known for years, molecules capable of
undergoing double ESIPT process (ESIDPT) were poorly
investigated. From the viewpoint of molecular design, two
classes of ESIDPT could be considered: a) Two proton
transfer processes occur independently (type A, Scheme 1a).
ESIPT or stepwise process via mono-ESIPT products D and
E, respectively. This fact could potentially increase the
probability of obtaining an energy profile suitable for the
[4]
[5]
[7]
Bis-flavonols, bis-2-(2’-hydroxyphenyl)benzazole and 2,2’-
development of novel bistable photoswitches. To the best of
[
6]
bipyridyl-3,3’-diol are prominent examples, the proton
transfer mechanisms of which have been examined in detail
by computation. b) Two proton transfer reactions are inter-
connected by a proton shutter (type B, Scheme 1b). This class
of molecules are of particular interest since there are three
possible ESIPT channels to convert B to C: concerted double
our knowledge, 3-hydroxy-pipecolic acid is the only molecule
belonging to this class that has been theoretically proposed
[
8]
and studied by ab initio methods. We hypothesized that 5-
amino-4-carboxamidothiazoles 1 could be suitable candidates
for the development of novel type B ESIDPT molecules.
Thiazole 1 is a known inhibitor of TBK1 and IKKe, useful for
[9,10]
the treatment of cancer and inflammatory diseases.
The
development of a new robust synthesis of 1 would therefore
be of general interests. We report herein a novel one-step
synthesis of 5-amino-4-carboxamidothiazoles 1 by an yttrium
triflate-catalyzed condensation of thiocarboxylic acids 2 with
isocyanides 3 and document its fluorescence properties as
[
*] Dr. S. Tong, S. Zhao, Dr. Q. Wang, Prof. Dr. J. Zhu
Laboratory of Synthesis and Natural Products
Institute of Chemical Sciences and Engineering
Ecole Polytechnique Fꢀdꢀrale de Lausanne
EPFL-SB-ISIC-LSPN, BCH 5304, 1015 Lausanne (Switzerland)
E-mail: jieping.zhu@epfl.ch
[
11]
potential ESIDPT chromophores (Scheme 1c).
Interested in modulating the generally accepted reactivity
Homepage: http://lspn.epfl.ch
[
12]
Dr. Q. He
of isocyanide,
carboxamidooxazole 4 by a zinc bromide-promoted conden-
we reported a synthesis of 5-amino-4-
Department of Chemistry, The University of Texas at Austin
Austin, TX 78712-1224 (USA)
[12a]
sation of carboxylic acid with isocyanide.
Compound 4
Prof. Dr. M.-X. Wang
displayed interesting fluorescence properties. However, its
relative instability limited its development as fluorescent
probe. We therefore turned our attention to thiazole and
thought to synthesize it by exploiting the reaction between
thiocarboxylic acids 2 and isocyanides 3 (Scheme 1c). How-
ever, thiocarboxylic acid acts in general as an S- rather than an
Key Laboratory of Bioorganic Phosphorous and
Chemical Biology (Ministry of Education), Tsinghua University
Beijing 100084 (China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.201702488.
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O-nucleophile and if this were also the case in our planned
reaction, it would afford 5-aminooxazole-4-thiocarboxamide
efficiently to afford 5-aminothiazole-4-carboxamides in good
to excellent yields (1b–1k). Functional groups such as Cl, Br,
I, CF , NO and N were well tolerated. Heteroaromatics such
5
(Scheme 1c). A closely related example in which 2 acted as
3
2
3
an S-nucleophile was the formation of N-alkyl-N-
acylthioformamide 6 from the reaction of 2 with 3 (Sche-
as indole, furan and thiophene were compatible (1l–1n) and
2-alkyl substituted thiazoles (1o, 1p) were accessible from
aliphatic thiocarboxylic acids. Adamantanylisocyanide and
(isocyanomethylene) dibenzene participated in the reaction
to afford the corresponding thiazoles 1q and 1r, respectively.
Finally, primary amino amides 7a and 7b were synthesized
from 1a and 1h (BF ·OEt , CH ClCH Cl, reflux) in yields of
[
13]
me 2a).
Therefore, we would have to alter the intrinsic
reactivity of both 2 and 3 in order to access thiazoles 1.
3
2
2
2
8
7
5% and 88%, respectively. Tertiary amino amides 7c and
d, prepared by N-methylation of 1b and 1h, respectively,
will be used as probes for the investigation of the fluorescent
mechanism.
A one-pot synthesis of thiazoles 1 was subsequently
developed by in situ generation of thiocarboxylic acids from
carboxylic acids 8. Refluxing a toluene solution of benzoic
Scheme 2. Reaction design and optimized reaction conditions.
[14]
acid 8a and Lawessonꢀs reagent followed by addition of
isocyanide 3a and Y(OTf) (0.1 equiv, 808C) afforded thia-
3
Using thiobenzoic acid (2a) and tert-butylisocyanide (3a)
as test substrates, the reaction conditions were optimized (cf.
the Supporting Information for details). Gratefully, the
desired thiazole 1a was formed in 83% isolated yield at the
expense of oxazole 5a by simply heating a toluene solution of
zole 1a in 78% overall yield (Scheme 4a). Carboxylic acids
2
a and 3a in the presence of a catalytic amount of Y(OTf)3
(
0.1 equiv) at 808C for 2 h (Scheme 2b). To the best of our
knowledge, this represented the first example in which
thiocarboxylic acid acted as O-nucleophile with complete
chemoselectivity.
As shown in Scheme 3, aromatic thiocarboxylic acids
bearing both electron withdrawing and donating groups
regardless of their positions participated in the reaction
Scheme 4. From carboxylic acids to thiazoles and selenazole.
bearing strong electron-donating (NPh ) and withdrawing
2
(
NO ) groups as well as an aryliodide function were readily
2
converted to the corresponding thiazoles (1s–1u, 1 f) in good
to excellent yields. Compound 1 f (X = I) was subsequently
converted to alkynylated derivatives 1v and 1w using the
[15]
Sonogashira coupling reaction. Selenazole 9,
which was
hardly accessible, was similarly synthesized from the carbox-
[16]
ylic acid with the assistance of Woollinsꢀ reagent
moderate yield.
in
Because of the pronounced S-nucleophilicity of 2, the
reaction of thiocarboxylic acid 2 with isocyanide 3 was
initially thought to give oxazole 5 as a major product
(Scheme 5). The exclusive formation of thiazole 1 was there-
fore intriguing as it indicated that the thiocarboxylic acid
acted as an O-nucleophile under our conditions. To under-
stand this abnormal chemoselectivity, a computational study
was performed using Gaussian 09 at B3LYP/GenECP level.
Assuming that the reaction went through a migratory inser-
tion mechanism, two complexes Fand G could be proposed as
Scheme 3. Scope of the thiazole synthesis. [a] 2 (0.1 mmol), 3
(
0.5 mmol), Y(OTf) , toluene (c=0.05m), 2 h. Yields refer to the
3
isolated products.
2
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Table 1: Photophysical data of thiazoles in toluene.
Dyes
labs
nm]
e
lex
[nm]
lem
[nm]
Dl
[nm]
Du
[cm
ff
[%]
À1
À1
À1
[
[m cm
]
]
[
[
[
[
[
[
[
[
[
a]
a]
a]
b]
a]
b]
a]
a]
b]
1
1
1
1
1
1
1
1
1
a
b
g
h
s
356
376
377
435
386
451
379
392
413
6920
333
376
376
442
390
460
376
392
420
393
422
425
532
444
558
422
436
544
37
46
48
97
58
107
43
44
131
4600
2900
3100
3800
3100
3800
2900
2600
5400
4.5
22676
19279
55799
13233
22327
16308
23740
25031
43.2
34.2
43.2
19.0
63.2
37.9
76.8
22.6
t
u
v
w
[a] The quantum yield was determined using quinine sulfate as reference,
F=0.55 in 1n H SO , l =366 nm. [b] The quantum yield was
2
4
ex
determined using coumarin 343 as reference, F=0.63 in ethanol,
lex =425 nm.
Scheme 5. Simplified mechanistic view and the results of DFT calcu-
lations. For the sake of clarity, triflates were omitted in complex F and
G. Selected bond distances of 10: dY-C1 =2.599 ꢁ, dY-C3 =2.591 ꢁ,
dY-O1 =2.299 ꢁ, dY-S1 =2.875 ꢁ, dO1-C1 =3.165 ꢁ, dO1-C3 =3.213 ꢁ,
dS1-C1 =3.353 ꢁ, dS1-C3 =3.338 ꢁ.
1h). Enlarging the conjugate system as in 1s–1w has the same
effect (76.8% for 1v). When a nitro group was introduced to
the 2-phenyl substituent (1h), an obvious bathochromic shift
was observed. The bond length of tBuN-H···O=C in 1h
(
1.96 ꢁ) is shorter than that in 1a (2.09 ꢁ) facilitating
[
19]
presumably the proton transfer. Inserting a multiple bond
between the two aromatic rings (1t, 1w) led to further red
shifts, with 1t emitting in the yellow region (Figure 1). Except
for 1a, all these thiazoles exhibited high extinction coeffi-
possible intermediates. Interestingly, structural optimization
of F and G converged to the same key complex (MeNC) Y-
2
(
OTf) (PhCOS) (10), in which both sulfur and oxygen atoms
3
4
À1
À1
of the thiocarboxylic acid coordinate to yttrium with a bond
length of 2.875 ꢁ and 2.299 ꢁ, respectively. The distances
between the coordinated divalent carbons of the two iso-
cyanides and the thiocarboxylic acid were calculated to be
cients (> 10 m cm ). We emphasize that nitro-substituted
thiazoles 1h, 1t, and 1w underwent visible light excitation,
a much sought after criterion for the development of novel
fluorescent probes. Compound 7c (Scheme 3) lacking the
intramolecular H-bonds was deprived of fluorescence (Fig-
ure 1b), indicating that thiazoles 1 could well be the ESIPT
molecules. Of note, the fluorescence quantum yields of these
dyes are much higher than that of the typical ESIPT
chromophores.
3
.165 ꢁ, 3.213 ꢁ for CH NꢀC···O and 3.353 ꢁ, 3.338 ꢁ for
3
CH NꢀC···S, respectively. The natural bond orbital (NBO)
3
charges analysis of the optimized complex 10 indicated that
the oxygen (À0.712e) of the thiocarboxylic acid acquired
more negative charge than sulfur (À0.163e, cf. the Supporting
Information). Furthermore, the highest occupied molecular
orbitals (HOMO) were largely localized on thiocarboxylic
acid. The p atomic orbitals of oxygen and the divalent carbon
of isocyanide were aligned, whereas those of sulfur and
isocyano carbon were in opposite phase. All these factors
would render the oxygen of thiocarboxylic acid in complex 10
more nucleophilic than sulfur. Therefore, the reaction would
be initiated by CÀO bond formation to afford the observed
product 1 via intermediates H, I and J (Scheme 5, see the
Supporting Information for detailed pathways leading to
1
and 5, respectively).
The X-ray structures of thiazoles 1a and 1h showed
clearly the existence of two intramolecular H-bonds, exactly
[
17]
the pattern desirable for the type B ESIDPT molecules. All
these thiazoles are stable under air and acidic conditions
(
TFA) and exhibited high photostability as indicated by the
photobleaching experiment (cf. the Supporting Information).
The key photophysical data, measured in toluene at room
temperature, are listed in Table 1. Upon UV excitation,
compound 1a emitted fluorescence in purple to blue
region. Introduction of an electron-donating (1b) or with-
drawing group (1g, 1h) at the para position of the 2-phenyl
moiety increased considerably the quantum yields (43.2% for
Figure 1. a) Normalized emission spectra of 1a, 1g, 1h and 1t in
toluene at RT and solvatochromism of 1h; b) Fluorescence emission
spectra of 1b, 1h versus N,N’-dimethylated derivatives 7c, 7d in
[
18]
À6
À1
toluene (c=2ꢂ10 molL ) at RT.
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Thiazole 1h showed pronounced positive solvatochrom- Conflict of interest
ism with large Stokes shifts in polar solvents (in toluene: lem
=
5
32 nm, Dl = 97 nm; in CH Cl : l = 629 nm, Dl = 189 nm,
The authors declare no conflict of interest.
2
2
em
quenched in MeOH). Lippert–Mataga plot of Stokes shift
Dn) versus the solvent polarity (Df) for dye 1h displayed
(
Keywords: fluorescence · homogeneous catalysis ·
hydrogen bonds · isocyanides · multicomponent reactions
a linear correlation with an excellent goodness of fit,
indicative of general solvent effect (cf. the Supporting
Information). The same phenomenon was observed for
compound 1t (in toluene: lem = 558 nm, Dl = 107 nm; in
CH Cl : l = 662 nm, Dl = 208 nm). It is conceivable that for
2
2
em
thiazoles 1h and 1t, the ESIPT may couple with the excited-
[
1] a) D. L. Williams, A. Heller, J. Phys. Chem. 1970, 74, 4473; b) J.
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of this class, that is, the occurrence of ESIPT is not
a prerequisite for the ESICT process. In contrary, the
ESICT of 1h could not only happen priori to ESIPT but
also facilitate the subsequent ESIPT process due to the
increased acidity of the proton in the 1h-CT* state
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3
reaction of thiocarboxylic acids 2 with isocyanides 3. In this
reaction, both 2 and 3 displayed altered reactivities due to
metal coordination. The thiazoles 1, designed to be novel
prototypical structures for the double ESIPT process, are
novel fluorescent dyes with interesting properties. Some of
them (1h, 1t, 1w) were excited by visible light irradiation and
emitted fluorescence at the NIR region with large Stokes
shift, high quantum yield and strong solvatochromism.
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Acknowledgements
Financial supports from Swiss State Secretariat for Education,
Research and Innovation, EPFL (Switzerland) and National
Natural Science Foundation of China (21320102002,
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4
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Angew. Chem. Int. Ed. 2017, 56, 1 – 6
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Angewandte
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structural changes, see for example: T. Iijima, A. Mototake, Y.
Manuscript received: March 9, 2017
Final Article published: && &&, &&&&
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Communications
Heterocylces
S. Tong, S. Zhao, Q. He, Q. Wang,
M.-X. Wang, J. Zhu*
&&&& — &&&&
Fluorophores for Excited-State
Intramolecular Proton Transfer by an
Yttrium Triflate-Catalyzed Reaction of
Isocyanides with Thiocarboxylic Acids
Colorful Reactivity: The reaction of thio-
carboxylic acids with isocyanides in the
presence of a catalytic amount of yttrium
triflate afforded 5-amino-4-carboxamido-
thiazoles in good to excellent yields.
Some of these heterocycles were excited
by visible light and emitted fluorescence
at the near-infrared region with large
Stokes shift, high quantum yield, and
strong positive solvatochromism.
6
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