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Cite this: Org. Biomol. Chem., 2012, 10, 2937
www.rsc.org/obc COMMUNICATION
Cascade synthesis of bis-N-sulfonylcyclobutenes via Cu(I)/Lewis acid-
catalyzed (3 + 2)/(2 + 2) cycloadditions: observation of aggregation-induced
emission enhancement from restricted CvN photoisomerization†
Kayambu Namitharan and Kasi Pitchumani*
Received 31st January 2012, Accepted 22nd February 2012
DOI: 10.1039/c2ob25226k
A remarkable role of Lewis acid additives in syntheses of bis-
N-sulfonylcyclobutenes via copper(I) catalyzed (3 + 2)/(2 + 2)
cycloaddition cascade of sulfonyl azides and terminal
alkynes is described. In addition, these cyclobutenes display
a unique aggregation-induced emission enhancement
(AIEE), reported for the first time, arising predominantly
from restricted rotation in CvN photoisomerization in the
solid state.
Synthetic strategies involving cascade or domino reaction
sequences that enable the construction of diverse and complex
molecular architectures, in particular, those functionalized with
variety of heteroatoms, present interesting and demanding chal-
lenges for the art of organic synthesis.
1
Cycloaddition cascades
2
are especially appealing by virtue of their ability to generate mul-
tiple C–C and/or C–heteroatom bonds with exquisite stereoche-
mical control. For example, N-sulfonylketenimine, a key
intermediate generated in situ from sulfonyl azides and terminal
alkynes via copper catalyzed azide–alkyne cycloaddition
process,
3
reported by Fokin and Meldal, has been utilized for a
variety of multicomponent cascade reactions.
4,5
Recently, we have developed an efficient one-pot synthesis of
imidazolidin-4-ones via N-sulfonylketenimine, generated in situ
from copper catalyzed cycloaddition of sulfonyl azides and term-
inal alkynes.
6
During the course of our investigation on further
reactions of N-sulfonylketenimines we found that, in the absence
of other nucleophiles,
7
4-toluenesulfonyl chloride enhances the
self (2 + 2) cycloaddition of N-sulfonylketenimines generated in
situ from copper catalyzed (3 + 2) cycloaddition of tosyl azide
and phenylacetylene in the presence of triethylamine (TEA) in
dichloromethane (DCM) (Scheme 1). Interestingly, the cyclo-
adduct is obtained as an ion pair namely triethylammonium-(E)-
N-2,4-diphenyl-3-(tosylimino)cyclobut-1-enyl-4-methylbenzene-
sulfonimidate, instead of the anticipated cyclobutenylsulfona-
mide. The structure and stereochemistry of the product are
unambiguously confirmed by single crystal X-ray analysis
(Fig. 1), which are in accordance with the
1
H NMR,
13
C NMR
and mass spectral data (see ESI†). Herein we report the copper/
Lewis acid catalyzed cycloaddition cascade of substituted sulfo-
nyl azides and terminal alkynes to variety of highly substituted
bis-N-sulfonylcyclobutenes under mild reaction conditions.
Details of optimization of reaction parameters are given in
Table 1.
Encouraged by the initial success using tosyl chloride as a
Lewis acid additive, various acid chlorides and metal halides
were tested (Table 1, entries 1–5). More promising results were
obtained with benzoyl chloride, leading to improved yields of
bis-N-sulfonylcyclobutenes and this was chosen for the remain-
ing optimization studies. The source of additive was found to be
critical for the transformation to proceed (Table 1, entry 6). Simi-
larly without a copper source, there was no reaction (Table 1,
entry 13) and this clearly highlights the specific role of copper(I)
in the (3 + 2) cycloaddition of sulfonyl azides and alkynes. The
use of CuCl or CuBr as alternate copper sources offered lower
yields (Table 1, entries 7 and 8). However, our interest in
Scheme 1 Generation and reactivity of N-sulfonylketenimine
†Electronic supplementary information (ESI) available: Experimental
methods, spectra, images and CIF data. CCDC 822410. For ESI and
crystallographic data in CIF or other electronic format see DOI: 10.1039/
c2ob25226k
School of Chemistry, Madurai Kamaraj University, Madurai-625021,
India. E-mail: pit12399@yahoo.com
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heterogeneous catalysis,
8
prompted us to use Cu(I)-modified
zeolites (Cu(I)–Y), as a heterogeneous copper source, which was
prepared according to a reported solid-state exchange procedure
and characterized by powder XRD, XPS and EDX (see ESI†).
Cu(I)–Y was found to be a better catalyst than other copper
sources and offered the highest yield (Table 1, entry 9). Among
the various solvents used (Table 1, entries 1, 10–12), DCM pro-
vided higher yield. Thus, the optimal conditions for this cyclo-
addition cascade involve Cu(I)-modified zeolite as a catalyst,
benzoyl chloride as a Lewis acid additive
9
and TEA as base in
DCM under N
2
atmosphere for 30 minutes (Table 1, entry 9).
This copper catalyzed cascade pathway is successfully
extended to different combinations of sulfonyl azides and term-
inal alkynes. As depicted in Table 2, this reaction works very
well for a wide range of substrates in short reaction times
(30 minutes) at room temperature, and the corresponding bis-N-
sulfonylcyclobutenes are obtained in fairly good yields. In most
of the cases, the products are purified by recrystallization and are
obtained as triethylammonium salts (3). Interestingly, wherever
purification by column chromatography (silica gel) is performed,
the product is obtained as an amide (4). Substituted arylsulfonyl
azides and arylacetylenes, containing electron-withdrawing
groups as well as electron-donating groups are well tolerated in
this reaction. In contrast, with an aliphatic sulfonyl azide or ali-
phatic alkynes, there is no product formation (Table 2, entries 9,
16 and 17). The reason may be the absence of stabilizing intra-
molecular aromatic π–πstacking interactions in the aliphatic
starting materials. Meanwhile, the recovery and reuse of Cu(I)–Y
are also investigated, and the recovered catalyst exhibits good
activity up to 4 consecutive cycles (Table 3).
In the proposed mechanism (Scheme 2), benzoyl chloride
added in catalytic amount (0.2 mmol ), combines with triethyl-
amine ( present in excess, 2 mmol) to form benzoyltriethylam-
monium chloride as the catalytic electrophilic species (E, 1).
Fig. 1 Single crystal X-ray crystal structure of compound 3a.
Table 1 Optimization of reaction conditions for the synthesis of
triethylammonium-(E)-N-2,4-diphenyl-3-(tosylimino)cyclobut-1-enyl-4-
methylbenzenesulfonimidate
a
Entry Catalyst Additive Solvent Yield
b
(%)
1 CuI TsCl DCM 66
2 CuI PhCOCl DCM 74
3 CuI 4-ClPhCOCl DCM 71
4 CuI AlCl
3
DCM 56
5 CuI SbCl
3
DCM 63
6 CuI —DCM 09
7 CuBr PhCOCl DCM 62
8 CuCl PhCOCl DCM 59
9 Cu(I)–Y PhCOCl DCM 81
10 Cu(I)–Y PhCOCl Toluene 63
11 Cu(I)–Y PhCOCl THF 68
12 Cu(I)–Y PhCOCl ACN 65
13 —PhCOCl DCM —
a
Reaction conditions: sulfonyl azide (1 mmol), alkyne (1 mmol), TEA
(2 mmol), catalyst (20 mg), additive (20 mol%) solvent (2 ml), rt, N
2
,
30 minutes.
b
Isolated yield.
Table 2 Copper(I)–Y zeolite catalyzed cascade synthesis of bis-N-
sulfonylcyclobutenes
a
Entry R
1
R
2
Yield
b
(%)
1 4-MeC
6
H
4
(1a)Ph(2a)3a,81
2Ph(1b)2a 4a,79
3 4-NO
2
C
6
H
4
(1c)2a 3b,71
4 2-NO
2
C
6
H
4
(1d)2a 3c,80
5 4-ClC
6
H
4
(1e)2a 3d,77
6 4-BrC
6
H
4
(1f)2a 4b,60
7 4-CF
3
C
6
H
4
(1g)2a 4c,74
8 2-Naphthyl (1h)2a 3e,85
9 Methanesulfonyl (1i)2a 0
10 1a 4-Pentyl C
6
H
4
(2b)4d,70
11 1b 2b 4e,69
12 1h 2b 4f,71
13 1b 4-CF
3
C
6
H
4
(2c)4g,64
14 1e 2c 3f,61
15 1h 2c 3g,66
16 1a n-Hexyl (2d)0
17 1a Cyclopropyl (2e)0
a
Reaction conditions: sulfonyl azide (1 mmol), alkyne (1 mmol), TEA
(2 mmol), Cu(I)–zeolite (20 mg), PhCOCl (0.2 mmol), DCM (2 mL), rt,
N
2
, 30 minutes.
b
Isolated yield.
Table 3 Reusability of Cu(I)–Y in the cascade synthesis of bis-N-
sulfonylcyclobutenes
a
Reuse 1st 2nd 3rd 4th
Yield
b
81 78 72 65
a
Reaction conditions: tosyl azide (5 mmol), phenylacetylene (5 mmol),
benzoyl chloride (1 mmol), TEA (6 mmol), Cu(I)–Y (100 mg), solvent
(8 mL), rt, N
2
, 30 minutes.
b
Isolated yield.
2938 |Org. Biomol. Chem., 2012, 10, 2937–2941 This journal is ©The Royal Society of Chemistry 2012
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This activates the in situ generated ketenimine to form an inter-
mediate 2. Simultaneously, the excess TEA, acting as a nucleo-
phile, polarizes another molecule of ketenimine to form 3.
Consequent (2 + 2) cycloaddition between these two electrophile
as well as nucleophile activated ketenimine generates the cyclo-
butane 5and releases the active electrophile E for subsequent
cycloadditions. The [2 + 2] dimer, 5, readily combines with
TEA forming the more stable triethylammonium imidate 6, with
extended conjugation, being the driving force for salt formation.
Aggregation-induced emission enhancement (AIE/AIEE) has
turned out to be the one of the most intriguing phenomena
10
to
achieve solid-state luminescent materials, with optoelectronic
applications, and also as highly selective chemosensors and bio-
imaging applications.
11
This aspect of AIEE-active materials pri-
marily resolves the problem of fluorescence quenching resulting
from aggregation. Possible reasons for this AIEE are formation
of specificJ-aggregates in the solid state and either single or
combined effect of, restricted intramolecular rotation (RIR) of
C–C, C–NorN–N single bond and CvC double bond as well
as molecular planarization. To have a better understanding
of this phenomenon and also to develop novel applications,
extensive efforts are being made to achieve new AIEE active
molecules by various research groups.
12
In molecules with an unbridged CvN structure, CvN iso-
merization is the predominant decay process of excited states
and so those compounds are often non-fluorescent. In contrast,
the fluorescence of their covalently bridged analogs increases
dramatically due to the suppression of CvN isomerization in the
excited state. A variety of fluorescent chemosensors with Schiff
base structures have been developed and restriction of CvN iso-
merization is responsible for the sensing mechanism.
13
To our
surprise, the synthesized bis-N-sulfonylcyclobutenes (3) of the
present study are found to exhibit AIEE involving CvN photoi-
somerization and also a strong pH dependent fluorescence To the
best of our knowledge, restricted CvN photoisomerization as a
source behind an AIEE system is unprecedented.
To study the AIEE phenomenon of the synthesised cyclobu-
tenes in detail, we investigated the UV-Vis absorption and emis-
sion behaviours of 3a and 3e, as selected examples. Both are
soluble in CHCl
3
, acetone, DMF, and DCM, but are insoluble in
n-hexane and water. 3a displays two distinct absorption bands at
280 and 350 nm in CHCl
3
medium (Fig. 2b). Based on, DFT
Scheme 2 Proposed mechanistic pathway.
Fig. 2 (a) PL spectra of 3a in different n-hexane–CHCl
3
ratios at a concentration of 7 × 10
−6
m, excited at 350 nm (b) UV-visible absorption spectra
of 3a in CHCl
3
solution (7 × 10
−5
m, black line) and in n-hexane–CHCl
3
(99 : 1) mixture (7 × 10
−5
m, dotted line).
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(B3LYP/6-31G) calculations, the band at 350 nm is assigned to
the (π–π*) of HOMO of diimine functionalized cyclobutene core
and LUMO of sulfonylaryl ring (Fig. g and h, ESI†). Stable
n-hexane dispersions of aggregates of 3a/3e are prepared using
CHCl
3
as an n-hexane-miscible solvent. Fig. 2a shows the corre-
sponding emission spectra of 3a in CHCl
3
(λ
exi
= 350 nm) with
different n-hexane–CHCl
3
ratios at a concentration of 7 × 10
−6
mol. Initially, the emission from the solution of 3a in CHCl
3
is
so weak that almost no photoluminescence (PL) signal is
noticed. However, a solution containing 70 : 30 (v/v) n-hexane–
CHCl
3
mixture displays a dramatic enhancement of lumines-
cence. When the n-hexane–CHCl
3
ratio reaches 99%, the emis-
sion intensity enhances very significantly with a regular shift
towards the blue region (Fig. 2a). A similar increase in photolu-
minescence of 3e upon addition of n-hexane to a CHCl
3
solution
is also observed (Fig. d, ESI†). The emission images of 3a in
pure CHCl
3
and, 99 : 1 (v/v) n-hexane–CHCl
3
under UV light
(365 nm) illumination at room temperature are shown in (Fig. e,
ESI†). Evidently, the emission of these cyclobutenes is signifi-
cantly enhanced in the solid state, indicating that, 3a and 3e are
AIE-active. This is further supported by the observation of a red
shift, due to strong intermolecular interactions (Fig. 2b), in the
absorption spectra of 3a (which is present as suspended particles
in 99% n-hexane).
The fluorescence spectra of 3a in the acetone–water mixture
(60 : 40) as a function of pH were also investigated (Fig. 3), and
display decreased intensity in acidic environment. This is attribu-
ted to protonation of imine groups which, in turn, reduces the
electron density making it weakly fluorescent. This is further
supported by
1
H NMR data. To facilitate the effective overlap of
π-orbitals, the central cyclobutene ring and the aryl ring (B)
were oriented in the same plane (see crystal structure of 3a,
Fig. 1). Consequently, ortho-hydrogens of aryl ring (B) are
placed very close to the imine nitrogens, shifted downfield and
come to resonance at 8.05 ppm as a doublet. However, in the
presence of trifluoroacetic acid (TFA), the deshielded ortho
protons are shifted up field, which may be due to the out of the
plane twisting of aryl ring (B) in the sterically hindered iminum
salts (Fig. 4).
Single crystal X-ray analysis also provides evidence for the
AIEE behaviour of 3a. The bond lengths of both the imine
groups (1.31 A°) were found to be higher than the normal
imines (1.27 A°). This decrease in bond order from the normal
value is attributed to the effective delocalization of electrons
between the two sulfonyl groups at the opposite diagonals of
cyclobutenes, Consequently, we believe photoisomerization is
easier, when compared to the normal unconjugated CvN double
bonds and this becomes the major non-radiative channel of
twisted intramolecular charge transfer (TICT) state in chloroform
solution. In contrast, as evident from crystal packing diagram
(Fig. 5), molecules are tightly packed in the solid state as a rigid
supramolecular network. Consequently, the CvN photoisomeri-
zation is greatly slowed down in solid state and thus, 3a in this
aggregated state exhibits significantly enhanced fluorescence
emission compared to its weak emission when molecularly dis-
persed in dilute solution.
In summary, a novel and efficient method was developed for
the facile synthesis of bis-N-sulfonylcyclobutenes via copper(I)/
PhCOCl catalyzed (3 + 2)/(2 + 2) cycloaddition cascade of sul-
fonyl azides and terminal alkynes in short reaction times. The
use of a copper(I)–Y zeolite as a heterogeneous copper(I) source
allows for the fast and easy isolation of the reaction products by
simple filtration in addition to other advantages such as catalyst
recyclability, ambient temperature and minimization of metallic
wastes. The observed AIEE of is attributed to the restricted
CvN photoisomerization in the aggregated state, based on pre-
liminary optical spectral measurements and single crystal XRD
data. To the best of our knowledge, this is the first report
wherein AIE/AIEE is attributed to restriction of rotation in CvN
photoisomerization, which is more pronounced in the solid state.
Fig. 3 PL spectra of 3a (5 × 10
−5
m) at different pH values in an
acetone–water (60 : 40) buffer excited at 350 nm.
Fig. 4
1
H NMR spectrum of 3a and 3a +TFA.
Fig. 5 Molecular packing diagram of 3a.
2940 |Org. Biomol. Chem., 2012, 10, 2937–2941 This journal is ©The Royal Society of Chemistry 2012
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Further studies towards a deeper mechanistic understanding of
this restricted rotation in CvN photoisomerization in the solid
state with different entities with varying molecular crystal struc-
tures are under way.
Acknowledgements
We thank the Department of Biotechnology (DBT), New Delhi
for financial support. We also thank Mr. V. Hakkim and
Dr. V. Subramanian, Central Leather Research Institute (CLRI),
Chennai for computational data.
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