Temporal control in tritylation reactions through light-driven variation in chloride ion binding catalysis-a proof of concept

Article abstract of DOI:10.1039/d0cy01090a

Tripodal triazole-linked azo(hetero)arene-based photoswitchable catalysts T1-5 have been designed, synthesized and optimized for the tritylation reaction of benzylamine (BzNH2). The tritylation reaction rates/yields achieved by light induced isomerization are compared between the native and photoswitched states of the catalyst T1. This concept of controlling the tritylation reaction rates with light has also been extended to additional substrates. The critical role of the triazole C-H?Cl- interactions has been confirmed by a combination of spectroscopic, calorimetric and computational studies. Also, the effect of variation in the binding affinities between the native and photoswitched states of the catalyst at room temperature in the temporal control of the catalysis has been demonstrated. This journal is

Full text of DOI:10.1039/d0cy01090a

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Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or gl oi ng sy
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DOI: 10.1039/D0CY01090A
Journal Name
ARTICLE
Temporal control in tritylation reactions through light-driven
variation in chloride ion binding catalysis – A proof of concept
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
b
Surbhi Grewal , Saonli Roy , Himanshu Kumar , Mayank Saraswat , Naimat K Bari , Sharmistha
b
a
Sinha* , Sugumar Venkataramani*
DOI: 10.1039/x0xx00000x
Tripodal triazole linked azo(hetero)arenes based photoswitchable catalysts T1-5 have been designed, synthesized and
optimized for the tritylation reaction of benzylamine (BzNH ). The tritylation reaction rates/yields achieved by light
www.rsc.org/
2
induced isomerization are compared between the native and photoswitched states of the catalyst T1. This concept of
controlling the tritylation reaction rates by light has also been extended to additional substrates. The critical role of the
triazole C-H…Cl interactions has been confirmed by a combination of spectroscopic, calorimatric and computational
studies. Also, the effect of variation in the binding affinities between the native and photoswitched states of the catalyst at
room temperature in the temporal control of the catalysis has been demonstrated.
Among the recent strategies in non-covalent catalysis,
activation of ion-pair through anion binding has started gaining
importance. Particularly, the utility of dual hydrogen donor
Introduction
1
0
Artificial control of functions such as catalysis, molecular
recognition, supramolecular assembly, etc. demand variation
in the environment or changes in selected parameters that can
act as a stimulus.1 Apart from the possibility of switching “on
and off”, stimuli responsive systems have also been reported
to offer spatially (selectivity) and/or temporally (kinetic)
control.3 Light, pH, coordination, redox properties and
supramolecular interactions have been considered to be
ability of thiourea and bis-triazole moieties have shown
1
1
remarkable results. Recently, Mancheño and co-workers
have utilized the concept of chloride ion binding as a catalysis
strategy in the tritylation of various nucleophiles and
,2
1
2
dearomatization of heterocycles. Leigh and co-workers
constructed a rotaxane based dual-function catalyst that can
remarkable pH driven switch from anion-binding catalysis to
1
3a
aminocatalysis.
Apparently, those catalysts demand
4
popular choices for external stimuli. Controlling reactions
reasonably higher temperature and/or long reaction time for
catalysis, presumably due to the structural rigidity and
restricted rotations. Indeed, such arguments have been
through light, and in particular, light-driven and/or
photoswitchable catalysis are of considerable interest in
recent times.5 Reversible control, the utility of ambient
conditions, subtle changes, and also the possibility of visible
light as a stimulus in the biomolecular window offer great
advantages but poses key challenges in light induced catalysis.
Several photo-controlled catalyst designs have been
reported that offer catalytic modulation predominantly
1
3b
validated by Flood and co-workers. Using a smartly designed
triazole based molecular cages, they demonstrated the
chloride ion capture with exceptional affinity in attomolar
range.
3
a,4b,6
through covalent and hydrogen bonding interactions.
Steric effects, electronic factors and the concepts of
aggregation/dissociation have also been reversibly tuned in
this regard.7 In spite of the recent surge on light-driven
8
covalent catalysis in synthetic organic chemistry, the
development of photoswitchable catalysis based on other
9
weak interactions have been less explored.
aDepartment of Chemical Sciences, Indian Institute of Science Education and
Research (IISER) Mohali, Sector 81, Knowledge City, SAS Nagar, Mohali, Punjab –
1
40306, India.
E-mail: sugumarv@iisermohali.ac.in.
b
Institute of Nano Science and Technology (INST), Sector 64, Mohali-160 062,
Punjab, India.
E-mail: sinhas@inst.ac.in
Fig. 1. Design and possible temporal control in photoswitchable
catalyst for tritylation reactions based on chloride ion binding.
Electronic
Supplementary
Information
(ESI)
available:
See
DOI: 10.1039/x0xx00000x
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Taking advantages of such works, we intended to design a
molecule with a tunable affinity toward chloride ions, which
can be utilized for light-controllable catalysts. We
hypothesized that catalytic activity (tritylation reaction rates)
can be modulated by varying anion binding ability of the
catalyst in its native and photoswitched states by light (Figure
Journal Name
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DOI: 10.1039/D0CY01090A
1
). Hence, as a proof of concept, we have combined
photoswitchable units and anion binding sites to alter the
reaction rates of the tritylation reactions by light. Considering
the thermal stability of the photoswitched states, and also for
efficiently utilizing the concept of photoswitching in catalysis,
it is necessary to perform the reactions at ambient
temperature and so, reasonably a weak binding and flexibility
are necessary. In this regard, we considered a tripodal 1,3,5-
tris-functionalized benzene (core unit) decorated with
azobenzenes (photoswitches) bearing triazole moieties (anion
binding sites) through a flexible tether. Although our initial aim
is to enable “fully activate or completely deactivate” the
catalyst by light, we observed a residual catalytic activity upon
photoisomerization of the catalyst. These residual activities are
in line with the inhibition of catalytic activity through
photoirradiation step, and reversible activation are known in
the hydrogen bonding based catalysis and photocontrol of
protein function.6 Herein, we report the design, synthesis,
utility of the catalyst, spectroscopic, isothermal calorimetric
and computational investigations in understanding the anion
binding strategy in photoswitchable catalysis in tritylation
reactions.
d,e
Results and discussion
Design and synthesis of the catalyst: The tripodal
photoswitchable catalysts are designed utilizing covalent
connections of photoswitches and triazole units with a core
moiety in 1,3,5-tris-functionalization mode. The choice of
triazole is made due to their exceptionally high binding affinity
-
towards chloride ions through C-Htriazole...Cl interactions, and
1
1c,12,13
their reported use in catalysis.
Azobenzenes have been
chosen as photoswitches due to the light-induced changes in
the molecular shape and size, and also the ease of
functionalization.14
Apparently,
the
direct
1,3-bis
functionalization of triazole moieties to a benzene ring led to
rigidity and restricted rotations, and so reasonably higher
o
12d
temperature (T ≥ 40 C) has been reported for catalysis. In
contrast, in our design, we have attached the triazole moieties
through -OCH - group to render flexibility and cooperativity to
2
Fig. 2. Photoswitching in catalyst T1. (a) Analysis using UV-vis
spectroscopy (DMSO, 9 M); (b) Photoswitching stability
(
alternative irradiations at 365 and 405 nm); NMR spectral
data (DMSO-d , 11 mM): (c) before irradiation; (d) after
irradiation at 365 nm for 40 min, and (e) after irradiation at
05 nm for 20 min; The PSS compositions have been estimated
using the normalized integral values corresponding to triazole-
Scheme 1. Synthesis of the target photoswitchable catalysts CH signal(s) and are tabulated (bottom); (f) Possible
6
4
T1-5. Conditions: (i) C1 (1.0 eq.), Az1-5 (3.3 eq.), CuSO
4
.5H
2
O
photoisomers of T1, and the composition of individual isomers
(
0.6 eq.), sodium (L)-ascorbate (1.2 eq.), 1:1 tBuOH-H O, RT.
2
at PSS.
2
| J. Name., 2012, 00, 1-3
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enable room temperature catalysis. The target catalysts T1-5 Table 1. Important entries in optimization of reaction
View Article Online
*
DOI: 10.1039/D0CY01090A
were synthesized by click chemistry using a core unit C1 and conditions :
1
5
the azidoazoarenes Az1-5 (Scheme 1). To synthesize the core
unit C1, phloroglucinol was tris-propargylated (see section S2
in ESI).1 In order to bring additional characteristics to the
catalyst, apart from parent Am1, four different amine
functionalized photoswitches Am2-5 with different solubility,
hydrogen bonding prospects and heterocycles have been
considered. 4-Aminoazoarenes and 4-aminoazopyridines have
been synthesized using standard procedures. Subsequently
all these amines Am1-5 were converted into their
corresponding azides Az1-5 by using diazotization procedure.
Using Cu(I) mediated 1,3-dipolar cycloaddition strategies, all
the five catalysts T1-5 have been successfully synthesized in
excellent yields.
2d
Catalyst
Catalyst loading Solvent
Time
(hrs)
#
Entry
Yield (%)
1
4b
(
mol%)
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
5.0
1
2
3
4
5
6
7
8
9
EEE-T1
EEE-T2
EEE-T3
EEE-T4
EEE-T5
ZZZ-T1a
ZZZ-T1b
ZZZ-T1c
ZZZ-T1d
ZZZ-T1e
ZZZ-T1f
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
9
9
9
9
9
9
9
9
9
9
9
80 (82)
40
60
68
Photoswitching studies: Photoswitching studies for all the
catalysts T1-5 are performed in DMSO or DMSO-d
6
, and
34
1
probed by UV-vis and H-NMR spectroscopic techniques,
respectively. At 365 nm, except the 4-hydroxy derivative T2, all
other catalysts showed excellent photoswitching (Figure 2a,
Figures S1-5 and Table S1 in ESI).
38 (38)
78
72
Upon irradiation at 405 nm, the reverse isomerization led
to regeneration of the native state in an excellent
conversion.16 Interestingly, all the derivatives except T2
showed a reasonable photoswitching even in solid state (KBr
medium). Also, the photoswitching stability of T1 was studied
by alternatively irradiating in forward and reverse direction at
68
1
1
0
1
55
33
*
Conditions: R1 (22.9 L, 0.21 mM), A1 (50 mg, 0.10 mM), 0.5
#
3
65 and 405 nm, respectively. The alternative irradiation steps mL solvent; Isolated yields; The catalyst was irradiated at 365
a
b
c
were conducted up to five cycles and no fatigue was observed nm in DMSO for 40 min; in DMSO for 1 min; in DMSO for 3
(
Figure 2b). Photoswitching experiments have also been min; in DMSO for 5 min; in DMSO for 30 min; in DMSO with
followed using NMR spectroscopy. For identification and continuous irradiation up to 9 h. (Irradiation was done in the
d
e
f
2 2
quantification of the individual isomers, shifts and signal presence of A1; The corresponding yields in CH Cl as a solvent
pattern of the triazole proton have been utilized (Figures 2c-f). are indicated in bold).
1
H-NMR spectroscopic studies showed that 365 nm irradiation
Although various solvents have been screened, we restricted
mainly to CH Cl and DMSO for the optimization based on the
factors such as solubility, and spectroscopic investigations.
led to a maximum of 83% ZZZ isomer in T1 at photostationary
state (PSS).1 Also, a small amount (17%) of EZZ isomer was
identified based on the two additional singlet signals at 1:2
ratio, and their chemical shifts. Upon 405 nm irradiation,
2
2
4a
1
7
Apart from the solvents, the catalyst loading, time,
nearly 84% EEE isomer has been formed back. This clearly catalysts T2-5 have also been screened. The complete
indicates that T1 can reversibly be converted between EEE- screening for the optimization and corresponding results are
and ZZZ- isomers by 365 (forward direction) and 405 nm available in the supporting information (see Table S2 in ESI).
(
reverse direction) irradiation conditions, respectively with Based on these optimization procedures, we have finalized our
excellent yields.
2 2
standard conditions as follows: DMSO or CH Cl as a solvent, 5
mol% catalyst T1, room temperature, and 9 hours (entry 1 in
Table 1). After optimization of conditions, we next tested the
effect of photoswitching on the catalyst T1 in the reaction
yield. Interestingly, with the photoswitched catalyst T1 (upon
irradiation at 365 nm in DMSO for 40 minutes), the yield has
been decreased to 38% (entry 7 in Table 1). Furthermore, we
evaluated time-dependent photoswitching of the catalyst T1
to understand the effect of irradiation times on the yield. The
catalyst solution is subjected to 1, 3, 5, and 30 min irradiation
times at 365 nm. Correspondingly, the yields are found to be
Reaction Optimization: All these targets T1-5 have been
utilized as a catalyst in the tritylation reaction of benzylamine
R1. In this regard, we have adopted the literature procedure
using DMAP-TrCl adduct A1 as a tritylating agent. In order to
optimize the reaction conditions, 1.0 eq. of salt A1 has been
1
2d
2
used along with 2.0 eq. of BzNH and stirred with catalyst T1 at
room temperature. The formation of the product has been
monitored by TLC, and was isolated and purified using column
chromatography. We have carried out all the reactions at
room temperature in duplicates (with and without irradiation
of the catalyst) in order to observe the effect of
photoswitching on the catalyst (Table 1 and Table S2 in ESI).
7
8, 72, 68 and 55%, respectively (entries 8-11 in Table 1). To
shed further light into this phenomenon, a separate
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a
Table 2. Additional illustrations for photo
D
s
O
w
I:it
1
c
0
h
.1
a
0
b
3
l
9
e
/D
c
0
a
C
taY0ly
1
s
0
i
9
s
0
:
A
S.
No.
1
Catalyst
EEE-T1
ZZZ-T1b
EEE-T1
ZZZ-T1b
EEE-T1
ZZZ-T1b
Substrate
Product
Yield (%)c
65 (80)
20 (38)
55 (65)
35 (48)
35
2
3
4
5
6
R2
P2
P3
P4
P5
R3
R4
R5
15
7
8
EEE-T1
ZZZ-T1b
54 (68)
35 (46)
a
For each substrate, both the reactions have been executed in
parallel; (Conditions: substrates R2-5 (0.21 mM), A1 (0.10
b
mM), 0.5 ml CH
CH Cl in the presence of A1. Isolated yields: Normal font –
2 2
Cl ); The catalyst was irradiated at 365 nm in
Fig. 3. Forward isomerization rate plots for the conversion of EEE-T1
c
2
2
1
to ZZZ-T1 up to the attainment of PSS using (a) H-NMR (Solvent:
DMSO-d
after 9 h; Bold – after 36 h.
6
; 11 mM) and (b) UV-Vis spectroscopy (Solvent: DMSO; 11
mM) (Both samples were irradiated with 365 nm LED light source of
intensity 7 mW at a fixed distance of 4 cm); (c) Plot depicting a T1 with the previously reported anion binding catalysts:
Table 3. Comparison of catalytic activity of the present catalyst
linear correlation between the %yield in the tritylation reaction of
Tunability/ Catalyst
Controlling loading
S.
No.
Time
(hrs)
T
( C)
BzNH and %ZZZ-T1 conversion (catalyst); The corresponding 365
2
Catalyst
o
nm irradiation times for obtaining the respective %ZZZ-T1
conversion are indicated; (d) Reaction profiles for the by EEE-T1 and
factor
(mol%)
Bis-
triazole
1
2
No
10.0
72
45
1
2d
1
ZZZ-T1. (The %conversion was obtained from the crude H-NMR
Rotaxane
bases
triazolium
2
using the normalized integral values of CH protons corresponding
to R1 and P1).
Yes
20.0
120
40
(pH)
1
3a
Dipyrrolyldik
-etone boron
complex
experiment has been performed with in situ irradiation at 365 nm,
which resulted in 33% yield (entry 11). This clearly confirmed the
influence of the ZZZ-isomer enriched catalyst in decreasing the
rate of the reaction despite an observation of residual catalytic
activity. Remarkably, the rates for the forward isomerisation step,
and a linear correlation between irradiation times (or %ZZZ-T1) vs
yields showed excellent consistency (Figures 3a-c and section S5
in ESI).
3
4
No
2.5
5.0
24
9
RT
RT
1
0b
T1
Yes
(Light)
(This work)
We observed that the irradiation of the catalyst T1 in the
presence of the adduct A1 reduced the yields further. All these
yields were estimated based on the isolation after 9 h. On
extending the reaction time up to 36 h, the yields were getting
better, however, due to a slow thermal reverse isomerization
of the photoswitched state of T1, it is quite unlikely to predict
the reaction rate solely by ZZZ-T1 (see Section S7 in ESI). Based
on these studies, the temporal control of the catalyst by light
has been successfully demonstrated. Besides the light
modulation, our current catalyst T1 showed excellent catalytic
activity in comparison to the previously reported catalysts for
the similar reactions involving anion binding (Table 3).
Next, two additional reactions (conversion of BzNH
BzNHTr P1) catalysed by EEE-T1 and ZZZ-T1, respectively in
CH Cl under identical conditions have been executed in
2
R1 to
2
2
1
parallel, and the reactions were followed by H-NMR. The NMR
spectral measurements have been done using the crude
samples collected at different intervals of time (Figure 3d and
section S6 in ESI). The reaction profile clearly proved that the
reaction rates under the influence of native and
photoswitched catalysts are quite different. More importantly,
the conversions were found to be comparable to the isolated
yields in both the cases.
Mechanistic studies: For a mechanistic understanding of the
underlying catalytic process, we have performed several
control experiments and additional studies (see section S4 in
ESI). The preliminary evidence for the T1-Cl binding was
obtained from the HRMS data of the solution containing T1
We have also extended this concept of photoswitchable
catalysis in tritylation reaction to additional substrates (Table
2
). Remarkably, the presence of 5 mol% of catalyst T1 in the
native state led to the tritylation reaction of benzyl alcohol
with a 65% yield. On the other hand, the photoisomerized
catalyst ZZZ-T1, lowered the yield by three times (20%). As
expected, the aliphatic cyclohexylamine, and secondary
amines such as, morpholine and pyrrolidine under identical
conditions led to moderate yields in the presence of EEE-T1.
and TBAC that confirmed the mass corresponding to the

[
M+Cl ] in both native as well as photoswitched states of the
catalyst T1 (see section S8 in ESI). In order to confirm the
cooperativity effect of the three triazole units, we have also
synthesized a bis-triazole based catalyst B1, and utilized for the
4
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catalysis (see section S2 in ESI). We have observed a variation
in the yields for the tritylation reaction in benzylamine using
the native (50%) and photoswitched (36%) catalysts B1 under
G
-3946.7
-4452.3
View Article Online
o
DOI: 10.1039/D0CY01090A
a
ITC studies were performed in DMSO at 20 C, and the

d
dissociation constant K , H, S and G are expressed in M ,
the optimized conditions in CH
2
Cl
2
. However, the relative

 

cal.M , cal.M .K , and cal.M , respectively.
yields were considerably lower than that of the reactions
catalysed by T1. Such evidence confirmed the existence of
binding as well as cooperativity of the catalyst with chloride
ions, which is necessary for the effectiveness in the catalysis.
In order to understand the binding sites and the binding
constants for the catalyst with chloride ions, we have
Fig. 4. Overall summary depicting photoswitchable catalysis in
tritylation reactions by light-driven variation in binding of
chloride Ions.
6
performed titrations between TBAC in DMSO-d . In this regard,
both the native and photoswitched states of T1 have been
utilized (see section S9 in ESI). Based on the H-NMR data for
Based on these observations, it is apparent that the higher
enthalpy change for the native state catalyst upon binding to
the ligand leads to efficient catalysis in contrast to the
photoswitched state. More importantly, the difference in the
dissociation constants between native and photoswitched
states is corroborating with the variation in the yields (or the
difference in the reaction rates) suggesting a crucial role of the
photoisomerization of T1 in determining the reaction yields.
1
T1 vs TBAC titrations, we observed downfield shifts in the
protons of triazole, and phenyl group (ortho protons of the
phenyl with respect to the triazole moiety) in both EEE and ZZZ
isomeric states of T1 indicating a plausible interaction sites
with the chloride ions. Whereas minor upfield shifts were
observed for the protons of the benzene core and methylene
group (see Figure S11 in ESI). Although the effect of ionic
strengths of the medium (at higher concentration of TBAC)
cannot be ruled out, only a restricted number of protons
exhibiting the downfield shifts could be an indicative of the
binding sites. Among the protons exhibiting shifts, only triazole
protons showed maximum chemical shift indicating it as a
major binding site. Interestingly, the binding sites did not
change even after isomerization of the catalyst that can be
realized once again by the signals exhibiting shifts and their
magnitude.
Computational studies:19 In order to confirm the
experimentally predicted binding modes as well as to
understand the difference upon photoswitching, computations
have also been performed at density functional theory.
Geometries have been optimized for EEE-, EEZ-, EZZ- and ZZZ-
photoisomers of the catalyst T1 and their respective

complexes with Cl ions at B3LYP/6-311G(d,p) level of theory.
The relative energies of all these isomers with and without the
binding of chloride ions were also calculated (see Figures S18-
Since the changes associated with chemical shifts in NMR
titration experiments were found to be too little, we have
estimated the binding constant using the standard isothermal
1
9 and section S11 in ESI). Based on the computations, the
native state of the catalyst is found to be more stable. The
progressive Z-isomerization increases the destabilization up to
a maximum of 45.6 kcal/mol. However, the chloride ion
binding increases the stability of the Z-component containing
photoisomers. It revealed that the anion binding enhances the
stability ranging between 1.5 and 2.3 kcals/mol. Furthermore,
the inspection of Mulliken charges revealed the involvement
of various atoms in the interactions with chloride ion (Table
S5). Incidentally, the triazole C-H, the ortho proton of the
phenyl relative to the triazole were found to gain maximum
positive charge upon interacting with the chloride ion.
Apparently, the same protons of the photoswitched states
exhibited only a marginal difference in the charges. However,
the chloride ion became more negative in the photoswitched
states compared to the native state. Furthermore, natural
bond orbital (NBO) analysis has been performed to quantify
the interaction energies at M06-2X/6-311G(d,p) level of
theory. In this regard, the interaction energies of chloride ion
with all the binding sites in native and the photoswitched
states of the catalyst have been estimated (Table S6). Based on
the analysis, we inferred that the triazole C-H is strongly
binding compared to the ortho proton of the phenyl attached
to the triazole in both the cases. However, the interaction
energies of all the binding sites showed a decrease in the
photoswitched state. All these results are consistent with the
experimentally observed NMR shifts. The difference in the
binding strength is attributed to the changes in the electronic
1
8
calorimetry (ITC) technique. The binding curves are fitted to
a one site binding model to determine the Kd, for the
o
interaction, H, S and G for the interactions. At 20 C, the
bindings are associated with negative G values indicating a
spontaneous interaction of the TBAC and the catalyst in both
the native (EEE-T1) and photoswitched (ZZZ-T1) states. A
positive enthalpy indicates an endothermic reaction in both
the cases. The positive entropy value suggests an entropically
driven desolvation binding mechanism. While the entropy
values are similar in both the cases, the binding of TBAC to the
native state of T1 is associated with higher enthalpy changes
compared to the photoswitched state. Our results indicate
d
that the dissociation constant, K for the binding of TBAC with


the photoswitched catalyst is 2070 ± 431 M  which is two-
fold greater than that of the native state catalyst, which is


estimated to be 877 ± 157 M (Table 4 and section S10 in ESI).
Table 4. Thermodynamic parametersa for the binding of
catalyst T1 with chloride ion before and after photoswitching:
Catalyst
EEE-T1
ZZZ-T1
K
d
877 ± 157
2763.0 ± 185.1
2070 ± 431
851.2 ± 42.5


H
S
22.9
18.1
effects arising due to the photoswitching (E to
Z
isomerization), and in turn, the lowering of electron
withdrawing character of azo group. The overall summary
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Journal Name
based on the experimental and computational results has been
portrayed as figure 4.
C. W. Bielawski, ACS Catal., 2013, 3, 8, 18741V8ie8w5A;r(tiecl)e OEn. lMine.
Broderick, N. Guo, C. S. Vogel, C. Xu, DJ.OSIu: 1t0te.1r0,3J9./TD.0MCYi0ll1e0r9, 0KA.
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In summary, we have designed and developed tris(triazole)
based photoswitchable catalysts that can exhibit light-
controlled catalysis through chloride ion binding. Based on
careful optimization of various parameters, and also the
isolated yields, we inferred that the catalyst T1 can control the
tritylation reaction rates by light – through photoswitching of
the catalyst. The native state of the catalyst T1 exhibited fast
catalysis, whereas upon photoisomerization of it inhibited the
reaction rates. Based on the isolated yields, we observed a
maximum of two times variation in the rates by controlling
photoisomerization in the catalyst. Such temporal control was
extended to additional substrates too. Through HRMS data,
NMR titrations, control experiments, and DFT computations,
we confirmed the mode of binding and the importance of
cooperative binding. Isothermal calorimetric measurements
revealed a two-fold difference in the dissociation constant
between chloride ion and the catalyst T1 in its native (EEE) and
photoswitched (ZZZ) states that is very well corroborated with
the differences in the isolated yields. Overall, the influence of
(
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Acknowledgements
We are grateful to the Science and Engineering Research
Board (SERB), New Delhi for the financial support
(EMR/2014/000780) and (SR/NM/NB-1082/2017). We are also 7. (a) M. D. Visco, J. Attard, Y. Guan and A. E. Mattson, Tett.
thankful to IISER Mohali for the financial support, the
departmental and central research facilities and also the NMR,
HRMS and other instrumentation support. We are thankful to
the INST central instrumentation facility for the use of ITC. SG,
SR and HK thank MHRD, MS thanks CSIR and NKB thanks DST
for their Research Fellowships, respectively. We thank Dr.
Sabyasachi Rakshit and Dr. Jino George for helping in
absorption spectroscopic and forward isomerization kinetics
measurments.
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Conflicts of interest
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4
05 nm irradiation, we obtained a maximum conversion in
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has limits due to attainment of
a
photostationary
equilibrium.
1
1
1
7. Surprisingly the uncatalyzed tritylation of BzNH reactions
2
in DCM and DMSO led to moderate product formation.
Indeed, similar observation was also reported in ref. 10b in
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