Cyclic (Alkyl)amino Carbene Complex of Aluminum(III) in Catalytic Guanylation Reaction of Carbodiimides

Article abstract of DOI:10.1021/acs.organomet.8b00358

Herein we report the synthesis of a cyclic (alkyl)amino carbene (cAAC) complex of AlMe₃. This complex was used as an efficient catalyst for the guanylation reaction of carbodiimides with primary arylamines and secondary amines to deliver guanidine derivatives in good to excellent yields. This catalytic protocol can tolerate a wide range of functional groups. Furthermore, the longevity of the catalyst was tested in successive catalytic cycles, which indicated a sustained catalytic activity over a multiple cycles. The mechanistic pathway was well understood with the help of stoichiometric reaction and DFT study.

Full text of DOI:10.1021/acs.organomet.8b00358

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Cyclic (Alkyl)amino Carbene Complex of Aluminum(III) in Catalytic
Guanylation Reaction of Carbodiimides
Pavan K. Vardhanapu, Varun Bheemireddy, Mrinal Bhunia, Gonela Vijaykumar,
and Swadhin K. Mandal*
Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741246, India
S
* Supporting Information
ABSTRACT: Herein we report the synthesis of a cyclic
(alkyl)amino carbene (cAAC) complex of AlMe3. This
complex was used as an efficient catalyst for the guanylation
reaction of carbodiimides with primary arylamines and
secondary amines to deliver guanidine derivatives in good to
excellent yields. This catalytic protocol can tolerate a wide
range of functional groups. Furthermore, the longevity of the
catalyst was tested in successive catalytic cycles, which
indicated a sustained catalytic activity over a multiple cycles. The mechanistic pathway was well understood with the help
of stoichiometric reaction and DFT study.
atalytic C−N bond formation by activation of amine N−
develop an efficient carbene-based aluminum catalyst for the
CH bonds to carbodiimides (RNCNR), known as guanylation reaction. As a part of our ongoing research
program using carbene as ligands,45−52 herein we used a strong
catalytic guanylation of amines with carbodiimides (CGAC
σ-donor ligand, such as cyclic (alkyl)amino carbene
(cAAC),53−60 to design an aluminum-based catalyst. In this
Article, we describe an efficient hydroamination reaction of
carbodiimides (Scheme 1) with a well-defined aluminum(III)
complex, [(cAAC)AlMe3] (1).
reaction), to construct N-containing compounds is one of the
most important chemical transformations in modern organic
synthesis.1−3 The catalytic guanylation reaction of amines is a
straightforward and atom-economical route to prepare multi-
substituted N-containing compounds, RNC(NR′R″)-
NHR,4−15 which have unique value in pharmaceutical
chemistry,16−18 agrochemicals, natural products, sweeteners,
explosives,19 organometallic and coordination chemistry,20−22
and organic synthesis.23,24 It was well established that direct
guanylation reaction of primary aliphatic amines with carbo-
diimides can be achieved under harsh conditions without using
any catalyst.25 However, because of lower nucleophilicity, the
guanylation reaction cannot be achieved with aromatic amines
or secondary amines without use of a suitable catalyst. In 2003,
the guanylation reaction of some aromatic amines with
activated N,N′-diaryl-substituted carbodiimides was reported
with tetrabutylammonium fluoride (TBAF).26 The first
catalytic guanylation of primary aromatic amines with
unactivated carbodiimides was reported by Richeson et al. in
2003.13 In recent years, the guanylation reaction of primary
aromatic amines has been explored catalytically with a large
number of transition metal8,13,27−30 and rare-earth metal
complexes,5,6,31−39 and sparse reports are known with main
group metal complexes.7,9,10,40−43 However, the most recent
trend deals with development of non-transition metal based,
earth-abundant, nontoxic, and inexpensive metals such as
aluminum for guanylation reaction.
Scheme 1. Hydroamination of Carbodiimides Catalyzed by
Complex 1 in C6D6
RESULTS AND DISCUSSION
■
The synthesis of 1 was accomplished by the treatment of free
cAAC in toluene with AlMe3 (2.0 M solution in toluene) at
−78 °C (Figure 1). Analytically pure off-white crystals of 1
were obtained in 70% isolated yield from a concentrated
solution of hexane at 0 °C after 3 days. Compound 1 readily
dissolves in toluene, benzene, hexane, THF, and Et2O.
Complex 1 was characterized by NMR spectroscopy (1H,
13C NMR), as well as by elemental analysis and single-crystal
1
X-ray diffraction study. The H and 13C NMR spectra of 1 in
C6D6 revealed resonances at δ −0.54 and −5.0 ppm,52
A thorough literature survey reveals that, though there are
several reports on aluminum-catalyzed guanylation reactions of
primary arylamines, well-designed aluminum carbene-based
catalysts are still scarce.42,44 This strongly motivated us to
Received: May 27, 2018
© XXXX American Chemical Society
A
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Table 1. Optimization Study for the Guanylation Reaction
a
of p-Toluidine with DCC
b
entry
1 (mol%)
solvent
C6D6
C6D6
C6D6
C6D6
time (h)
yield (%)
1
2
3
4
5
5
5
2
1.5
1
6
1
1
1
>99
>99
>99
81
C6D6
1
59
6
7
8
9
10
11
12
−
2
2
2
2
C6D6
6
1
1
1
1
1
−
>99
52
27
15
>99
>99
toluene-d8
THF-d8
DMSO-d6
CDCl3
C6D6
Figure 1. Synthesis of complex 1 and ORTEP diagram of 1 with
thermal ellipsoids drawn at 50% probability. The hydrogen atoms are
omitted for clarity. Selected bond lengths [Å] and angles [°]: Al1A−
C1A 2.1241(16), Al1A−C23A 1.9895(19), Al1A−C21A 1.9979(19),
Al1A−C22A 1.9958(18), N1A−C1A 1.3042(19), C2A−C1A
1.526(2), N1A−C4A 1.5345(19), C4A−C3A 1.528(2); C21A−
Al1A−C1A 100.96(7), C3A−C2A−C1A 104.33(12), N1A−C1A−
Al1A 128.98(11), N1A−C1A−C2A 108.12(13).
c
1
2
d
C6D6
15 min
a
Reaction conditions: p-toluidine (0.2 mmol), N,N′-
dicyclohexylcarbodiimide (0.2 mmol), 1 (x mol%), room temper-
b
ature, 0.6 mL of solvent. Yields determined by 1H NMR
c
spectroscopy using hexamethylbenzene as internal standard. Hy-
droamination reaction was performed at 60 °C. Catalytic hydro-
amination reaction was performed with 2 mol% catalyst loading at 60
d
respectively, which may be assigned to the methyl group
bound to the Al(III) ion. Also the 13C NMR spectrum exhibits
a resonance at δ 254.0 ppm, corresponding to the aluminum−
carbene carbon resonance which is significantly upfield shifted
upon metal coordination from the corresponding free cAAC
(304.2 ppm)52,60 but downfield shifted in comparison to the
reported NHC−AlMe3 adduct (174.3 ppm, NHC = 1,3-di-tert-
butylimidazol-2-ylidene).61 To unambiguously ascertain the
expected bond connectivities, a suitable crystal was analyzed
via single-crystal X-ray diffraction (Figure 1).62 As anticipated,
the X-ray crystal structure of 1 exhibits an aluminum(III)
trapped in a distorted tetrahedral environment and bonded to
the cAAC and three methyl groups. The Al−CcAAC bond
distance in 1 is 2.1241 (16) Å, which is comparable with
previously crystallographically analyzed Al−CNHC [2.162(2) Å,
NHC = 1,3-di-tert-butylimidazol-2-ylidene]61 and Al−CaNHC
bond distances [2.033 and 2.104 Å] observed in AlMe3−
aNHC adducts.52,63
Knowing the structure of the complex, we next used 1 as a
precatalyst for hydroamination reaction of primary arylamines
with unactivated carbodiimides such as N,N′-dicyclohexyl-
carbodiimide (DCC). When a solution of complex 1 (5 mol%)
and DCC in C6D6 was stirred in the presence of 1 equiv of p-
toluidine at room temperature, >99% of substituted derivative
4 was realized (Table 1, entry 1−2). Upon decreasing the
catalyst loading to 2 mol%, >99% of product 4 was observed
within 1 h (Table 1, entry 3). Further decreases in catalyst
loading to 1.5 and 1 mol% resulted in decreasing the
conversion (Table 1, entries 4 and 5). The role of catalyst 1
in the guanylation of primary arylamine is eminent: as in a
blank reaction, no product was detected by 1H NMR
spectroscopy even after 6 h (Table 1, entry 6). Upon further
optimization of solvent (Table 1, entries 7−11) and temper-
ature, quantitative yield of the guanylated product 4 was
achieved in benzene-d6 and toluene-d8, though 2 mol% catalyst
loading afforded the full conversion within 15 min at 60 °C
(Table 1, entry 12).
°C for 15 min.
With the optimized reaction conditions in hand (Table 1,
entries 3 and 7), we explored the scope of the guanylation
reaction of various primary arylamines with carbodiimides to
give their corresponding guanidine derivative (Table 2). Under
the standardized conditions, the presence of an electron-
donating group at the para or ortho position of an aniline
moiety such as 4-methylaniline, 4-isopropylaniline, and 2-
methoxyaniline afforded high yields of 98%, 92%, and 90% of
the corresponding guanidine derivatives 5a, 5b, and 5c,
respectively (Table 2). Under similar reaction conditions, the
aniline moieties containing electron-withdrawing substituents
at the para-position offered nearly quantitative yields of the
corresponding products, 89−96% (Table 2). 4-Bromo-, 4-
chloro-, 4-fluoro-, and 4-nitroaniline afforded excellent isolated
yields, such as 96%, 92%, 90%, and 89%, of the corresponding
guanidine derivatives 5d−5g, respectively. Under the same
reaction conditions, when 2-bromoaniline was subjected to
react with carbodiimide 3, almost quantitative yield (97%) of
the corresponding product 5h was observed (Table 2). In
contrast, 2-hydroxyaniline did not react at all with carbo-
diimide 3 to afford the guanidine product 5i (Table 2).
Further, we extended the substrate scope of the catalytic
guanylation reaction of various aniline derivatives with N,N′-
diisopropylcarbodiimide (DIC) to realize the corresponding
guanidine derivatives (Table 2). In a fashion similar to that
observed with N,N′-DCC, 4-methylaniline, 4-isopropylaniline,
or aniline was reacted under the standardized conditions with
DIC to obtain the corresponding product 6a, 6b, or 6c in
excellent yields of 97%, 89%, and 98%, respectively. Under the
standardized conditions, 2-methoxy-, 2,6-dimethyl-, and 2,6-
diisopropylaniline yielded 93%, 86%, and 81% of the
corresponding product 6d, 6e, or 6f (Table 2). This result
suggests that introduction of a more sterically hindered group
in aniline moiety somewhat suppresses the yield of the
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guanidine product 7a or 7b was realized in 97% or 95%
isolated yield (Table 2). Under the same reaction conditions,
pyrrolidine afforded 97% yield of 7c with DIC, whereas 81%
yield of 7d was obtained with DCC. It implies that steric
interaction lowers the yield of the guanylation product for
DCC.
Table 2. Catalytic Guanylation Reaction of Primary
Aromatic Amines or Secondary Aliphatic Amines with
Carbodiimides
a
Furthermore, to check whether the catalyst 1 remains active
over several catalytic cycles, we assessed its longevity for the
catalytic guanylation reaction. In this study, we carried out five
successive catalytic runs using 2 mol% of catalyst loading in an
NMR tube, and to maintain the homogeneity, this reaction was
performed at 60 °C. After every 15 min time interval, we
1
checked the conversion to the guanidine product through H
NMR in C6D6 and added a fresh batch of both the substrates
DCC and p-toluidine under an inert atmosphere without
adding any additional catalyst into the reaction mixture. The
1H NMR spectrum after every 15 min time interval indicates a
complete consumption of the substrates for three successive
catalytic runs (Figure 2), decreasing after the third cycle, and
a
Reaction conditions unless specified otherwise: primary aromatic
amines (2 mmol), N,N′-dicyclohexylcarbodiimide or N,N′-
diisopropylcarbodiimide (2 mmol), complex 1 (2 mol%), toluene
(5 mL), 25 °C, 1 h, N2 atmosphere. All products were isolated by
b
c
extracting with ether. NMR yield. Secondary amines (2 mmol),
N,N′-dicyclohexylcarbodiimide or N,N′diisopropylcarbodiimide or
N,N’−ditertiarybutylcarbodiimide, complex 1 (2 mol%), neat
condition, 60 °C, 6 h, N2 atmosphere. All products were isolated
by extracting with ether.
guanidine product (6d−6f). Moreover, when aniline contain-
ing electron-withdrawing substituents such as 4-bromo, 4-
chloro, 4-fluoro, or 4-nitro was subjected to react with DIC
under the same reaction conditions, the catalytic guanylation
products 6g, 6h, 6i, and 6j, respectively, were achieved in high
yield (91−97%, Table 2). The incorporation of an electron-
withdrawing substituent such as bromo at the o-position does
not alter the yield of the corresponding guanidine product 6k
(98%). When a bulkier carbodiimide such as N,N′-di-tert-
butylcarbodiimide (DTC) was subjected to react with p-
toluidine or aniline under the optimized conditions for
catalytic guanylation reaction, a very good isolated yield of
the corresponding product 6′a (89%) or 6′b (82%) was
realized. Overall the catalytic activity of complex 1 for primary
amines is by and large comparable to that reported earlier with
AlMe3,42 except in the case of bulkier amines such as 2,6-
dimethylaniline. A control experiment under identical con-
ditions using 2 mol% AlMe3 revealed that a sterically hindered
substrate such as 2,6-dimethylaniline leads to only 25% yield of
the desired product, whereas the present catalyst 1 leads to an
86% yield.
Given the importance of guanidine derivatives as essential
structural moieties in various drugs and natural products, next
we probed the catalytic guanylation reaction of secondary
aliphatic amines with different carbodiimides.64−66 It is well
established that secondary amines are less reactive than
primary amines in guanylation reaction of carbodiimides.67
Here, we have investigated the substrate scope for cyclic
secondary amines with 2 mol% of 1 at higher temperature (60
°C) under neat conditions. When cyclohexylamine or
morpholine was reacted with DIC for 6 h, the corresponding
Figure 2. Longevity of the catalyst, tested by monitoring the time
required for complete consumption of substrates by 1H NMR
spectroscopy in five consecutive catalytic cycles. Reaction conditions:
2 mol% complex 1, N,N′-dicyclohexylcarbodiimide (0.2 mmol), p-
toluidine (0.2 mmol), and 600 μL of benzene-d6 as solvent at 60 °C.
After completion of each catalytic cycle, fresh N,N′-dicyclohexylcarbo-
diimide (0.2 mmol) and p-toluidine (0.2 mmol) were added without
addition of any catalyst. Yields were determined for each cycle after 15
1
min by recording the H NMR spectrum of the reaction mixture.
reaching almost 60% after the fifth run. This longevity test
clearly demonstrates that the catalyst stays active for at least
three consecutive catalytic runs without losing its efficacy.
Next, the catalytic guanylation reaction was performed
between DCC and p-toluidine in the presence of catalyst 1 at
room temperature in C6D6 in an NMR tube, and the
conversion was monitored by 1H NMR spectroscopy at
every 5 min time interval. The plot of NMR yield of
guanylation product (%) vs time (min) clearly suggests that
the reaction proceeds very fast in the initial 5 min, and then the
rate of conversion gradually decreases until the maximum is
reached within 1 h (Figure 3).
To explore the mechanistic course of the hydroamination of
carbodiimide catalyzed by 1, we embarked on a tandem
experimental and computational approach. In this study, we
performed several stoichiometric reactions. At first, when
C
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this step. DFT study suggests that path b, leading to formation
of intermediate 11, is more favorable thermodynamically as
compared to the path a, where the intermediate complex 10 is
proposed (Figure 4). The Gibbs free energy difference
Figure 3. Plot of the yield of the guanylation product (%) vs time
1
(min) as monitored by H NMR spectroscopy.
Figure 4. Computationally optimized structures of intermediates 10
and 11 for nucleophilic attack of arylamido N atom to carbodiimide.
All hydrogens except for arylamido N−H have been omitted for
clarity.
catalyst 1 was subjected to react with DCC in a 1:1 ratio in
1
benzene-d6, no change in the H NMR spectrum was found,
which suggests no reaction between catalyst 1 and DCC (SI,
Figure S55). However, the reaction between complex 1 and p-
toluidine in a 1:1 ratio in benzene-d6 revealed a sharp change
1
in the H NMR spectrum within 10 min, and we observed a
between 11 and 10 was calculated as −0.33 eV, indicating a
preference for 11 which does not necessitate the cleavage of an
Al−amido bond. Intermediate 11, upon further consumption
of both the substrates carbodiimide and p-toluidine, yields the
product 4 through a 1,3-H shift, and the intermediate 9 is
regenerated.
new peak at δ 0.19 ppm, indicating the formation of methane
gas42,68 (SI, Figure S56). Furthermore, the 1:1 mixture of 1
and p-toluidine showed a resonance at −0.39 ppm
corresponding to Al-Me peak, shifted to the deshielded region
as compared to the Al-Me resonance in 1 (δ −0.54 ppm).
Based on this observation, we proposed that the first step of
the catalytic cycle is formation of an Al-amido complex under
evolution of methane via the four-centered TS-8, which
subsequently reacts with DCC to form the intermediate 9,
where carbodiimide is coordinated with Al (Scheme 2). Such
CONCLUSIONS
■
In conclusion, we have developed an efficient cyclic (alkyl)-
amino carbene complex of aluminum(III) which can guanylate
various primary arylamines and secondary aliphatic amines.
The catalytic reactions were accomplished under mild reaction
conditions with a wide range of amine substrates. The catalyst
remained active for three successive catalytic runs without any
loss of its catalytic activity. A combined experimental and
theoretical investigation discloses that the hydroamination
reaction of carbodiimide proceeds through the elimination of
methane. The present methodology may be considered as an
inexpensive and convenient way to prepare guanidine.
Scheme 2. Plausible Mechanism for Catalytic
Hydroamination of Carbodiimides
EXPERIMENTAL SECTION
■
General Methods and Instrumentation. Unless stated
otherwise, reactions were performed in flame-dried glassware under
an oxygen-free atmosphere (N2) using standard Schlenk techniques or
inside an MBraun glovebox maintained below 0.1 ppm of O2 and
H2O level. All solvents were distilled from Na/benzophenone prior to
use. Trimethylaluminum (2.0 M in toluene) solution was purchased
from Sigma-Aldrich. All other chemicals were purchased from
commercial sources and used as received. The liquid carbodiimides
and amines were distilled from CaH2 twice and stored over 4 Å
molecular sieves before use. Elemental analysis was carried out using a
PerkinElmer 2400 CHN analyzer, and sample was prepared by
keeping them under reduced pressure (10−2 mbar) overnight. The
melting point was measured in a sealed glass tube on a Buchi B-540
̈
Al-amido bond formation under methane evolution was
proposed in earlier reports.19,42 The computationally opti-
mized (M06/6-31G(d)/IEFPCM level of theory) structure of
TS-8 clearly exposes the elongation of Al−Me bonds of 1 upon
addition of p-toluidine, and it favors the evolution of methane
gas (see SI, Figure S58 and Table S2).
The intermediate 9 can subsequently undergo nucleophilic
attack by the amido N to the DCC carbon, resulting in two
possible intermediates, 10 and 11. We undertook a density
functional theory (DFT) study to understand further details on
melting point apparatus and was uncorrected. Deuterated solvents
were purchased from Cambridge Isotope Laboratories, dried by
sodium/potassium alloy, and stored over 4 Å molecular sieves prior to
use. Crystallographic data for the structural analysis of 1 have been
deposited at the Cambridge Crystallographic Data Center (CCDC
No. 1571838). These data can be obtained free of charge from the
Cambridge Crystallographic Data Center. NMR spectra were
recorded on a JEOL ECS 400 MHz spectrometer and on a Bruker
Avance III 500 MHz spectrometer. All chemical shifts were reported
in ppm using tetramethylsilane as a reference. Chemical shifts (δ)
downfield from the reference standard were assigned positive values.
D
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Synthesis of Complex 1, (cAAC)AlMe3. In a glovebox, an oven-
dried 50 mL Schlenk flask was charged with cyclic (alkyl)amino
carbene (300 mg, 1.04 mmol), and dry toluene (10 mL) was added
through a cannula at 25 °C in nitrogen atmosphere. Subsequently,
AlMe3 solution (2.0 M in toluene; 0.62 mL, 1.1 mmol, 1.06 equiv)
was added dropwise in the reaction mixture at −78 °C. The reaction
mixture was then stirred overnight, and during the course of the
reaction, the color changed to pale yellow. After removal of all the
volatiles under high vacuum, the pale yellow colored compound was
extracted in dry hexane (25 mL) followed by filtration through a
Celite pad, and the reaction mixture was concentrated to ca. 8 mL.
Storage of this reaction mixture at 0 °C for 2−3 days afforded
colorless crystals (262 mg, 70%). Complex 1 decomposes at 125 °C.
1H NMR (400 MHz, C6D6) δ 7.13 (t, J = 8.1 Hz, 1H), 7.01 (d, J = 7.7
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00358.
■
S
Spectroscopic data, scanned spectra, X-ray crystallo-
graphic data for 1, and details on DFT calculations
(PDF)
Cartesian coordinates of calculated structures (XYZ)
JMol structure for TS-8 (MOL)
JMol structure for intermediate 10 (MOL)
JMol structure for intermediate 11 (MOL)
Hz, 2H), 2.71−2.61 (m, 2H), 1.42 (s, 6H), 1.36−1.32 (m, 9H), 1.06
(d, J = 6.9 Hz, 6H), 0.82 (s, 6H), −0.54 (s, 9H); 13C NMR (100
MHz, C6D6) δ 254.0, 145.4, 134.7, 129.9, 125.4, 80.9, 56.5, 50.9, 29.1,
28.8, 28.7, 26.0, 24.3, −5.0 ppm. Anal. Calcd for C23H41AlN (358.56):
C, 77.04; H, 11.53; N, 3.91. Found: C, 76.95; H, 11.48; N, 3.92.
General Procedure for the Catalytic Hydroamination of
Primary Arylamines. A 25 mL Schlenk tube was charged with
complex 1 (0.0143 g, 2 mol%, 0.04 mmol), carbodiimide (2.0 mmol),
and aromatic primary amine (2.0 mmol) followed by the addition of
toluene (5.0 mL) under N2 atmosphere. The resulting mixture was
stirred at 25 °C for 1 h. After the reaction was completed, the reaction
mixture was extracted with ether and filtered to give a clean solution.
After removal of the solvent under vacuum, the final products were
further purified by washing with diethyl ether or hexane.
General Procedure for the Catalytic Hydroamination of
Secondary Amines. A 25 mL Schlenk tube was charged with
complex 1 (0.0143 g, 2 mol%, 0.04 mmol), carbodiimide (2.0 mmol),
and secondary amine (2.0 mmol) under N2 atmosphere. The resulting
mixture was stirred at 60 °C for 6 h under neat conditions. After the
reaction was completed, the reaction mixture was extracted with ether
and filtered to give a clean solution. After removal of the solvent
under vacuum, the final products were further purified by washing
with diethyl ether or hexane.
Accession Codes
CCDC 1571838 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: swadhin.mandal@iiserkol.ac.in.
■
ORCID
Varun Bheemireddy: 0000-0002-5267-124X
Gonela Vijaykumar: 0000-0002-1249-7347
Swadhin K. Mandal: 0000-0003-3471-7053
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the CSIR, India (Grant No. 01(2779/14/EMR-II),
for financial support. P.K.V., M.B., and G.V. thank the UGC,
New Delhi, India, for research fellowships.
Procedure for Catalyst Longevity Experiment. A screw-
capped NMR tube was charged with catalyst 1 (2 mol%), p-toluidine
(0.2 mmol), DCC (0.2 mmol), and hexamethylbenzene as an internal
standard (0.033 mmol) in C6D6 (0.6 mL). The catalytic reaction was
performed at 60 °C by applying in situ recycling methodology. We
monitored the longevity of catalyst 1 by performing several catalytic
runs within the same reaction pot to test whether the catalyst
remained live for several catalytic cycles or not. We repeated the
catalysis up to five times by charging a fresh batch of substrates after
each catalytic run in the same reaction vessel but without adding more
catalyst into the reaction mixture. After full consumption of the
substrate, a fresh batch of substrates was introduced in the reaction
medium. After each 15 min interval, conversion into the product was
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checked by recording a H NMR spectrum of the reaction mixture.
The catalytic runs were repeated for five successive cycles. It was
found that catalyst 1 remained catalytically active up to three
consecutive runs (Figure 2) without losing any catalytic activity as
1
evidenced from H NMR spectroscopy. This result clearly indicates
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Computational Details. Gaussian 09 software69 was used to
perform numerical calculations at the DFT level with the M06
functional70 in toluene medium within the IEFPCM model.71 The 6-
31G(d)72 basis set was employed for optimizing the molecular
structures without any symmetry constraints. The real harmonic
vibrational wavenumbers obtained for each optimized structure
confirmed the energy-minimized geometry. An isosurface value of
0.03 has been used to visualize the frontier Kohn−Sham molecular
orbitals.
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