Protonated trititanate nanotubes: an efficient catalyst for one-pot three-component coupling of benzothiazole amines, heterocyclic aldehydes, and dialkyl/diaryl phosphites with a greener perspective

Article abstract of DOI:10.1016/j.tetlet.2016.01.001

Nano-size catalysts of TiO₂, ZnO, CuO, and protonated trititanate nanotubes (H₂Ti₃O₇) have been investigated for the one-pot three component synthesis of novel α-aminophosphonates from benzothiazole amines, hete

Full text of DOI:10.1016/j.tetlet.2016.01.001

Accepted Manuscript
Protonated trititanate nanotubes: An efficient catalyst for one-pot three-compo-
nent coupling of benzothiazole amines, heterocyclic aldehydes and dialkyl/
diaryl phosphites with a greener perspective
Bhoomireddy Rajendra Prasad Reddy, Motakatla Venkata Krishna Reddy,
Peddiahgari Vasu Govardhana Reddy, Dharani Praveen Kumar, Muthukonda
V. Sankar
PII:
S0040-4039(16)30001-6
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.01.001
TETL 47165
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
21 November 2015
29 December 2015
2 January 2016
Please cite this article as: Reddy, B.R.P., Reddy, M.V.K., Reddy, P.V.G., Kumar, D.P., Sankar, M.V., Protonated
trititanate nanotubes: An efficient catalyst for one-pot three-component coupling of benzothiazole amines,
heterocyclic aldehydes and dialkyl/diaryl phosphites with a greener perspective, Tetrahedron Letters (2016), doi:
http://dx.doi.org/10.1016/j.tetlet.2016.01.001
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Graphical Abstract
Protonated trititanate nanotubes: An efficient catalyst
for one-pot three-component coupling of benzothiazole
amines, heterocyclic aldehydes and dialkyl/diaryl
phosphites with a greener perspective
Leave this area blank for abstract info.
Bhoomireddy Rajendra Prasad Reddya, Motakatla Venkata Krishna Reddya, Peddiahgari Vasu Govardhana
Reddy a, *, Dharani Praveen Kumar b and Muthukonda V. Shankar b, *

1
Tetrahedron Letters
Protonated trititanate nanotubes: An efficient catalyst for one-pot three-
component coupling of benzothiazole amines, heterocyclic aldehydes and
dialkyl/diaryl phosphites with a greener perspective
Bhoomireddy Rajendra Prasad Reddya , Motakatla Venkata Krishna Reddya, Peddiahgari Vasu Govardhana
Reddya, , Dharani Praveen Kumarb and Muthukonda V. Sankarb, 
a Department of Chemistry, Yogi Vemana University, Kadapa, A.P, India- 516003.
b Nano Catalysis and Solar Fuels Research Laboratory, Department of Material Science and Nanotechnology, Yogi Vemana University, Kadapa, A.P, India-
516003.
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
Nano-size catalysts of TiO2, ZnO, CuO and protonated trititanate nanotubes (H2Ti3O7) have
been investigated for the one-pot three component synthesis of novel α-aminophosphonates
from benzothiazole amines, heteroaldehydes and dialky/diaryl phosphites via Kabachnik-Fields
reaction. This methodology provides a new and convenient approach to multicomponent
reaction and the H2Ti3O7 nanotubes catalyst is recyclable upto seven cycles.
Available online
Keywords:
Heterogeneous catalysis
Protonated trititanate nanotubes
Kabachnik-Fields reaction
Acidic sites
2015 Elsevier Ltd. All rights reserved.
Benzothiazole amines
1. Introduction
The synthesis of α-aminophosphonates have witnessed
tremendous growth due to broad spectrum of biological and
pharmaceutical properties such as anticancer,5a-c antileishmanial
chemo type,5d antiproliferative,5e antioxidant,5f antimicrobial,5g, 5h
antiviral5i activities and inhibitors of protein tyrosine
phosphatases.5j Owing to the potential importance of α-
aminophosphonate derivatives in life sciences, pharmaceuticals
and agriculture; world-wide efforts have been made in the last
few decades by researchers and various protocols have been
developed for their synthesis using various Brønsted/Lewis acids,
heteropoly acids and heterogeneous catalysts.6 But, some of the
methodologies endure few setbacks such as unavailable starting
materials, usage of corrosive catalysts, multi step process, high
temperature, long reaction time, non-compatible solvent and low
yields. Moreover several research groups focused on the
synthesis of α-aminophosphonates using simple aromatic
amines/aldehydes and phosphites, but limited with heterocyclic
amines/aldehydes and phosphites. Hence, the development of
simple, effective, high-yielding and environmentally friendly
approaches using new catalysts for the synthesis of heterocycle
derived α-aminophosphonates is an important task for organic
chemists. Among various heterocycles known benzothiazole
skeleton is considered to be the most “privileged” structure due
to its biological and pharmaceutical properties.7
In recent years, nano-catalysis has emerged as a sustainable
and competitive alternate to conventional catalysis since the
nanoparticles (NPs)/nanostructers (NS) possess selectivity and
reactivity, while at the same time maintain the intrinsic features
of a heterogeneous catalyst. The ease of separation, recovery and
reuse of these NPs/NS further enhance their attractiveness as
green and sustainable catalysts.1 Among various nanostructured
materials, the TiO2 nanostructures have been widely investigated
over the past few decades due to multiple potential applications
in Friedel-Crafts alkylation,2a-d Michael reaction,2e Biginelli
condensation,2f
Strecker
reaction,2g
Kabachinick-Fields
reaction,2h synthesis of 5-hydroxymethyl-furfural,2i α-hydroxy
phosphonates,2j nucleic bases,2k multi component synthesis of 2-
oxo dihydropyrroles2land polysubstituted pyrrolidinones.2m
Now a days, one dimensional (1-D) TiO2 nanotubes, nanorods
and nanowires attracted much attention due to their unique
properties viz., large surface-to-volume ratio, better dispersibility,
decreased inter-crystalline contacts and increased accessibility of
acid sites.2a-c &3 In this connection, we succeeded in synthesis of
tertiary α-aminophosphonates through the catalytic application of
H2Ti3O7 nanotubes (TNT) and nanorods (TNR) with high yields.4
———
 Corresponding author. Tel.: +91-998-520-0965; fax: +91-856-222-5419; e-mail: pvgr@yogivemanauniversity.ac.in
 Co-corresponding author. Tel.: +91-996-684-5899; fax: +91-856-222-5419; e-mail: shankar@yogivemanauniversity.ac.in

2
Tetrahedron Letters
In continuation of our research interest on the introduction of
Besides this, higher surface area and porous nature of TNT may
also contribute to its catalytic performance.
recoverable TNT catalysts, we surmised that novel benzothiazole
derived α-aminophosphonates (4a-q) (Scheme 1) can be
synthesized from 5/6-aminobenzothiazoles with various
heterocyclic aldehydes and dialkyl/diaryl phosphites via
Kabachnik-Fields reaction in the presence of environmentally
benign, easily separable, recyclable, and highly effective catalyst
system TNT. Further the reaction conditions like optimization of
catalyst and solvent were carried out and listed in Fig. 1 and
Table .2
Table 1. NH3 adsorption capacities equal to total amounts of
acid sites.
Amount of acid sites
Entry
Catalyst
Nano CuO
Nano ZnO
Bulk TiO2
Nano TiO2
TNT
(mL.g-1 of NH3)Ref
14.52e
1
2
3
4
5
15.62e
0.12e
16.22e
H
e
t
H
N
84.6
H
TNT(15 mg)
B
RO
P
OR
O
RO
+
P
O
t
OHC
NH2
+
Het
Bt
Neat, rt, 15 min
OR
As far as the effect of solvent concerned, the model reaction
was carried out in several solvents as well as solvent free
condition to investigate the efficiency of the catalyst. From Table
2, it is evidenced that the best yield was obtained when the
reaction was performed in toluene. It is quite interesting to note
that the reaction also took place smoothly under solvent free
conditions and generated the corresponding α-aminophosphonate
in excellent yield. To avoid the use of volatile solvents and to
reduce the environmental hazards, all the three-component
coupling reactions were performed under solvent free conditions.
Further investigation revealed that the result was affected by the
amount of catalyst. The catalyst loading varied from 5 to 20 mg
and observed that maximum yield obtained with just 15 mg.
However no significant improvement in yield was observed when
the catalyst load is increased to more than 15 mg.
4a-q
3a-c
1a-c
2a-k
Het : Heterocyclic; Bt: Benzothiazole; R: Alkyl/Aryl
Scheme 1. TNT catalyzed Kabachnik-Fields reaction of
benzothiazole amines, different heterocyclic aldehydes and
dialky/diaryl phosphites.
2. Results and discussion
2.1. Catalytic activity of TNT
The catalytic performance of the TNT was examined through
Kabachnik-Fields reaction of benzo[d]thiazol-5-amine (1b), 5-
methoxybenzofuran-2-carbaldehyde (2a) and diphenyl phosphite
(3c) in toluene at room temperature, which afforded diphenyl
(benzo[d]thiazol-5-ylamino)(5-methoxybenzofuran-2-yl)methyl-
phosphonate (4d). For comparison of the results, we tested the
catalytic activity using commercially available material such as
bulk/nano TiO2 as well as nano ZnO and nano CuO. The
formation of 4d was observed with all the catalysts (Fig. 1). In
the case of bulk TiO2, the yields of 4d did not reach 25% even
after 30 min. While in the case of nano TiO2, nano ZnO and nano
CuO the results were obtained in moderate yields. In contrast to
the bulk/nano TiO2, nano ZnO and nano CuO; TNT exhibited
remarkable catalytic performance. However, the reaction did not
occur in the absence of the catalyst even with prolonged reaction
time (Table 2, Entry 8).
Table 2. Screening of reaction conditions for the synthesis of α-
aminophosphonate (4d) by TNT catalyst
H3CO
H
O
P
H2N
H3CO
+
TNT, rt
N
N
O
C6H5O
N
C6H5O
H
+
Solvent, 15 min
P O
S
1b
O
2a
CHO
OC6H5
3c
S
OC6H5
4d
Catalyst loading
Entry
1
Solvent
(mg)
15
Yielda (%)
96
Toluene
Ethanol
DMSO
DMF
2
15
15
50
10
15
75
0
3
4
15
5
6
Acetonitrile
Water
Neat
15
15
7
15
95
---
68
82
90
95
95
8b
Neat
---
9
Neat
5
10
11
12
13
Reaction
Neat
10
Neat
12.5
17.5
20
Neat
Neat
Fig.1 Time courses of α-aminophosphonate (4d) formation using
conditions:
Benzo[d]thiazol-5-amine
(1
mmol),
5-
various solid acid catalysts.
methoxybenzofuran-2-carbaldehyde (1 mmol), diphenyl phosphite (1 mmol),
solvent (1 mL), TNT catalyst, r.t and quenched after 15 min. a Isolated yield. b
Reaction continued up to 6 h.
Reaction conditions: Catalyst (15 mg), benzo[d]thiazol-5-amine (1 mmol), 5-
methoxybenzofuran-2-carbaldehyde (1 mmol), diphenyl phosphite (1 mmol),
toluene (1 mL) and at room temperature.
Under optimal conditions, we chose a variety of structurally
different benzothiazole amines heterocyclic aldehydes, and
dialkyl/diaryl phosphites to understand the scope and efficiency
of the TNT promoted one-pot three component Kabachnik-Fields
reaction and the results are compiled in Table 3. The reaction of
To understand the reason behind the remarkable activity of
TNT over the other samples used, we investigated acidity of the
catalysts using NH3-TPD (Table 1). In NH3-TPD test TNT
showed greater desorption of NH3 over bulk/nano TiO2, nano
CuO and nano ZnO. Therefore, the outstanding activity of TNT
over the other samples tested may be due to the increased acidity.
benzo[d]thiazol-6-amine
(1a),
5-methoxybenzofuran-2-
carbaldehyde (2a) and dialkyl/diaryl phosphites (3a-c) was
carried out to determine the influence of the phosphite. In case of

3
dialkyl phosphites, the yield was decreased with increase in the
alkyl chain (4a-b). Diphenyl phosphite gave better yield than
dialkyl phosphites and moreover the products obtained as solids
while brown colour oils were observed with dialkyl phosphites.
We then selected benzo[d]thiazol 2/5/6 amines, instead of simple
aromatic amines to carry out the reaction. Benzo[d]thiazol-5-
amine (1b) afforded better yield when compare to
benzo[d]thiazol-6-amine (1a), on the other hand benzo[d]thiazol-
2-amine (1c) was found to be ineffective partner in this three-
component reaction. To expand the scope of aldehyde substrates,
we studied the reaction with a variety of heterocyclic aldehydes.
In all the cases, the corresponding α-aminophosphonates were
obtained in good to excellent yields except with 1H-pyrrole-2-
carbaldehyde (2d) and 1H-imidazole-4-carbaldehyde (2k).
Thiophene-2-carbaldehyde (2b) was a better substrate than furan-
2-carbaldehyde (2c) in the case of simple 5-membered
Dimethoxynicotinaldehyde (2g) afforded superior yield than 5-
chloro-2-fluoronicotinaldehyde (2f) in nicotinaldehyde series.
1H-Indole-3-carbaldehyde (2h) gave low yield i.e 79% due to its
less solubility. The remaining aldehydes 5-methoxybenzofuran-
2-carbaldehyde
(2a),
picolinaldehyde
(2e),
2-
cyclopropylpyrimidine-4-carbaldehyde (2i) and 3-(pyrimidin-5-
yl)benzaldehyde (2j) gave the promising yields. All products
were characterized by IR, NMR (1H, 13C & 31P) and mass spectral
(HRMS) analysis, which for known compounds were found to be
identical with the literature data (4m, 4q)6a and the remaining
new compounds were analysed spectroscopically and described
in the supporting information. The NH protons were confirmed
by D2O exchange experiments. It is of quite interest to observe
the NH proton resonating at 5.00-4.96 ppm as quartet as seen in
proton NMR of 4d and its disappearance was found in D2O
exchange experiments.
heterocycles
for
this
coupling
reaction.
2,6-
Table 3. TNT catalyzed synthesis of α-aminophosphonates (4a-q)
Entry
1
Amine
(1a-c)
Aldehyde
(2a-k)
Phosphite
(3a-c)
Product
(4a-q)
Yielda (%)
OC2H5
C2H5O
O
P
O
N
N
N
H3CO
H3CO
O
N
N
C2H5O
P H
N
S
S
S
84
1aS
H2N
H
O
CHO
CHO
4a
3a OC2H5
H3CO
H3CO
2a
OC4H9
C4H9O
O
P O
O
C4H9O
P
H
N
2
3
4
5
80
90
95
0
1aS
H2N
H
O
4b
4c
OC4H9
3b
2a
O
OC6H5
P
C6H5O
O
O
C6H5O
P H
H3CO
H3CO
N
N
OC H
3c
6
5
N
1aS
H2N
H2N
H
O
CHO
CHO
H3CO
H3CO
2a
O
P
H
N
C6H5O
H
O
O
N
1bS
OC H
3c
6
5
2a
C6H5O
P
O
S
OC6H5
4d
H3CO
O
P
OC6H5
N
C6H5O
H
C6H5O
O
P
N
S1cNH
2
O
CHO
O
OC H
3c
6
5
N
H
S
2a
4e
H2N
H2N
H2N
H2N
O
P
H
N
N
S
CHO
CHO
S
C6H5O
H
N
N
N
1bS
1bS
1bS
1bS
C6H5O
P
O
2b
6
7
8
OC H
95
89
0
3c
6
5
S
OC6H5
4f
O
P
H
N
N
N
N
O
2c
C6H5O
H
O
C6H5O
P
O
OC H
3c
6
5
4gS
OC6H5
O
P
H
N
C6H5O
H
N
H
N
H
CHO
CHO
P
O
C6H5O
OC H
3c
6
5
S
2d
OC6H5
4h
O
P
H
N
C6H5O
H
N
2e
N
N
OC H
3c
6
5
9
95
81
C6H5O
P
O
S
OC6H5
4i
N
F
Cl
CHO
H2N
O
P
N
H
N
C6H5O
H
Cl
N
N
F
1bS
OC H
3c
6
5
10
C6H5O
P
O
2f
4jS
OC6H5

4
Tetrahedron Letters
H3CO
N
OCH3
H
N
H2N
H2N
O
CHO
N
N
C6H5O
P H
N
H3CO
N
OCH3
1bS
1bS
OC H
3c
6
5
11
12
89
79
C6H5O
P
O
2g
4kS
OC6H5
H
N
CHO
O
P
C6H5O
H
H
N
N
N
H
2h
OC H
3c
6
5
P
O
S
C6H5O
N
4l
OC6H5
H2N
H2N
H2N
N
N
N
N
H
N
O
P
N
N
N
CHO
CHO
1bS
1bS
1bS
13
14
90
96
C6H5O
H
C6H5O
P
O
2i
OC6H5 4mS
OC H
3c
6
5
H
N
O
P
N
N
N
N
C6H5O
H
C6H5O
P
O
2j
N
OC6H5 4nS
OC H
3c
6
5
HN
HN
H
N
N
O
N
CHO
O
P
N
2k
C6H5O
15
16
17
trace
88
P
C6H5O
H
4oS
OC H
OC6H5
OC6H5
3c
6
5
O
P
N
N
C6H5O
S
S
CHO
C6H5O
H
P
O
N
H2N
H2N
1aS
1aS
2b
OC H
3c
6
5
N
4pS
H
O
P
OC6H5
N
C6H5O
N
C6H5O
H
P
O
N
N
CHO
OC H
90
3c
6
5
2i
N
4qS
H
N
Reaction conditions: Benzothiazole amine (1 mmol), heterocyclic aldehyde (1 mmol), dialkyl/diaryl phosphite (1 mmol), solvent free, r.t and 5 mol% TNT
catalyst quenched after 15 min. a Isolated yields
From these results, plausible mechanism is proposed for the
process of TNT catalyzed Kabachnik-Fields reaction (Fig. 2).
The mechanism involves the activation of aldehyde by TNT (I)
followed by the nucleophilic addition of amine to afford imine
(II) and the subsequent activation of imine (II) by TNT
facilitates the addition of phosphite to give an activated
phosphonium intermediate (III), which then gave the desired
product (IV) (Fig. 2). The acid-base interaction between the
TNT-aldehyde (in imine formation step) and TNT-imine (in
phosphite addition step) facilitated the reaction by shifting the
equilibrium towards product formation.
Encouraged by the highly efficient catalytic outcome of TNT
as depicted in Table 4, we tested its ability to perform as a
recyclable catalyst with the coupling reaction of benzo[d]thiazol-
5-amine (1b), 5-methoxybenzofuran-2-carbaldehyde (2a) and
diphenyl phosphite (3c). After each catalytic cycle, the reaction
mixture was dissolved in EtOAc (2mL) and the catalyst was
separated by centrifugation, thoroughly washed with EtOAc and
o
finally dried at 100 C for 6 h before performing the next cycle.
We checked the recyclability test up to seven consecutive cycles
and examined that first four cycles resulted in quantitative yields
of 4d without any loss of catalytic efficiency, however, from the
5th cycle onwards a slight decrease of catalytic efficiency was
observed (Table 4).
To understand the reason behind decrease in catalytic
efficiency, we carried out the NH3-TPD experiment for fresh and
reused TNT (after 7th cycle) to determine acid sites concentration.
The NH3-TPD curve (Fig. 3) of the fresh and reused TNT sample
consists of two unresolved peaks (at around 160.9 and 302 oC for
o
fresh TNT; at around 148.5 and 294.3 C for reused TNT). The
low and high temperature peaks could be attributed to NH3
desorbtion from weak and strong acidic sites, respectively.8The
total amount of NH3 adsorption on fresh and reused TNT is 84.6
mL.g-1 and 52.2 mL.g-1, respectively. The decrease in
concentration of acid sites may be one of the main reasons for the
drop in yield of the product.
Table 4. Recyclability studies of TNT on the synthesis of α-
aminophosphonate (4d)
Run
1
2
95
3
95
4
95
5
93
6
88
7
81
Yielda(%) 95
Reaction
conditions:
Benzo[d]thiazol-5-amine,
(1
mmol)
5-
methoxybenzofuran-2-carbaldehyde (1 mmol) and diphenyl phosphite (1
a
mmol), solvent free, TNT catalyst (15 mg), r.t and quenched after 15 min.
Fig.
2
A
possible mechanism for the synthesis of
Isolated yield.
α-aminophosphonates over a TNT catalyst.

5
Fig. 4 Characterization of the TNT (a) HR-TEM image, (b) XRD pattern, (c) N2-adsorption-desorption Isotherm and (d) BJH pore size
distributions.
(JCPDS No.47-0561 and 31-1329). In the N2-adsorption-
desorption isotherm (Fig. 4c), TNT showed a typical type IV
isotherm with a hysteresis loop, confirming the mesoporous
nature of TNT.9 The BET surface area of the sample was
calculated to be 286 m2.g-1. The corresponding pore size
distribution curve (Fig. 4d) by the BJH method consists of two
portions with peak values of ca. 2.1-4.2 nm and 4.2-23 nm.
Taking into account of the morphology of TNT, the smaller pores
relates to the inner hollow spaces of the nanotubes; while the
larger pores refers the pores between the nanotubes.10 The
surface area increases from 5.1 m2.g-1 (bulk TiO2 which used as
precursor for the preparation of TNT) to 286 m2.g-1 confirming
the tubular structure of TNT.
3. Conclusions
In conclusion, H2Ti3O7 nanotubes as a highly efficient catalyst
Fig. 3 NH3-TPD profile of (a) Fresh TNT and (b) Reused TNT
in the synthesis of novel α-aminophosphonates from
benzothiazole amines, heteroaldehydes and dialky/diaryl
after seventh cycle.
phosphites has been demostrated. This developed green protocol
2.2. Characterization of TNT
offers several advantages such as requirement of small amount of
reusable catalyst, high reaction rates, operational simplicity and
mild reaction conditions. The high rate of H2Ti3O7 nanotubes is
explained by enhanced acidic sites, surface area and
mesoporosity.
Based on the high catalytic activity of TNT, the physio-
chemical properties were analyzed and discussed in detail. The
morphology, crystal structure and surface properties are
displayed in Fig. 4. TEM image reveals (Fig. 4a) that random
orientation of open tube-like bundles have hollow inner space
with openings at both the ends. The cylindrical nanotubes have
length > 100 nm with inner and outer diameters of the nanotubes
ranging 3-5 nm and 8-12 nm, respectively. In XRD patterns (Fig.
4b), the characteristic diffraction peak of TNT exhibited at 2 =
10.2⁰ (001) indicates the presence of typical layered crystal
structure. All other peaks centred at 2 = 24.1 (202), 28.3 (11-2)
and 48.2⁰(303) can be well indexed as the monoclinic structure
4. Experimental
4.1. Hydrothermal synthesis of H2Ti3O7 nanotubes
As received chemicals were used for catalyst synthesis
without further purification. In a typical synthesis process, TiO2
fine particles (2.5 g) dispersed in 10 M NaOH aqueous solution
(200 mL) and transferred to Teflon-lined autoclave (capacity 250
mL) and heated in electric oven at 130 C/20 h.11 The white

6
Tetrahedron Letters
precipitate obtained was subjected to washing (twice) with
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(d) Kantam, M. L.; Laha, S.; Yadav, J.; Sreedhar, B.
Tetrahedron Lett. 2006, 47, 6213-6216; (e) Kantam, M.
L.; Laha, S.; Yadav, J.; Choudary, B. M.; Sreedhar, B.
Adv. Synth. Catal. 2006, 348, 867-872; (f) Baghbanian,
S. M.; Farhang, M.; Baharfar, R. Chin. Chem. Lett.
2011, 22, 555-558; (g) Sarvari, M. H. Tetrahedron
2008, 64, 5459-5466; (h) Kassaee, M. Z.; Masrouri, H.;
Movahedi, F.; Mohammadi, R. Helv. Chim. Acta. 2010,
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distilled water (800 mL) followed by dil. HCl (0.1 N, 200 mL),
finally washed with ethanol (200 mL), and dried at 80 C for 12
h. The obtained material was denoted as TNT.
4.2. General procedure for synthesis of α-aminophosphonates
(4a-q)
Dialkyl/diaryl phosphite (1.0 mmol) was added portion wise
over a period of 5 min to the stirred mixture of heterocyclic
aldehyde (1.0 mmol) and benzothiazole amine (1.0 mmol) at
room temperature. Further 5 mol % of TNT was added to the
reaction mixture and the stirring was continued for 15 min. After
completion of the reaction as monitored through TLC, the
reaction mixture was dissolved in EtOAc (2mL) and the catalyst
was separated by centrifugation followed by subsequent
washings with EtOAc. The recovered catalyst was reused for the
next cycle. The filtrate was washed with brine, dried over
anhydrous Na2SO4, filtered and concentrated on a rotary
evaporator and the resulting residue was purified by silica gel
column chromatography (70:30, hexane/EtOAc) to afford the
corresponding
pure
α-aminophosphonate.
The
novel
α-aminophosphonates were structurally assigned by their IR,
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NMR (1H, 13C & 31P) and mass spectral (HRMS) analyses.
Acknowledgments
We are grateful for financial support from the Department of
Atomic Energy (DAE) Board of Research in Nuclear Sciences
(BRNS), Mumbai, India (2012/37C/33/BRNS). B. R. P. Reddy
thanks to the UGC-CSIR, New Delhi, India for the award of
Junior Research Fellowship.
[4] Reddy, B. R. P.; Reddy, P. V. G.; Reddy, B. N. New J.
Chem. 2015, DOI: 10.1039/C5NJ01914A
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Supplementary data
General experimental details, characterization data and copies
of the NMR (1H, 13C & 31P) spectra of all synthesized compounds
and HRMS spectra of 4b, 4c, 4j, 4l and 4n is associated with this
article can be found, in the online version at
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