First sonochemical, simple and solvent-free synthesis of chiral tert-butanesulfinimines using silica supported p-toluenesulfonic acid

Article abstract of DOI:10.1080/00397911.2018.1540047

A solvent-free, versatile procedure has been developed for the effective synthesis of tert-butanesulfinylimines of a variety of aldehydes using chiral tert-butanesulfinamides under green, sonochemical conditions. This method utilizes silica supported p-to

Full text of DOI:10.1080/00397911.2018.1540047

Synthetic Communications
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First sonochemical, simple and solvent-free
synthesis of chiral tert-butanesulfinimines using
silica supported p-toluenesulfonic acid
Rama Moorthy Appa, Jangam Lakshmidevi, Sana Siva Prasad, Baitinti
Muralidhar, Bandameeda Ramesh Naidu, Manchala Narasimhulu & Katta
Venkateswarlu
To cite this article: Rama Moorthy Appa, Jangam Lakshmidevi, Sana Siva Prasad,
Baitinti Muralidhar, Bandameeda Ramesh Naidu, Manchala Narasimhulu & Katta
Venkateswarlu (2019): First sonochemical, simple and solvent-free synthesis of chiral tert-
butanesulfinimines using silica supported p-toluenesulfonic acid, Synthetic Communications, DOI:
10.1080/00397911.2018.1540047
To link to this article: https://doi.org/10.1080/00397911.2018.1540047
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SYNTHETIC COMMUNICATIONSV
https://doi.org/10.1080/00397911.2018.1540047
First sonochemical, simple and solvent-free synthesis
of chiral tert-butanesulfinimines using silica supported
p-toluenesulfonic acid
Rama Moorthy Appaa, Jangam Lakshmidevia, Sana Siva Prasada,
Baitinti Muralidharb, Bandameeda Ramesh Naidua, Manchala Narasimhulua, and
Katta Venkateswarlua
aLaboratory for Synthetic and Natural Products Chemistry, Department of Chemistry, Yogi Vemana
b
University, Kadapa, India; Department of Chemistry, School of Advanced Sciences, Vellore Institute
of Technology, Vellore, India
ABSTRACT
ARTICLE HISTORY
Received 11 August 2018
Accepted 21 October 2018
A solvent-free, versatile procedure has been developed for the
effective synthesis of tert-butanesulfinylimines of a variety of alde-
hydes using chiral tert-butanesulfinamides under green, sonochemi-
cal conditions. This method utilizes silica supported p-toluenesulfonic
acid (pTSAꢀSiO2) as an efficient, safer and inexpensive catalyst under
aerobic conditions. The practicable simplicity, easy preparation of the
catalyst from readily available substances, high substrate scope,
excellent yields of products in short reaction times and environmen-
tally benign (solvent-free sonochemical) conditions are the excep-
tional assets of this finding.
KEYWORDS
pTSAꢀSiO2; tert-
butanesulfinimines;
aldehydes; sonochemical
synthesis; solvent-
free condition
GRAPHICAL ABSTRACT
Introduction
The synthesis induced by the application of ultrasound has been evidenced as promin-
ent green chemical technology in most of the studies. This synthesis frequently associ-
ates with the saving of energy (about 92% of energy saving over conventional
heating[1]), large increase in the yield of the products in shorter times, decrease or elim-
ination of the use of toxic, volatile and expensive organic solvents, and formation of
cleaner products with little or no by-products.[2] Besides easy handling, unique
CONTACT Venkateswarlu Katta
kvenkat@yogivemanauniversity.ac.in
Laboratory for Synthetic and Natural
Products Chemistry, Department of Chemistry, Yogi Vemana University, Kadapa 516 005, India.
Supplemental data for this article can be accessed on the publisher’s website.
ß 2019 Taylor & Francis Group Group, LLC

2
R.M. APPA ET AL.
mechanism and large scope for the invention of potential synthetic methods are the
inborn futures of ultrasound-assisted synthesis.
The chiral tert-butanesulfinylimines developed by Ellman et al. become prevailed sub-
strates in organic synthesis. These are proved as effective auxiliaries in several reports
for making chiral amine-containing precursors and biologically significant complex mol-
ecules.[3] Moreover, (S)- and (R)-tert-butanesulfinamides and (S)- and (R)-a-methylben-
zylamines are widely accepted chiral ammonia equivalents among all the chiral amine
substrates.[4] In the beginning, Ellman et al. synthesized tert-butanesulfinimines by
MgSO4 catalyzed condensation of chiral tert-butanesulfinamides with aldehydes.[5] In
addition to the condensation of sulfinamides with carbonyl compounds, oxidative con-
densation of alcohols, thiols, ethers, sulfides and disulfides with tert-butanesulfina-
mides[6] are the other and less widely used approach for the construction of chiral tert-
butanesulfinimines. But, due to the wide scope of catalysts and substrates, the condensa-
tion of aldehydes with sulfinamides is a simple and routine practice to access these imi-
nes.[6b] In this connection, CuSO4,[7] Ti(OEt)4,[7,8] Cs2CO3,[9] KHSO4,[10] Yb(OTf)3,[11]
NaOH/tBuOK,[12] BnZnBr,[13] Amberlist-15,[14] pyrrolidine-4Å MS,[15] TiCl4,[16]
[21]
B(OCH2CF3)3,[17] B(OiPr)3,[18] TPA,[19] HBF4ꢀDEE[20] and I2
were also developed as
catalysts in addition to MgSO4. Nevertheless, these methods required toxic and expen-
sive organic solvent as media and many of these require a large quantity of catalysts (up
to 3 mol ratios). Hence, it is highly inevitable to develop mild and convenient protocols
for the synthesis of tert-butanesulfinimines.
Solid acid catalysts especially those based on silica support are widely used in organic
synthesis and have substantial benefits like easy preparation from inexpensive materials,
simple separation of the catalyst after the completion of the reaction by filtration, the pos-
sibility of recyclability and high substrate selectivity.[22] In the green chemistry point-of-
view, the synthetic process may be green when no (toxic) solvent is employed.[23] The
ecological pollution may be decreased by the application of solvent-free conditions in syn-
thesis since about 85% of the chemicals used in pharmaceutical industry are solvents and
their recovery rate is hardly about 50–80%.[24] Inspired by this ideology, we report herein
a convenient and effective method for the synthesis of chiral tert-butanesulfinyl aldimines
in the presence of silica supported p-tlouenesulfonic acid (pTSAꢀSiO2) as an efficient cata-
lyst under solvent-free and aerobic conditions. Furthermore, this is the first report to use
ultrasound for the synthesis of tert-butanesulfinimines under solvent-free conditions. For
the best of our knowledge, there are only a few reports exists for the use of pTSAꢀSiO2 as
a sole catalyst and all of them deals the dehydration of secondary and tertiary alcohols.[25]
This protocol further establishes the scope of the catalysis of pTSAꢀSiO2.
Results and discussion
Initially we have studied the imine formation reaction of benzaldehyde (1a) (1 mmol)
with (R)-tert-butanesulfinamide (2) (1.05 mmol) under ultrasound irradiation using
pTSAꢀSiO2 (100 mg, 1.6 mol%)[25] in 5 ml of THF, DCM or methanol and found 73%,
50% and 35% of (R)-tert-butanesulfinylimine (3a) in 1 h (Table 1, entries 1–3). No
improvement was observed by increasing catalyst load to 2.0 mol% (Table 1,
entries 4–6).

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SYNTHETIC COMMUNICATIONSV
3
Table 1. Influence of solvent and load of catalyst.a
Sl. No.
Catalyst load
Solvent
THF
Time (min)
Isolated yield
1
1.6 mol%
1.6 mol%
1.6 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
2.0 mol%
0.8 mol%
1.2 mol%
1.6 mol%
2.0 mol%
60
60
60
60
60
60
60
60
60
60
60
60
180
180
90
60
60
90
60
60
30
30
73
50
35
74
53
35
32
15
43
15
15
18
15
12
79
88
89
78
38
62
98
98
2
3
4
5
6
7
8
9
10
11
12
13b
14b
15c
16d
17e
18d
19
20
21
22
DCM
Methanol
THF
DCM
Methanol
Et2O
EtOAc
Toluene
DMSO
DMF
Ethanol
THF
–
THF
THF
THF
–
–
–
–
–
aReaction conditions: 1a (1.0 mmol) and 2 (1.05 mmol), catalyst – pTSAꢀSiO2, solvent (5 mL) under ultrasound irradiation.
bReaction was conducted under no ultrasound irradiation at room temperature.
cReaction was conducted 70 ꢁC without ultrasound irradiation.
dReaction was conducted 80 ꢁC with no ultrasound irradiation.
eReaction was conducted 90 ꢁC with no ultrasound irradiation.
We further investigated the influence of the solvents on present conversion by using
non-polar solvents such as diethyl ether, ethyl acetate and toluene (Table 1, entries 7–9)
and polar solvents such as DMSO, DMF and ethanol (Table 1, entries 10–12). These
results indicated that the solvent, THF is suitable among the studied using 1.6 mol% of
pTSAꢀSiO2 (Table 1, entry 1). To understand the influence of ultrasound on the current
conversion we have conducted the reaction of 1a with 2 in THF or without THF at
room temperature and at different temperatures in the absence of ultrasound (Table 1,
entries 13–18). Very low yields of chiral sulfinylimine, 3a (15% and 12%) was observed
at room temperature in THF and solvent-free conditions in 3 h (Table 1, entries 13, 14).
The formation of high yield of 3a (88%) was observed at 80 ꢁC in THF in 1 h and no
improvement of the yield was observed by an increase of temperature (Table 1, entries
16, 17). This reaction was formed 78% of the yield of 3a in 1.5 h in solvent-free and
ultrasound-free conditions at 80 ꢁC (Table 1, entry 18).
The reaction was found to proceed to give excellent yield (98%) of 3a in the absence
of solvent in 0.5 h in the presence of 1.6 mol% of pTSAꢀSiO2 under ultrasound irradi-
ation (Table 1, entry 21). No improvement of the yield of 3a was found when increased
the catalyst load to 2.0 mol% (Table 1, entry 22) and incompleteness of the reaction was
observed with 0.8 mol% and 1.2 mol% of catalyst (Table 1, entries 19, 20). Furthermore,
the literature procedures require 10 mol% to 3 mol ratio of the catalysts for the conden-
sation of tert-butanesulfinamides with aldehydes[5,7–21] and hence, the present method
is remarkable by the use of very low catalyst load as 1.6 mol%.

4
R.M. APPA ET AL.
Table 2. Synthesis of (R)-tert-butanesulfinimines (3).a
Sl. No.
R
Time (min)
Product (3)
Isolated yield (%)
1
2
3
4
5
6
7
8
4-fluorophenyl (1b)
4-nitrophenyl (1c)
2-nitrophenyl (1d)
4-chlorophenyl (1e)
4-cyanophenyl (1f)
4-isopropylphenyl (1g)
4-bromophenyl (1h)
4-hydroxyphenyl (1i)
4-methoxyphenyl (1j)
2-hydroxyphenyl (1k)
4-N,N-dimethylaminophenyl (1l)
3,4-dimethylphenyl (1m)
1-naphthyl (1n)
25
3b
98
97
92
95
95
95
95
92
92
90
91
80
89
96
92
87
95
94
93
30b
45b
30b
30
3c[20]
3d[21]
3e[21]
3f[20]
3g[6a]
3h[20]
3i
35
30b
50
9
40
3j[7]
10
11
12
13
14
15
16
17
18
19
50
3k[20]
3l
45b
50
3m[6a]
3n[21]
3o[21]
3p[20]
3q
50
30
trans-styryl (1o)
2-phenylethyl (1p)
9-anthracenyl (1q)
2-furyl (1r)
2-pyridyl (1s)
3-indolyl (1t)
35
50b
40
3r[7]
3s[7]
3t
30
50b
aReaction conditions: 1 (1.0 mmol) and 2 (1.05 mmol), catalyst – pTSAꢀSiO2 (1.6 mol%), under ultrasound irradiation.
bFew drops of water was added to make the reaction contents wet.
With the optimized conditions in hand, the method was tested for an array of alde-
hydes (1) and the results are listed in Table 2. The aromatic aldehydes with electron
withdrawing groups such as fluoro, nitro, chloro, and cyano show excellent yields
(92–98%) of imines under present experimental conditions (Table 2, entries 1–5).
Aromatic aldehydes with electron releasing groups such as isopropyl, bromo, hydroxyl,
methoxy, N,N-dimethyl groups also displayed high yields of products (90–95%) in
30–50 min (Table 2, entries 6–11). Disubstituted benzaldehyde (3,4-dimethylbenzalde-
hyde, 1m) provides good yield (80%) of imine, 3 m in 50 min (Table 2, entry 12).
Aliphatic aldehydes such as trans-cinnamaldehyde (1o) and 3-phenylpropanal (1p)
show high conversion into imines under pTSAꢀSiO2 catalysis (Table 2, entries 14, 15).
Naphthalene-1-carbaldehyde (1n) and anthracene-9-carbaldehyde (1q) were also found

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5
Table 3. Synthesis of (S)-tert-butanesulfinimines (5).a
Sl. No.
R
Time (min)
Product (3)
Isolated yield (%)
1
2
3
4
5
6
7
8
9
4-nitrophenyl (1c)
4-cyanophenyl (1f)
4-N,N-dimethylaminophenyl (1l)
2,4-dimethoxyphenyl (1u)
4-hydroxyphenyl (1i)
2-phenylethyl (1p)
9-anthracenyl (1q)
2-furyl (1r)
30b
30
5a[10]
5b
96
95
90
82
90
92
89
95
90
40b
50b
50
5c[10]
5d
5e[19]
5f[10]
5g[19]
5h[10]
5i[10]
50
50b
35
2-pyridyl (1s)
35
aReaction conditions: 1 (1.0 mmol) and 4 (1.05 mmol), catalyst – pTSAꢀSiO2 (1.6 mol%), under ultrasound irradiation.
bFew drops of water was added to make the reaction contents wet.
as best substrates and yielded 3n and 3q with 89% and 87% (Table 2, entries 13,16).
Heteroaromatic aldehydes such 2-fural (1r), pyridine-2-carbaldehyde (1s) and indole-3-
carbaldehyde (1t) gave excellent yields of their corresponding imines (3r–3t) in
30–50 min (Table 2, entries 17–19). In some cases like CuSO4,[7] Yb(OTf)3,[11] and
I2[21], the yields of imines of heteroaryl aldehydes was found low.
The reaction for perfect solid system (e.g. in the case of 1c–1e, 1 h, 1 l, 1q, and 1t)
was found to give very low yields of imines under solvent-free conditions, but by mak-
ing reaction mixture wet by an addition of few drops of water the reaction proceeds to
give the best results (Table 2, entries 2–4, 7, 11, 16, 19). This observation evidences the
acoustic cavitation mechanism. The mechanism of acoustic cavitation is well studied
and is similar to that of reported in Ref.[2b,26]. The mechanism of the condensation of
an aldehyde with sulfinamides in the presence of pTSAꢀSiO2 is assumed to be identical
to that of acid catalyzed imine formation from amines and aldehydes, but it can be
accelerated by the adiabatic heating of the reactants in acoustic cavitation mechanism.
We then applied this method for the preparation of (S)-tert-butanesulfinyl aldimines.
In this connection a variety of aldehydes such as aromatic aldehydes [4-nitrobenzalde-
hyde (1c), 4-cyanobenzaldehyde (1f), 4-N,N-dimethylaminobenzaldehyde (1l), 2,4-dime-
thoxybenzaldehyde (1u), 4-hydroxybenzaldehyde (1i) and anthracene-9-carbaldehyde
(1q)], aliphatic aldehyde [3-phenylpropanal (1p)] and heteroaromatic aldehydes [2-fural
(1r) and pyridine-2-carbaldehyde (1s)] are found to reacts rapidly with (S)-tert-butane-
sulfinamide (4) to provide high yields of imines, 5a–5i (Table 3, entries, 1–9).

6
R.M. APPA ET AL.
Table 4. Synthesis of Schiff base (8).a
Sl. No.
Carbonyl compound
Amine (7)
Product (8)
Isolated yield (%)
1
2
3
4-cyanobenzaldehyde (1f)
4-cyanobenzaldehyde (1f)
cyclohexanone (6)
aniline (7a)
p-anisidine (7b)
m-toluidine (7c)
8a[27]
8b
70
74
65
8c[28]
a1f or 6 (1.0 mmol), 7 (1.05 mmol), pTSAꢀSiO2 (1.6 mol%) under ultrasound irradiation, time – 2h.
To ensure the formation of chiral enriched sulfinylimines, we measured the optical
25
26
rotation values of 3c, 3j and 3 s {[a]D – 64.64 (c ¼ 0.1, CHCl3), [a]D – 67.62 (c ¼ 0.1,
25
CHCl3) and [a]D – 163.00 (c ¼ 0.1, CHCl3)}, and are found to be very near to that of
20
20
23
reported {[a]D – 70 (CHCl3)[21], [a]D – 68 (CHCl3)[21] and [a]D – 172 (CHCl3)[7]}.
It indicates the formation of (R)-tert-butanesulfinylimines with optical purity. The
25
optical rotation value of enantiomer of 3c, that is, 5a was found to be [a]D þ 65.30
(c ¼ 0.1, CHCl3) with almost similar value but with opposite sign indicates the forma-
tion of tert-butanesulfinylimine with (S)-configuration.
The influence of the pTSAꢀSiO2 catalysis under ultrasound irradiation was also
studied for the preparation of imines of carbonyl compounds (Schiff bases). The reac-
tion of carbonyl compounds such as 4-cyanobenzaldehyde (1f) or cyclohexanone (6)
with aromatic amines (7a–7c) took comparatively extended time (2.0 h) to give good
yields (65–74%) of Schiff bases, 8a–8c (Table 4, entries 1–3).
A poor reusability of the catalyst was observed (only 32% of the product was formed)
when the catalyst reused on separation by dissolving the reaction mixture of 1a and 2
in ethyl acetate and filtering the catalyst followed by drying. The reaction mixture was
thus added a little amount of silica gel to get free-flowing solid, was loaded directly on
the silica gel packed glass column and chromatographed to get the product 3a with no
alteration of yield. This step avoids the catalyst separation step and hence saves from
[17]
the necessity of toxic organic solvent. In the case of pyrrolidine,[15] B(OCH2CF3)3,
[18]
and B(OiPr)3
the catalyst separation step is critical but an additional reagent (hypo)
is necessary for I2 catalysis.[21]
Conclusion
In summary, a versatile, simple and sonochemical method has been developed for the
construction of tert-butanesulfinylimines of aldehydes and imines from aldehydes and
tert-butanesulfinamides under solvent-free and aerobic conditions. The current method
obeys several principles of green chemistry, such as saving the energy by an application
of ultrasound, avoiding the toxic, flammable and expensive organic solvents as reaction
media, formation of products with excellent yields by eliminating the formation of by-
products (waste), unnecessitating catalyst separation step there by saving the organic
solvents and the application of low amounts (1.6 mol%) of solid acid catalyst. This find-
ing further establishes the scope of ultrasound and pTSAꢀSiO2 in the synthesis of chiral
tert-butanesulfinyl aldimines.

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SYNTHETIC COMMUNICATIONSV
7
Experimental
General experimental procedure for the preparation of imines
Aldehyde (1) (1.0 mmol), chiral tert-butanesulfinamide (2 or 4) (1.05 mmol) and
pTSAꢀSiO2 (1.6 mol%) in a round-bottomed flask were sonicated in a bath sonicator.
An increase of temperature up to 48 ꢁC was observed during irradiation. After comple-
tion (as indicated by TLC), the reaction mixture was added to a little amount of silica
gel to obtain a free flowing gel. The gel obtained was loaded on top of a small pad of
silica gel loaded in a glass column, which was eluted with a mixture of n-hexane and
ethyl acetate at varying ratios to obtain pure imine (3 or 5 or 8). The structures of the
imines were ascertained from their spectral (1H & 13C NMR and MS) data. The data of
reported imines were found identical with the literature values. The characterization
data of some of the representative compounds are provided here.
Compound 3t
1
Pale yellow solid. 93% yield. H-NMR (400 MHz, CDCl3) d: 9.14 (1 H, brs), 8.76 (1 H, s),
8.32 (1 H, d, J ¼ 8.6 Hz), 7.66 (1 H, d, J ¼ 2.8 Hz), 7.48ꢂ 7.42 (1 H, m), 7.36ꢂ 7.27 (2 H,
m), 1.31 (9 H, s). 13C-NMR (100 MHz, CDCl3) d: 156.3, 137.0, 132.5, 124.8, 124.0, 122.3,
122.2, 115.2, 111.6, 57.1, 22.5. HRMS (ESI) m/z: Calcd for C13H17N2OS [Mþ H]þ:
249.1056. Found: 249.1056.
Compound 5d
1
Light Yellow solid. 82% yield. H-NMR (400 MHz, CDCl3) d: 8.95 (1 H, s), 7.95 (1 H, d,
J ¼ 8.7 Hz), 6.55 (1 H, d, J ¼ 8.7 Hz), 6.45 (1 H, s), 3.87 (6 H, s), 1.24 (9 H, s). 13C-NMR
(100 MHz, CDCl3) d: 164.6, 161.1, 157.8, 129.7, 116.3, 105.9, 97.7, 57.3, 55.5, 55.4, 22.5;
HRMS (ESI) m/z: C13H20NO3S [M þ H]þ: 270.1158. Found: 270.1157.
Funding
The authors would like to thank SERB, New Delhi for financial support with [grant no. SB/FT/
CS-009/2014] and J. Lakshmidevi thanks SERB, DST, New Delhi for INSPIRE fellow-
ship [IF150772].
ORCID
Katta Venkateswarlu
http://orcid.org/0000-0002-1475-1126
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