A simple and rapid route to novel tetra(4-thiaalkyl)ammonium bromides

Article abstract of DOI:10.1039/c3ra44042g

A simple approach for the preparation of symmetrical quaternary ammonium bromides employing thiol-ene click chemistry is used to synthesize tetra(4-thiaalkyl)ammonium bromides. This approach allows the incorporation of a variety of alkyl moieties onto the nitrogen center with a one-step synthesis involving easy work-up, no side reactions and environmentally friendly reagents. To elucidate information regarding the behaviour of this novel class of compounds, comparisons to tetraalkylammonium analogues have been made. These include melting points, activity as phase-transfer catalysts, and conformational predictions from computational modelling. All results are consistent in indicating stronger bonding between the quaternary cation and the anion for the salts with 4-thiaalkyl chains as compared to those with n-alkyl chains.

Full text of DOI:10.1039/c3ra44042g

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A simple and rapid route to novel tetra(4-thiaalkyl)-
ammonium bromides†
Cite this: RSC Adv., 2013, 3, 24612
a
b
b
Richard A. O'Brien,* Christy Wheeler West,* Brian E. Hollingsworth,
a
a
c
a
Alexandra C. Stenson, Codey B. Henderson, Arsalan Mirjafari, Niloufar Mobarrez,
Kevin N. West, Kaila M. Mattson, E. Alan Salter, Andrzej Wierzbicki
and James H. Davis, Jr.
b
d
a
a
a
A simple approach for the preparation of symmetrical quaternary ammonium bromides employing thiol–
ene click chemistry is used to synthesize tetra(4-thiaalkyl)ammonium bromides. This approach allows the
incorporation of a variety of alkyl moieties onto the nitrogen center with a one-step synthesis involving
easy work-up, no side reactions and environmentally friendly reagents. To elucidate information
regarding the behaviour of this novel class of compounds, comparisons to tetraalkylammonium
analogues have been made. These include melting points, activity as phase-transfer catalysts, and
conformational predictions from computational modelling. All results are consistent in indicating
stronger bonding between the quaternary cation and the anion for the salts with 4-thiaalkyl chains as
compared to those with n-alkyl chains.
Received 31st July 2013
Accepted 7th October 2013
DOI: 10.1039/c3ra44042g
www.rsc.org/advances
structures based on availability of amines and alkyl halides.
Under neutral or acidic environments, quat salts are generally
Quaternary ammonium salts or “quat salts” have been the stable up to 150 C. Conversely, under basic conditions, their
Introduction
ꢀ
18
mainstay of research and industrial use for phase transfer
catalysis for over forty years beginning with the pioneering work
susceptibility to Hofmann elimination and thermal decompo-
ꢀ
sition at temperatures above 100 C limit the utility of quat salts
1
2
3
19
of Starks, Makosza and Brandstrom. In addition, the pub-
lished literature since then has described a multitude of uses
in applications requiring alkaline environments.
20
Previous work in our group described the thiol–ene click
4
5
for quat salts, including preparation of ionic liquids, antimi- reaction with tetraallylammonium bromide and 1-thiogylcerol
crobial agents, disinfectants, surfactants, fabric soeners,
to prepare a polyhydroxylated tetra(4-thiaalkyl)ammonium
antistatic agents, wood preservation, osmolytes, chemical bromide with subsequent metathesis to form the correspond-
capacitors, electrolytes for batteries, capping agents for ing ionic liquid. The use of the thiol–ene reaction to add four
copper nanoparticles, and plant growth retardants.
6
7
8
9
10
11
12
13
14
15
16
alkyl appendages to the ammonium cation was modelled aer
the approach by Hawker et al. for the thiol–ene based assembly
Quaternary ammonium compounds are typically prepared
by quaternization of tertiary amines with alkyl halides and by
21
of functionalized dendrimers.
Since the introduction of the principles of click chemistry by
17
anion exchange reactions on commercially available salts.
22
Such a synthetic route poses a limit on the variety of quat salt Kolb et al. in 2001, researchers in the areas of synthetic and
materials science have strived to incorporate these into drug
discovery, and into syntheses of small molecules, polymers, and
advanced materials. According to these guiding principles,
click chemistry includes syntheses that: (a) are modular, (b)
have high yields, (c) have by-products that are removable by
a
Department of Chemistry, University of South Alabama, Mobile, Alabama, USA,
23
3
6688. E-mail: robrien@southalabama.edu; Fax: +1 251-460-6370; Tel: +1 251-460-
427
7
b
Department of Chemical & Biomolecular Engineering, University of South Alabama,
Mobile, Alabama, 36688, USA. E-mail: cwwest@southalabama.edu; Fax: +1 251- non-chromatographic processes, (d) are stereospecic, (e) are
4
c
60-1485; Tel: +1 251-460-7463
insensitive to oxygen or water, (f) can be accomplished with
readily available starting materials and reagents, (g) can be
performed without solvent or in environmentally benign media,
and, (h) are wide in scope. Our intent was to incorporate as
many of these principles as possible into a synthetic route for a
new class of quaternary ammonium salts and to investigate
their use as phase transfer catalysts.
Department of Chemistry and Physics, Florida Gulf Coast University, Fort Myers,
Florida, 33965, USA
d
Department of Chemistry & Biochemistry and Materials Research Laboratory,
University of California-Santa Barbara, Santa Barbara, California, 93106, USA
†
Electronic supplementary information (ESI) available: Chemical shi data for
the NMR and ESI-MS analyses performed on the compounds synthesized. See
DOI: 10.1039/c3ra44042g
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Table 1 Preparation of tetra(4-thiaalkyl)ammonium salts
Experimental
Synthesis of tetra(4-thiaalkyl)ammonium bromides
For the present study, tetraallylammonium bromide was
selected as the starting precursor due to its ease of synthesis
from triallylamine with allyl bromide in a one-step process. As
in our previous work, we found methanol to be well-suited as Entry
the reaction solvent with a 1 : 0.5 : 2 equivalent ratio of alke-
20
a
R group
Yield (%)
1
2
3
4
5
6
7
8
9
–C
2
–C
3
–C
3
H
5
H
7
H
7
71
54
66
51
64
67
77
47
61
61
57
74
50
98
56
ne : photoinitiator : thiol being combined, followed by UV-
irradiation at ambient temperature for 6–8 hours in a sealed
(isopropyl)
quartz tube. The photochemical reactions were carried out
without a nitrogen purge of the quartz tube before irradiation.
The thiol–ene chemistry proceeded to completion via an anti-
Markovnikov addition to the allyl moieties and without obser-
–C H (n-butyl)
4
4
4
4
6
7
9
9
9
9
–C
–C
–C
–C
–C
H
H
H
H
H
(tert-butyl)
(isobutyl)
(anteisobutyl)
13
15
17
24
vation of any side reactions.
The photoinitiator (2,2-dimethoxy-2-phenylacetophenone) 10
and corresponding thiols were easily removed by washing with 11
hexane or diethyl ether, in which none of the starting or product
salts were soluble. No further purication was required to
obtain the desired salts. All of the thiols employed for our
studies were commercially available in high purity.
8
–C H
–C14
–C18
–CH
–CH
H
29
1
1
1
1
2
3
4
5
H
37
2
CH
C H
6 5
2
(CF
(benzyl)
11 (cyclohexyl)
2 7 3
) CF
2
–C
6
H
a
Isolated yield aer purication and drying, no optimization.
Spectroscopic characterization
Reactions were carried out in a 50 mL round-bottom ask
containing 2.0 mL of a saturated aqueous solution of potassium
cyanide (solid always visible), 10.00 mL of a 0.200 M solution of
Characterization of the tetra(4-thiaalkyl)ammonium bromides
1
13
19
using H, C and F NMR was performed on a JEOL 500 MHz
NMR with multi-nuclear capabilities using CDCl as the
3
1
5
-bromopentane in toluene, and an amount of catalyst equal to
deuterated solvent. Chemical shis, provided as ESI† to this
.0 mol% of the amount of 1-bromopropane in the system.
paper, are reported relative to TMS as the internal reference at
1
19
ꢀ
Temperature was controlled at 60 C, and the system was stirred
at 1400 rpm. Reaction analysis was carried out by taking peri-
odic 1 mL samples of the organic layer and analysing by an
Agilent 7820 gas chromatograph equipped with a thermal
conductivity detector. Each reaction was performed at least
three times with reproducible results.
0
.00 ppm for H and F NMR and 77.23 ppm (middle peak,
1
3
CDCl ) for C NMR. ESI-MS analyses were performed by ow-
3
injection on a Thermo Scientic mass spectrometer using HPLC
grade acetonitrile. As our interest was in the cations of these
salts, data was collected in positive ion mode.
Differential scanning calorimetry
As part of the characterization of these novel quaternary salts, Molecular modelling
we have examined the thermal behaviour of tetra(4-thiadodecyl)
ammonium bromide (entry 10, Table 1) and compared it with
that of a tetraalkylammonium bromide of equivalent chain
length, tetradodecylammonium bromide. The DSC curves were
obtained using a TA Instruments Q2000 with sample sizes
between 8 and 15 mg. Each compound was characterized using
both a conventional DSC method (solid line) with a scan rate of
Gas-phase geometry optimizations and the generation of elec-
trostatic potential/isodensity surfaces were performed using
Spartan '08 (Wavefunction, Inc., Irvine, CA). The isolated cations
and cation/bromide complexes were optimized using the B3LYP
density functional method and the 6-31G** basis set. All
structures were conrmed as stable by computing analytic
vibrational frequencies. The thiaalkyl-based cation required
tighter convergence criteria than default values in order to
eliminate all imaginary modes. The relative strength of binding
of bromide or cyanide to one cation over another was estimated
25
26
ꢀ
ꢁ1
ꢀ
1
0 C min and modulated DSC utilizing a scan rate of 3 C
ꢁ
1
ꢀ
min with a 1 C amplitude modulation at a period of 60
seconds.
by computing DE of the gas-phase exchange: DE ¼ E(cation
1
) +
Phase-transfer catalysed reactions
E(cation ) + E(cation $anion)).
2
$anion) ꢁ (E(cation
2
1
For the purposes of this preliminary study, our goal was to
establish that these tetra(4-thiaalkyl)ammonium bromides
exhibit activity as phase-transfer catalysts and to compare the
activity to that of a traditional tetraalkylammonium analog. For
Results and discussion
Synthesis
these reactions, we chose tetradodecylammonium bromide and In all but two salts, the new tetra(4-thiaalkyl)ammonium
its tetra(4-thiaalkyl)ammonium analogue (entry 10), having a bromides were isolated as solids, while viscous liquids were
sulfur atom replacing the fourth carbon from the nitrogen in obtained for entries 7 and 14 as shown in Table 1. The range
each chain.
of alkyl thiols employed shows the versatility of the thiol–ene
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reaction. Alkyl chains of various lengths ranging from ethyl
to octadecyl were incorporated, as well as branched and
cyclic groups and even peruorinated chains. Entry 14
employs benzyl mercaptan as the thiol and provides an
opportunity for incorporation of aryl rings into the quater-
nary framework as well as future substitution at the ortho,
meta and para positions. According to NMR analysis, the
reactions proceeded to completion, and the losses resulting
in reported yields result from a work-up method that has not
been optimized.
Entries 4 through 7 in Table 1 provide an interesting series,
based on the differences in structure of the butyl chains as part
of the overall thioether chain. The salts of entries 4, 5 and 6
Fig. 1 DSC curves for (A) tetradodecylammonium bromide and (B) tetra(4-
yielded solids while entry 7 was isolated as a viscous liquid. As thiadodecyl)ammonium bromide (entry 10). The solid lines were obtained using
2
7
we have shown in previous work with ionic liquids,
a
at simple temperature ramp and the dashed lines were obtained using modu-
lated DSC. The curves are offset on the y-axis to distinguish them from one
another, but are not scaled.
perturbation in the alkyl chain leads to lower packing efficiency
28
and lower surface area contact for entry 7 and thus to an
increased uidity – most probably as an entropy-dominated
effect.
The compound is observed to decompose upon melting, and
samples were observed to change colour from white to brown
Spectroscopic characterization
ꢀ
aer scans at temperatures higher than 210 C, even when the
1
13
The H and C NMR of each new salt fully comport with its
melting point was not reached. Because of this, the scans shown
in Fig. 1 were each performed using separate samples. This
decomposition temperature is signicantly higher than that of
the tetraalkyl analogue, which can be seen to exhibit endo-
1
13
proposed structure and formulation. Both H and C NMR
spectra of entries 1–15 unambiguously show complete removal
of the photoinitiator and thiols with complete thiolation, which
is readily ascertained on the basis of peak integration ( H) and
the disappearance of the olenic peaks ( H, C) of the starting
tetraallylammonium bromide. Of interest is the F NMR of
entry 13, which yielded peaks in the ꢁ80 to ꢁ126 ppm region for
1
ꢀ
thermic degradation beginning at around 160 C.
1
13
The higher melting point of tetra(4-thiadodecyl)ammonium
bromide relative to tetradodecylammonium bromide is indica-
tive of greater cation–anion interactions in the sulfur-contain-
ing salt. It is unclear whether the phase transition, which
certainly disrupts solid-phase interactions between ion pairs,
additionally disrupts internal ion-pair interactions or interac-
tions between a bromide of one ion pair and a hydrogen alpha
to the sulfur on another ion pair. Regardless, the higher melting
point is consistent with other measurements and calculations
in this work indicating enhanced interactions between the ions,
including the predictions made using quantum-based compu-
tational methods and the experimentally-observed reduction of
reaction rate. Furthermore, the greater thermal stability may be
due to the sulfur's proximity to the central nitrogen, electroni-
cally or sterically preventing degradation reactions. We are
currently investigating the thermal behaviour of these salts in
greater detail.
1
9
29
the presence of the C–F bonds for the uorinated tether.
Further elucidation of entries 1–15 using ESI-MS yielded
interesting results with fragmentation of the salts, though the
expected parent ions were not seen. As a representative
example, entry 10, having four dodecyl chains (m/z ¼ 763),
produces a high-mass ion with m/z ¼ 471, consistent with a
structure in which two of the four dodecyl chains have been
cleaved, leaving two intact while re-generating two of the orig-
inal N-tethered allyl moieties. This retro-thiolation is likely
promoted by severe steric crowding around the central nitrogen
of these ions. This type of fragmentation pattern was observed
20
in our previous work.
Differential scanning calorimetry
The curves obtained using differential scanning calorimetry are
provided in Fig. 1. The most prominent features of each curve
are the sharp melting points, which are followed by broad
Phase-transfer catalysed reactions
thermal decomposition. For the tetradodecylammonium For the novel quaternary ammonium salts, phase-transfer
bromide, the melting point as measured by the transition onset catalysis (PTC) offers both a potential application and a tool for
ꢀ
from the modulated DSC scan was 86.3 C. This value is in good evaluating behaviour. Because the mechanism of the PTC cycle
ꢀ
30
32
agreement with a previously reported melting point (89 C), as is generally well understood, the activity of these salts
are the two lower temperature solid/solid mesophase transi- compared to their traditional analogues provides a means of
31
tions. For the tetra(4-thiadodecyl)ammonium bromide, the characterizing them. A commonly used reaction for screening
melting point is signicantly higher, 234 C from the modu- the activity of phase transfer catalysts is the nucleophilic
lated scan and 238 C from the traditional scan. This range is substitution of cyanide for bromide. In this work, we have
ꢀ
ꢀ
33
consistent with visual determination of the melting point of chosen as a model reaction the cyanide displacement on
ꢀ
2
34–235 C.
1-bromopentane:
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This reaction takes place by a well-established mechanism in
+
ꢁ
ꢁ
which the Q exchanges the Br for the CN in the aqueous
phase, transfers it to the organic phase where a second-order
nucleophilic substitution takes place, generating the product
+
ꢁ
and regenerating the Q Br , which partitions back into the
aqueous phase to restart the cycle.
34
As shown in Fig. 2, results indicate that the quaternary
ammonium salts containing sulfur yield slower reaction rates
than those without. However, the reaction rate is signicantly
increased compared to that without a catalyst present. With no
catalyst, approximately 3% conversion of the 1-bromopentane
was achieved in a 24 hour period.
Fig. 3 Pseudo-first-order integrated rate expression [ln(1/1 ꢁ X)] (X ¼ conver-
ꢀ
sion) for the reaction of 1-bromopentane to hexanenitrile at 60 C using (A)
tetradodecylammonium bromide and (-) tetra(4-thiadodecyl)ammonium
bromide (entry 10) as phase-transfer catalysts.
According to the established mechanism for this reaction,
the rate law should be
Table 2 Rate constants for the reaction of 1-bromopentane to hexanenitrile at
ꢀ
6
0
C using tetradodecylammonium bromide and tetra(4-thiadodecyl)ammo-
+
ꢁ
org
rate ¼ k [Q CN ] [C H Br]
2
5
11
nium bromide (entry 10) as phase-transfer catalysts. The second order constants
(
k
2
) are evaluated by accounting for the concentration of catalyst that is factored
0
Because the catalyst concentration is constant, pseudo-rst- into the pseudo-first order constants (k
1
)
order reaction kinetics are expected:
k0
(s
ꢁ1
)
k
(L mol
ꢁ1 ꢁ1
s )
Catalyst
1
2
0
rate ¼ k
1
[C
5
H11Br]
ꢁ5
ꢁ3
ꢁ3
Tetradodecylammonium bromide
Tetra(4-thiadodecyl)ammonium
9.5 ꢂ 10
3.9 ꢂ 10
9.5 ꢂ 10
3.9 ꢂ 10
ꢁ
5
In this pseudo-rst-order rate law, a constant catalyst bromide
concentration has been factored into the rate constant.
The relevant plots for the integrated form of the pseudo-

rst-order rate expression are provided in Fig. 3. The linearity
order of magnitude. As the rate-limiting step involves the
attack of the cyanide ion on the 1-bromopentane, the rate of
reaction depends upon the nucleophilicity of the cyanide ion.
We believe that a tighter ion pairing reduces the charge
density on the cyanide anion, rendering it less nucleophilic
and thus reducing the reaction rate. This is consistent with the
results of preliminary quantum calculations performed on the
smaller pentyl and 4-thiapentyl model systems shown in Fig. 4
and discussed below.
of these plots is consistent with the mechanistic prediction.
Rate constants, evaluated using the slope of the best linear t
lines in Fig. 3, are shown in Table 2. The rate constant for the
tetra(4-thiadodecyl)ammonium bromide (entry 10) is less than
that for tetradodecylammonium bromide, but is of the same
We conclude that while the reactions employing the
4-thiaalkylammonium salts as phase-transfer catalysts are
slower than those with the traditional quat salts, the reduction
in rate is not signicant enough to disqualify compounds of this
novel class as candidates for PTC. There may be cases where
their increased stability makes them more attractive, or exam-
ples where the increased binding can be manipulated to control
reaction selectivity. Using the insight gained in this study, we
are continuing to explore potential applications exploiting both
their reactivity in exchange reactions and their stability relative
to traditional quat salts.
Molecular modelling
ꢀ
Fig. 2 Conversion of 1-bromopentane to hexanenitrile at 60 C using (A)
Molecular modelling was performed using the model cations
tetrapentylammonium and tetra(4-thiapentyl)ammonium. The
tetradodecylammonium bromide and (-) tetra(4-thiadodecyl)ammonium
bromide (entry 10) as phase-transfer catalysts.
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C–S bonds permits these hydrogen–bromide contacts and yet
another hydrogen–bromide contact from the terminal methyl
group at right. At the same time, the sulfur lone pair regions are
directed away from the bromide ion. The resulting charge
delocalization of the bromide ion is far more pronounced in
this complex, wherein the sulfur atoms serve as sinks for the
anion's negative charge, conveyed through several contacting
thiaalkyl hydrogens.
Our calculations show that the bromide ion binds more
favorably to the model 4-thiapentyl cation than to the
ꢁ
1
pentyl cation by 2.5 kcal mol . The stronger thiaalkyl–anion
interaction explains the signicantly higher melting point of
tetra(4-thiadodecyl) ammonium bromide over that of tetrado-
decylammonium bromide. Follow-up model calculations in
which cyanide is the complexed counter-ion yield completely
analogous results, again with greater charge delocalization of
the anion in the 4-thiapentyl complex as compared to the pentyl
ꢁ1
complex, and stronger binding by 3.8 kcal mol . The observed
reduction in the intrinsic reaction rate for the phase-transfer
catalyzed displacement reaction is explained both by the tighter
binding of cyanide and the reduced negative charge on cyanide
accomplished by the thiaalkyl groups, which result in a weak-
ening of cyanide's nucleophilicity.
Fig. 4 Optimized geometries and electrostatic potential energy maps of model
pentyl- and 4-thiapentyl-based quaternary ammonium cations (top) and bromide
salts (bottom). Minima, maxima, and other selected values of the electrostatic
ꢁ1
potential energy (kJ mol ) are indicated. Maxima for the bromide complexes are
located on the backside surfaces.
Conclusions
In conclusion, we have demonstrated a rapid and simple
approach for the construction of symmetrical tetra(4-thiaalkyl)-
ammonium bromides using the thiol–ene click reaction, which
offers considerable promise as a means to fabricate structurally
elaborate quat salts. The reactions proceed quickly and cleanly,
and provide acceptable yields of products without optimization.
Based on the observed thermal behaviour, the PTC activity, and
our quantum-based computations, the tetra(4-thiaalkyl)ammo-
nium cations exhibit stronger interactions with their associated
anions than do their tetraalkylammonium counterparts.
Further studies involving the thermal stability of these novel
salts, their partitioning between aqueous and organic phases,
and their utility in various applications are ongoing in our
laboratories.
lowest-energy conformations of these model cations and their
bromide complexes are shown in Fig. 4. At the le side of Fig. 4
are the tetrapentylammonium cation (top) and its bromide
complex (bottom). At right are the tetra(4-thiapentyl)ammo-
nium cation (top) and its bromide complex (bottom). The
depictions of the isolated cations show a quasi-tetrahedral
arrangement of the pentyl (or 4-thiapentyl) chains about the
nitrogen center. As expected, there is a gradual falling off of
positive charge along the pentyl chains. In contrast, the
electronegative sulfur atom of the thiapentyl chains causes the
C3 methylene and terminal C5 methyl regions to be far more
positive than their alkyl counterparts; the conformation about
ꢀ
the C2–C3 bonds (d(C1–C2–C3–S) ¼ 70 ) allows each sulfur
atom to interact with a C2 hydrogen of a neighbour thialkyl
chain.
Acknowledgements
The bottom images of Fig. 4 indicate substantial differences
between the corresponding bromide complexes. On the le, the
bromide ion is electrostatically bound to the most positive
region of the pentyl-based cation, contacting two C1 and one C2
hydrogens; one pentyl moiety has what is usually a higher-
energy rotational conformation about its C2–C3 bond to achieve
contact between a C4 hydrogen and the bromide ion. Higher-
quality calculations (B3LYP/6-311+g(d,p)) indicate that this
This material is based upon work supported by the National
Science Foundation under Grant number #1133101.
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ꢁ
1
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)
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can approach the bromide ion; the rotational exibility of the
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