Synthesis and properties of l-valine based chiral long alkyl chain appended 1,2,3-triazolium ionic liquids

Article abstract of DOI:10.1039/c4ra05037a

The increasing importance of ionic liquids (ILs) in various strata of chemical sciences is largely due to the fact that modification in the architecture of the cation and/or the anion imparts favorable and specific properties to an IL. Consequently, there is a need to develop new ILs with different functionalities. A series of l-valine based alkyl chain-appended 1,2,3-triazolium ILs (alkyl = hexyl, octyl, dodecyl, cetyl and octadecyl) with iodide and hexafluorophosphate anions, respectively, are synthesized and characterized. These new ILs show optical activity and hence are termed chiral ILs (CILs). All ten CILs are room temperature ILs (RTILs) as their melting points, obtained from differential scanning calorimetry (DSC), are found to be below ambient temperature. Thermogravimetric analysis indicates these CILs to have adequate thermal stability. The longer alkyl chain containing CILs exhibit facile self-aggregation when dissolved in ethanol. There is a hint of pre-micellar aggregation by short alkyl chain possessing CILs along with the long alkyl chain containing ones. These CILs demonstrate weak absorbance and emission of UV-Vis radiation and hence can be considered ideal solvents for photochemical applications. These CILs, consisting of different functionalities, possess interesting properties and have potential to be used in many areas of chemistry. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra05037a

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Synthesis and properties of L-valine based chiral
long alkyl chain appended 1,2,3-triazolium ionic
liquids†
Cite this: RSC Adv., 2014, 4, 33478
*
*
Roli Mishra, Shubha Pandey, Shruti Trivedi, Siddharth Pandey
*
and Pramod S. Pandey
The increasing importance of ionic liquids (ILs) in various strata of chemical sciences is largely due to the fact
that modification in the architecture of the cation and/or the anion imparts favorable and specific properties
to an IL. Consequently, there is a need to develop new ILs with different functionalities. A series of L-valine
based alkyl chain-appended 1,2,3-triazolium ILs (alkyl ¼ hexyl, octyl, dodecyl, cetyl and octadecyl) with
iodide and hexafluorophosphate anions, respectively, are synthesized and characterized. These new ILs
show optical activity and hence are termed chiral ILs (CILs). All ten CILs are room temperature ILs (RTILs)
as their melting points, obtained from differential scanning calorimetry (DSC), are found to be below
ambient temperature. Thermogravimetric analysis indicates these CILs to have adequate thermal stability.
The longer alkyl chain containing CILs exhibit facile self-aggregation when dissolved in ethanol. There is
a hint of pre-micellar aggregation by short alkyl chain possessing CILs along with the long alkyl chain
containing ones. These CILs demonstrate weak absorbance and emission of UV-Vis radiation and hence
can be considered ideal solvents for photochemical applications. These CILs, consisting of different
functionalities, possess interesting properties and have potential to be used in many areas of chemistry.
Received 28th May 2014
Accepted 18th July 2014
DOI: 10.1039/c4ra05037a
www.rsc.org/advances
are reported to result in widely varying properties of ILs.3
Therefore, various functionalized ILs, termed ‘task-specic ILs
Introduction
(TSILs)’, may be obtained from a given parent structure by
generating wide range of compounds by simply varying the
cation, its substitution pattern and/or the anion.25 Conse-
quently, efforts on designing and developing ILs with modied
functionalities have increased over the last few years. In this
regard, ILs possessing one or more chiral centers on their cation
or anion, called chiral ILs (CILs), are of special interest as
chirality plays an important role in chemistry. Due to potential
additional applications associated with CILs in analytical
separation, in chiral discrimination, in asymmetric synthesis,
in chiral chromatography, and as chiral solvents; a growing
number of CILs have been designed, synthesized and utilized
recently.22,23 For synthesis of CILs, among others, amino acids
offer a facile platform as amino acid-based CILs are not only
biodegradable but are also biocompatible and less toxic.22,23 It is
proposed that amino acid-based 1,2,3-triazolium CILs may help
enhance the greener aspects of these versatile substances.26
Application potential of ILs can be signicantly enhanced by
combining their interesting and useful aspects with surface-
active properties. Subsequently, a subclass of ILs, where the
anion or the cation of an IL consists of a charged hydrophilic
head group and a long chain hydrophobic tail, shows some
differences from common ILs and are coined surface-active ILs
(SAILs).27 These ILs possess surface-active properties similar to
traditional surfactants and can assemble into various aggregates/
Ionic liquids (ILs) have attracted increasing interest as versatile
alternate substances in chemical sciences over the last decade
and a half,1–7 especially in synthesis, in catalysis, in separation
processes, in biotechnology, as lubricant additives, as electro-
lytes, etc.8–20 Various applications of ILs arise from the unique
combination of properties they offer, such as, near nonvolatility,
low toxicity, excellent thermal stability, high ionic conductivity,
excellent reusability, good solubility, and a broad electrochemical
stability window.21–23 They are unique in the sense that they are
organic salts composed entirely of ions that are liquid at ambient
conditions (or below 100 ꢀC).24 As members of a subclass, the ILs
that are liquid at ambient conditions, called room temperature
ionic liquids (RTILs), are obviously more desirable and are better
projected as solubilizing media.
ILs are called ‘designer solvents’ because their physico-
chemical properties can be tuned for a specic application by
different combination of cations and anions.22,24b,c Structural
changes/modications in both cation and anion architecture
Department of Chemistry, Indian Institute of Technology Delhi, Hauz Khas, New Delhi –
110016, India. E-mail: shubhapandey@hotmail.com; sipandey@chemistry.iitd.ac.in;
pramod@chemistry.iitd.ac.in
† Electronic supplementary information (ESI) available: Spectral characterization
data includes 1H NMR, 13C NMR, mass, UV-Vis absorbance and uorescence
emission spectra. See DOI: 10.1039/c4ra05037a
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from L-valine amino acid according to reported literature proce-
dures (Scheme 1).32,33 Alkylazides of hexyl, octyl, dodecyl, cetyl and
octadecyl (3a–e), respectively, were synthesized from stirring their
bromides with sodium azide in DMF at 60 C.34 Click reaction35
ꢀ
of compound
2 with various azides (3a–e) in equimolar
amounts resulted in their 1,2,3-triazole derivatives (Boc-val-alkyl-
Tr): N-(tert-butoxycarbonyl)-L-valinyl-4-aminomethyl-1-hexyl-1,2,3-
triazole (4a), N-(tert-butoxycarbonyl)-L-valinyl-4-aminomethyl-1-
octyl-1,2,3-triazole
(4b),
N-(tert-butoxycarbonyl)-L-valinyl-4-
aminomethyl-1-dodecyl-1,2,3-triazole (4c), N-(tert-butoxycarbonyl)-
L-valinyl-4-aminomethyl-1-cetyl-1,2,3-triazole (4d) and N-(tert-
butoxycarbonyl)-L-valinyl-4-aminomethyl-1-octadecyl-1,2,3-triazole
(4e) in good yields (Scheme 1). Methylation of Boc-val-alkyl-Tr
compounds (4a–e) with methyl iodide resulted in triazolium-
based chiral ionic liquids (CILs) (Boc-val-[Cn-Tr][I], 5a–e) in nearly
quantitative yields and high purity.36 To understand the effect of
anion on the properties of CILs, anion exchange reactions using
ammonium hexauorophosphate on compounds 5a to 5e were
performed to achieve desired CILs with hexauorophosphate
anion (Boc-val-[Cn-Tr][PF6], 6a–6e).37 As the PF6 anion based ILs can
hydrolyze and form HF over the time,38 we have used freshly
prepared 6a–6e in all subsequent work reported in this paper. All
synthesized CILs (5a–e and 6a–e) are yellowish-orange, highly
viscous liquids at room temperature except dodecyl CILs (5c and
6c) that are light yellow in color. The fact that all the ILs synthe-
sized are chiral in nature is conrmed by measuring their optical
rotation (see Experimental section). They all show appreciable
optical rotation in accordance with the values reported for L-valine
and related compounds in the literature (vide infra).33a
Fig. 1 Structures of the new CILs.
morphologies, such as, micelles, vesicles, bilayers, liquid crystals,
etc.28,29 In some instances, the SAILs have exhibited improved
surface-active properties as compared to their more traditional
counterparts having similar chain lengths.29 As a result, there is a
constant need to develop newer SAILs with better properties.
Among all classes of ILs, imidazolium cation containing ILs
have dominated the eld due to their high conductivity, low
viscosity and wide liquid range deemed important for various
applications.21,30,31 Despite their wide applications, these ILs have
some clear limitations. Apart from the minor toxicity issues,26 1,3-
dialkylimidazolium salts undergo deprotonation at carbon-2
under strong basic conditions. 1,2,3-Triazolium-based
compounds, in this regard, may turn out to be advantageous as
they may not have hydrogen with such high acidity attached to
ring-carbon anked by two nitrogen atoms.21,30a Further, 1,2,3-
triazoliumm-based compounds are highly desired as energy-rich
materials and are alternative to many conventional materials
used in explosive and propellant industry as molecular nitrogen
is the primary decomposition product of 1,2,3-triazole ring
rendering them as environmentally-acceptable alterantives.31 In
this paper, we report a class of new CILs synthesized starting from
L-valine, an a-amino acid (Fig. 1). These CILs possess alkyl chains
of variable length that may impart surface-active properties to
these ILs, connected through 1,2,3-triazolium moiety. We have
investigated thermal and optical properties of these new CILs. We
also report the aggregation behavior of these CILs. These new
long chain containing CILs may show extended applications in
separation and chromatography as well as in colloid chemistry.
The structures of the synthesized CILs (Boc-val-[Cn-Tr][X]) as
well as azides (3a–e) and triazoles (4a–e) were established by the
analysis of their 1H and 13C NMR spectra, and IR and mass
spectra (Fig. S1 to S45, ESI†). For example, in the IR spectrum of
4d the formation of 1,2,3-triazole ring was conrmed by
appearance of characteristic peaks of –N]N– and –C]C– at
1526 cmꢁ1 and 1468 cmꢁ1, respectively, as well as disappear-
ance of alkyne peak of 2 at 2117 cmꢁ1 and azide peak of 3d at
2092 cmꢁ1 (please refer to ESI†). The formation of 1,2,3-triazole
ring was further conrmed by appearance of a new peak of
1
triazole ring proton at 7.47 ppm in H NMR spectrum and two
peaks at 139 and 144 ppm in 13C NMR spectrum of 4d. As soon
as 3d was converted to its 1,2,3-triazole derivative, the peak at
1
2.21 ppm which corresponded to the alkyne proton in the H
NMR spectrum of 3d, completely disappeared from 1H NMR of
4d. When 4d was converted to its triazolium derivative 5d, a
distinct downeld shi in the triazole ring proton from 7.47
ppm to 8.85 ppm was observed along with the appearance of a
new peak of methyl proton (–N+–CH3) at 4.28 ppm as singlet
indicating the methylation of 1,2,3-triazole ring (Fig. 2).
Further, anion exchange of 5d with hexauorophosphate
caused 1,2,3-triazolium ring proton to again shi upeld from
1
8.85 to 8.57 ppm in the H NMR spectrum of 6d (Fig. 2).
Results and discussion
Synthesis and characterization of novel CILs
Thermal properties of CILs
Starting materials, 1 [N-(tert-butoxycarbonyl)-L-valine] and 2 [N- One of the most crucial aspects of an IL is its thermal stability.
(tert-butoxycarbonyl)-L-valine N0-propargylamide] were synthesized Thermal stabilities of the ten newly synthesized CILs (5a–e and
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Scheme 1 Details of the synthesis of triazolium-based CILs.
Table 1 Thermal properties of Boc-val-[Cn-Tr][X] CILs determined
from TGA and DSC analysis
a
b
CIL
Tdcp /ꢀC
Tdcp /ꢀC
Tm/ꢀC
Boc-val-[C6-Tr][I]
Boc-val-[C6-Tr][PF6]
Boc-val-[C8-Tr][I]
Boc-val-[C8-Tr][PF6]
Boc-val-[C12-Tr][I]
Boc-val-[C12-Tr][PF6]
Boc-val-[C16-Tr][I]
Boc-val-[C16-Tr][PF6]
Boc-val-[C18-Tr][I]
Boc-val-[C18-Tr][PF6]
93
90
91
80
93
102
110
210
90
132
142
157
120
130
129
180
226
164
213
ꢁ9
ꢁ5
ꢁ7
ꢁ6
+7
ꢁ9
ꢁ6
+18
ꢁ5
ꢁ10
110
a
Tdcp is the temperature obtained from the TGA traces at which 10%
b
total weight loss had occurred. Is the temperature obtained from
1
Fig. 2 Partial H NMR spectra of compounds 4d, 5d and 6d showing
TGA at which 19% total weight loss, corresponding to the loss of BOC
protecting group, had occurred.
shifting in triazole and triazolium group protons.
6a–e) are investigated using thermogravimetric analysis (TGA). higher thermal stability than ILs with iodide and TFOꢁ
The thermal decomposition temperature (Tdcp) for each CIL is anions.36,39 Considering the fact that BOC is a good leaving
obtained at the onset of the decomposition dened by 10% group, Tdcp is also estimated at 19% mass loss which corre-
weight loss. The Tdcp thus obtained are reported in Table 1. A sponds to the loss of the protective BOC group (Table 1). It is
careful examination of the data reveals that among all CILs, interesting to note that our CILs when functionalized with free
interestingly, the two cetyl chain containing CILs exhibit better amine show considerably improved thermal stabilities as
thermal stability (Fig. 3). Further, Boc-val-[C16-Tr][PF6] not only compared to the BOC-protected CILs.40 The thermal stabilities
shows highest thermal stability among all ten CILs (Tdcp near- of the free amine functionalized CILs may be considered
ing 210 ꢀC) but it also shows well-behaved mass loss prole signicant.
occurring in a single step. Further, except for cetyl CILs, the
Different scanning calorimetry (DSC) is perhaps the most
thermal stabilities of the other CILs appear to be not too appropriate and used technique to obtain quantitative infor-
affected by the identity of the anion – Tdcp of Boc-val-[C16-Tr] mation regarding important phase transitions, such as, glass
[PF6] is almost double that of Boc-val-[C16-Tr][I]. Thermal transition, melting/freezing, crystallization/recrystallization,
stabilities of higher chain length (octadecyl) as well as lower etc. The DSC traces of the ten CILs, in general, do not show
chain length (hexyl, octyl, and dodecyl) containing CILs are any glass transition within the temperature range of investiga-
clearly not as good as that of Boc-val-[C16-Tr][PF6]. It is generally tion (ꢁ80–150 ꢀC). In order to nd whether our CILs can be
observed that ILs with bulkier anions (BF4, PF6, TF2N) show classied as RTILs as opposed to just ILs, we estimated the
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chain containing CIL, Boc-val-[C8-Tr][I], and two long alkyl
chain appended CILs, Boc-val-[C16-Tr][I] and Boc-val-[C16-Tr]
[PF6]. Due to the fact that our CILs are only sparingly soluble in
water, we investigated their self-aggregation behavior, if any,
when dissolved in ethanol. Self-aggregation of surfactants in
non-aqueous media is a topic of active research as water is not
always the media of interest in several applications.41 We
investigated concentration-dependent behavior of the three
aforementioned CILs dissolved in ethanol at ambient condi-
tions using uorescence response of pyrene, a well-known
uorescence solvatochromic probe.
Molecular uorescence from an appropriate uorophore is
well-suited to furnish information regarding self-aggregation of
long-chain molecules owing to the higher sensitivity and
orthogonality of information inherent to uorescence tech-
niques.42 Pyrene is one of the most widely used neutral uo-
rescence probes for polarity studies.43–45 The pyrene solvent
polarity scale (Py I1/I3) is dened by its I1/I3 emission intensity
ratio, where I1 is the intensity of the solvent-sensitive band
arising from the S1(v ¼ 0) / S0(v ¼ 0) transition and I3 corre-
sponds to the solvent-insensitive S1(v ¼ 0) / S0(v ¼ 1) transi-
tion. The I1/I3 ratio increases with increasing solvent dipolarity
and is a function of both the solvent dielectric (3) and the
refractive index (n) via the dielectric cross term, f(3, n2). In
ethanol, in the absence of any self-assembling moieties, the Py
I1/I3 is fairly high due to the high polarity of ethanol. As self-
assembling molecules are added to ethanol the I1/I3 starts to
decrease rapidly till the critical aggregation concentration (cac)
is reached aer which the decrease in I1/I3 is not so rapid
anymore Fig. 4. Once the aggregates are formed, non-polar
pyrene partitions into the aggregates resulting in gradual
decrease in I1/I3. The inection point is considered to be the cac.
Pyrene I1/I3 as a function of CIL concentration is presented in
Fig. 5. While pyrene I1/I3 in the higher concentration regime do
not show any inection point for Boc-val-[C8-Tr][I], the short
alkyl chain CIL; for Boc-val-[C16-Tr][I] and Boc-val-[C16-Tr][PF6],
the inection points are clearly present. From the inection
points, the cac for Boc-val-[C16-Tr][I] and Boc-val-[C16-Tr][PF6]
are estimated to be 12.5(ꢂ1.5) mM and 8.7(ꢂ1.2) mM, respec-
tively. It is clear that these CILs self-aggregate at fairly low cac
values in ethanol. The lower cac for Boc-val-[C16-Tr][PF6] as
compared to that for Boc-val-[C16-Tr][I] implies more efficient
and favorable aggregation for the cetyl chain appended
compounds when [PF6]ꢁ as opposed to [I]ꢁ is the counter ion.
Fig. 3 TGA profile of Boc-val-[C16-Tr][I] and Boc-val-[C16-Tr][PF6]
under nitrogen at a scanning speed of 10 ꢀC minꢁ1
.
melting points (Tm) of the ten Boc-val-[Cn-Tr][X] CILs using DSC.
The Tm values are summarized in Table 1. A careful examination
of the Tm of all ten CILs reveals them all to be RTILs as all the
CILs have melting points below ambient temperature of 25 ꢀC.
The CIL Boc-val-[C16-Tr][PF6], which is the most stable ther-
mally, is observed to have the highest Tm. Except for Boc-val-
[C16-Tr][PF6] (Tm ¼ +18 ꢀC) and Boc-val-[C12-Tr][I] (Tm ¼ +7 ꢀC),
all other CILs have fairly similar Tm values that vary from ꢁ10 ꢀC
to ꢁ5 ꢀC. This is surprising as it is usually conceived that more
energy is required to overcome increased van der Waals inter-
actions as the mass and chain length of the compound is
increased.31 We believe the complexity of the interactions
present in our CILs translates into observation of no clear-cut
trend in the melting point of our CILs as the appended chain
length is increased. The extent of coiling of alkyl chain may
contribute to the observed melting behavior. The similar
melting points may imply the L-valine along with the 1,2,3-tri-
azolium functionalities to control the interactions within the
CILs as opposed to the alkyl carbon chains as far as phase
transition involving melting is concerned. The inherent
complexity of the interactions presents in our CIL system on
molecular level is revealed nonetheless.
Self-aggregation of CILs
Many such long alkyl chain appended ILs have gained addi- These cac values are similar to those reported for similar alkyl
tional popularity due to their surface active properties. As chain containing conventional cationic surfactants dissolved in
mentioned earlier, the SAILs show conventional surfactant like ethanol.41e,f It is clear that while the short alkyl chain appended
behavior.27,28 However, selected properties of some of the SAILs CIL does not self-aggregate within ethanol at higher concen-
are found to be superior to the conventional surfactants.29 Many trations, the long alkyl chain containing CILs do, and thus, can
such SAILs are known to form various self-aggregated assem- effectively be called SAILs.
blies, such as, normal micelles, reverse micelles, bilayers,
Interestingly, a careful examination of the data presented in
microemulsions, etc.28 Effective coupling of conventional Fig. 5 reveals presence of inection points for all three CILs at
advantages of RTILs with their surface-active capabilities much lower concentrations as well. The conventional cationic
enhances the overall applications of such SAILs. In order to surfactants of similar chain lengths are not known to self-
explore whether our CILs self-aggregate at ambient conditions aggregate in nonaqueous media at such low concentrations.41
and what are the roles of the alkyl chain length as well as the In general, amphiphilic molecules are known to form aggre-
counter ion (i.e., the anion of the CIL), we chose a short alkyl gates above a well-dened critical aggregation concentration
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Fig. 4 Normalized fluorescence spectra of pyrene (10 mM) dissolved in
Boc-val-[C16][PF6] solution in ethanol at ambient conditions. lex ¼ 337
nm, slits ¼ 1/1, spectrum in ethanol is in the absence of Boc-val-[C16
]
[PF6], pre-cac is 4.0 mM Boc-val-[C16][PF6], and post-cac is 9.4 mM
Boc-val-[C16][PF6].
(cac). However, during the years, there have been several
experimental indications, as well as theoretical ones, for the
appearance of aggregates at concentrations well below the cac –
a phenomenon referred to as premicellar aggregation. Presence
of premicellar aggregates has been conrmed, in particular, by
uorescence correlation spectroscopy (FCS) experiment, which
appears to have afforded direct observation of premicellar
aggregates at concentrations four times lower than the macro-
scopically determined cac.46 Such premicellar aggregates are
considered metastable and are rather short-lived. There is
ample evidence of the presence of premicellar aggregates in
several cases in current literature.46 It appears our CILs,
including the short chain containing Boc-val-[C8-Tr][I] which
does not show cac, form premicellar aggregates as evident by
the inection in the pyrene I1/I3 at much lower concentrations
(Fig. 5A). For, Boc-val-[C8-Tr][I], Boc-val-[C16-Tr][I] and Boc-val-
[C16-Tr][PF6], the concentrations for the onset of the premicellar
aggregation is estimated to be 3.8(ꢂ0.5) mM, 1.1(ꢂ0.3) mM,
and 1.3(ꢂ0.3) mM, respectively. While these concentrations are
similar for the cetyl containing CILs, it is relatively higher for
the short octyl chain containing CIL. The presence of L-valine
functionality may tentatively be proposed to be the reason for
the formation of premicellar aggregates in ethanol by our CILs.
Fig. 5 Pyrene (10 mM) band 1-to-band-3 emission intensity ratio (Py I1/I3)
within CILs dissolved in ethanol at ambient conditions: Boc-val-[C8-Tr][I]
(panel A); Boc-val-[C16-Tr][I] (panel B); Boc-val-[C16-Tr][PF6] (panel C).
[C16-Tr][PF6] dissolved in two different solvents – ethanol (polar-
protic) and chloroform (chlorinated relatively nonpolar) – rep-
resenting two major classes of solvents. The UV-Vis absorbance
spectra of aforementioned CILs are presented in Fig. S46 and
S47† and a representative spectrum for Boc-val-[C16-Tr][PF6]
dissolved in ethanol is depicted in Fig. 6. It is clear from the
absorbance spectra that CILs weakly absorb UV-Vis radiation
and exhibit two bands as shown by all four CILs irrespective of
the solvent. The higher energy band maxima appears in the
range 282–292 nm, whereas the lower energy band maxima
appears in the vicinity of 360(ꢂ4) nm. The band at lower energy
can be tentatively assigned to the Boc-NH- and/or L-val func-
tionalities, while the 1,2,3-triazolium moiety can be held
responsible for the higher energy absorbance feature. These
UV-Vis absorbance and uorescence of CILs
Unwanted absorbance and, in some cases, subsequent unde- absorbance spectral features are in good agreement with those
sired emission of radiation are associated to certain class of ILs reported in the literature for cationic triazolium macrocycles.48
in the current literature.47 The inherent ability of an IL to absorb Due to the nature of the functionalities involved, the associated
and/or emit UV-Vis radiation mostly hampers its application molar absorptivities of our CILs are not too high. The low molar
potential. We measured UV-Vis absorbance spectra of Boc-val- absorptivities of our CILs are advantageous as lower UV cut-offs
[C8-Tr][I], Boc-val-[C8-Tr][PF6], Boc-val-[C16-Tr][I], and Boc-val- of solvent media are preferred in most chemical analyses.
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triethylamine, and alkyl bromides (hexyl, octyl, dodecyl, cetyl
and octadecyl) were purchased from Aldrich Chemical
Company. Fluorescence probe pyrene (Py) was purchased from
Sigma-Aldrich in the highest purity possible. ACS spectropho-
tometric grade chloroform ($99.8%) was purchased from
Sigma-Aldrich. Ethanol (99.9%) was obtained from Merck Ltd.
All other reagents used in the study were purchased from
Sigma-Aldrich or Merck and were chemically pure. All chem-
icals were obtained in the highest purity grade possible and
were used as received unless otherwise stated. All synthesized
compounds were dried under high vacuum at 60 ꢀC for 48 hours
to remove water.
The solvents used were dried and distilled. HPLC grade
solvents were used for uorescence spectroscopy and UV-Vis
spectroscopic studies. Column chromatography was per-
formed on silica gel (60–120 mesh) obtained from Merck. The
eluting solvent indicated in parentheses for each purication
was determined by TLC that was performed on pre-coated silica
gel on aluminum sheets (kieselgel 60, F254, Merck). TLC plates
were visualized in an iodine chamber. 1H and 13C NMR spectra
were recorded on a 300 MHz Bruker DPX 300 instrument at
room temperature using tetramethylsilane (TMS) at 0.00 as an
internal standard. Mass spectra were acquired on a triple-
quadrupole Finnigan TSQ7000 mass spectrometer equipped
with electrospray ionization (ESI) in both positive and negative
ion modes. The Infrared spectra were recorded on a Agilent
Technology Cary 600 series FTIR Spectrometer and band posi-
tions were given in reciprocal centimeters (cmꢁ1). Decomposi-
tion temperature was measured using model TGA Q50 from TA
Instruments, Inc. at a ramp rate of 10 ꢀC minꢁ1. Melting points
were measured with model DSC Q200 instrument purchased
from TA Instruments. Optical rotation (a) were measured on
Rudolph Research Polarimeter Model Autopol-V Analytical at 25
ꢀC with concentration (g per 100 cm3 of CHCl3). A UV-Vis double
beam spectrophotometer (model UV-2450, Shimadzu) with
variable band width was used for acquisition of the UV-Vis
absorbance spectra. Fluorescence spectra were acquired on a
model FL 3-11 Fluorolog-3 modular spectrouorometer
(Horiba-Jobin Yvon, Inc.) with single Czerny-Turner grating
excitation and emission monochromators as wavelength selec-
tion devices, a 450 W Xe-arc lamp as the excitation source, and a
PMT as the detector. Fluorescence spectra of the probes pyrene
(Py) were collected with the following excitation/emission slit
widths (in nm): Py: 1/1 excited at 337 nm and CILs: 3/3 excited at
360 nm. All reported spectroscopic values were averages based
on performing triplicate measurements on independently
prepared samples. The spectral responses from appropriate
blanks were subtracted before data analysis in each case. All
absorbance and uorescence data were acquired using 10.0 mm
pathlength quartz cuvettes. All data analysis was performed
using SigmaPlot v12.0 soware.
Fig. 6 UV-Vis absorbance (green, 100 mM) and fluorescence emission
spectra (pink, 25 mM) of Boc-val-[C16-Tr][PF6] dissolved in ethanol at
ambient conditions [slits: 3/3, lex ¼ 360 nm].
We also collected the uorescence emission spectra of CILs
Boc-val-[C8-Tr][I], Boc-val-[C8-Tr][PF6], Boc-val-[C16-Tr][I] and
Boc-val-[C16-Tr][PF6] dissolved in ethanol and chloroform,
respectively. As in case of UV-Vis absorbance, the uorescence
emission characteristics of the CILs are found to be indepen-
dent of the nature of the solvent. The uorescence emission
spectrum of Boc-val-[C16-Tr][PF6] dissolved in ethanol is
included in Fig. 6 (the emission spectra of all four CILs are
presented in Fig. S48†). Most importantly, the emission signal
from the CILs are found to be very weak implying them to
possess very low uorescence quantum yields. This is again
advantageous and preferred of a medium as low uorescence
background renders a solvent useful in chemical applications
where various optical spectroscopic techniques are to be
employed. Weak UV-Vis radiation absorbing and emitting
ability makes our CILs useful solvents in chemical analyses
where low background signals are desired.
Conclusions
We have successfully synthesized a series of new L-valine based
chiral alkylated-1,2,3-triazolium ILs consisting of two different
anions (iodide and hexauorophosphate). These long chain
triazolium-based CILs show adequate thermal stability and they
are all in liquid state at room temperature hence can be clas-
sied as RTILs. Due to the presence of long alkyl chains, they
easily self-aggregate and show surface-active behavior and can
thus be assigned to the SAIL family. They absorb and emit UV-
Vis radiation very weakly and hence are desirable as solvents in
photochemical applications. This new class of CILs with several
interesting and useful properties is bound to nd use in various
applications in chemistry.
Requisite amounts of materials were weighed on a Mettler-
Toledo AB104-S balance with a precision of ꢂ0.1 mg. Stock
solution of pyrene and CILs were prepared in ethanol and
Experimental section
Materials and methods
Starting materials, amino acid L-valine, Boc-anhydride, stored in pre-cleaned amber glass vial at 4 ꢂ 1 ꢀC. To acquire the
1-hydroxylbenzotriazole (HOBt), sodium azide, propargyl- uorescence emission spectra of pyrene in CILs dissolved in
amine, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), ethanol, appropriate aliquot of the probe was taken from the
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stock and ethanol was added to achieve the desired nal CDCl3, d in ppm); 0.87–0.89 (m, 3H), 1.29 (br, s, 10H), 1.48–1.62
concentration. These solutions contained 10 mM pyrene such (m, 2H), 3.25 (t, J ¼ 6.9 Hz, 2H).
that the pyrene concentration remained constant during the
Dodecyl azide (3c). Colorless liquid, yield 89%, IR (ATR, nmax
/
1
titrations. Solutions of different CILs were prepared separately cmꢁ1); 2921, 2852, 2092, 1465, 1257, 721; H NMR (300 MHz,
in chloroform and ethanol. Solutions thus prepared were sub- CDCl3, d in ppm); 0.85–0.88 (m, 3H), 1.29 (br, s, 18H), 1.58 (t, J ¼
jected to uorescence and UV-Vis absorbance spectroscopic 6.9 Hz, 2H), 3.23 (t, J ¼ 6.9 Hz, 2H).
acquisitions.
Cetyl azide (3d). Colorless liquid, yield 90%, IR (ATR, nmax
/
1
cmꢁ1); 2921, 2852, 2092, 1465, 1257, 721; H NMR (300 MHz,
CDCl3, d in ppm); 0.88 (t, J ¼ 6.9 Hz, 3H), 1.06–1.26 (m, 26H),
1.45–1.61 (m, 2H), 3.24 (t, J ¼ 7.2 Hz, 2H).
Synthesis of various compounds
Boc-L-val-OH (1). A solution of L-valine (2.34 g, 20 mmol) in a
mixture of THF (30 mL), water (15 mL) and 1 M NaOH (20 mL)
Octadecyl azide (3e). Colorless liquid, yield 89%, IR (ATR,
n
max/cmꢁ1); 2921, 2852, 2092, 1465, 1257, 894, 721; 1H NMR (300
was stirred and cooled in an ice bath. Aerwards di-tert-butyl- MHz, CDCl3, d in ppm); 0.88–0.90 (m, 2H), 1.28–1.37 (s, 30H),
pyrocarbonate (4.8 g, 22 mmol) was added and reaction mixture 1.59–1.64 (m, 2H), 3.27 (t, J ¼ 6.9 Hz, 1H).
was further stirred for 12 h at room temperature. It was then
General procedure for synthesis of N-(tert-butoxycarbonyl)-L-
acidied with 1 M HCl and extracted with ethyl acetate (3 ꢃ 100 valinyl-4-aminomethyl-1-alkyl-1,2,3-triazole (Boc-val-Cn-Tr) (4a–
mL). The organic layer was dried over anhydrous sodium sulfate e). To a solution of corresponding azides (3a–e, 1 mmol) and
and evaporated under reduced pressure to give a gummy mass, compound 2 (1 mmol) in t-BuOH–H2O (9 : 1), sodium ascorbate
yield 95%, 1H NMR (300 MHz, CDCl3, d in ppm); 0.92–0.89 (m, (40 mg, 20 mol%), CuSO4 (24 mg, 10 mol%) were added. The
ꢀ
6H, CH(CH3)2), 1.44 (s, 9H, (CH3)3), 2.07–2.10 (m, 1H, b-CH), solution was stirred at 60 C for 24 h. The solution was evapo-
4.19–4.23 (m, 1H, a-CH), 8.10 (br s, 1H, NH); 13C NMR (75 MHz, rated under reduced pressure. The residue was washed with
CDCl3, d in ppm); 17.42, 18.99, 28.27, 31.01, 58.39, 80.00, 155.85, ethyl acetate to remove the starting material. The combined
176.63.
organic layers were washed with brine (25 mL) and dried over
Boc-L-valine-propargylamide (2). Boc-L-val-OH (1) (1.086 g, 5 Na2SO4. The crude product was puried by column chroma-
mmol) was dissolved in CH2Cl2 (10 mL). To this solution, 1- tography on silica-gel (ethyl acetae/hexane) to give respective
hydroxy benzotriazole (HOBt) (743 mg, 5.5 mmol, 1.1 equiv.), 1- compounds (4a–e).
ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC) (1.054 g,
N-(tert-Butoxycarbonyl)-L-valinyl-4-aminomethyl-1-hexyl-
5.5 mmol, 1.1 equiv.) was added and the resulting mixture was 1,2,3-triazole (Boc-val-C6-Tr) (4a). Off-white solid, yield 82%, mp
stirred at 0 C for 20 minutes. Propargylamine (0.352 mL, 5.5 68 C, IR (KBr, nmax/cmꢁ1); 3443, 3337, 2959, 2931, 1682, 1649,
ꢀ
ꢀ
1
mmol) and triethylamine (1.53 mL, 11 mmol, and 2.5 equiv.) 1523, 1465, 1368, 1248; H NMR (300 MHz, CDCl3, d in ppm)
were subsequently added. The reaction mixture was allowed to 0.77–0.83 (m, 9H, CH(CH3)2, CH3 (hex)), 1.18–1.22 (m, 6H, CH2),
stir overnight at 25 ꢀC. Reaction mixture was extracted with 1.33 (s, 9H, (CH3)3), 1.79 (m, 2H, CH2), 2.02–2.13 (m, 1H, b-CH),
ethyl acetate and subsequently washed with 5% cold HCl, 3.95 (br, s, 1H, a-CH), 4.23 (t, J ¼ 6.9 Hz, 2H, CH2) 4.43 (s, 2H,
saturated aq. NaHCO3, water and brine. The organic phase was CH2), 5.40 (d, J ¼ 7.2 Hz, 1H, NHCH), 7.55 (s, 1H, NHCH2), 7.70
dried over Na2SO4, concentrated under reduced pressure, and (s, 1H, Tr-CH); 13C NMR (75 MHz; CDCl3, d in ppm); 13.4, 17.2,
puried by column chromatography. Solid, yield 90%, [a]2D0 18.8, 21.9, 25.6, 27.8, 29.2, 29.7, 30.6, 30.6, 34.3, 49.9, 59.3, 79.3,
ꢁ13.3 (c 1.0, CHCl3), (lit33a [a]D22 ꢁ11.4 (c 1.5, CHCl3)), 1H NMR 155.4, 171.6; HRMS (ESI) m/z: calcd for C19H35N5O3 [M + Na]+
(300 MHz, CDCl3, d in ppm); 0.91–0.97 (m, 6H, CH(CH3)2), 1.44 404.2632, found 404.2640.
(s, 9H, (CH3)3), 2.09 (m, 1H, b-CH), 2.19 (t, J ¼ 2.4 Hz, 1H, HC^),
N-(tert-Butoxycarbonyl)-L-valinyl-4-aminomethyl-1-octyl-
3.96–4.12 (m, 3H, CH2, a-CH), 5.28 (d, 1H, J ¼ 9 Hz, NHCH), 6.90 1,2,3-triazole (Boc-val-C8-Tr) (4b). Pale yellow solid, yield 89%,
(br, s, 1H, NHCH2); 13C NMR (75 MHz, CDCl3, d in ppm); 18.0, mp 84 ꢀC, IR (KBr, nmax/cmꢁ1); 3350, 2956, 2925, 1679, 1656,
1
18.9, 28.1, 28.6, 29.1, 29.4, 31.1, 59.5, 71.1, 79.5, 79.8, 155.8, 1523, 1466, 1370, 1303, 1246; H NMR (300 MHz, CDCl3, d in
171.1; HRMS (ESI) m/z: calcd for C13H22N2O3 (M + Na)+ ppm); 0.70–0.98 (m, 6H, CH(CH3)2), 1.01–1.34 (m, 10H, CH2),
277.1528, found 277.1528.
1.43 (s, 9H, (CH3)3), 1.78–1.97 (m, 2H, CH2), 2.00–2.83 (m, 1H, b-
General synthetic procedure for alkylazides (3a–e). To a CH), 3.88–3.97 (m, 1H, a-CH), 4.03–4.26 (m, 2H, CH2), 4.44–4.94
solution of alkyl bromides in DMF, solid NaN3 was added. The (m, 2H, CH2), 5.06 (d, 1H, NHCH), 6.99 (br, s, 1H, NHCH2), 7.48
solution was stirred at room temperature for overnight before it (s, 1H, Tr-CH); 13C NMR (75 MHz, CDCl3, d in ppm); 13.8, 17.5,
was quenched with excess amount of water. It was extracted 19.0, 22.3, 26.2, 28.1, 28.7, 28.8, 29.1, 29.4, 30.0, 31.0 31.4, 31.6,
with ethyl acetate. The combined organic layer was dried over 34.5, 50.1, 59.5, 79.3, 122.2, 155.7, 172.0; HRMS (ESI) m/z: calcd
anhydrous Na2SO4 and evaporated under reduced pressure to for C21H39N5O3 [M + Na]+ 432.2945, found 432.2945.
get the colorless liquid.
N-(tert-Butoxycarbonyl)-L-valinyl-4-aminomethy-1-dodecyl-
1,2,3-triazole (Boc-val-C12-Tr) (4c). Pale yellow solid, yield 88%,
Hexyl azide (3a). Colorless liquid, yield 87%, IR (ATR, nmax
/
cmꢁ1); 2959, 2931, 2098, 1464, 1253, 720; H NMR (300 MHz; mp 87–89 ꢀC, IR (KBr, nmax/cmꢁ1); 3364, 2956, 2918, 2850, 1679,
1
1
CDCl3, d in ppm); 0.83 (t, J ¼ 6.6 Hz, 3H), 1.16–1.35 (m, 6H), 1658, 1448, 1371, 1300; H NMR (300 MHz, CDCl3, d in ppm);
1.47–1.57 (m, 2H), 3.17 (t, J ¼ 6.9 Hz, 2H).
0.85–0.876 (m, 9H, CH(CH3)2, CH3 (dodecyl)), 1.25–1.34 (m,
18H, CH2), 1.43 (s, 9H, (CH3)3), 1.84–1.86 (m, 2H, CH2), 2.07–
Octyl azide (3b). Colorless liquid, yield 92%, IR (ATR, nmax
/
cmꢁ1); 2925, 2856, 2090, 1464, 1258, 723; H NMR (300 MHz, 2.10 (m, 1H, b-CH), 4.03 (d, J ¼ 6 Hz, 1H, a-CH), 4.30 (t, J ¼ 6 Hz,
1
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2H, CH2), 4.51 (d, J ¼ 3 Hz, 2H, CH2), 5.52 (d, J ¼ 9 Hz, 2H, 1H, b-CH), 3.99 (t, J ¼ 6.3 Hz, 1H, a-CH), 4.42 (s, 3H, CH3), 4.51
NHCH), 7.62 (d, J ¼ 6 Hz, 1H, NHCH2), 7.86 (s, 1H, Tr-CH); 13
C
(t, J ¼ 7.3 Hz, 2H, CH2), 4.80–4.81 (m, 1H, CH2), 5.23–5.26 (d, J ¼
NMR (75 MHz, CDCl3, d in ppm); 13.8, 17.5, 19.0, 22.4, 26.2, 7.2 Hz, 1H, NHCH), 8.53 (br, s, 1H, NHCH2), 8.92 (s, 1H, Tr-CH);
28.7, 29.0, 29.1, 29.2, 29.3, 30.0, 31.0, 31.6, 34.5, 50.0, 59.5, 79.2, 13C NMR (75 MHz, CDCl3, d in ppm); 13.8, 17.7, 19.2, 22.2, 25.8,
122.2, 144.6, 155.7, 172.0; HRMS (ESI) calcd for C25H47N5O3 [M 28.1, 28.5, 28.6, 29.0, 29.3, 30.6, 31.3, 32.2, 39.3, 54.1, 59.9, 79.5,
+ Na]+ 488.3571, found 488.3571.
130.3, 141.7, 155.7, 173.0; HRMS (ESI) m/z: calcd for C22H48N5O3
N-(tert-Butoxycarbonyl)-L-valinyl-4-aminomethyl-1-cetyl- 424.3282 [M ꢁ I]+, found 424.3294.
1,2,3-triazole (Boc-L-val-C16-Tr) (4d). Orange solid, yield 85%,
Boc-val-[C12-Tr][I] (5c). Pale yellow liquid, [a]2D0 + 2.2 (c 0.56,
mp 78 ꢀC, IR (KBr, nmax/cmꢁ1); 3343, 3295, 2920, 2957, 2849, CHCl3), IR (ATR, nmax/cmꢁ1); 3433, 3230, 2893, 2859, 1687, 1478,
1688, 1651, 1526, 1468, 1301, 1247; 1H NMR (300 MHz, CDCl3, d 1368, 1257, 1165, 743; H NMR (300 MHz, CDCl3, d in ppm);
1
in ppm); 0.77–0.80 (m, 9H, CH(CH3)2, CH3 (cetyl)), 1.18 (m, 26H, 0.89–0.97 (m, 9H, CH(CH3)2, CH3 (dodecyl)), 1.25–1.34 (m, 18H,
CH2), 1.33 (s, 9H, (CH3)3), 1.79–1.97 (m, 2H, CH2), 2.05–2.11 (m, CH2), 1.43 (s, 9H, (CH3)3), 2.04 (s, 2H, CH2), 2.17 (m, 1H, b-CH),
1H b-CH), 3.88 (br, s, 1H, a-CH), 4.24 (t, J ¼ 6 Hz, 2H, CH2), 4.45 4.01 (s, 1H, a-CH), 4.45 (s, 3H, CH3), 4.53 (s, 2H, CH2), 4.83 (s,
(d, J ¼ 6 Hz, 2H, CH2), 5.03 (m, 1H, NHCH), 6.88 (s, 1H, NHCH2), 2H, CH2), 5.29 (br, s, 1H, NHCH), 8.54 (s, 1H, NHCH2), 8.93 (s,
7.47 ( s, 1H, Tr-CH); 13C NMR (75 MHz, CDCl3, d in ppm); 13.9, 1H, Tr-CH); 13C NMR (75 MHz, CDCl3, d in ppm); 13.9, 17.8,
17.5, 19.0, 22.4, 26.3, 28.1, 28.8, 28.9, 29.1, 29.2, 29.3, 29.4, 29.5, 19.3, 22.4, 25.9, 28.2, 28.6, 28.8, 29.1, 29.3, 30.6, 31.7, 32.3, 39.4,
30.0, 31.0, 31.7, 33.6, 34.5, 50.1, 59.6, 79.3, 113.9, 122.2, 139.0, 54.1, 60.0, 79.5, 130.3, 141.8, 155.8, 173.1; HRMS (ESI) m/z: calcd
144.6, 155.7, 172.0; HRMS (ESI) m/z: calcd for C29H55N5O3 (M + for C26H50N5O3 480.3908 [M ꢁ I]+, found 480.3926.
Na)+ 544.4197, found 544.4210.
Boc-val-[C16-Tr][I] (5d). Yellow liquid, [a]2D0 + 2.4 (c 1.0,
N-(tert-Butoxycarbonyl)-L-valinyl-4-aminomethyl-1-octadecyl- CHCl3), IR (ATR, nmax/cmꢁ1); 3435, 3227, 2923, 2853, 1703, 1668,
1,2,3-triazole (Boc-val-C18-Tr) (4e). Light yellow solid, yield 86%, 1499, 1464, 1365, 1240, 1164, 731; 1H NMR (300 MHz, CDCl3, d
mp 86 C, IR (nmax/cmꢁ1); 3339, 3293, 2956, 2925, 2853, 1689, in ppm); 0.78–0.98 (m, 9H, CH(CH3)2, CH3 (cetyl)), 1.01–1.29 (m,
ꢀ
1
1656, 1651, 1526, 1469, 1366, 1247, 1178; H NMR (300 MHz, 26H, CH2), 1.35 (s, 9H, (CH3)3), 1.79–1.82 (m, 2H, CH2), 2.09–
CDCl3, d in ppm); 0.85–0.97 (m, 9H CH(CH3)2, CH3 (octadecyl)), 2.14 (m, 1H, b-CH), 3.93 (m, 1H, a-CH), 4.44 (s, 3H, CH3), 4.46 (d,
1.24 (m, 30H, CH2), 1.42 (s, 9H, (CH3)3), 1.70 (s, 2H, CH2), 1.87 (t, J ¼ 5.4 Hz, 2H, CH2), 5.20 (m, 1H, NHCH), 8.48 (br, s, 1H,
J ¼ 6.0 Hz, 1H, b-CH), 3.96 (t, J ¼ 6.0 Hz, 1H, a-CH), 4.27 (m, 2H, NHCH2), 8.85 (s, 1H, Tr-CH); 13C NMR (75 MHz, CDCl3, d in
CH2) 4.30 (t, J ¼ 6.0 Hz, 2H, CH2), 4.52 (d, J ¼ 5.7 Hz, 2H, CH2), ppm); 13.6, 17.6, 19.0, 22.2, 25.7, 27.9, 28.4, 28.8, 29.0, 29.1,
5.03 (br, s, 1H, NHCH), 6.72 (br, s, 1H, NHCH2), 7.51 (s, 1H, 29.2, 29.8, 30.4, 31.4, 32.1, 39.1, 53.9, 59.7, 79.2, 130.1, 141.4,
Tr-CH); 13C NMR (75 MHz, CDCl3, d in ppm); 14.0, 17.5, 19.2, 155.5, 172.8; HRMS (ESI) m/z: calcd for C30H58N5O3 536.4534
22.6, 26.4, 28.2, 28.9, 29.2, 29.3, 29.4, 29.5, 29.6, 30.2, 31.9, 34.7, [M ꢁ I]+, found 536.4540.
50.3, 59.7, 79.7, 122.1, 144.5, 155.8, 171.8; HRMS (ESI) m/z: calcd
Boc-val-[C18-Tr][I] (5e). Yellow liquid, [a]2D0 + 2.5 (c 0.55,
CHCl3), IR (ATR, nmax/cmꢁ1); 3435, 3232, 2922, 2852, 1703, 1668,
for C31H60N5O3 (M + H)+ 550.4691, found 550.4704.
General procedure for methylation of N-(tert-butoxy- 1499, 1464, 1365, 1164, 729; 1H NMR (300 MHz, CDCl3, d in
carbonyl)-L-valinyl-4-aminomethyl-1-alkyl-1,2,3-triazole (5a–e). ppm); 0.86–0.98 (m, 9H, CH(CH3)2, CH3 (octadecyl)), 1.25–1.35
The solution of Boc-val-alkyl-triazole (4a–e) (0.1 mol) and MeI (m, 30H, CH2), 1.43 (s, 9H, (CH3)3), 1.64–1.99 (m, 2H, CH2),
ꢀ
(0.6 mol) was stirred overnight at 80 C. The reaction mixture 2.01–2.22 (m, 1H, b-CH), 4.00 (t, J ¼ 6.4 Hz, 1H, a-CH), 4.43 (s,
was evaporated under reduced pressure. The residue was 3H, CH3), 4.52 (t, J ¼ 7.2 Hz, 2H, CH2), 4.82 (d, J ¼ 5.7 Hz, 1H,
diluted with CHCl3 and washed with H2O, dried over Na2SO4 CH2), 5.25 (d, J ¼ 7.2 Hz, 1H, NHCH), 8.53 (br, s, 1H, NHCH2),
and evaporated under reduced pressure to give 5a–e in quan- 8.94 (s, 1H, Tr-CH); 13C NMR (75 MHz, CDCl3, d in ppm); 14.1,
titative yield.
18.0, 19.4, 22.6, 26.1, 28.3, 28.7, 29.2, 29.3, 29.4, 29.5, 29.6, 30.5,
Boc-val-[C6-Tr][I] (5a). Yellow liquid, [a]2D0 + 3.3 (c 0.73 in 31.9, 32.4, 39.1, 54.3, 60.5, 79.9, 130.6, 142.5, 156.0, 173.4;
CHCl3), IR (ATR, nmax/cmꢁ1); 3435, 3258, 2960, 2929, 1667, 1500, HRMS (ESI) m/z: calcd for C32H62N5O3 564.4847 [M ꢁ I]+, found
1458, 1365, 754, 640; 1H NMR (300 MHz; CDCl3, d in ppm); 0.77– 564.4847.
0.96 (m, 9H, CH(CH3)2), CH3 (hexyl), 1.15–1.27 (m, 6H, CH2),
General procedure for synthesis of Boc-val-[Cn-Tr][PF6] (6a–
1.33 (s, 9H, (CH3)3), 1.92–1.97 (m, 2H, CH2), 2.05–2.09 (m, 1H, b- e). A solution of triazolium salts (5a–e; 100 mg in 5 mL MeOH)
CH), 3.90–3.95 (m, 1H, a-CH), 4.35 (s, 3H, CH3), 4.44–4.47 (m, was added to saturated methanolic solution of NH4PF6 (3 mL)
2H, CH2), 4.73–4.76 (m, 2H, CH2), 5.18–5.23 (m, 1H, NHCH), and stirred for 2 h. White precipitate of product obtained was
8.52 (br, s, 1H, NHCH2), 8.87 (s, 1H, Tr-CH); 13C NMR (75 MHz; ltered and washed with methanol and then dried under
CDCl3, d in ppm); 13.3, 17.5, 18.9, 21.7, 25.2, 27.8, 28.7, 29.0, reduced pressure followed by vacuum in desiccators over P2O5
30.3, 31.9, 39.1, 53.8, 56.4, 59.6, 79.1, 130.0, 141.3, 155.4, 172.7; and KOH Pellets, yield 85–90%.
HRMS (ESI) m/z: calcd for C20H38N5O3 396.2969 [M ꢁ I]+, found
Boc-val-[C6-Tr][PF6] (6a). Yellow colored liquid, [a]2D0 + 1.48 (c
396.2969.
1.08, CHCl3), IR (ATR, nmax/cmꢁ1); 3421, 3298, 2962, 2931, 1671,
Boc-val-[C8-Tr][I] (5b). Yellow liquid, [a]2D0 + 2.6 (c 0.53 in 1500, 1366, 1162, 840; H NMR (300 MHz, CDCl3, d in ppm);
CHCl3), IR (ATR, cmꢁ1); 2959, 2927, 2856, 1702, 1668, 1499, 0.81–0.89 (m, 9H, CH(CH3)2), CH3 (hexyl), 1.18–1.26 (m, 15H,
1365, 1162, 729, 641; 1H NMR (300 MHz, CDCl3, d in ppm); (CH3)3, CH2), 1.92 (s, 2H, CH2), 2.06–2.08 (m, 1H, b-CH), 3.92 (t, J
0.851–0.97 (m, 9H, CH(CH3)2), CH3 (octyl), 1.26–1.33 (m, 10H, ¼ 6 Hz, 1H, a-CH), 4.30 (s, 3H, CH3), 4.40–4.45 (m, 2H, CH2),
CH2), 1.42 (s, 9H, (CH3)3), 1.98–2.05 (m, 2H, CH2), 2.13–2.18 (m, 4.65 (m, 2H, CH2), 5.19 (m, 1H, NHCH), 8.04 (br, s, 1H, NHCH2),
1
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Paper
8.57 (s, 1H, Tr-CH); 13C NMR (75 MHz; CDCl3 d in ppm); 13.8, Assistantships. ShP thanks Department of Science & Tech-
17.8, 19.3, 22.2, 25.7, 28.2, 29.0, 29.6, 30.5, 30.8, 32.2, 38.6, 54.2, nology, Government of India for the Fast-Track Fellowship
60.4, 80.0, 130.0, 142.2, 156.1, 173.4; HRMS (ESI) m/z: calcd for [grant no. SB/FT/CS-021/2012] and SiP thanks CSIR, Govern-
C
20H38N5O3 396.2969 (M ꢁ PF6)+, found 396.2969.
ment of India [grant no. 01(2767)/13/EMR-II] for partially
Boc-val-[C8-Tr][PF6] (6b). Yellow color liquid, [a]2D0 + 2.0 (c funding this work.
0.70, CHCl3), IR (ATR, nmax/cmꢁ1); 3425, 3237, 2961, 2928, 1671,
1499, 1465, 1366, 1241, 1162, 842; 1H NMR (300 MHz, CDCl3, d
in ppm); 0.79–0.87 (m, 9H, CH(CH3)2, CH3 (octyl)), 1.91–1.25 (m,
10H, CH2), 1.34 (s, 9H, (CH3)3), 1.91–2.03 (m, 2H, CH2), 2.05–
2.09 (m, 1H, b-CH), 3.91 (t, J ¼ 6 Hz, 1H, a-CH), 4.32 (s, 3H, CH3),
4.45 (t, J ¼ 6.9 Hz, 2H, CH2), 4.64 (d, J ¼ 4.5 Hz, 2H, CH2), 5.26
(d, J ¼ 6 Hz, 1H, NHCH), 8.14 (s, 1H, NHCH2), 8.59 (s, 1H, Tr-
CH); 13C NMR (75 MHz, CDCl3, d in ppm); 13.9, 17.8, 19.3, 22.4,
26.0, 28.2, 28.6, 28.8, 29.1, 29.5, 30.6, 31.5, 31.8, 32.1, 33.7, 38.8,
54.1, 60.1, 79.8, 129.9, 141.8, 155.9, 173.2; HRMS (ESI) m/z: calcd
for C22H42N5O3 424.3282 (M ꢁ PF6)+, found 424.3298.
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26H, CH2), 1.39 (s, 9H, (CH3)3), 1.93–1.97 (m, 2H, CH2),
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C
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C
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33486 | RSC Adv., 2014, 4, 33478–33488
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