Theoretical study of DABCO-based ionic liquid: Synthesis and reaction mechanism

Article abstract of DOI:10.1007/s11224-013-0348-4

Quaternary ammonium salt obtained from the Menshutkin reaction between DABCO and benzyl chloride has been used in the synthesis of a novel Bronsted acidic ionic liquid (IL), namely 1-benzyl-4-(sulfobutyl)-diaza-bicyclo-octane hydrogen sulfate. The reaction of DABCO with benzyl chloride is a crucial step in the synthesis of this IL. Density functional theory calculations at B3LYP/6-31G(d,p) level have been employed to investigate the mechanism of Menshutkin reaction by calculating the energy barriers through possible transition states i.e., five-membered ring transition state and S_N2 transition state in gas phase and in diethyl ether as a solvent. It was found that while DABCO reacts with benzyl chloride through the well-known S _N2 transition state mechanism, the corresponding reaction with chlorodiphenylmethane can proceed through both S_N2 and five-membered ring transition state mechanism. However, S_N2 transition state mechanism is still the strongly preferred one out of the two possible mechanisms. The electronic structure analysis shows that solvent effects and enhanced resonance stabilization may play a decisive role in guiding the reaction pathway.

Full text of DOI:10.1007/s11224-013-0348-4

Struct Chem
DOI 10.1007/s11224-013-0348-4
ORIGINAL RESEARCH
Theoretical study of DABCO-based ionic liquid: synthesis
and reaction mechanism
•
Amritpal Singh Paramjit Singh Neetu Goel
•
Received: 14 August 2013 / Accepted: 10 September 2013
Ó Springer Science+Business Media New York 2013
Abstract Quaternary ammonium salt obtained from the
Menshutkin reaction between DABCO and benzyl chloride
has been used in the synthesis of a novel Bronsted acidic
ionic liquid (IL), namely 1-benzyl-4-(sulfobutyl)-diaza-
bicyclo-octane hydrogen sulfate. The reaction of DABCO
with benzyl chloride is a crucial step in the synthesis of this
IL. Density functional theory calculations at B3LYP/6-
31G(d,p) level have been employed to investigate the
mechanism of Menshutkin reaction by calculating the
energy barriers through possible transition states i.e., five-
membered ring transition state and SN2 transition state in
gas phase and in diethyl ether as a solvent. It was found
that while DABCO reacts with benzyl chloride through the
well-known SN2 transition state mechanism, the corre-
sponding reaction with chlorodiphenylmethane can pro-
ceed through both SN2 and five-membered ring transition
state mechanism. However, SN2 transition state mechanism
is still the strongly preferred one out of the two possible
mechanisms. The electronic structure analysis shows that
solvent effects and enhanced resonance stabilization may
play a decisive role in guiding the reaction pathway.
Introduction
Ionic liquids (ILs) are a class of novel compounds generally
composed of organic cations and inorganic or organic anions
that are capable as green alternative solvents for extraction,
purification, catalysis, and synthesis. Because of their
appealing features such as negligible vapor pressure, non-
volatile, non-flammable, powerful dissolving power, and
high thermal stability [1–5], ILs have elicited great attention
in both academia and research and their catalytic properties
have been investigated extensively over the last several
years [6–8]. ILs can be termed as designer solvents owing to
the easy tailoring of their appealing features by varying
cations, anions, or alkyl substituents on cation for specific
purposes [9]. Bronsted acidic ILs have the potential to
replace acidic catalysts in industry and can catalyze various
reactions such as esterification, etherification, and Pinacol
rearrangement. Synthesis of ILs and predicting their prop-
erties that are tailor made for specific purposes require a
molecular level understanding of the reaction mechanism.
Density functional theory (DFT) is a reliable and widely
used tool to obtain information about the energetics, struc-
ture, transition states, and reaction pathways at low com-
putational cost and without accuracy loss. The synthesis
mechanism of ILs has been studied at molecular level by
using B3LYP functional [10–12]. The mechanisms of for-
mation of imidazolium halide and pyridinium halide salts via
Menshutkin reaction [10, 11, 13–15] between N-alkylimi-
dazole/pyridine and alkyl halide were studied by Liu and co-
workers [11, 12]. They reported that the SN2 mechanism is
energetically much more favorable [11, 12] than the five-
membered ring transition state mechanism proposed in early
literature by Li and co-workers [10] because the large steric
hindrance annihilates the stabilization due to hydrogen
bonding in five-membered ring transition state. The solvents
Keywords DFT ꢀ Menshutkin reaction ꢀ Transition
state ꢀ Ionic liquid
Electronic supplementary material The online version of this
article (doi:10.1007/s11224-013-0348-4) contains supplementary
material, which is available to authorized users.
A. Singh ꢀ P. Singh ꢀ N. Goel (&)
Department of Chemistry and Centre of Advanced Studies in
Chemistry, Panjab University, Chandigarh 160014, India
e-mail: neetugoel@pu.ac.in
123

Struct Chem
have considerable effect on mechanism of Menshutkin
reaction through favorable interactions with polar transition
states [11, 16, 17].
such as oxathioacetalization, conjugate addition of
amines to electron deficient alkenes, preparation of
dibenzo[a,j] xanthenes, etc. [18–22]. Herein we report
the synthesis of a novel Bronsted acidic ILs having
DABCO framework, namely 1-benzyl-4-(sulfobutyl)-di-
aza-bicyclo-octane hydrogen sulfate ([benzylDAB-
COC4H8SO3H]HSO4) III. It was prepared by reaction
of 4-aza-1-benzylazoniabicyclo-[2.2.2]-octane chloride
I with 1,4-butane sultone in dry DCE (dichloroethane)
followed by acidification with sulfuric acid in methanol
(Scheme 1). The salt I was prepared by the reaction of
These previous theoretical studies have focused on the
mechanism of Menshutkin reaction of formation of aromatic
heterocyclic alkyl halide salts. We have extended this study
to saturated heterocyclic system by investigating the mech-
anism of formation of N-substituted DABCO (diaza-bicy-
clo-[2.2.2]-octane) chlorides. It is pertinent to mention here
that properties of non-aromatic saturated heterocyclic rings
are quite different from the planar, electron-rich aromatic
heterocyclic rings studied previously in literature.
1
DABCO with benzyl chloride in dry diethyl ether. The H
In the present work, we synthesized a novel Bronsted
acidic IL having DABCO framework. The microcosmic
mechanism of Menshutkin reaction between DABCO and
benzyl chloride/chlorodiphenylmethane was then investi-
gated within the framework of DFT in gas phase, and
solvent effects were studied in diethyl ether. The interac-
tions of solvent with both the transition states, i.e., five-
membered ring transition state and SN2 transition state, are
crucial as both are polar in nature, SN2 being more polar.
NMR spectrum of I showed a singlet at d 5.13 due to
benzylic protons while two triplets were present at d 3.18
and d 3.78 corresponding to six protons each of the
DABCO rings. In the 1H NMR spectrum of II, the benzylic
proton singlet appears at d 4.36 while signals for aliphatic
chain protons appeared at the expected positions.
Preparation of Bronsted acidic IL III
To a solution of DABCO (0.6 g, 5.4 mmol) in dry diethyl
ether (15 ml), benzyl chloride (1.2 ml, 10.8 mmol) was
added and reaction mixture was stirred at room temperature
overnight. The solid that separated out was filtered and
thoroughly washed with diethyl ether to remove excess
benzyl chloride to furnish I (1.07 g, 85 %) [23]. 1H NMR
(300 MHz, CDCl3): d 3.18 (t, J = 7.5 Hz, 6H), 3.78
(t, J = 7.5, 6H), 5.13 (s, 2H, Ar–CH2), 7.35-7.45 (m, 3H,
Experimental
Synthesis of novel Bronsted acidic IL
In recent years, the catalytic activity of DABCO-based
Bronsted acidic ILs has been investigated for reactions
Scheme 1 Synthesis of IL
Cl
N
1-benzyl-4-(sulfobutyl)-diaza-
bicyclo-octane hydrogen sulfate
(III)
N
Dry Et2O
+
N
overnight stirring
N
Cl
Benzyl chloride
DABCO
4-Aza-1-benzyl-1-azonia-
bicyclo[2.2.2]octane chloride
I
85% yield
O
O
O
700C, Stirring
S
Dry DCE,
8h
SO3H
HSO4
SO3
N
N
2 H2SO4
(0.624 N)
N
MeOH,
700C, Stirring
N
8h
HSO4
Cl
III
86% yield
IL
II
81% yield
123

Struct Chem
ArH) 7.66–7.68 (m, 2H, ArH). 13C NMR (75 MHz,
CDCl3): 45.39, 51.76, 96.05, 129.71, 129.13, 130. 46,
133.47.
Results and discussion
Synthesis mechanism
The salt I {4-aza-1-benzylazoniabicyclo[2.2.2]octane
chloride} (1.07 g, 4.5 mmol) was taken in dry DCE
(60 ml) and 1,4-butane sultone (0.46 ml, 4.5 mmol) was
added dropwise in 20 min and the reaction mixture was
heated at 70–80 °C for 8 h. After completion of reaction,
DCE was removed under reduced pressure and the residual
white solid was washed with dry diethyl ether and dried
under vaccum to furnish II (1.36 g, 81 %). 1H NMR
(300 MHz, D2O): d 1.66–1.87 (m, 4H), 2.76–2.86 (m, 2H),
3.03 (t, J = 7.8 Hz, 2H), 3.32 (t, J = 7.8 Hz, 2H),
3.45–3.53 (m, 2H), 3.85–3.98 (m, 8H), 4.36 (s, 2H, Ar–
CH2), 7.39–7.53 (m, 5H).
The Menshutkin reaction between DABCO and benzyl
chloride (Scheme 1) is a crucial step for the synthesis of ILs
having DABCO framework. The current work attempts to
provide the molecular level understanding of the reaction
mechanism. The optimized structures involved in the reac-
tion and the calculated electronic energy profiles along
reaction coordinates are shown in Figs. 1 and 2,
respectively.
Cl
N
N
+
A stoichiometric amount of sulfuric acid (0.642 N in
methanol, 21.05 ml, 7.2 mmol) was added to the salt II
(1.36 g, 3.6 mmol) and the solution was refluxed for
8 h and methanol was evaporated. The residual mass
was crystallized from ethanol to furnish Bronsted acidic
N
N
Cl
Benzyl chloride
DABCO
4-Aza-1-benzyl-1-azonia-
bicyclo[2.2.2]octane chloride
Hydrogen bonding in reactant-like precursor R and
product P is shown in Fig. 1 with C1–HꢀꢀꢀN H-bond dis-
1
ILs III (1.65 g, yield 86 %). H NMR (400 MHz, D2O):
d 1.71–1.77 (m, 2H), 1.87–1.93 (m, 2H) 2.89 (t,
J = 8 Hz, 2H), 3.52 (t, J = 8 Hz, 2H), 3.89–3.94 (m,
12H), 4.72 (s, 2H, Ar–CH2), 7.47–7.57 (m, 5H). 13C
NMR (100 MHz, D2O): 132.89, 131.59, 129.64, 124.84,
68.87, 64.46, 51.23, 50.71, 49.66, 43.95, 20.78, 20.29.
HRMS (m/z) = 339.1737 (calculated for M?-H–
2HSO4 = 339.1737).
˚
tance of 2.40 A in R and bifurcated H-bonds in P i.e., C1–
˚
HꢀꢀꢀCl and C5–HꢀꢀꢀCl with a distance of 2.40 and 2.32 A,
respectively. The overall reaction was found to be endo-
thermic by 1.83 kcal/mol.
In five-membered ring transition state, labeled as TS1,
nucleophilic nitrogen atom in DABCO attacks the posi-
tively charged C1 atom at side of Cl atom in R which leads
˚
to C1–N15 (2.76 A) bond formation and simultaneous bond
˚
cleavage between C1–Cl (from 1.84 A to 2.89 A). This can
Computational details
be confirmed from transition vector corresponding to
imaginary frequency (222.07i cm-1). The two centered
H-bond distances of C1–HꢀꢀꢀCl and C5–HꢀꢀꢀCl H-bond
The geometry optimization calculations were carried out
at B3LYP/6-31G(d,p) [24–28] level of DFT as imple-
mented in Gaussian 03 package [29]. The standard
6-31G(d,p) basis set employed here maintains a balance
between accuracy and computational cost. No symmetry
constraints were imposed on initial structures. Fre-
quency calculations were carried out at the same level
of theory to verify all stationary points as minima (zero
imaginary frequency) or the first-order saddle point (one
imaginary frequency) and to provide zero-point vibra-
tional energy (ZPE) corrections. Reaction pathways
were traced using intrinsic reaction coordinates (IRC)
calculations to confirm that the transition state actually
connects the two corresponding minima [30]. Atomic
charges were calculated using electrostatic potential
(ESP) method. The solvent effects were considered by
re-optimizing the gas-phase structures in diethyl ether
(dielectric constant = 4.3) using a self-consistent reac-
tion field (SCRF) [31] based on polarizable continuum
model (PCM) of Tomasi group [32] at B3LYP/6-
31G(d,p) level of theory.
˚
distances are found to be 2.31 and 2.57 A, respectively.
The high-energy barrier (38.72 kcal/mol higher than the
reactant precursor R, see Fig. 2) to reach the TS1 is not
surprising and is attributed to steric hindrance as attack by
N atom on the C1 is from the same side where Cl is
attached. But this is contrary to easy synthesis of N-
substituted DABCO halide salts. The present calculations
envisaged an alternate lower energy pathway to reaction
that led to the optimization of a new transition state TS2
where the N15 atom in DABCO attacks the C1 atom center
behind C1–Cl bond and a typical SN2-type structure TS2
was obtained as shown in Fig. 1. The geometrical param-
˚
eters of TS2 suggest C1–Cl (2.61 A from 1.84 A) bond
˚
˚
breaking and N15–C1(1.88 A) bond formation as confirmed
by change of bond lengths and transition vector corre-
sponding to imaginary frequency (299.86i cm-1). The
Walden inversion for the two hydrogen atoms on C1 was
observed from the relative dihedral angles, DH3–C1–C5–H6
changes from 28.15° in isolated benzyl chloride to -17.71°
123

Struct Chem
Fig. 1 The optimal structures of the reactant-like precursor (R), transition state TS1, transition state TS2, and product (P) along two reaction
pathways. The distances are in angstroms (Color figure online)
The ESP charge analysis of the optimized transition
structures showed that partial negative charges on C1 and
Cl in five-membered ring transition state (TS1) are -0.16C
and -0.58C and in SN2 transition state (TS2) are -0.46C
and -0.71C, respectively. Thus while SN2 mechanism
involves more polar transition state TS2, the reduced
charges in five-membered ring transition state TS1 indicate
that the charge has been dispersed on the neighboring
atoms. Equal C–H-bond lengths in the CH2Cl group and
the adjoining benzene ring suggested the role of resonance
stabilization that has probably led to charge dispersal. This
hypothesis needed further justification and prompted us to
investigate if resonance stabilization provided by the phe-
nyl ring plays a role and may become the decisive factor
over the steric hindrance in controlling the reaction
Fig. 2 The calculated electronic energy profile for the reaction of
DABCO with benzyl chloride along reaction coordinates. The relative
energies are in kcal/mol
mechanism. With this objective, the mechanism of reaction
of DABCO with chlorodiphenylmethane (Scheme 2) was
studied. The optimized structures of the reactant-like pre-
cursor (R0) and the product (P0) and the calculated elec-
tronic energy profiles along reaction coordinate are given
in Figs. 3 and 4, respectively.
in SN2 transition state. The barrier to cross TS2 is only
27.73 kcal/mol. It is thus clear that the formation mecha-
nism of N-benzyl DABCO halide salts follows SN2
mechanism i.e., the nucleophilic N15 atom in DABCO
attacks the positively charged C1 atom behind the Cl atom
in R to give N-benzyl DABCO cation resulting in hetero-
lytic cleavage of C1–Cl. This result explains the easy
synthesis of N-benzyl DABCO halide salts.
Hydrogen bonds are formed in these two structures: C1–
0
˚
HꢀꢀꢀN bond with a distance of 2.40 A in R , and a two-
0
˚
center hydrogen bond in P with a distance of 2.29 A for
˚
C1–HꢀꢀꢀCl and 2.46 A for C5–HꢀꢀꢀCl. The overall reaction
is found to be endothermic by 5.36 kcal/mol.
123

Struct Chem
˚
The two hydrogen bond distances are found to be 2.22 A in
The five-membered ring transition state obtained by DFT
˚
calculations, labeled as TS3, offers an energy barrier of
28.66 kcal/mol. It involves the attack by nucleophilic
nitrogen atom in DABCO on the positively charged C1 atom
C1–HꢀꢀꢀCl and 2.46 A in C5–HꢀꢀꢀCl.
The other possible transition state suggested by the
present calculations follows SN2-type mechanism where
the N atom in DABCO attacks the C1 from opposite side of
chlorine atom and is labeled as TS4. The geometrical
parameters of TS4 show that C1–N15 bond is being formed
and C1–Cl bond is breaking as confirmed by change of
in R0 from the side of Cl atom, leading to formation of C1–
˚
15 bond (3.36 A) and breaking the C1–Cl bond (from 1.86
N
˚
to 3.03 A) as confirmed by the transition vector corre-
sponding to negative imaginary frequency (86.50i cm-1).
Scheme 2 Menshutkin reaction
between DABCO and
chlorodiphenylmethane
N
Cl
N
N
H
+
H
Cl
N
1-diphenylmethyl-4-aza-bicyclo-
[2.2.2]octane chloride
DABCO
chlorodiphenylmethane
Fig. 3 The optimal structures
of reactant-like precursor (R0),
transition state TS3, transition
state TS4, and product (P0). The
distances are in angstroms
(Color figure online)
123

Struct Chem
involved in reaction mechanism are likely to have strong
interactions with solvent and are considered in the calcu-
lations by including solvent effects.
Solvent effects
For reaction of DABCO with benzyl chloride in diethyl
ether as solvent, the structures of reactant precursor, five-
membered ring transition state, SN2 transition state, and the
product are labeled as R00, TS5, TS6, and P00, respectively.
While in gas phase, the energy barrier to cross TS1 is
11 kcal/mol higher than that for TS2; in the solvent the gap
widens and the barrier to cross TS5 is 21.48 kcal/mol
higher than that for TS6 (see Fig. 5). This result is unsur-
prising as diethyl ether being a polar solvent stabilizes the
polar SN2 transition state more than the five-membered
ring transition state [as already discussed, SN2 mechanism
involves highly polar transition state; TS2 has higher par-
tial negative charges on C1 and Cl (-0.46C and -0.71C)
than that in TS1 (-0.16C and -0.58 C)]. These calcula-
tions show that the reaction between DABCO and benzyl
chloride in diethyl ether proceeds through SN2 transition
state mechanism. The overall reaction is found to be exo-
thermic by 14.86 kcal/mol in diethyl ether due to the sol-
vation of polar product contrary to gas phase in which
reaction is endothermic by 1.83 kcal/mol.
Fig. 4 The calculated electronic energy profile for the reaction of
DABCO with chlorodiphenyl methane along reaction coordinates.
Energies are in kcal/mol
˚
˚
bond lengths (2.53 A for C1–N15 and from 1.86 to 2.95 A
for C1–Cl bond) and transition vector corresponding to
negative imaginary frequency (79.13i cm-1). The barrier
to cross TS4 is 28.79 kcal/mol.
The negligible energy difference between the two tran-
sitions states (see Fig. 4) indicates that Menshutkin reac-
tion between chlorodiphenyl methane and DABCO may
proceed through both the five-membered ring transition
state mechanism (TS3) as well as SN2 transition state
mechanism (TS4). It is an important conclusion in light of
the earlier discussed fact that SN2-type mechanism (TS2) is
energetically favorable over the five-membered ring tran-
sition state mechanism (TS1) for the reaction between
benzyl chloride and DBACO. Thus, the presence of two
phenyl rings in the reactant plays a decisive role in the
reaction mechanism and it has lowered the energy required
to reach TS3. This observation has been rationalized in
terms of the calculated ESP charges that indicate resonance
stabilization of partial negative charge on C1 in TS3. Two
phenyl groups lead to enhanced resonance stabilization of
five-membered ring transition state for the reaction
between DABCO and chlorodiphenylmethane (TS3 for
Scheme 2) in comparison to the dispersal of negative
charge on one phenyl group attached to C1 in TS1
(Scheme 1) for the reaction of DABCO with benzyl chlo-
ride. As a result, there is a higher partial negative charge on
Cl atom (-0.65 C) in TS3 than on Cl atom (-0.58 C) in
TS1. Since hydrogen bonding strengthens with increase in
electronegativity of atom involved in hydrogen bonding,
the extent of hydrogen bonding is more in TS3 than in TS1.
The present calculations have elucidated the reaction
mechanism and role of phenyl groups for the Menshutkin
reaction having DABCO framework. However, only gas-
phase calculations are not sufficient for fixing the mecha-
nism of Menshutkin reaction. The polar transition states
For the reaction of DABCO with chlorodiphenylme-
thane in diethyl ether, energy barrier to cross TS7 (five-
membered ring transition state) is 6.28 kcal/mol higher
than that of TS8 (SN2-type transition state) as shown in
Fig. 6. This is contrary to the gas-phase reaction where
enhanced resonance stabilization and hydrogen bonding
made five-membered ring transition state (TS3) as facile as
the SN2-type transition state (TS4). The contradiction in
polar dielectric medium such as diethyl ether w.r.t. gas-
Fig. 5 The calculated electronic energy profile for the reaction of
DABCO with benzyl chloride along reaction coordinates in diethyl
ether. The relative energies are in kcal/mol
123

Struct Chem
transition state mechanism, the reaction with chloro-
diphenylmethane is possible through both five-membered
ring transition state and SN2 transition state mechanism.
However, SN2 mechanism is still strongly preferred as
compared to five-membered ring transition state mecha-
nism. The role of resonance stabilization and solvent
interactions in guiding the reaction pathway has been
established. The molecular level understanding of the
commercially important Menshutkin reaction gained in the
present study is believed to go a long way in planning the
synthesis of DABCO based-ILs with desired catalytic
activity.
Acknowledgments A.P.S. thanks the Grant from Council of Sci-
entific and Industrial Research (No. 09/135(0650)/2011-EMR-I) for
this research work.
Fig. 6 The calculated electronic energy profile for the reaction of
DABCO with chlorodiphenyl methane along reaction coordinates in
diethyl ether. Energies are in kcal/mol
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In this work, synthesis mechanism of DABCO-based IL,
electronic structure, and both gas-phase and solvent-phase
reaction routes through IRC calculations have been inves-
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charges and solvent interactions for the Menshutkin reac-
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DABCO and benzyl chloride proceeds through polar SN2
123

Struct Chem
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