ORIGINAL RESEARCH

Theoretical study of DABCO-based ionic liquid: synthesisand reaction mechanism

Amritpal Singh • Paramjit Singh • Neetu Goel

Received: 14 August 2013 / Accepted: 10 September 2013

� 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.

Keywords DFT � Menshutkin reaction � Transition

state � Ionic liquid

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

Electronic supplementary material The online version of thisarticle (doi:10.1007/s11224-013-0348-4) contains supplementarymaterial, 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

DOI 10.1007/s11224-013-0348-4

have considerable effect on mechanism of Menshutkin

reaction through favorable interactions with polar transition

states [11, 16, 17].

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.

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.

Experimental

Synthesis of novel Bronsted acidic IL

In recent years, the catalytic activity of DABCO-based

Bronsted acidic ILs has been investigated for reactions

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

DABCO with benzyl chloride in dry diethyl ether. The 1H

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,

N

N

+Dry Et2O

overnight stirring N

N

Cl

Cl

I85% yield

4-Aza-1-benzyl-1-azonia-bicyclo[2.2.2]octane chloride

OS

O

O Dry DCE,

700C, Stirring

8h

N

N

SO3

Cl

81% yield

2 H2SO4(0.624 N)

MeOH,700C, Stirring 8h

N

N

SO3H

HSO4

HSO4

III IL II

86% yield

DABCO Benzyl chloride

Scheme 1 Synthesis of IL

1-benzyl-4-(sulfobutyl)-diaza-

bicyclo-octane hydrogen sulfate

(III)

Struct Chem

123

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.

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).

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

ILs III (1.65 g, yield 86 %). 1H 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).

Computational details

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.

Results and discussion

Synthesis mechanism

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.

Hydrogen bonding in reactant-like precursor R and

product P is shown in Fig. 1 with C1–H���N H-bond dis-

tance of 2.40 A in R and bifurcated H-bonds in P i.e., C1–

H���Cl and C5–H���Cl 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

be confirmed from transition vector corresponding to

imaginary frequency (222.07i cm-1). The two centered

H-bond distances of C1–H���Cl and C5–H���Cl H-bond

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�

N

N

+N

N

Cl

Cl

4-Aza-1-benzyl-1-azonia-bicyclo[2.2.2]octane chloride

DABCO Benzyl chloride

Struct Chem

123

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.

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

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.

Hydrogen bonds are formed in these two structures: C1–

H���N bond with a distance of 2.40 A in R0, and a two-

center hydrogen bond in P0 with a distance of 2.29 A for

C1–H���Cl and 2.46 A for C5–H���Cl. The overall reaction

is found to be endothermic by 5.36 kcal/mol.

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)

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

Struct Chem

123

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

in R0 from the side of Cl atom, leading to formation of C1–

N15 bond (3.36 A) and breaking the C1–Cl bond (from 1.86

to 3.03 A) as confirmed by the transition vector corre-

sponding to negative imaginary frequency (86.50i cm-1).

The two hydrogen bond distances are found to be 2.22 A in

C1–H���Cl and 2.46 A in C5–H���Cl.

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

N

N+

N

N

Cl

Cl

chlorodiphenylmethane1-diphenylmethyl-4-aza-bicyclo-[2.2.2]octane chlorideDABCO

HH

Scheme 2 Menshutkin reaction

between DABCO and

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)

Struct Chem

123

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

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.

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. 4 The calculated electronic energy profile for the reaction of

DABCO with chlorodiphenyl methane along reaction coordinates.

Energies are in kcal/mol

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

Struct Chem

123

phase mechanism can be explained considering the inter-

actions of the polar solvent medium with the highly polar

transition state involved in the SN2 mechanism. This causes

stabilization of the transition state, and the reaction

between DABCO and chlorodiphenylmethane in diethyl

ether preferably proceeds through TS8 as the solvation

interactions with the polar diethyl ether solvent are suffi-

ciently strong to overcome the resonance stabilization

dominant in the gas-phase reaction mechanism.

However, it is noteworthy that TS7 offers energy barrier

which is only 6.28 kcal/mol higher than that of TS8 in

comparison to the energy barrier difference of 21.48 kcal/

mol between TS5 and TS6. This implies that effective charge

dispersal in case of reaction with chlorodiphenylmethane

due to the presence of two phenyl rings is prominently evi-

dent in gas-phase reaction and sufficiently lowers the barrier

for five-membered ring transition state. Though this effect

persists in the polar solvent medium, the course of reaction is

governed by solvent interactions with the polar SN2 transi-

tion state. The exothermic nature of the reaction between

DABCO and chlorodiphenylmethane in diethyl ether is also

attributed to solvation effects of diethyl ether.

Conclusion

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-

tigated at the B3LYP/6-31G(d,p) level of theory. The

favorable pathway considering the energetic, partial atomic

charges and solvent interactions for the Menshutkin reac-

tion mechanism is elucidated and reported here for the first

time, to the best of our knowledge. While reaction between

DABCO and benzyl chloride proceeds through polar SN2

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.

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