Solvolysis of 2-adamantyl p-toluenesulfonate in ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate

Article abstract of DOI:10.1016/j.tetlet.2015.09.056

The solvolytic rate constants of 2-adamantyl p-toluenesulfonate were determined in [BMIM][BF₄] in the presence/absence of 2,6-lutidine by NMR measurements. The activation parameters suggest that the reaction proceeded by k_c mechanism. The solvolysis quantitatively gave 2-adamantanol as a product by the reaction with the water present in the ionic liquid. The solvent ionizing power of [BMIM][BF₄] was found to be similar to that of ethanol.

Full text of DOI:10.1016/j.tetlet.2015.09.056

Tetrahedron Letters 56 (2015) 6066–6068
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Solvolysis of 2-adamantyl p-toluenesulfonate in ionic liquid
1-butyl-3-methylimidazolium tetrafluoroborate
⇑
⇑
Takao Okazaki , Koji Ito, Toshikazu Kitagawa
Department of Chemistry for Materials, Graduate School of Engineering, Mie University, Tsu, Mie 514-8507, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The solvolytic rate constants of 2-adamantyl p-toluenesulfonate were determined in [BMIM][BF ] in the
4
Received 12 August 2015
Revised 9 September 2015
Accepted 16 September 2015
Available online 16 September 2015
presence/absence of 2,6-lutidine by NMR measurements. The activation parameters suggest that the
reaction proceeded by k mechanism. The solvolysis quantitatively gave 2-adamantanol as a product
c
by the reaction with the water present in the ionic liquid. The solvent ionizing power of [BMIM][BF
was found to be similar to that of ethanol.
4
]
Ó 2015 Elsevier Ltd. All rights reserved.
Keywords:
Ionic liquids
Solvent ionizing power
Solvolysis
2
-Adamantyl
Kinetics
Grunwald–Winstein relationship
Introduction
S
N
1 solvolysis,4 which is accelerated by polar solvents. Grunwald
and Winstein defined Y using Eq. 1,
logðk=k Þ ¼ Y
where k is the first-order rate constant of the solvolysis of tert-
Ionic liquids have attracted much attention as a new type of sol-
vents with properties interesting to many scientists and engineers
0
ð1Þ
1
–3
in recent years.
Ionic liquids are fluid salts composed of stable
0
organic cations and either organic or inorganic anions. Commonly
used types of the ionic liquids include 1,3-dialkylimidazolium,
alkylammonium, tetraalkylammonium, tetraalkylphosphonium,
and N-alkylpyridinium salts formed with bis(trifluoromethylsul-
fonyl)imide, tetrafluoroborate, hexafluorophosphate, and perfluo-
roalkylsulfonate ions. They are also considered ‘green solvents’ in
electrochemistry and organic synthesis due to their non-volatility,
non-flammability, reusability, and high polarity. 1,3-Dialkylimida-
butyl chloride in 80% ethanol at 25 °C, and k is the corresponding
rate constant in another solvent. Later, Bentley and Schleyer defined
the solvent ionizing power for tosylates, YOTs, using the rate con-
stants for 2-adamantyl p-toluenesulfonate (1, Fig. 1) as a reference
substrate. Although it is a secondary substrate, 1 solvolyzes by an
N c
S 1 (limiting) or k mechanism without nucleophilic solvent assis-
5
tance at the rate determining step.
Creary reported the solvolytic rates for 1-adamantyl methane-
sulfonate and 2-adamantyl trifluoromethanesulfonate in 1-butyl-
3-methylimidazolium bis(trifluoromethylsulfonyl)imide, [BMIM]
zolium-based ionic liquids such as [BMIM][BF
BMIM][PF ], and [BMIM][TfO] are popular among the known ionic
liquids (Fig. 1).
4 2
], [BMIM][Tf N],
[
6
1
–3
The liquids have been used for solvolysis,
[Tf
higher than those found in common polar protic solvents. The ion-
izing power of [BMIM][Tf N] for the mesylate was comparable to
that of ethanol. Okamura and Usui investigated the solvolysis of
2-methyl-2-(p-methoxyphenyl)-1-propyl p-fluorobenzenesul-
fonate, which reacted via an aryl-assisted route (a k process), in
imidazolium-based ionic liquid [BMIM][PF ] and [EMIM][BF
Chiappe reported the reaction of 1-iodoadamantane with NaN
2
N] (Fig. 1). The rates in the ionic liquid were not extraordinarily
nucleophilic reactions, electrophilic reactions, Diels–Alder
reactions, and catalytic reactions.
2
6
Polarity, which is one of the most fundamental and important
parameters for characterizing the solvent effect in chemical reac-
tions, has been quantified by solvent polarity scales based on
physicochemical properties such as rate constants, equilibrium
constants, and spectral absorption shifts.1 These scales include
the solvent ionizing power, Y, derived from the rate constants of
D
7
6
4
].
,2
3
in
8
[BMIM][PF
6
] and [BMIM][Tf
2
N]. 2-Bromoadamantane did not
react with NaN
3
in ionic liquids due to low nucleofugality of the
leaving group. Some other solvolytic studies have reported the
effect of added ionic liquids in methanol and in benzyl alcohol.9
⇑
Corresponding authors. Tel./fax: +81 59 231 9418.
E-mail addresses: okazaki@chem.mie-u.ac.jp (T. Okazaki), kitagawa@chem.
mie-u.ac.jp (T. Kitagawa).
http://dx.doi.org/10.1016/j.tetlet.2015.09.056
0
040-4039/Ó 2015 Elsevier Ltd. All rights reserved.

T. Okazaki et al. / Tetrahedron Letters 56 (2015) 6066–6068
6067
2
characterized by negative activation entropy, while nearly zero
entropy is usually observed for a unimolecular reaction.12
The first-order rate constant of 1 at 25 °C was estimated to be
OTs
OH
1
1
2
ꢀ10 ꢀ1
8
.0 ꢁ 10
The solvent ionizing power of YOTs for [BMIM][BF
be around ꢀ1.5, which is only slightly larger than that of ethanol
s
by extrapolation of the data at other temperatures.
4
] is calculated to
Me
N
Bu
N
N
Me
Bu Me
Et
N
N
N N
(Y
OTs = ꢀ1.75). These results are in line with the data for [BMIM]
SO CF3
2
BF4-
-
[Tf
N] reported by Creary.
Similarities to ethanol have been also reported in the research
BF
4
2
SO CF3
2
[
BMIM][Tf N] [BMIM][BF4] [EMIM][BF
4
]
1b,13
2
T
using the Dimroth–Reichardt E (30) scale.
This scale is one of
the most popular empirical polarity scales employing changes in
Figure 1. Structures of 2-adamantyl compounds (1 and 2), [BMIM][Tf
BF ], and [EMIM][BF ].
2
N], [BMIM]
⁄
the
p–p
absorption band of solvatochromic 2,6-diphenyl-4-
[
4
4
(
3
2,4,6-triphenyl-1-pyridino)phenolate (Reichardt’s betaine dye
0). The absorption is shifted to shorter wavelength with
increasing solvent polarity. Systematic studies have
The value of the solvent ionizing power generally depends on
the reference substrates and leaving groups. Herein we report a
solvolytic study of 1, which has been used as the most fundamental
reference compound for the determination of the solvent ionizing
demonstrated that 1,3-dialkylimidazolium salts are less polar
than water, and are similar to polar organic solvents (methanol,
1
,13
ethanol, 1-propanol, and acetonitrile).
BMIM][Tf N], ethanol, [BMIM][BF ], 80% ethanol, and methanol
have been determined to be 51.5, 51.9, 52.5, 53.7, and
T
The E (30) scales of
power, in ionic liquid [BMIM][BF
solvent polarity on the rates of the k
estimation of the solvation effect of an ionic liquid. [BMIM][BF
one of the most commonly used ionic liquids in organic
4
] to investigate the effect of its
route. This is a novel kinetic
] is
[
2
4
c
4
ꢀ1
5
5.8 kcal mol , respectively. [BMIM][BF
4
] is only slightly more
2
,3
polar than [BMIM][Tf N] and ethanol.
synthesis.
2
The Y values are determined from kinetic data, which are corre-
lated with microscopic change in solvation during the ionization.14
Although it has been uncertain that the solvatochromic scale can
apply to the heterolytic reactions in the ionic liquids, similar aspect
Results and discussion
4
Ionic liquid [BMIM][BF ] was prepared by a method similar to
6,10,11
was confirmed between YOTs and E
nol. Therefore, the effect of solvation by the ionic liquids on the
heterolytic reactions could be interpreted using the E (30) scale.
T 4
(30) for [BMIM][BF ] and etha-
those described in the literature.
[BMIM][Cl] was synthesized
by the reaction of 1-methylimidazole with 1-chlorobutane in ace-
tonitrile and was purified by recrystallization in acetonitrile-ethyl
T
The rate constant of 1 only slightly decreased in the presence of
a 2,6-lutidine buffer. An addition of 3% water resulted in a 4-fold
rate enhancement. The addition of 2,6-lutidine and the increase
in water concentration did not influence effectively the solvolytic
rates of this system.
Creary has found the large enhancement of the solvolysis rate
2
for 1-adamantyl mesylate in [BMIM][Tf N] by addition of small
amounts of water, whereas the enhancement effect is small for
acetate. [BMIM][BF
BMIM][Cl] with NaBF
checked by NMR.
Kinetic studies were carried out by following the solvolysis with
H NMR spectroscopy. Aliquots of a solution of 1 in [BMIM][BF
4
] was prepared by exchanging the anion of
[
4
twice. The purity of the ionic liquid was
1
4
]
were sealed in ampules and were heated at 75, 100, and 125 °C
for various reaction times. The products were extracted with ether
and were analyzed by 1H NMR. Tosylate 1 was found to give
the solvolysis of 1-(tert-butyl)-3,3-dimethyl-2-oxobutyl triflate
cleanly only 2-adamantanol (2) as the product by the reaction with
the water present in the ionic liquid. Rate constants were deter-
6
that reacts via a k
D
route. Since both 1 and 1-adamantyl mesylate
c
are widely accepted to solvolyze through k process, the significant
mined from plots of ln(1 ꢀ A
p t p t
/A ) versus time, where A and A
enhancement effect might be attributed to the mesylate leaving
group. The ion pair return might be suppressed by increasing the
rate for product formation process by the solvent attack at the
ion-pair stage for the mesylate.
are the area of the H-2 signal for 2 and the total area of the H-2 sig-
nals for both 1 and 2, respectively. The formation of 2 was found to
follow the first order kinetics, and the rate constants and activation
parameters are summarized in Table 1. The activation entropy
à
(D
S ) was small and slightly positive, suggesting that the reaction
proceeds by the S
N
1 mechanism, and that 2 is unlikely to have
Conclusion
formed by the nucleophilic attack on the sulfur atom of the
p-toluenesulfonate group with S–O bond cleavage. The hydrolysis
of sulfonate esters that proceeds via S–O bond fission is
In summary, the solvent ionizing power of [BMIM][BF
on kinetic measurements was found to be similar to that of
4
] based
Table 1
Kinetic data for the solvolysis of 2-adamantyl p-toluenesulfonate (1) in [BMIM][BF
4 2
] (0.2 wt % H O)
k (s 1
ꢀ
)
D
H
à
(kcal mol
ꢀ1
)
DS (cal K mol )
à
ꢀ1
ꢀ1
Run no.
2,6-Lutidine (M)
Temp (°C)
ꢀ
4
1
2
3
4
None
None
None
0.068
0.068
None
None
None
125
125
125
125
125
100
100
75
3.1 ꢁ 10
4.0 ꢁ 10
ꢀ
4
ꢀ
ꢀ
4
4 ꢁ 10
2 ꢁ 10
4
a
ꢀ4
5
8.2 ꢁ 10
3.2 ꢁ 10
2.8 ꢁ 10
1.3 ꢁ 10
ꢀ
ꢀ
ꢀ
5
6
7
8
5
6
10b
2
5
8.0 ꢁ 10ꢀ
30
0.8
a
3
% (v/v) of water was added.
b
Extrapolated from data at the higher temperatures.

6
068
T. Okazaki et al. / Tetrahedron Letters 56 (2015) 6066–6068
ethanol. The solvolysis of 1 in [BMIM][BF
4
] was found to give quan-
References and notes
titatively 2 through the S 1 mechanism by the NMR analysis.
N
1
.
(a) Welton, T., 2nd ed. In Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T.,
Eds.; Wiley-VCH: Weinheim, 2008. Chapter 3.5; (b) Reichardt, C.; Welton, T.
Solvents and Solvent Effects in Organic Chemistry; Wiley-VCH: Weinheim, 2011;
(c) Zhang, S.; Chen, Z.; Qi, X.; Deng, Y. New J. Chem. 2012, 36, 1043–1050; (d)
Chiappe, C.; Pomelli, C. S.; Rajamani, S. J. Phys. Chem. B 2011, 115, 9653–9661;
The present and further study of Y scale of ionic liquids would
elucidate the microscopic effect of solvation of the ionic liquids
during the heterolytic reactions that are used in synthesis of vari-
ous functional compounds. Since the polarity of the ionic liquids
can be tuned by changing the counter anion, comparison of the Y
scale of the imidazolium-based ionic liquids with various anions
will reveal the role of the anions, and the research will provide
the fundamental data to develop fine-tuned ionic liquids as envi-
ronmentally friendly (green) solvents with desired solvation effect
for enhancement of product yield and selectivity.
(
e) Chiappe, C.; Pieraccini, D. J. Phys. Org. Chem. 2005, 18, 275–297.
2
.
Greaves, T. L.; Drummond, C. J. Chem. Rev. 2008, 108, 206–237.
3. (a) Chiappe, C., 2nd ed. In Ionic Liquids in Synthesis; Wasserscheid, P., Welton, T.,
Eds.; Wiley-VCH: Weinheim, 2008. Chapter 5.1; (b) Earle, M., 2nd ed. In Ionic
Liquids in Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim,
2008. Chapter 5.2; (c) Wasserscheid, P.; Schulz, P., 2nd ed. In Ionic Liquids in
Synthesis; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, 2008.
Chapter 5.3; (d) Welton, T. Chem. Rev. 1999, 99, 2071–2084; (e) Hallet, J. P.;
Welton, T. Chem. Rev. 2011, 111, 3508–3576; (f) Martins, M. A. P.; Frizzo, C. P.;
Moreira, D. N.; Zanatta, N.; Bonacorso, H. G. Chem. Rev. 2008, 108, 2015–2050;
(g) Wasserscheid, P.; Keim, W. Angew. Chem., Int. Ed. 2000, 39, 3772–3789; (h)
Okazaki, T.; Laali, K. K.; Bunge, S. D.; Adas, S. K. Eur. J. Org. Chem. 2014, 1630–
Experimental
1
644; (i) Hubbard, A.; Okazaki, T.; Laali, K. K. J. Org. Chem. 2008, 73, 316–319;
NMR spectra were recorded on a 300 MHz spectrometer at
room temperature. 2-Adamantyl p-toluenesulfonate (1) was pre-
pared by the reaction of 2-adamantanol (2) with p-toluenesulfonyl
(
j) Laali, K. K.; Okazaki, T.; Bunge, S. D. J. Org. Chem. 2007, 72, 6758–6762.
4. Grunwald, E.; Winstein, S. J. Am. Chem. Soc. 1948, 70, 846–854.
5. (a) Schadt, F. L.; Bentley, T. W.; Schleyer, P. v. R. J. Am. Chem. Soc. 1976, 98,
7667–7674; (b) Bentley, T. W.; Llewellyn, G. Prog. Phys. Org. Chem. 1990, 17,
15
10
chloride in the usual manner. Ionic liquids [BMIM][Cl] and
121–158.
1
1
[
BMIM][BF
4
]
were prepared by methods similar to those reported
6
. Creary, X.; Willis, E. D.; Gagnon, M. J. Am. Chem. Soc. 2005, 127, 18114–18120.
in literature. The other reagents were commercially available and
were used as received.
7. Okamura, M.; Nagashima, M.; Ookami, K.; Usui, S. In Proceedings of the 55th
Symposium on Organic Reactions in Niigata, Japan, 2005, 97.
8
9
.
.
Chiappe, C.; Pieraccini, D.; Saullo, P. J. Org. Chem. 2003, 68, 6710–6715.
(a) Man, B. Y. W.; Hook, J. M.; Harper, J. B. Tetrahedron Lett. 2005, 46, 7641–
7645; (b) Kochly, E. D.; Citrak, S.; Gathondu, N.; Amberchan, G. Tetrahedron Lett.
Kinetics
2
014, 55, 7181–7185.
0. Dupond, J.; Consorti, C. S.; Suarez, P. A. Z.; Souza, R. F. Org. Synth. 2002, 79, 236–
43.
11. Creary, X.; Willis, E. D. Org. Synth. 2005, 82, 166–169.
1
The solvolytic rates were determined using the usual sealed
ampule technique. A solution of 1 (0.020 M) in [BMIM][BF ] was
4
2
1
2. (a) Benkovic, S. J.; Benkovic, P. A. J. Am. Chem. Soc. 1966, 88, 5504–5511; (b)
Anslyn, E. V.; Dougherty, D. A. Modern Physical Organic Chemistry; Springer:
Sausalito, 2006. Chapter 7.
prepared, and glass ampules were filled with 0.5 mL aliquots of
the solution and then sealed. These ampules were left in oil ther-
mostats for desired periods of time. The contents of the ampules
were extracted five times with ether. The combined ether layers
13. (a) Muldoon, M. J.; Gordon, C. M.; Dunkin, I. R. J. Chem. Soc., Perkin Trans. 2 2001,
33–435; (b) Reichardt, C. Green Chem. 2005, 7, 339–351; (c) Poole, C. F. J.
Chromatogr. A 2004, 1037, 49–82; (d) Poole, C. F.; Poole, S. K. J. Chromatogr. A
010, 1217, 2268–2286.
4
1
were concentrated and analyzed by H NMR. Only 2 was detected
2
as a product. The first-order rate constants were calculated using
standard least-squares procedures.
14. Bently, T. W.; Carter, G. E.; Roberts, K. J. Org. Chem. 1984, 49, 5183–5189.
15. (a) Schleyer, P. v. R.; Nicholas, R. D. J. Am. Chem. Soc. 1961, 83, 182–187; (b)
Fieser, L.; Fieser, M. Reagents for Organic Synthesis; Wiley: New York, 1967.
1179.
Acknowledgment
This work was supported in part by a Grant-in-Aid for Scientific
Research on Innovative Areas ‘Reaction Integration’ (21106013),
Japan.