Alkyl and dialkylammonium tetrafluoroborate catalyzed cis-trans isomerization of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane

Article abstract of DOI:10.1016/j.tet.2004.07.074

Alkyl and dialkylammonium tetrafluoroborate promoted cis-trans isomerization of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane (1) in DMSO-d₆ were studied. The isomerization equilibrium constant K are within the range of 3.74-3.30 from 22 to 4

Full text of DOI:10.1016/j.tet.2004.07.074

Tetrahedron 60 (2004) 9205–9212
Alkyl and dialkylammonium tetrafluoroborate catalyzed cis–trans
isomerization of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane
a
Chiou-Ling Chang, Man-kit Leung
a,b,*
a
and Mei-Hui Yang
a
Department of Chemistry, National Taiwan University, Taipei 106, Taiwan, ROC
b
Institute of Polymer Science and Engineering, National Taiwan University, Taipei 106, Taiwan, ROC
Received 27 April 2004; revised 9 July 2004; accepted 23 July 2004
Available online 27 August 2004
Abstract—Alkyl and dialkylammonium tetrafluoroborate promoted cis–trans isomerization of 1,3,5-trimethyl-1,3,5-triphenylcyclotrisiloxane
1) in DMSO-d were studied. The isomerization equilibrium constant K are within the range of 3.74–3.30 from 22 to 47 8C. Thermodynamic
parameters of DH8 and DS8 for the isomerization were K0.95 kcal/mol and K0.59 cal/mol-K respectively. The isomerization rate is first order in
(
6
C
K
[cis-1] and second order in [R
catalytic strength follows the decreasing order of
n
NH4KnBF
4
]. Both components of R
H
n
4
NH4Kn and BF are essential for the catalytic cis–trans isomerization. The
C
C
3
C
C
C
C
3
N(CH
2
)
6
NH
On-C
8
H
17NH
3
On-C16
3 3 3 2 3
H33NH OMe CNH OPhCH NH O
C
C
C
2 2 2 3 3 f
Et NH [Ph CHNH , Et NH . Inversion region was observed in the plot of ln(k /T) versus (1/T) with the ceiling located at around 38 8C.
The positive activation enthalpy of 9 kcal/mol was estimated at 22–32 8C. The activation enthalpy turns to be slightly negative at TO38 8C.
q 2004 Elsevier Ltd. All rights reserved.
1. Introduction
cis–trans Isomerization of cis-1 by ZnCl or FeCl as the
2 3
catalyst in CH NO at elevated temperature has been first
3
2
7
Because of their special physical properties and chemical
stability, polysiloxanes have been widely used in electrical,
textile, plastic, paper, automobile, with minor amounts
reported by Spielvogel and Frye. Later on, the use of
pyridinium chloride or PhNH Cl in DMF as a catalyst for
3
8
the isomerization has also been reported. However,
pyridinium acetate was found to be inactive under similar
conditions, implying that the counter anion indeed plays an
important role on the above isomerization process. A ring
opening reaction mechanism was proposed by the authors in
that case.
1
going into food and medical products. Among the synthetic
approaches for polysiloxanes, cationic and anionic ring
opening polymerization of cyclic siloxanes attract many
2
research teams to study. In particular, the ring-opening
process has been considered as a key step to control the
3
kinetics and mechanisms of the polymerization. Many
research results suggested that chelation of metal cations by
the siloxane moieties would importantly affect the polym-
1.1. R NH
n
cis-1
BF promoted cis–trans isomerization of
4Kn 4
4
erization process. Explorations of the interactions between
C
siloxane monomers and metal cations or R N ions have
The synthesis of cis-1 has been reported in previous
literature. The structure of cis-1 was further confirmed by
4
5
been reported. However, systematic studies on the
9
1
0,11 1
C
X-ray crystallographic analysis before use.
cis-1 in DMSO-d (Fig. 1) shows a singlet of methyl protons
at d 0.55 ppm, which is corresponding to the resonance of
the methyl protons. cis-1 (14.9 mM) is stable in DMSO-d6
H NMR of
interactions between cyclic siloxanes and R NH
n
are
4Kn
rare. As our continuous interest in hydrogen bonding
6
interactions as well as cation binding, we have recently
investigated the possibility of using hydrogen bond donor as
a catalyst to activate cyclic siloxanes for reaction. Herein we
report novel conditions of alkyl or dialkylammonium
tetrafluoroborate promoted cis–trans isomerization of cis-
6
1
(
,3,5-trimethyl-1,3,5-triphenyl-cyclotrisiloxane (cis-1)
Scheme 1).
Keywords: cis–trans Isomerization; Isomerization; Polymerization.
*
Corresponding author. E-mail: mkleung@ntu.edu.tw
Scheme 1.
0040–4020/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2004.07.074

9206
C.-L. Chang et al. / Tetrahedron 60 (2004) 9205–9212
for several days without causing any change on the
spectrum. On addition of NH (CH ) NH (BF ) or
n-C H NH BF (5 mM), two additional methyl singlets
of the trans-isomer at d 0.46 and 0.41 ppm, with the
integration ratio of 2:1, gradually appeared. These results
suggested that the ammonium salt acts as a catalyst to
promote the cis–trans isomerization.
1.2. Kinetics and thermodynamics
3
2 6
3
4 2
Kinetic studies revealed that the isomerization of cis-1 to
trans-1 is first order in [cis-1]. The equilibrium constants
8
17
3
4
8
cat
f
for the cis–trans isomerization of 1, defined as KZk /
cat
b
k
tures in
Z[trans]eq/[cis]eq, were evaluated at various tempera-
cat
kf
To elucidate the role of the salt in this reaction, contrast
experiments were run for comparison. When cis-1 (0.1 M)
was treated with n-C H NH Cl (0.1 M) at 25 8C for three
cis-1#trans-1
cat
k
b
8
17
3
days, no reaction was observed. On the other hand,
polymerization of cis-1 (0.1 M), instead of cis–trans
isomerization, slowly proceeded in the presence of LiBF4
DMSO-d6 and are listed in Table 1. The equilibrium
constants K are within the range of 3.74–3.30 from 22–
47 8C. Thermodynamic parameters of DH8 and DS8 were
determined from a linear plot of ln K versus 1/T and were
found to be K0.95 kcal/mol and K0.59 cal/mol-K respect-
ively. Although the sign of DH8 is consistent with the
(
n-C H NH Cl and LiBF , cis–trans isomerization occurred
0.1 M). When cis-1 (0.1 M) was treated with a mixture of
8
17
3
4
along with some degree of polymerization. On the contrary,
LiCl (0.08 M) would lead to cis–trans isomerization but in
much slower rate. Even in the presence of relatively high
concentration of LiCl, only 5% of trans-1 was observed
after one day.
prediction based on the ring strain energy (E ) measure-
s
ments, in which cis-cyclotrisiloxane isomer has slightly
1
2
higher ring strain than the trans one, the DH8 of
K0.95 kcal/mol is relatively small in comparison to the
reported DE of K2.1 kcal/mol. The negative entropy of
s
All these results indicate that the cis–trans isomerization
C
K0.59 cal/mol-K may be attributed to the siloxane-DMSO
interactions that restricted the degree of freedom of the
solvent molecules. Noteworthy to mention are the facts that
the literature values of [trans]/[cis] were found to be nearly
K
requires the coexistence of both n-C H NH and BF4 as
8
17
3
C
3
K
the catalysts. Either n-C H NH or BF4 alone does not
8
17
effectively promote the cis–trans isomerization. The same
reaction occurred in CD CN, along with some degree of
1
3
equal to 3 either in methylcyclohexane or in nitro-
methane, indicating that the solvent effects on the
equilibrium are small.
3
7
K
4
polymerization. On the other hand, when BF had been
K
replaced by ClO4 ion, a complicated reaction mixture was
K
obtained. All this indicated that BF4 should have a specific
role for this isomerization.
The mathematical expression for the first order kinetics
described above is shown as the following equation.
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
½
cis-1ꢀt
1
1
K
cat
¼ Kkf 1 þ
ln
K
t þ ln
½
cis-1ꢀ
K þ 1
K
K þ 1
0
In the kinetic expression, [cis-1] is the initial concentration
o
cat
of [cis-1], kf is the rate constant for the forward reaction, t
is the reaction time elapsed, [cis-1] is the concentration of
t
1
4
cis-1] at time t. Linear regression of ln{[cis-1] /[cis-1] K
[
t
0
cat
1
equilibrium constant K had been measured in the previous
/(KC1)} versus t gave a slope of Kk (1C1/K). Since the
f
cat
equilibrium experiments, the values of kf could then be
calculated. In most of the kinetic runs, the data were
collected up to 80% conversion and the linearity of the plots
are usually good with the correlation coefficient larger than
cat
0
function of concentration of the catalyst, we tentatively
.99. Since the value of kf obtained above should be a
cat
f
P
proposed an empirical formula of k Zk [R NH
BF ] ,
4 o
f
n
4Kn
in which [R NH
BF ] is the concentration of the
4 o
n
4Kn
alkylammonium tetrafluoroborate used in the reaction, and
the original first order kinetic equation could be re-
expressed as follows.
Table 1. Equilibrium constants KZ[trans]eq/[cis]eq and the Gibbs free
energy change for the cis–trans isomerization of cis-1 at various
temperatures
Temperature (8C)
K
DG8 (kcal/mol)
22.0G0.2
26.0G0.2
37.0G0.2
47.0G0.2
3.74
3.65
3.44
3.30
K0.774
K0.770
K0.761
K0.759
1
Figure 1. H NMR spectra of NH
3
2
(CH )
6
NH
3
(BF
4
)
2
(4.46 mM) catalyzed
at:
cis–trans isomerization of cis-1 (14.9 mM) at 25.0G0.1 8C in DMSO-d
a) 26 min; (b) 93 min; (c) 177 min; (d) 244 min.
6
(

C.-L. Chang et al. / Tetrahedron 60 (2004) 9205–9212
9207
cat
f
Table 2. Determination of the reaction order P from a plot of ln[k ] versus
n 4
ln[R NH4KnBF ]
C
NH4Kn
R
n
Reaction order
C a
17NH
n-C
8
H
3
C a
2.53
2.07
2.05
2.14
PhCH
C
2
NH
2 6 3
N(CH ) NH
3
C a
H
n-C
3
C b
17NH
8
H
3
a
At TZ25.0G0.1 8C.
At TZ47.0G0.1 8C.
b
d½cis-1ꢀ
dt
cat
f
p
4 o
K
¼ k ½cis-1ꢀ ¼ k ½R NH BF ꢀ ½cis-1ꢀ:
f
n
4Kn
Figure 2. A plot of ln(k /T) versus 1/T.
f
To further understand the reaction mechanism of the cis–
trans isomerization, temperature variation experiments
Table 2 summarized the reaction orders p obtained from the
BF ] . For
the ammonium salts we studied, the p values at 25 8C were
found to be or slightly larger than 2 with an average of 2.2G
cat
slope of the plots of ln(k ) versus ln[R NH
were carried out, in which n-C
the catalyst. The order p in [n-C
H
17NH
BF
BF
was used as
] in the kinetic
f
n
4Kn
4 o
8
3
4
H
17NH
8
3
4
expression changed slightly from 2.53 to 2.14 when the
temperature was varied from 25 8C to 47 8C. Although
variation of the reaction order with temperature could be
attributed to the intrinsic behavior of this reaction,
experimental errors arising from independent kinetic runs
as well as linear regression may also lead to similar extents
of uncertainty. Since the variation of the reaction order is
relatively small, we keep therefore our assumption that the
0.2. Fractional order in pyridinum chloride catalyzed cis–
trans isomerization has been previously reported in
literature.
8
Although the reaction order obtained above is not exactly
equal to 2, particularly in the case of C H NH BF where
the p value was found to be 2.53 in the regression, their
8
17
3
4
cat
2
second order plots of kf versus [R NH
n
BF ] are very
4
order of p is equal to 2 for the following evaluation. The k
for n-C BF were measured at seven different
temperatures and the data are shown in Figure 2. Inversion
region was observed in the plot of ln(k /T) versus (1/T) with
f
4Kn
linear with the correlation coefficient larger than 0.99.
Therefore, we tentatively assigned the reaction order p as 2.
To compare the catalytic strength of different alkylammo-
8
H
17NH
3
4
f
nium tetrafluoroborates, their k values (Table 3) were
f
the ceiling at around 38 8C. The positive activation enthalpy
of 9 kcal/mol was estimated at the temperature region of
evaluated at 25G0.1 8C with [R NH
n
BF ] around
4 o
4Kn
8
22–32 8C. The activation enthalpy turns to slightly
6
mM. Their reactivity follows the decreasing order of
C
C
C
C
H N(CH ) NH On-C H NH On-C H NH OMe
3
negative at the temperature region higher than 38 8C. This
observation suggested a complex reaction mechanism for
the catalytic isomerization. Nevertheless, the existence of
the inversion region could possibly be rationalized by an
associative mechanism with a pre-complexation step
followed by a rate determining cis–trans isomerization
step. When the temperature increases, the rate determining
cis–trans isomerization step is accelerated while the pre-
complexation step is retarded. When the temperature is high
enough, the entropically less favored pre-complex for-
mation may become unfavorable that the pre-equilibrium
kinetics is no longer valid. This will lead to a non-linear plot
3
2 6
3
8
17
3
16 33
3
C
3
C
3
C
2
C
C
CNH OPhCH NH OEt NH [Ph CHNH , Et NH .
2
2
2
3
3
Less hindered primary alkylammonium cations such as
n-C H NH , n-C H NH and H N(CH ) NH show
C
3
C
3
C
C
3
8
17
16 33
3
2 6
highest reactivity for the cis–trans isomerization. For
ammonium cations bearing larger substituent such as
C
C
Me CNH and PhCH NH , or secondary alkylammonium
3
3
2
C
3
cations such as Et NH , their k values drop significantly by
2
2
f
almost one order of magnitude. In addition, very sterically
hindered Ph CHNH and Et NH or tetraalkylammonium
C
3
C
2
3
tetrafluoroborates such as Et NBF basically show no
4
4
catalytic reactivity in our study.
of ln(k /T) versus (1/T) in the temperature variation
f
The cis–trans isomerization also proceeds in CD CN but in
3
experiments.
much slower rate. The isomerization kinetics was found to
be second order in [C H NH BF ] with the k value of
Although the explanation for the slightly negative activation
enthalpy at the high temperature region may not be
immediately obvious, the associative mechanistic argument
of pre-complex formation is consistent with at least two
8
17
3
4
f
K1
K2
1
.6 min
M
mM, which is 2 order of magnitude slower than that in
under the condition of [R NH
BF ] Z
n
4Kn 4 o
6
DMSO-d6.
Table 3. Catalytic rate constants k
f
for various alkylammonium tetrafluoroborates
C
NH4Kn
K3
] (10 M)
cat
f
K1
K1
K2
R
n
[R
n
NH4KnBF
4
k
(min
)
k
f
[krel] (min
M
)
C
K2
K2
K3
K3
K3
K4
n-C
C
8
H
17NH
N(CH
3
(2)
NH
5.37
5.78
5.99
6.37
6.18
5.86
5.83
7.73
5.89
1.04!10
1.53!10
5.07!10
1.23!10
1.12!10
6.00!10
360 [1]
C
H
3
2
)
6
3
(3)
459 [1.3]
141 [0.39]
30 [0.103]
29 [0.078]
18 [0.046]
—
C
n-C16
Me CNH
PhCH
3
H33NH (4)
C
3
3
(5)
C
3
2
NH
(6)
C
2
Et
Et
2
NH
(7)
C
3
NH (8)
Very slow
Very slow
Very slow
C
CHNH
3
Ph
2
(9)
—
—
C
PheNH
3
(10)

9208
C.-L. Chang et al. / Tetrahedron 60 (2004) 9205–9212
features: (1) Sterically hindered alkylammonium salts that
do not favor the pre-complex formation usually show low
catalytic activity for the reaction; (2) Heat releasing but
entropically unfavored pre-complex formation may be a key
reason for the slightly negative activation enthalpy at higher
Et NBF to the solution of RNH BF does not significantly
4 4 3 4
alter the reaction rate of isomerization. We tentatively
attributed this result to the ion pair association of
Et NBF . In particular, in the absence of hydrogen bond
2
0
4
4
C
interactions, solvation of the bulky Et N with DMSO is
4
1
5
temperature region.
expected to be poor. Furthermore, we have observed the
reactivity-suppression effects of Et NCl on RNH BF . In an
independent experiment, we discovered that the catalytic
4
3
4
Since hydrogen bonding between organic acids and
1
6
cyclotrisiloxane has long been proposed in literature, it
is reasonable to suggest that hydrogen bond complexation
may involve in this cis–trans isomerization. However, the
chemical instability of cis-1 in the ammonium salt solution
hampered the direct study of the hydrogen bond intermedi-
ate. To elucidate the possibility of hydrogen bond complex
formation, we selected a system of hexamethylcyclotrisi-
loxane (D )/PhCH NH BF in CD CN as the model to
strength of n-C H NH BF (5.9 mM) was reduced by
1
6
33
3
4
60% in the presence of Et NCl (14 mM). This result is
4
consistent with the above assumption of ion-pair formation.
Addition of Et N salt to a solution of RNH BF would
C
4
3
4
lead to Et NBF ion-pair formation, reducing the effective
4
4
K
concentration of free BF4 ions, and hence suppressing the
catalytic reactivity of the RNH BF solution.
3
4
3
2
3
4
3
investigate. D is chemically stable and would not lead to
3
Since ion pair association in organic solvents is compli-
cated, detailed study of the reaction order would be
2
1
other products in the presence of RNH BF . In addition,
3
4
K
PhCH NH BF is the least reactive primary alkylammo-
2
relatively difficult. Although it is known that BF would
it is
3
4
4
19,22
K
dissociate into BF CF at high temperature,
nium salt in the list we have studied. Furthermore, the
reactivity of the isomerization in CD CN is lower according
to previous results.
3
unlikely to undergo this process in our case because the
reactions were carried out only at room temperature or
slightly above room temperature. Furthermore, the present
3
K
23
In CD CN, up-field shift of the N–H signal of PhCH NH BF
4
of F would lead to polymerization that has not been
observed during cis–trans isomerization. On the other hand,
3
2
3
(
0.09 M), from 6.5 ppm approaching to 5.9 ppm, was
observed when the added amount of D gradually increased.
K
BF4 has been proposed to act as a nucleophile to coordinate
directly and react with group IV compound. Although
mechanistically we do not have evidence for the complexa-
tion of cis-1 with BF , our results again indicated that BF
3
2
4
The data is summarized in Table 4. These results suggested
C
3
that hydrogen bond complexation of RNH /D occurs in
3
K
4
K
4
this case. The unusual direction of upfield-shift of the N–H
signal may be due to the change of the coordination shell
throughout complexation. Hydrogen bond accepting proper-
ties of nitriles have recently been studied by combining
is not truly innocent.
Although the details of the mechanism are not completely
understood, pre-complexation involving R NH
1
7
C
K
crystallographic data as well as theoretical calculation.
Before complexation, the RNH should be coordinated
, BF4
4Kn
n
C
3
and cis-1 before isomerization to the trans isomer is
plausible. We therefore proposed a mechanistic schematic
diagram shown in Scheme 2. We believed that a push–pull
mechanism involving hydrogen bond interaction between
K
4
with either CD CN or BF . Since both of them will have
3
C
hydrogen bonding with RNH , the N–H signal is relatively
3
down-field shift. On complexation, the coordination shell of
RNH3 will be partially occupied by a D molecule, leading
C
3
to an up-field shift of the N–H signal. Similar results were
obtained for n-C H NH BF . Since DMSO and aceto-
nitrile are hydrogen bond acceptors with similar polarity,
1
6
33
3
4
1
8
C
3
we expected that formation of RNH /cis-1 as a reactive
intermediate in DMSO is possible.
K
The role of BF4 is also an interesting target to study.
Perfluorinated complex anions such as BF , or PF have
K
4
K
6
been regarded as non-nucleophilic and inert of innocent
anions. If BF4 involves in the catalytic process, one may
1
9
K
K
expect that changing the concentration of BF4 would lead
to significant effect on the reaction rate. Therefore, we have
attempted using Et NBF as independent BF source to
K
4
4
4
K
control the BF4 concentration. However, addition of
Table 4. N–H Chemical shifts of PhCH
amounts of D
2
NH
3
BF
4
in the presence of various
3
[D
3
]/[PhCH
2
NH
3
BF
4
]
d (N–H) (ppm)
0
0
1
3
4
6
6.51
6.39
6.18
6.05
5.98
5.93
.21
.85
.15
.64
.33
At TZ25.0G0.1 8C with [PhCH
2
NH
3
BF
4
]Z0.09 M.
Scheme 2.

C.-L. Chang et al. / Tetrahedron 60 (2004) 9205–9212
9209
2. Experimental
All reactions were performed under nitrogen. Acetonitrile
1
l3
was dried over CaH2. H and C NMR spectra were
recorded on a Varian Unity plus (400 MHz). All 400 MHz
H NMR spectra were recorded in CDCl3 deuterated
1
acetonitrile (CD CN) and deuterated dimethylsulfoxide
3
Scheme 3.
(DMSO-d ) and are reported in ppm as d. The kinetic
6
1
experiments were carried out in DMSO-d . The H NMR
assignments for cis-1 and trans-1 were previously reported
6
C
and the oxygen atom of the Si–O–Si units, and
4Kn
R NH
n
K
probably coordination of BF4 with the silicon atoms are
operating for the cis–trans isomerization.
7,8,13
in literature.
using Nicolet MAGNA-IR 550 spectrometer series type FT-
Infrared spectra were obtained with KBr
K1
IR and are reported in n as cm . Elemental analysis was
performed by Heraeus CHN-O Rapid.
On the basis of this assumption, a putative mechanism is
proposed as shown in Scheme 3. We have already discussed
the possibility of R NH
n
C
4Kn
(cis-1) complex formation.
2.1. General procedure for the kinetic measurements
C
Since there is a positive charge on R NH
n
(cis-1),
4Kn
1
The kinetic measurements were performed by using H
further formation of a reactive ion pair intermediate of
R NH
C
4Kn
K
(cis-1)BF4 is not impossible. On the other hand,
n
NMR spectroscopy (Varian Unity plus (400 MHz)) with
temperature control. Solution of cis-1 and solution of the
ammonium salt with known concentrations in DMSO-d6
were prepared before measurement. Both solutions were
pre-warmed to the desired temperature, mixed, and
subjected to the NMR spectrometer for data collection.
The concentrations of cis-1 after mixing were set within the
range of 8.48 to 25.5 mM. The concentrations of the
ammonium salts were set within the range of 1.19 to
C
K
the possibility of the formation of R NH
n
through the ion pair of R NH
n
eliminated. Once the R NH
n
pseudorotational isomerization of R NH
n
4Kn
(cis-1) BF4
C
K
BF4 could not be
4Kn
C
4Kn
K
(cis-1)BF is formed,
4
C
4Kn
(cis-1)BF at
4
the pentacoordinated silicon atom may occur to give
R NH
C
4Kn
K
(trans-1)BF , followed by dissociation of the
4
n
ion-pair complex would give trans-1 as the product. This
type of pseudorotational mechanism is known as the Berry
pseudorotation and turnstile twists that have been studied
12.6 mM. The progress of the isomerization of cis-1 to
trans-1 was monitored by taking the NMR spectrum for
2
5
and reviewed in literature.
1
every 15 to 30 min. H NMR of cis-1 in DMSO-d shows a
6
A rate equation could then be derived on the basis of the
reaction scheme, in which K Z[R NH
singlet of methyl protons at d 0.55 ppm, which is
corresponding to the resonance of the methyl protons. On
addition of the ammonium salt solution, two additional
methyl singlets of the trans-isomer at d 0.46 and 0.41 ppm
gradually appeared. The ratio of [trans-1]/[cis-1] at a time
could be determined on the basis of the methyl proton
integrations. For the temperature variation experiments, the
temperature range of 25–47 8C was used. The rate constant
was determined according to the equation
0
][BF ][cis-1] is the overall equilibrium constant
C
4Kn
K
(cis-1)BF ]/
4
n
C
4Kn 4
[
for R NH
tion rate constant from R NH
n
R NH
n
C
4Kn
K
(cis-1)BF4 formation, and k is the isomeriza-
I
n
C
K
(cis-1)BF4 to the trans
4Kn
isomer.
d½cis-1ꢀ
þ
K
¼ k ½R NH ðcis-1ÞBF ꢀ
K
I
n
4Kn
4
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
dt
½
cis-1ꢀt
1
1
K
cat
¼ Kkf 1 þ
ln
K
t þ ln
½
cis-1ꢀ
K þ 1
K
K þ 1
0
0
þ
K
k K ½R NH ꢀ½BF ꢀ½cis-1ꢀ
¼
I
n
4Kn
4
By using linear least square fitting of ln([cis-1] /[cis-1] K
t
o
cat
(
1/KC1)) versus t, a slope of Kk (1C1/K)could be
f
cat
However, this equation does not straightly explain for the
observeation of the second reaction order in [R NH BF ] ,
unless we assumed that dissociation of R NH BF in
obtained. The value of kf could then be calculated.
n
4Kn
4 o
n
4Kn
4
2.2. General procedure for the preparation of
alkylammonium tetrafluoroborates
DMSO is extensive. This may be plausible due to strong
hydrogen bond interactions between R NH
.
BF If the
4
n
4Kn
assumption is valid, each equivalent of R NH BF would
Following is a typical preparative procedure for alkylam-
monium tetrafluoroborates. Alkyl or phenyl ammonium
tetrafluoroborates were prepared according to the literature
procedures of Kukhar. They were prepared by reaction of
the corresponding alkylammonium chlorides with triethyl-
oxonium tetrafluoroborate in acetonitrile. Since most of the
salts are hygroscopic, they are collected under nitrogen,
sealed and sent for microanalysis.
n
4Kn
4
C
dissociatetogiverespectivelyoneequivalentofR NH
n
and
4Kn
K
C
K
BF , and therefore [R NH ] and [BF ] are both equal to
n
4
4Kn
4
2
6
[
would give the following equation,
RnNH4KnBF ] . Substitution these into the above equation
4 o
d½cis-1ꢀ
2
4 0
K
¼ k ½R NH BF ꢀ ½cis-1ꢀ
f
n
4Kn
dt
2
.2.1. Octylammonium tetrafluoroborate (2). To a
solution of 1-octylamine (10 mL, 56.9 mmol) in CH Cl
2
where k K could be combined and denoted as the k . The
I
f
2
second order kinetics in [R NH BF ] could then be
4 o
explained.
(20 mL) in an ice bath were slowly added concentrated
hydrochloric acid (4.75 mL, 56.9 mmol). The mixture was
n
4Kn

9210
C.-L. Chang et al. / Tetrahedron 60 (2004) 9205–9212
1
stirred for another 5 h. White suspension was collected by
filtration and further purified by recrystallization from
CH Cl . The crystals were collected under nitrogen and
give colorless solid. (69%): H NMR (400 MHz, DMSO-d )
6
1
d 8.21 (s, 3H), 1.28 (s, 9H); C NMR (100 MHz, DMSO-
3
K1
d6) d 51.2, 27.4; IR (KBr) n 3200–2900 cm (NH , br, m).
C
3
2
2
dried in vacuum to give colorless octylammonium chloride
(
tert-Butylammonium tetrafluoroborate was recrystallized
from acetonitrile to give colorless solid (39%): H NMR
1
6.69 g, 71%): H NMR (400 MHz, DMSO-d ) d 8.08 (s,
1
6
1
3
3
H), 2.70 (t, JZ7.6 Hz, 2H), 1.55–1.50 (m, 2H), 1.35–1.20
(400 MHz, DMSO-d ) d 7.74 (s, 3H), 1.23 (s, 9H);
6
C
1
3
(m, 10H), 0.84 (t, JZ6.8 Hz, 3H); C NMR (100 MHz,
DMSO-d ) d 38.7, 31.2, 28.5, 26.9, 25.9, 22.1, 14.0; IR
NMR (100 MHz, DMSO-d ) d 51.7, 27.7; IR (KBr) n 3300–
6
K1
C
3
K1
3100 cm (NH , br), 1300–1000 cm (B–F, br). Anal.
6
K1
C
(
KBr) n 3400–2700 cm
(NH , br, s). To a solution of
calcd for C H BF N: C, 29.85; H, 7.51; N, 8.70. Found: C,
4
3
12
4
octylammonium chloride (3 g, 18 mmol) in dried aceto-
nitrile (18 mL) under nitrogen at room temperature was
added triethyloxonium tetrafluoroborate (Et O BF ,
29.79; H, 7.65; N, 8.43.
C
K
4
3
2.2.5. Benzylammonium tetrafluoroborate (6). Benzyl-
ammonium chloride obtained was crystallized from MeOH
18 mmol). The solution was refluxed for 24 h until out-
gassing of alkyl chloride was no longer observed. In some
1
to give colorless solid (87%): H NMR (400 MHz, DMSO-
1
occasion, the completion of reaction was traced by H
13
d6) d 8.46 (s, 3H), 7.50–7.36 (m, 5H), 3.99 (s, 2H);
C
NMR. The solid obtained after evaporation of the solvent
was further purified by recrystallization from ethyl acetate.
The product was collected by filtration under nitrogen, and
NMR (100 MHz, DMSO-d ) d 134.1, 128.9, 128.5, 128.4,
6
K1
42.1; IR (KBr) n 3200–2700 cm (NH , br), 1600 cm
C
3
K1
(C]C). Benzylammonium tetrafluoroborate obtained was
crystallized from acetonitrile to give colorless solid (75%):
1
dried in vacuum to give colorless crystals. (2.76 g, 71%): H
NMR (400 MHz, DMSO-d ) d 7.60 (s, 3H), 2.75 (t, JZ
1
6
H NMR (400 MHz, CD CN) d 7.48–7.43 (m, 5H), 6.46 (s,
3
1
3
3H), 4.15 (s, 2H); C NMR (100 MHz, CD CN) d 133.1,
130.4, 130.4, 130.1, 44.9; IR (KBr) n 3300–3100 cm
(NH , br), 1615 cm (C]C), 1200–1000 cm
Anal. calcd for C H BF N: C, 43.12; H, 5.17; N, 7.18.
7
.6 Hz, 2H), 1.52–1.46 (m, 2H), 1.35–1.20 (m, 10H), 0.85
3
1
3
K1
(
t, JZ6.8 Hz, 3H); C NMR (100 MHz, CD CN) d 41.4,
3
C
3
K1
K1
3
3
2.5, 29.8, 29.7, 27.7, 26.9, 23.4, 14.4; IR (KBr) n 3400–
200 cm (NH , br), 1200–1000 cm (B–F, br). Anal.
(B–F).
K1
C
3
K1
7
10
4
calcd for C H BF N: C, 44.27; H, 9.29; N, 6.45. Found: C,
8
Found: C, 43.44; H, 5.15; N, 7.24.
20
4
4
4.79; H, 9.42; N, 6.50.
2
.2.6. Diethylammonium tetrafluoroborate (7). Diethyl-
2
.2.2. 1,6-Hexanediammonium bistetrafluoroborate (3).
,6-Hexanediammonium dichloride obtained was purified
ammonium chloride obtained was recrystallized from
acetonitrile and followed by washing with hexane and
ether to give colorless solid (68%): H NMR (400 MHz,
1
by washing with methanol and CH Cl to give colorless
1
2
2
1
solid (75%): H NMR (400 MHz, DMSO-d ) d 8.10 (s, 6H),
6
DMSO-d ) d 9.01 (s, 2H), 2.90–2.81 (m, 4H), 1.17 (t, JZ
6
1
7.6 Hz, 6H); C NMR (100 MHz, DMSO-d ) d 41.2, 10.9;
3
2
.73 (t, JZ7.6 Hz, 4H), 1.57–1.53 (m, 4H), 1.31–1.28 (m,
6
1
H); C NMR (100 MHz, DMSO-d ) d 39.2, 27.4, 26.0; IR
3
K1
IR (KBr) n 3300–2700 cm (NH , br). Diethylammonium
C
2
4
6
K1 C
KBr) n 3400–2700 cm (NH , br). 1,6-Hexanediammo-
(
nium bistetrafluoroborate obtained was recrystallized from
tetrafluoroborate obtained was recrystallized from ethyl
acetate to give colorless solid (44%): H NMR (400 MHz,
3
1
1
acetonitrile to give colorless solid. (46%): H NMR
DMSO-d ) d 8.20 (s, 2H), 3.02 (q, JZ7.2 Hz, 4H), 1.15 (t,
6
1
3
(
400 MHz, CD CN) d 5.82 (s, 6H), 2.95 (t, JZ7.6 Hz,
JZ7.6 Hz, 6H); C NMR (100 MHz, DMSO-d ) d 42.0,
6
K1 C
3
1
H), 1.66–1.59 (m, 4H), 1.41–1.31 (m, 4H); C NMR
3
4
11.7; IR (KBr) n 3200–2900 cm
900 cm (B–F, br). Anal. calcd for C H BF N: C, 29.85;
4 12 4
(NH , br), 1300–
2
K1
(
100 MHz, CD CN) d41.2, 27.3, 26.1; IR (KBr) n 3300–
3
K1
200 cm (NH , br), 1200–1000 cm (B–F, br). Anal.
C
3
K1
3
calcd for C H B F N : C, 24.69; H, 6.22; N, 9.60. Found:
H, 7.51; N, 8.70. Found: C, 29.83; H, 7.67; N, 8.58.
6
18 2 8 2
2
7
C, 25.02; H, 6.22; N, 9.43.
.2.3. Hexadecylammonium tetrafluoroborate (4). Hex-
adecyl-ammonium chloride obtained was recrystallized
2.2.7. Triethylammonium tetrafluoroborate (8).
Triethylammonium chloride prepared was recrystallized
1
from acetonitrile to give colorless solid (48%): H NMR
2
(400 MHz, DMSO-d ) d 10.60 (s, 1H), 3.06–3.00 (m, 6H),
6
1
13
1.19 (t, JZ7.6 Hz, 9H); C NMR (100 MHz, DMSO-d ) d
from CHCl3 to give colorless solid (79%): H NMR
(
6
K1
C
(NH , br).
400 MHz, DMSO-d ) d 7.88 (s, 3H), 2.72 (t, JZ7.6 Hz,
45.2, 8.4; IR (KBr) n 3200–2900 cm
Triethylammonium tetrafluoroborate obtained was precipi-
6
2
6
2
H), 1.53–1.48 (m, 2H), 1.35–1.15 (m, 26H), 0.84 (t, JZ
1
.8 Hz, 3H); C NMR (100 MHz, DMSO-d ) d 38.7, 31.4,
3
1
tated from ethyl acetate to give colorless solid (38%): H
6
9.1, 29.1, 29.0, 28.9, 28.8, 28.6, 27.0, 25.9, 22.2, 14.1; IR
NMR (400 MHz, DMSO-d ) d 9.01 (s, 1H), 3.08 (q, JZ
6
K1
C
13
(
ammonium tetrafluoroborate was recrystallized from THF
KBr) n 3200–2700 cm
(NH , br). Hexadecyl
7.2 Hz, 6H), 1.16 (t, JZ7.6 Hz, 9H); C NMR (100 MHz,
3
K1 C
DMSO-d ) d46.0, 8.9; IR (KBr) n 3200–2900 cm (NH ,
6
1
to give colorless solid (53%): H NMR (400 MHz, DMSO-
K1
br), 1300–900 cm (B–F, br). Anal. calcd for C H BF N:
6
16
4
d ) d 7.56 (s, 3H), 2.7 (t, JZ7.6 Hz, 2H), 1.51–1.45 (m, 2H),
C, 38.13; H, 8.53; N, 7.41. Found: C, 38.30; H, 8.73; N,
7.37.
6
1
3
1
.36–1.18 (m, 26H), 0.84 (t, JZ6.8 Hz, 3H); C NMR
(
2
100 MHz, DMSO-d ) d 38.9, 31.3, 29.1, 29.0, 28.9, 28.8,
8.6, 27.1, 25.8, 22.2, 14.1; IR (KBr) n 3300–3100 cm
6
K1
2.2.8. (1,1-Diphenylmethyl)ammonium tetrafluorobo-
rate (9). (1,1-Diphenylmethyl)ammonium chloride
C
K1
(
NH , br), 1200–1000 cm
(B–F, br). Anal. calcd for
C H BF N: C, 58.36; H, 11.02; N, 4.25. Found: C, 58.68;
3
1
6
36
4
obtained was recrystallized from ethanol to give colorless
solid (91%): H NMR (400 MHz, DMSO-d ) d 9.25 (s,
1
H, 11.00; N, 4.26.
6
3
H), 7.56 (d, JZ7.2 Hz, 4H), 7.40 (t, JZ7.6 Hz 4H), 7.33 (t,
13
2.2.4. tert-Butylammonium tetrafluoroborate (5). tert-
Butylammonium chloride was crystallized from ethanol to
JZ7.6 Hz, 2H), 5.61 (s, 1H); C NMR (100 MHz,
DMSO-d ) d 138.8, 129.2, 128.7, 127.8, 57.6; IR (KBr) n
6

C.-L. Chang et al. / Tetrahedron 60 (2004) 9205–9212
K1
(C]C, s). (1,1-
9211
K1
C
(NH , br), 1610 cm
3
300–2600 cm
Bond; Plenum: New York, 1978; pp 203–212. (c) Kendrick,
T. C.; Parbhoo, B. M.; White, J. W. Comprehensive Polymer
Science; Allen, G., Bevington, J. C., Eastmond, G. C.,
Ledwith, A., Russo, S., Sigwalt, P., Eds.; Pergamon: Oxford,
1989; Vol. 4, pp 459–492. (d) Mazurek, M. Recent Advances
in Anionic Polymerization; Hogen-Esch, T. E., Smid, J., Eds.;
Elsevier: New York, 1987; pp 329–349.
3
Diphenylmethyl)ammonium tetrafluoroborate obtained was
recrystallized from CH Cl to give colorless solid (57%): H
1
2
2
NMR (400 MHz, CD CN) d 7.49–7.26 (m, 10H), 7.26 (s,
3
1
3
H), 5.69 (s, 1H); C NMR (100 MHz, CD CN) d 137.2,
3
1
3
K1
30.3, 130.3, 128.4, 60.0; IR (KBr) n 3300–3100 cm
C
K1
K1
(
Elemental analysis suggested that the salt is a mono-
hydrate. Anal. calcd for C H BF N$(H O): C, 54.01; H,
2
NH , br), 1597 cm (C]C), 1200–900 cm (B–F, br).
3
4. (a) Laita, Z.; Jel ´ı nek, M. J. Polym. Sci. 1966, B4, 739–745.
(b) Mazurek, M.; Chojnowski, J. Makromol. Chem. 1977, 178,
1005–1017. (c) Ricard, A. Eur. Polym. J. 1979, 15, 1–4.
1
3
14
4
5
.58; N, 4.85. Found: C, 54.87; H, 5.38; N, 4.92.
(
d) Grubb, W. T.; Osthoff, R. C. J. Am. Chem. Soc. 1955, 77,
405–1411. (e) Hurd, D. T.; Osthoff, R. C.; Corrin, M. L.
J. Am. Chem. Soc. 1954, 76, 249–252.
1
2.2.9. L-Phenylalanine methyl ester ammonium tetra-
fluoroborate (10). To a solution of L-phenylalanine (5.0 g,
5
6
. Olliff, C. J.; Pickering, G. R.; Rutt, K. J. J. Inorg. Nucl. Chem.
980, 42, 1201–1202.
2
0
4
9.8 mmol) in distilled MeOH (50 mL) under nitrogen at at
8C was added distilled thionyl chloride (3.28 mL,
4.7 mmol) dropwise. After addtion, the mixture was stirred
1
. (a) Leung, M.; Mandal, A. B.; Wang, C.-C.; Lee, G.-H.; Peng,
S.-M.; Cheng, H.-L.; Her, G.-R.; Chao, I.; Lu, H.-F.; Sun,
Y.-C.; Shiao, M.-Y.; Chou, P.-T. J. Am. Chem. Soc 2002, 124,
at room temperature for 24 h. The reaction was followed by
TLC, using BAW (butanol–acetic acid–H OZ4:1:1) as the
mobile phase. After completion of the reaction, methanol
was removed by distillation. Ether (400 mL) was added to
precipitate the product. The solid product was collected by
suction filtration under nitrogen, followed by washing with
iced ether, and dried under vacuum to give L-phenylalanine
2
4
287–4297. (b) Chen, J.-A.; Lai, J.-L.; Lee, G. H.; Wang, Y.;
Su, J. K.; Yeh, H.-C.; Lin, W.-Y.; Leung, M.-k. Org. Lett.
001, 3, 3999–4002. (c) Liao, C.-F.; Lai, J.-L.; Chen, J.-A.;
2
Chen, H.-T.; Cheng, H.-L.; Her, G.-R.; Su, J. K.; Wang, Y.;
Lee, G. H.; Leung, M.; Wang, C.-C. J. Org. Chem. 2001, 66,
1
2
566–2571.
. Spielvogel, D. E.; Frye, C. L. J. Organomet. Chem. 1978, 161,
65–169.
. Tsyganov, Yu. V.; Mileshkevich, V. P. Zh. Obshch. Khim.
982, 52, 1849–1854. CA 97: 163084.
methyl ester ammonium chloride (5.91 g, 91%): H NMR
(
7
8
400 MHz, DMSO-d ) d 8.53 (s, 3H), 7.35–7.23 (m, 3H),
6
1
7
3
7
1
3
1
.22 (d, JZ6.8 Hz, 2H), 4.28 (t, JZ6.8 Hz, 1H), 3.66 (s,
H), 3.14 (dd, JZ14.0, 6.0 Hz, 1H); 3.06 (dd, JZ13.8,
1
.0 Hz, 1H); C NMR (100 MHz, DMSO-d ) d 169.1,
3
1
6
34.4, 129.2, 128.4, 127.1, 53.2, 52.5, 35.9; IR (KBr) n
300–2700 cm
600 cm
9. Lewis, R. N. J. Am. Chem. Soc. 1948, 70, 1115–1117.
10. Shklover, V. E.; Bokii, N. G.; Struchkov, Yu. T.; Andrianov,
K. A.; Zavin, B. G.; Svistunov, V. S. Zh. Strukt. Khim. 1974,
15, 841–849. CA 81:178259 b.
K1
C
(NH , br), 1766 cm
K1
(C]O).
(C]C). L-phenylalanine methyl ester
3
K1
ammonium tetrafluoroborate obtained was precipitated
from CH Cl to give colorless solid. (18%): H NMR
1
11. Voronkov, M. G.; Mileshkevich, V. P.; Yuzhelevskii, Yu. A.
The Siloxane Bond; Plenum: New York, 1978, p 12.
2
2
(
400 MHz, DMSO-d ) d 8.33 (s, 3H), 7.37–7.22 (m, 3H),
6
7
3
1
.21 (d, JZ6.8 Hz, 2H), 4.32 (t, JZ6.8 Hz, 1H), 3.68 (s,
12. The values of E are 82 kJ/mol (19.6 kcal/mol) for the cis-
s
1
H), 3.12–3.02 (m, 2H); C NMR (100 MHz, DMSO-d ) d
3
isomer and 73 kJ/mol (17.5 kcal/mol) for the trans isomer.
For reference, see: Voronkov, M. G.; Klyuchnikov, V. A.;
Mironenko, E. V.; Shvets, G. N.; Danilova, T. F.; Khudobin,
Yu. I. J. Organomet. Chem. 1991, 406, 91–97.
6
69.5, 134.5, 129.4, 128.7, 127.4, 53.2, 52.7, 36.0; IR (KBr)
K1
C
K1
n 3300–2800 cm
(NH , br), 1747 cm
(C]O),
(B–F, br). Anal.
3
K1
K1
1 (C]C), 1200–900 cm
calcd for C H BF NO : C, 44.98; H, 5.28; N, 5.25.
609 cm
13. Dias, F. B.; Lima, J. C.; Ma c¸ anita, A. L.; Horta, A.; Pi e´ rola,
I. F. J. Phys. Chem. A 2000, 104, 17–24.
1
0
14
4
2
Found: C, 44.78; H, 5.25; N, 5.32.
14. Jeffers, P. M. J. Phys. Chem. 1972, 76, 2829–2832.
15. For discussion of negative enthalpy, see (a) Houk, K. N.;
Rondan, N. G. J. Am. Chem. Soc. 1984, 106, 4293–4294.
Acknowledgements
(
b) Gorman, A. A.; Lovering, G.; Rodgers, M. A. J. J. Am.
Chem. Soc. 1979, 101, 3050–3055, and references cited
therein.
We thank the National Science Council of Republic of
China for the financial support. (NSC 92-2113-M-002-020).
1
6. (a) Tsyganov, Yu. V.; Mileshkevich, V. P. J. Gen. Chem.
USSR. (Engl. Transl.) 1980, 1465–1468. (b) Wilczek, L.;
Chojnowski, J. Macromolecules 1981, 14, 9–17.
7. Le Questel, J.-Y.; Berthelot, M.; Laurence, C. J. Phys. Org.
Chem. 2000, 13, 347–358.
1
1
References and notes
8. The hydrogen bond acceptor basicities b of DMSO and
acetonitrile are 0.76 and 0.31 respectively, which indicates that
DMSO and acetonitrile both are hydrogen bond acceptors. For
reference, see Kamlet, M. J.; Abboud, J. L. M.; Taft, R. W.
Prog. Phys. Org. Chem. 1981, 13, 485–630.
1
. Burger, C.; Kreuzer, F.-H. Silicon in Polymer Synthesis;
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. (a) Brook, M. A. Silicon in Organic, Organometallic, and
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New Jersey, 1993.
2
19. Farooq, O.; Tiers, G. V. D. J. Org. Chem. 1994, 59, 2122–2124
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3
. (a) Kendrick, T. C.; Parbhoo, B.; White, J. W. The Chemistry
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B. R. J. Organomet. Chem. 1970, 21, 171–180.
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Quaternary Ammonium Salts; Academic: London, 2001, pp 8–
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to DMSO, see Marcus, Y. Ion Properties; Marcel Dekker: New
York, 1997, pp 228–229.
2
5. (a) Johnson, S. E.; Payne, J. S.; Day, R. O.; Holmes, J. M.;
Holmes, R. R. Inorg. Chem. 1989, 28, 3190–3198. (b) Ugi, I.;
Marquarding, D.; Klusacek, H.; Gillespie, P.; Ramirez, F. Acc.
Chem. Res. 1971, 4, 288–296.
2
1. (a) Miyabe, K.; Taguchi, S.; Kasahara, I.; Goto, K. J. Phys.
Chem. B 2000, 104, 8481–8490. (b) Szwarc, M. Acc. Chem.
Res. 1969, 87–96.
2
2
2. (a) Farooq, O. J. Organomet. Chem. 2000, 613, 239–243.
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3. Tartakovskaya, L. M.; Kopylov, V. M.; Zhdanov, A. A.
2
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