Dynamics for reactions of ion pairs in aqueous solution: Reactivity of tosylate anion ion paired with the highly destabilized 1-(4-methylphenyl)-2,2,2- trifluoroethyl carbocation

Article abstract of DOI:10.1002/poc.1642

The sum of the rate constants for solvolysis and scrambling of carbon bridging and nonbridging oxygen-18 at 4-MeC₆H₄CH(CF ₃)OS(¹⁸O₂)Tos in 50/50 (v/v) trifluoroethanol/water, (k_solvRk_i

Full text of DOI:10.1002/poc.1642

Research Article
Received: 7 August 2009,
Revised: 30 September 2009,
Accepted: 6 October 2009,
Published online in Wiley InterScience: 13 January 2010
(
www.interscience.wiley.com) DOI 10.1002/poc.1642
Dynamics for reactions of ion pairs in aqueous
solution: reactivity of tosylate anion ion
paired with the highly destabilized
1-(4-methylphenyl)-2,2,2-trifluoroethyl
carbocation
Minami Teshima , Yutaka Tsuji and John P. Richard *
a
a
b
The sum of the rate constants for solvolysis and scrambling of carbon bridging and nonbridging oxygen-18 at
1
8
S6 S1
4
k
-MeC
6
H
4
CH(CF
3
)OS( O
2
)Tos in 50/50 (v/v) trifluoroethanol/water, (ksolv R kiso) ¼ 5.4 T 10
s
, is 50% larger than
S6
solv ¼ 3.6 T 10 for the simple solvolysis reaction of the sulfonate ester. This shows that the ion-pair intermediate of
S1
-1
1
0
solvolysis undergoes significant internal return to form reactant. These data give a value of k ¼ 1.7 T 10
s
for
internal return of the carbocation-anion pair to the substrate. This rate constant is larger than the value of
9
S1
k
-1 ¼ 7 T 10 s reported for internal return of an ion pair between the 1-(4-methylphenyl)ethyl carbocation and
pentafluorobenzoate anion to the neutral ester (4-MeC CH(CH )O CC ) in the same solvent. The partitioning of
6
H
4
3
2
6 5
F
ion pairs to the 1-(4-methylphenyl)ethyl carbocation and to the highly destabilized 1-(4-methylphenyl)2,2,2-
trifluoroethyl carbocation is compared and contrasted. Copyright ß 2010 John Wiley & Sons, Ltd.
Keywords: Solvolysis; Carbocation; Ion-Pair; Dynamics
the results of a study of the reorganization of the ion-pair
INTRODUCTION
R
S
intermediate 4 ꢀ OSO
2 6 4
C H -4-Me of solvolysis of a ring-
substituted 1-phenyl-2,2,2-trifluoroethyl tosylate, where the
We are interested in using the experimental protocols developed
for the detection of the reaction of ion pair intermediates of sol-
carbocation is destabilized by the strongly electron-withdrawing
[
1–4]
[12–16]
volysis
to determine absolute rate constants for the very fast
a-CF
3
substituent.
This work was initiated for the following
[
5–7]
reactions of ion pairs;
mechanism.
and also to probe changes in reaction
We have prepared several chiral or isotopically
reasons.
[
8–11]
(
(
1) The a-CF for a-CH substitution has been shown to destabilize
3 3
labeled substrates for solvolysis reactions, and used these to
obtain data that provide estimates of absolute rate constant ratios for
partitioning of ion pair intermediates between competing pathways:
one pathway that leads to formation of solvolysis products, and the
second to the products of racemization of a chiral benzoate ester,
isomerization of a thionobenzoate, or exchange of oxygen-16 and
oxygen-18 between bridging and nonbridging positions at an
isotopically labeled benzoate or tosylate ester. These rate constant
ratios were combined with the appropriate absolute rate constant for
carbocation capture by solvent to obtain estimates of absolute
rate constants for reactions of carbocation anion pairs.
ring substituted 1-phenylethyl carbocations by 8–12kcal/mol
[
14–16]
relative to neutral substrate.
examining the effect of this strongly electron-withdrawing
a-CF substituent on the partitioning of ion pair intermedi-
We were interested in
[5]
3
[7]
ates of solvolysis of 1-(4-methylphenyl)ethyl derivatives.
R
S
2) The ion pair 1 ꢀC
6 5
F CO2 has been generated as an inter-
[6,9]
mediate of solvolysis of 1-(4-methylphenyl)ethyl pentafluor-
[
5,6]
obenzoate.
By comparison, carbocation-anion pairs to the
much more unstable 1-(4-methylphenyl)-2,2,2-trifluorethyl
carbocation can only be generated using a more weakly basic
[12,14,15]
leaving group anion, such as tosylate anion.
There is little
or no data for addition of tosylate anion to carbocations in
aqueous solution, so that it is not clear whether addition of this
*
Correspondence to: J. P. Richard, Department of Chemistry, University at
Buffalo, Buffalo, NY 14260, USA.
E-mail: jrichard@buffalo.edu
a
b
M. Teshima, Y. Tsuji
These earlier experiments focused on the putative ion-pair
intermediates of solvolysis of ring-substituted 1-phenylethyl
Department of Biochemistry and Applied Chemistry, Kurume National College
of Technology, Komorinomachi, Kurume 830-8555, Japan
R
S
[5]
R
S [7]
derivatives such as 1 ꢀC
6
F
5
CO ,
2 ꢀC
6
H
5
CSO
, and
2
J. P. Richard
R
S
[9,11]
3
ꢀ O
3
SC
6
H
4
-4-Me.
We now extend this work and report
Department of Chemistry, University at Buffalo, Buffalo, NY 14260, USA
J. Phys. Org. Chem. 2010, 23 730–734
Copyright ß 2010 John Wiley & Sons, Ltd.

REACTIONS OF ION PAIRS IN AQUEOUS SOLUTION
weakly basic anion competes effectively with addition of the
relatively nucleophilic solvent water. We were therefore inter-
ested in determining whether the ion pair intermediate of the
reaction of 1-(4-methylphenyl)- 2,2,2-trifluorethyl tosylate would
undergo internal return of the weakly nucleophilic sulfonate
anion in the nucleophilic solvent water.
(I ¼ 0.5, NaClO ) to give a final substrate concentration of 1.4 mM.
4
At specified reaction times the remaining substrate and reaction
products were extracted into 500 mL of toluene, the organic layer
was washed with water, dried over MgSO4 and the solvent
removed on a rotary evaporator. The remaining residue was
1
3
6 6
dissolved in 0.6 mL of C D and saved for C-NMR analysis.
Product analyses
EXPERIMENTAL
The products of the solvolysis and of the azide anion nucleophilic
substitution reactions of 4-OS(O) C H -4-Me in 50/50 (v/v)
2
6 4
All organic and inorganic chemicals were reagent grade from
commercial sources and were used without further purification.
The oxygen-18 labeled water, 99 atom % excess, was purchased
from ISOTEC, Inc. H and C NMR spectra were recorded on a
JEOL AL-400 FT-NMR spectrometer operating at 400 MHz and
2
,2,2-trifluoroethanol/water (I ¼ 0.5, NaClO
HPLC analysis, as described in previous work.
methods were also used to calculate the product rate constant
4
) were determined by
[
15]
Published
1
13
ꢁ
1
[15,21]
ratio kas/k
s
(M ) from these product yields.
1
3
100.4 MHz, respectively.
C-NMR analyses
1
3
Proton-decoupled C-NMR spectra were recorded on a JEOL
AL-400 FT-NMR spectrometer operating at 100.4 MHz. The
spectra were centered at 78.3 ppm, and collected using a sweep
width of 605.4 Hz with 8000 data points (0.076 Hz/pt), an 8 s
relaxation delay time, and a pulse angle of 458. Chemical shifts
were measured in ppm relative to the peak at d ¼ 77.0 ppm for
Syntheses
2,2,2-Trifluoro-1-(4-methylphenyl)ethanone was synthesized by
[
17] 1
the procedure of Stewart and coworkers.
H NMR (400 MHz,
CDCl ): d 7.93 (AB, 2H, J ¼ 8.0 Hz, ArH), 7.34 (AB, 2H J ¼ 8.0 Hz, ArH),
3
2.46 (s, 3H, CH
3
). 1-(4-Methylphenyl)-2,2,2-trifluoroethanol (4-OH)
1
[
3
3
C]CDCl .
was synthesized by reduction of the ketone with sodium boro-
[
18]
hydride.
chromatography eluting with n-hexane: yield 78%. H NMR
400 MHz, CDCl
): d 7.36 (AB, 2H, J ¼ 8.0 Hz, ArH), 7.22 (AB, 2H,
J ¼ 8.0 Hz, ArH), 4.98 (dq, 1H, J ¼ 4.4 Hz, J ¼ 6.4 Hz, CHCF
d, 1H, J ¼ 4.4 Hz, CHOH), 2.37 (s, 3H, CH ).
The crude alcohol was purified by silica-gel column
1
RESULTS
(
3
2
18
2 6 4
The oxygen-18 labeled substrate 4-OS( O) C H -4-Me was
synthesized by adaptation of published procedures (Experimen-
tal section). Earlier studies of the solvolysis and oxygen-18
scrambling of ring-substituted 1-phenylethyl derivatives used
3
), 2.60
(
3
1
8
18
[
ClS( O)
2 2 6 4
]-p-Toluenesulfonyl chloride (ClS( O) C H
-4-Me)
was prepared by bubbling chlorine gas through a mixture of
1
0 g of p-thiocresol and 5 mL of oxygen-18 labeled water for two
substrates enriched in carbon-13 at the benzylic carbon and
[
19]
[6,9]
hours.
The reaction products were extracted with ether,
oxygen-18 at the leaving group.
The substrate used in this
purified by silica-gel column chromatography and recrystallized
from benzene/n-hexane; yield 27%, mp. 66.0–67.58C.
work contains only natural abundance of carbon-13 at the
benzylic carbon. This saves time in preparing the solvolysis
reaction substrate, but substantially increases the time required
for C NMR analysis of the distribution of oxygen label at the
sulfonate substrate.
1
-(4-Methylphenyl)-2,2,2-trifluoroethyl tosylate (4-OS(O)
2
1
3
C
6
H
4
-4-Me) was synthesized from 4-OH and p-toluenesulfonyl
[
20]
chloride by following a published procedure.
The crude
ꢁ
6
ꢁ1
reaction product was purified by recrystallization from ether/
A first-order rate constant of ksolv ¼ 3.6 ꢂ 10
determined for solvolysis of 4-OS(O) -4-Me in 50/50 (v/v)
) at 258C by monitoring the
s
was
n-hexane: mp. 81.5-83.08C. The same procedure was used to
C H
2 6 4
1
8
synthesize oxygen-18 labeled [( O
2
)SO]-1-(4-methylbenzyl)-
trifluoroethanol/water (I ¼ 0.5, NaClO
4
1
8
2
,2,2-trifluoroethyl tosylate (4-OS( O) C H -4-Me) from 4-OH
progress of the reaction by UV spectroscopy. The values of ksolv
2
6 4
1
8
(
1g, 5.3 mM) and ClS( O)
2
- C
6
H
4
-4-Me (1g, 5.3 mM); yield 59%,
): d 7.65 (AB, 2H,
remain constant as the azide anion concentration is increased to
[
20]
1
ꢁ1
mp. 84-858C.
H NMR (400 MHz, CDCl
3
0.50 M. A product rate constant ratio of kaz/k
s
¼ 1.1 ꢃ 0.1 M was
J ¼ 8.4 Hz, ArH), 7.22 (d, 4H, J ¼ 8.0 Hz, ArH), 7.11 (AB, 2H,
calculated from the yields, determined by HPLC analysis, of the
solvent and azide anion adducts formed in five reactions at [N3 ]
2
ꢁ
J ¼ 8.4 Hz, ArH), 5.63 (q, 1H, J ¼ 6.4 Hz, CHCF
3
), 2.40 (s, 3H, CH
) d 145.14, 140.33, 133.06,
26.66 (1C, quaternary), 129.57, 129.22, 127.95, 127.83 (2C, tertiary),
22.21 (q, JCF ¼ 280.2 Hz, CF ), 78.05 (q, JCF ¼ 34.2 Hz, CHCF ).
3
),
13
2
1
1
.33 (s, 3H, CH
3
); C NMR (100.4 MHz, CDCl
3
between 0.10 and 0.50 M. These values of ksolv and kaz/k are in fair
s
ꢁ
6
ꢁ1
¼ 0.80 Mꢁ1
agreement with ksolv ¼ 3.2 ꢂ 10
s
and kaz/k
s
reported in earlier work.
3
3
1
3
Figure 1 shows C NMR spectra in the region of the benzylic
C H -4-Me (Fig. 1A) and of the unreacted
2 6 4
substrate recovered during solvolysis of 4-OS( O)
1
8
carbon of 4-OS( O)
Kinetic analyses
The reaction of 4-OS(O) Tos in 50/50 (v/v) trifluoroethanol/water
1
8
2 6 4
C H -4-Me in
50/50 (v/v) trifluoroethanol/water (I ¼ 0.5, NaClO ) at 258C
2
4
(
I ¼ 0.5, NaClO
in absorbance at 265 nm. The reaction was initiated by making a
00-fold dilution of a solution of 4-OS(O) -C H -4-Me in
4
) at 258C was monitored by following the decrease
(Figures 1B – 1D). The signal for the benzylic carbon at
78.342 ppm is split into a quartet by fluorines from the a-CF
group. Figures 1B – 1D show the appearance of the a second
3
1
2
6 4
1
8
18 16
acetonitrile into 3.0 mL of 50/50 (v/v) trifluoroethanol/water
I ¼ 0.5, NaClO ) to give a final substrate concentration of 3 mM.
2 6 4
The isomerization reaction of 4-OS( O) C H -4-Me was also
quartet for the isomerization reaction product 4- OS( O, O)
-4-Me, which is shifted upfield by 0.027 ppm to 78.315 when
the oxygen-18 moves from a nonbridging to a bridging
(
4
6 4
C H
1
8
[
22]
monitored in 50/50 (v/v) trifluoroethanol/water (I ¼ 0.5, NaClO ).
position.
4
The sulfonate ester (100 mg) was dissolved in 1 mL of acetonitrile
and mixed with 200 mL of 50/50 (v/v) trifluoroethanol/water
Figures 1C and 1D show small peaks for an unknown impurity
at ca 78.6 ppm, which is close to the most downfield peak of the
J. Phys. Org. Chem. 2010, 23 730–734
Copyright ß 2010 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

M. TESHIMA, Y. TSUJI AND J. P. RICHARD
1
3
Figure 1. Partial C spectra of unreacted substrate in C
6
D
6
recovered
-4-Me in 50/50 (v/v) trifluor-
) at 258C. The signal for the benzylic
carbon is split by the a-fluorines (J ¼ 34 Hz). (A) Initial spectrum of
18
during the solvolysis of 4-OS( O)
oethanol/water (I ¼ 0.5, NaClO
2 6 4
C H
4
1
8
2 6 4
substrate 4-OS( O) C H -4-Me. (B) Spectrum of substrate recovered
after a 36 hour reaction time (3220 transients). (C) Spectrum of substrate
recovered after a 48 hour reaction time (3560 transients). (D) Spectrum of
substrate recovered after a 69 hour reaction time (10590 transients). The
origin of the small peak at ca 78.6 ppm observed in Figures 1C and 1D is
not known
Figure 2. Time course for the formation of isomerized reaction product
18
18 16
18
4
6 4 2 6 4
- OS( O, O)C H -4-Me during the solvolysis of 4-OS( O) C H -4-Me
quartet for the benzylic carbon. The relative area of the peaks for
1
8
18 16
in 50/50 (v/v) TFE/H O at 258C. The solid line shows the nonlinear
2
the isomerization reaction product 4- OS( O, O) C
6
H
4
-4-Me
1
8
least-squares fit of the data to Eqn (1) derived for Scheme 1 using
(
A
iso) and the remaining substrate 4-OS( O)
2
C
6
H
4
-4-Me (A ) was
S
ꢁ
6
ꢁ1
ꢁ6 ꢁ1
k
solv ¼ 3.6 ꢂ 10
s
and kiso ¼ 1.8 ꢂ 10
s . The upper and lower
therefore determined from the integrated areas of the two most
upfield shifted peaks for the quartet for the benzylic carbon.
dashed lines show the fits obtained using values of kiso that are 10% higher
and 10% lower, respectively, than the value from least-squares analysis
1
8
Figure 2 shows the fractional conversion of 4-OS( O)
2
1
8
18 16
C H -4-Me to 4- OS( O, O)Tos [A /A ] at different reaction
6
4
iso
T
times t, where Aiso is the area of the carbon-13 NMR peak for the
have little effect on the rate constants for carbocation-
1
8
18 16
[14–16]
benzylic carbon of 4- OS( O, O)C
6
H
4
-4-Me and A
T
is the total
nucleophile addition.
This shows that the increase in the
1
8
reactivity of 4 compared to 1R toward nucleophile addition
caused by the large effect of the a-CF3 substituent on the
thermodynamic reaction driving force is essentially completely
R
area of the peak for 4-OS( O) C H -4-Me at t ¼ 0. Values of A
2
6
4
T
were calculated as A ¼ (A þ A )/exp(-k t), where A and A
T
S
iso
solv
S
iso
are the observed peak areas for the benzylic carbons of
1
8
18
18 16
R
4
-OS( O)
2
C
6
H
4
-4-Me and 4- OS-( O, O)C
6
H
4
-4-Me, respect-
offset by a decrease in reactivity of 4 due to an increase in the
Marcus intrinsic barrier for carbocation-nucleophile addition.
ꢁ
6
ꢁ1
[23–25]
ively, and ksolv ¼ 3.6 ꢂ 10
s
.
1
8
The solid line in Fig. 2 shows the nonlinear least squares fit of
the experimental data to eq 1 derived for Scheme 1 using
The sum of the rate constants for solvolysis and O-scrambling
1
8
ꢁ6 ꢁ1
of 4-OS( O)
2
C
6
H
4
-4-Me, ksolv þ kiso ¼ 5.4 ꢂ 10
s is larger
ꢁ
6
ꢁ1
ꢁ6 ꢁ1
ksolv ¼ 3.6 ꢂ 10
s
determined by monitoring solvolysis by UV
than ksolv ¼ 3.6 ꢂ 10
s
for solvolysis of the unlabeled ester.
ꢁ
6
ꢁ1
spectroscopy and kiso ¼ 1.8 ꢂ 10
s , which is treated as a
variable parameter. The upper and lower dashed lines in Fig. 2
show that the experimental data is accommodated by values of
k
iso that lie within ꢃ10% of the rate constant determined by least
squares analysis.
ꢀ
ꢁ
ꢀ ꢁ
Aiso
2
3
2
ꢁksolvt
ꢁð1:5kisoþksolvÞt
e
¼
e
ꢁ
(1)
AT
3
DISCUSSION
R
It is surprising that the a-CF
3
for a-CH
3
substitution at 1 , which
R
strongly destabilizes 4 towards nucleophile addition, should
Scheme 1.
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Copyright ß 2010 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2010, 23 730–734

REACTIONS OF ION PAIRS IN AQUEOUS SOLUTION
Scheme 2.
þ
0
9
ꢁ1
This scrambling of oxygen-18 label between bridging and
nonbridging positions may occur by rearrangement of these
oxygen at an ion pair reaction intermediate followed by internal
return to reactant (k and k , Scheme 2), or the reaction may
the tosylate anion to 4 is larger than k ¼ 6 ꢂ 10 s for addition
s
of the solvent 50/50 water/trifluoroethanol. This suggests that the
sulfonate anion should show a modest selectivity towards
nucleophile addition to carbocations in aqueous solution.
r
-1
proceed by an uncoupled concerted pathway that avoids
formation of a highly unstable reaction intermediate.
0
s
[
8,11,26]
k1ðk þ kꢁdÞ
ksolv ¼
(1)
(2)
(3)
0
s
There is good evidence that isomerization of esters that
exchanges the position of carbon-bridging and nonbridging
oxygen proceeds by a stepwise mechanism (Scheme 2), when the
carbocation reaction intermediate has a lifetime of longer than
k þ kꢁd þ kꢁ1
k1k
ꢁ
1
kiso ¼ 1
0
s
:5ðk þ kꢁd þ kꢁ1Þ
ꢁ
11 [8,26]
10
s.
The ion pair intermediate of solvolysis of 1-O CC F
2 6 5
kiso
kꢁ1
¼
was shown in earlier work to undergo deprotonation to form
0
ksolv
1:5ðk þ kꢁdÞ
[
21]
[6]
s
alkene, internal return to reform substrate, and racemization
[
5]
followed by internal return to substrate. A similar stepwise
mechanism is strongly favored for the isomerization of
To the best of our knowledge the nucleophilic selectivity of
tosylate anion towards addition to carbocations in aqueous
solution has not been measured because: (1) Tosylate salts are
relatively insoluble in water. (2) Tosylate carbocation anion pairs
must be generated by solvolysis of a reactive substrate with a
leaving group that is better than tosylate (e. g., triflate) in order to
give a tosylate ester product that is sufficiently stable to isolate.
However, organic triflates are only stable when attached to
electron deficient carbon (e.g. methyl triflate), and their reactions
in water tend to proceed by a concerted-type mechanism that
avoids formation of carbocation reaction intermediates.
1
8
4
2 6 4
-OS( O) C H -4-Me in 50/50 (v/v) water/trifluroethanol
R
R
because the lifetimes determined for 1 and 4 in this solvent
are similar.
solvent to 1 and 4 is estimated to be slightly larger than the
[
21,27]
0
9
ꢁ1
The value of k
s
ꢄ 6 ꢂ 10 s for addition of
R
R
9
ꢁ1
[27,28]
value of 4 ꢂ 10 s calculated using the azide ion ‘‘clock’’,
because a significant fraction of the azide ion adduct is formed by
a preassociation mechanism that does not involve diffusion-
[
21,27]
controlled trapping of the carbocation.
Equations 1–3 give the relationships between the experimen-
tal rate constants ksolv and kiso and the microscopic rate constants
There is less internal return of the ion-pair intermediate during
0
18
for the individual steps in Scheme 2, where k
s
is the rate constant
6 5
solvolysis of 1- OC(O)C F in 50/50 trifluoroethanol/water (kiso/
[
6]
for direct addition of solvent to the ion pair reaction intermediate,
and k is rate constant for irreversible diffusional separation of the
ion pair to the free carbocation which then undergoes addition of
ksolv ¼ 0.13) compared with internal return during solvolysis of
18
2 6 4
d
4-OS( O)
0
two carbocations 1 and 4 show similar reactivity (k
C
H
-4-Me in the same solvent (kiso/ksolv ¼ 0.50). The
R R
s
¼ 6 ꢂ
[
6]
9
ꢁ1
[15,21,27]
solvent.
These rate laws were derived by making the
10 s ) toward addition of solvent.
constant ratio of kiso/ksolv ¼ 0.13 for the reaction of 1- OC(O)C
The smaller rate
1
8
assumption that there is a single rate constant k-1 for collapse
of the two ion pairs in Scheme 2, and that the reorganization of
the ion pair that exchanges any of the three equivalent sulfonate
6 5
F
18
2 6 4
compared with the reaction of 4-OS( O) C H -4-Me requires a
smaller rate constant k 1 for internal return of the pentafluor-
ꢁ
1
1
ꢁ1 [7]
18
oxygen (k ꢄ 10
s
)
is much faster than the other reactions of
obenzoate anion to form rearranged product 1-OC( O)C F .
r
6 5
0
0
the ion pair, so that k
both the steady state concentrations and their rates of internal
return of the two ion pairs to give the neutral esters will be equal.
r
>> (k
s
þ kꢁd), kꢁ1 . Under these conditions
Combining the rate constant ratio kiso/ksolv ¼ 0.13 determined for
the isomerization and solvolysis reactions of the pentafluor-
1
8
0
9
ꢁ1
obenzoate ester 1- OC(O)C F with k ¼ 6 ꢂ 10 s
and
gives k-1 ¼ 7 ꢂ 10 s for unimolecular collapse
of the ion pair intermediate of solvolysis to the neutral benzoate
6
5
s
1
0
ꢁ1
10 ꢁ1
9
ꢁ1
A value of k-1 ꢄ 1.7 ꢂ 10
s
for unimolecular collapse of
k
-d ¼ 1.6 ꢂ 10
s
R
ꢀ
S18
18 16
18
18 16
4
OS-( O, O)C
6 4
H -4-Me to form 4- OS( O, O)
1
0
ꢁ1
C H -4-Me can be calculated (eq 3) from the experimental rate
ester, which is ca. 2-fold smaller than k ꢄ 1.7 ꢂ 10 estimated
in this work for collapse the intermediate of solvolysis of
4-OS( O) C H -4-Me (Scheme 3).
2 6 4
s
6
4
ꢁ1
0
9
ꢁ1
constant ratio kiso/ksolv ¼ 0.50 and using k
s
¼ 6 ꢂ 10 s and
1
0
ꢁ1
18
[6]
k
-d ¼ 1.6 ꢂ 10
s
. The estimated rate constant for addition of
J. Phys. Org. Chem. 2010, 23 730–734
Copyright ß 2010 John Wiley & Sons, Ltd.
www.interscience.wiley.com/journal/poc

M. TESHIMA, Y. TSUJI AND J. P. RICHARD
Scheme 4.
Acknowledgements
We acknowledge the National Institutes of Health (GM 39754) for
generous support of this work.
REFERENCES
Scheme 3.
[
[
1] J. M. Harris, Prog. Phys. Org. Chem. 1974, 11, 89–173.
2] D. J. Raber, J. M. Harris, P. v. R. Schleyer, In: Ions and Ion Pairs in Organic
Reactions, (Ed.: M. Szwarc), John Wiley & Son, New York, 1974. Vol. 2.
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[
[
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Yatsugi, J. Am. Chem. Soc. 1995, 117, 8974–8981.
The similar rate constants for collapse of the ion pairs shown in
Scheme 3 is interesting. The two carbocations show the same
intrinsic reactivity towards addition of nucleophlic solvent
[5] Y. Tsuji, T. Mori, M. M. Toteva, J. P. Richard, J. Phys. Org. Chem. 2003, 16,
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6] Y. Tsuji, T. Mori, J. P. Richard, T. L. Amyes, M. Fujio, Y. Tsuno, Org. Lett.
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[8] T. L. Amyes, M. M. Toteva, J. P. Richard, In: Reactive Intermediate
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Hoboken, NJ, 2004. p 41-68.
4
0
9
ꢁ1 [15,21,27]
8
[
[
(
k ¼ 6 ꢂ 10 s ).
However, tosylate anion is ca 10 -more
s
2
reactive as a leaving group than pentafluorobenzoate anion in
solvolysis reactions with rate determining bond cleavage of
[
23]
a-substituted 4-methoxybenzyl derivatives. This leaving group
is therefore expected to show a smaller nucleophilicity for
reaction in the reverse bond synthesis direction. The observed
similar reactivity of tosylate and pentaflurobenzoate towards
[9] Tsuji, Y. M. M. Toteva, T. L. Amyes, J. P. Richard, Org. Lett. 2004, 6,
3633–3636.
[10] J. P. Richard, T. L. Amyes, M. M. Toteva, Y. Tsuji, Adv. Phys. Org. Chem.
2004, 39, 1–26. Y. Tsuji, J. P. Richard, Chem. Rec. 2005, 5, 94–106.
11] Y. Tsuji, J. P. Richard, J. Am. Chem. Soc. 2006, 128, 17139–17145.
12] A. D. Allen, C. Ambidge, C. Che, H. Michael, R. J. Muir, T. T. Tidwell, J.
Am. Chem. Soc. 1983, 2343–2350.
R
R
addition to carbocations 1 and 4 which show similar intrinsic
electrophilic reactivity almost certainly reflects a Hammond-type
leveling in the selectivity of these anions in nucleophilic addition
[
[
[
29]
to reactive carbocations.
We have made the simplifying assumption that k
addition of aqueous/trifluoroethanol to the ion pair
[
13] T. T. Tidwell, Angew. Chem., Int. Ed. Engl. 1984, 23, 20–32.
0
s
ꢄ k
s
for
[14] J. P. Richard, J. Am. Chem. Soc. 1986, 108, 6819–6820.
15] J. P. Richard, J. Am. Chem. Soc. 1989, 111, 1455–1465.
[16] J. P. Richard, T. L. Amyes, L. Bei, V. Stubblefield, J. Am. Chem. Soc. 1990,
12, 9513–9519.
17] R. Stewart, K. C. Teo, Can. J. Chem. 1980, 58, 2491–2496.
[
R
S18
18 16
R
4
ꢀ
OS-( O, O)C H -4-Me and to free carbocation 4 .
6
4
1
We note, however, that there is good evidence that addition of
the strongly electron-withdrawing a-CF group to benzylic
carbocations is accompanied by an increase in delocalization of
[
3
[18] A. D. Allen, I. C. Ambidge, C. Che, H. Micheal, R. J. Muir, T. Tidwell, J.
Am. Chem. Soc. 1983, 105, 2343–2350.
[19] S. Oae, T. Kitao, Y. Kitaoka, Tetrahedron 1963, 19, 827–832.
[
15,30]
charge into the phenyl ring (Scheme 4A).
This attenuates the
[
20] A. D. Allen, M. P. Jansen, K. M. Koshy, N. N. Mangru, T. T. Tidwell, J. Am.
destabilizing inductive substituent effect by moving the center of
Chem. Soc. 1982, 104, 207–211.
[21] J. P. Richard, W. P. Jencks, J. Am. Chem. Soc. 1984, 106, 1373–1383.
[22] J. M. Risley, R. L. V. Etten, J. Am. Chem. Soc. 1979, 101, 252–253.
positive charge away from the electron-withdrawing a-CF3
dipole. This increased delocalization in charge is the underlying
[
23] T. L. Amyes, I. W. Stevens, J. P. Richard, J. Org. Chem. 1993, 58,
057–6066.
24] J. P. Richard, T. L. Amyes, M. M. Toteva, Acc. Chem. Res. 2001, 34,
81–988.
cause of the large Marcus intrinsic barriers for nucleophile
addition to ring-substituted phenyl 2,2,2-trifluoroethyl carbo-
6
[
[
23,24]
cations.
It is interesting to speculate that placement of a
9
leaving group anion next to the carbocation might favor partial
relocalization of charge onto the benzylic carbon (Scheme 4B).
This would reduce the Marcus intrinsic reaction barrier and cause
an increase in the rate constants for carbocation nucleophile
addition that could partly explain the high observed reactivity of
the tosylate anion in internal return to neutral substrate.
[25] J. P. Richard, Tetrahedron 1995, 51, 1535–1573.
[26] W. P. Jencks, Chem. Soc. Rev. 1981, 10, 345–375.
[27] J. P. Richard, M. E. Rothenberg, W. P. Jencks, J. Am. Chem. Soc. 1984,
1
06, 1361–1372.
[
[
28] J. P. Richard, W. P. Jencks, J. Am. Chem. Soc. 1982, 104, 4689–4691.
29] G. S. Hammond, J. Am. Chem. Soc. 1955, 77, 334–338.
[30] I. Lee, S. C. Dong, J. J. Hak, Tetrahedron 1994, 50, 7981–7986.
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