Ion-Molecule Pairs in Leaving-Group-Promoted Solvolytic Elimination Reactions

Article abstract of DOI:10.1021/jo9619645

Solvolysis of 1-(1-methyl-1-phenylethyl)pyridinium cation (1-P⁺) in 25 vol % acetonitrile in water at 60 °C provides the substitution product 2-hydroxy-2-phenylpropane (1-OH) and the elimination product 2-phenylpropene (3). The fraction of elimination is about half of that obtained with an acetate leaving group of similar basicity. The total rate of reaction is dependent on the basicity of the leaving group, 1-(1-methyl-1-(4-cyanophenyl)ethyl)pyridinium cation (1-P-CN⁺) reacts 1100 times faster than 1-P⁺ which corresponds to a Bronsted parameter of β_1g = -0.93. Also the fraction of elimination is leaving-group dependent as expressed by a Bronsted parameter of β= 0.12 for the dehydronation of the ion-molecule pair by the leaving group. Addition of substituted pyridines has only a minor effect on the product ratio. The reactions are concluded to occur stepwise through a common carbocation intermediate of ion-molecule pair type. The overall kinetic deuterium isotope effects using the hexadeuteriated analogs were measured as k_obs^H/k_obs^D6 = 1.85 ± 0.10 (60 °C) with the pyridinium ion 1-P⁺and as k_obs^H/k_obs^D6 = 1.53 ± 0.06 (40 °C) with the 4-cyanopyridinium ion 1-P-CN⁺. The kinetic deuterium isotope effects on the elimination step (assuming the reaction from intermediate to alcohol is insensitive to isotopic substitution) were measured as k_e^H/k_e^D6 = 2.7 ± 0.2 for 1-P⁺ (60 °C) and 3.4 ± 0.2 for 1-P-CN⁺ (40 °C).

Full text of DOI:10.1021/jo9619645

J . Org. Chem. 1997, 62, 1079-1082
1079
Ion -Molecu le P a ir s in Lea vin g-Gr ou p -P r om oted Solvolytic
Elim in a tion Rea ction s
Alf Thibblin* and Yoshihiro Saeki
Institute of Chemistry, University of Uppsala, P.O. Box 531, S-751 21 Uppsala, Sweden
Received October 21, 1996X
+
Solvolysis of 1-(1-methyl-1-phenylethyl)pyridinium cation (1-P ) in 25 vol % acetonitrile in water
at 60 °C provides the substitution product 2-hydroxy-2-phenylpropane (1-OH) and the elimination
product 2-phenylpropene (3). The fraction of elimination is about half of that obtained with an
acetate leaving group of similar basicity. The total rate of reaction is dependent on the basicity of
+
the leaving group, 1-(1-methyl-1-(4-cyanophenyl)ethyl)pyridinium cation (1-P -CN ) reacts 1100
+
times faster than 1-P which corresponds to a Brønsted parameter of âlg ) -0.93. Also the fraction
of elimination is leaving-group dependent as expressed by a Brønsted parameter of â ) 0.12 for
the dehydronation of the ion-molecule pair by the leaving group. Addition of substituted pyridines
has only a minor effect on the product ratio. The reactions are concluded to occur stepwise through
a common carbocation intermediate of ion-molecule pair type. The overall kinetic deuterium isotope
H
D6
effects using the hexadeuteriated analogs were measured as kobs /kobs ) 1.85 ( 0.10 (60 °C) with
+
H
D6
the pyridinium ion 1-P and as kobs /kobs ) 1.53 ( 0.06 (40 °C) with the 4-cyanopyridinium ion
+
1
-P -CN . The kinetic deuterium isotope effects on the elimination step (assuming the reaction
from intermediate to alcohol is insensitive to isotopic substitution) were measured as k
.7 ( 0.2 for 1-P (60 °C) and 3.4 ( 0.2 for 1-P -CN (40 °C).
H
D6
e
/k
e
)
+
+
2
In tr od u ction
initially formed carbocationic intermediate is not just an
encounter complex but an intermediate with a significant
We are interested in the role of ion-molecule pairs as
lifetime.2
,5-8
These reports include solvolysis of hydro-
intermediates in solvolytic elimination and substitution
2
nated ethers in which an allylic rearrangement reaction
and an “aromatization” reaction,5 respectively, were
employed as ion-molecule pair probes.
reactions.1
-10
Ion-molecule pairs are the initially formed
complexes in stepwise solvolytic reactions of substrates
with neutral leaving groups such as water, alcohols,
thioethers, and amines (eq 1). Such encounter complexes
also are formed when carbocations react with solvent in
nucleophilic substitution reactions (eq 2). However, most
often ion-molecule pairs have been ignored in discus-
sions of reaction mechanisms. The “free”, solvent-
equilibrated carbocation generally has been considered
to be the only intermediate in these reactions. Thus,
There seems to be only one previous report on the
intermediacy of an ion-molecule pair in an elimination
reaction in solution.8 An irreversibly formed, very short-
lived ion-molecule pair was concluded to be a common
intermediate for the solvolytic elimination and substitu-
tion reactions of 9-(2-phenoxy-2-propyl)fluorene in acidic,
highly aqueous media.8 The mechanistic conclusions
were based upon measured kinetic isotope effects and
product compositions. Some of the results reported in
reaction of the complex (k
slower than diffusional separation (k-d) to free ions (eq
). However, recent studies of some acid-catalyzed
p
) has been assumed to be much
1
0
the present paper have been communicated previously.
1
Resu lts
(1)
The solvolysis of 1-(1-methyl-1-phenylethyl)pyridinium
+
(
1-P ) perchlorate or 1-(1-methyl-1-(4-cyanophenyl)eth-
+
yl)pyridinium (1-P -CN ) perchlorate in 25 vol % aceto-
nitrile in water provides the alcohol 2-hydroxy-2-phenyl-
propane (1-OH) and the alkene 2-phenylpropene (3,
Scheme 1).
(2)
The kinetics of the reactions were studied by a sam-
pling high-performance liquid chromatography procedure
and, with 1-P -CN at 40 °C, by following the decrease
in absorbance at 226 nm by UV spectrophotometry. The
measured rate constants and reaction conditions are
shown in Table 1. The effect of solvent on the total
reaction rate is small but there is a significant effect on
solvolysis reactions of ethers have revealed that the
+
X
Abstract published in Advance ACS Abstracts, J anuary 15, 1997.
(
1) Sneen, R. A.; Felt, G. R.; Dickason, W. C. J . Am. Chem. Soc.
1
973, 95, 638.
(
2) Thibblin, A. J . Chem. Soc., Perkin Trans. 2 1987, 1629.
(3) Katritzky, A. R.; Brycki, B. E. J . Phys. Org. Chem. 1988, 1, 1
and references cited therein.
4) (a) Szele, I.; Zollinger, H. J . Am. Chem. Soc. 1978, 100, 2811.
b) Hashida, Y.; Landells, R. G. M.; Lewis, G. E.; Szele, I.; Zollinger,
H. J . Am. Chem. Soc. 1978, 100, 2816.
the elimination-to-substitution ratio k
E S
/k .
(
(
Addition of pyridine (17 mM) and 4-cyanopyridine (17
+
+
mM) to the reaction solutions of 1-P and 1-P -CN ,
respectively, has no effect on total reaction rates. Small
effects on the product compositions are observed. Owing
to difficulties in accurately quantifying the amount of
alcohol product 1-OH in the presence of a large amount
of pyridines, the reaction solutions in these product
(
(
(
(
(
5) Thibblin, A. J . Chem. Soc., Chem. Commun. 1990, 697.
6) Thibblin, A. J . Org. Chem. 1993, 58, 7427.
7) Thibblin, A. J . Phys. Org. Chem. 1993, 6, 287.
8) Sidhu, H.; Thibblin, A. J . Phys. Org. Chem. 1994, 578.
9) Bleasdale, C.; Golding, B. T.; Lee, W. H. L.; Maskill, H.;
Riseborough, J .; Smits, E. J . Chem. Soc., Chem. Commun. 1994, 93.
10) Thibblin, A.; Sidhu, H. J . Phys. Org. Chem. 1993, 6, 374.
(
S0022-3263(96)01964-0 CCC: $14.00 © 1997 American Chemical Society

1
080 J . Org. Chem., Vol. 62, No. 4, 1997
Thibblin and Saeki
Sch em e 1
Sch em e 2
discrimination is larger for 4-cyanopyridine as leaving
group than for 4-nitrobenzoate and acetate anions.
Ta ble 1. Kin etic Da ta for th e Solvolysis Rea ction s of
Discu ssion
+
+
1
-P a n d 1-P -CN a n d th e Cor r esp on d in g Deu ter ia ted
Com p ou n d s
The solvolysis of the cumyl derivatives 1-P + and 1-P -
CN in the aqueous solvent comprises S
substratea
temp, °C
10 (kE + kS), s
6
b
-1
3
10 kE/kSc
+
N
1-E1 reactions.
The reactions are zero-order in nucleophile since no effect
of added strong nucleophile, like azide anion, on the rate
of disappearance of the substrates is observed. Further-
more, the elimination reactions are not solvent-promoted
E2 reactions11 since added hydroxide anion does not
2
5 vol % Acetonitrile in Water
1
-P +
d 6-1-P
-P -CN
60
60
60
60
60
60
40
40
1.79
0.97
2030
1422
27
10
11
3.0
66
4
+
+
1
+
+
d 6-1-P -CN
1
1
1
-OAc
-OMe
-P -CN
+
12
+
d
increase the rate of reaction of 1-P
or 1-P -CN , or
+
124.7
81.3
9
2.6
increase the fraction of elimination.
The nature of the solvent has a very minor effect on
the rate of solvolysis as expected for the solvolysis of
d 6-1-P -CN
Methanol
1
1
-P +
-P -CN
60
40
59
8
substrates with neutral leaving groups. A logarithmic
+
+
+
plot of the solvolysis rate of 1-P versus the polarity (E
T
)
Ethanol
12
of the solvent has a slope of about zero. This is in sharp
contrast to cumyl derivatives with negatively charged
leaving groups which show greatly enhanced solvolysis
rates with increasing polarity of the solvent.
1
1
-P +
-P -CN
60
40
99
16
a
b
Substrate concentration 3 mM. Estimated maximum errors
3% at 40 °C and ( 5% at 60 °C. Estimated maximum error (
In acidic solution, reference 10.
c
(
.
d
a
The difference in basicity of the leaving groups (pK )
1
1
3
5
.17 and 1.90) is reflected in the solvolysis rates; 1-P -
Ta ble 2. Nu cleop h ilic Discr im in a tion betw een Azid e
An ion a n d Meth a n ol in 25 vol % Aceton itr ile in
Wa ter a t 40 °C
+
+
CN reacts about 1100 times faster than 1-P . This
corresponds to a Brønsted parameter of âlg ) -0.93.
It has been concluded that the solvolysis of cumyl
halides and esters, which gives competing substitution
and elimination, occurs by a branched mechanism with
substratea
kN /kMeOH
b
3
1
1
1
-P -CN+
-P NB
-OAc
25
14
17
14
a common ion-pair intermediate. The alkene is formed
predominantly from the ion pair with the leaving group
acting as the hydron-abstracting base. Reaction via the
ion pair is the main route to elimination product in
a
Substrate concentration 3 mM. b Ratio of second-order rate
constants (eq 3).
solvolysis in highly aqueous media, even for such a
composition experiments contained a low concentration
+ 15,16
2
relatively stable carbocation as Ph C(Me) .
(0.48 M) of methanol. Thus, the amount of alkene 3 was
+
We conclude that the eliminations from 1-P and 1-P -
compared with the amount of methyl ether product
+
CN occur in a similar manner; the basic pyridine leaving
1
-OMe. The measured product ratios of alkene 3 to
group abstracts a hydron within an ion-molecule pair
(k , Scheme 2). The leaving group functions as a general
e
+
substitution product 1-OMe were 1.0 and 0.23 with 1-P
and 1-P -CN , respectively, without pyridine addition and
+
base. The Brønsted parameter for the elimination from
the ion-molecule pair calculated with the two pyridine
leaving groups, assuming that the substitution reaction
rate with the solvent is not affected, is â ) 0.12 (Figure
1
4
.0 and 0.27 with 0.40 M of pyridine or 0.36 M of
-cyanopyridine added. Hydroxide anion (0.2 M) does not
+
increase the rate of reaction of 1-P -CN or increase the
E S
product ratio k /k (Table 1).
1
). The value is similar to that for catalysis by added
Azide anion does not increase the total reaction rate
but gives rise to one further substitution product, 2-azido-
substituted acetate anions (â ) 0.13) reported for sol-
14
volysis of cumyl chloride.
2
3
-phenylpropane (1-N ). The nucleophilic discrimination
between azide (0.25 M) and methanol (1.8 M), as ex-
pressed by ratios of second-order rate constants, was
calculated from the measured product ratios by means
of eq 3.
(
11) (a) Meng, Q.; Thibblin, A. J . Am. Chem. Soc. 1995, 117, 1839.
(
b) Meng, Q.; Thibblin, A. J . Am. Chem. Soc. 1995, 117, 9399. (c) Meng,
Q.; Thibblin, A. J . Chem. Soc., Chem. Commun. 1996, 345. (d) Meng,
Q.; Gogoll, A.; Thibblin, A. J . Am. Chem. Soc. 1997, in press.
(
(
12) Katritzky, A. L.; Brycki, B. J . Am. Chem. Soc. 1986, 108, 7295.
13) J encks, W. P.; Regenstein, J . Handbook of Biochemistry and
) ([1-N ]/[1-OMe])([MeOH]/[N -]) (3)
3 3
Molecular Biology, 3rd ed.; Fasman, G. D., Ed.; CRC Press: Cleveland,
k /k
N
MeOH
1976.
3
(
(
14) Thibblin, A. J . Phys. Org. Chem. 1989, 2, 15.
15) Thibblin, A. J . Phys. Org. Chem. 1992, 5, 367.
The data are collected in Table 2. The nucleophilic
(16) Thibblin, A. Chem. Soc. Rev. 1993, 22, 427.

Ion-Molecule Pairs in Solvolytic Elimination Reactions
J . Org. Chem., Vol. 62, No. 4, 1997 1081
Ta ble 3. Kin etic Deu ter iu m Isotop e Effects for th e
+
+
Rea ction s of 1-P a n d 1-P -CN in 25 vol % Aceton itr ile in
Wa ter
H
D6
H
D6
substrate temp, °C (kE + kS) /(kE + kS)
(kE/kS) /(kE/kS)
1
1
1
-P +
-P -CN
-P -CN
60
60
40
1.85 ( 0.10
1.43 ( 0.10
1.53 ( 0.06
2.7 ( 0.2
3.5 ( 0.2
3.4 ( 0.2
+
+
The kinetic deuterium isotope effects on the elimina-
H
D6
tion step k
e
/k
e
can be approximated to the measured
H
D6
isotope effect (k
the isotope effect on the formation of the alcohol is
negligible. The isotope effect on k is composed of a
E
/k
S
) /(k
E
/k
S
)
(Table 3) by assuming that
e
primary kinetic isotope effect corresponding to the hydron
transfer to the abstracting base (i.e., the leaving pyridine
within the ion-molecule pair) and a small secondary
isotope effect from the five nonreacting hydrogen atoms.
The isotope effect on the elimination promoted by the
F igu r e 1. Brønsted plot for the dehydronation of the car-
bocation 1 by the leaving group in 25 vol % acetonitrile in
+
H
D6
pyridine leaving group (k
e
/k
e
) 2.7 ( 0.2) is smaller
a
water at 60 °C. The pK values of the leaving groups refer to
1
3
H/k
D6
water.
than with 4-cyanopyridine (k
e
e
) 3.5 ( 0.2) and is
in accord with an early transition state in which the
extent of hydron transfer from the carbocation is rela-
tively small. Consistently, the Brønsted parameter is
small, â ) 0.12.
Addition of pyridine and 4-cyanopyridine, respectively,
to the reaction solutions has only a minor effect on the
elimination-to-substitution ratio. For example, 0.40 M
pyridine does not increase significantly the alkene-to-
ether ratio in a 0.48 M solution of methanol in 25 vol %
+
The isotope effect on the total reaction rate of 1-P -CN ,
H
D6
(k + k ) /(k + k ) ) 1.53 ( 0.06 at 40 °C (Table 3), is
E
S
E
S
acetonitrile in water. This indicates that formation (k
d
)
slightly larger than that measured for cumyl 4-nitroben-
zoate 1-P NB (1.40 ( 0.06 at 25 °C). The isotope effect
for 1-P is somewhat larger, 1.85 ( 0.10 at 60 °C (Table
1
4
of the ion-molecule pair by diffusional association of the
carbocation and pyridine is slow compared to the reaction
of the carbocation with a nucleophilic solvent molecule
+
3). The difference in magnitude for these three leaving
groups may be attributed to a requirement for more
hyperconjugative stabilization of the transition state of
the rate-limiting ionization step with decreasing reactiv-
ity of the substrate. The maximal ionization isotope
(k
sub, Scheme 2). This result is not surprising since the
unsubstituted cumyl carbocation is relatively short-lived;
-
10
17
the lifetime is about 10
s in highly aqueous media.
Bunton and co-workers have studied hydron transfer
from relatively stable carbocations in 50 vol % acetoni-
trile-water.18 They found that amines with little steric
hindrance are efficient bases for the abstraction of a
hydron from 1-ferrocenylalkyl carbocations. For ex-
ample, acetate anion was found to have about the same
reactivity as pyridine toward the carbocation. The more
unstable cumyl carbocation intermediates exhibit similar
reactivity differences, as concluded from the data of Table
H
D
19
H
effect is k /k ) 1.15 per deuterium at 25 °C, i.e., k /
D6
k
e 2.3. A significant amount of internal return seems
likely since solvation of the nucleophilic pyridine leaving
group in the ion-molecule pair should be minor.
Our results are consistent wih the mechanism shown
in Scheme 2. As discussed above, the elimination occurs
mainly directly from the ion-molecule pair. The elimi-
nation from the “free” solvent-equilibrated carbocation
is very slow as indicated by the very small elimination-
1
. Acetate anion in a cumyl ion pair gives about two
times more alkene than pyridine in the cumyl ion-mole-
cule pair. Thus, after correction for the statistical factor
of two for the acetate anion, the bases show very similar
reactivity.
It was found that steric hindrance is a more important
factor in tertiary-amine promoted hydron-transfer from
to-substitution ratio for the acid-catalyzed solvolysis of
-3
the methyl ether 1-OMe of k
E
/k
S
) 4 × 10 (Table 1).
The carbocationic intermediates show relatively low
selectivity toward different nucleophiles (Table 2). A
selectivity of kN3/kHOH ) 42 has been measured for cumyl
14
chloride at 25 °C. The value of kN3/kMeOH ) 25 for 1-P -
1
-ferrocenylalkyl carbocations than in ordinary hydron-
+
CN (Table 2) combined with the value of kMeOH/kHOH
)
1
8
transfer reactions. Accordingly, it was concluded that
the carbocation and the base are in close proximity in
the transition state. Ion-dipole interactions in the en-
counter complex were suggested to be of importance.
Such interactions are presumably responsible for the
relative stability of the cumyl ion-molecule pairs as well
as ion-molecule pairs in general formed in solvolysis
reactions.
14
2
.9 measured previously for cumyl chloride gives kN3
/
k
HOH ) 25 × 2.9 ) 74. Thus, the reaction with solvent
water (concentration 41.7 M) has a rate constant of about
k
upon the assumption that the reaction with azide anion
9
9
-1
HOH′ ) 5 × 10 × 41.7/74 ) 3 × 10 s . This is based
9
-1
is diffusion controlled with a rate constant of 5 × 10 M
-1 20
s
.
The large rate constant for reaction of the inter-
mediate with water as well as the different selectivities
measured with different leaving groups (Table 2) indicate
that a significant part of the substitution occurs at the
ion-molecule pair stage. This should occur by a preas-
The nature of the solvent affects the competition
between elimination and substitution. The elimination-
+
to-substitution ratio k
E
/k
S
for 1-P increases in the
solvent series 25 vol % acetonitrile in water, methanol,
and ethanol (Table 1).
(19) Westaway, K. C. Isotopes in Organic Chemistry; Buncel, E., Lee,
C. C., Eds.; Elsevier: Amsterdam, 1987; Vol. 7, Ch. 5.
(20) (a) McClelland, R. A.; Kanagasabapathy, V. M.; Steenken, S.
J . Am. Chem. Soc. 1988, 110, 6913. (b) McClelland, R. A.; Kanagasa-
bapathy, V. M.; Banait, N. S.; Steenken, S. J . Am. Chem. Soc. 1991,
113, 1009.
(
17) Richard, J . P.; Amyes, T.; Vontor,T. J . Am. Chem. Soc. 1991,
13, 5871.
18) Bunton, C. A.; Carrasco, N.; Davoudzadeh, F. J . Chem. Soc.,
Perkin Trans. 2 1981, 924.
1
(

1
082 J . Org. Chem., Vol. 62, No. 4, 1997
Thibblin and Saeki
sociation mechanism in which the azide ion is already
present when the bond to the leaving group is cleaved
nitromethane (50 mL) with stirring over 40 min at -5 to 0
°
C. The mixture was stirred further for 1 h, and then an
additional portion of silver perchlorate (0.01 mol) in dry
nitromethane (50 mL) was added in the same way. After
stirring for 1 h, the silver salt was centrifuged off and ether
added which resulted in a cloudy solution in which the product
eventually precipitated out as a white solid. Recrystallization
several times from acetonitrile-ether gave pure material in
(
this is not explicitly indicated in Scheme 2).
Substituents on the phenyl group change the lifetime
-7
-13
of the cumyl carbocation from about 10 s to about 10
1
7
s in highly aqueous media. The longer lifetime corre-
sponds to a solvent-equilibrated carbocation, and the
shorter to an extremely short-lived ion pair that does not
have a significant lifetime in the presence of an efficient
nucleophile, i.e., an uncoupled concerted reaction or, if
the nucleophile stabilizes the transition state, to a
coupled concerted reaction.
Is there any other mechanistic alternative for the
elimination reaction? A solvent-promoted E2 reaction
should not be a reasonable mechanism because such a
reaction should require an acidic â-hydron.11 Moreover,
a strong base, such as hydroxide anion, does not increase
the reaction rate. A plausible mechanistic alternative
is concerted pericyclic elimination. However, such a
mechanism involves a four-centered transition state and
this is not very favored energetically. The elimination
reactions of cumyl derivatives with electron-withdrawing
1
2
10% yield: mp 126-128 °C (lit. mp 124 °C).
2
2
1
-(1-(1,1,1-( H
p yr id in iu m (d -1-P ) perchlorate was prepared from d
Cl as described above.
-(1-Meth yl-1-(4-cya n op h en yl)eth yl)p yr id in iu m (1-P -
3
)-Me t h yl)-1-p h e n yl-2,2,2-( H
3
)e t h yl)-
+
6
6
-1-
1
4
1
+
CN ) perchlorate was prepared from 1-Cl by the same method
+
as used for 1-P .
2
2
1-(1-(1,1,1-( H
eth yl)p yr id in iu m (d
3
)-Meth yl)-1-(4-cya n op h en yl)-2,2,2-( H
3
)-
+
6
-1-P -CN ) perchlorate was prepared
from d -1-Cl as described above.
6
1
-(1-Met h yl-1-p h en ylet h yl)a cet a t e (1-OAc) was pre-
pared by ZnCl
2
-catalyzed acetylation of 1-OH with acetic
anhydride.14
1
-(1-Meth yl-1-ph en yleth yl)-4-n itr oben zoate (1-P NB) was
14
prepared from 1-OH and 4-nitrobenzoyl chloride in pyridine.
Kin etics a n d P r od u ct Stu d ies. The reaction solutions
were prepared by mixing the organic solvent with water at
room temperature, ca. 22 °C. The reaction vessel was a 2-mL
HPLC flask, sealed with a gas-tight PTFE septum, which was
placed in an aluminum block in the water thermostat bath.
The reactions were initiated by fast addition of a few microli-
tres of the substrate dissolved in acetonitrile by means of a
syringe. The concentration of the substrate in the reaction
solution was usually 3 mM but in some experiments 0.3 mM.
At appropriate intervals, samples were analyzed using the
HPLC apparatus. The rate constants for the disappearance
of the substrates were calculated from plots of substrate peak
area, or substitution product area, versus time by means of a
nonlinear regression computer program. Very good pseudo
first-order behavior was seen for all of the reactions studied.
The separate rate constants for the elimination and substitu-
tion reactions were calculated by combination of product
composition data, obtained from the peak areas, and the
relative response factors were determined in separate experi-
ments, with the observed rate constants.
substituents on the phenyl group has been proposed to
be of this type.21 However, the results for these elimina-
tions may be explained alternatively by an ion-pair
mechanism which is supported by the measured values
of the Hammett and Grunwald-Winstein parameters.21
Exp er im en ta l Section
Gen er a l P r oced u r es. The high-performance liquid chro-
matography (HPLC) analyses were carried out with a liquid
chromatograph equipped with a diode-array detector on a
Asahipak ODP-50 reversed-phase column (5 µm, 4 × 150 mm).
The mobile phase was a solution of acetonitrile in water. The
reactions were studied at constant temperature in a water
thermostat bath. The UV spectrophotometry was performed
with a spectrophotometer equipped with an automatic cell
changer kept at constant temperature with water from the
thermostat bath.
The relative response factors were measured for 1-OH and
3 by analysis of mixtures of the two components prepared by
weighing. The response factors for all other substitution
products were assumed to be the same as that of 1-OH.
Experimental support for this assumption has been reported.
Ma ter ia ls. Acetonitrile (Riedel de Haen or J . T. Baker) and
methanol (J . T. Baker or Merck) were of HPLC grade and were
used without further purification. All other chemicals were
of reagent grade and were used without further purification.
+
The kinetics of 1-P -CN at 40 °C was also studied by
2
-Hyd r oxy-2-p h en ylp r op a n e (1-OH) was purified by re-
following the decrease in absorbance at 226 nm using UV-
spectrophotometry. The reactions, which were run in 3-mL
quartz cells, were followed for at least 10 half-lives. The rate
constants were calculated by means of a nonlinear regression
computor program in which the measured infinity absorbance
value was kept constant. Very good pseudo first-order behav-
ior was observed.
crystallization of commercially available material (Aldrich)
from pentane.
2
-P h en ylp r op en e (3) (Fluka) was purified by fractional
distillation at reduced pressure.
2
1
,1,1,3,3,3-( H
6 6
)-2-Hyd r oxy-2-p h en ylp r op a n e (d -1-OH)
2
2
was synthesized from 1,1,1,3,3,3-( H
and phenylmagnesium bromide.
-(1-Meth yl-1-p h en yleth yl)p yr id in iu m (1-P ) perchlo-
rate was prepared from 1-Cl by a slight modification of a
previously published method. To dry pyridine (0.06 mol) and
-methyl-1-phenylethyl chloride (0.02 mol) in dry nitromethane
was added silver perchlorate (0.01 mol) dissolved in dry
6
)-acetone (Ciba, 99.5% H)
1
4
+
The estimated errors are considered as maximum errors
derived from maximum systematic errors and random errors.
1
1
4
1
2
Ack n ow led gm en t. We thank Mr. Q. Meng for
assistance and the Swedish Natural Science Research
Council for supporting this work.
1
(21) Amyes, T.; Richard, J . P. J . Am. Chem. Soc. 1991, 113, 8960.
J O9619645