Acid-catalyzed solvolysis of allylic ethers and alcohols. Competing elimination and substitution via a thermodynamically 'stable' carbocation

Article abstract of DOI:10.1021/ja9807800

Specific acid-catalyzed solvolysis of 1-methoxy-1,4-dihydronaphthalene (1-OMe) or 2-methoxy-1,2-dihydronaphthalene (2-OMe) in 25 vol % acetonitrile in water yields mainly the elimination product naphthalene, which is accompanied by a trace of 2-hydroxy-1,2-dihydronaphthalene (2-OH). No intramolecular rearrangement or formation of the alcohol 1-OH from 1-OMe was found. The nucleophilic selectivity between added azide ion and solvent water was measured as k(N₃)/k(HOH) = 2.1 x 10⁴ (ratio of second-order rate constants). The results indicate a relatively stable benzallylic carbocation toward trapping by nucleophiles (k(w) = 1 x 10⁷ s^-1). However, the elimination-to-substitution ratio with solvent water as base/nucleophile is large. Thus, in contrast to other carbocations of similar reactivity toward nucleophiles, the barrier to dehydronation is very low, k(e)= 1.6 x 10¹⁰ s^-1, and accordingly, this step does not show any catalysis from added general bases. The heats of reaction of the solvolytic eliminations of 1-OH and 2-OH are ΔH = -23.7 and -18.4 kcal mol^-1, respectively, as measured by microcalorimetry.

Full text of DOI:10.1021/ja9807800

6512
J. Am. Chem. Soc. 1998, 120, 6512-6517
Acid-Catalyzed Solvolysis of Allylic Ethers and Alcohols.
Competing Elimination and Substitution via a Thermodynamically
†
“
Stable” Carbocation
Necmettin Pirinccioglu and Alf Thibblin*
Contribution from the Institute of Chemistry, UniVersity of Uppsala, Box 531, S-751 21 Uppsala, Sweden
ReceiVed March 9, 1998
Abstract: Specific acid-catalyzed solvolysis of 1-methoxy-1,4-dihydronaphthalene (1-OMe) or 2-methoxy-
1
,2-dihydronaphthalene (2-OMe) in 25 vol % acetonitrile in water yields mainly the elimination product
naphthalene, which is accompanied by a trace of 2-hydroxy-1,2-dihydronaphthalene (2-OH). No intramolecular
rearrangement or formation of the alcohol 1-OH from 1-OMe was found. The nucleophilic selectivity between
4
added azide ion and solvent water was measured as kN /kHOH ) 2.1 × 10 (ratio of second-order rate constants).
3
7
The results indicate a relatively stable benzallylic carbocation toward trapping by nucleophiles (kw ) 1 × 10
-
1
s ). However, the elimination-to-substitution ratio with solvent water as base/nucleophile is large. Thus, in
contrast to other carbocations of similar reactivity toward nucleophiles, the barrier to dehydronation is very
10
-1
low, ke ) 1.6 × 10 s , and accordingly, this step does not show any catalysis from added general bases.
The heats of reaction of the solvolytic eliminations of 1-OH and 2-OH are ∆H ) -23.7 and -18.4 kcal
-
1
mol , respectively, as measured by microcalorimetry.
Introduction
Scheme 1
We are interested in the factors that govern the competition
between substitution and elimination from carbocation inter-
mediates. Reaction directly from an ion pair may provide
predominately elimination owing to facile hydron abstraction
1
by the leaving group. Even very weakly basic leaving groups,
such as halide ions, show significant kinetic basicities in many
solvolytic reactions.1 In contrast, solvent-equilibrated car-
bocations generally yield predominately substitution in nucleo-
,2
1
-3
philic solvents.
We now report results on a system which provides nearly
exclusively elimination product despite the intermediacy of a
+
relatively stable benzallylic carbocation (R ). Thus, the acid-
that are responsible for the large elimination-to-substitution ratio.
These reactions are of biochemical interest since naphthalene
hydrates are thought to be intermediates in the mammalian
metabolism of naphthalene and dihydronaphthalenes.6
catalyzed solvolysis of the ethers 1-OMe and 2-OMe provide
naphthalene as the major product as shown in Scheme 1. A
study of the dehydration of the corresponding alcohols 1-OH
and 2-OH was recently reported by the groups of More O’Ferrall
and Boyd.4 They discussed the reasons for the much higher
reactivity of these alcohols compared to that of 1-phenylethanol.
However, it is not clear whether the predominate elimination
is attributable to the large thermodynamic stability of the
naphthalene product or whether there are other important factors
Reaction mechanistic studies of these allylic ethers are also
interesting because they may provide insight into the reactivity
7
and stability of carbocation-molecule pairs. Such species are
,5
formed, for example, when hydronated ethers undergo carbon-
oxygen heterolysis. We have found previously that they are
intermediates with significant lifetimes, i.e., they are not only
encounter complexes. Thus, intramolecular rearrangement of
allylic ethers in aqueous solvents has been observed,7b and the
substitution reaction of a tertiary ether with solvent water is
†
Presented in part at the Sixth European Symposium on Organic
7
f
faster than diffusional separation.
Reactivity (ESOR VI), Louvain-la-Neuve, Belgium, 1997.
(
(
1) Thibblin, A. Chem. Soc. ReV. 1993, 22, 427.
2) (a) Thibblin, A.; Sidhu, H. J. Am. Chem. Soc. 1992, 114, 7403. (b)
(6) (a) Boyd, D. R.; McMordie, R. A. S.; Sharma, N. D.; Dalton, H.;
Williams, P.; Jenkins, R. O. J. Chem. Soc., Chem. Commun. 1989, 339. (b)
Boyd, D. R.; Hand, M. V.; Sharma, N. D.; Chima, J.; Dalton, H.; Sheldrake,
G. N. J. Chem. Soc., Chem. Commun. 1991, 1630. (c) Boyd, D. R.; Dorrity,
M. R. J.; Hand, M. V.; Malone, J. F.; Sharma, N. D.; Dalton, H.; Gray, D.
J.; Sheldrake, G. N. J. Am. Chem. Soc. 1991, 113, 666.
Meng, Q.; Thibblin, A. Submitted.
(
(
3) Richard, J. P.; Jencks, W. P. J. Am. Chem. Soc. 1984, 106, 1373.
4) Boyd, D. R.; McMordie, R. A. S.; Sharma, N. D.; More O’Ferrall,
R. A.; Kelly, S. C. J. Am. Chem. Soc. 1990, 112, 7822.
5) More O’Ferrall, R. A.; Rao, S. N. Croat. Chem. Acta 1992, 65, 593.
(
S0002-7863(98)00780-X CCC: $15.00 © 1998 American Chemical Society
Published on Web 06/18/1998

Acid-Catalyzed SolVolysis of Allylic Ethers and Alcohols
J. Am. Chem. Soc., Vol. 120, No. 26, 1998 6513
Table 1. Second-Order Rate Constants for the Acid-Catalyzed
Reactions of 1-OMe, 2-OMe, 1-OH, and 2-OH at 25 °C
ethers
alcohols
1
-OMe
2-OMe
-1
1-OH,
s
2-OH,
s
k
H
,
k
M
H
,
k
M
H
k
M
H
-
1
s-1
s-1
-1 -1
-1 -1
solvent
M
MeCN-watera
53.2b,e
0.128b,f
47
c,g
0.173b,h
d,g
46
glycerol-watera
254
58
240
c,i
d,i
k
1.22c,j
d,j
2
1.22
water
1.8k
a
5 vol % organic solvent in water. Measured by HPLC. c Mea-
b
2
d
sured by UV spectrophotometry (UV). Measured by microcalorimetry
MC). Cf. Figure S1a. Cf. Figure S1b. 0.1 M acetate buffer, pH
.18 (UV) and pH 5.19 (MC). 0.1 M formate buffer, pHs 3.38 and
e
f
g
(
4
4
h
Figure 1. Effect of added salts on the rate of solvolysis of 1-OMe in
the presence of 0.10 M acetate buffer (buffer ratio 1.0) in 25 vol %
acetonitrile in water at 25 °C.
i
.15. 0.05 M phosphate buffer, pH 6.60. The rate constants at 15 and
-1
4
5 °C were measured (UV) as 126 and 1475 M-1
s , respectively.
j
0
.1 M acetate buffer, pH 4.18. The rate constants at 15 and 45 °C
were measured (UV) as 0.328 and 9.33 M
from ref 4.
-
1
-1
s
, respectively. k Data
The results of the present work are consistent with a common
solvent-equilibrated carbocation intermediate, but some reaction
through ion-molecule pairs cannot be excluded. The mecha-
nistic details of the reactions are discussed.
Results
The acid-catalyzed solvolysis of 1-methoxy-1,4-dihydronaph-
thalene (1-OMe) or 2-methoxy-1,2-dihydronaphthalene (2-OMe)
in 25 vol % acetonitrile in water at 25 °C yields mainly
naphthalene (3) along with a small amount of 2-hydroxy-1,2-
dihydronaphthalene (2-OH) (Scheme 1). No traces of the
rearranged ether product 2-OMe or the alcohol 1-hydroxy-1,4-
dihydronaphthalene (1-OH) were found in the reactions of
Figure 2. Effect of added salts on the rate of solvolysis of 1-OMe in
25 vol % acetonitrile in water (and in 50 vol % methanol in water,
lower line) at 25 °C in the presence of 0.50 M NaCl, 0.075 M phosphate
buffer at pH 6.23.
1
-OMe. The kinetics of the reactions were studied by a
sampling high-performance liquid chromatography procedure
or by following the appearance of 3 as a function of time by
UV spectrophotometry. The same methods were used to study
the kinetics of the acid-catalyzed solvolysis of 1-OH and 2-OH.
The rate of solvolysis is, as expected, proportional to the
+
acidity of the medium, i.e., [H3O ], which is shown for 1-OMe
and 2-OMe in Figure S1. The rate of solvolysis at constant
buffer ratio is not a function of buffer concentration (Figure
S2), which indicates that the acid catalysis is of specific rather
than of general type. Moreover, the absence of buffer catalysis
also indicates that there is no general base catalysis involved
in the rate-limiting step. The measured rate constants are shown
in Table 1.
The effect of salts on the reactions is relatively large in 25
vol % acetonitrile in water as shown in Figure 1 for 1-OMe
with acetate buffer. Salt effects have also been studied at a
higher pH using phosphate buffer at a constant concentration
of 0.5 M NaCl (Figure 2). Similar behavior was observed for
3
Figure 3. Increase in the elimination-to-substitution ratio [3]/[2-N ]
for 1-OMe and 2-OMe, respectively, with reaction time. Reaction
-
conditions: 3.00 M azide buffer, [HN
3
]/[N
3
] ) 4.71.
2
5
-OMe (plots not shown). The effect of salt concentration in
0 vol % methanol in water is much smaller, which is shown
Addition of azide ion gives rise to the substitution product
-N3 (Scheme 1), but no trace of 1-N3 is observed. However,
2
in Figure 2 for azide ion. As expected, one more product,
-OMe, is formed in this solvent mixture.
it is not possible to exclude that it is formed initially since allylic
2
8
azides are known to isomerize very rapidly. Addition of 0.3
(
7) (a) Sneen, R. A.; Felt, G. R.; Dickason, W. C. J. Am. Chem. Soc.
973, 95, 638. (b) Thibblin, A. J. Chem. Soc., Perkin Trans. 2 1987, 1629.
c) Katritzky, A. R.; Brycki, B. E.; J. Phys. Org. Chem. 1988, 1, 1 and
references therein. (d) Szele, I.; Zollinger, H. J. Am. Chem. Soc. 1978, 100,
811. (e) Hashida, Y.; Landells, R. G. M.; Lewis, G. E.; Szele, I.; Zollinger,
M NaN3 yields only 5% of 2-N3. The same initial fraction was
obtained from all the ethers and alcohols.
1
(
The azide product 2-N3 is not stable but slowly reacts to give
naphthalene (3) as the final product. The rate constant of
decomposition of 2-N3 was measured in a separate experiment
2
H. J. Am. Chem. Soc. 1978, 100, 2816. (f) Thibblin, A. J. Chem. Soc.,
Chem. Commun. 1990, 697. (g) Thibblin, A. J. Org. Chem. 1993, 58, 7427.
-
6
-1
as 470 × 10
s
(see the Experimental Section). Figure 3
(
h) Thibblin, A. J. Phys. Org. Chem. 1993, 6, 287. (i) Thibblin, A.; Sidhu,
H. J. Phys. Org. Chem. 1993, 6, 374. (j) Bleasdale, C.; Golding, B. T.;
Lee, W. H. L.; Maskill, H.; Riseborough, J.; Smits, E. J. Chem. Soc., Chem.
Commun. 1994, 93. (k) Sidhu, H.; Thibblin, A. J. Phys. Org. Chem. 1994,
shows the variation of the product ratio [3]/[2-N3] for 1-OMe
(8) Gagneux, A.; Winstein, S.; Young, W. G. J. Am. Chem. Soc. 1960,
82, 5956.
7
, 578. (l) Thibblin, A.; Saeki, Y. J. Org. Chem. 1997, 62, 1079.

6514 J. Am. Chem. Soc., Vol. 120, No. 26, 1998
Pirinccioglu and Thibblin
Table 3. Effect of Salt on the Elimination-to-Substitution Ratio
[
0
3]/[2-OMe] for Acid-Catalyzed Solvolysis of 1-OH in 50 vol %
Methanol in Water at 25 °C
salt (0.50 M)a [3]/[2-OMe]
b,c
salt (0.50 M)a [3]/[2-OMe]
0
b,c
0
none
NaClO
NaOAc
NaCl
1100
1315
1205
1454
NaBr
NaSCN
NaN
3
1522
1782
2356
4
d
a
.05 M phosphate buffer, pH 6.23. b Measured after 20-30 min
0
of reaction. No significant amount of solvolysis of the product 2-OMe
c
has occurred after this short reaction time. Estimated maximum error
(
10%. d 0.30 M.
Table 4. Nucleophilic Selectivities Expressed as Ratios of
Second-Order Rate Constants (Eq 1)
substrate
k
N3/kHOH
k
MeOH/kHOH
k
EtOH/kHOH
4
a
b
c
Figure 4. Increase in the substitution-to-elimination ratio [2-N
i.e., ratios extrapolated to zero reaction time, with NaN concentration
for reaction of 1-OH and 1-OMe, respectively. Reaction conditions:
.50 M NaCl, 25 °C, 0.075 M phosphate buffer in 25 vol % acetonitrile
3
]/[3]
0
,
1-OMe
2.1 × 10
∼3.6
∼2.8
3
a
5 vol % acetonitrile in water. b Calculated as the ratio of [3]/[2-
2
OH] and [3]/[2-OMe] measured in aqueous acetonitrile and in 50 vol
0
methanol in water, respectively. c Calculated as the ratio of [3]/[2-
%
in water at pH 6.30 and in 50 vol % methanol in water at pH 6.13.
OH] and [3]/[2-OEt] measured in aqueous acetonitrile and in 50 vol
ethanol in water, respectively.
%
Table 2. Effect of Salt on the Elimination-to-Substitution Ratio
[
3]/[2-N
3 0
] for Acid-Catalyzed Solvolysis of 1-OH in 25 vol %
Table 5. Heat of Reactions for the Dehydration of 1-OH and
Acetonitrile in Water at 25 °C
2
-OH at 25 °C
salt (0.75 M)a
none
[3]/[2-N ]
3 0
b
salt (0.75 M)a
[3]/[2-N
]
3 0
b
,a µW
P
0
∆H, kcal mol-1
substrate
9.0 ( 0.6
11.0 ( 1.2
12.0 ( 1.2
NaCl
NaBr
NaSCN
12.5 ( 1.4
13.3 ( 1.5
16.0 ( 1.6
-OHb
d
1
2
20.0
12.0
-23.7 ( 0.4
-18.4 ( 0.2
NaClO
NaOAc
4
-OHc
e
a
Heat effect at time zero (extrapolated). b Concentration of 0.76 mM.
a
.075 M phosphate buffer, pH 6.23. b Extrapolated to time zero;
0
c
e
Concentration of 0.64 mM. d Average of 12 runs at pH 6.10-6.80.
Average of six runs at pH 4.18.
0
.25 M NaN
3
.
and 2-OMe as a function of time. The product ratio was
Activation Parameters. Activation parameters were calcu-
lated for the solvolyses of 1-OH and 2-OH in 25 vol % glycerol
in water at constant pH, rate constants measured at 15, 25, and
5 °C (Table 1). The activation enthalpies and entropies were
extrapolated to time zero to obtain the reaction rate ratio kN -
3
-
-
[
N3 ]/kE for nucleophilic substitution with azide, kN [N3 ], and
3
elimination, kE. The intercepts of the two plots are 5.0 and
.5, i.e., the same within experimental error, which suggests a
4
5
calculated from slopes of Arrhenius plots and the free energies
common intermediate for the reactions of these isomeric
substrates. Extrapolated ratios for 1-OMe and 1-OH are plotted
versus azide ion concentration in Figure 4, which also includes
data for 1-OH in 50 vol % methanol in water. The slopes of
these plots, which are very similar, correspond to the ratios of
the second-order reaction rate constants of addition of azide
ion and the pseudo-first-order rate constants of dehydronation
of the carbocation intermediate. The slope of the plot for 1-OMe
q
of activation at 25 °C. The results for 1-OH are ∆G ) 14.2
-
1
q
-1
q
kcal mol , ∆H ) 14.5 kcal mol , and ∆S ) 1 eu and for
q
-1
q
-1
2
∆
-OH are ∆G ) 17.3 kcal mol , ∆H ) 19.6 kcal mol , and
q
S ) 7 eu.
Calorimetric Measurements. The acid-catalyzed solvolyses
of the alcohols have also been studied by heat-flow microcalo-
rimetry. These reactions, as described above, yield exclusively
naphthalene and naphthalene along with a trace of 2-OH,
respectively. Thus, the heats produced from both reactions are
attributable to formation of the elimination product.
-
1
is 0.32 M .
No substitution product with any other strong nucleophile
was detected, not even with thiocyanate ion. The reason is
presumably that 2-SCN is formed but that it is not stable and
rapidly yields naphthalene and a small amount of 2-OH as the
final product. Another possibility is that the small amount of
The reactions were carried out in 25 vol % glycerol in water.
Glycerol was used instead of acetonitrile to minimize distur-
bances from trace amounts of vaporization of the organic solvent
from the sealed reaction flasks. A time period of 40-60 min
after initiation of the reaction was required for thermal
equilibration. The observed decreases in heat flow reflect
pseudo-first-order kinetic behavior of the reactions. The
measured rate constants agree well with those obtained with
the spectrophotometric technique (Table 1). The measured
reaction enthalpies are given in Table 5.
2
-SCN is not detected owing to coelution with the large
naphthalene product peak during the HPLC analysis.
The effect of salt on the product ratio [3]/[2-N3] extrapolated
to time zero was studied with 1-OH. The ratio increases with
more nucleophilic salts as shown in Table 2. The product ratio
[3]/[2-OMe] for solvolysis of 1-OH in 50 vol % methanol in
water was also found to change by addition of salt (Table 3).
The nucleophilic selectivity toward azide ion and solvent
components is very large. The results for 1-OMe are shown in
Table 4. The alcohols 1-OH and 2-OH show the same results.
The rate-constant ratios are ratios of second-order rate constants
calculated by means of eq 1.
Discussion
Rate-Limiting Carbocation Formation. The alcohols 1-OH
and 2-OH and the corresponding ethers 1-OMe and 2-OMe are
highly reactive. For example, acid-catalyzed solvolysis of 1-OH
k /k
) ([2-Nu]/[2-OH])/([Nu]/[H O])
(1)
11
Nu HOH
2
yielding naphthalene is almost 10 times faster than the

Acid-Catalyzed SolVolysis of Allylic Ethers and Alcohols
J. Am. Chem. Soc., Vol. 120, No. 26, 1998 6515
corresponding reaction of 1-phenylethanol.4,9 The formation
of the benzallylic carbocation (R ) from the ethers is rate-
Scheme 2
+
limiting as shown by the absence of general acid-base catalysis
(Figure S2). This is in accord with the results of More O’Ferrall
and co-workers, who did not find any general catalysis for 1-OH
and 2-OH in aqueous acetic acid buffers.4 One-step concerted
elimination directly from the hydronated substrates, or rate-
limiting elimination via the carbocation, would require such
catalysis. The higher reactivity of 1-OMe compared to that of
the isomeric 2-OMe is consistent with stepwise reactions of the
corresponding hydronated ethers since concerted base-promoted
elimination reactions are unfavored compared to their 1,2-
counterparts due to the requirement for a large number of
changes in bonding occurring at a single reaction step.
naphthalene product, or is the addition of nucleophiles to the
carbocation slower than normal? This issue is addressed in
the following.
The rate constant (kw) for reaction of the carbocation with
water to give 2-OH (Scheme 2) has been measured by the
1
1
“
azide-clock” method assuming that the reaction with azide ion
9
-1
is diffusion-controlled with a rate constant of 5 × 10 M
The faster reaction of 1-OH than of 1-phenylethanol reflects
that the acid-catalyzed dehydration of the latter does not involve
rate-limiting formation of the cation but undergoes rate-limiting
elimination. However, this difference in rate-limiting step has
been estimated to account for only a factor of about 700 of the
-
1 3,12
s .
The rate constant measured in this manner is kw ) 1.0
7
-1
×
10 s . The same result was obtained by starting from either
1
-OMe, 1-OH, or 2-OMe. This rate constant is not unusually
small and is consistent with a diffusion-limited reaction with
azide anion since direct measurements of rate constants using
flash photolysis have shown that carbocations with kw > 1 ×
11 4
total factor of ca. 10 . Other factors such as enhanced
resonance in the cyclic cation and the effect of a vinyl substituent
5
-1
4
10 s react with azide ion in a diffusion-controlled process in
have been suggested to be of importance.
1
2
+
aqueous acetonitrile. The reactivity of R is on the same order
Salt effects on acid-catalyzed solvolysis reactions are more
complex than those on uncatalyzed solvolysis since they consist
of the effect on the fast hydronation step combined with the
effect on the ionization to the carbocation-molecule complex.
The observed effect of salts on the reaction rate is relatively
large in aqueous acetonitrile but is small in 50 vol % methanol
in water (Figures 1 and 2). Similar behavior has been seen
+
as that of (4-MeC6H4)2CH , for which a rate constant of kw )
7
-1
12
3
.2 × 10 s was directly measured.
Only one azide product is observed in the solvolysis of 1-OMe
and 1-OH. A plausible trace of 1-N3 formed is expected to
rapidly isomerize to 2-N3 since allylic azides are known to
8
isomerize quickly. A single allylic azide isomer product has
13
been observed previously in solvolytic reactions. The car-
bocation shows discrimination between the nucleophiles water,
methanol, and ethanol of kMeOH/kHOH ∼ 3.6 and kEtOH/kHOH ∼
before for uncatalyzed solvolysis reactions of substrates with
charged leaving groups.10 The absence of an increase in the
rate of reaction of 1-OMe in the presence of added azide ion in
aqueous methanol suggests that bimolecular reactions are not
significant.
2
.8, ratios of second-order rate constants (eq 1 and Table 4).
1
The values are “normal” for a relatively stable carbocation. In
summary, there is no indication for an unusually slow reaction
of the benzallylic carbocation with nucleophiles but, of course,
conclusive evidence could only be obtained through direct
measurement of the rate constants.
The pKR for the pseudo acid-base equilibrium for cation
hydration (eqs 2 and 3) is a measure of the thermodynamic
+
stability of R .
kw
In accord with the results discussed above, we attribute the
unusually large elimination-to-substitution ratio to an unusually
fast dehydronation of the carbocation intermediate. The azide-
trapping method yields a rate constant for hydron abstraction
+
+
R + H O {
\
} ROH + H
(2)
(3)
2
kH
+
+
K ) k /k ) [ROH][H ]/[R ]
R
w
H
+
10 -1
from R of ke ) 1.6 × 10
s
in the aqueous acetonitrile
solvent (Figures 3 and 4). This rate constant is unusually large
for a thermodynamically stable carbocation such as the benza-
The rate constant (kH, Table 1) for the acid-catalyzed solvolysis
of 2-OH in 25 vol % acetonitrile in water is 0.173 M s .
This rate constant, combined with the rate constant for addition
-
1
-1
+
4
llylic R . For example, it is 10 times larger than the elimination
rate constant for 1-(4-methylphenyl)ethyl carbocation measured
+
7 -1
of water to R of kw )1.0 × 10 s (vide infra) yields pKR )
3
+
in 50 vol % trifluoroethanol in water, despite the fact that the
-
7.8. The corresponding rate constant (kw) for reaction of R
difference in pKR values (vide supra) indicates a great difference
in thermodynamic stability of the cations.
to give 1-OH is not known, but it is reasonable to assume that
it is at least 10 times smaller than that for giving 2-OH, which
yields pKR g -6.3. These values could be compared with the
corresponding parameter for the 1-(4-methylphenyl)ethyl car-
The fast hydron abstraction reflects the large thermodynamic
stability of the naphthalene product (see next section) and should
correspond to a very small amount of hydron transfer in the
transition state. Thus, the transition state of the hydron-
abstraction step is expected to be very reactant-like, i.e., close
to the benzallylic carbocation in structure (Brønsted â ∼ 0).
Accordingly, the effect of added general bases on the elimina-
tion-to-substitution ratio is not large. For example, the presence
of 0.75 M acetate ion in the aqueous acetonitrile gives only a
bocation of pKR ) -12.8, measured in 50 vol % trifluoroethanol
in water.3
Reactivity of the Benzallylic Carbocation. In contrast to
most other solvolysis reactions through relatively stable car-
bocation intermediates, the dehydronation of the carbocation is
much faster than the addition reaction with solvent or added
powerful nucleophiles. The elimination-to-substitution ratio kE/
kS with solvent water is as large as 1600. Is this abnormal
relative reactivity solely due to the large stability of the
(11) Slow nucleophilic addition has previously been reported for R-car-
bonyl- and R-thiocarbonyl-substituted benzyl carbocations: Richard, J. P.;
Lin, S.-S.; Buccigross, J. M.; Amyes, T. L. J. Am. Chem. Soc. 1996, 118,
12603.
(12) McClelland, R. A. Tetrehedron 1996, 52, 6823.
(13) (a) Rosenberg, A. M.; Sneen, R. A. J. Am. Chem. Soc. 1961, 83,
900. (b) Thibblin, A. J. Chem. Soc., Perkin Trans. 2 1986, 313.
(
9) (a) Schubert, W. M.; Keefe, J. R. J. Am. Chem. Soc. 1972, 94, 559.
b) Modena, G.; Rivetti, F.; Scorrano, G.; Tonellato, U. J. Am. Chem. Soc.
977, 99, 3392.
10) Toteva, M. M.; Richard, J. P. J. Am. Chem. Soc. 1996, 118, 11445.
(
1
(

6516 J. Am. Chem. Soc., Vol. 120, No. 26, 1998
Pirinccioglu and Thibblin
Scheme 3
Scheme 4
The failure to observe formation of 1-OH and 2-OMe from
1
-OMe is presumably due to the fact that the cleavage of the
carbon-oxygen bond is accompanied by extensive delocaliza-
tion of charge in this system. This delocalization is expected
+
to decrease the stability of 1 OHMe, which favors diffusional
separation by increasing the barrier for collapse of the ion-
molecule pair back to covalent material as well as increasing
the barrier for direct substitution of the ion-molecule pair with
a water molecule.
small increase in the ratio [3]/[2-N3] (Table 2) compared to no
salt addition. However, this effect is not significantly larger
than that for perchlorate ion. Nevertheless, there is a small
increase with more powerful nucleophiles (Table 2). A similar
increase in the amount of elimination was also observed in 50
vol % methanol in water where the ratio of naphthalene-to-
ether formation was studied (Table 3).
Experimental Section
General Procedures. NMR spectra were recorded for CDCl
solutions with a Varian XL 300 spectrometer. Chemical shifts are
indirectly referenced to tetramethylsilane via the solvent signal
3
1
(chloroform-d , 7.26 and 77.0 ppm). The high-performance liquid
chromatography analyses were carried out with a Hewlett-Packard 1090
liquid chromatograph equipped with a diode-array detector on an Inertsil
5 OSD-2 (3 × 100 mm) reversed-phase column. The chromatography
was performed isocratically using acetonitrile in water as the mobile
phase. The reactions were studied at constant temperature in a HETO
In principle, the catalysis could be due to either hydron
abstraction within the ion pair or a first formation of the covalent
material (2-Nu) which undergoes a concerted one-step 1,2-
elimination with the solvent acting as the hydron-abstracting
base. Catalysis from added chloride ion has been observed
previously for the dehydronation of ferrocenyl-stabilized car-
01 PT 623 thermostat bath. The UV spectrophotometry was performed
on a Kontron Uvicon 930 spectrophotometer equipped with an
automatic cell changer kept at constant temperature using a thermostated
water bath. The reaction solutions for the kinetic experiments were
prepared by mixing acetonitrile, glycerol, or methanol with water at
room temperature, ca. 22 °C. The pH was measured before and after
reaction using a Radiometer PHM82 pH meter with an Ingold glass
microelectrode. The pH values given are those measured before mixing
with the organic solvent. The microcalorimetric experiments were
carried out with a dual-channel calorimeter (Thermometrics Thermal
Activity Monitor 2277). The signals were recorded on both a two-
channel potentiometric recorder (LKB 2210) and a HP 3390 digital
device.
14
bocations in 50 vol % acetonitrile in water.
Mapping of Reaction Energies. The microcalorimetric
measurements show large heats of reaction: ∆H ) -23.7 and
-
1
-
18.4 kcal mol for 1-OH and 2-OH, respectively. These
values agree well with the estimated change in free energy of
q
-1
15
∆G
∼ -20 kcal mol
(K ∼ 10 ) for dehydration of
4,5
naphthalene hydrates in aqueous solution. In contrast, 1-phe-
-
1
nylethanol is more stable than styrene by about 2 kcal mol
9a
(K ) 0.025).
Materials. Previously reported procedures were followed for
preparation of the substrates 1-hydroxy-1,4-dihydronaphthalene (1-
The difference in enthalpy of ∆∆H ) 5.3 kcal mol-1 for
-OH and 2-OH is close to the difference in activation enthalpy,
∆H ) 5.1 kcal mol , but the difference in reactivity is
smaller (Table 1; 208-fold in glycerol-water, ∆∆G ) 3.2 kcal
mol ). The difference in energy of the rate-limiting transition
states of these reactions may be attributable to a more delocal-
ized transition state of the reaction of 2-OH. This more
advanced transition state corresponds to a larger entropy of
reaction. The measured energies are shown in Scheme 3.
1
∆
15
15
OH), 1-methoxy-1,4-dihydronaphthalene (1-OMe), and 2-hydroxy-
q
-1
16
1,2-dihydronaphthalene (2-OH). 2-Methoxy-1,2-dihydronaphthalene
q
(2-OMe) was prepared from 2-OH by the same procedure as was used
for synthesis of 1-OMe. After purification the substrates were free
from traces of isomeric impurities, and the ethers did not contain any
of the corresponding alcohol. Methanol and acetonitrile were of HPLC
grade. All other chemicals were of reagent grade and used without
further purification.
-
1
Kinetics and Product Studies: HPLC Procedure. The reactions
were initiated by addition of a few microliters of the substrate dissolved
in acetonitrile to a 2-mL HPLC flask containing 1.2 mL of the
thermostated reaction solution to give a substrate concentration of 0.1-
0.2 mM. The reaction flask was sealed with a gastight PTFE septum
and placed in a thermostated aluminum block in the HPLC apparatus.
At appropriate intervals, samples were automatically injected onto the
column and analyzed. The rate constants for the disappearance of the
substrates were calculated from plots of substrate peak area versus time
by means of a nonlinear regression computer program. Very good
Ion-Molecule Pair Intermediates. The complexes initially
formed by the carbon-oxygen cleavage are the carbocation-
methanol pairs as indicated in Scheme 4. However, our results
do not give any information as to whether these complexes are
discrete intermediates, i.e., if they have significant lifetimes.
We have previously found indications in several systems for
the intermediacy of ion-molecule pairs with methanol as the
7
b,f,g
leaving group.
For example, results were reported that
strongly suggest that delocalization of charge in the carbocation
7
f
lags behind the cleavage of the carbon-oxygen bond.
(15) Moss, R. J.; Rickborn, B. J. Am. Chem. Soc. 1986, 51, 1992.
16) (a) Bamberger, E.; Lodter, W. Justus Leibigs Ann. Chem. 1895,
(
(
14) Bunton, C. A.; Carrasco, N.; Cully, N.; Watts, W. E. J. Chem. Soc.,
288, 100. (b) Jeffrey, A. M.; Jerina, D. M. J. Am. Chem. Soc. 1972, 94,
4048.
Perkin Trans. 2 1980, 1859.

Acid-Catalyzed SolVolysis of Allylic Ethers and Alcohols
J. Am. Chem. Soc., Vol. 120, No. 26, 1998 6517
pseudo-first-order behavior was seen for all of the reactions studied.
The separate rate constants for the elimination and substitution reactions
were calculated by combination of product composition data, obtained
from the peak areas and the relative response factors determined in
separate experiments, with the observed rate constants.
The azide product 2-N is not stable but slowly reacts to give
3
naphthalene as the final product. The rate constant for this decomposi-
tion was measured after complete solvolysis of 1-OH with azide buffer
at low pH. After neutralization, the disappearance of 2-N was followed
3
chambers. After a total time of 30-45 min it was possible to start
recording the first-order heat-flow decay. The reactions were followed
for at least 10 half-lives.
The microcalorimeter was statically calibrated after a kinetic
experiment using the reaction solutions. The time constant of the
instrument was found to be 240 s, i.e., no correction for this parameter
was necessary for calculation of the rate constants of the reaction heat
decay. Very good first-order rate constants (kobs) were measured. These
agree well with those measured in the kinetics using the UV spectro-
photometric technique (Table 1).
by HPLC.
UV Spectrophotometric Procedure. The reactions were run in
0
The extrapolated heat flow at time zero (P ) was used for calculation
3
-mL standard quartz cells using the above-mentioned equipment.
of the reaction heat (∆H) according to eq 4:
Addition of a few microliters of a concentrated solution of the substrate
in acetonitrile to 2.5 mL of reaction solution gave an initial concentra-
tion of the substrate in the reaction flask of 0.1 mM. The increases in
absorbance at 275 and 300 nm, respectively, were followed as a function
of time and the pseudo-first-order rate constant calculated by a nonlinear
regression computer program.
P0 ) ∆Hkobsn
(4)
where n is the amount of substrate (mol) in the reaction vial.
The estimated errors are considered as maximum errors derived from
maximum systematic errors and random errors.
Microcalorimetric Procedure. This technique has the advantage
that both kinetic data and reaction heats are obtained from the same
kinetic experiment. The reactions were run in parallel in the two
channels, both composed of a sample compartment and a reference
compartment. Glass vials (3 mL) were used as reaction and reference
vessels. All four vessels were filled at the same time with 2.5 mL of
premixed buffer solution (glycerol-water). After this step, 20 µL of
substrate in acetonitrile was added to the two reaction vessels while
Acknowledgment. We thank Prof. Rory More O’Ferrall for
comments and the Swedish Natural Science Research Council
for supporting this work.
Supporting Information Available: The effect of pH and
buffer concentration on the rate of solvolysis of 1-OMe and
2
-OMe (Figures S1 and S2) (2 pages, print/PDF). See any
2
0 µL of pure acetonitrile was added to the reference vessels. The
current masthead page for ordering information and Web access
instructions.
vials were sealed with gastight PTFE septa and slowly introduced into
the compartments of the instrument for about 15 min of prethermo-
stating. They were than lowered further down into the detection
JA9807800