Acid-catalyzed hydrolysis of bridged bi- and tricyclic compounds. XXXVII. Kinetics and mechanisms of 1- and 3-acetoxynortricyclanes

Article abstract of DOI:10.1002/(SICI)1099-1395(200002)13:2<133::AID-POC225>3.0.CO;2-3

The disappearance of 3- and 1-acetoxynortricyclanes (1 and 2) in aqueous perchloric acid was followed by capillary gas chromatography at different temperatures and acid concentrations. According to the activation parameters, solvent deuterium isotope effe

Full text of DOI:10.1002/(SICI)1099-1395(200002)13:2<133::AID-POC225>3.0.CO;2-3

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2000; 13: 133–138
Acid-catalyzed hydrolysis of bridged bi- and tricyclic
compounds. XXXVII. Kinetics and mechanisms of 1- and
3-acetoxynortricyclanes
Martti Lajunen, Rita Yli-Mannila and Outi Salminen
Department of Chemistry, University of Turku, FIN-20014 Turku, Finland
Received 6 September 1999; revised 4 November 1999; accepted 4 November 1999
ABSTRACT: The disappearance of 3- and 1-acetoxynortricyclanes (1 and 2) in aqueous perchloric acid was followed
by capillary gas chromatography at different temperatures and acid concentrations. According to the activation
parameters, solvent deuterium isotope effects and parameters of excess acidity equations, the AAC2 ester hydrolysis
with two water molecules in the transition state is dominant at the lower acid concentrations studied (1–5.5 M HClO4)
and the AdE2 hydration of the cyclopropane ring is dominant at higher acid concentrations (6–8 M HClO4) at 298 K. 3-
Nortricyclanol (3) is formed via hydrolysis from 1, whose hydration products were not analyzed. 2-Norbornanone (4)
is formed via both hydrolysis and hydration from 2. Copyright 2000 John Wiley & Sons, Ltd.
KEYWORDS: nortricyclanes; 2-norbornanone (norcamphor); kinetics; acid catalysis; excess acidity; ester
hydrolysis; hydration; reaction mechanism
INTRODUCTION
EXPERIMENTAL
The hydrolysis of 3-acetoxynortricyclane or 3-acetoxy-
tricyclo[2.2.1.02,6]heptane (1) follows the AAC2 mechan-
ism (the acid-catalyzed bimolecular reaction via acyl-
Materials. The syntheses of 3- and 1-acetoxynortricy-
clanes (1 and 2) have been presented previously.6,7 The
purities by gas chromatography (GC) were >99.5 and
97% (3% of 4-acetoxynortricyclane?), respectively. The
acetates were identified from their 13C NMR and GC–
FTIR spectra.5,6
3
oxygen fission)1,2 in 1 mol dm HClO4 in 60 vol.%
dioxane–water, and variations in temperature, dioxane:
water ratio (1 M HClO4) or acid concentration (60%
dioxane–water) do not alter the mechanism.3 However,
a comparison of the rate constants of protonation (hy-
dration via the AdE2 or A SE2 mechanism) of the
cyclopropane ring of 3-substituted nortricyclanes hints
that the hydration may be observable in aqueous
HClO4.4,5 An increase in acid concentration should
increase the level of hydration and decrease that of the
AAC2 ester hydrolysis because of decreasing water
activity. Therefore, the kinetics of 3-acetoxynortricy-
clane (1) and of its interesting isomer, 1-acetoxynortri-
cyclane (2), in which the substituent is situated on a
cyclopropane carbon atom, were studied in this work in
aqueous perchloric acid at different acid concentrations.
Kinetic measurements. The disappearance of the sub-
strates in HClO4(aq.) was followed by GC (with an FFAP
capillary column) using nitrobenzene as the internal
standard and dichloromethane as the extracting solvent.
The CH2Cl2 solution (2 cm3) was neutralized by one drop
of concentrated NH3(aq.). The pseudo-first-order rate
constants were calculated from the slopes of the strictly
linear (r = 0.9998–0.999 995) correlation log (St S?) vs
t, where St is the ratio of the GC integrals of the substrate
and the internal standard at the time t. Each rate constant
was measured twice and the values were equal within 3%
at least (average 0.5%).
Product analyses. About 0.2 g of the substrate was stirred
magnetically with 50 cm3 of 1 M HClO4(aq.) for 2.7 h (ca
1 t1/2) and 24 h (ca 10 t1/2) in the case of 1 and with the
same volume of 5 M HClO4(aq.) for 4 h (>10 t1/2) in the
case of 2 at room temperature in a tightly stoppered
Erlenmeyer flask. The solution was extracted several
times with CH2Cl2 and the combined organic solution
*Correspondence to: M. Lajunen, Department of Chemistry, Uni-
versity of Turku, FIN-20014 Turku, Finland.
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 133–138

134
M. LAJUNEN, R. YLI-MANNILA AND O. SALMINEN
Table 1. Rate constants of disappearance for 3- and 1-acetoxynortricyclanes (1 and 2) in aqueous perchloric acid at different
temperatures and acid concentrations and in DClO4 (D2O)
3
b
c
1 d
Substrate
T (K)
c(HClO4)a (mol dm
)
X0
Log aw
kÉ (10 4 s
)
1
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
278.7
288.2
298.2
298.2
298.2
308.2
298.2
298.2
298.2
298.2
298.2
308.2
318.2
328.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
298.2
278.7
288.2
298.2
298.2
308.2
298.2
298.2
0.998
1.000
1.492
1.968
2.479
3.020
3.496
3.914
4.472
4.970
5.483
6.185
6.508
6.468
6.428
6.428
6.422
6.384
6.993
7.563
1.009
1.000
1.003
1.006
1.001
0.995
1.457
2.045
2.459
2.993
3.476
3.840
4.472
4.970
5.483
6.183
6.428
6.422
6.458
7.084
7.038
6.993
6.993
6.942
7.563
8.045
0.271
1.648
0.857 Æ 0.003
0.868 Æ 0.009e,f
1.226 Æ 0.003
1.564 Æ 0.003
1.959 Æ 0.003
2.388 Æ 0.006
2.797 Æ 0.007
3.228 Æ 0.009
4.000 Æ 0.007
5.093 Æ 0.009
6.727 Æ 0.013
13.30 Æ 0.07
1.582 Æ 0.019
5.29 Æ 0.06
0.381
0.487
0.611
0.758
0.906
1.052
1.271
1.491
1.740
2.118
1.626
1.572
1.522
1.474
1.430
1.388
1.322
1.254
1.174
1.048
2.259
1.000
17.52 Æ 0.13
17.63 Æ 0.12e
17.77 Æ 0.10g
54.39 Æ 0.14
41.67 Æ 0.16
102.9 Æ 0.3
2.604
2.977
0.273
0.880
0.748
1.650
2
1.169 Æ 0.009
1.170 Æ 0.018e
1.90 Æ 0.06g
3.10 Æ 0.02
7.56 Æ 0.04
17.1 Æ 0.3
0.373
0.505
0.606
0.750
0.900
1.025
1.271
1.491
1.740
2.117
2.259
1.630
1.564
1.524
1.476
1.432
1.396
1.322
1.254
1.174
1.048
1.000
1.671 Æ 0.008
2.403 Æ 0.004
2.954 Æ 0.006
3.732 Æ 0.010
4.536 Æ 0.010
5.324 Æ 0.010
6.59 Æ 0.03
8.125 Æ 0.014
9.86 Æ 0.05
14.26 Æ 0.05
16.61 Æ 0.06
22.18 Æ 0.06g
17.13 Æ 0.12
2.83 Æ 0.04
2.276
0.994
8.97 Æ 0.05
2.604
0.880
26.43 Æ 0.06
26.6 Æ 0.3e
74.7 Æ 0.3
2.977
3.312
0.748
0.629
47.6 Æ 0.2
107.0 Æ 0.4
a
Temperature corrected.
b Excess acidity.
13
c
12
Logarithm of water activity.
d Error limits are standard deviations.
e
Calculated from the activation parameters (Table 2).
f M. Lajunen and V. Nieminen, unpublished results (1998).
g Measured in DClO4 (D2O).
was neutralized and dried by allowing it to flow through
anhydrous K2CO3. The solvent was evaporated and the
residue analyzed by GC and GC–FTIR spectroscopy, and
the products were identified by comparing their retention
times and spectra with those of the authentic 3-
nortricyclanol (3) and 2-norbornanone (4).6
RESULTS AND DISCUSSION
Rate constants, activation parameters, isotope
effects and products. Reaction mechanisms
Rate constants of disappearance for 3- and 1-acetoxynor-
Copyright 2000 John Wiley & Sons, Ltd.
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HYDROLYSIS OF BRIDGED BI- AND TRICYCLIC COMPOUNDS. XXXVII
135
Table 2. Parameters of activation and solvent deuterium isotope effects and parameters of the excess acidity equations [Eqns
(5)±(7)] at 298 K for the hydrolysis of 3- and 1-acetoxynortricyclanes (1 and 2) in LClO4 (L2O) (L = H or D), with standard
deviations
3
1
1
Substrate
c(LClO4) (mol dm
)
DH≠ (kJ mol
)
DS≠ (J mol 1 K
)
kH/kD
1
1.00
6.43
1.01
6.43
6.99
69.6 Æ 0.4a
83.7 Æ 0.3c
70.7 Æ 0.6
77.1 Æ 0.3c
89.3 Æ 1.2a
32.3 Æ 0.8c
83.1 Æ 1.8
51.7 Æ 1.1c
0.68 Æ 0.09b
0.985 Æ 0.013
0.61 Æ 0.03
2
0.748 Æ 0.005
Log
[k0,2 (M
1
m≠d
Log (k0/KSH
)
s
1)]
‡
Substrate
Mechanism
AAC
m*
pKSH
‡
1
1
2
0.73 Æ 0.04
0.79 Æ 0.04
0.76 Æ 0.05
0.80 Æ 0.04
0.82 Æ 0.03
0.77 Æ 0.05
2.61 Æ 0.10
2.19 Æ 0.20
2.99 Æ 0.12
2.36 Æ 0.12
2.42 Æ 0.09
2.52 Æ 0.07
0.96 Æ 0.06
1.02 Æ 0.05e
1.91 Æ 0.06
1.02 Æ 0.06
0.82 Æ 0.04e
1.76 Æ 0.05
7.593 Æ 0.006
AdE2
AL1f
AC2
AdE2
AL1f
7.25 Æ 0.07
6.66 Æ 0.07
A
6.80 Æ 0.11
2
A
7.458 Æ 0.006
A
6.53 Æ 0.10
a
M. Lajunen and V. Nieminen, unpublished results (1998).
b Ref. 5.
c
Apparent values due to two competing reactions.
d m≠1, m≠2 or m≠3 depending on the mechanism.
e
m≠2 = m* m≠2/1.80. 13
2
f The dominant reaction at the highest acid concentrations is assumed to be entirely the AAL1 ester hydrolysis.
tricyclanes (1 and 2) in aqueous perchloric acid at
different temperatures and acid concentrations and in
DClO4(D2O) are listed in Table 1. The rate constant
(8.7 Â 10 5 s 1) for 1 as measured in 1.00 M HClO4(aq.)
ment with the earlier studies,3 but its level was clearly
reduced at higher acid concentrations, when acetoxy-
substituted norborneols and norbornanediols (not identi-
fied) were probably formed (Scheme 1; only one route,
but evidently the most important,5 has been presented). 2-
Norbornanone (norcamphor, 4) was the only observed
product of 1-acetoxynortricyclane (2) at all acid con-
centrations studied (Scheme 2). Its formation from 2 and
from 1-hydroxynortricyclane (5; Scheme 2) has been
reported earlier in both alkaline and acidic media.7,8
at 298.2 K is in good agreement with that measured
5
earlier under similar conditions (9.1 Â 10
s
1)3. The
rate constants for the two isomers are generally fairly
close to each other, although 1 is a secondary and 2 a
tertiary acetate. The activation parameters and solvent
deuterium isotope effects calculated from the second-
order rate constants [ka = k /c(HClO4)] are presented in
Table 2. The entropies of activation ( 89 and 83 J
1
K
1 mol for 1 and 2, respectively), the enthalpies of
activation (70 and 71 kJ mol 1) and the isotope effects
(kH/kD = 0.68 and 0.61) in 1 M LClO4(L2O) (L = H or D)
at 298 K are typical of the AAC2 mechanism.1 However,
the same parameters when measured at a higher acid
Separation of two reactions by the excess acidity
method
The separation of the two competing reactions can be
made quantitatively by applying the excess acidity
theory,9,10 according to which the pseudo-first-order rate
concentration (6.4 or 7 M), DS≠ = 32 and 52 J
1
K
1 mol 1, DH≠ = 84 and 77 kJ mol and kH/kD = 0.99
constant, k , obeys the equation
and 0.75, respectively, are clearly higher. The differences
are evidently due to a change of reaction from the ester
hydrolysis (AAC2 mechanism; Schemes 1 and 2) to the
hydration of the cyclopropane ring (AdE2 mechanism;
Schemes 1 and 2) with increasing acid concentration.5 If
a common change of the mechanism of ester hydrolysis
from AAC2 to AAL1 (the acid-catalyzed unimolecular
reaction via alkyl-oxygen fission)1 took place with
increasing acid concentration, the entropy of activation
would become less negative than observed and the
isotope effect would decrease rather than increase.1,5
3-Nortricyclanol (3) was the only product observed for
3-acetoxynortricyclane (1) in 1 M HClO4(aq.), in agree-
‡
‡
logk
log‰cS=ꢀcS ‡ cSH †Š logcH
nlogaw
ˆ m 1mà X0 ‡ logꢀk0;1=K
†
ꢀ1†
ꢀ2†
ˆ
‡
1
SH
in the case of AAC2 hydrolysis and the equation
ˆ
‡
logk
logcH ˆ m 2mà X0 ‡ logk0;2
2
in the case of AdE2 hydration. In the latter reaction,
partial protonation of the carbonyl oxygen, however,
causes curvature of the ideally linear plot, log k
‡
log cH vsX0. This ‘side reaction’ can be taken into
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 133–138

136
M. LAJUNEN, R. YLI-MANNILA AND O. SALMINEN
Scheme 2
Scheme 1
constant of the substrate protonated on the carbonyl
oxygen.
account, when a linear equation11
AAC2 hydrolysis is practically the only reaction at the
three or four lowest acid concentrations. Thus the
‡
‡
logk
log‰cS=ꢀcS ‡ cSH †Š logcH
intercept parameter, log(k0,1/KSH ), can be estimated
‡
ˆ m 2mà X0 ‡ logk0;2
ꢀ3†
)],
ˆ
2
with Eqn. (1) from the rate constants measured under
these conditions (Table 1) and from the following
approximate values of the excess acidity parameters:
is obtained. The correction term, log[cS/(cS ‡ cSH
‡
can be calculated with the equation9
m*1 = 0.64,14–16 m≠1 = 1.009 and pKSH
‡
=
3.43 (for
isopropyl acetate).17 The approximate rate constant
,app(AAC2) can be calculated at every acid concentra-
tion, and its subtraction from the corresponding observed
value gives the approximate k ,app(AdE2). The
logꢀcSH =cS† logcH ˆ mà X0 ‡ pK
ꢀ4†
‡
‡
‡
SH
1
k
In these equations, cS and cSH
‡
are the concentrations of
k
the unprotonated and protonated (on the carbonyl
oxygen) substrate in the aqueous acid with concentration
hydration reaction is dominant at the five or six highest
acid concentrations, when k ,app(AdE2) obeys well the
linear Eqn. (3) (e.g. r = 0.9996 for 1 and 0.998 for 2, if
n = 2). By extending the linear dependence to all the acid
concentrations studied the improved approximations of
cH [= c(HClO4)], water activity aw (the recent molarity-
‡
based log aw values of Cox12 are used, because their
3
standard state is correct, ‘a hypothetical ideal 1 mol dm
solution of water in water’) and excess acidity X0,13 and n
is the number of water molecules in the transition state.9
The slope parameters m≠ and m≠ are indicative of the
k ,imp(AdE2) can be evaluated and their subtraction from
the observed k values gives improved k ,imp(AAC2)
values. These rate constants can be used for the non-
linear least-squares minimization (NLSM) according to
the equation18
1
2
transition states of the two mechanisms and the slope
parameters m*1 and m*2 depend on the site of proton
attack, the former on the carbonyl oxygen and the latter
on the cyclopropane ring (m*2 = 1.80 Æ 0.10)12,13 and
k0,1 and k0,2 are the medium-independent rate constants
of the rate-limiting steps (r.l.s. in Schemes 1 and 2) of the
‡
logk ꢀAAC2† logcH
nlogaw
ˆ
mÃ1X0
ˆ m 1mà X0 log‰1 ‡ ꢀcH =KSH †10
Š
‡
‡
1
‡
‡ logꢀk0;1=KSH
†
ꢀ5†
two reactions. KSH is the thermodynamic dissociation
‡
Copyright 2000 John Wiley & Sons, Ltd.
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HYDROLYSIS OF BRIDGED BI- AND TRICYCLIC COMPOUNDS. XXXVII
137
where improved values of the parameters m≠ , m*1,
1
pKSH
‡
and log(k0,1/KSH ) can be obtained by iteration.
‡
They can be used as better approximate values (see
above) and the whole iteration process is repeated until
the values of the parameters no longer change markedly.
The final best values are given in Table 2.
The NLSM iteration according to the equation11,19
ˆ
log kÉꢀAdE2† log cH ˆ m 2mÃ2X0 log‰1
‡
mÃ1X0
‡
‡
‡ ꢀcH =KSH †10
Š
‡ log k0;2
ꢀ6†
was also made with the final kÉ(AdE2) values
[= kÉ,obs. kÉ(AAC2)NLSM] in order to estimate the
values of parameters m≠ m*2, m*1, pKSH
and log k0,2.
‡
2
Figure 1. Logarithms of pseudo-®rst-order rate constants vs
acid concentration for 3-acetoxynortricyclane (1) in
These parameters are also shown in Table 2. In this case
the iterated values are, however, rather dependent on the
initial estimated values of the parameters, probably
owing to the small number of acid concentrations where
the level of the AdE2 hydration is significant enough.
Accordingly, these values may be less reliable than those
for the AAC2 hydrolysis in Table 2.
HClO4(aq.) at 298.2 K: &, total disappearance; *, AAC
hydrolysis; and ~, AdE2 hydration
2
The iterative procedures worked well when the number
of water molecules in the transition state of the AAC
2
hydrolysis was fixed at two, i.e. n = 2 in Eqns (1) and (5),
but did less well or the excess acidity parameters obtained
were exceptional values if n = 1 or 3. Two is the
generally accepted number of water molecules in the
transition state of the AAC2 ester hydrolysis.1,2,9,10,12
Excess acidity parameters
Figure 2. Logarithms of pseudo-®rst-order rate constants vs
acid concentration for 1-acetoxynortricyclane (2) in
The values of the excess acidity parameters in Table 2
seem, in the main, reasonable. The slope parameter
m*1 for the protonation of the carbonyl oxygen, aver-
age 0.78 Æ 0.04, is in agreement with that measured
HClO4(aq.) at 298.2 K: &, total disappearance; *, AAC
hydrolysis; and ~, AdE2 hydration
2
earlier.14–16,18 The slope parameter m≠ for the AAC
2
1
hydrolysis, average 0.99 Æ 0.03, is practically unity, as it
should be.9 The slope parameter m≠ for the AdE2
2
hydration of 2, 0.82 Æ 0.04, is typical of the acid-
catalyzed protonation of the cyclopropane ring of
nortricyclanes,20 but that, for 1 1.02 Æ 0.05, seems too
high, because generally 0 ꢁ m≠ ꢁ 1.9 One reason may be
the existence of some AAL1 ester hydrolysis1,2,12 in
addition to the AdE2 hydration of the cyclopropane ring at
high acid concentrations. The rate constant kÉ(AAL1)
obeys the equation
ˆ
log kÉꢀAAL1† log cH ˆ m mÃ1X0 log‰1
‡
3
mÃ1X0
‡
‡
‡ ꢀcH =KSH †10
Š
‡
‡ logꢀk0;3=KSH
†
ꢀ7†
Figure 3. Logarithms of pseudo-®rst-order rate constants vs
substituent constant sIq of X for the AdE2 hydration of 3-X-
substituted nortricyclanes in 1.00 M HClO4(aq.) at 298.2 K:
X = 1, H; 2, CH2OH; 3, CH2Cl; 4, COCH3; 5, OH; 6, OCH3; 7,
OCOCH3; 8, CN; and 9, NO2
which is formally close to Eqn. (6), suitable for the AdE2
hydration, but generally gives m≠ values higher than
3
unity (Table 2).9,12 The activation parameters and the
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 133–138

138
M. LAJUNEN, R. YLI-MANNILA AND O. SALMINEN
solvent deuterium isotope effects in 6.4 M LCIO4(L2O)
(Table 2), however, show that the level of the AAL
hydrolysis cannot be large. Another reason may be some
exceptional behavior of the acetyl group (enolization?),
as was recently observed in the hydration of 3-
Mr Jaakko Hellman for recording the GC–FTIR and
NMR spectra, respectively, and to Mr Ville Nieminen for
his assistance with kinetic measurements.
1
acetylnortricyclane [preliminary values: m*1 = 0.87 Æ
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=
1.55 Æ 0.08] (M.
‡
Lajunen and V. Mikkola, unpublished results). The
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average pKSH
‡
value, 2.4 Æ 0.2, evaluated for both 1
and 2, shows that 1- and 3-nortricyclyl acetates are as
expected more basic than isopropyl acetate (pKSH =
‡
3.43),17 although the difference seems too great.
The parameters in Table 2 allow the estimation of the
rate constants for both significant reactions, the ester
hydrolysis and the hydration of the cyclopropane ring,
with Eqns (5) and (6) at all acid concentrations used.
These are presented in Figs 1 and 2 together with the
observed total rate constants. The curves in Figs 1 and 2
seem reasonable. It is also possible to check the
correctness of the rate constant kÉ(AdE2) for 1, because
the corresponding values have been measured for several
3-X-substituted notricyclanes in HClO4(aq.).5,11,21–25
They are presented, as extrapolated to an acid concentra-
tion of 1.00 mol dm 3 and a temperature of 298.2 K, as a
function of the substituent constant ꢀqI of X in Fig. 3.
26
This shows that the kÉ(AdE2) value evaluated for 1 is in
fair agreement with the others, although its level is only
0.2% of the total disappearance rate under these
conditions.
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Acknowledgements
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We are grateful to Dr Robin A. Cox for supplying a copy
of Ref. 12 prior to publication, to Dr Martti Dahlqvist and
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 133–138