Rate and product studies with 2-adamantyl fluoroformate under solvolytic conditions

Article abstract of DOI:10.1002/poc.1194

The specific rates of solvolysis of 2-adamantyl fluoroformate have been measured at 25.0°C in 20 pure and binary solvents. These are well correlated using the extended Grunwald-Winstein equation, with incorporation of the N _T solvent nucleophilicity scale and the Y_Cl solvent ionizing power scale. The sensitivities (l = 2.15±0.17 and m = 0.95±0.07) toward the changes in solvent nucleophilicity and solvent ionizing power, and the k_F/k_Cl values are very similar to those previously observed for solvolyses of n-octyl fluoroformate, consistent with the addition step of an addition-elimination pathway being rate-determining. For aqueous ethanol, measurement of the product ratio allowed selectivity values (5) to be determined. The results are compared with those reported earlier for 2-adamantyl chloroformate and mechanistic conclusions are drawn. Copyright

Full text of DOI:10.1002/poc.1194

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2007; 20: 525–531
Published online 29 April 2007 in Wiley InterScience
(www.interscience.wiley.com) DOI: 10.1002/poc.1194
Rate and product studies with 2-adamantyl
fluoroformate under solvolytic conditions
1
1
1
2
*
*
Jin Burm Kyong, Chan Joo Rhu, Yong-Gun Kim and Dennis N. Kevill
1Department of Chemistry and Applied Chemistry, Hanyang University, Ansan-si, Gyeonggi-do 426-791, Korea
2Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115-2862, USA
Received 13 January 2007; revised 14 March 2007; accepted 16 March 2007
ABSTRACT: The specific rates of solvolysis of 2-adamantyl fluoroformate have been measured at 25.0 8C in 20 pure
and binary solvents. These are well correlated using the extended Grunwald–Winstein equation, with incorporation of
the NT solvent nucleophilicity scale and the YCl solvent ionizing power scale. The sensitivities (l ¼ 2.15 ꢀ 0.17 and
m ¼ 0.95 ꢀ 0.07) toward the changes in solvent nucleophilicity and solvent ionizing power, and the kF/kCl values are
very similar to those previously observed for solvolyses of n-octyl fluoroformate, consistent with the addition step of
an addition-elimination pathway being rate-determining. For aqueous ethanol, measurement of the product ratio
allowed selectivity values (S) to be determined. The results are compared with those reported earlier for 2-adamantyl
chloroformate and mechanistic conclusions are drawn. Copyright # 2007 John Wiley & Sons, Ltd.
KEYWORDS: 2-adamantyl fluoroformate; addition–elimination; Grunwald–Winstein equation; product selectivity
INTRODUCTION
chloride ion or solvent. The corresponding 1-adamantyl
fluoroformate7 showed a very much-reduced tendency
toward the ionization and only in solvents of high ionizing
power and low nucleophilicity does the behavior parallel
to that of the chloroformate. In other solvents, the
predominant pathway involves addition–elimination.
The overall picture for solvolyses of primary,
secondary and tertiary chloroformates in hydroxylic
solvents can be expressed according to Scheme 1, with
pathways involving a bimolecular substitution at the acyl
carbon shown to the left and pathways involving a
rate-determining unimolecular reaction with accompany-
ing loss of carbon dioxide shown to the right.
A recently published study of the solvolysis-decom-
position of 2-adamantyl chloroformate (2-AdOCOCl)1 is
extended to 2-adamantyl fluoroformate (2-AdOCOF).
Although the solvolyses of alkyl and aryl haloformates
had been extensively studied kinetically, less was known
about the kinetics and mechanism of the solvolyses of the
above fluoroformate and chloroformate. Accordingly, a
study of the reaction mechanism for 2-adamantyl
fluoroformate under solvolytic conditions is a subject
of continuing interest.
In hydroxylic solvents, chloroformates with a primary
alkyl group usually solvolyze with rate-determining
attack at the carbonyl carbon. Only in solvents of very
low nucleophilicity and very high ionizing power can, in
some instances, an ionization pathway be detected.2–4
Secondary alkyl chloroformates follow the ionization
pathway in all but the more nucleophilic and less ionizing
solvents (100% and 90% EtOH and MeOH).1,5 For
tertiary alkyl chloroformates,6 an ionization process, with
loss of carbon dioxide, is favored. The ionization pathway
with solvolysis-decomposition is the only one operating
for solvolyses of 1-adamantyl chloroformate. Loss of
carbon dioxide leads to the relatively stable 1-adamantyl
cation, which can be captured by simultaneously formed
A recent report concerning the solvolyses of n-octyl
fluoroformate and chloroformate8 found the kF/kCl ratio to
be only slightly less than unity in 100%EtOH and
100%MeOH and to be somewhat above unity for
solvolyses in mixtures of water with ethanol, acetone,
dioxane, or 2,2,2-trifluoroethanol (TFE). These ratios
were considered to be consistent with the addition step
of an addition–elimination mechanism being rate
determining.
The extended Grunwald–Winstein equation (Eqn (1))9–12
has been found to be a very powerful tool for the study of
a solvolysis reaction:
logðk=koÞ ¼ lNT þ mYCl þ c
(1)
*Correspondence to: J. B. Kyong, Department of Chemistry and
Applied Chemistry, Hanyang University, Ansan-si, Gyeonggi-do
426-791, Korea.
In Eqn (1), k and ko represent the specific rates of
solvolysis in a given solvent and in the standard solvent
(80% ethanol), respectively; l is the sensitivity towards
E-mail: jbkyong@hanyang.ac.kr
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 525–531

526
J. B. KYONG ET AL.
Scheme 1
changes in solvent nucleophilicity (NT);11m is the
sensitivity towards changes in solvent ionizing power
(YCl);12 and c is a constant (residual) term. As the
mechanism moves from an ionization pathway to a
bimolecular pathway, the l-value should increase and the
m-value should decrease. The utility of the YCl scale for a
process in which nucleophilic attack is accompanied by
movement of electron associated with the carbonyl group
onto the oxygen of the group has been discussed
previously.8
previously reported1 for the solvolyses of 2-adamantyl
chloroformate are also tabulated.
DISCUSSION
The variance of kF /kCl ratios has suggested14 differences
in mechanism and a useful additional probe will be to
apply the extended Grunwald–Winstein equation (Eqn 1)
and compare the l and m values with those previously
Table 1. Specific rates of solvolysis (with standard devi-
ations) of 2-adamantyl fluoroformate in pure and aqueous
solvents at 25.0 8C together with the appropriate solvent
nucleophilicity (NT) and solvent ionizing power (YCl) values
and the specific rate ratio relative to 2-adamantyl chloro-
formate (kf/ kCl)
RESULTS
The specific rates of solvolysis of 2-adamantyl fluor-
oformate at 25.08C are reported in Table 1. The solvents
consisted of ethanol, methanol, binary mixtures of water
with ethanol, methanol, 2,2,2-trifluoroethanol (TFE),
acetone, and three binary mixtures of TFE and ethanol.
The NT and YCl values are also reported in Table 1,
together with the kF/kCl ratios. A determination was also
made in methanol-d (MeOD).
In methanol, ethanol, and 80% ethanol, specific rates
were determined at three additional temperatures, and
these values, together with calculated enthalpies and
entropies of activation, are reported in Table 2.
For the reactions in ethanol, 90ꢁ50% ethanol, and 50%
acetone, product studies were carried out using gas
chromatography with response-calibrated flame ioniz-
ation detection, and those results are reported in Table 3.
For the aqueous ethanol, selectivity values (S) were
calculated according to Eqn 2,
b
c
d
Solvent(%)a
105k (sꢂ1
)
NT
YCl
kF/kCl
100 MeOHe
90 MeOH
80 MeOH
70 MeOH
60 MeOH
100 EtOH
90 EtOH
80 EtOH
70 EtOH
60 EtOH
50 EtOH
80 Me2CO
70 Me2CO
60 Me2CO
50 Me2CO
70 TFE
2.27 ꢀ 0.09
22.1 ꢀ 0.6
0.17
ꢂ0.01
ꢂ0.06
ꢂ0.40
ꢂ0.54
0.37
ꢂ1.17
ꢂ0.18
0.67
1.46
2.07
ꢂ2.52
ꢂ0.94
0.00
0.78
1.38
0.42
2.40
2.46
2.32
1.84
0.37
2.71
3.48
3.94
3.01
1.86
0.65
0.82
0.61
41.8 ꢀ 1.0
64.9 ꢀ 3.5
103 ꢀ 3
0.441 ꢀ 0.012
8.04 ꢀ 0.07
14.9 ꢀ 0.4
0.16
0.00
24.4 ꢀ 0.3
ꢂ0.20
ꢂ0.38
ꢂ0.58
ꢂ0.37
ꢂ0.42
ꢂ0.52
ꢂ0.70
ꢂ1.98
ꢂ1.73
ꢂ0.94
ꢂ0.34
0.08
29.9 ꢀ 0.5
38.9 ꢀ 1.0
2.02
ꢂ0.83
0.646 ꢀ 0.023
2.21 ꢀ 0.03
4.28 ꢀ 0.06
11.9 ꢀ 0.2
0.17
0.95
1.73
2.96
3.16
0.63
0.931 ꢀ 0.03
3.21 ꢀ 0.2
0.011
0.026
0.053
0.25
50 TFE
60T–40Ef
40T–60Ef
20T–80Ef
0.180 ꢀ 0.02
0.367 ꢀ 0.02
0.425 ꢀ 0.02
½2ꢂadamantyl ethyl etherꢃ½H2Oꢃ
S ¼
(2)
ꢂ0.48
½2ꢂadamantanoꢃ½EtOHꢃ
ꢂ1.42
0.45
In Eqn 2, the product concentrations are divided by the
concentration of the component of the solvent producing
them. The 5.0% 2-adamantanol observed after reaction in
100% ethanol is deducted from the percentages of this
product observed in aqueous ethanol mixtures before
calculation of the S values. The selectivity values (S) are
presented in Table 3. For comparison, S2-AdOCOCl values
a Volume/volume basis at 25.08C, except for TFE–H2O mixtures, which are
on a weight/weight basis.
b Values from Ref. 11.
c Values from Ref. 12.
d Values relative to those for the corresponding solvolysis of 2-adamantyl
chloroformate (values from Ref. 1).
e Value in 100% MeOD of 1.32(ꢀ0.10) ꢄ 10ꢂ5 sꢂ1, leading to a kMeOH
/
kMeOD value of 1.72 ꢀ 0.19.
f T–E represents TFE–ethanol mixtures.
Copyright # 2007 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2007; 20: 525–531
DOI: 10.1002/poc

STUDIES WITH 2-ADAMANTYL FLUOROFORMATE
527
¼
Table 2. Specific rates for solvolysis of 2-adamantyl fluoroformate at various temperatures and enthalpies DH (kcal molS1
)
¼
and entropies DS (cal molS1KS1) of activation
¼
b
¼
b
Solvent (%)a
100% MeOH
T (8C)
105k (sꢂ1
)
DH
(kcal molꢂ1
)
DS
(cal molꢂ1Kꢂ1
)
298.158C
298.15 8C
25.0
50.0
60.0
70.0
25.0
50.0
60.0
70.0
25.0
30.0
40.0
50.0
2.27 ꢀ 0.09
10.2 ꢀ 0.9
15.4 ꢀ 0.4
26.9 ꢀ 1.4
0.441 ꢀ 0.013
2.36 ꢀ 0.05
4.03 ꢀ 0.11
6.09 ꢀ 0.03
14.9 ꢀ 0.4
20.0 ꢀ 0.4
26.1 ꢀ 0.3
35.2 ꢀ 0.6
10.8 ꢀ 0.3
11.8 ꢀ 0.5
10.3 ꢀ 0.8
ꢂ44.2 ꢀ 1.0
ꢂ44.2 ꢀ 2.0
ꢂ42.2 ꢀ 3.0
100% EtOH
80% EtOH
a Volume-volume basis at 25.0 8C.
b With associated standard error.
obtained for octyl fluoroformate8 and benzyl fluorofor-
mate.15
The l/m ratio has been suggested as a useful
mechanistic criterion and the values of Table 4 divide
nicely into two classes with values of 1.9 to 3.4 for those
entries postulated to represent addition–elimination
(A–E) and values below 0.84 for those believed to
represent ionization (I).
An analysis using the extended Grunwald–Winstein
equation of the data for the specific rates of solvolysis of
2-adamantyl fluoroformate leads to a very good linear
correlation with values of 2.15 ꢀ 0.17 for l, 0.95 ꢀ 0.07
for m, ꢂ0.06 ꢀ 0.07 for c, and 0.959 for the correlation
coefficient (Fig. 1).
The data points for solvolyses in TFE-ethanol mixtures
fell below the best fit line, as is often observed in
treatments of this type. This phenomenon was very
recently discussed16 and it will not be considered again in
this report. Correlations were carried out both with and
without the TFE-ethanol data. The results of the
correlation are presented in Fig. 1 and reported in
Table 4, together with the corresponding parameters
obtained in the analyses of earlier studied substrates.
The higher m-values for the solvolyses of fluorofor-
mates, relative to chloroformates, may reflect the need for
increased solvation of the developing negative charge on
the carbonyl oxygen in the presence of the more
electronegative fluorine attached at the carbonyl carbon.
For 2-adamantyl fluoroformate, the value for the ratio
l/m of 2.3 is very similar to those previously observed for
the solvolyses of benzyl fluoroformate,15n-octyl fluor-
oformate.8 and benzoyl fluoride,17 which have been
shown to solvolyze with the addition step of an
addition–elimination pathway being rate determining.
The consideration of kF /kCl ratios in nucleophilic
substitution reactions has long been recognized as a
useful tool in studying the reaction mechanism.19 This is
especially so when the attack is at an acyl carbon. For SN1
reaction, a value as low as 10ꢂ7 was observed in
4-(N,N-dimethylamino)benzoyl halide solvolyses20 and a
low value of 1.3 ꢄ 10ꢂ4 was also observed for acetyl
halide solvolyses in 75% acetone.19 These values reflect
an appreciable ground-state stabilization for the fluoride21
and the need to break a strong carbon–fluorine bond in the
Table 3. Percentages of the products present after the solvolysis of 2-adamantyl fluoroformate in a binary hydroxylic solvent at
25.0 8C and the calculated selectivity values
2-AdOH
(18.6)b
2-AdOCOOEt
(40.1)b
c
d
Solvent(%)a
S2-AdOCOF
S2-AdOCOCl
100EtOH
90EtOH
80EtOH
70EtOH
60EtOH
50EtOH
50Me2CO
5.0
25.1
37.9
49.1
57.0
62.2
100
95.0
74.9
62.1
50.9
43.0
37.8
—
—
—
1.33
1.52
1.59
1.78
2.13
—
1.15
1.20
1.04
0.89
0.56
—
a Volume/volume basis at 25.0 8C,
b Retention time (min) under the GC conditions and with a calibration correction factor (C.C.F) for the ester of 0.646.
c Selectivity involved in solvent attack at the acyl carbon.
d Selectivity values for 2-adamantyl chloroformate (Ref. 1).
Copyright # 2007 John Wiley & Sons, Ltd.
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DOI: 10.1002/poc

528
J. B. KYONG ET AL.
Figure 1. Plot of log(k/ko) for solvolyses of 2-adamantyl fluoroformate in pure and binary solvents at 25.0 8C against
(2.15NT þ 0.95YCl)
rate determining step. In contrast, if the addition step is
rate-determining, values of close to unity (and frequently
above it), reflecting a large electron deficiency at the
ionization pathway for the solvolyses of the chloride in
these solvents.
The solvent deuterium isotope effect has previously
been studied for several solvolyses of chloroformate
esters. In 100% water, the kH2O /kD2O ratio was in the
range of 1.8 to 2.0 at 7–25.08C for a series of substrates
believed to react by the bimolecular mechanism. The
value for isopropyl chloroformate,5 in the ionization
range, was somewhat lower at 1.25 and the value for
diphenylcarbamoyl chloride was lower again at 1.1.22
More recently, values have become available for the
corresponding ratio for methanolysis. Values have been
reported for the kMeOH /kMeOD ¼ 2.1–2.5 for a series of
substituted phenyl chloroformates,23,24 2.4 for
p-nitrobenzyl chloroformate,13 and 1.88 for 2-adamantyl
chloroformate.1 In the latter value, however, there is
probably a contribution from the ionization pathway. The
ionization contribution is 12% in ethanol, and this would
be expected to increase somewhat in methanol.
carbonyl carbon of
a
haloformate incorporating
fluorine,17 are frequently observed. This situation has
recently been discussed in a consideration of n-octyl
haloformate solvolyses,8 where kF/kCl specific rate ratios
of 0.6 to 15 were observed.
Due to the previous study of 2-adamantyl chlorofor-
mate1 involving 19 of the 20 solvent compositions of the
present study, a wide range of kF/kCl values are available.
For a meaningful consideration of the specific rate ratio at
25.08C (kF /kCl) for solvolyses of 2-adamantyl fluor-
oformate and 2-adamantyl chloroformate (Table 1), it is
important that the kF and kCl values are for the same
reaction pathway. The values for the ratio vary from a low
of 0.4 in 100% ethanol (similar to the 0.6 for n-octyl
haloformates) to a high of 3.9 in 70% ethanol (similar to
the 2.9 for n-octyl haloformates in 80% acetone). The
very low values for 70% TFE, 50% TFE, and
60%TFE-40% ethanol (0.011 to 0.053) are to be expected
because of the previously demonstrated1 dominance of an
The solvent deuterium isotope effect values (Table 1)
for methanolysis of 2-adamantyl fluoroformate of kMeOH
/
kMeOD ¼ 1.72 is of a magnitude usually taken to indicate
J. Phys. Org. Chem. 2007; 20: 525–531
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STUDIES WITH 2-ADAMANTYL FLUOROFORMATE
529
Table 4. Correlationsa of the specific rates of the solvolyses by the addition–elimination mechanism for 2-adamantyl
fluoroformate and by this mechanism or an ionization mechanism for several other chloroformate and fluoroformate esters,
and benzoyl fluoride
Substrate
Mechb
nc
ld
md
cd
Re
l/m
PhOCOClf
MeOCOClg
EtOCOClh
EtOCOClh
i-PrOCOCli
BzOCOClj
BzOCOClj
BzOCOFk
A-E
A-E
A-E
I
21
19
28
7
1.68 ꢀ 0.10
1.59 ꢀ 0.09
1.56 ꢀ 0.09
0.69 ꢀ 0.13
0.28 ꢀ 0.05
1.95 ꢀ 0.16
0.25 ꢀ 0.05
1.57 ꢀ 0.20
1.43 ꢀ 0.13
1.80 ꢀ 0.13
1.67 ꢀ 0.07
1.58 ꢀ 0.09
0.57 ꢀ 0.06
0.58 ꢀ 0.05
0.55 ꢀ 0.03
0.82 ꢀ 0.16
0.52 ꢀ 0.03
0.57 ꢀ 0.05
0.66 ꢀ 0.06
0.76 ꢀ 0.08
0.70 ꢀ 0.05
0.79 ꢀ 0.06
0.76 ꢀ 0.03
0.82 ꢀ 0.05
0.47 ꢀ 0.03
0.95 ꢀ 0.07
0.84 ꢀ 0.06
0.12 ꢀ 0.41
0.16 ꢀ 0.17
0.973
0.977
0.967
0.946
0.979
0.966
0.976
0.933
0.974
0.959
0.988
0.953
0.970
0.959
0.968
2.95
2.74
2.84
0.84
0.54
3.42
0.38
2.07
2.04
2.28
2.20
1.93
0.19 ꢀ 0.24
ꢂ2.40 ꢀ 0.27
ꢂ0.12 ꢀ 0.05
0.16 ꢀ 0.15
I
20
15
11
16
13
23
19
41
19
20
17
A-E
I
ꢂ2.05 ꢀ 0.11
ꢂ0.13 ꢀ 0.27
ꢂ0.09 ꢀ 0.17
0.13 ꢀ 0.34
A-E
A-E
A-E
A-E
A-E
I
BzOCOFk
OctOCOFl
OctOCOFl
C6H5COFm
2-AdOCOCln
2-AdOCOFo
2-AdOCOFo
ꢂ0.08 ꢀ 0.18
ꢂ0.09 ꢀ 0.10
0.11 ꢀ 0.19
ꢁ0
ꢁ0
2.27
2.28
A-E
A-E
2.15 ꢀ 0.17
1.92 ꢀ 0.15
ꢂ0.06 ꢀ 0.07
ꢂ0.02 ꢀ 0.06
a Using Eqn 1.
b Addition–elimination (A–E) and ionization (I).
c Number of solvent systems included in the correlation.
d With associated standard errors, those associated with the c values being the standard errors of the estimate.
e Correlation coefficient.
f Values from Ref. 18.
g Values from Ref. 2.
h Values from Ref. 3.
i Values from Ref. 5.
j Values from Ref. 13.
k Values from Ref. 15 and with the second entry omitting the three TFE–ethanol data points.
l Values from Ref. 8 and with the second entry omitting the three TFE–ethanol data points.
m Values from Ref. 17.
n Values from Ref. 1.
o This study and with the second entry omitting the three TFE–ethanol data points.
that nucleophilic attack by a methanol molecule is
assisted by general–base catalysis by a second methanol
molecule.25,26
For solvolyses in ethanol, methanol, and 80% ethanol,
the values of the enthalpy and the entropy of activation for
the solvolysis of 2-adamantyl fluoroformate (Table 2) are
10.3 to 11.8 kcal molꢂ1 and ꢂ42.2 to ꢂ44.2 cal
molꢂ1Kꢂ1, respectively. The very negative entropies of
activation are consistent with the bimolecular nature of
the proposed rate-determining step.
Product studies were based on determinations by gas
chromatography at 10 half lives, estimated from the
specific rates of Table 1. The percentage compositions
and the selectivity values calculated using Eqn 2 are
reported in Table 3. The 5.0% of alcohol after solvolysis
in 100% ethanol is similar to the 2.7% found after
ethanolysis of 1-adamantyl chloroformate6 and the 3.3%
found after ethanolysis of 1-adamantyl fluoroformate.7
For the 2-adamantyl chloroformate in 100% ethanol, a
smaller amount of 2-adamantanol (1.3%) was found, and
since number 2-adamantanol was found for the reaction in
100% TFE, this would appear to be due to the presence of
a small concentration of water in the ethanol.1 As
previously proposed,6,7 this product can also result from
the reaction of the substrate with moisture during
manipulation. The product studies for the solvolyses of
2-adamantyl fluoroformate (Table 3) are also consistent
with an addition–elimination pathway. The products can
be rationalized in terms of mixed carbonate esters being
formed from reaction with either a pure alcohol or with
the alcohol component of a mixed solvent and
2-adamantanol resulting from an initial water attack
being followed by carbon dioxide loss from the initially
formed hydrogen carbonate (Scheme 1a). In particular,
there was no evidence for either 2-adamantyl fluoride,
from a competing decomposition, or the appropriate
mixed ether, which would have been formed by an
ionization pathway involving loss of carbon dioxide,
followed by reaction of the carbocation with an alcohol
component of the solvent (Scheme 1b).
In aqueous ethanol, the selectivity values (S) increase
slightly from a value of 1.33 in 90% ethanol to 2.13 in
50% ethanol (Table 3). These values are not very different
to those which have been observed for other solvolyses of
carbonyl chlorides and fluorides believed to follow the
addition–elimination pathway. Previous S values for
attack at the acyl carbon of chloroformate and fluoro-
formate esters, which are believed to follow an addition–
elimination pathway, with the addition step being rate
determining, have usually been somewhat larger, with
values of 2.0 to 4.1 for benzyl chloroformate in
90%ꢁ50%ethanol,13 2.2 to 3.6 for benzyl fluoroformate
Copyright # 2007 John Wiley & Sons, Ltd.
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530
J. B. KYONG ET AL.
in 90%ꢁ50%ethanol,15 4.1 and 6.1 for p-nitrobenzyl
chloroformate in 80% and 60%ethanol, 13 and 2.6 and 4.6
for p-nitrophenyl chloroformate in 80% and 60%etha-
nol.27 The lower S values for nucleophilic attack at the
acyl carbon in the present study and in the study of the
corresponding chloride could be a consequence of a
higher steric hindrance associated with the 2-adamantyl
cage slightly increasing the extent of reaction with the
smaller water molecules at the expense of the bulkier
ethanol molecules.
The falling of the S value to 0.56 for solvolysis in 50%
ethanol, previously reported for solvolyses of 2-adamantyl
chloroformate,1 is consistent with the change over to an
ionization pathway in all but the more nucleophilic and
least ionizing solvents (100% and 90% EtOH and
MeOH). In the more aqueous solvents, S values for
bimolecular reaction of the chloride are not available for
comparison.
The selectivity values (S ¼ 1.33 to 2.13) for solvolyses
of 2-adamantyl fluoroformate in 90%–50% aqueous
ethanol are slightly lower but similar to those which have
been observed for other solvolyses of carbonyl chlorides
and fluorides believed to follow the addition–elimination
pathway.
In the present study, unlike the solvolyses of
2-adamantyl chloroformate, where, in most solvents,
solvolysis-decomposition (ionization pathway) was
observed, the solvolyses of 2-adamantyl fluoroformate
have a pathway involving bimolecular attack by solvent at
acyl carbon, and it is suggested that the addition step of an
addition–elimination pathway is rate determining
(Scheme 1a).
EXPERIMENTAL
2-Adamantyl fluoroformate (b.p. 118–120 8C/1.2 mmHg)
was prepared from 2-adamantyl alcohol via reaction with
1-chloroethyl chloroformate by a procedure as described
earlier.29 Solvents were purified as previously described.6
The kinetic procedures were as described earlier,7,28
using a substrate concentration of about 7.0 ꢄ 10ꢂ3 M and
with 5 ml aliquots removed for titration.
The percentages of products formed during the
solvolyses were determined by response-calibrated
GLPC, as previously described,6,7 using a 2.1 m glass
column containing 10% Carbowax 20M on Chromosorb
WAW 80/100 with an injection temperature of 150 8C and
column temperature 100 8C. The retention times (in min)
of observed products are reported in Table 3.
CONCLUSIONS
The solvolyses of 2-adamantyl fluoroformate give a
satisfactory extended Grunwald–Winstein correlation
(Eqn 1) over a wide range of NT and YCl values. The
sensitivities to changes in NT and YCl (l ¼ 2.15 and
m ¼ 0.95) are very similar to those for several fluor-
oformate and chloroformate esters (Table 4), which have
been shown to solvolyze with the addition step of an
addition–elimination pathway being rate determining.
The kF /kCl values obtained in a comparison with the
corresponding solvolysis of 2-adamantyl chloroformate
are similar to those for solvolyses of n-octyl fluorofor-
mate relative to n-octyl chloroformate, consistent with a
bimolecular addition–elimination mechanism, proceed-
ing through a tetrahedral intermediate. Favoring the
explanation of a relatively low value (when both react by
the addition–elimination pathway) in terms of alkyl
variation is the observation28 that the kF /kCl ratio for
solvolyses of haloformate esters in 70% aqueous acetone
at 30.18C decreases from methyl (7.16), ethyl (5.46) or
n-propyl (4.95) to isopropyl (1.09), consistent with the
value of slightly less than unity for the 2-adamantyl
haloformate solvolyses in this solvent (at 25.08C).
The solvent deuterium isotope effect value for
methanolysis, kMeOH /kMeOD ¼ 1.72 is of a magnitude
usually taken to indicate that nucleophilic attack by a
methanol molecule is assisted by general-base catalysis
by a second methanol molecule.
Acknowledgements
This work was supported by the research fund of Hanyang
University made in the program year 2006.
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