1
342 J . Org. Chem., Vol. 64, No. 4, 1999
Notes
Ta ble 2. k70Tfe/k97Tfe Ra te Ra tios a n d Typ es of
like arenesulfonates (2, 4a , 5a , 5c) respond abnormally
to the solvent change, i.e., their rates of solvolysis
decrease with increasing ionizing power of the solvent.
Among these, 5c, the neophyl-like compound with the
most highly activated phenyl group, shows the largest
decrease in rate when the solvent is changed from 97%
to 70% TFE. More significantly, 4b and 5b, the two
neophyl-like substrates with highly deactivated phenyl
groups, respond normally to the solvent changesa sol-
vent response which is consistent with that of the
cycloalkylcarbinyl compounds.
a
Rea r r a n ged P r od u cts
compound
k70TFE/k97TFE
temp, °C
rearrangementb
nonec
phenyl
cyclopropyl
phenyl
1
2
3
1.5
0.8
1.2
0.9
30
30
25
30
45
45
55
35
45
55
65
30
35
45
45
45
55
d,e,f
f,g
f,h
4
4
5
5
5
a
b
a
b
c
0
.7
1.4
.4
0.7
.7
2.0
.1
0.5
.5
phenylf,h
phenyli
1
0
cyclopentyli
The solvolysis products listed in Table 2 also show a
trend consistent with the k70TFE/k97TFE ratios. For ex-
ample, the cycloalkylcarbinyl (3, 6, 7) and neopentyl
arenesulfonates (8), which respond normally to the
2
phenyli
0
6
7
8
1.8
1.9
1.8
cyclobutylj
cyclopentyl
methyl
j
aqueous TFE solvent effect, yield over 99% rearranged
f,k,l
1a,d,7,11e-f
products.
While the neophyl-like compounds (2,
1
.7
4
a , 5a , and 5c), which respond abnormally to the aqueous
a
Solvolysis products of all listed substrates yielded almost
TFE solvent effect, yield over 99% phenyl-rearranged
exclusively rearranged products. b Rearrangement by phenyl means
that a phenyl (or aryl) group has migrated from a neighboring
carbon to the reaction site carbon; rearrangement by cycloalkyl
means that an alkyl ring has expanded from either a three- to a
four-membered ring, or a four- to a five-membered ring, or a five-
1c,d,11b,16
products.
Interestingly, 5b, the deactivated neo-
phyl-like compound which shows a normal response to
the aqueous TFE solvent effect, also yields exclusively
ring-expanded products.1d Taken altogether, these results
suggest that an aryl group rearrangement step accounts
for the abnormal response of 4a and 5a to the aqueous
TFE solvent effect. Equally important, the evidence,
c
d
e
to a six-membered ring. Reference 11a. Reference 11b. Ref-
f
g
h
i
erence 11c. In acetic acid. Reference 26. Reference 1c. Ref-
erence 1d. Reference 11d. Reference 11e. l Reference 11f.
j
k
OMe NO
1d
+
1d
based on k /k
demonstrates that these same solvolysis reactions (in
acetic acid or TFE) undergo ionization by a k process
2
rate ratios and F values, clearly
measuring response to changing solvent ionizing power.
The type of rearranged product, either cycloalkyl or
phenyl, which was previously reported1c,d,11a-f for sub-
strates 2-8 is also given in Table 2.
∆
involving σ-participation by the cycloalkyl groups in the
developing carbocation center.
This somewhat complicated picture is understandable
in terms of a two-step mechanistic scheme:1 (1) ioniza-
tion of the arenesulfonate to a tight ion pair via a
transition state, stabilized largely by σ-bond delocaliza-
tion of charge into its cycloalkyl ring, and (2) dissociation
of the tight ion pair to a solvent-separated ion pair. More
specifically, we propose that the tight ion pair undergoes
Discu ssion
b,d
On the basis1d,12 that primary solvolysis occurs by two
s
, nucleophilically solvent assisted
which leads to only unrearranged products, and k
discrete pathwayssk
∆
,
neighboring group assisted which leads to only rear-
ranged productssthe observation that all the primary
substrates13 listed in Table 2 yield almost exclusively
both solvent and structural rearrangement to give a
1
5
10,17
rearranged products supports a k
mechanistic speculation has been further corroborated
by the linear correlation of log k
(3, 6, 7, 8)5b with log k
2) in a series of solvents and by the linear correlation of
∆
pathway. This
solvent-separated ion pair.
Then, upon nucleophilic
attack by solvent, the solvent separated ion pair yields
the titrable arenesulfonic acid as well as the aryl-
rearranged products.
Such a proposal, that is, solvolysis of a neophyl-like
substrate via a transition state with little phenonium ion
character followed by significant structural reorganiza-
tion before eventual capture by solvent, explains well the
solvolytic behavior of both the 1-arylcyclopropylcarbinyl
and 1-arylcyclbutylcarbinyl arenesulfonates.
Given the normal solvolytic behavior of 3, 6, and 7 in
the aqueous TFE solvents, it seems very unlikely that
reduced charge delocalization into the cycloalkyl rings
would account for the reverse order of solvolysis of 4a ,
t
t
(
b,d
log k
t
(4, 5)1 with log k
t
of the correspondingly substi-
tuted neophyl arenesulfonates in acetic acid and aqueous
TFE.
The k70TFE/k97TFE ratios listed in Table 2 cover a small
range but show a consistent and significant pattern.
Those for neopentyl (8) and the cycloalkylcarbinyl com-
pounds (3, 6, 7) respond normally to the solvent change,
i.e., their rates of solvolysis increase with increasing
ionizing power of the solvent. In contrast, the neophyl-
(
11) (a) Schadt, F. L.; Bentley, T. W.; Schleyer, P. v. R. J . Am. Chem.
5
a , and 5c in the same two solvents. Nor does it seem
Soc. 1976, 98, 7667-7674. (b) Heck, R.; Winstein, S. J . Am. Chem.
Soc. 1957, 79, 3432-3438. (c) Saunders: W. H., J r.; Pine, R. H. J . Am.
Chem. Soc. 1961, 83, 882. (d) Roberts, D. D.; Wu, C.-H J . Org. Chem.
1
8
likely that ion pair return would be enhanced in 70%
aqueous TFE over that in 97% aqueous TFE. On the
other hand, it does seem likely that solvation effects upon
carbocations stabilized by aryl bridging and those stabi-
lized by the inductive effects of aliphatic substrates [such
1
974, 39, 3937-3939. (e) Reich, I. L.; Diaz, A.; Winstein, S. J . Am.
Chem. Soc. 1969, 91, 5635-5637. (f) Heidke, R. L.; Saunders: W. H.,
J r. J . Am. Chem. Soc. 1966, 88, 5816.
(
12) Dukes, M. D.; Harris, J . M.; Mount, D. L. J . Am. Chem. Soc.
978, 100, 8137-8146.
13) 2-Adamantyl tosylate is the major exception. Also, although the
1
(
solvolysis of cyclopropylcarbinyl arenesulfonates yields nonrearranged
esters and alcohols as the major products, it is generally accepted that
both the rate-controlling and product-determining steps occur via a
(16) T.; Ishitobi, H.; Irie, T. J . Org. Chem. 1969, 34, 1086-1089.
(17) Shiner, V. J ., J r.; Imhoff, M. A. J . Am. Chem. Soc. 1985, 107,
2121-2124.
1
4
19
σ-bond delocalization of charge into the cyclopropane ring.
14) For a review, see: Roberts, D. D. J . Org. Chem. 1991, 56, 5661-
665.
15) See ref 1d and references therein.
(18) (a) It is known that ion pair return is higher in less aqueous
(
trifluoroethanol solvents. (b) Kevill, D. N.; D’Sousa, M. J . J . Phys. Org.
Chem. 1992, 5, 287-294. (c) Bunton, C. C.; Mhala, M. M.; Moffatt, J .
R. J . Org. Chem. 1984, 49, 3684.
5
(