On the significance of electrophilic pull in solvolysis. Abnormally low tosylate/bromide rate ratio and unusually high reactivity for 1-(2,6-dimethylphenyl)-2,2-dimethyl-1-propyl bromide

Full text of DOI:10.1021/jo951776z

J . Org. Chem. 1996, 61, 1523-1525
1523
Ta ble 1. Solvolysis Ra te Con sta n ts for Br om id es 1B-4B
On th e Sign ifica n ce of Electr op h ilic P u ll in
Solvolysis. Abn or m a lly Low Tosyla te/
Br om id e Ra te Ra tio a n d Un u su a lly High
Rea ctivity for 1-(2,6-Dim eth ylp h en yl)-2,2-
d im eth yl-1-p r op yl Br om id e
a n d Tosyla te 4T
k (s-1), 25 °C
solvent
1B
2B
3B
4B
4T
90E
80E
70E
60E
80A
70A
60A
50A
100M
1.11 × 10-6
1.04 × 10-4
4.18 × 10-7 3.87 × 10-6 4.83 × 10-7 1.38 × 10-6 3.08 × 10-4
1.33 × 10-6 1.23 × 10-5 1.70 × 10-6 4.06 × 10-6 7.88 × 10-4
4.59 × 10-6 3.94 × 10-5 5.65 × 10-6 1.21 × 10-5 1.96 × 10-3
Kwang-Ting Liu*,†,‡ and Chung-Shin Tang†
6.22 × 10-8 4.94 × 10-7
3.48 × 10-5
Department of Chemistry, National Taiwan University,
Taipei 107, Taiwan, Republic of China, and Department of
Chemistry, National Central University, Chung-Li 320,
Taiwan, Republic of China
2.87 × 10-7 2.65 × 10-6 4.10 × 10-7 1.13 × 10-6 1.68 × 10-4
1.42 × 10-6 1.43 × 10-5 1.87 × 10-6 5.05 × 10-6 6.36 × 10-4
6.50 × 10-6 6.62 × 10-5 9.53 × 10-6 2.59 × 10-5 2.65 × 10-3
2.78 × 10-6 2.72 × 10-7 1.18 × 10-6 2.64 × 10-4
90M 8.63 × 10-7 1.03 × 10-5 1.20 × 10-6 4.94 × 10-6 8.78 × 10-4
80M 3.22 × 10-6 3.58 × 10-5 4.09 × 10-6 1.74 × 10-5 2.49 × 10-3
60M 3.00 × 10-5 3.04 × 10-4 3.85 × 10-5 1.55 × 10-4 1.60 × 10-2
100T 6.92 × 10-5 8.31 × 10-4 3.96 × 10-4 1.04 × 10-3 1.26 × 10-1
8T2E 9.37 × 10-6 9.46 × 10-5 3.72 × 10-5 8.51 × 10-5 1.88 × 10-2
6T4E 1.51 × 10-6 1.45 × 10-5 4.21 × 10-6 9.25 × 10-6 2.76 × 10-3
Received September 29, 1995
The importance of solvation of leaving groups on
reactivities has been known from the rate enhancement
due to the presence of a small amount of water in the
ethanolysis of benzhydryl chloride ever since the begin-
ning of the study of solvolytic mechanisms.1 A “push-
pull mechanism” for solvolysis has been proposed,2 but
it was considered to be controversial.3 Although atten-
tion has been paid to electrophilic assistance by acid or
by metal ions,4 specific electrophilic solvation is often
included in the scales of solvent-ionizing power, such as
Y.5 A four-parameter equation has been used for the
correlation analysis of solvolytic reactivities,6 from which
the contribution of the electrophilic assistance might be
evaluated if a sufficient number of data, 20 or more, are
available.7 A double-difference method using four sol-
vents was also suggested in order to estimate electrophilic
solvent assistance.8 However, in the latter case, the
highly ionizing solvent hexafluoro-2-propanol should be
employed, and thus this method was not applicable to
many reactive substrates. Now we would like to report
evidence of significant influence on reactivities due to the
electrophilic pull of leaving anions from the solvolysis of
1-aryl-2,2-dimethyl-1-propyl bromides 1B-4B and tosyl-
ates 1T-4T and to propose the use of low tosylate/
bromide rate ratio as a criterion for the presence of this
effect.
4T6E 2.16 × 10-7
4.96 × 10-4
Ta ble 2. Cor r ela tion An a lyses of log k for 1-4 a ga in st
YBn X
substance
m
R
standard deviation
n
1B
2B
3B
4B
2T
3T
4T
0.897
0.908
1.05
1.03
1.00
1.14
1.16
0.994
0.997
0.991
0.994
0.996
0.987
0.977
0.028
0.018
0.043
0.033
0.024
0.051
0.061
14
15
13
13
16
16
16
and experimental trend of ∆Hq, although the trend of ∆Gq
was the same in both cases. It was considered to be the
result of the neglect of significant solvation of leaving
anion in calculations.9 To verify this proposal, 1-phenyl-,
1-(2-methylphenyl)-, 1-(2,2-dimethylphenyl)-, and 1-(2-
isopropylphenyl)-2,2-dimethyl-1-propyl bromides (1B-
4B) and the tosylate 4T were prepared and solvolyzed.
Conductimetrical rate constants were measured at least
in duplicate ((2%). Pertinent rate data are given in
11-13
Table 1. Results of correlation analyses with YBnX
are listed in Table 2. Rearranged product was not
observed.
The observation of excellent linear log k - YBnX
correlations for 1B-4B and 2T but slight deviations from
linearity for 3T and 4T suggested limiting SN1 mecha-
nisms and small steric retardation to resonance in the
solvolytic transition state for tosylates 3T and 4T.
Bromides, however, seem to be less sensitive to the
bulkiness of ortho substituents. In addition, contrary to
what was observed for the corresponding tosylates,9 3B
showed higher reactivity than that of the unsubstituted
bromide 1B in spite of steric inhibition of resonance due
(5) Bentley, T. W.; Schleyer, P. v. R. Adv. Phys. Org. Chem. 1977,
14, 43-45.
(6) Abraham, M. H.; Grellier, P. L.; Abboud, J .-L. M.; Doherty, R.
M.; Taft, R. W. Can. J . Chem. 1988, 66, 2673.
(7) For example: Abraham, M. H.; Doherty, R. M.; Kamlet, M. J .;
Harris, R. M.; Taft, R. W. J . Chem. Soc., Perkin Trans. 2 1987, 913.
(8) Doherty, R. M.; Abraham, M. H.; Harris, J . M.; Taft, R. W.;
Kamlet, M. J . J . Org. Chem. 1986, 51, 4872.
Resu lts a n d Discu ssion
In a previous study, the solvolysis of tosylates 1T-3T
was found to show discrepancy9 between the theoretical10
(9) Liu, K.-T.; Tang, C.-S.; Chin, C.-P. J . Chin. Chem. Soc. (Taipei)
1994, 41, 351.
(10) Lee, I.; Kim, N. D.; Kim, C. K. J . Phys. Org. Chem. 1993, 6,
499.
(11) YBnOTs was derived form the rate of solvolysis for 1T; see: Liu,
K.-T.; Yang, J .-S.; Chang, S.-M.; Lin, Y.-S.; Sheu, H.-C.; Tsao, M.-L.
J . Org. Chem. 1992, 57, 3041.
(12) YBnOTs for 80% methanol, 0.750, was obtained from k ) 6.02 ×
10-4/s; see: Tang, C.-S. M. S. Thesis, National Taiwan University, J une
1994.
† National Taiwan University.
‡ National Central University.
(1) Farinacci, N. T.; Hammett, L. P. J . Am. Chem. Soc. 1937, 59,
2542.
(2) Swain, C. G. J . Am. Chem. Soc. 1948, 70, 1119.
(3) Thornton, E. R. Solvolytic Mechanisms; Ronald Press: New York,
1964; pp 12-13.
(4) For review, see: Kevill, D. N. In The Chemistry of Functional
Groups, Supplement D; Patai, S., Rappoport, Z., Eds.; Wiley-Inter-
science: New York, 1983; Part 2, The Chemistry of Halides, Pseudo-
halides and Azides, Chapter 20.
(13) For revised and extended YBnBr values, see: Liu, K.-T. J . Chin.
Chem. Soc. (Taipei) 1995, 42, 607.
0022-3263/96/1961-1523$12.00/0 © 1996 American Chemical Society

1524 J . Org. Chem., Vol. 61, No. 4, 1996
Notes
Ta ble 3. Activa tion P a r a m eter s for th e Solvolysis of
only about 1/10 of that for 1. Since k for 3 has been
considered to be abnormally large (vide supra), the lower
rate ratio is likely due to the fast reaction of bromides.
Moreover, OTs/Br rate ratios for 4 are invariably higher
than those for 2, which could be attributed to the relief
of ground-state strain,18 not to the separation of charges
or the stability in the transition state.19,20 It is interesting
to note that the smallest ratio was observed in the case
of 3, despite an increasing strain introduced by the
second o-methyl group. In addition, for each substrate
decreased kOTs/kBr ratios were observed in specific solvent
mixtures, such as aqueous ethanol or ethanol-trifluoro-
ethanol, containing more water or more trifluoroethanol.
Since the literature data21 for free energies of transfer
of ions indicated better solvation of anions in water and
in trifluoroethanol than in common organic solvents, the
influence of solvation of leaving anions on the solvolytic
reactivities should be taken into consideration.
Su bstr a tes 1B-3B
∆Hq (kcal/mol)
∆Sq (eu)
∆Gq (kcal/mol)
solvent 1B 2B 3B
1B
2B
3B
1B 2B 3B
70E
60E
70A
60A
50A
90M
24.5 22.9 25.2 -3.3 -4.3 -0.3 25.5 24.1 25.3
23.7 22.1 24.5 -3.5 -4.6 -0.2 24.7 23.4 24.6
24.3 22.8 25.2 -6.7 -7.4 -3.3 26.4 25.0 26.1
23.5 21.1 24.5 -6.3 -9.9 -2.4 25.4 24.0 25.2
22.8 20.5 23.0 -5.6 -8.9 -4.2 24.5 23.1 24.3
24.1 22.9 25.2 -5.5 -4.4 -1.2 25.7 24.2 25.5
8T2E 23.1 22.3 24.8 -6.0 -3.9 -0.1 24.9 23.5 24.8
80E
80E
21.0 20.3 21.2 -10.9 -8.9 -7.8 24.3 22.9 23.5
21.7 20.7 22.3 -12.3 -11.1 -8.2 25.4 24.0 24.8
Sch em e 1
The heat of hydration of benzenesulfonate ion was
found to be smaller than that of iodide ion,22 and thus,
should be even smaller than that of bromide ion.21
Consequently, it is likely that the high reactivity for 3B
and the low kOTs/kBr ratio for 2 and 3 in all solvent
systems are due to the better solvation of bromide ions.23
The observation of a “normal” rate ratio for 4 could be
attributed to the balance of a rate enhancement by
solvation of leaving anion for the bromide and an ac-
celeration due to the relief of steric strain for the tosylate.
Generally speaking, for benzylic substrates in which the
resonance stabilization of cationic transition state of
solvolysis is less effective due to steric factors, the
significance of solvation of leaving group will be more
profound. In other words, the importance of electrophilic
pull2 of the leaving bromide ion, not tosylate, might be
very influential on the solvolytic reactivity.25 Moreover,
the observation of a low tosylate/bromide rate ratio may
be used as a diagnosis for indicating the significance of
electrophilic pull in solvolysis. Further studies are in
progress.
Ta ble 4. Tosyla te/Br om id e Ra te Ra tios for 1-4
solvent
1T/1B
2T/2B
3T/3B
4T/4B
80E
70E
60E
70A
60A
50A
90M
80M
60M
100T
80T20E
60T40E
256
217
166
199
175
166
246
187
147
181
265
299
114
106
76.7
122
96.5
82.9
122
94.4
79.6
115
149
166
29.8
23.2
17.4
22.1
19.1
16.3
27.8
22.4
20.0
18.4
24.6
30.2
223
194
162
149
126
103
178
143
103
122
221
298
Exp er im en ta l Section
Gen er a l Rem a r k s. Capillary melting points are uncor-
rected. Elemental analyses were done in the Microanalytical
Laboratory at the Department of Chemistry, National Taiwan
University.
Ma ter ia ls. Solvents for the kinetic studies were spectral
grade or anhydrous solvents were freshly distilled before use.26
Literature procedures were used for the preparations of tosylate
4T11 and bromides 1b -4b .27 The products were purified by
to 2,6-dimethyl substituents. Another effect is likely to
be involved to account for the difference between the
order of reactivity observed in the present study and in
the case of tosylates.9 On the other hand, experimental
activation parameters for 1B-3B (Table 3) reveal that
neither ∆Gq nor ∆Hq is consistent with the trends
obtained from AM1 calculations (Scheme 1).14,15 Neglect
of solvent effect16 is again the most probable explanation.
Analysis of the tosylate/bromide rate ratios in Table 4
is worthwhile. The kOTs/kBr values for 1 and 4 are close
to each other, within 30% difference in general, and are
of the same order of magnitude observed for 2-adamantyl
analogs.17 However, the ratios are comparatively smaller
for 2 and 3, especially in the latter case where kOTs/kBr is
(17) Fry, J . L.; Lancelot, C. J .; Lam, L. K. M.; Harris, J . M.;
Bingham, R. C.; Raber, D. J .; Hall, R. E.; Schleyer, P. v. R. J . Am.
Chem. Soc. 1970, 92, 2538.
(18) Bingham, R. C.; Schleyer, P. v. R. J . Am. Chem. Soc. 1971, 93,
3189.
(19) Hoffmann, H. M. R. J . Chem. Soc. 1965, 6762.
(20) Bentley, T. W.; Christl, M.; Kemmer, R.; Llewellyn, G.; Oakley,
J . E. J . Chem. Soc., Perkin Trans. 2 1994, 2531.
(21) Marcus, Y. Pure Appl. Chem. 1983, 55, 977.
(22) Morris, D. F. C. Tetrahedron 1958, 4, 423.
(23) One reviewer pointed out that for the 2-adamantyl system a
higher tosylate/bromide rate ratio in acetic acid than in 80% ethanol
was recorded.17 However, the rate constant for the bromide at 25 °C
was extrapolated from data at 150 and 174.6 °C24 and might be
uncertain.
(24) Fry, J . L.; Lam, L. K. M.; Harris, J . M.; Bingham, R. C.;
Schleyer, P. v. R. J . Am. Chem. Soc. 1970, 92, 2542.
(25) In a recent paper, bromide was considered as an intermediate
type concerning hydrogen bond susceptibility. See: Mitsuhashi, T.;
Hirota, H.; Yamamoto, G. Bull. Chem. Soc. J pn. 1994, 67, 824.
(26) Perrin, D. D.; Armarogo, W. L. F. Purification of Laboratory
Chemicals, 3rd ed.; Pergamon Press: New York, 1988.
(14) Although no general trend of ∆Sq could be realized, the least
negative value was invariably found for 3B and might be attributed
to a relatively more dissociated transition state.15
(15) Schaleger, L. L.; Long, F. A. Adv. Phys. Org. Chem. 1963, 1, 1.
(16) Kollman, P. Chem. Rev. 1993, 93, 2395.

Notes
J . Org. Chem., Vol. 61, No. 4, 1996 1525
recrystallization from n-hexane or by chromatography over
amine-washed alumina. Spectral data were found to compatible
with the assigned structures, and acceptable elemental analyses
(C and H) were obtained for new compounds (see the supporting
information).
Kin etic Mea su r em en ts. Conductimetric rate constants
were measured by using the system developed in this labora-
tory.28 Solutions of 10-4-10-5 M were employed. In some cases,
the addition of a small amount of 2,6-lutidine to the solution
was found to be necessary to prevent curvature of the rate
constant plot.
ature were used to calculate activation parameters and to
estimate the rate constant, which could not be measured directly
at 25 °C (see the supporting information).
P r od u ct An a lyses. The product of solvolysis in aqueous
acetone was isolated and was identified as the corresponding
alcohol by its proton NMR spectrum.
Ack n ow led gm en t. We thank the National Science
Council of the Republic of China (NSC 84-2113-M-002-
003) for financial support.
All reactions were followed to at least 2 half-lives and gave
excellent first-order behavior with correlation coefficients of
greater than 0.995. Rate constants measured at other temper-
Su p p or tin g In for m a tion Ava ila ble: Characterization
data and rate constants for prepared compounds (4 pages).
This material is contained in libraries on microfiche, im-
mediately follows this article in the microfilm version of the
journal, and can be ordered from the ACS; see any current
masthead page for ordering information.
(27) Liu, K.-T.; Sheu, H. C. J . Chin. Chem. Soc. (Taipei) 1991, 38,
29.
(28) Sheu, H.-C.; Liu, K.-T. Proc. Natl. Sci. Counc. ROC (A) 1993,
17.
J O951776Z