J . Org. Chem. 1997, 62, 3711-3714
3711
Rea ctivity of Su bstitu ted P h en yld im eth ylsu lfon iu m Ion s w ith
Me
Com m on Nu cleop h iles. A Test of p Klg for
P h en yld im eth ylsu lfon iu m Sa lts a n d a Com p a r ison w ith Meth yl
Ar en esu lfon a tes
K. R. Fountain,* Timothy W. Dunkin, and Kamlesh D. Patel
Division of Sciences, Truman State University, Kirksville, Missouri 63501
Received September 24, 1996X
The correlations of nucleophilic rate data for phenyldimethylsulfonium ions with common
Me
nucleophiles with pKlg values shows that the slopes of the line, âlgMe, correlate qualitatively with
Edwards hardness parameter for the nucleophile, and not with the Swain-Scott no parameter.
Comparison with substituted methyl arenesulfonates shows different leaving group behavior in
the two systems. These results support Shaik’s hypothesis that leaving group behavior consists of
some SET character.
In tr od u ction
attack from known cases in the same solvent that we
previously used, methanol-d4. We can make a claim that
these reactions are not reversible at least in the cases of
the phenolate ions. Mixtures of the corresponding sub-
stituted methyl phenyl ethers, sodium fluoroborate, and
methylphenyl thioethers in methanol-d4 gave no 1H NMR
spectra corresponding to the phenyldimethylsulfonium
fluoroborate substrates. The correlations, shown below,
indicate that a good case can be made that all of the
reactions, under our experimental conditions, are not
readily reversible on the time scale of our reactions (1-2
days). The expected small solvent kinetic isotope effects
are ignored. The kinetics for reaction 3 (where Ar )
Quantitative expressions of leaving group behavior are
useful for mapping transition states for nucleophilic
reactivity.1,2 A particularly useful parameter1 is pKlg
Me
which is defined using equilibrium data provided by
Lewis3 et al. for exchange of methyl groups between
arenesulfonate ions in sulfolane-dimethylsulfone eutec-
Me
Me
tic, eq 1. The slopes of plots of pKlg vs log knuc (âlg
)
Ar1SO3Me + Ar2SO3- h Ar1SO3- + Ar2SO3Me (1)
are limited to values1 between 0.0 and 1.0. They thus
form a counterpart to ânuc values of the nucleophile side
of nucleophilic reactivity.
1
C6H5-) were followed by our published H NMR method,2
where the disappearance of the S+Me2 signals is mea-
sured. The internal standard in this case was 1,1,1-
trichloromethane. The temperature control was held at
30° ((0.3) by the variable temperature probe of the
Varian XL200 NMR. The rate constants were deter-
mined in at least duplicate, many times triplicate or
more.
Our group has recently reported4 on pKlg values for
Me
methyl group transfer between methyl phenyl thioethers,
defined analogously to pKlgMe for methyl arenesulfonates,
from Lewis data on these compounds.3 These pKlg
Me
values in both the methyl arenesulfonates and the
phenyldimethylsulfonium salt series correlate well with
their reduction potentials and with the energies of lowest
unoccupied molecular orbitals (ELUMO).4 The present
G-ArS+Me2 + Nu:- f G-ArSMe + NuMe (3)
Me
paper reports on the quantitative use of the pKlg from
phenylmethyl thioethers, eq 2, in measuring nucleophilic
activity.
The F-, N3-, and SCN- nucleophiles were from reagent
grade sodium and potassium salts, and the kinetics runs
were pseudo-first-order with a 10-fold excess of the anion.
Pyridine was Fisher Reagent grade and was used as
received in pseudo-first-order reactions. Second-order
rate constants were obtained by division of the pseudo-
first-order rate constants by the known concentrations
of the excess nucleophiles.
Ar1S+Me2 + Ar2SMe h Ar1SMe + Ar2S+Me2 (2)
Exp er im en ta l Resu lts
Nucleophiles from the Swain-Scott5 list of nucleophiles
in methanol were used to represent typical nucleophilic
Phenoxide salts were made by treating a known
amount of the substituted phenol with a stoichiometric
amount of a standardized 0.5 M solution of methoxide
in methanol. Evaporation gave the crude salts, which
were recrystallized from acetone-methyl tert-butyl ether.
The methyl arenesulfonates have all previously been
reported.2
The substituted phenyldimethylsulfonium salts have
all been previously characterized.6 The physical proper-
ties of the salts were all consistent with the literature
X Abstract published in Advance ACS Abstracts, May 1, 1997.
(1) (a) Hoffman, R. V.; Shankweiler, J . M. J . Am. Chem. Soc. 1986,
108, 5536. (b) McManus, S. P.; Smith, M. R., J r.; Shankweiler, J . M.;
Hoffman, R. V. J . Org. Chem. 1988, 53, 141, and references therein.
(2) (a) Fountain,K. R.; Fountain, D. P.; Michaels, B.; Meyers, D. B.;
Salmon, J . K.; Van Galen, D. A.; Yu, P. Can. J . Chem. 1991, 69, 798.
(b) Fountain, K. R.; Hutchinson, L. K.; Mulhearn, D. C.; Xu, Y.-B. J .
Org. Chem. 1993, 58, 7883.
(3) (a) Lewis, E. S.; Hue, D. D. J . Am. Chem. Soc. 1984, 106, 3392.
(b) Lewis, E. S.; Vanderpool, S. R. Ibid. 1977, 99, 1946. (c) Ibid. 1978,
100, 6421. (d) Lewis, E. S.; Kukes, S.; Slater, C. D. Ibid. 1980, 102,
1619. (e) Lewis, E. S.; Smith, M. J .; Christie, J . J . J . Org. Chem. 1983,
48, 2527.
(4) Fountain, K. R.; Patel, K. D.; Dunkin, T. W.; Powers, J . A.; Van
Galen, D. A. J . Org. Chem. 1997, 62, 853.
(5) Isaacs, N. S. Physical Organic Chemistry; Longman Scientific
& Technical, (J ohn Wiley & Sons): New York, 1987; p 243.
(6) Saeva, F. D.; Breslin, D. T.; Martie, P. A. J . Am. Chem. Soc. 1989,
111, 1328, and references therein; also ref 3. (b) Saeva, F. D.; Morgan,
B. P. J . Am. Chem. Soc. 1984, 106, 4124. (c) Saeva, F. D. Tetrahedron
1986, 42, 6123. (d) Saeva, F. D. Top. Curr. Chem. 1990, 156, 61.
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