SULPHONIUM SALT SOLVOLYSIS: PART III. NUCLEOPHILIC ROLE OF ANIONS IN SOLVOLYSIS OF DIMETHYL-t-BUTYL SULPHONIUM SALTS

Abstract

The elTect of variation of added anion type (I-, C1-, NOa-, CIO.I-, CE13COO-) on the kinetics of solvolysis of climethyl-t-butyl sulphonium ions is interpreted as evidence supporting a nucleophilic role for the anion in low-polarity solve~~ts. The specific case of acetate is discussed in terms of its relation to biochemical trans~llethylatiotl reactions involving methionine. ISTRODUCTIONIn the previous paper of this series (1) it was suggested that tlie anion plays an increasingly important role as a n~~cleophilic species in the solvolysis of s~~lphonium salts as the polarity of the solveilt mediuin is reduced. In tlie particular case of solvolysis of dimethyl-t-butyl sulphonium salts in ethanol-water mixtures, it has bee11 shown (1, 2 ) that in highly aqueous n~eclia the rate of solvolysis is independent of ailion character, in keeping \vitli the SN1-type mechanism involving the production and subsequent fast solvolysis of the t-but171 carboilium ion. As the solvent polaritj~ is decreased by increasing the ethanol content the observed rate of solvolysis becoilles increasingly dependent on anion type. The form of this dependence has beell analyzed in terms of a lriiletic expressioil involving the oilset of both ion pairing ancl a n~~cleophilic role of the anion (2) as solvent polarity climinishes. PI-evious experimental study of such solvolyses has been limited to a narro\v range of anions of relatively sinlilar nucleophilicities (e.g. the halides). In order to investigate further the possible n~~cleophilic role oi the salt anioils in solvolysis we report here a series of kinetic s t~~d i e s of the solvolysis of dimethyl-t-butj~l sulphonium salts in highly ethanolic solvent inedia with variation of anion over the series I-, C1-, KOa-, C10.,-, and CH;IC00-..?\I1 kinetic studies were carried out by the radiocheinical gas flow counting technique used in previous work (1, 2) ancl described by 1-1 yne and Wolfgang (3). Labelled (C1.'-methyl) dimethyl-t-butyl sulphonium iodide was prepared as described previously (3) and nletathesized with the corresponding silver salts. In a few cases this metathesis was not carried out so that the kinetic systenl contained ill iodide in addition to the anion concentration of the added salt. It was found that the effect of this iodide ion was very s~nall a t the high co~icentrations of added salt used and was within the experimental uncertainty of the kinetic data. .All \vorking solutions were ill sulphoni~im salt in 0.945 mole fraction ethanol-water mistures. Acldecl anions were I-, C1-, CI-I3COO-, SOX-, and C10-4in the form of their lithium salts. In a few selected runs potassium salts were used instcad of lithium to check the independence of rate on cation type. This \\;as foulid to be true in every case tested. All salts were vacu~lm-dried and assayed for water of crystallization. Since our primary interest in this work was in the dependence of ki,' (see equation [I.]) on anion nucleophilicity, rate investigations were limited to the linear portion of the kOb, vs. [S-] relations...

Solvent effects on the intramolecular conversion of trimethylsulfonium chloride to dimethyl sulfide and methyl chloride

“…13,14,[17][18][19] The effective barrier is the result of weighing the relative transition state energies by the molar fraction of the corresponding reactants. 13,14,[17][18][19] The effective barrier is the result of weighing the relative transition state energies by the molar fraction of the corresponding reactants.…”

“…[17][18][19][20][21] The solvolysis proceeds via a solvent stabilised t-butyl cation 17 and the ion pair concentration increases as the permittivity of the solvent decreases. [17][18][19][20][21] The solvolysis proceeds via a solvent stabilised t-butyl cation 17 and the ion pair concentration increases as the permittivity of the solvent decreases.…”

“…The solvolysis of various sulfonium salts R 3 0 SX in a series of X À + R 3 0 S + + ROH -R 2 0 S + R 0 OR + HX (2) has been subjected to many kinetic investigations. 11,[13][14][15][17][18][19][20] The solvolysis of dimethyl-t-butylsulfonium halides R 2 R 0 SX and the role of ion pairs R 2 R 0 S + ÁX À in the solvolysis process…”

“…where K A is the equilibrium constant, have been studied in great detail. [17][18][19][20][21] The solvolysis proceeds via a solvent stabilised t-butyl cation 17 and the ion pair concentration increases as the permittivity of the solvent decreases. The value of K A continuously decreases as the solvent becomes more polar, but is still greater than one for pure water.…”

Sulfonium Salt Solvolysis: Part V. Model Systems for Biological Transmethylation

“…T h e effect of actually attaching the carboxyl group to the sulfonium salt, as opposed to having i t available as a separate molecular entity, may be assessed b y comparing t h e rate of solvolysis of the sulfonium salt I (13.5 X lop4 s-I a t 60') with the rate constant for the internal ion-pair attack b y acetate ion (1.5-3.0 X lop4 s-I a t 60') a s estimated from the value obtained in the previous paper of this series (1). T h e favorable entropy effect of actually attaching the participating acetate group to the reacting ~nolecule results in a rate enhancement of some 4 t o 8 times.…”