Secondary Sulfonium Ions

Secondary Sulfonium Ions. Protonated aliphatic sulfides have been studied at low temperatures by NMR spectroscopy in strong acid systems149 [Eq. (4.38)]. They show well-resolved NMR spectra, with the proton on sulfur being observed at about 81 H 6.0. 

The NMR spectrum of protonated thiane-3,3,5,5-c/ has also been studied in HS03F-SbF5 to determine the conformational position of the proton on sulfur in the six-membered ring and to study the ring inversion process.162 The proton on sulfur resides exclusively in the axial position. 

The protonated sulfides are less susceptible to cleavage than the corresponding protonated ethers and also more stable than the protonated thiols.149 Protonated tert-butyl methyl ether is completely cleaved to tot-butyl cation and protonated methanol even at 70°C. On the other hand, protonated tert-butyl methyl sulfide 60 is stable even at —60°C. When the temperature is increased to — 15°C, protonated fert-butyl methyl sulfide very slowly cleaves to tert- butyl cation and protonated methyl thiol149 [Eq. (4.39)]. 

Protonated di-terf-butyl sulfide 61 shows very little cleavage at 60°C. At —35°C it cleaves slowly (f1/2 - 1 h) to tot-butyl cation and protonated hydrogen sulfide [Eq. (4.40)], with the latter showing the H NMR peak at 81H 6.60. 

Protonated secondary sulfides show extraordinary stability toward the strongly acidic medium. Protonated isopropyl sulfide shows no appreciable cleavage up to +70°C in a solution of HS03F-SbF5 (1 1). 

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Cyclic ethers, acetals and cyclic amines belong to the group of hard bases while cyclic sulfides can be considered as soft bases. On the other hand proton is the hardest acid whereas alkyl cations are much softer. Thus, in good agreement with the concept of hard and soft acids and bases (the alike ones react faster giving more stable products), secondary oxonium ions react slower with monomer than their tertiary oxonium counterparts. The same is observed for cyclic amines whereas secondary sulfonium ions apparently react faster with cyclic sulfides than the corresponding tertiary sulfonium ions do l. 

Thiols, the sulfur analogs of alcohols, are usually prepared by Sjv 2 reaction of an alkyl halide with thiourea. Mild oxidation of a thiol yields a disulfide, and mild reduction of a disulfide gives back the thiol. Sulfides, the sulfur analogs of ethers, are prepared by an Sk2 reaction between a thiolate anion and a primary or secondary alkyl halide. Sulfides are much more nucleophilic than ethers and can be oxidized to sulfoxides and to sulfones. Sulfides can also be alkylated by reaction with a primary alkyl halide to yield sulfonium ions. 

Protonic acids are efficient initiators for the polymerization of both sulfides and amines. The polymerization of thiiranes initiated with perchloric acid proceeds without induction periods. Induction periods are present, however, with methyl fluorosulfonate initiator 11). Secondary sulfonium salts are more reactive than tertiary ones (the opposite is true with oxonium ions)12) and induce rapid polymerization ... 

Organic Cations The organic cations, whose dilute solution can be used in wellbore treatments to minimize polymer plugging, constitute a large class of compounds. They have two common characters a cationic end group and some hydrocarbon substitution on the cation. The cation can be an ammonium ion, a phosphonium ion, pyridinum ion, sulfonium ion, chromium ion or oxonium ion. The hydrocarbon substitution in the case of an ammonium ion can be primary, secondary, tertiary or quaternary. The number of carbons must be smaller than 10 for anionic polymers. When the carbon number is too large, the organic cation precipitates with an anionic polymer such as 30% hydrolyzed polyacrylamide. 

It is well known empirically, and not difficult to understand in terms of a simple electrostatic model, that the solvolysis rates of halides and aryl sulfonates are much more solvent dependent than those of ammonium or sulfonium ions, which solvolyze by losing a neutral leaving group. In these systems, for reasons given at the end of Sec. VB, 3(b), the secondary 8-isotope effects should be smaller, since for equivalent bond extension, the incipient carbonium ion must carry a larger net positive chaige. 

Protonic initiation is also the end result of a large number of other initiating systems. Strong acids are generated in situ by a variety of different chemistries (6). These include initiation by carbenium ions, eg, trityl or diazonium salts (151) by an electric current in the presence of a quartenary ammonium salt (152) by halonium, triaryl sulfonium, and triaryl selenonium salts with uv irradiation (153—155) by mercuric perchlorate, nitrosyl hexafluorophosphate, or nitryl hexafluorophosphate (156) and by interaction of free radicals with certain metal salts (157). Reports of "new" initiating systems are often the result of such secondary reactions. Other reports suggest standard polymerization processes with perhaps novel anions. These latter include (Tf)4Al (158) heteropoly acids, eg, tungstophosphate anion (159,160) transition-metal-based systems, eg, Pt (161) or rare earths (162) and numerous systems based on tri flic acid (158,163—166). Coordination polymerization of THF may be in a different class (167). 

Some 5-(alkyloxy)thianthrenium perchlorates (15) have been prepared in which the alkyl group may be primary or secondary. Reaction with iodide ions may result in 5 n2 reaction at the alkyl group or NAr reaction at the sulfonium sulfur atom leading to the formation of thianthrene. ... 

A study of reaction of weakly basic nucleophiles (X ) with 5-(alkoxy)thianthrenium ions (18a-e) in MeCN and DMSO has revealed that E2C elimination competes effectively with 5n2 reaction and reaction at sulfonium sulfur when X = r is used. The proportion of E2C product (21, 53, 52, 35 and 6.6% cycloalkene, respectively) is much higher for (18a-e) than found previously for reaction at primary and acyclic secondary alkyloxy groups (RO). 

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