Primary Sulfonium Ions

Primary Sulfonium Ions. Aliphatic thiols are completely protonated in HS03F-SbF5 diluted with S02 at -60°C [Eq. (4.30)].149 

Protonated secondary thiols are stable even at higher temperatures. Protonated isopropyl thiol cleaves slowly at 0°C in HS03F-SbF5 (1 1 M) solution. No well-identified carbocations were found in the NMR spectra due to the instability of the isopropyl cation under these conditions. Protonated. sec-butyl thiol 55 cleaves to tert-butyl cation at this temperature [Eq. (4.33)]. 

Protonated primary thiols are stable at much higher temperatures. Protonated w-butyl thiol 56 cleaves to ferf-butyl cation only at +25°C [Eq. (4.34)]. 

Mercaptosulfonium salts H3S2+MF6 (M = As, Sb), analogs of protonated hydrogen peroxide, have been prepared [Eq. (4.37)] and characterized by vibrational spectroscopy by Minkwitz et al.161 According to calculations (ab initio, general force field), the cation 59 has a conformation with the lone pair of H2S+ antiperiplanar to the S—H bond. 

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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. 

The numerous straightforward examples of internal displacement reactions leading to isolable cyclic products will not be discussed here, but only, for the most part, those ionization reactions in which a cyclic intermediate or transition state is deduced from the rearranged structure of the product. A well-known example is mustard gas and other alkyl chlorides with sulfur on the /3-carbon atom. Although mustard gas is a primary and saturated alkyl chloride, its behavior is like that of a typical tertiary alkyl chloride. It reacts so fast by a first order ionization that the rate of the usual second order displacement reaction of primary alkyl halides is not measureable. Only the ultimate product, not the rate, is determined by the added reagent.228 Since the effect of the sulfur is too large to be explicable in terms of a carbon sulfur dipole or similar explanation, a cyclic sulfonium ion has been proposed as an... 

One of the best-known toxic alkylating agents is mustard gas, an early chemical warfare agent that caused an estimated 400,000 casualties in World War I. A primary halide, mustard gas is highly reactive toward 8 2 displacements by nucleophilic amino groups in proteins. It is thought to act through an intermediate sulfonium ion in much the same manner as S-adenosylmethionine. 

The hydrolysis of 2,2 -bischloroethyl sulfide (mustard gas) was one of the first neighboring-group-accelerated reactions to get widespread attention. Initially the hydrolysis of this compound is purely first order in the sulfide, and the rate is unaffected by added alkali or other nucleophiles the increasing amount of chloride ion, however, slows the rate with time. Since formation of the primary carbocation is not expected, these data suggest an anchimerically assisted solvolysis [Eq. (1)], leading to the formation of the intermediate sulfonium ion (1), which may react with water, chloride ion, or any other nucleophile present. A second anchimerically assisted process that leads to displacement of the second chloro group can occur. 

The driving force for participation also more than offsets the transition-state strain in the case of 2-chloromethylthiirane (24). The acetolysis of (24) proceeds 1000 times faster than cyclopropylmethyl chloride and gives, as one product, 3-acetoxythietane via the sulfonium ion (25) [Eq. (8)]. The oxygen analog of (24), epichlorohydrin, reacts 3 times as fast as cyclopropylmethyl chloride, which is already accelerated relative to primary derivatives (see Chapter 2). 

Step 1 Make a new bond between a nucleophile and an electrophile and simultaneously break a bond to give stable molecules or ions. The reason for the extremely rapid hydrolysis of the sulfur mustards is neighboring group participation by sulfur in the ionization of the carbon-chlorine bond to form a cyclic sulfonium ion. This is the rate-determining step of the reaction although it is the slowest step, it is much faster than reaction of a typical primary chloroalkane with water. At this point, you should review halogenation of alkenes (Sections 6.3D and 6.3F) and compare the cyclic halonium ions formed there with the cyclic sulfonium ion formed here. 

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. 

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. ... 

It is probable that a primary reason for the lower stability of complexes formed between dicyclohexyl-18-crown-6 and cations larger than the optimum size (e.g. Cs+) is that these cations are too large to "fit into the ligand cavity. On the other hand, as cation size decreases from that affording maximum stability, the hydration energy of the cation becomes predominant and little or no reaction is found, as in the case of Ca2+. Very large cations such as di, tri, and tetramethylammo-nium and trimethyl sulfonium do not appear to form complexes wi h dicyclohexyl-18-crown-6 in aqueous solution (4). Also, tetramethyl-ammonium ion (radius = 3.47 A (30)) complexes less strongly (log K =... 

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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