Thiols, Sulfides, and Sulfonium Salts

Starch thiols can have the SH group directly on the pyranose ring or in a side chain. There are several methods of synthesizing starch thiols of the first type. One of them is based on the pyrolysis of starch xanthates (32), but the reaction proceeds in two parallel routes one producing thionates (35) and the other producing thiols (37).2675,2677 Reduction of starch xanthates with NaBPU in alkali is another approach to thiols. Thiols prepared in this manner were subjected to graft polymerization with vinyl polymers. Nucleophilic substitution of the chlorine 

Starch dialdehyde reacts with thiols to form dithioacetals.525 Thiol groups in thiol acids undergo oxidation in air into disulfides.2681 

The reaction of thiols with alkyl halides produces sulfides, which are converted into sulfonium halides in the presence of excess alkyl halides.2680 Dimethyl sulfoxide (42) and acetic anhydride (41) react with starch365 to cause oxidation at the C-3 and C-6 positions, and methylthiomethyl ethers are formed at 0-6, according to the following reaction (43-46)  

On the other hand, it has been demonstrated that amylose having the 6-OH group protected by the trityl group reacts with dimethyl sulfoxide in the presence of acetic anhydride to effect oxidation at C-2 and conversion of the 3-hydroxyl group into the methylthiosomethyl ether.2289 

Dispersions of starch in dimethyl sulfoxide become stiff within 20 min when brought into contact with water.2684 Sulfonium salts can be produced by the reaction of reagents already bearing the sulfonium moiety, as when starch is treated with 2-chloroethylmethylethyl sulfonium iodide in the presence of calcium hydroxide.2685 The resulting formulations can be applied as paper coatings, paper adhesives, sizes, and thickeners.2685,2686 

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Reactions of thiols, sulfides, and sulfonium salts (Section 12.10). 

Thia-[2,3]-Wittig sigmatropic rearrangement of lithiated carbanions 47, obtained by deprotonation of the S-allylic sulfides 46, affords the thiols 48 or their alkylated derivatives 49. The corresponding sulfonium ylides 51, prepared by deprotonation of the sulfonium salts 50 also undergoes a [2,3]-sigmatropic shift leading to the same sulfides 49 [36,38] (Scheme 13). As far as stereochemistry is concerned, with crotyl (R R =H,R =Me) and cinnamyl (R, R =H,R =Ph) derivatives, it has been shown that the diastereoselectivity depends on the nature of the R substituent and on the use of a carbanion or an ylide as intermediate. 

Thiols, RSH, the sulfur analogs of alcohols, are usually prepared by Sn2 reaction of an alkyl halide with thiourea. Mild oxidation of a thiol yields a disulfide, RSSR, and mild reduction of a disulfide gives back the thiol. Sulfides, RSR, the sulfur analogs of ethers, are prepared by a Williamson-type Sn2 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 (R2SO) and to sulfones (R2S02). Sulfides can also be alkylated by reaction with a primary alkyl halide to yield sulfonium salts, RaS. ... 

Since sulfides tend to react with electrophiles, a method for protection could be quite useful. Sulfoxides can be used to protect sulfides and are easily formed by a variety of oxidants. Sulfides can be regenerated with thiols, SiCLi (0 C, 15 min, TFA, anisole) LiBat/McjSiCl DMF-S03/HSCH2CH2SH (DMF, Pyr, rt, 85% yield) dithiane, NBS (CHCI3, rt, 89-96% yield) Bii4NBr, TFA, thioanisole, anisole, ED T Catecholborane, benzene. Sulfides can also be protected as sulfonium salts. 

One solution is a [2,3] sigmatropic rearrangement of a sulfonium ylid. A stable allylic thiol reacts cleanly with an a-haloketone 64 to give a sulfide and hence the sulfonium salt after ethylation. Only a weak base is need to make the ylid 65 as this is also an enolate. Notice the regioselectivity here the alternative ylid would also be stabilised but only by an alkene. The [2,3] shift joins the allylic fragment to the enolate of the ketone and at the same time turns it inside out. This automatically produces the more difficult product 63 after desulfurisation with Raney nickel. We shall see alternative solutions to this problem later in the chapter.9... 

Thiols are sulfur analogs of alcohols. They are stronger acids and have lower boiling points than alcohols. Thiolate ions are weaker bases and better nucleophiles in protic solvents than alkoxide ions. Sulfur analogs of ethers are called sulfides or thioethers. Sulfides react with alkyl halides to form sulfonium salts. 

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