Thiolate ions Thiols

Thiol—Disulfide Interchange Reactions. The interchange between thiols and disulfides has been reviewed (50). This reaction is base-catalyzed. It involves the nucleophihc attack of a thiolate ion on a disulfide. This is shown in equations 35, 36, and 37. 

Treatment of thiiranes with lithium aluminum hydride gives a thiolate ion formed by attack of hydride ion on the least hindered carbon atoms (76RCR25), The mechanism is 5n2, inversion occurring at the site of attack. Polymerization initiated by the thiolate ion is a side reaction and may even be the predominant reaction, e.g. with 2-phenoxymethylthiirane. Use of THF instead of ether as solvent is said to favor polymerization. Tetrahydroborates do not reduce the thiirane ring under mild conditions and can be used to reduce other functional groups in the presence of the episulfide. Sodium in ammonia reduces norbornene episulfide to the exo thiol. 

Treatment of a thiol with a base, such as NaH, gives the corresponding thiolate ion (RS-), which undergoes reaction with a primary or secondary alkyl halide to give a sulfide. The reaction occurs by an Sn2 mechanism, analogous to the Williamson synthesis of ethers (Section 18.2). Thiolate anions are among... 

Thioethers (sulfides) can be prepared by treatment of alkyl halides with salts of thiols (thiolate ions). The R group may be alkyl or aryl and organolithium bases can be used to deprotonate the thiol. As in 10-37, RX cannot be a tertiary halide, and sulfuric and sulfonic esters can be used instead of halides. As in the Williamson... 

The conversion (19) of thiols to disulphides coupled with reduction of flavin (vitamin B2 family) is a topic of import in connection with coenzyme reactivity in flavoenzymes. Since flavin oxidation of thiols involves nucleophilic attack of thiolate ion in the rate-determining step (Loechler and Hollocher, 1975 Yokoe and Bruice, 1975), this biologically important reaction would be markedly affected by hydrophobic environments. 

TABLE 3. Second-order rate constants for the reaction of alkyl nitrites with the thiolate ions derived from three thiols in water at 25 °C... 

Thioethers (sulfides) can be prepared by treatment of alkyl halides with salts of thiols (thiolate ions).7S2 R may be alkyl or aryl. As in 0-35, RX cannot be a tertiary halide, and sulfuric and sulfonic esters can be used instead of halides. As in the Williamson reaction (0-12), yields are improved by phase-transfer catalysis.753 Instead of RS ions, thiols themselves can be used, if the reaction is run in benzene in the presence of DBU (p. 1023).754 Neopentyl bromide was converted to Me3CCH2SPh in good yield by treatment with PhS in liquid NH3 at -33°C under the influence of light.755 This probably takes place by an SrnI mechanism (see p. 648). Vinylic sulfides can be prepared by treating vinylic bromides with PhS in the presence of a nickel complex,756 and with R3SnPh in the presence of Pd(PPh3)4.757 R can be tertiary if an alcohol is the substrate, e.g,758... 

Similar arguments have been used to show that the attack of thiols upon esters is not general-base-catalyzed but involves the direct attack of the thiolate ion even at 10 pH units below the pKa of the thiol, where only 1 part in 1010 is ionized.46... 

The reaction of OH with thiolate ions, taken as an overall reaction, is an ET reaction [reaction (36)]. One must, however, again take into account that a three-electron bonded intermediate is formed in the first step (Chap. 7). In semi-de-protonated dithiothreitol reaction (32) dominates over the H-abstraction reaction (33) (Akhlaq and von Sonntag 1987), although the rate constant for the reaction of OH with a thiol and a thiolate ion are both diffusion controlled (k = 1.5 x 1010 dm3 mol-1 s 1). This is another example of the potentially high regio-selectivity of OH reactions. 

The (oxidizing) a-carboxyalkyl radicals do not react readily with thiols (Table 6.4). They are, however, rapidly reduced by thiolate ions [reaction (20)]. The reactions of thiols with DNA radicals play a very important role in the chemical repair of DNA radicals in cells (Chaps 12.10 and 12.11). The reversibility of the H-donation of thiols, that is, H-abstraction by thiyl radicals, is discussed in Chapter 7.4. 

Hydrogen and Electron-Transfer Reactions by Thiols/Thiolate Ions 144... 

The (oxidizing) peroxyl radicals behave in a similar fashion. They do not react readily with thiols, not even with thiophenols whose H atoms are very weakly bound, but they are readily reduced by the corresponding thiolate ion in contrast to alkyl radicals which are poor electron acceptors and hence do not react with thiolate ions (Simic and Hunter 1986). 

Thiols are more acidic than water. Therefore, thiolate ions are easily generated by treating thiols with aqueous sodium hydroxide. 

Thiol sulfide mercapto group alkylthio group disulfide thiolate ion trialkylsulfonium salt sulfoxide sulfone... 

Sec. 18.8) from thiols by Sn2 reaction of thiolate ions with primary alkyl halides... 

Thiolate ions are also useful for the demethylation of certain ethers, esters, amines, and quaternary ammonium salts. Aryl methyl ethers can be cleaved by heating with EtS in the dipolar aprotic solvent DMF ROAr + EtS ArO +EtSR. Carboxylic esters and lactones are cleaved (the lactones give co-alkylthio carboxylic acids) with a thiol and AICI3 or AlBrs. Esters and lactones... 

---