Thiolation Thiols

Complexes Containing S-, Se- and Te-donor Ligands 6.14.3.3.1 Thiolates, thiols, thioethers, and related ligands... 

The initiator system 2a/DBU was formed by addition of DBU ([2a]/[DBU] = 1 1), which leads to the thiolate/thiol equilibrium through proton exchange The polymerization of MT using this initiator system was carried out in DMF at 0 °C and was quenched by l-(chloromethyl)naphthalene, which has been reported to be effective for termination of thiirane polymerization. With a feed ratio of [MT]o/[2a/DBU]o = 30, the reartion was complete within 5 min to afford the corresponding star-shaped polymer the nrrmber-average molecular weight (M ) and MWD (Mw/MJ, which were 3500 and 1.05, respectively. 

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. 

Perfluoroalkylaiion of thiols by perfluoroalkyl iodides can be performed in liquid ammonia under UV irradiation [59, 60], This photochemical reaction can also occur with thiolates in acetomtrile or under phase-transfer conditions [60. 61] (equations 51 and 52). 

In fact, perfluoroalkyl iodides can react with thiolates even in the absence of UV irradiation [55, 62, 63] However, the photochemical reaction is particularly useful for the transformation of aliphatic thiols [59], because the spontaneous condensation gives a considerable quantity of disulfide in that case (equations 53-55). 

The final step in the /3-oxidation cycle is the cleavage of the /3-ketoacyI-CoA. This reaction, catalyzed by thiolase (also known as j8-ketothiolase), involves the attack of a cysteine thiolate from the enzyme on the /3-carbonyI carbon, followed by cleavage to give the etiolate of acetyl-CoA and an enzyme-thioester intermediate (Figure 24.17). Subsequent attack by the thiol group of a second CoA and departure of the cysteine thiolate yields a new (shorter) acyl-CoA. If the reaction in Figure 24.17 is read in reverse, it is easy to see that it is a Claisen condensation—an attack of the etiolate anion of acetyl-CoA on a thioester. Despite the formation of a second thioester, this reaction has a very favorable A).q, and it drives the three previous reactions of /3-oxidation. 

FIGURE 24.17 The mechanism of the thiolase reaction. Attack by an enzyme cysteine thiolate group at the /3-carbonyl carbon produces a tetrahedral intermediate, which decomposes with departure of acetyl-CoA, leaving an enzyme thioester intermediate. Attack by the thiol group of a second CoA yields a new (shortened) acyl-CoA. 

A thiol, usually under basic catalysis, can undergo Michael addition to an activated double bond, resulting in protection of the sulfhydryl group as a substituted S-ethyl derivative. Displacement of an ethyl tosylate by thiolate also affords an S-ethyl derivative. 

Quite a number of asymmetric thiol conjugate addition reactions are known [84], but previous examples of enantioselective thiol conjugate additions were based on the activation of thiol nucleophiles by use of chiral base catalysts such as amino alcohols [85], the lithium thiolate complex of amino bisether [86], and a lanthanide tris(binaphthoxide) [87]. No examples have been reported for the enantioselective thiol conjugate additions through the activation of acceptors by the aid of chiral Lewis acid catalysts. We therefore focussed on the potential of J ,J -DBFOX/ Ph aqua complex catalysts as highly tolerant chiral Lewis acid catalyst in thiol conjugate addition reactions. 

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... 

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. 

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... 

EPR investigations are necessarily carried out in frozen solution at low temperature. Room temperature binding of thiols to FeMoco has been monitored by F NMR spectroscopy using /J-CF3C6H4S as the reporter ligand. These experiments revealed that the binding of thio-late is characterized by a dynamic equilibrium between the FeMoco and thiolate (159) and that cyanide and methyl isocyanide can bind to isolated FeMoco complexed with thiol (160). 

The significance of this discovery is that control of the degree of polymerization and of coordination modes of the thiolate precursors is hkely to affect the shape of the final nanoproduct in a solventless method. Since traditional thiol-metal coordination chemistry is well established, this affords a great opportunity to join the traditional coordination chemistry with an increased variety of nano-... 

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