Thioesters from alkyl halides

Homologous thioesters Besides complete hydrolysis to give esters, 1,1,1-tris-(methylthio)alkanes derived from alkyl halides can also be a source of methylthio esters. The transformation is accomplished by heating the tris(methylthio)alkanes with aqueous HBF4 in DMSO at 130°C. 

The thioesters (70) are available both by action of alkyl halides upon the corresponding dithiols (72), and by alkylation with diazo compounds.18 They are not attacked by hot dilute sodium hydroxide, but dissolved in hot acids giving yellow solutions from which they are precipitated unchanged on dilution with water. On the reactivity see Refs. 14, 15, 17, and 125. 

The proposed mechanism, which is based on the double carbonylation of styrene oxide, is shown in Scheme 6.2. The generation of an acylcobalt carbonyl complex from the reaction of cobalt tetracarbonyl anion with an alkyl halide is followed by reaction with a thiirane. This species can undergo carbonylation, the thioester function can undergo hydrolysis to reveal a sulfido nucleophile, and intramolecular cydization then produces thietan-2-one. The thietan-2-one can undergo ring cleavage and the mercapto acid results by protonahon. 

A superior and relatively versatile procedure for the synthesis of unsymmetrical dialkyl thioethers, which avoids the unattractive direct use of thiols, utilizes the stable l-alkylthioethaniminium halides, which are readily obtained from thioacet-amidc [32] (Scheme 4.4). The reaction has also been used for the synthesis of alkyl aryl thioethers from activated aryl halides [33], but it cannot be used for the synthesis of cyclic thioethers, as polymeric sulphides are formed from a,co-dihaloalkanes. A similar sequence to that which leads to the thioethers has been used for the synthesis of S-alkyl thioesters [34] (see 4.1.26). 

The reverse reaction in which thioacetamide is initially alkylated and then reacted under phase-transfer catalytic conditions with the acyl halide results in the formation of A-acylthioamidates (Scheme 4.15), with only trace amounts of the S-alkyl thioesters [35], S-Alkyl thioacetates have also been obtained from trifluoro-methylsulphonyloxy compounds upon reaction with potassium thioacetate in the presence of TDA-1 [61]. It is probable that tetraalkylammonium salts would be equally good catalysts. 

Sodium mercaptides are prepared from the mercaptans and aqueous or alcoholic solutions of sodium hydroxide or alcoholic sodium eth-oxide. The sodium mercaptide reacts with halides, chlorohydrins, esters of sulfonic acid, or alkyl sulfonates [6] to give sulfides in yields of 70% or more. A recent report describes a general procedure for synthesizing aryl thioesters by a nucleophilic displacement of aryl halide with thiolate ion in amide solvents. No copper catalysis is necessary as in an Ullmann-type reaction. 

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