Thio- and Dithiocarboxylation

Olah et al.457 have developed a mild method for the preparation of methyl and ethyl thio(dithio)benzoates. They prepared 5-methyl (5-ethyl) thiocarboxonium and dithio-carboxonium fluoroantimonates 121a and 121b by methylating (ethylating) carbonyl 

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S. Scheithauer and R. Mayer, Thio- and Dithiocarboxylic Adds and Their Derivatives (Topics in Sulfur Chemistry, ed. A. Senning, vol. 4), Thieme, Stuttgart, 1979. 

S. Scheithauer u. R. Mayer in A. Sennlng, Topics in Sulfur Chemistry, Bd. 4 Thio- and Dithiocarboxylic... 

Historically, the first chalcogenocarboxylic acid discovered was thiocarboxylic acid - thioacetic acid - reported by Kekule in 1854 [1]. Since then, chalcogenocarboxylic acids, and particularly numerous thio- and dithiocarboxylic acid esters, have been synthesized and summarized in several review articles [2-9]. In contrast, until very recently, little has been known about the chemistry of the congeners containing heavier chalcogen atoms, such as selenium and tellurium, probably due to their instability and the handling difficulties associated with them. In this chapter, the chemistry of chalcogenocarboxylic acids, their syntheses, structures, spectral features and reactions are reviewed. 

The structures of thio- and dithiocarboxylic acids have been studied extensively using IR, UV/Vis and NMR spectra, and molecular orbital calculations for many decades now [3, 7-9]. Thiocarboxylic acids are considered to exist as fast tautomeric equilibrium mixtures of thiol (I) and thioxo (II) forms... 

The acidities of thio- and dithiocarboxylic acids have been discussed in an earlier review [3]. For example, the values of PhCOOH, PhCOSH and PhCSSH are 4.20,2.48 and 1.92, respectively. The acidities of some chalcogeno-carboxylic acids have been estimated through theoretical studies. The gas-phase acidities (AH) of formic acid and its sulfur congeners are 342.1 for HC(0)0H, 332.2 for HC(0)SH, 328.8 for HC(S)OH and 325.8 kcal/mol for HC(S)SH. Therefore, the acidity appears to increase roughly in proportion to the number of sulfur atoms present [23]. For selenocarboxylic acids, gas-phase acidities are 340.4 for HC(0)0H, 327.6 for HC(0)SeH, and 321.9 kcal/mol for HC(Se)OH, indicating that selenocarboxylic acids are more acidic than their parent carboxylic acids [24]. It was also predicted that selenocarboxylic acids maybe more acidic than their corresponding thioic acids [14]. 

Chalcogenocarboxylic acids, especially thio- and dithiocarboxylic acids, have been used as starting materials for synthesizing corresponding chalcogenocarboxylic acid derivatives, and this topic is discussed in greater detail in Chapters 2 and 3. However, at this point we will review some of the most distinctive reactions reported over the last ten years. 

Thione-S-oxides react regiospecifically with allyl and benzylsilanes in the presence of a stoichiometric amount of tetra-n-butylammonium fluoride to produce allyl and benzyl sulphoxides [8], cf. the analogous fluoride initiated reaction of thio-ketones and dithiocarboxylic esters with silanes [9, 10]. The yields of sulphoxides... 

Little crystallographic information is available on chalcogenocarboxylic acids. Few (if any) thio- [25,26] and dithiocarboxylic acids [25-28] are known, and no congeners containing selenium and tellurium have been found. 

In 1994, successful isolations of various alkyl and aryl selenocarboxylic acids [14] enabled them to undergo exact reactions. Reactions of selenocarboxylic acids with dicyclohexylcarbodiimide (DCC) yield the corresponding diacyl selenides 25 and selenourea 26 quantitatively (Scheme 15) [14], as in reactions of thio- [44] and dithiocarboxylic acids [45,46]. In air,selenocarboxylic acids are immediately oxidized to afford the corresponding diacyl diselenides [13, 14]. Also, thio- and dithio-carboxylic acids readily react with aryl isocyanates to give acyl carbamoyl [47,48] and thioacyl carbamoyl sulfides [49, 50], respectively. 

Piperazine-2,5-dione treated with sodium hydride in A,A-dimethylacetamide and carbon disulfide followed by esterification with methyl iodide gave l-methylthio(thio-carbonyl)piperazine-2,5-dione (methyl 2,5-dioxopiperazine-l-dithiocarboxylate) and l,4-bis[methylthio(thiocarbonyl)]piperazine-2,5-dione (107), but when the reaction was carried out in dimethyl sulfoxide containing tetrahydrofuran it afforded (107) plus 3-[bis(methylthio)methylene]-l-methylthio(thiocarbonyl)piperazine-2,5-dione (108) (1630). 

In general, thiocarbonyl halides (59) function as thioacylation reagents with a variety of nucleophiles to yield the appropriate thio derivatives (60) (Scheme 31). For example, (59) on condensation with thiols, amines, potassium cyanide or potassium thiocyanide yields the corresponding thio compounds (60). Thiolocarboxylic acids (50) characteristically acylate alcohols and amines with desulfuration (Scheme 32). Dithiocarboxylic acid esters (54b) react with organolithium or Grignard reagents to give the dithioketals (61) after treatment with an alkyl halide (Scheme 33). 

The alkylidyne complexes, L Mo(CR) (Lra = Tp(CO)2, (bpy)Br(CO)2 R, e.g., Ph derivatives), react with one equivalent of propylene sulfide to form thioacyl complexes, L Mo(r/2-SCR). In the case of Tp(CO)2Mo(r/2-SCCf,114Me-4) conversion into dithiocarboxylate, mixed thioselenocarboxylate/thio-latocarbene, cc-thiolatoalkyl, and a,a-bis(thiolato)alkyl species has been demonstrated 897 More recently, related thiocarboxamide and alkoxythiocarbonyl chemistry has been reported 898 Moreover, Mo(CO)L2 (L = depe, dppe) react with HC(S)NMe2 to produce thiocarboxamide hydrides (389) that convert at 125 °C to aminomethylidyne hydrosulfides (390).8"... 

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