Desulfurization thioesters

Desulfuration of thiols.2 Aromatic, benzylic, and aliphatic thiols are converted into hydrocarbons when heated in acetic acid with molybdenum carbonyl. The bridgehead adamantane-l-thiol is an exception and gives instead a thioester, RSCOCHj. Yields are 50-90%. The actual reagent is probably tetrakis(acetato)-dimolybdenum.3 Molybdenum carbonyl deposited on silica can also be used. 

In the synthesis of a-amino acids [290] through addition of the carbanion of MMTS to nitriles the overall process involves three other steps frequently encountered in sulfur-mediated chemistry a Pummerer-type rearrangement, with a less common migration of a methylthio group, and a Raney nickel desulfurization following transesterification of the thioester function. 

Another indole/oxindole synthesis achieves the critical ortho-substitution by Sommelet-Hauser rearrangement of an anilinosulfonium ion intermediate. Use of p-thioketones (G = R, an alkyl group) generates 2-substituted indoles, whereas p-thioesters (G = OR) lead to oxindoles. In each case, a 3-thio substituent must be removed by desulfurization. 

Hauptmann et al. found that aromatic disulfides, thioesters, and thiols were transformed into thioethers when refluxed in xylene (140°C) in the presence of Raney Ni degassed at 200°C.130,134,135 However, when these compounds were heated at 220°C with degassed Raney Ni or reduced Ni, biphenyls were obtained in good yields. The yields of the products with Raney Ni degassed at 500°C were never lower and were sometimes higher than those obtained with Raney Ni degassed at 200°C, as seen from an example shown in eq. 13.64. On the basis of these results, it was concluded that the presence of hydrogen was not essential for these desulfurizations.136... 

Desulfurization of thioesters can be used in the synthesis of ethers and particularly of crown ethers, although yields are rather low (equation 3). 

Several other metal salts have been proposed to react with acid halides to give acid anhydrides in good yield. Thus, thallium carboxylates in etheral suspension may be treated with acid halides to lead to acid anhydrides in high yield. The method has the advantage that thallium chloride can be separated by filtration and the product is isolated after evaporation of the solvent. Another synthetic possibility exists in the desulfurization of thioesters with mercury(II) carboxylates. Mercuiy(II) carboxylates can also be reacted with triethyl phosphite or tri-n-butylphosphine to provide acid anhydrides in yields of 60-80%. Another possibility is offered by the reaction of 1 mol of iV,Af-dicyclohexylthiourea and 2 mol of silver or mercury(II) carboxylates. Mixed carboxylatocarbamato anhydrides follow from the room temperature reaction of iV, -dialkylcarbamato metal derivatives with acid chlorides. Either Na, Cu", Ti" or V" are employed. ... 

A way out to the problems of conventional reductive acylation of Kamiya disulfides was devised after the observation that thioacids displace mercapto-benzothiazole from these intermediates rapidly and quantitatively at room temperature in organic solvents [55]. Obtained acyldithioazetidinones 147 were smoothly desulfurized by PPh3 to afford target thioesters 127. In this process, complete retention of C4 configuration was observed with substrates 147a d... 

SOCI2) underwent conversion to 147 water and the symmetrical diacyl disulfide RCOSSCOR were liberated in the process. Initial formation of an acyl sulfinyl thioanhydride, 163, immediately suffering S-S cleavage from the second molecule of the thioacid, has been suggested [102] the sulfenic acid (formula 109) liberated by this reaction would react with the third thioacid equivalent to yield the observed product. Obtained disulfides 147 were then desulfurized with PPhj to thioesters 127 [55, 93], as described in the previous paragraph. 

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