Abstract
We described herein a selective method to prepare α-organylthio esters and α-organylthio ketones by the reaction of β-keto esters with sodium S-benzyl sulfurothioate or sodium S-alkyl sulfurothioate (Bunte salts) under basic conditions in toluene as the solvent at 100 °C. When 4 equivalents of a base were used, a series of differently substituted α-thio esters were obtained with up to 90% yield. On the other hand, employing 2 equivalents of a base, α-thio ketones were achieved after 18 h under air. Furthermore, after a shorter reaction time, the isolation of keto–enol tautomers was possible, revealing them as significant intermediates for the mechanism elucidation.
Highlights
During the last ten years, sodium S-organyl sulfurothioates, known as Bunte salts, were rediscovered by many researchers as stable, nonhygroscopic, and moisture-resistant thiolating agents
This work demonstrated an easy access to α-sulfenylated amides (Scheme 1C) [52]. Inspired by those elegant pioneering studies, we explored the reaction of 1,3-dicarbonyl compounds with Bunte salts mediated by a base (Scheme 1D)
Based on the above results and on the information from the literature, we propose the following mechanism for the synthesis of 3 (Scheme 4) [56,57,58,59,60] and 4 (Scheme 5) [61,62,63,64]: We believe that initially, an acidic hydrogen atom bonded to the methylene group between the two carbonyl groups in the β-keto ester 1 is removed by a HO− anion to generate the stabilized carbanion intermediate A
Summary
During the last ten years, sodium S-organyl sulfurothioates, known as Bunte salts, were rediscovered by many researchers as stable, nonhygroscopic, and moisture-resistant thiolating agents. We chose ethyl acetoacetate (1a) and sodium S-benzyl sulfurothioate (2a) as the standard starting materials to establish the best reaction conditions under an air atmosphere (Table 1). As such, intrigued by this result and aiming to find selective methods to prepare both α-thio carbonyl compounds (3a and 4a), we decided to verify the influence of different inorganic and organic bases on the reaction (Table 1, entries 2–9).
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