Davys reagent R2 Me

Conversion of propanoic acid into methyl propanedithioate with Davy s reagent (R2 - Me) 

A solution of propanoic acid (5.92g, 80mmol) in 1,2,4-trichlorobenzene (80 ml) was heated at 80-100°C. Davy s methyl reagent (12.6 g, 44 mmol), prepared according to [142], was added portionwise in 2 min. The mixture was heated at 140°C during 10 min (caution operate under a hood and trap obnoxious vapours by bleach). It was then cooled to room temperature. The liquid which separated was distilled to give yellow methyl propanedithioate (5.93 g, 49mmol, 61%),b.p. 57°C/12 torr. 

Better yields are obtained when the carboxylic acid is heated before addition of the reagent to minimize the formation of trialkyl tetrathiophosphate [142]. 

Davy s reagents with R2 = Me, Et, i-Pr or PhCH2 have been prepared [142] and used [140, 141]. The first two are commercially available from Fluka and Aldrich. 

Dithioacids themselves (R CSSH) could be prepared in about a 40-50% yield by addition of Grignard reagents to caibou disulfide in THF as the solvent, followed by acidification in the presence of ether or pentane [143]. Hartke has reported that the yields of dithioacids could be improved by reverse addition of the Grignard reagent to carbon disulfide [144], a protocol previously used by Julia [145] in the preparation of dithioesters from allylic organometallics. A 70-80% yield of dithioacetic acid was thus achieved on a molar scale. 

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