Substrate (2.8 mmoles)
Lawessons reagent (1.4 mmoles)
THF (170 ml)

Dissolve Lawessons reagent (580 mg, 1.43 mmoles) in THF (120 ml) and add THF (50 ml) solution of the amide (500 mg, 2.86 mmoles) to this reagent at room temperature. Stir at room temperature until reaction is complete by tlc (in this case 30 minutes but you can leave the reaction overnight if more convenient). Evaporate solvent and then carry out a standard aqueous work-up using water and extraction with ether. Purification by silica-gl chromatography (petroleum ether: ether gradient, 3:1, 2:1, 1:1) to give the product. Yield 472 mg, 86%.

Lawessons reagent can be really very useful for this transformation. The original procedure used toluene as solvent and the reaction carried out at elevated temperature over prolonged periods. However carrying out the reaction in THF allows it to be conducted at room temperature with reduced reaction times, but it is important that the Lawessons dissolves in the reaction medium and that does take a great deal of THF. It is critical that a good aqueous work-up is used prior to chromatography, do not be tempted to simply remove the solvent and carry out chromatography as impurities will persist. The non-polar product tends to run high up the tlc plate. Note also that Lawessons and impurities are unpleasant smelling species. In my opinion this works better than procedure using P₄S₁₀.

¹H NMR 7.3, 5H, m, Ph; 5.0, 2H, s, PhCH₂; 3.6, 2H, t, CH₂N; 3.1, 2H, t, CH₂CS; 2.0, 2H, m, CH₂;

Pederson, Lawesson, Bull. Soc. Chim. Belg. 1978, 87, 223, 229 & 293. See also Bull. Soc. Chim. Belg. 1981, 90, 103