Benzoic acid can be a troublesome byproduct of m-CPBA oxidation. If reaction is done in refluxing DCE (for example), cool reaction mixture to 0oC to precipitate out all m-CPBA/benzoic acid. Then add 10% aqueous solution of Na 2SO 4. Two layers form. Separate and extract with DCM. Repeat the addition of Na 2SO 4 several times then combine all organic layers, wash with saturated solution of NaHCO 3, then saturated aqueous solution of NaCl, then dry (MgSO 4) and evaporate under reduced pressure. If purifying product by flash column chromatography, any m-CPBA/benzoic acid still present is easily separated as it is UV active, extremely polar and it tends to deposit on column tip during purification. This is easily taken care of as m-CPBA is soluble in EtOAc.
Lawesson’s reagent: Adsorb the reaction mixture onto Celite and load it on a column. Purification works well if your substrate isn’t too polar.