Lawesson's reagent

Lawesson's reagent, or LR, is a chemical compound used in organic synthesis as a thiation agent. It is made by the reaction of anisole with phosphorus pentasulfide (P4S10).

History
Lawesson's reagent was first made popular by Sven-Olov Lawesson, who did not, however, invent it. Lawesson's reagent was first made many years ago during a systematic study of the reactions of arenes with P4S10. In later years a series of inorganic chemists (Maingroup chemists) have explored the general chemistry of Lawesson's reagent and related compounds. Please see for details of the general chemistry of this class of compound the pages on naphthalen-1,8-diyl 1,3,2,4-dithiadiphosphetane 2,4-disulfide and the 1,3,2,4-dithiadiphosphetane 2,4-disulfides.

Preparation
A mixture of anisole with phosphorus pentasulfide (P4S10) should be heated until the mixture is clear and no more hydrogen sulfide is formed. Recrystallization from toluene or xylene will yield pure LR.

As Lawesson's reagent has a strong and unpleasant smell, it is best to prepare the compound within a fume-hood and to treat all glassware used with a decontamination solution before taking the glassware outside the fume-hood. One common and effective method of destroying the foul smelling residues is to use an excess of sodium hypochlorite (Chlorine bleach).

Mechanism of action
Lawesson's reagent has a four membered ring of alternating sulfur and phosphorus atoms. With heating, the central phosphorus/sulfur four-membered ring can open to form two reactive dithiophosphine ylides (R-PS2). Much of the chemistry of lawessons's reagent is in fact the chemistry of these reactive intermediates. The existence of these reactive intermediates can be demonstrated simply by recording a 31P-{{{sup|1}}H} NMR spectrum of a mixture of two different 1,3,2,4-dithiadiphosphetane 2,4-disulfides, the spectrum will have the two singlets for the original 1,3,2,4-dithiadiphosphetane 2,4-disulfides and two doublets for the 1,3,2,4-dithiadiphosphetane 2,4-disulfide formed by the combination of dithiophosphine ylides which bear different ancillary groups.

Uses
The chemistry of Lawesson's reagent and related substances has been reviewed by several groups. A common use of LR is to convert a carbonyl into a thiocarbonyl. For instance, an amide can be converted into a thioamide using Lawesson's reagent. In addition LR has been used for the synthesis of thioesters and thioketones.

In general, the more electron rich a carbonyl is, the faster the carbonyl group will be converted into the corresponding thiocarbonyl by LR.

A combination of silver perchlorate and Lawesson's reagent is able to act as an oxophilic Lewis acid with the ability to catalyze the Diels-Alder reaction of dienes with α,β-unsaturated aldehydes.

In one study reaction of maltol with LR results in a selective oxygen replacement in two positions.