Lawesson's reagent

Lawesson's reagent
General
Systematic name	2,4-bis(4-methoxyphenyl)- 1,3,2,4-dithiadiphosphetane 2,4-disulfide
Other names	Lawesson's reagent, LR
Molecular formula	C14H14O2P2S4
SMILES	COC1=CC=C(P2(SP(S2) (C3=CC=C(OC)C=C3) =S)=S)C=C1
Molar mass	404.47 g/mol
Appearance	Slightly yellow powder
CAS number	19172-47-5
Properties
Density and phase	? g/cm3, solid
Solubility in water	Insoluble
Melting point	228 - 231 °C (? K)
Hazards
MSDS	External MSDS
Main hazards	Irritant Harmful (XN)
NFPA 704
Flash point	? °C
R/S statement	R: 15/29 20/21/22 S: 22 45 7/8
RTECS number	?
Supplementary data page
Structure and properties	n, εr, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Related thiation agents	H2S, P2S5
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

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 sulfide (P₄S₁₀).

History

Lawesson's reagent was not invented by S.O. Lawesson, he was a chemist who made it popular as a thionation reagent. Lawesson's reagent was first made many years ago during a systematic study of the reactions of arenes with P₄S₁₀.^[1] 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 napthalen-1,8-diyl 1,3,2,4-dithiadiphosphetane 2,4-disulfide and the 1,3,2,4-dithiadiphosphetane 2,4-disulfides.

Preparation

File:Lawessons.jpg

The structure of Lawesson's reagent

A mixture of anisole with phosphorus sulfide (P₄S₁₀) should be heated until the mixture is clear and no more hydrogen sulfide is formed.^[2] 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 simple by recording a ³¹P-{¹H} 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.^[3][4][5] 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 alpha,ß-unsaturated aldehydes.

In one study ^[6] reaction of Maltol with LR results in a selective oxygen replacement in two positions.

Maltol reaction with LR

References

1. ^ Lecher, H. Z.; Greenwood, R. A.; Whitehouse, K. C.; Chao, T. H. J. Am. Chem. Soc. 1956, 78, 5018.
2. ^ Thomsen, I.; Clausen, K.; Scheibye, S.; Lawesson, S.-O. Org. Syn., Coll. Vol. 7, p.372 (1990); Vol. 62, p.158 (1984). (Article)
3. ^ Foreman and Woollins, Dalton 2000, 1533-1543.
4. ^ Jesberger, Synthesis 2003, 1929-1958. (Review)
5. ^ Cava and Levinson, Tetrahedron 1985, 5061-5087. (Review)
6. ^ A novel heterocyclic atom exchange reaction with Lawesson's reagent: a one-pot synthesis of dithiomaltol Daniel Brayton, Faith E. Jacobsen, Seth M. Cohen and Patrick J. Farmer Chemical Communications, 2006, 206-208. DOI Abstract