Lawesson's reagent

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Template:Chembox header | Lawesson's reagent
Lawesson's reagent
3D model of the Lawesson's reagent molecule
Template:Chembox header | 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
Template:Chembox header | Properties
Density and phase ? g/cm³, solid
Solubility in water Insoluble
Melting point 228 - 231 °C (? K)
Template:Chembox header | 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 ?
Template:Chembox header | Supplementary data page
Structure and
properties
n, εr, etc.
Thermodynamic
data
Phase behaviour
Solid, liquid, gas
Spectral data UV, IR, NMR, MS
Template:Chembox header | Related compounds
Related thiation
agents
H2S, P2S5
Template:Chembox header | 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 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.[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 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.[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 simply 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 α,β-unsaturated aldehydes.

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

References

  1. Lecher, H. Z.; Greenwood, R. A.; Whitehouse, K. C.; Chao, T. H. (1956). "The Phosphonation of Aromatic Compounds with Phosphorus Pentasulfide". J. Am. Chem. Soc. 78: 5018. doi:10.1021/ja01600a058.
  2. Template:OrgSynth
  3. Foreman, M.S.; Woollins, J.D. (2000). "Organo-P-S and P-Se heterocycles". J. Chem. Soc. Dalton Trans. 2000: 1533–1543.
  4. Martin Jesberger, Thomas P. Davis, Leonie Barner (2003). "Applications of Lawesson's Reagent in Organic and Organometallic Syntheses". Synthesis. 2003: 1929–1958. doi:10.1055/s-2003-41447.
  5. Cava, M. P.; Levinson, M. I. (1985). "Thionation reactions of Lawesson's reagents". Tetrahedron. 41 (22): 5061–5087.
  6. Daniel Brayton, Faith E. Jacobsen, Seth M. Cohen and Patrick J. Farmer (2006). "A novel heterocyclic atom exchange reaction with Lawesson's reagent: a one-pot synthesis of dithiomaltol". Chemical Communications. 2006: 206–208. doi:10.1039/b511966a.

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