1.3.2.4- Dithiadiphosphetane 2,4-disulphides

N.m.r. spectroscopic evidence has been provided to suggest the dissociation of relatively simple 1,3,2,4-dithiadiphosphetane 2,4-disulphides (i.e. those with non-bulky groups) in solution this evidence comprises the observed inversion at one of the phosphorus atoms (i.e. cis/trans interconversion) and the exchange reactions... 

The interaction of trialkyl or triaryl esters of phosphorotetrathioic acid with P S q at 180° leads to a scrambling of P-SP and P-SC bonds. The products consist of mixtures of 1,3,2,4-dithiadiphosphetane 2,4-disulphides (76) and the perthiodiphosphates (77), the former being formed in large excess. The same products are obtained in essentially the same ratios from P S q and the thiols RSH. In the case of alkane-a,u-dithiols, the products are the dithiaphosphorinanes (78) and (79), both of which are isolable. This last reaction, during which the cyclic acids (80) appear not to be involved, proceeds well for n=2 or 3, but poorly when n=4. ... 

Several of the procedures employed for the synthesis of dithiophosphinic acids have also been developed for the preparation of a remarkable group of compounds already referred to as phosphonotrithioic bisanhydrosulphides or thionophosphine disulphide dimers but which, structurally, are 1,3,2,4-dithiadiphosphetane 2,4-disulphides. Their importance is such as to warrant separate discussion, and this is to be found in a later section. 

Before going on to consider some of the very valuable reactions of anhydrosulphides, both in the general context of organophosphorus chemistry and also from the viewpoint of more conventional organic synthesis, it is worth considering them briefly from the structural point of view. The so-called thionophosphine disulphides are (generally) dimeric and possess the 1,3,2,4-dithiadiphosphetane 2,4-disulphide structure. As normally prepared, the compounds exist in the trans form 207, as evidenced by X-ray analyses of, for example the 2,4-dimethyl , 2,4-diphenvl (powder structure) 2,4-bis(4-methoxyphenyl) and 2,4-bis(2,4,6-triisopropylphenyl) compounds. Evidence has been obtained which clearly demonstrates the ability of the cyclic structure to dissociate and re-form under varied con-... 

On the other hand, reactions of the thionophosphine disulphides (1,3,2,4-dithiadiphosphetane 2,4-disulphides) have been reported in abundance. Many of these reactions have been known for some years and have been reviewed more fully else-... 

Dithiadiphosphetans.—2,4-Disubstituted 1,3,2,4-dithiadiphosphetan 2,4-disulphides (155) are the anhydrides of dithiophosphonic acids. They have been prepared by the reaction of (156) with The di-t-butyl derivative (155 ... 

Thiations of phosphoryl groups have occasionally be carried out with 2,4-bis(4-methoxyphenyl)-l,3,2,4-dithiadiphosphetane 2,4-disulphide (55). This, known as Lawesson s reagent, is a powerful reagent, particularly for the thiation of carbonyl groups, and it has been extensively investigated in this respect (the preparation and properties of... 

Lawesson s reagent [2,4-bis(4-methoxyphenyl)-l, 3,2,4-dithiadiphosphetan 2,4-disulphide] has been employed as a thiating agent it also acts as an amino-... 

Peptide Synthesis.—2,4-Bis-(4-methoxyphenyl)-1,2,3,4-dithiadiphosphetan 2,4-disulphide (Lawesson s reagent) is a new racemization-free coupling reagent for peptide synthesis. Dimethylphosphinothioyl (MPT) mixed anhydrides of N-protected amino-acids, e.g, (299), are useful for the synthesis of peptides that contain hydroxyamino-acids coupling occurs without protection of the side-chain (Scheme 146). ... 

---