Thiometaphosphate Anions

The reaction of P4S10 with KCN and H2S in acetonitrile leads to the formation of the anion (9.638). The Ph4As+ salt is reported as probably containing a pyramidal PS3 anion [14]. 

Hydrolysis studies with the thioated anions (9.639a,b) indicate that the monomeric thiometaphosphate anion (9.639c) is formed as a short-lived intermediate during the course of the reaction. 

Using bulky groups such as Bu, SiMc3,2,4,6 BU3C6H2 and so on, stable imino-thioxophosphoranes and methylenethioxophosphoranes have been synthesized, for example. 

Some of these compounds appear to exist as dithiaphosphetanes, while others may exist as diazaphosphetidines. 

Phosphinylidene thioxophosphoranes are obtained by direct addition of S to diphosphenes, and methylene(thioxo)phosphoranes occur as intermediates in the formation of thiaphosphirene sulphides from methylene phosphoranes (9.642). Only a few of these compounds have so far been found stable enough for isolation. 

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As noted previously, studies of the mechanisms of phosphate monoester solvolysis have been extended to the mechanisms of the analogous phosphorothioate ester solvolysis because the thiometaphosphate anion is believed to be more stable than the metaphosphate anion. Thus, a general method based upon P NMR spectroscopy for the configurational analysis of chiral thiophosphate monoesters (see Fig. 10) was described recently by Cullis and co-workers (38). 

Cullis, Lowe, and co-workers have independently described reaction conditions that allow the formation of free thiometaphosphate anion, as evidenced by... 

Burgess J, Blundell N, Cullis PM, Hubbard CD, Misra R. Evidence for free monomeric thiometaphosphate anion in aqueous-solution. J Am Chem Soc. 1988 110 7900-7901. Cullis PM, lagrossi A. Thiophosphoryl-transfer reactions - stereochemical course of solvolysis of para-nitrophenyl thiophosphate in protic solvent and the possible role of thiometaphosphate. J Am Chem Soc. 1986 108 7870-7871. 

In summary, Cullis and Lowe have demonstrated that the generation of stable thiometaphosphate in solution is possible. The next section describes analogous studies on probing for the existence of free metaphosphate anion, since it is this intermediate that has potential significance in enzyme-catalyzed phosphoryl transfer reactions. 

The pH dependent chemical hydrolyses of the phosphorothioate analogues 2 -, 3 - and 5 -UMPS have been investigated. Between pH 2 and 5 dethio-phosphorylation is between 200 and 300 times faster than dephosphorylation due to the higher stability of the intermediate thiometaphosphate compared to the metaphosphate anion. At pH values less than 1, no migration of the thio-phosphoryl group takes place but rather, desulfurisation occurs. 

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