Phosphorofluoridates

At the begiiming of the war we synthesized, by methods de-scribed below and also in Chapter rv, a series of dialkyl phosphorofluoridates (I). In general, these compounds were colourless, stable and almost odourless liquids. With them we carried out tests (a) on ourselves, (6) on animals, (c) on enzyme systems. The very close collaboration of the Departments of Chemistry, Physiology, Biochemistry and Pathology at Cambridge permitted of the initial screening of a compound often within a few 

At higher concentrations the toxicity was such as to cause a quick knock-out action. For these observations small animals were used and standard techniques employed. Inhalation 

Saunders, Ministry of Supply Meeting, London, II December 1941 McCombie and Saunders, Nature, Land, 1946, 157, 287 Saimders and Stac, J. Chem. Soc. 1948, p. 695 Saimders et al. B.P. 601,210. 

The compound is also toxic by injection thus for intra-venous injection into rabbits the l.d. 50 in normal saline was about 0-5 mg./kg. Pupil constriction began 2 min. after injec-tion, followed by loss of muscular co-ordination and then by lespiratory collapse. 

We carried out a great deal of work on the relationship between the above physiological effects and chemical constitution, and it was shown conclusively that the more potent compounds were those derived from secondary alcohols. Thus, for example, di-Mopropyl phosphorofluoridate is very much more potent than diethyl phosphorofluoridate or di- -propyl phosphorofluoridate and the toxicity of the dicyc/ohexyl ester is of a high order (l.c. 50 for mice, rats and rabbits was 0-11 mg./l.). Di- -butyl phosphorofluoridate had low toxicity and produced only feeble 

The ester fluorides of phosphoric acid and phosphonic adds were the earliest known examples of biologically active phosphorus compounds. Sarin (2) and soman (3), previously mentioned, did not attain practical importance because of their high toxicity, but they were the starting point for the development of other derivatives more suitable for agricultural purposes. Within this framework Schrader developed N,N,N, N -tetramethyl phosphorodiamidofluoridate (14), which is used under the name dimefox. It is produced by the reaction of phosphoryl chloride with dimethylamine and the subsequent interaction of the phosphoro-chloridate formed with sodium fluoride (Schrader, 1947). 

Dimefox is very toxic its lOj for rats being 3-5 mg/kg. It has a weak contact-insecticidal effect but at the same time excellent systemic properties. It is mainly used in hop cultures against plant lice and mites. The concentration of the insecticide in the hop plant attains its maximum one week after treatment of the soil and from this time on degradation causes a continuous decrease in concentration (Dejonckheere, 1974). 

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The esters of monofluorophosphoric acid are of great interest because of their cholinesterase inhibiting activity which causes them to be highly toxic nerve gases and also gives them medical activity (see Enzyme inhibitors). The most studied is the bis(l-methylethyl)ester of phosphorofluoridic acid also known as diisopropyl phosphorofluoridate [155-91 DFP (5), and as the ophthalmic ointment or solution Isoflurophate USP. It is used as a... 

Chemical Name Phosphorofluoridic acid bis( 1-methylethyljaster Common Name Fluostigmine... 

Aryl phosphates and thiophosphates, and alkyl dithiophosphates are important agrochemicals, while phosphorofluoridates have been prepared as chemical warfare agents. Concern over the persistence and the biodegradability of organophosphate and organophosphorothioates, which are nsed as agrochemicals, has stimnlated stndies into their degradation. Considerable attention has been... 

DeFrank JJ, WE White (2002) Phosphorofluoridates biological activity and biodegradation. Handbook Environ Chem 3N 295-343. 

Brief notes are added on phosphorofluoridates even though their destruction by microbial activity— though clearly possible—is limited by their toxicity to the requisite microorganisms. One of the motivations for their inclusion is the fact that the hydrolytic enzyme(s) responsible for defluorination—organophosphorus acid anhydrase (OPA)—is widespread, and is found in a number of bacteria (Landis and DeFrank 1990). The microbial hydrolysis of organophosphorus pesticides and cholinesterase inhibitors is accomplished by several distinct enzymes, which are collectively termed organophosphorus acid anhydrases (OPAs). These have been reviewed (DeFrank 1991), so that only a few additional comments are necessary. 

Furthermore, in the particular type of phosphate molecule under discussion (VI) we showed that when X is fluorine, compounds of high toxicity result whereas myotic effect is absent and toxicity of a low order if X = H, Et, OH, OEt, OCH2CH2C3, OCH2CH2F, Cl, NH2, NHMe, NHPh, CH2CH2F, CN, SON, etc.1 In Chapters rv and vi, however, we consider in more detail cases where X is not fluorine, but nevertheless toxicity results. Toxicity is also of a low order in the aromatic series for example, diphenyl phosphorofluoridate is relatively non-toxic and devoid of myotic properties. We also showed that ethyl phosphoro-difluoridate, (C2H50)P0F2, had neither myotic nor toxic action.1... 

Concurrently with experiments on animals, the action of the phosphorofluoridates on enzymes was investigated in Cambridge.3 It was shown in 1942 that esters of phosphorofluoridic acid inhibit4 the action of the enzyme cholinesterase, which is present in tissue fluids and hydrolyses acetylcholine to the much less active choline. 

As subsequent work showed, this discovery had a marked effect on the course of phosphorofluoridate chemistry. It now became possible to prepare phosphorofluoridates in excellent yield and from cheap and readily accessible materials in particular, no tertiary base was required. 

Further modifications were then made in this hydrogen phosphite method of preparing di-isopropyl phosphorofluoridate in order to put it on an industrial scale. 

After a large number of experiments, we found that the preparation could be run virtually as a one-stage process. The whole process consists simply in adding phosphorus trichloride to isopropyl alcohol, dissolved in a solvent such as carbon tetrachloride, without external cooling. The crude product (still in the solvent) is chlorinated and then heated with an inorganic fluoride, e.g. sodium fluoride. After filtration, the solvent is distilled off and the pure di-isopropyl phosphorofluoridate distilled. ... 

Many of these compounds were toxic for example, tetra-methyl phosphorodiamidic fluoride (dimethylaminofluorophos-phine oxide (VII)) had a l.c. 50 of 0-1 mg./l. Unlike the phosphorofluoridate esters, however, they were devoid of myotic action. 

As a matter of interest, we obtained a compound, diethyl phosphorofluoridite (IX), of a lower state of oxidation than the corresponding phosphorofluoridate by the action of phosphorus dichlorofluoride on ethyl alcohol. The new compound, unlike the phosphorofluoridate, was readily hydrolysed by water, was relatively non-toxic and did not produce myosis.1... 

Di 2 fluoroethyl phosphorofluoridate (XVIII) was prepared with the idea of combining the toxic principles of the fluoro-aoetates and of the phosphorofluoridates. It was readily obtained by the action of phosphorus oxydichlorofluoride on... 

Until this work began in Cambridge at the beginning of the war, the alkyl phosphorofluoridates had received practically no attention. Lange1 had given a tedious and laborious method for preparing dimethyl and diethyl phosphorofluoridates in very poor yield as follows. 

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