Toxic esters of phosphoric and methylphosphonic acids

Parathion under the trade name E605 was produced by the German company IG Farben as an insecticide and acaricide

The production of organophosphorus chemical warfare agents with high purity and long shelf life has proven to be an unsolvable challenge even for countries with advanced chemical industries, such as Iran, Iraq, Lebanon, and Syria. Methylphosphonic difluoride (DF) is the main precursor for the production of G-type chemical warfare agents (sarin, soman, cyclosarin). Its production is a multiple-step, difficult, unsafe, and complex set of chemical engineering processes. Only the United States possessed the capability to produce high-purity DF (methylphosphonic difluoride) that maintained stability after 50 years of storage, for application in binary munitions.

The removal of fluorine from molecular structures dramatically reduces toxicity characteristics of organophosphorus compounds. However, this reduction in toxicity can be counterbalanced by the introduction of specific molecular substituents, including, 4-nitrophenoxy, azide, oximino functional groups,[43] and nitrogen-containing heterocycles.[4] Moreover, a notable increase in toxicity is observed during the transition from phosphate to methylphosphonate (or corresponding pyrophosphonate) derivatives.[43]

Many countries have sought alternatives to G-gases, such as CWAs that are equally toxic but easier to produce. Esters of phosphoric and methyl phosphonic acids are just one of many groups of "alternative CWAs." An important feature of this group of candidate chemical agents is their lack of typical G- and V-type agent pharmacophore groups such as -CN, -F, or thiocholine. Corrosive and toxic fluorine compounds are not necessary for their production, and the industrial process does not require equipment made of expensive silver and nickel alloys. Phosphate precursors are often employed in the manufacture of a range of items, including pesticides, plasticizers, and flame retardants.

The production of some phosphates and methylphosphonates can be easily organized in agrochemical or pharmaceutical plants since they were used as insecticides and ophthalmic drugs until the end of the 20th century. In the '40s, eye drops with paraoxon, which was synthesized by German chemist G. Schrader, and diisopropyl fluorophosphate (DFP), produced by British chemist B.C. Saunders were used to treat glaucoma. Both Schrader and Saunders were developing chemical weapons for their countries during World War II.

In the USSR, the eye drops Fosarbin, Phosphacol, and Armin[74], were developed under the direction of A. E. Arbuzov, was used for the treatment of glaucoma. All of these pharmaceutical drugs are highly toxic. A. E.  Arbuzov is well known as the founder of the Kazan school of organophosphorus chemistry; it is less well known that he headed the work on the development of nerve agents during World War II and in the postwar period. In 1943, his group, independently of German chemists, synthesized sarin and 18 of its homologs.[44] As usual, in the USSR, the development of organophosphorus pesticides, drugs, and CWAs was carried out by the same research institutes.

Head of the Kazan School of Organic Chemists A. Arbuzov (1877–1968) and his two disciples G. Kamay (1901–1970) and A. Razumov (1897–1987) synthesized sarin and its homologues at the end of 1943.

The early days of organophosphate pesticides saw the development of extremely hazardous compounds like TEPP, Mevinphos, and Paraoxon. These chemicals were quickly withdrawn from production due to their severe toxicity to warm-blooded organisms, including humans. Parathion, relatively less dangerous than its predecessors, remained commercially available in Europe even into the early 2000s. In the Soviet Union, Parathion (known locally as Thiofos) was manufactured from 1950 to 1972 at the Volgograd Chemical Plant, notably using some equipment that had previously been used in the production of the nerve agent sarin.[41]

Phosphate esters

In Great Britain, toxicological studies on TEPP (Tetraethyl pyrophosphate) were carried out during World War II. TEPP had a significant advantage over hydrocyanic acid and phosgene, as it could cause lethal poisoning not only by inhalation but also by skin contact. The estimated lethal cutaneous dose for humans ranges from 350 to 600 milligrams.[12] However, further research has shown that TEPP hydrolyzes quickly and is unsuitable for military applications.

In 1943, the first organophosphorus contact insecticide based on TEPP, Bladan, was introduced by the German company Farbenfabriken Bayer, as a cheaper alternative to nicotine.[28]

TEPP LD50 — 0.71 mg/kg (mouse, ip) (X0.7 sarin)[71]	Fosarbin LD50 — 0.45 mg/kg (mouse, sc)[21]	Seleno-TEPP LD50 — 0.48 mg/kg (mouse, ip) (X0.9 sarin)[20]	Thio-TEPP LD50 — 0.056 mg/kg (mouse, ip) (X7,5 sarin)[20]

Data presented by scientists from East Germany suggest that Seleno-TEPP is highly toxic, while Thio-TEPP toxicity is comparable to V-type nerve agents. However, there is no supporting evidence for these findings.[20]

During World War II, German chemist G. Schrader synthesised a very powerful contact insecticide, Paraoxon (Fosfakol, E-600). Experiments conducted at Edgewood Arsenal in 1958 showed that the inhalation toxicity of paraoxon aerosol was three times less than that of sarin vapor.[23] Despite high toxicity to warm-blooded animals and humans, the relatively low volatility make paraoxon unsuitable as a chemical warfare agent.[28]

Chemically pure Paraoxon is a tasteless and odorless liquid, is highly resistant to hydrolysis and penetrates well through the skin.[23] These properties make it a nearly perfect sabotage poison. In the '80s, South African secret police tried to use the paraoxon to assassinate opponents of apartheid. The poison was planned to be spiked into lipstick, roller deodorants, cigarettes, drinks, and apply to underwear. Police officers were instructed on how to correctly apply paraoxone to clothing:

The results of research at the U.S. Chemical Warfare Laboratory in 1962 with substance VX showed that the skin of the head, neck, and groin area is best suited for the application of organophosphorus CWAs, since it is from these areas of the body that the most complete and rapid entry of the poison into the body occurs.[18]

In 1989 apartheid police, using Paraoxon, attempted to assassinate Frank Chikane, then general-secretary of the South African Council of Churches. The poison was applied to underwear while he was being screened at the Johannesburg airport. F. Chikane suffered from vomiting, loss of muscle control, and acute respiratory problems.[17] It was only by sheer luck that the FBI discovered the paraoxon metabolites in urine while visiting the USA.[24]

It is hypothesized that the paraoxon used in the poisoning of F. Chikane was obtained through oxidation of parathion, a commercially available but less toxic thiono analog.[18] An identical reaction occurs within the human organism during parathion poisoning.

Parathion (E-605)	Paraoxon (E-600)

During Bush War in Rhodesia (1965–1980), the Rhodesian Central Intelligence Organization contaminated the clothing of insurgents with parathion. The poison-treated uniforms were distributed among the guerrillas in various ways. Parathion has been applied to clothing in the crotch and under the armpits.[26] In 2020, the Russian Federal Security Service (FSB) used a similar method to poison Russian opposition activist A. Navalny, The organophosphorus chemical agent A-262 was applied to the inner surface of the underwear.[39]

In 2015, Russian intelligence agencies attempted to poison Bulgarian businessman Emilian Gebrev. Analyses conducted at the Finnish research institute Verifin were unable to detect the active substance. However, metabolites found in Gebrev's blood and urine suggested the possible breakdown products of highly toxic pesticides, with Parathion being the most likely candidate.[75] After the poisoning of Sergei and Yulia Skripal in 2018 — an operation involving the same agents of Federal Security Service (FSB) as in Bulgaria — it became clear that Russian intelligence services were not using low-toxicity pesticides but rather far more lethal agents from the "Novichok" group.

The highly toxic insecticide Mevinphos (Fosdrin) was first synthesized by German chemists during World War II, but the method of production was not patented. This could have been for two reasons: the view of German toxicologists that the limit of mammalian toxicity for safe employment in plant protection corresponds to a LD₅₀ of the order of 5 mg/kg![25] or it was included in a secret list of 200 potential chemical agents.[26]

Paraoxon / Phosphacol LD50 — 0.6 mg/kg (mice, ip)[20] (X0.67 sarin)[76]	cis-Mevinphos LD50 — 2,0 mg/kg (mice, ip) (X0.25 sarin)[71]	Ro 3-0346 LD50 — 0.14 mg/kg (mice, i/v)[4] (0.8X sarin)[76]	Ro 3-0422 LD50 — 0.02 mg/kg (mice, i/v)[1,4] (X5 sarin)[76]

In 1954, K. Andrews synthesized 3-pyridine and 3-quinoline esters of diethylphosphic acid Ro 3-0346 and Ro 3-0422.[4] More Active Ro 3-0422 «to possess an anticholinesterase potency greater than that of any other organic phosphate so far described», however, it was a crystalline substance, rapidly hydrolysed and therefore unsuitable as a chemical agent.[1] the tertiary base Ro 3-0346 was a colorless oil; however, it demonstrated lower toxicity.[4]

In 1953, British chemists synthesized the highly toxic pesticide Amiton, which became the first representative of a new class of nerve agents V-series compounds.[42] In the USSR, N.N. Godovikov from the Institute of Organoelement Compounds (INEOS) conducted the first synthesis of amiton. He later joined one of the Soviet V-series research groups led by academician M.I. Kabachnik.

Amiton (VG)	GT-165	Compounds Godovikov

Besides amiton, N.N. Godovikov also studied its quaternary ammonium (GT-165) and cyclic analogs.[38]

Methylphosphonate esters

It was Schrader who first noticed that “phosphonic acid esters are significantly more toxic than the corresponding phosphoric acid esters. In many cases, the toxicity of phosphonic acid esters is about an order of magnitude greater than that of the corresponding phosphoric acid ester”.[28]

Pyrophosphonate analogues of TEPP were synthesized in the 1950s by the brilliant British chemist A. H. Ford-Moore from the Chemical Defence Experimental Establishment (CDEE). These compounds exhibit high toxicity upon inhalation and dermal exposure, though they are less potent than sarin. "Sarin anhydride" similarly does not demonstrate pronounced toxicity.[40]

Diisopropyl dimethylpyrophosphonate	Methylphosphonofluoridic anhydride

The substitution of one oxyalkyl group with a methyl group, as observed in chemical agents VP (phosphonic analog of Ro3-0346) and EA 1576 (phosphonic analog of Mevinphos), predictably resulted in increased toxicity. Both compounds were synthesized by T.P. Dawson and C.E. Williamson in the mid-1950s.[31,32]

In 1954–1955, the U.S. Chemical Corps conducted experiments to study the toxicity of a compound codenamed VP.[31] This substance belongs to the group of V-gases, but contains in its structure a 3-hydroxypyridine fragment rather than a thiocholine fragment –S–C–C–N< as V-gases. Described in classified documents as a "a new percutaneous supertoxic material," candidate chemical agent VP was inferior to sarin in inhalation toxicity but 420 times more toxic when applied to the skin.[2] It was also well suited for long-term storage because it did not react with steel, even at elevated temperatures. The only downside of the agent GP was its inability to penetrate uniforms, resulting in its rejection.[13]

At the same time as the agent EA 1576, was being investigated at the Edgewood Arsenal[3], but there is very little information about this substance. More toxic isomer of EA 1576 has the “trans” configuration and the less toxic the “cis” form.[32] In the USSR, this group was studied by the famous military chemist L. Soborovsky, who synthesized trifluoromethyl analogs of EA 1576.[19] Exchanging methyl group for CF₃ is to enhance the desired biological or physical properties of a compound without making significant changes in chemical structure.[29] L. Soborovsky led a project to develop Soviet V-gas, and in 1960, he received the Lenin Prize for developing an industrial method of producing sarin.[30]

EA 1576	Soborowsky's substance	Agent VP LD50 — 0.036 mg/kg (rabbits, i/v)[2] (X0.4 sarin)	7-MEPQ LD50 — 0.03 mg/kg (mice, i/v)[6] (X3 sarin)[71]

In 1986, scientists from the Israel Institute for Biological Research (IIBR) published an article on the synthesis of a highly toxic anticholinesterase agent 7-MEPQ[5], which had a toxicity comparable to Ro 3-0422.[6]

Armine (Armine) was synthesized in 1957 at the Kazan Institute of Chemistry and Technology. Its creator was A. Razumov, one of the developers of Soviet sarin. Armine was utilized in the USSR as a treatment for glaucoma; however, it also possessed characteristics typical of chemical warfare agents: it was a liquid resistant to hydrolysis and high temperatures, maintaining its activity during prolonged storage. In terms of toxicity, Armine is 2.5–5 times less potent than sarin, but its production process is less complex.[74]

A study of 12 homologs of Armine, conducted jointly with the US Army Chemical Center in 1964, showed that the phenylbutyl derivative has the highest activity.[15] The O-isopropyl homolog of Armine is two times more toxic than, Armine[15] and the isobutyl homolog is five times more toxic.[20]

Armine LD50 — 0.81 mg/kg (mice, ip) (X0.6 sarin)[20]	O-Isobutylarmine LD50 — 0.16 mg/kg (mice, ip) (X2.6 sarin)[20]

In Yugoslavia, at a chemical plant near Mostar during 1980-1984, research was conducted on organophosphorus warfare agents, including Armine. Laboratory-scale production of Armine was also established at this facility.[14] In Yugoslav scientific studies investigating antidotes for nerve agents, Armine was utilized as a reference anticholinesterase compound.

Apart from their simpler production process, all compounds examined in this section do not possess significant advantages over G-agents, and particularly V-agents. Following the inclusion of their precursors in the list of controlled substances, they have completely lost their military significance.