Clayton-Jensen chlorophosphonation

What is Clayton-Jensen chlorophosphonation?

Initially, three research groups, namely Clayton and Jensen, Soborovsky, Zinovyev, and Englin in pre-Soviet Union, and Graf in Germany, studied this reaction almost simultaneously. However, Clayton and Jensen were the first to report this reaction in 1948, while Graf might have been the first to study it but published his results last.

Clayton-Jensen chlorophosphonation reaction involves the direct chlorophosphonation of aliphatic hydrocarbons with phosphorus trichloride and oxygen (benzene is unreactive).

Clayton-Jensen chlorophosphonation
Clayton-Jensen chlorophosphonation

Due to contamination by the oxidation of phosphorus trichloride to phosphorus oxychloride, the yield of phosphonyl chloride is usually around 25 % based on the amount of PCl3 used. Although a large amount of catalysts have been tested for this reaction, none of them seem to have any apparent catalytic effect, and some substances, including sulfuric acid H2SO4, iodine I2, iron powder, and boron trifluoride BF3, actually inhibit the reaction.

Additionally, light does not significantly affect the yield. The steric effect is evident in this reaction, as shown by the 13 % yield obtained from the toluene reaction, the 2.3 % yield obtained from the diphenylmethane reaction, and the lack of product from the triphenylmethane reaction. Furthermore, this reaction only works with phosphorus trichloride PCl3 and not with other phosphorus halides such as PBr3 and PI3. It is interesting to note that a higher yield is obtained when more hydrocarbon is used. For example, the yield increases to the 20-29 % range when the molar ratio of cyclohexane to PCl3 increases from 0.5 to 2.0. However, there is no noticeable increase in yield when more cyclohexane is used beyond that point. When sufficient pure reagents are used, the reaction is not spontaneous, and the apparent spontaneity of the reaction is due to the presence of trace amounts of an initiator in the hydrocarbon. Furthermore, there is no significant change in yield when the reaction is carried out in a temperature range of -40 to 70 ºC.

Clayton-Jensen chlorophosphonation is commonly used to produce alkyl phosphonyl chlorides, which serve as intermediates for the synthesis of alkyl phosphonic acid and its corresponding esters.