Through the Rearrangement of Aryl Phosphates

The rearrangement of the aryl phosphate esters 254 into esters 255 of (2-hydroxyaryl)-phosphonic acids under the influence of a strongly basic agent was discovered by Melvin who employed Ida and independently, and almost by accident, by Cambie and Palmer, who treated the phosphate ester 258 with BuLi and so obtained the phosphonate ester 259. 

Dhawan and Redmore have explored the scope of the isomerization. Thus, the (2-hydroxyaryl)phosphonate ester 255 may itself be 0-phosphorylated and a second rearrangement carried out, with the resultant formation of 257 from 256 in addition, the rearrangement of 260 into 261 and that of 262 into 263 were carried out. The diaryl phosphates 264 rearrange to the bis(2-hydroxyaryl)phosphinates 265 whilst the use of mixed alkyl aryl phenylphosphonates (266) affords mixed diarylphosphinates 

The scope of the reaction has been extended to include sulphur-containing substrates (S-aryl phosphorothiolates). When treated with Ida, diethyl and diisopropyl AS-phenyl phos-phorothiolate yield diethyl and diisopropyl (2-mercaptophenyl)phosphonates in yields of 16 and 60%, respectively  

Finally, it may be noted that the treatment of dialkyl 2-chlorophenyl phosphates with metallic sodium and the formation of the Grignard reagent from dialkyl 2-bromophenyl phosphates both yield dialkyl (2-hydroxyphenyl)phosphonates after work-up in the case of dialkyl 2,4-dibromophenyl phosphate, the elimination of bromine is restricted to the 2-position .  

Two important syntheses of aromatic phosphonic and phosphinic acids are based on classical aromatic chemistry. The use of aryldiazonium salts has the distinct advantage over Friedel-Crafts reactions in that the entering phosphorus is placed in a position of certainty and, additionally, only one step is required in the final stage, whereas two are sometimes needed in the Friedel-Crafts procedure. 

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