The reactivity of organophosphorus compounds. Part XXVII. Reactions of alkyl hydrogen alkylphosphonates with p-nitrobenzonitrile oxide: anchimerically assisted P–O fission in acidic hydrolysis of the resulting α-hydroxyimino-p-nitrobenzyl alkylphosphonates

Abstract

p-Nitrobenzonitrile oxide reacts readily with alkyl hydrogen alkylphosphonates (I; R¹= Me, R²= Me, Et, Pr, Prⁱ, Bu^tCH₂, or Bu^tCHMe) and (I; R¹= Et, Bu^t, or Ph; R²= Et), diethyl hydrogen phosphate, and hydrogen diethylphosphinate to give the corresponding 1:1-adducts, alkyl α-hydroxyimino-p-nitrobenzyl alkylphosphonates (II; R¹= Me; R²= Me, Et, Pr, Prⁱ, Bu^tCH₂, or Bu^tCHMe) and (II; R¹= Et, Bu^t or Ph; R²= Et), diethyl α-hydroxyimino-p-nitrobenzyl phosphate (II; R¹= EtO, R²= Et), and α-hydroxyimino-p-nitrobenzyl diethyl-phosphinate (III), respectively. Except for the last case, which is stable in acid, the adducts undergo very fast, anchimerically assisted, P–O fission at pH 2–3·5 in dilute aqueous dioxan (ca. 10⁷ faster, as shown by a kinetic study, than corresponding hydrolysis of simple esters which undergo alkyl–oxygen fission under similar conditions). Transesterification, also very fast, proceeds similarly, while the O-methyl derivative of (II; R¹= Me; R²= Bu^tCH₂) is stable at pH 2–4·5. The results point to a novel neighbouring-group acceleration involving the proton of the oxime group in (II).