NMR of terminal oxygen. 9—17O NMR of the PO ‘double bond’: Phosphine oxides, phosphinates, phosphonates, acylphosphonates and related compounds

The 17O NMR spectra of phosphorus‐bound oxygen atoms in secondary phosphine oxides (R2HPO, 1), alkyl phosphinates [RHP(O)OR′, 2 and R2P(O)OR′, 3], dialkyl phosphonates [RP(O)(OR′)2, 4], dialkyl acylphosphonates [RCOP(O)(OR′)2, 5] and some related compounds were measured. The signals of terminal O appear at high field, only slightly downfield from those of bridge O. Compared with carbonyl O, terminal PO shows lower sensitivity to structural variations, notably to geminal groups (with the exception of Cl and Br), and a very low sensitivity to arene ring substituents. The difference is discussed in terms of π‐bond character; the carbonyl oxygen of RCOP(O)OR′)2 is sensitive to variations in structure.