α-Oxyiminophosphonates: chemical and physical properties. Reactions, theoretical calculations, and X-ray crystal structures of (E) and (Z)-dimethyl α-hydroxyiminobenzylphosphonates
Dialkyl α-oxyiminoalkylphosphonates, obtained by treatment of dialkyl acylphosphonates with hydroxylamine or methoxylamine, are mostly mixtures of E and Z isomers. Structural assignments of the oximes were based on X-ray crystallographic analysis of representative compounds: (E)- and (Z)-dimethyl α-hydroxyiminobenzylphosphonates [(E)- and (Z)-(1a)]. The 31P n.m.r. chemical shifts of the E isomers always appear at lower field than those of the corresponding Z isomers. Thermal fragmentation of (1a) affords benzonitrile and dimethyl hydrogen phosphate, with (Z)-(1a) reacting faster than the E isomer. α-Oxyiminophosphonates undergo E⇌Z isomerization catalysed either by acid or by base under certain conditions, the E isomer being the thermodynamically more stable one. An E+Z mixture of dialkyl α-oxyiminophosphonates can be mono-de-alkylated by non-basic nucleophiles (e.g. NaI) to afford monoalkyl oxyiminophosphonates of unchanged isomeric composition. The geometrical isomers of α-hydroxyiminophosphonates differ in their behaviour under basic conditions. While treatment of (E)-(1a) with NaOH in boiling methanol leads, by mono-de-alkylation, to sodium methyl α-hydroxyiminobenzylphosphonate [(E)-(2a)], under the same conditions (Z)-(1a) undergoes fragmentation, by C–P bond cleavage, to benzonitrile and dimethyl hydrogen phosphate. Control experiments established that the fragmentation of (Z)-α-hydroxyiminophosphonates involves an intramolecular attack on the phosphorus atom by the ionized Z oriented oxime oxygen. Similar differences in behaviour are noted between the isomers of the monoanions of α-hydroxyiminophosphonates. MNDO/H Calculations demonstrate the feasibility of forming internal hydrogen bonds in Z isomers, and their possible contribution to conformational preferences. Single-crystal X-ray diffraction studies of (E)- and (Z)-(1a), and (E)-(2b), clearly identified the geometric isomers and correlated them with the 1H and 31P n.m.r. resonances.