Hydroxyphosphoranes as intermediates in the isomerisation of o-hydroxyphenyl phosphinates
Abstract
The interconversions of isomeric esters of 3-methylcatechol and 1-hydroxyphosphetan 1-oxides (phosphetinic acids) have been studied by variable temperature 1H n.m.r. spectroscopy (ΔG*ca. 17–18 kcal mol–1) and the intermediate hydroxyspirophosphoranes trapped as methoxyspirophosphoranes on treatment with diazomethane. The more stable ester from 3-methylcatechol and diphenylphosphinic acid has been obtained essentially pure; it isomerises to the less stable isomer with ΔG* 24.4 ± 0.2 kcal mol–1. No methoxyphosphorane was detected on treatment with diazomethane. The corresponding phosphinothioate esters have higher free energies of activation for interconversion and the intermediate hydrothiospirophosphoranes are not trapped with diazomethane. P-Hydroxy-2,2′-spirobi-(1,3,2-benzodioxaphosphole) has been obtained in solution in THF by treatment of the corresponding P-chloro-compound with one mole equivalent of water. With chlorotrimethylsilane and triethylamine it gave the P-trimethylsilyloxyphosphorane; with o-hydroxyphenyl o-phenylene phosphite (36) in DMF it gave the tris-(o-phenylene) phosphate anion (37) and o-phenylene phosphonate.