Acylphosphonic acids and methyl hydrogen acylphosphonates: Physical and chemical properties and theoretical calculations

Acylphosphonic acids (5) and methyl hydrogen acylphosphonates (3) were synthesized by di- and mono-demethylation of dimethyl acylphosphonates (1). Spectroscopic data (i.r., ³¹P and ¹H n.m.r.) are reported for these types of compounds for the first time. Examination of their hydrolytic stability under acidic and basic conditions revealed that except for methyl hydrogen acylphosphonates (3) that are unstable under highly alkaline pH conditions, the C-P bond in these types of compounds is stable in most cases. Nucleophilic reagents, e.g. amines, borohydride, or hydroxylamine react with the carbonyl group of ionized acylphosphonates with the preservation of the C-P bond, to yield α-imino-, α-hydroxy-, or α-oxyimino- alkylphosphononate anions, respectively. Semi-empirical quantum mechanical (MNDO/H) calculations were performed on benzoylphosphonic acid (5c) and on the esters and anions derived from it, as representatives of their classes, in order to assess bond lengths and preferred conformations, and to estimate charges on the carbonyl and phosphoryl groups. Calculations show that for both neutral and ionized (anions) compounds free rotation around the C-P bond is expected due to the low energy barriers.