A study on the acidic hydrolysis of cyclic phosphinates: 1-Alkoxy-3-phospholene 1-oxides, 1-ethoxy-3-methylphospholane 1-oxide, and 1-ethoxy-3-methyl-1,2,3,4,5,6-hexahydrophosphinine 1-oxide

The hydrochloric acid-catalyzed hydrolysis of phosphinates was studied on 1-alkoxy-3-phospholene 1-oxides, 1-ethoxy-3-methylphospholane 1-oxide, and 1-ethoxy-3-methyl-1,2,3,4,5,6-hexahydrophosphinine 1-oxide as the model compounds. Under the conditions applied, the isomerization of the 3-phospholene moiety to the 2-phospholene ring also occurred leading to mixtures of the corresponding 1-hydroxy-3-phospholene oxide and 1-hydroxy-2-phospholene oxide. According to our optimized method, using 3 equivalents (0.5 mL) of concentrated hydrochloric acid in 1 mL of water per ca. 2 mmol of the substrate at reflux, the completion required 3-10 hour. The hydrolyses were characterized by pseudo-first-order rate constants and the isomerizations by rate constants. The application of p-toluenesulfonic acid under microwave irradiation at 140°C in the hydrolysis of 1-alkoxy-3-methyl-3-phospholene oxides associated with reaction times of 1-3 hour. The reactivity order of the 5- and 6-ring phosphinates in hydrolysis was set up.