[13598-52-2] · H3O5P · Monoperoxyphosphoric Acid · (MW 114.00)
Alternate Names: peroxymonophosphoric acid; PMPA.
Solubility: sol acetonitrile, dioxane.
Form Supplied in: not available commercially.
Analysis of Reagent Purity: assay using iodometry.6
Preparative Methods: (a) to a stirred suspension of 14.2 g of Phosphorus(V) Oxide in 60 mL of acetonitrile at 0 °C, 10 g of 70% Hydrogen Peroxide (caution!) in 20 mL of acetonitrile was added over a period of 45 min. The mixture was stirred at rt for 16 h.6 (b) 87% H2O2 is used;7 the latter is the more common procedure.
Handling, Storage, and Precautions: should be used immediately after preparation; reagent stored at 5 °C for 26 days lost 30% of active oxygen. Since H3PO5 is a peroxy acid, care should be exercised while carrying out the reactions; during workup, check for peroxides before solvent removal. This reagent should be handled in a fume hood.
Epoxides which are not readily opened by acids can be prepared by reacting alkenes with H3PO5. trans-Stilbene oxide (1) has been prepared in 74% yield through epoxidation of trans-stilbene with H3PO5 in dioxane.2 Oxidation of diphenylacetylene with H3PO5 at rt in acetonitrile furnishes the diketone (2) in 48% yield;3 this reaction can be carried out with Trifluoroperacetic Acid (TFPAA) but not with the normal peroxycarboxylic acids.
H3PO5 is an effective reagent for aromatic hydroxylation, the reactivity being comparable to that of TFPAA; oxidation of mesitylene in acetonitrile (25 °C, 4 h) furnishes mesitol in 77% yield.4
Reaction of acetophenone (3) with H3PO5 in acetonitrile at 30 °C furnished the aryl acetate (6) in 91% yield. The ketones (4) and (5) were also oxidized to (7) and (8) in yields of 95% and 98%, respectively; the corresponding methyl benzoates were not obtained. Baeyer-Villiger reaction of acetophenones with H3PO5 has nearly 100-fold higher rate than that with peroxycarboxylic acids such as Perbenzoic Acid.8
Baeyer-Villiger reaction of (9) with H3PO5 furnished the phenol (10) (eq 1).6 However, the reagents Peracetic Acid or TFPAA furnished the carboxylic acid (11).
Oxidation of N,N-dimethylaniline in aqueous acetonitrile at 30 °C afforded the N-oxide in 82% yield.5 Oxidation of THF with H3PO5 in acetonitrile or without solvent at 25 °C furnishes g-butyrolactone in 40-45% yield.9 This oxidation cannot be performed with percarboxylic acids such as MeCO3H.
A. Somasekar Rao & H. Rama Mohan
Indian Institute of Chemical Technology, Hyderabad, India