[943-39-5] · C7H5NO5 · p-Nitroperbenzoic acid · (MW 183.13)
(electrophilic reagent which delivers oxygen to alkenes2)
Alternate Names: p-nitroperoxybenzoic acid; PNPBA.
Physical Data: white crystalline solid; mp 136-137 °C.
Solubility: sol THF, ether, CHCl3, benzene.
Form Supplied in: white powder, 80% purity.
Analysis of Reagent Purity: iodometric titration.
Preparative Method: the reaction between Na2O2 and p-Nitrobenzoyl Chloride in THF in the presence of small quantities of water below -5 °C furnishes the sodium salt of PNPBA. p,p´-Dinitrobenzoyl peroxide and p-nitrobenzoic acid are removed from the reaction mixture during workup. PNPBA (purity 94-96%) is obtained in 85% yield (0.1 mol scale).2
Purification: recrystallization from ether-petroleum ether.
Handling, Storage, and Precautions: at room temperature, PNPBA is stable for long periods. It should be stored in polyethylene containers in a refrigerator. Since PNPBA is a peroxide and potentially explosive, care should be exercised when carrying out reactions with it; during workup, check for the presence of peroxide before evaporating solvent.
Due to the presence of the electron withdrawing NO2 group, PNPBA epoxidizes alkenes about ten times more rapidly than Perbenzoic Acid (PBA); m-Chloroperbenzoic Acid is only slightly more reactive than PBA. trans-Stilbene is thus oxidized to trans-stilbene oxide (eq 1).3 A solution of 5.4 g of stilbene (30 mmol) in 60 mL of CHCl3 is added dropwise with stirring to a suspension of 7.5 g (33 mmol) of 80% PNPBA in 100 mL of CHCl3; the reaction mixture is kept at rt for 24 h with occasional shaking, and filtered. The filtrate is washed with 10% aq Na2CO3, dried, and the solvent is evaporated. The residue is recrystallized from petroleum ether to give 5.5 g (93%) of the epoxide.
The monosubstituted alkene (1) reacts with PNPBA (22 h, rt) to furnish the epoxidation product in 75% yield.4 The epoxide (2), an intermediate for the synthesis of cyclobutanone, has been prepared in 90% yield by treating methylenecyclopropane with PNPBA in CH2Cl2 at -10 °C.5
Reaction of the a,b-unsaturated ketone (3) with PNPBA in benzene (24 h, rt) furnishes the 5b,10b-epoxide in 91% yield.6 This epoxide cannot be prepared from (3) using m-CPBA or H2O2/MeOH/NaOH. Reaction of the alkene (4) with PNPBA in CHCl3 (reflux, 3 days) in the presence of the radical inhibitor 4,4´-thiobis(6-t-butyl-3-methylphenol) furnishes the 4a,5a-epoxide in 30% yield.7 m-CPBA and benzonitrile/H2O2 are not useful reagents for the epoxidation of (4); TBHP/Mo(CO)6 furnishes the epoxide in low yields.
Reaction of alicyclic hydrocarbons with PNPBA results in regio- and stereoselective hydroxylation (eq 2).8 Tertiary alcohols are obtained with high selectivity (>97%); hydroxylation takes place with retention (>98%) of configuration. Dioxiranes are excellent reagents for the hydroxylation of alicyclic hydrocarbons under mild conditions (see Dimethyldioxirane).9
For an example, see eq 3.10
A. Somasekar Rao & H. Rama Mohan
Indian Institute of Chemical Technology, Hyderabad, India