[93-59-4] · C7H6O3 · Perbenzoic Acid · (MW 138.13)
Alternate Name: peroxybenzoic acid; PBA.
Physical Data: mp 41-42 °C.
Solubility: sol CHCl3, CH2Cl2, benzene, ethyl acetate, ether; slightly sol water.
Form Supplied in: long white needles, but usually handled in solution; not available commercially.
Preparative Methods: Sodium Methoxide reacts readily with Dibenzoyl Peroxide (1) (eq 1); complete experimental details are available for preparing perbenzoic acid based on this reaction.4 Alkaline perhydrolysis (HOO-) of (1) in aqueous organic solvents has been employed for preparing PBA in 90% yields.5 PBA has been prepared in 85-90% yield by adding 70% Hydrogen Peroxide (Caution!) to a suspension of benzoic acid (2) in methanesulfonic acid (eq 2).6 PBA has been prepared in 90% yield by passing oxygen containing catalytic quantities of ozone through a solution of benzaldehyde in ethyl acetate.7
Purification: PBA can be purified by recrystallization at -20 °C from a 3:1 mixture of petroleum ether-diethyl ether.
Handling, Storage, and Precautions: analytically pure PBA can be stored for long periods in a refrigerator without significant loss of active oxygen. Since PBA is a peroxide and potentially explosive, care must be exercised in carrying out the reactions with it; during workup, check for peroxides before evaporating the solvent. This reagent should be handled in a fume hood.
This reagent oxidizes simple alkenes, including alkene substrates incorporating a variety of functional groups (ethers, alcohols, esters, etc.), amines, and sulfides. Ketones undergo oxygen insertion reactions (Baeyer-Villiger oxidation).
PBA, like other organic peracids, reacts with alkenes readily under mild conditions to furnish epoxides in good yields (see, for example m-Chloroperbenzoic Acid).8 Styrene is thus oxidized to styrene oxide (3) (eq 3).9 To PBA (0.33 mol) in 500 mL of CHCl3 is added at 0 °C, with stirring, 0.3 mol of styrene. The reaction mixture is kept at 0 °C for 24 h. Assay of PBA at this stage will show that one equivalent of PBA has been consumed. The reaction mixture is shaken with aq 10% NaOH solution, water, and then dried (Na2SO4). The solvent is evaporated and the residue is fractionated using a Vigreux column to furnish the epoxide (3) in 70-75% yield.
Oxidation of the alkene (4) furnishes the epoxide (5) (eq 4).10 The disubstituted cis-alkene (6) gave in quantitative yield the corresponding cis-epoxide (7).11 Epoxidation of cycloheptene in CHCl3 (3.2% solution, 48 h, 0 °C) furnished cycloheptene oxide in 78% yield.12
Reaction of the diene (8), having two trisubstituted double bonds, with 1 equiv of PBA proceeded regio- and stereoselectively to give monoepoxide (9) (eq 5).13
The benzylic epoxide (10) has been obtained in 77% yield by epoxidizing 1-phenylcyclohexene with a CHCl3 solution of PBA at 0 °C.14 A pure sample of (10) was obtained by distilling the crude reaction product over powdered KOH; this epoxide is very sensitive to acids but is stable in basic medium. It has been observed that 7a,8a-epoxides of steroids and triterpenes are stable in basic media but these epoxides are readily cleaved even with traces of acids.15
Epoxidation of the diene (11) takes place regioselectively at the tetrasubstituted double bond to furnish the monoepoxide (12) in 61% yield.16 3b-Acetoxy-8a,9a-epoxyergostane was prepared in 85% yield by epoxidizing the alkene (13) using benzene as solvent.17
Epoxidation of 3-acetoxycholest-2-ene (14) in CHCl3 solution (-12 °C, 42 h) is stereoselective; the 2a,3a-epoxide is isolated in 80% yield.18 Epoxidation of the vinyl chloride (15) (CHCl3 solution, 12 h, rt) furnished the corresponding chloro epoxide in 68% yield.19
The sensitive epoxide (17) has been prepared from the enol ether (16) in 90% yield (eq 6).20 An ether solution of (16) and PBA is allowed to react for 30 s; benzoic acid is removed by passing the reaction mixture rapidly (30 s) through an alumina column.
Epoxidation of the a,b-unsaturated ketone pulegone (18) (5-10 °C, 24 h) is not stereoselective; a 2:1 mixture of a,b-epoxy ketones (19) and (20) is obtained in 86% yield.21
Hydroxy-directed stereoselective epoxidation has been observed with allylic cyclohexenols (eq 7).22
Fe2O3-catalyzed oxidation of ketones with molecular oxygen in the presence of benzaldehyde at rt gives the lactones efficiently (eq 8).23 The reacting species may be PBA or the radical PhCO3&bdot;. Baeyer-Villiger reaction of (-)-(21) furnishes the ester (-)-(22) in 85% yield with retention of configuration.24
Aromatic primary amines containing electron-withdrawing groups are oxidized to nitroso compounds.2 Oxidation of (23) thus furnishes the nitroso compound (24) in 85% yield. This reaction cannot be carried out with peracetic acid. The oxaziridine (25) has been prepared in 56-74% yield by reacting the corresponding aldimine with PBA.25 Episulfides are oxidized to episulfoxides.3 The episulfoxide (26) has been obtained in 77% yield by oxidizing the episulfide in CH2Cl2 at -20 °C.
In the epoxidation of alkenes which are moderately or highly reactive, the yields obtained with PBA are comparable to the yields obtained using m-Chloroperbenzoic Acid or Peracetic Acid. However, m-CPBA and MeCO3H are available commercially; PBA is not available commercially, but can be prepared conveniently in a short time. For the epoxidation of alkenes which react sluggishly (e.g. a,b-unsaturated esters) m-CPBA and Trifluoroperacetic Acid are preferred.
A. Somasekar Rao & H. Rama Mohan
Indian Institute of Chemical Technology, Hyderabad, India