Peroxyimidic acid

A process that is effective for epoxidation and avoids acidic conditions involves reaction of an alkene, a nitrile, and hydrogen peroxide.82 The nitrile and hydrogen peroxide react, forming a peroxyimidic acid, which epoxidizes the alkene, by a mechanism similar to that for peroxyacids. An important contribution to the reactivity of the peroxyimidic acid comes from the formation of the stable amide carbonyl group. 

These ketones can also be used in kinetic resolutions.107 The carbohydrate-derived ketones have been used in conjunction with acetonitrile and H202. The reactions are believed to proceed through dioxiranes generated by a catalytic cycle involving a peroxyimidic acid.108... 

Peroxyacetic acid generated in situ from sodium perborate and glacial acetic acid has been used for oxime to nitro group conversion. Peroxyimidic acid generated from acetonitrile and hydrogen peroxide has found similar use. An Mo(IV) peroxy complex has been reported for the oxidation of both ketoximes and aldoximes. 

The researchers proposed mechanisms for the reaction which involved either in situ formation of performic acid or peroxyimidic acid (Figure 2.45). 

An efficient, high-yield synthesis of A-alkyl and A-aryl oxaziridine by oxidation of aldimines with buffered Oxone (potassium peroxymonosulfate) has been introduced by Hajipour and Pyne (Equation (47)) <92JCR(S)388>. Oxidation of the aldimine is accomplished in aqueous NaHC03/ acetonitrile or acetone affording the oxaziridine within 15-30 minutes in excellent yield (95-98%). The active oxidizing species in acetone and acetonitrile are thought to be dimethyldioxirane and peroxyimidic acid [MeC(OOH)=NH), respectively. 

The addition of a perhydroxyl anion to a nitrile group generates a peroxyimidic acid (Figure 16.9). This highly reactive and extremely unstable compound does not allow isolation, but is safe to handle when prepared in situ. In chemical reactions, peroxyimidic acids differ significantly in reactivity and selectivity from aromatic or aliphatic peroxycarboxylic acids. 

FI G U RE 16.15 pKa values of peracids. Data for peroxyimidic acids (from Eilers, B., Mechanistische Untersu-chungen zur nucleophilen Aktivierung von Wasserstoffperoxid mit Hilfe der Sigulettsauerstoff-Phosphoreszenz, Thesis, University Frankfurt am Main, 2002.)... 

To improve the cold-water performance of a European HDPD, the addition of a nitrile quat is most beneficial. Whereas the peroxyimidic acid formed is active at 20-40 C, the peroxyacetic acid covers the high temperature rauge of 50-90 C. The detergent can therefore be claimed as suitable for use at all tempaatures rauging from 20 to 90°C. In such mixtures, synergistic benefits arise mainly between 40 and fiO C. 

While Oxone has been commonly used to generate dioxiranes from ketones, Shi s studies have shown that epoxidation with ketone 2 or 5c can be carried out with a nitrile and H2O2 as the primary oxidant, giving high enantioselectivity for a variety of olefins. Peroxyimidic acid 55 is likely to be the active oxidant that reacts with the ketone to form dioxirane 10. Mixed solvents, such as CH3CN-EtOH-CH2Cl2, improve the conversions for substrates with poor solubilities. This epoxidation system is mild and provides conversion and enantioselectivity similar to that using Oxone as oxidant. 

Dioxiranes for alkene epoxidation may be prepared in situ from a catalytic amount of a ketone and Oxone (potassium peroxymonosulfate triple salt). )V,)V-Dimethyl-and A, A -dibenzylalloxans (20a) and (20b) (Figure 3) have been prepared and used as novel dioxirane catalysts for the epoxidation of a range of di- and tri-substituted alkenes in good to excellent yield. H2O2 (rather than the usual Oxone) has been successfully used as primary oxidant in asymmetric epoxidations with Shi s fructose-derived ketone (21) in acetonitrile. The ketone is converted into the dioxirane, which is responsible for epoxidation and the active oxidant responsible for dioxirane formation is proposed to be peroxyimidic acid formed by combination of H2O2 with acetonitrile. ... 

Scheme 1.5 Formation of Peroxyimidic acid, the reactive intermediate in the Payne epoxidation of alkenes...

Shi has also recently reported asymmetric epoxidation mediated by alkahne hydrogen peroxide [44,45]. High yields and ees were obtained under these reaction conditions with up to 95% ee for 1-phenylcyclohexene oxide using the original fructose derived catalyst 10. Peroxyimidic acid 11 is postulated to be the active oxidant (Scheme 1.15). 

Scheme 1.15 Postulated formation of peroxyimidic acid in the asymmetric epoxidation reaction using hydrogen peroxide as primary oxidant...

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