Oxazirane

Thermal isomerization of 2-tert-butyl-3-phenyloxazirane in diethylcarbitol from 60-100 °C to iV-rerr-butylbenzalnitrone 

The energies of activation increase from about 21.5 to 34 kcal.mole and the heats of reaction vary from —11.84 kcal.mole to about +2.20 kcal.mole For aryl substituted amino-1,2,3-triazoles there is a good correlation between the logarithm of the rates of isomerization and Hammett s a value for substituents. 

In contrast thermolysis of substituted 1-acetoxy-benzotriazoles results in a carbon-to-oxygen migration of an alkyl group, viz. 

The thermal isomerization of substituted 5-amino-l,2,3,4-tetrazoles in ethylene glycol, viz. 

At somewhat higher temperature (180-185 °C) 5-aryltetrazoles undergo thermal decompositions viz. 

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Peracid oxidation of imines is the most general synthesis of oxaziranes (Section 5.08.4.1.1). Other peroxides and metal catalysis have also been employed. 

By means of perbenzoic acid oxidation, a bicyclic oxazirane (233,234) (152) is formed from 5,5-dimethyl-4-phenyl-/l -pyrroline (151). 

In the last 10 years many three-membered rings with two hetero atoms have been synthesized. Their formation occurs surprisingly smoothly with an ease comparable to that of the three-membered rings with one hetero atom. The oxaziranes (I), diaziridines (2), and diazirincs (3) have become, in a very short time, classes of compounds with an extensive literature. 

Preparation of Oxaziranes jrom SchiiJ s Bases and Peracids... 

Oxaziranes (1) are produced by the action of peracids on Schiff s bases fEq. (2)], a reaction first described by Krimm in patents. ... 

The preparation of oxaziranes is almost always carried out by reacting a solution of peracetic acid in a volatile inert solvent at temperatures near 0 0 with the Schiff s base. Without any significant variation in the reaction conditions, a large number of oxaziranes has thus been prepared. Table I gives a selection of the fifty or so known oxaziranes. 

The process can be still more simplified. It is not always necessary to use a pre-formed Schiff s base. Often it is sufficient to bring the carbonyl compound and the amine together in an inert solvent and to add the peracid to the mixture later, - In this way oxaziranes can be obtained in good yield even if the Schiff s base is unknown or can only be obtained in poor yield. For example, formaldehyde gives with aliphatic amines usually only trimers of the Schiff s bases (4). On the other hand, the synthesis of 2-cyclohexyl-oxazirane (5) from cyclohexylamine, formaldehyde, and peracetic acid proceeded in 66%... 

Compounds which contain two oxazirane rings are obtainable from Schiff s bases of glyoxal or terephthalic dialdehyde, e.g. (6). A bifunctional oxazirane is also obtained from ethylene diamine and cyclohexanone (7)d ... 

Isolated reports have appeared of the use of other sources of oxygen for the synthesis of oxaziranes. Belew and Person obtained an oxazirane (8) by the reaction of ozone on isobutylidene-icrf-butyl-... 

According to a report by Japanese authors, oxazirane rings are also formed by the action of silver oxide on perhydro-nitrogen heterocycles, e.g., decahydroquinoline (10) [Eq. (5)]. ... 

The light sensitivity of nitrones in solution was observed by Kamlet and Kaplan, " who postulated the formation of oxaziranes. Krohnke formulated the rearrangement caused by light on a nitrone by using an oxazirane as an intermediate. Splitter and Calvin successfully isolated the rearranged products (9, 11, 12) and identified them as known oxaziranes [Eq. (6)]. 

The rearrangement of phenyl- er -butyl nitrone to the isomeric oxazirane (9) occurred in 95% yield on irradiation in acetonitrile for 2 hr. Because 2-ieri-butyl-3-phenyloxazirane (9) can be reconverted into the more stable nitrone, the photochemical reaction involves the conversion of radiation energy into chemical energy. 

The same rearrangement was observed somewhat later by the irradiation of 5,5-dimethyl-A -pyrroline oxide (13)/ The product (11% yield) was shown to be identical with the oxazirane (14) synthesized from 5,5-dimethyl-A -pyrroline and hydrogen peroxide. 

The Preparation of Oxaziranes from Carbonyl Compounds and Derivatives of Hydroxylamine and Chloramine... 

Mixtures of benzaldehyde, iV-methylhydroxylamine-O-sulfonic acid and dilute sodium hydroxide react to give oxaziranes [Eq. (7)]. The 2-mcthyl-3-phenyloxazirane (15) obtained (40% yield) is iden-... 

A -Chloromethylamine attacks ketones in alkaline solution with formation of oxaziranes with cyclohexanone, compound 17 is produced in 50% yield. The reaction with aldehydes with zV-chloromethyl-amine yields predominantly acid amides. However, oxaziranes are also produced here as by-products. From benzaldehyde and A -chloro-methylamine, 2-raethyl-3-phenyloxazirane (15) was obtained in 10% yield. 

The oxaziranes are in the majority of cases distillable liquids with boiling points somewhat higher than the corresponding Sehiff s bases. During distillation, temperatures above 100 C should be avoided. In distinction to the isomeric nitrones, the less polar oxaziranes are usually noncrystalline. They have a characteristic unpleasant smell and are nonbasic. Attempts to force salt formation results in fission of the three-membered ring. ... 

The oxaziranes are colorless and they do not absorb in the UV region. Aryl-substituted oxaziranes show only the absorption of the aryl group. The oxaziranes are also transparent in the double bond region of the infrared spectrum, but they show a well developed band near 1400 cm" which has been considered characteristic for oxa-ziranes. ... 

The UV and IR spectra eliminate structures with a CN double bond. The isomerism of nitrones and oxaziranes thus cannot be a result of CIS or traris arrangement of substituents about a double bond. The carbon atoms of an oxazirane are still at the oxidation level of the carbonyl compound used in its syntheses. By acid hydrolysis, for example, 2-terf-butyl-3-phenyloxazirane (9) can be split into benzaldehyde and tert-butylhydroxylamine fEq. (8)]. ... 

Thus the structure of the oxazirane must formally involve elimination of water from one molecule each of the carbonyl compound and of an alkyl hydroxylamine. (In the synthesis of oxazirane from N-methylhydroxylamine-O-sulfonic acid and benzaldehyde, this method... 

The molecular refraction of oxaziranes is in accordance with the values calculated from the individual refractions. For the isomeric nitrones, exaltation occurs. ... 

Two pieces of chemical evidence support the three-membered ring formulation. The bifunctional oxazirane prepared from glyoxal, tert-butylamine, and peracetic acid (6) can be obtained in two crystalline isomeric forms. According to the three-membered ring formula there should be two asymmetric carbon atoms which should allow the existence of meso and racemic forms. A partial optical resolution was carried out with 2-7i-propyl-3-methyl-3-isobutyloxazirane. Brucine was oxidized to the N-oxide with excess of the oxazirane. It was found that the unused oxazirane was optically active. 

Further indications of the structure of the oxaziranes are obtained by reduction and isomerization. None of the reactions described in the following sections is incompatible with the three-membered ring structure. 

The most characteristic property of the oxaziranes is tlieir strong oxidizing character which is approximately equal to that of hydrogen peroxide. Oxaziranes react with hydrochloric acid the chlorine thus liberated is, however, used up in secondary reactions. " Two equivalents of iodine are formed from acid iodide solutions according to Eq. Titration of the free iodine allows a simple estimation of... 

Similarly the active oxygen of oxaziranes can be transferred to triphenylphosphine with the formation of ]ihosphine oxide and to tertiary amines yielding amine oxides. ... 

The behavior of oxazirane toward acids could not be predicted from the experience w ith other classes of compounds. Oxaziranes possess an astonishing resistance even toward strong acid. In this respect they show a clear difference from. V,0-acetals or compounds which contain an OCN group in a larger ring. 

Here a typical property of three-membered rings wdth two hetero atoms is shown and this property is also found in the diaziridines. Only with the oxaziranes which are substituted by aryl groups in the 3-position does the hydrolysis by acids occur according to Eq. (14) with formation of an aromatic aldehyde and alkyl hydroxylamine. 

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