Oxazirane, formation

Cis-trans isomerization of a,JV-diphenylnitrones can be photosensitized but does not result upon direct irradiation.457 Instead, oxazirane formation occurs. Presumably, then, the nitrone -> oxazirane photorearrangement proceeds from excited singlets. 

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. 

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)]. 

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. ... 

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

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. 

The almost quantitative formation of free ammonia can be used for an estimation of the oxazirane. With pure oxaziranes, ammonia yields of 93-96% are obtained. 

Emmons proposes as the chain starting reaction a direct attack of the ferrous ion on the oxazirane ring with the formation of an 0-radieal (24) Eq. (20)]. This radical (24) starts a reaction chain fEq. (21 ). By the attack of a further molecule of oxazirane, forma-... 

In this sequence a radical, possibly a btradical derived from unpairing the electrons of the oxazirane oxygen-nitrogen bond, abstracts the a-hydrogen atom of the A -alkyl group to form (XXI) which subsequently isomerizes to (XXII). Alternatively the formation of (XXII) may take place directly by a concerted, reaction. In either event the iminoaikoxy radical (XXII) carries the chain. The ammonia which is formed presumably comes from aldol-like condensations of the imine (XXIV). The fact that vapor-phase pyrolysis does not take this course simply reflects tbo low probability of a chain reaction in the vapor phase. 

The formation of azine is not a means of trapping the hydrazine formed in the Raschig process with acetone, the reaction proceeding by way of two distinct intermediates dimethyl oxazirane and acetone hydrazone, as follows ... 

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