The Reaction of 1-Azirines With Acid Chlorides. A Potential Route to the 2-Azirine Ring System

Benzoyl chloride reacted with 3-methyl-2-phenyl-1-azirine (3) in acetone at 5° to give ring-expanded oxazole 4 and ring-opened dichloro amide 5. By carrying out the reaction in refluxing benzene, it was possible to obtain the intermediate N-benzoyl-2-chloroaziridine 6 resulting from addition of the reagent to the C═N. Benzoyl chloride in refluxing benzene reacted with 2,3-diphenyl-1-azirine (9) and fused azirine 14 to give chloroaziridines 10 and 15. Chloroaziridines 6 and 10 were solvolyzed in polar solvents giving oxazoles 4 and 11 and smaller amounts of the dichloro amides 5 and 12. Benzenesulfonyl chloride reacted with azirine 3 to yield the ring-opened sulfonamides 17 and 18, whereas azirine 9 was inert to benzenesulfonyl chloride in pyridine. Azirines 3 and 9 were unreactive toward alkyl halides in refluxing acetone and were insoluble in dilute hydrochloric acid. The low basicity of azirines was found to be in accord with the high percentage of s character (35.6) of exocyclic bonds in this ring system as measured by the carbon-13-hydrogen coupling constant (176 Hz) of 3-phenyl-1-azirine. Attempted dehydrochlorination of 6 and 10 using either 1,4-diazabicyclo[2.2.2]octane (DABCO), potassium t-butylate, or sodium hydride did not give the anticipated 2-azirine derivative. Chloroaziridine 6 was unreactive toward DABCO and sodium hydride, whereas chloroaziridines 6 and 10 reacted with potassium t-butylate to produce 1-azirines 3 and 9 and t-butyl benzoate. These results along with the anomalous reaction of 1-azirines with Grignard reagents to give aziridines are discussed in terms of the antiaromaticity of the 2-azirine ring system.