Azirines, from alkynes

The exploration of the chemistry of azirines has led to the discovery of several pyrrole syntheses. From a mechanistic viewpoint the simplest is based upon their ability to behave as a-amino ketone equivalents in reactions analogous to the Knorr pyrrole synthesis cf. Section 3.03.3.2.2), as illustrated in Schemes 91a and 91b for reactions with carbanions. Parallel reactions with enamines or a-keto phosphorus ylides can be effected with electron-deficient 2//-azirines (Scheme 91c). Conversely, electron-rich azirines react with electron deficient alkynes (Scheme 91d). 

The addition of phthalimidylnitrene (374) to simple alkynes affords 1-azirines in yields of 1-15% (Scheme 10). In this reaction, which is of no real preparative value, the symmetrical 2-azirines (375) were suggested as the most plausible intermediates and unequivocal proof of the existence of such species was demonstrated from a series of 1,2,3-triazole pyrolysis reactions <71CC1518). Extrusion of nitrogen from the regioisomeric 4,5-disubstituted 1,2,3-triazoles (376) during flash vacuum pyrolysis furnished identical product mixtures which included both regioisomeric 1-azirines (377). 

Attempts to prepare 1 //-azirines by the reaction of JV-aminonitrenes with alkynes have been unsuccessful, although convincing evidence for the intermediacy of 1//-azirines has been obtained by analysis of the products from such reactions <69CC147,71AHC(13)45, B-83MI 101-02,84CHEC-i(7)47>. 

In addition, the Chiba group reported that the reaction of vinyl azides 72 and internal alkynes 73 in the presence of a catalytic amount of [Cp RhCl2]2 and Cu(OAc)2 with AcOH in dimethylformmide (DMF) could provide isoquinolines 74 in moderate to good yields (Scheme 16.33) [54]. It was speculated that the reaction could be initiated by the formation of N—H imines B from vinyl azides 72 via the Cu-mediated reductive ring opening of highly strained three-membered cyclic imines, 2//-azirines... 

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