Carbanions, reaction with azirines

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). 

Reactions of Azirines.—Two groups of workers have investigated the preparation of 2H-pyrroles by the reaction of carbanions with azirines. Laurent etal. obtained azirines from the action of base on methiodides of AW-dimethyl-hydrazones the azirines could be used, without isolation, to provide overall yields of 65—70% of pyrroles, based on the quaternized hydrazones. The azirine (286 R = = Me, R" = Ph) reacted with (287 R = H or Me, R = H) in... 

There are a few reports on the use of oximes as electrophilic amination reagents. Since 1984, ketone O-sulfonyloxknes have found applicability as amino transfer reagents to car-banions. In the reaction of organometaUic compounds with oximes, carbanions attack the carbonyl carbon of the oxime, giving Af-substituted hydroxylamines as addition products (Scheme 53, path a). However, a number of scattered reports have been also published on the formation of aziridines by a-deprotonation, followed by addition (path b) or formation of azirines by a-deprotonation before addition (path c). Addition of carbanions to azirines also yields aziridines, which are hydrolyzed to a-aminoalcohols. 

Phenyl-l-azirine (42a) reacts with acetophenone in the presence of the dimethylsulfinyl carbanion to give 2,4-diphenylpyrrole (150).65 This reaction probably involves initial attack of the enolate anion on the carbon-nitrogen double dond, to give intermediate 147 and 148 which ring-opens to 149 and loses hydroxide ion giving pyrrole (150). A similar reaction of ethyl benzoylacetate with 2-phenyl-l-azirine yields 3-benzoyl-4-phenyl-2-oxopyrroline (151). 

Succinimide synthesis.3 Reaction of 2-phenylazirine (1) and Mo(CO)6 (1 equiv.) with diethyl sodiomalonate furnishes the succinimide 2 in 46% yield. This reaction is applicable generally to 2-aryl-substituted azirines and carbanions of 0-dicarbonyl com-... 

An interesting Darzens-like formal cycloaddition of azirines has also been reported <2004EJ02421>. While this is not a true cycloaddition reaction, two bonds are formed in the same reaction sequence. Reaction of sulfone 114 with lithium diisopropylamide (LDA) generates a carbanion which then adds to the azirine to generate intermediate 115. Cyclization of this intermediate with the allyl halide provides the product aziridine 116. This reaction proceeds in only 13% yield but is nonetheless an interesting route to fused-ring aziridines (Equation 27). 

The use of potassium <-butoxide as the base directly produced a low yield of 1-azirine contaminated with propiophenone. However, Nair has reported that azirine (37) can be prepared from hydrazone (34) in 63% yield by using the dimethylsulfinyl carbanion as the base. Even if the a-hydrogen is tertiary, this does not always ensure success of the reaction. Treatment of 38 with sodium isopropoxide... 

Like the pyrrole synthesis, the assembly of 4-methylene-3-oxa-l-azabicyclo[3.1.0]hexanes is likely triggered by the formation of O-vinyl oxime (detected by GLC and NMR). The deprotonation of O-vinyl oxime in a-position relative to the oxime function and the further intermolecular nucleophilic substitution of the vinyloxy group can lead to azirine A (Scheme 1.178). The latter reacts with acetylene (in the form of carbanion) to give acetylenic ethynyl aziri-dine B (the nitrogen analog of the Favorsky reaction), which is added to the third... 

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