Azirinium ylides

The 5,7-diaryl-2-fluoro-4/7-l,3-diazepines have been synthesized from 3-aryl-substituted 277-azirines and difluoro-carbene (Scheme 13). The reaction involves isomerization of azirinium ylide into a 2-aza-l,3-diene, which undergoes [4+2] cycloaddition with the starting azirine, followed by ring expansion and dehydrofluorination <2006TL639>. 

The thermocatalytic Rh(ll) decomposition of diazo malonate in the presence of 3-phenyl-2/7-azirine 772d was proposed to give rise to an azirinium ylide 803 <2004TL6003>. This reactive ylide is preferentially transformed into 2-azabuta-l,3-diene derivative 804 or, with excess diazo compound, via reaction with the Rh-carbenoid, forms the 3,4-dihydro-2/7-pyrrole derivative 806 via intermediate 805 (Scheme 196). 

Interestingly, the reaction of the more heavily substituted diphenyl 2//-azirine afforded azetine 808 in 73% yield when it was allowed to react with diazo malonate in the presence of Rh2(OAc>4. The structure of azetine 808 was established by reduction to diol 809 under the action of LLAIH4 (Scheme 197). It would appear as though the reactivity of the initially formed azirinium ylide is dependent on the degree of substitution about the 2//-azirine ring. 

The Rh2(OAc)4-catalysed reaction of 2/7-azirine-2-carbaldehydes with dimethyl diazomalonate proceeds via azirinium ylide formation, isomerization to 2-azabuta-1,3-dienes followed by l,6- r-electrocyclization to give 2//-l,3-oxazines (Scheme 47). ... 

Ring Expansion. Formation of sulfur ylides from cyclic sulfides and dimethyl diazomalonate in the presence of Rh catalyst is accompanied by 1,2-ring expansion. This reaction was used for preparation of dihydrothiopyranes (eq 30). Ring expansion of azirinium ylides under similar conditions afforded corresponding 2,3-dihydroazete (eq 31). ... 

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