✍ Scribed by Makosza, Mieczyslaw ;Golinski, Jerzy ;Ostrowski, Stanislaw ;Sahasrabudhe, Arvind B. ;Rykowski, Andrzej
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Halo carbanions / Vicarious Nucleophilic Substitution (VNS) / Annulation / Quinoxalines / Naphthyridines Carbanions of a-haloalkyl aryl sulfones, sulfonates, and sulfonamides react with bicyclic heteroaromatic compounds (qui- noxalines, naphthyridines, and 5-azaquinoxalines) according to two general pathways: vicarious nucleophilic substitution of hydrogen and/or bisannulation. In some cases other com-petitive reactions such as S,Ar are observed. Factors governing the direction of these reactions are discussed in terms of the charge distribution in the anionic o adducts and the reaction conditions.
Carbanions with a leaving group X at the carbanionic center can replace hydrogen in electrophilic aromatic and heteroaromatic compounds, particularly in nitroarenes. This reaction, named "Vicarious Nucleophilic Substitution of Hydrogen" (VNS), proceeds by addition of the carbanion to nitroarenes in the orthoor para-positions bearing hydrogen, followed by base-induced p-elimination of HXZp4). This process is of general character with respect to nitroarenes and carbanions. Many electrophilic heterocycles without a nitro group such as 1,2,Ctriazine 5,6), benzothiazole ~t e r i d i n e ~~" ) etc. also undergo this reaction as exemplified by the reaction of a 1,2,4-triazine derivative in Scheme 1.
Scheme 1 R X leaving group, Y : carbanlon stabilizing group However, some bicyclic heterocycles such as quinoxalines and naphthyridines react with a-halo carbanions in a different way to afford bisannulated products8). The dramatic difference in the course of the reaction of 1,2,4-triazines,
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