Electrofugal fragment

Only on rare occasions is the electrofugal fragment a carbonium ion, as most often appropriate structures undergo substitution or elimination reactions more easily. However, ions such as the very stable tropylium cation are exceptions (279) . 

The basic mechanism of fragmentation (equation 1) is a concerted one, with a contribution from each of the five centers to the transition state. Two-step carbonium ion or, very rarely, carbanion mechanisms mean in all cases that substitution, elimination or ring closure can take place rather than fragmentation. However, the one-step synchronous fragmentation is allowed only when the compound can adopt a conformation where the leaving group X as well as the electron pair of the electrofuge Y are antiperiplanar to the C(2)—C(3) bond (Scheme 5). ... 

Fragmentation A reaction in which the carbon moiety is the positive leaving group (i.e. the electrofuge) in an elimination reaction. 

In a number of fragmentations, the electrofugal group is generated only after an initial addition of a nucleophilic species. Typical of this class of fragmentation are the reactions of /3-halogenocarbonyl compounds, e.g. ... 

In a number of olefin-forming fragmentations, the electrofuge is the lone pair of an oxygen or nitrogen atom. One of the most well-known examples is the solvolysis of 4-bromoquinuclidine, wz. ... 

There are three potential options for the mechanism of Grob fragmentation reactions, one concerted and two stepwise. The stepwise reactions, which result from initial loss of either the nucleofuge or electrofuge, are less useful in synthesis since they often promote side reactions. However, one highly efficient stepwise transformation was reported by Kato. Treatment of diol 6 with pyridinium chloride results in formation of benzyl carbocation 8. Cleavage of this alcohol yields monocycle 9 and, ultimately, the ketone product 10. ... 

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