Alkenes reactive carbanions

Conjugate Additions of Reactive Carbanions to Activated Alkenes and Alkynes... 

DI- and Tri-anions.—Frequently, the formation of a multi-ion is necessary in order that a specific site in a molecule can be rendered active. This is especially so where that site is less easily lithiated than others within the molecule. Such a case presents itself with the dianions (36), where reaction occurs at the more reactive carbanionic centre to give access to various useful p-hydroxy-sub-stituted compounds from conventional electrophiles. " The requisite dianions can be formed from a-chloro-alcohols and a-chloro-ketones " or alternatively by lithiation of the corresponding mercurial compounds (S ). Since the mercurial compounds can in turn be obtained from an alkene by addition of Hg(OAc)a-H2X, in excellent yield, the method provides a very versatile synthesis of p-hydroxy-compounds from alkenes. The same authors have used the new trianions (38), again generated from a mercury compound by lithium-mercury... 

Thus with dihalocarbenes we have the interesting case of a species that resem bles both a carbanion (unshared pair of electrons on carbon) and a carbocation (empty p orbital) Which structural feature controls its reactivity s Does its empty p orbital cause It to react as an electrophile s Does its unshared pair make it nucleophilic s By compar mg the rate of reaction of CBi2 toward a series of alkenes with that of typical electrophiles toward the same alkenes (Table 14 4) we see that the reactivity of CBi2... 

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