Nucleophilic reactions 1- vinyllithium

The Shapiro reaction has been used as a method for the initiation of anionic cyclisations, as described in section 7.2. Hydrazones derived from 23 are particularly valuable in this area, not only as the source of vinyllithiums but also as carbene equivalents. Nucleophilic addition to 26 promotes the collapse of the product lithioamine to give the organolithium 27. 

Some typical reactions of 1,1 -difluoroethene with nucleophiles are summarized in Scheme 2.18. Alkoxides [3], trialkylsilyl anion [4], ester enolates [5], and diphenylphosphinyl anion [6] attack the gem-difluorinated carbon of 5. However, it is noteworthy that nucleophilic substitution and proton abstraction are in some cases competitive, and thus s -butyl lithium abstracts the (3 -vinylic proton predominantly to generate vinyllithium. The lithium species can be trapped with an aldehyde, providing difluoroallyl alcohol, which is then hydrolyzed to a, (3-unsaturated carboxylic ester (11) [ 7 ] (Scheme 2.19). Some synthetically useful examples are shown in Schemes 2.20 and 2.21. Tetrathiafulvalene derivative (14) is prepared from difluorinated derivative (13) [8]. An elegant intramolecular version was demonstrated by Ichikawa, which provided a range of cyclized compounds (17), including dihydrofurans, thiophenes, pyrroles, and cyclopentenes, and also corresponding benzo derivatives (20) [2]. 

The stereochemistry of the direct attack can be expected to resemble the corresponding reactions at a saturated carbon (Sections 5.1.1.1 and 5.1.1.2)—inversion for nucleophilic substitution, and retention, or perhaps occasionally inversion, for electrophilic substitution. In practice, SN2 reactions at trigonal carbon are rare,503 and their stereochemistry, where inversion is known,504 barely established. Electrophilic attack, like the reactions of vinyllithium reagents with protons, aldehydes and carbon dioxide, takes place invariably... 

The equilibrium between a-silyl alkoxides and silojq carbanions can be shifted toward the carbanion side by introduction of a conjugating group into either or both the acylsilane and the nucleophile. In 1980, Reich et al. reported that treatment of all l-substituted acylsilanes with vinyllithium followed by a variety of electrophiles affords a-substituted enol silyl ethers 22 via a siloxy allyllithium intermediate 21 fScheme 8.1Similar reactions using phenyllithium give products in which electrophilic quenching occurred at the benzylic position. 

In a similar manner, certain bis(arene)iron(II) salts react easily with nucleophiles and may be used in organic synthesis. For example, (mesityIene)2Fe(PF6)2 reacts with phenyl, terf-butyl, and vinyllithium to give either the compound XLVIII or the pseudo-ferrocene derivative XLIX, according to the stoichiometry of the reaction (Helling and Braitsch, 1970). 

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