Gem-dimetal compound

This prominent name reaction looks perfect, but still has some serious drawbacks. The ylide is strongly basic, which often leads to enolization of the starting carbonyl compound. At the same time, it often suffers from a lack of nudeophi-lidty. For example, treatment of a-tetralone with methylenetriphenylphosphorane will not give any methylenated product as the substrate is deprotonated to form the enolate. Treatment of esters with an ylide often ends with recovery of the starting ester because of insuffident nudeophilicity of the ylide. These problems, however, can be overcome by the use of a gem-dimetal compound. The reaction of carbonyl compounds with gem-dimetal compounds is outlined in Scheme 5.2. 

Scheme 5.2. Methylenation of carbonyl compounds with a gem-dimetal compound.

As a relative of the gem-dimetal compound, a metal-carbene complex should also be considered (Scheme 5.3) [4]. In this case, the reaction proceeds via a 1-metalla-2-oxacycIobutane. 

Treatment of aldehydes with these gem-dimetal compounds led to alkylidenation. In these cases, the addition of BFa-OEta as a Lewis add was required to activate the aldehyde (Scheme 5.21). 

Scheme 5.20. Preparation of gem-dimetal compounds by carbometallation. n-CgHis MgBr...

Nakamura et al. reported that zincated hydrazones react with vinyl Grignard reagent to afford gem-dimetal compounds. These were treated with benzaldehyde to give alkenyl hydrazones (Scheme 5.22) [29]. 

Scheme 5.22. Alkene synthesis using a hydrazone-derived gem-dimetal compound.

Not only a zinc-copper couple, but also a zinc-lead couple forms a gem-dimetal species from diiodomethane, according to the 1975 Nysted patent5. He also insisted that treatment of dibromethane with a zinc-lead couple in THF at 80 °C forms a characteristic gem-dizinc species 1 (equation 3). However, there was no further evidence concerning the structure except H NMR data, which was not enough for the complete structural determination. The obtained compound was definitely a gem-dizinc species, but the written structure 1 was not fully characterized. The white solid 1 is obtained as a dispersion in THF, and does not dissolve in DMF and DMI (l,3-dimethyl-2-imidazolidinone). This THF dispersion is commercially available from Aldrich Co. as Nysted reagent. Nysted also showed that this dizinc compound is effective for the methylenation of a-hydroxy ketone moiety in steroid derivatives5. 

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