Gem-dizinc species

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. 

Following the same strategy as used for methylenation, general alkylidenation of carbonyl compounds would be realized by preparation of the corresponding gem-dizinc species. The preparation of these gem-dizinc species, however, is con-... 

The silyl group may promote reduction of the C-Br bond at the a-position, as it stabilizes a radical species at this a-position. The obtained gem-dizinc species is fairly stable as it has no y8-hydrogen. In the same way, gem-dizinc species bearing germyl and boryl groups at the a-position have also been prepared in good yields [31, 32]. 

Reactions of carbonyl compounds with these gem-dizinc species gave the corresponding heteroatom-substituted alkenes. In these reactions, the addition of a titanium salt was necessary. jS-TiCls was used, except in the case of reaction of a-boryl-substituted gem-dizinc, where TiCU was used instead (Table 5.4). 

In these reports, the zinc powder that had been used by Nozaki and coworkers in Kyoto University was pyrometallurgy zinc. It contains 0.04—0.07% lead (Pb) originally. This lead played an important role for the acceleration of further reduction of Simmons-Smith reagent into gem-dizinc species, according to Takai and coworkers report in 1994 [11], This effect was consonant with Nysted s result [9]. 

The problem about the aging period for preparation of reactive species 3, argued by Lombardo, should not be attributed only to the formation of gem-dizinc species. As titanium(IV) chloride is also reduced with zinc powder, [17] the titanium salt that works as a mediator would be the low-valent one. The reduction process of titanium(IV) may also sometimes cause the problem of reproducibility of methylenation reaction. In 1998, Matsubara and coworkers reported a general... 

Following the same strategy with methylenation, any alkylidenation of carbonyl compounds would be realized by preparation of the corresponding gem-dizinc species. The preparation of these gem-dizinc species, however, presents some difficulty, compared to the preparation of bts(iodozmcio)methane. The simple reduction of gem-dihaloalkane carrying /3-hydrogen will suffer from /8-elimination and result in the formation of elimination product. In addition, depending on the substrate, the intermediary a-haloalkylzinc compound is less stable than a-halo-methyl zinc to take a route to a-elimination (Scheme 8.17). 

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