Dizinc compounds

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. 

Eisch and Piotrowski reported the preparation of gem-dizinc compounds from diiodomethane and zinc powder in the titanocene chloride mediated methylenation of ketones in 1983 (equation 5)7. In this case, it was not mentioned that they had used pyrometallurgy zinc. The Tebbe-type reagent 2 was shown as an intermediary species. Before addition of titanocene chloride, the amount of methane was measured after hydrolysis of the reaction mixture to determine the formation of gem-dizinc species7. 

These side reactions, however, can be suppressed by addition of TMEDA. As shown in equations 9-11, several types of gem-dizinc compounds are prepared in the presence of... 

At 2 F mol-1, the mono- and dizinc compounds are obtained simultaneously in variable amounts. 2,5-Dibromothiophene behaves basically like the corresponding benzene reagent. At 4 F mol-1, only the dizinc compound is obtained but the yield is not satisfactory (28%). 

On the contrary, dichlorobenzene reagents give a mixture of mono- and dizinc compounds at 4 Fmol-1. In fact, the monozinc compound is the main product at 2 Fmol-1 whereas at more than 2 Fmol-1 the reaction does not proceed further, even when all the starting reagent is not consumed in the case of non-activated dichlorobenzenes. 

The first progress was made by Takai and Lombardo, who developed an in situ entry to titanium-alkylidene chemistry starting from the reagent combinations 5 and 6 (Scheme 4) [9]. These reactions proceed via a gem-dizinc compound 7 (its formation is catalyzed by traces of lead or lead(II) salts), which is subsequently transmetalated with TiCl4 to the titanium-alkylidene species 8, the actual olefination reagent. To date, 8 has not been characterized in detail [10]. These in situ reagents exhibit chemoselectivities similar to those of the structurally defined methylenation reagents 1-3. 

Takai, K. Kakiuchi,T. Kataoka,Y. Utimoto, K., A Novel Catalytic Effect of Lead on the Reduction of a Zinc Carbenoid with Zinc Metal Leading to a Geminal Dizinc Compound. Acceleration of the Wittig-Type Olefination with the RCHX2-TiCl4-Zn Systems by Addition of Lead. /. Org. 

Treatment of ketones wifh a mixture of CH2I2, TiCl4, and zinc in THF at 0°C for 90 min produced fhe mefhylenation product in 5-8% yield whereas when lead dichloride was added the reaction proceeded smoothly at 0 °C to give the product in 81% yield wifhin 30 min (Scheme 13.68) [85]. A catalytic amount of lead dichloride probably promotes furfher reduction of zinc carbenoid (ICH2ZnI) by zinc in THF to give a geminal dizinc compound (CH2(ZnI)2) which is a key intermediate for the methylenation of carbonyl compounds. 

Takai, K. et ah, A novel catalytic effect of lead on the reduction of a zinc carbenoid with zinc metal leading to a geminal dizinc compound. Acceleration of the Wittig-type olefination with the RCHX2-TiChj-Zn systems by addition of lead, J. Org. Chem., 59, 2668, 1994. 

RCHO, the dizinc compound (3 or 4), and additive were mixed in... 

These side reactions, however, can be suppressed by the addition of TMEDA. As shown in Scheme 5.16, several types of gem-dizinc compounds have been prepared. In the reaction of 1,1-diiodoethane, the addition of TMEDA proved unnecessary [18]. These dizinc species are not as stable as bis(iodozincio)methane 4. They can be stored under Ar at 25 °C for just a few hours, and gradually decompose through j8-elimination. 

Treatment of trimethylsilyldibromomethane with zinc in THF affords the corresponding dizinc compound in good yield [30]. Again, the zinc dust used for these preparations was pyrometallurgy grade zinc, containing about 0.04-0.07% of lead. 

Scheme 5.23. Preparation of silyl-, germyl-, and boryl-substituted gem-dizinc compounds.

Scheme 8.22 Preparation of silyl-substituted gem-dizinc compounds.

In the same procedure, dichlorobenzene reagents give a mixture of mono and dizinc compounds at 4 F mol of CljAr. The entire starting reagent is consumed only when dichlorobenzene is activated with an electron-withdrawing group (Eq. (21)) (with dichlorobenzene, 51% of the starting product is recovered at 4 F mol ). 

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