Lombardo-Takai reagent

The advantage of the Takai-Lombardo reagents is the possibility of transferring substituted alky-... 

Scheme 4. Takai-Lombardo reagents TMEDA= N.iV,Af, N -tetramethyl-ethylenediamine.

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. 

Indeed, the reproducibility of this method was questioned by Lombardo in 1982 [14]. He attempted to apply Takai and Oshima s procedure to the methylenation of a gibberellin derivative with Zn (pure, without lead)-CH2Br2-TiCl4, but only decomposition of the substrate was observed. Lombardo later demonstrated an improved procedure. According to his report, the requisite ageing period for the preparation of the reagent was three days (Scheme 5.7). 

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