Lombardo-Takai olefination

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. 

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