Titanocene reagents, titanium dichloride

An alternative, but related, route to allenic titanium reagents from propargylic esters has been reported recently. Reaction of titanocene dichloride with BuMgCl and Mg yields a reactive titanocene intermediate, formulated as Cp2Ti. This reduced Ti species reacts in situ by oxidative addition to propargylic acetates. The allenyltitanium reagents thus produced add to aldehydes and ketones, as expected, to afford homopropargylic alcohols (Table 9.27) [43]. 

The nucleophilic addition of a hydride to the exocyclic double bond of fulvenes, using LiBEt3H as the hydride transfer reagent, resulted in the formation of the appropriately substituted lithium cyclopentadienide intermediates, which is insoluble under the reaction conditions chosen and can be isolated for purification purposes. Two equivalents of the substituted lithium cyclopentadienide undergo a transmetallation reaction when reacted with 1 mol equivalent of titanium tetrachloride in THF under reflux to give the appropriate non-bridged substituted titanocene dichloride in overall yields of up to 77% as seen in Scheme 2. 

Alkylidenation Several attempts have been made to prepare titanium alkylidene complex 17. The reaction of titanocene dichloride with triethylaluminum does not afford an alkylidene-bridged complex similar to the Tebbe reagent, probably due to the presence of a jS-hydrogen, and produces instead the aluminotitanium hydride... 

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