Titanium cyclopentadienide

A useful alternative to phosphorus ylids are the titanium reagents, such as, 71, prepared from dicyclopentadienyltitanium dichloride and trimethylaluminum. Treatment of a carbonyl compound with the titanium cyclopentadienide complex 71 Tebbe s reagent) in toluene-THF containing a small amount of pyridine " leads to the alkene. Dimethyltitanocene (Me2TiCp2), called the Petasis reagent, is a convenient and highly useful alternative to The mechanism of Petasis... 

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. 

Within this review we describe a second reaction pathway, which includes a carbolithiation reaction and leads to achiral-substituted titanocene dichlorides also. Therefore, aryl lithium species are added to the identical substituted 6-aryl fulvenes. This leads to the formation of highly substituted, but achiral diarylmethyl-functionalised lithium cyclopentadienides as seen in Scheme 5, which can still be used in the transmetallation reaction with titanium tetrachloride. 

Volpin et al. (92) reported that a mixture of sodium cyclopentadienide, titanium tetrachloride, and diphenylacetylene gave a derivative of bis(cyclo-pentadienyl)titanium for which they suggested structure (XCVII) or, less probably, (XCVIII). 

Ring-substituted derivatives of nickelocene, cobaltocene, and of molybdenum and titanium 71-cyclopentadienyls, which would be difficult to make by direct substitution of the parent metal-ring complex, have been made starting with these substituted cyclopentadienides [17, 18]. 

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