Titanium compounds transmetalation

Mechanistically, the pre-catalyst Cp2TiCl2 104 is reduced to a [Ti]-H 105 species which is subsequently able to hydrotitanate the triple bond 106. A transmetallation from titanium to aluminum regenerated the [Ti]-H species to generate the (yy )-hydroaluminated compound 107 (Scheme 13). 

Covalently bonded chiral auxiliaries readily induce high stereoselectivity for propionate enolates, while the case of acetate enolates has proved to be difficult. Alkylation of carbonyl compound with a novel cyclopentadienyl titanium carbohydrate complex has been found to give high stereoselectivity,44 and a variety of ft-hydroxyl carboxylic acids are accessible with 90-95% optical yields. This compound was also tested in enantioselective aldol reactions. Transmetalation of the relatively stable lithium enolate of t-butyl acetate with chloro(cyclopentadienyl)-bis(l,2 5,6-di-<9-isopropylidene-a-D-glucofuranose-3-0-yl)titanate provided the titanium enolate 66. Reaction of 66 with aldehydes gave -hydroxy esters in high ee (Scheme 3-23). 

Lithium-titanium transmetallation of intermediate 374 made possible the chemoselec-tive discrimination between aldehydes and ketones, reacting mainly or exclusively with the first type of carbonyl compounds, as observed for the phthalan-derived intermediate 331. ... 

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. 

When the ligand system consists of chlorine atoms, monoalkyl- and monoaryl-titanium(IV) compounds are readily made from TiCU by a chlorine atom replacement. The parent compound in this series, MeTiCb, is made conveniently from the reaction of TiCU and dimethylzinc without the use of ethereal solvents, the preparation being carried out in n-pentane or dichloromethane. Other methylating species can be used, such as MeMgBr, MeLi and MeAlCh. When MeLi is used in diethyl ether, an equilibrium takes place between different complexation states of Ti in the resulting MeTiCU with the solvent (equation 8). Transmetallation from organotin compounds like (15 equation 9) has also been utilized to make trichlorotitanium compounds. ... 

The oxidation of titanium enolato complexes, derived from a transmetallation reaction of the corresponding lithium enolates with Cp2TiCl2, by dimethyldioxirane has been investigated as a general, convenient, effective, and chemo- and diastereoselective synthesis of cr-hydroxy carbonyl compounds. The Cp derivative shown in Scheme 601 results in much higher diastereoselectivities than other enolato complexes studied.1556... 

Transmetallation is important in forming a transition metal catalyst such as titanium alkyl by alkylating the titanium chloride with alkylaluminum compound in Ziegler type catalysts. 

A transmetallation to titanium derivatives also promotes the addition of various zinc organo-metallics to cailxrnyl compounds. However, the functional group tolerance seems to be limited (see Scheme 10). ° Interestingly, diethylzinc reacts with aromatic 1,2-diketones to give a-ethoxy ketones (see equation 10). ... 

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