Titanocene dichloride carbonyl compounds

Dimethyltitanocene (213), called the Petasis reagent, can be used for alkenation of carbonyls (aldehydes, ketones, esters, thioesters and lactones). This reagent is prepared more easily than the Tebbe reagent by the reaction of titanocene dichloride with MeLi. However, this reagent may not be a carbene complex and its reaction may be explained as a nucleophilic attack of the methyl group at the carbonyl [67], Alkenylsilanes are prepared from carbonyl compounds. Tri(trimethylsilyl)titanacyclobutene (216), as a... 

The Petasis reagent, dimethyl titanocene (4.93) can also be used for the methylenation of carbonyl compounds. The Petasis reagent (4.93) is prepared by the reaction of methyl magnesium chloride or methyllithium with titanocene dichloride (Cp2TiCl2). Carbonyl compounds on heating with 4.93 at 60-65° C in a toluene solution give the corresponding alkenes or enol ethers. 

Benzylidenation Dibenzyltitanocene 35 is prepared by the reaction of titanocene dichloride with benzylmagnesium chloride [84] it reacts with carbonyl compounds upon heating at 45-55 °C to afford styrene derivatives [85] (Scheme 4.32). The benzylidenation was found to be best accomplished with 3-4 equivalents of 35 in toluene THF and hexane proved to be less effective reaction media. Aldehydes react rather sluggishly with 35, whereas benzylidenation proceeds well with ketones. Effective olefination also proceeds with esters, lactones, and amides giving enol ethers and enamines. In most benzylidenations with 35, 1,2-diphenylethane is... 

As described in the previous section, a-elimination of dialkyltitanocenes is of limited use for the preparation of titanocene-alkylidenes. Since thioacetals are readily available from carbonyl compounds or through alkylation of bis(phenylthio)methane and related organosulfur compounds, a thioacetal-titanocene(II) system enables the use of different types of carbene complexes. Use of appropriate thioacetals is of crucial importance in this system for the preparation of alkylidene complexes 48, the corresponding diphenyl thioacetals are the starting materials of choice. No carbene complexes are formed from dialkylthio-acetals. To generate vinylcarbene complexes 49, trimethylene thioacetals of aji-unsaturated aldehydes or l,3-bis(phenylthio)propene derivatives are employed (Scheme 4.41). The low-valent titanium species 44 is also easily prepared by the reduction of titanocene dichloride with magnesium in the presence of triethyl phosphite at room temperature. The presence of molecular sieves 4A is essential for the reproducibility of this preparation. In relatively large-scale preparations, care should be taken to control the reaction temperature [92, 93g]. 

Carbonyl Olefination Using Zirconium, Tantalum, Niobium, Molybdenum 18S Tab. 4.15. Olefination of carbonyl compounds with a gem-dichloride-titanocene(ll) system. 

Carbonyl alienation proceeds quite efficiently with titanocene alkenyUdene intermediate, formed in situ from alkenyltitanocene precursors. Indeed, these compounds are easily prepared from titanocene dichloride 65 and 2 equiv of alkenyl-magnesium bromide 66 followed by the addition of carbonyl derivatives 68 (Scheme 12.46) [66]. 

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