Titanocene chlorides, preparation

Eisch and Piotrowski reported the preparation of gem-dizinc compounds from diiodomethane and zinc powder in the titanocene chloride mediated methylenation of ketones in 1983 (equation 5)7. In this case, it was not mentioned that they had used pyrometallurgy zinc. The Tebbe-type reagent 2 was shown as an intermediary species. Before addition of titanocene chloride, the amount of methane was measured after hydrolysis of the reaction mixture to determine the formation of gem-dizinc species7. 

Benzenedisulfenyl chloride, prepared by chlorination of benzenedithiol, reacted with the titanocene complex (Cp 2TiCl)2S3 at low temperature in carbon disulfide to give the known 1,2,3,4,5-benzopentathiepin 105 in 73% (Equation 18) <2000EJI921>. 

It has been reported recently that organotitanium compounds prepared conveniently from the corresponding Grignard reagents and titanocene chloride cyclize with an appropriately positioned internal alkene in the presence of ethylaluminium chloride (equation 119) ... 

In the initial description of this method triethylamine, diethylamine, pyridine, and sodium ethylate were reported to be effective, but yields of ferrocene and titanocene dichloride were only 3-4% (33). These low yields probably resulted from the use of unreactive ferrous chloride prepared by high temperature dehydration. 

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. 

Titanocene pentasulfide (1) has been used as a transfer reagent for the S5 unit to prepare numerous inorganic homo- and heterocycles. More recently, cyclic organic polysulfanes have also been obtained from (1) by reaction with bis(sulfenyl chlorides) (equation 99). ... 

The Tebbe reagent 3 is prepared as reddish-orange crystals by the reaction of two equivalents of trimethylaluminum with titanocene dichloride [5]. The mechanism of the formation of 3 has been well investigated [6]. Theoretical studies based on ah initio calculations on a model complex suggest that the appropriate representation of the Tebbe reagent is in terms of an intramolecular complex, in which chlorine acts as the electron donor and aluminum as the acceptor [7]. Since the reagent is sensitive to air and moisture, the use of a standardized solution in benzene or toluene is recommended. Such solutions of 3 can be currently purchased from various commercial sources. The titanocene-methylidene 4 is formed by the action of a Lewis base such as pyridine or 4-dimethylaminopyridine on 3, with the expulsion of dimethylaluminum chloride (Scheme 4.3). The carbene complex 4 itself... 

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... 

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