Titanocene methylene

A thioformaldehyde complex of titanium (55) was similarly obtained from the titanocene methylene complex 54 and either elemental sulfur or sulfur sources such as propene sulfide, cyclohexene sulfide, styrene sulfide, or Ph3P = S. Best yields were achieved with propene sulfide [Eq. (II)].67... 

Titanocene methylene-Zinc iodide complex, CpjTiCHj ZnI, (1). The reagent is prepared by reaction of CpjTiCE with CHjfZnl), in THF. It is related to the Tebbe reagent (8, 83-84 10, 87-88), but is easier to prepare. 

METHYLENATION Alkyidimesitylboranes. p,-Chlorobis(cyclopentadienyl)(dimethyl-aluminum)-p.-methylene-titanium. Methylene bromide-Zinc-Titanium(IV) chloride. Organomolybdenum reagents. Titanocene methylene-Zn halide complex. N,N,P-Tri-methyl-P-phenylphosphinothioic amide. Trimethylstannylmethyllithium. 

Chloro[(trimethylsilyl)methyl]ketene. Formaldehyde. Titanocene methylene-Zinc halide. 

Stille, J. R., Grubbs, R. H. Synthetic applications of titanocene methylene complexes selective formation of ketone enolates and their reactions. J. Am. Chem. Soc. 1983, 105, 1664-1665. 

Moreover, it is noteworthy that the highly reactive titanocene methylene has been applied to a pyrrole synthesis [106]. 

Interestingly theoretical treatment has established that the energetics of the titanacyclobutane is more stable than the olefin-carbene complex resulting of its rearrangement [108] and a titanacyclobutane is in fact postulated as a titanocene methylene precursor in reaction (43). 

Reactions of titanocene-methylidene generated from titanacyclobutanes with acyl chlorides 55 [46] or acid anhydrides 56 [47] lead initially to the titanium enolates 57 (Scheme 14.24), which then afford aldols upon treatment with the carbonyl compounds. On the other hand, five-membered cyclic anhydrides are methylenated with dimethyltitanocene (Table 14.5, entry 7) [45]. 

Another titanium-based reagent for the methylenation of carbonyl compounds is that prepared from dibromomethane/zinc/titanium tetrachloride and related systems (Scheme 14.25) [48]. These systems transform a wide variety of carboxylic acid derivatives to terminal olefins in the same way as titanocene-methylidene does. 

Table 14.5. Carbonyl methylenation utilizing titanocene-methylidene...

Methylenative dimerization takes place when terminal alkynes are treated with the tita-nocene/methylidene/zinc halide complex generated from titanocene dichloride and CH2(ZnI)2. The process is believed to involve the formation of a titanacyclobutene intermediate [75],... 

Table 3.8. Examples of methylenations with the Tebbe reagent and with dimethyl titanocene.

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. 

The Tebbe Reagent is a metal carbenoid prepared from the dimetallomethylene species derived by the reaction of trimethyl aluminium with titanocene dichloride this reagent exhibits carbenoid behaviour after the addition of a catalytic amount of pyridine. The Tebbe Reagent reacts with various carbonyl partners to give the product of methylenation ... 

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. 

Petasis, N. A., Lu, S.-P. Methylenations of heteroatom-substituted carbonyls with dimethyl titanocene. Tetrahedron Lett. 1995, 36, 2393-2396. 

Similarly, treatment of 17 with 1-alkenes and alkynes affords the corresponding metallacyclobutane and metallacyclobutene compounds, respectively [57,59]. Heating the dimethyl complex of titanocene generates the methylene species [44-46,71-73]. 

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