Carbonyl compounds Tebbe reagent

Similarly to alkenes, alkynes react with various titanium-methylidene precursors, such as the Tebbe reagent [13,63], titanacydobutanes [9b, 64], and dimethyltitanocene [65] to form the titanium-containing unsaturated cyclic compounds, titanacydobutenes 67 (Scheme 14.29). Alternatively, 2,3-diphenyltitanacydobutene can be prepared by the reaction of the complex titanocene(II) bis(trimethylphosphine) with 1,2-diphenylcyclopropene [66]. Substituent effects in titanacydobutenes [67], the preparation of titanocene-vinylke-tene complexes by carbonylation of titanacydobutenes [68], and titanacyclobutene-vinylcar-bene complex interconversion [69] have been investigated. 

Transformation of a carbonyl compound to the corresponding cjco-olefin using Tebbe s reagent. 

Petasis, et al. have discovered that dimethyltitanocene is an excellent substitute for the Tebbe reagent" for the methylenation of heteroatom-substituted carbonyl compounds. ... 

Dormond ef rrZ. have exploited compounds 30 and particularly 32 as synthetically useful reagents. The uranium-mediated methylenation of carbonyl compounds (cf.. the Wittig, Peterson or Tebbe reactions) is illustrated in Equation (5.1). The instantaneous formation under ambient conditions of the insertion product 33 was essentially quantitative (e.g./ =... 

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 so-called Tebbe reagent 272 reacts with carbonyl compounds to form olefinic products 139a>, a reaction which is of particular synthetic value in case of esters (Equation 87) 139b). 

The Tebbe reagent (4.85) is used for methylenation of carbonyl compounds. ... 

In compounds containing both ketone and ester groups, the ketone selectively reacts in the presence of 1 equiv. of the Tebbe reagent (4.85), but with excess amount of the Tebbe reagent (4.85) both carbonyl groups are alkenated. 

The titanium-aluminum methylidene (3), commonly known as the Tebbe reagent,was the first well-characterized compound in this series. The Tebbe reagent (3) and related metallacycles (4) have been used principally as methylidene sources toward carbonyl groups. Both are believed to provide the titanium methylidene (5). ... 

Zirconium-aluminum analogs of the Tebbe reagent have been reported to react with carbonyl compounds slowly, in low yield or not at all. However, zirconium aUcylidenes which are stabilized by a phosphorus rather than an aluminum ligand do transfer their alkylidene moiety to carbonyl. These results provide some of the few examples of long chain alkylidene transfer to ca nyl from an organo-metallic reagent (equation 31). 

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... 

Tebbe s reagent, Cp2TiCH2Al(CH3)2Cl, converts carbonyl compounds to methy-lenes. This reagent when applied to ot,j6-unsturated aldehydes and ketones generates dienes (equation 106). Synthetic utility of the reagent for generation of dienes and polyenes is limited because of the difficulty in the preparation and incompatibility with other functional groups such as esters etc. 

The active species in the Tebbe olefination is believed to be the nucleophilic (Schrock-type) titanocene methylidene, which is formed from the Tebbe reagent upon coordination of the aluminum with a Lewis base (e.g., pyridine). This methylidene in its uncomplexed form, however, has never been isolated or observed spectroscopically owing to its extreme reactivity. The same intermediate can also be generated by other means." The titanocene methylidene reacts with the carbonyl group to form an oxatitanacyclobutane intermediate that breaks down to titanocene oxide and the desired methenylated compound (alkene). The driving force is the formation of the very strong titanium-oxygen bond. 

For a comprehensive review detailing the scope and utility of methylenation of carbonyl compounds using Tebbe s reagent, see S. H. Pine, Org. React., 1993, 43, 1 for a more recent review, see R. H. Hartley and G. J. McKiernan, J. Chem. Soc., Perkin Trans. 1, 2002, 2763. 

---