Titanium methylidene

Another approach to synthetically useful olefin metathesis involves the utilization of higher homologues of titanium-methylidene 15, as shown in Scheme 14.11. If the resulting titanium carbene complex 20 is more stable than the starting alkylidene complex 15, this reaction can be employed for the generation of various titanocene-alkylidenes and as a method for the preparation of unsaturated compounds. 

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. 

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). ... 

In the case of the Tebbe reagent, the titanium methylidene is presumably released by some Lewis base. That base is often the heteroatom of the substrate. A Lewis base such as tetrahydrofuran or pyridine may be added to the reaction and usually enhances reactivity relative to reaction with the substrate alone- ... 

The metallacycles (4) generate the requisite titanium methylidene thermally. Consistent with this is the observation that reaction with a carbonyl compound is first order in the metallacycle and zero order in the carbonyl compound. Metallacycle stability depends on the alkene moiety that has replaced the aluminum-chlorine portion of (3) and thus could provide a series of reagents whose reactivity is temperature dependent. ... 

The titanium methylidene interacts with the carbonyl carbon-oxygen double bond in a sequence that is believed to resemble the metathesis process. With the metallacycle (4), a titanium oxametallacycle is believed to be formed, which then decomposes to generate the new carlxm-carbon double bond and a titanium-oxygen product. The driving force for the reaction has been attributed to the oxophilicity of titanium (equation 18). ... 

More recently, Petasis has shown that titanocene dimethyl, " which is easily generated from titanocene dichloride and methyllithium, also generates the same titanium methylidene upon thermolysis (Scheme 13.5, bottom). Thermolysis of titanocene dimethyl is a convenient way to generate the titanocene-methylidene complex because titanocene dimethyl is formed in high yield and is stable to water, -although it is sensitive to light and can undergo autocatalytic decomposition. This complex has been used in synthesis... 

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