Mechanism dimethyltitanocene

Hartwig65 reported that dimethyltitanocene is an efficient catalyst for the hydroboration of alkenes and examined the mechanism of titanocene dicarbonyl-catalyzed the hydroboration of alkynes (Scheme 5).66... 

Titanacyclobutene complexes result from the reaction of dimethyltitanocene with alkynes (Scheme 29). The reaction can be envisioned to proceed either by initial methane extrusion to form the methylidene, followed by [2+2] cycloaddition, or by initial alkyne insertion to generate the vinylic intermediate followed by 7-elimination. The evidence suggests that the latter mechanism dominates. Diphenylacetylene and bis(trimethylsilyl)acetylene both provide the corresponding titanacyclobutene complex, preferentially abstracting an allylic 7-hydrogen over the... 

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... 

In 1999, Doye disclosed that dimethyltitanocene is a catalyst widely applicable to intermolecular hydroamination of alkynes with primary aryl- and alkylamines [302]. In the case of unsymmetrically substituted alkynes, the reaction occurs with high re-gioselectivity, forming the anti-Markovnikov products exclusively (Scheme 14.127). Kinetic studies suggest that the reaction mechanism involves the formation of a Ti-imido complex as the catalytically active species. Doye further developed a tandem Ti-catalyzed protocol of alkyne hydroamination and imine reduction, affording secondary amines in a fully catalytic one-pot reaction [303]. 

The Petasis reagent (Me2TiCp2, dimethyltitanocene) undergoes similar olefi-nation reactions with ketones and aldehydes. The originally proposed mechanism [3] was very different from that of Tebbe olefmation. However, later experimental data seem to suggest that both Petasis and Tebbe olefmation share the same mechanism, i.e. the carbene mechanism involving a four-membered titanium oxide ring intermediate [6]. 

The Petasis reagent (Me2TiCp2, dimethyltitanocene) undergoes similar olefination reactions with ketones and aldehydes [5]. However, the mechanism is very different. 

The mechanism of carbonyl methylenation with dimethyltitanocene 30 is one of the major subjects of discussion in titanium-carbene chemistry. Two reaction pathways have been proposed. Based on the observation of H/D scrambling in reactions using a deuterated ester and Cp2Ti(CD3)2, Petasis proposed that the reaction proceeds by methyl transfer to form the adduct 31 and subsequent elimination of methane and titanocene oxide (Scheme 4.29, Path A) [64]. Later, a detailed study by Hughes and co-workers using and D-labeled compounds showed that the methylenation of esters with 30 proceeds via a titanium carbene mechanism (Path B) [82]. 

Scheme 4.29. Plausible mechanisms for carbonyl methylenation with dimethyltitanocene.

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