Ketenes transition metal catalysis

Free organic ketenes are very reactive molecules. They can be stabilized by transition metal complexes. Transition metal ketene complexes are important intermediates in catalysis, such as in the Fischer-Tropsch process . Ketenes can be bonded to transition metal conq>lexes in a wide variety of ways . The two common bonding modes for mononuclear ketene complexes are r -(C, O) and ti -(C, C). The ii -(C, O) mode is considered to be favored by early transition metal complexes due to the electrophiUc properties of the metals, while the r] -(C, C) mode is preferred by late transition metals. Indeed, several nickel ketene conq)lexes have been characterized spectroscopically to have the C) bonding mode, and these ketene complexes have demonstrated nmch... 

Books have been published on cyclic polyolefin complexes and metal vapour synthesis, while a review on metal vapour cryochemistry also contains information relevant to the latter. Reviews have also been published on organometallic derivatives of alkenes and ketenes, cationic Rh diolefin complexes, and nucleophilic addition to cationic hydrocarbon complexes. More personalized reviews have appeared by Maitlis on Pd -acetylene chemistry, by Jonas and Kruger on alkali-metal-transition-metal w-complexes, and by Werner on bridged allyl and cyclopentadienyl complexes. Articles by Wilke on homogeneous catalysis, by Vollhardt on metal-mediated approaches to steroid synthesis, by Schrock > on organotantalum chemistry, and by Grubbs on nickel metallocycles also contain material of relevance to r-complexes. 

At the time the chemistry of main group enolates flourished already for a while, that of late transition metals had a shadowy existence in synthetic organic chemistry. Their stoichiometric preparation and the sluggish reactivity - tungsten enolates, for example, required irradiation to undergo an aldol addition [24a] - did not seem to predestine them to become versatile tools in asymmetric syntheses [27]. The breakthrough however came when palladium and rhodium enolates were discovered as key intermediates in enantioselective catalyses. After aldol reactions of silyl enol ethers or silyl ketene acetals under rhodium catalysis were shown to occur via enolates of the transition metal [8] and after the first steps toward enantioselective variants were attempted [28], palladium catalysis enabled indeed aldol additions with substantial enantioselectivity... 

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