Titanium chemistry

A clear limitation of titanium chemistry has to do with the observation that such sterically hindered ketones as 58 do not react well with the less reactive n-alkyltitanium reagents 77). Of course, this does not apply to sterically non-hindered steroidal C22-aldehydes, which react smoothly and with high degrees of asymmetric induction 77). 

An interesting example of an application of this method pertains to the synthesis of pharmacologically active synthetic A1 -tetrahydrocannabinoids 134) of the type 267 which have a lipophilic tertiary alkyl side-chain. Equation 84 shows that organo-titanium chemistry provides a versatile means to prepare the precursors 264 (65-80 %)133). Demethylation of 264 using trimethylsilyl chloride and sodium iodide affords the resorcinol derivatives 265 ( 95%)133>. Compounds of this type have been previously condensed with 266 in the presence of acids to form the A1(6)-isomers of 267, which in turn can be converted into 267135). It should be mentioned that the meta-substitution pattern of 265 prohibits simple Friedel-Crafts alkylation of resorcinol, which is the reason why alternative multistep syntheses of 264 have had to be developed l34 136>. 

A28. R. Feld and P. L. Cowe, The Organic Chemistry of Titanium. Butterworth, London, 1965. Summary of descriptive organo-titanium chemistry, including many references to patent literature little theoretical treatment. 

Optically active propargylstannanes have been prepared by this route from the optically active phosphates, and then reduced to the corresponding allylstannanes, again using titanium chemistry.8... 

Monoanionic boratabenzene ligands can formally be derived from benzene by replacement of GH by BRT. Boratabenzene ligands are less basic and less nucleophilic than Cp rings, but replacement of Cp by boratabenzene ligands in titanium chemistry has been studied. 

This chapter reviews a range of recent calculations on several different problems involving titanium chemistry, performed primarily by this group. We begin, in Section 2, by considering the theoretical and computational methods that have been used. This is followed, in Section 3, by a discussion of unusual structures and associated potential energy surfaces that occur in titanium chemistry due in large part to the electron-deficient nature of this element. In Section 4, the potential use of divalent Ti as a catalyst is discussed. A summary and discussion of future topics is presented in Section 5. 

An interesting linkage reaction between ethene and carbon dioxide is reported in titanium chemistry [1]. Bis(pentamethylcyclopenta-dieny1(ethene)titanium(II) reacts rapidly with one equivalent of carbon dioxide in a toluene solution at -78°C affording a titanalac-tone in yields up to 80 % (Equation 1). The complex is thermally robust both as a solid and in solution but decomposes slowly upon exposure to visible light. 

Very related to the titanium chemistry described above are experiments with resembling molybdenocene complexes. If the molybdenum-diphenyl-acetylene complex Cp2Mo(PhC=CPh) is used as starting material, no C-C linkage reaction takes place with carbon dioxide. Instead, a crystalline solid was isolated whose x-ray proved the formation of a molybdenum-carbon dioxide complex [85] (Equation 15). 

Transition-metal catalysed asymmetric synthesis has again seen further developments over the past year. Of particular note are stereoselective aldol condensations, and asymmetric epoxidation reactions. Hydrogenation methods have seen few significant advances, and are considered only briefly this year. The important area of carbon-carbon bond-forming reactions continues to attract considerable attention although much work still uses palladium, iron, or copper, nickel and titanium chemistry is also being increasingly used, a trend that will no doubt continue. 

The chemistry of titanium is of considerable importance, primarily because of its roles as a catalyst in various chemical reactions (e.g silane polymerization (1), hydrosilation (2), and Ziegler-Natta (3) polymerization), as materials and materials precursors, and as the basis for electronic and magnetic devices. In the past several years, the interest in titanium chemistry in this group has focused on its fundamental molecular and electronic structure in a variety of chemical environments, on its function as a catalyst in the hydrosilation and bis-silylation reactions, and on the nature of the structure, bonding, and mechanism of formation of metallocarbohedrenes. 

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