Titanium oxide, stereochemistry

Table 21.2 Oxidation states and stereochemistries of titanium, zirconium and hafnium...

Although the reaction of a titanium carbene complex with an olefin generally affords the olefin metathesis product, in certain cases the intermediate titanacyclobutane may decompose through reductive elimination to give a cyclopropane. A small amount of the cyclopropane derivative is produced by the reaction of titanocene-methylidene with isobutene or ethene in the presence of triethylamine or THF [8], In order to accelerate the reductive elimination from titanacyclobutane to form the cyclopropane, oxidation with iodine is required (Scheme 14.21) [36], The stereochemistry obtained indicates that this reaction proceeds through the formation of y-iodoalkyltitanium species 46 and 47. A subsequent intramolecular SN2 reaction produces the cyclopropane. 

The most common coordination number of titanium is six (recognized for all oxidation states of the metal), although compounds are known in which the coordination number is four, five, seven or eight. The common oxidation states of titanium with the associated coordination numbers and stereochemistries are summarized in Table 3. The properties of these molecules will be discussed in the appropriate sections. In brief, however, titanium compounds in the +III or lower oxidation states are readily oxidized to the +IV state. Furthermore, titanium compounds can usually be hydrolyzed to compounds containing Ti—O linkages. 

The titanium trichloride-diethylaluminum chloride catalyst converted butadiene to the cis-, trans,-trans-cyclododecatriene. Professor Wilke and co-workers found that the particular structure is influenced by coordination during cyclization between the transition metal and the growing diene molecules. Analysis of the influence of the ionicity of the catalyst shows effects on the oxidation and reduction of the alkyls and on the steric control in the polymerization. The lower valence of titanium is oxidized by one butadiene molecule to produce only a cis-butadienyl-titanium. Then the cationic chain propagation adds two trans-butadienyl units until the stereochemistry of the cis, trans, trans structure facilitates coupling on the dialkyl of the titanium and regeneration of the reduced state of titanium (Equation 14). 

The most common coordination number of titanium is six, although four-, five-, seven-, and eight-coordinate compounds are known (Table 2). Table 3 summarizes the common oxidation states of titanium with the associated coordination numbers and stereochemistries. Zirconium shows a similar range of oxidation states (see Zirconium Hafnium Inorganic Coordination Chemistry), however, Zr and Flfr are much less stable, relative to Zr and Hf, than is the case for titanium. 

Transition metal catalysts not only increase the reaction rate but may also affect the outcome of the oxidation, especially the stereochemistry of the products. Whereas hydrogen peroxide alone in acetonitrile oxidizes alkenes to epoxides [729], osmic acid catalyzes syn hydroxylation [736], and tungstic acid catalyzes anti hydroxylation [737]. The most frequently used catalysts are titanium trichloride [732], vanadium pentoxide [733,134], sodium vanadate [735], selenium dioxide [725], chromium trioxide [134], ammonium molybdate [736], tungsten trioxide [737], tungstic acid [737],... 

Interesting synthetic approaches for the construction of the tricyclo[5.2.2.0k5]undecane skeleton of the eremanes have been developed, but only two have been successful. The synthesis of ( )-eremolactone relied on an acid-catalysed double Michael addition on the silyloxydiene (Scheme 45) (127). This on treatment with mesityl oxide in the presence of titanium (IV) chloride gave, inter alia, an inseparable 1 2 mixture of diastereisomers (189) in 64% yield. Reduction with NaBH4 gave the separable hydroxy ketones (190 and 191, 1 2), the relative stereochemistry of which was secured from an X-ray study of 190. Following the introduction of the double bond, the side chain was elaborated on each of the two diastereoisomers as shown in Scheme 45. This synthesis has a number of problems. A complex mixture of isomers is generated in the first step, the cyclohexene... 

The X-ray Absorption Near Edge Structure (XANES) spectral features have been used as an additional method for elucidating the stereochemistry as well as the oxidation state of the metal. For example, based on the information available from XANES studies on titanium tetra-alkoxides, the most probable coordination state of titanium in trimeric ethoxide and butoxide [Ii(OR)4]3 (R = Et, Bu") has been suggested to be five. 

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