Titanocene complexes isomerization

In addition to phosphine ligands, a variety of other monodentate and chelating ligands have been introduced to functionalized polymers [1-5]. For example, cyclo-pentadiene was immobilized to Merrifield resins to obtain titanocene complexes (Fig. 42.13) [102]. The immobilization of anionic cyclopentadiene ligands represents a transition between chemisorption and the presently discussed coordinative attachment of ligands. The depicted immobilization method can also be adopted for other metallocenes. The titanocene derivatives are mostly known for their high hydrogenation and isomerization activity (see also Section 42.3.6.1) [103]. 

The reactions of titanium-alkylidenes prepared from thioacetals with unsymmetrical olefins generally produce complex mixtures of olefins. This complexity arises, at least in part, from the concomitant formation of the two isomeric titanacyclobutane intermediates. However, the regiochemistry of the titanacyclobutane formation is controlled when an olefin bearing a specific substituent is employed. Reactions of titanocene-alkylidenes generated from thioacetals with trialkylallylsilanes 30 afford y-substituted allylsilanes 31, along with small amounts of homoallylsilanes 32 (Scheme 14.16) [28]. 

Titanocene(n) species promote the conversion of unsaturated thioacetals to cyclic compounds. This cyclization proceeds with the loss of the terminal alkene carbon. Treatment of the thioacetal 83 with the low-valent titanium species Cp2Ti[P(OEt)3]2 (3 equiv) in refluxing THF afforded benzoxocines 86 and 87 (by isomerization of 86) in 61% yield (Scheme 14) <1999SL354>. Using 4 equiv of the titanocene(n), the yield is higher (70%) but the selectivity is lower (the ratio 86 87 becomes 82 18). The mechanism or the reaction probably involves the formation of the titanium carbene complex 84, its intramolecular reaction with the double bond to form titanocyclobutane 85, and the subsequent elimination of methylidenetitanocene <1999SL354>. 

Ring inversion has been reported in the 1,2,4,5-benzBnetetrathiolato binuclear complexes of titanocene, zirconocene, and hafnoc ne.< Isomerization in dinuclear iron complexes includes dsjtrans exchange in [( / -Cp)2Fe2(CO)2(/<-CO)(/2-SiX Bu)] (X = C1, Br, I, Me)< and an inversion of the butterfly-like bent skeleton of an (oxodimethylidenemethane)iron dimer (Scheme 25). ... 

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