Titanium complexes ligand metathesis reactions

Bis(cyclopentadienyl)titanium(II) dicarbonyl complexes, preparation and reactivity, 4, 250 Bis(cyclopentadienyl)titanium(II) dinitrogen complexes, preparation and reactivity, 4, 250 Bis(cyclopentadienyl)titanium halides ligand metathesis reactions, 4, 537 olefin polymerization, 4, 538 organic reactions, 4, 540 properties, 4, 530 reductions, 4, 532 synthesis, 4, 510... 

Also, switterionic titanium-imide complexes mediate the reaction of carbon dioxide with primary amines to give the corresponding isocyanates and symmetrical carbodiimides via a ligand metathesis reaction . 

Imido zwitterionic titanium complexes, Ti=NAr[CH3B(C6F5)3], undergo reaction with carbon dioxide to give isocyanates and symmetrical carbodiimides via a ligand metathesis... 

The macrocyclic calix[4]arene complex (Ti(Bu -calix[4]arene))2 was prepared from Ti(NMe2)4 and the neutral ligand. One of the four O atoms of each ligand bridges the two titanium centers.673 The related complex TiCl(But-calix[4]arene(0Me)(0)3) (61) was synthesized by a salt metathesis reaction. Subsequent displacement of the Cl by alkyl or aryl groups was readily accomplished.674... 

Complexes with /i -methylidenes are also known. Complex 2.41 in which methylidene and chloride bridges are present between titanium and aluminum is known as Tebbe s complex. Note that had we not known the structure, Tebbe s complex might have been formulated as Cp TiCCH ) plus Me AlCl. As will be seen, Tebbe s complex has relevance in alkene polymerization and metathesis reactions. Complex 2.42 is a unique example of a monometallic complex with a single carbon atom as one of the ligands. From the earlier examples it is clear that representative organometallic complexes with M-CR (n = 0-3) have all been isolated and fully characterized. 

As mentioned in the introduction, early transition metal complexes are also able to catalyze hydroboration reactions. Reported examples include mainly metallocene complexes of lanthanide, titanium and niobium metals [8, 15, 29]. Unlike the Wilkinson catalysts, these early transition metal catalysts have been reported to give exclusively anti-Markonikov products. The unique feature in giving exclusively anti-Markonikov products has been attributed to the different reaction mechanism associated with these catalysts. The hydroboration reactions catalyzed by these early transition metal complexes are believed to proceed with a o-bond metathesis mechanism (Figure 2). In contrast to the associative and dissociative mechanisms discussed for the Wilkinson catalysts in which HBR2 is oxidatively added to the metal center, the reaction mechanism associated with the early transition metal complexes involves a a-bond metathesis step between the coordinated olefin ligand and the incoming borane (Figure 2). The preference for a o-bond metathesis instead of an oxidative addition can be traced to the difficulty of further oxidation at the metal center because early transition metals have fewer d electrons. 

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