Titanium imido complexes, reaction with

Titanium imido complexes supported by amidinate ligands form an interesting and well-investigated class of early transition metal amidinato complexes. Metathetical reactions between the readily accessible titanium imide precursors Ti( = NR)Cl2(py)3 with lithium amidinates according to Scheme 84 afforded either terminal or bridging imido complexes depending on the steiic bulk of the amidinate anion. In solution, the mononuclear bis(pyridine) adducts exist in temperature-dependent, dynamic equilibrium with their mono(pyiidine) homologs and free pyridine. 

The formation of a bis(guanidinate)-supported titanium imido complex has been achieved in different ways, two of which are illustrated in Scheme 90. The product is an effective catalyst for the hydroamination of alkynes (cf. Section V.B). It also undergoes clean exchange reactions with other aromatic amines to afford new imide complexes such as [Me2NC(NPr )2]2Ti = NC6F5. ... 

Titanium imido complexes [Ti(NBu-f)(L)(py)] (I MeC(2-C5H4N) (CH2N(3,5-C6H3Mc2))2) wdth aryl acetylenes afford [2+2] cycloaddition products 76 (Ar = Ph, p-Tol) (08OM2518). Reaction with a second equivalent of alkyne affords azatitanacyclohexadienes 77 (07OM5522). 

The kinetic and thermodynamic selectivity for reactions of a titanium-imido complex with different types of C-H bonds has been determined. Reactions with substrates that possess primary and secondary C-H bonds occur selectively at the primary C-H bond. In addition, reactions with mixtures of alkanes and arenes occur selectively at the arene C-H bond. Like the stabilities of most low-valent, late metal complexes, the primary alkyl complex is thermodynamically more stable than the secondary alkyl complex, and the aryl complexes are more stable than the alkyl complexes. Activation of olefins at the ally-lie position occurs more slowly than reaction at the vinyl position, but when it does occur, the reaction generates a stable Ti -allyl complex. 

Several other studies have been reported on the utilization of P=N motif eontaining ligands. Thus, a titanium-imido complex (32) was stabilized by bis(iminophosphoranyl)methanide ligands. This complex undergos dynamic isomerization. Also, complex (32) displayed modest catal3rtic activity in the reaction of aniline with phenyl acetylene. ... 

The nudeophile is activated by the formation of a titanium(IV)-imido complex 19. The next step is a [2 + 2] cydoaddition with one of the jt-bonds of the allene, depending on the regioselectivity leading to either 20 or 22. Compound 20 then delivers 21 by twofold stepwise proto-demetallation and the latter enamine tau-tomerizes to the imine 24 (Scheme 15.3). Compound 22, on the other hand, should provide allylamines 23, but as we shall see, there are no examples of that mode of reaction known so far. 

Inter- and intramolecular (cyclometallation) reactions of this type have been ob-.served, for instance, with titanium [408,505,683-685], hafnium [411], tantalum [426,686,687], tungsten [418,542], and ruthenium complexes [688], Not only carbene complexes but also imido complexes L M=NR of, e.g., zirconium [689,690], vanadium [691], tantalum [692], or tungsten [693] undergo C-H insertion with unactivated alkanes and arenes. Some illustrative examples are sketched in Figure 3.37. No applications in organic synthesis have yet been found for these mechanistically interesting processes. 

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 first hydroaminations by this mechanism were reported by Bergman with zircono-cene complexes and by Livinghouse with monocyclopentadienyl titanium and zirconium complexes. Bergman reported the intermolecular addition of a hindered aniline to an alkyne. The hindrance of the aniline was important to prevent formation of stable dimeric complexes containing bridging imido groups. Livinghouse reported intramolecular reactions that occurred at lower temperatures over shorter times. The intramolecularity of this process allows the [2+2] cycloaddition of the imido complex with the alkyne to be faster than the dimerization. 

Scheme 14.138). A ligand accelerating effect has been observed with the titanium bis (sulfonamide)imido complex. This complex can promote the reaction at room temperature with reaction rate comparable to that of its precursor, supporting the hypothesis of a catalytically active titanium imido intermediate. 

Imido complexes of titanium, supported by cyclooctatetfaene ligands, react with phenyl isothiocyanate across the C=S bond via a [2-t-2] cycloaddition reaction to give a four-membered ring metallacycle . 

The insertion products of diphenylcarbodiimide into Ti(0-i-Pr)4 carry out metathesis reactions at elevated temperatures by insertion of an equivalent of carbodiimide in a head to head fashion followed by an extrusion reaction Also, insertion into a Ti-C bond is observed in the reaction of CpTiMcs with carbodiimides to form CpTiMe2[NRC(Me)NR] Insertion into Ti-C bonds is also observed in the reaction of diisopropylcarbodiimide with imido titanium cations Carbodiimides also insert into Ta-N bonds in a mixed tantalium amido/imido complex... 

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