Titanium-imido complex

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. 

A range of chloride metathesis reactions of the monomeric titanium N,N -bis(trimethylsilyl)benzamidinato-imido complexes have been described. These... 

The Cp Ti(amidinate) fragment provides a particularly useful platform for the synthesis of novel titanium imido complexes and the study of their unusual reactivity. Numerous single-, double-, and cross-coupling and imido-transfer reactions were investigated using these compounds. The synthetic routes leading to CpTi and Cp Ti amidinate derivatives are outlined in Scheme... 

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. ... 

The guanidinate-supported titanium imido complex [Me2NC(NPr02l2Ti = NAr (Ar = 2,6-Me2C6H3) (cf. Section IILB.2) was reported to be an effective catalyst for the hydroamination of alkynes. The catalytic activity of bulky amidinato bis(alkyl) complexes of scandium and yttrium (cf. Section III.B.l) in the intramolecular hydroamination/cyclization of 2,2-dimethyl-4-pentenylamine has been investigated and compared to the activity of the corresponding cationic mono(alkyl) derivatives. 

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. 

Isocyanate formation through multiple bond metathesis of C02 with carbodiimide has been also demonstrated [112]. This transformation can be promoted by titanium isopropoxide, at 383 K, in THF as solvent. It is worth noting that the reverse process, which opens an entry into carbodiimide synthesis, is a well-known process that is catalyzed by several other systems, including trialkylphosphine oxides [113] or vanadium-oxo or -imido complexes [114]. 

Imidazolium ligands, in Rh complexes, 7, 126 Imidazolium salts iridium binding, 7, 349 in silver(I) carbene synthesis, 2, 206 Imidazol-2-ylidene carbenes, with tungsten carbonyls, 5, 678 (Imidazol-2-ylidene)gold(I) complexes, preparation, 2, 289 Imidazopyridine, in trinuclear Ru and Os clusters, 6, 727 Imidazo[l,2-a]-pyridines, iodo-substituted, in Grignard reagent preparation, 9, 37—38 Imido alkyl complexes, with tantalum, 5, 118—120 Imido-amido half-sandwich compounds, with tantalum, 5,183 /13-Imido clusters, with trinuclear Ru clusters, 6, 733 Imido complexes with bis-Gp Ti, 4, 579 with monoalkyl Ti(IV), 4, 336 with mono-Gp Ti(IV), 4, 419 with Ru half-sandwiches, 6, 519—520 with tantalum, 5, 110 with titanium(IV) dialkyls, 4, 352 with titanocenes, 4, 566 with tungsten... 

The metathesis of symmetrical aliphatic carbodiimides is also catalyzed by tungsten imido complexes above 140 Other carbodiimide metathesis catalysts include Cr(ll)/Si02/ iminophosphoranes, imido circonocenes, guanidine supported titanium complexes and group 14 amide complexes. ... 

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 chloro imido-amidinato titanium complex TiGl(NBut)[PhC(NSiMe3)2](py)2 reacts with LiCH2SiMe3 or with LiCH(SiMe3)2 in benzene to give rare examples of terminal imido monoalkyltitanium derivatives TiR(NBut)[PhG(NSiMe3)2](py) (Scheme 36). The molecular structure of the trimethylsilylmethyl derivative has been determined by X-ray diffraction.90... 

Titanium amido and imido complexes are of increasing interest as reagents in organic transformations and catalysis.1 1 For ease of representation, the terminal titanium-imido or titanium-oxo linkages are generally drawn as Ti=X (X = NR, O). Nevertheless, the formal metal-ligand multiple bonds in these complexes are better described as a2-tt4 triple bonds. 

Scheme 11 Preparation of catalytically competent terminal titanium imido complex...

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. 

---