Amido complexes late-transition-metal

The cleavage of allcylamine N-H bonds by late transition metals to form metal amido complexes is also rare [69, 70]. When the transition metal is a low valent, late metal, the resulting amido complexes are highly reactive [71, 72]. It appears that the amination of aryl halides can involve an unusual N-H activation process by a palladium alkoxide to form a highly reactive palladium amide [65, 73]. 

The 1,2-addition of C—H bonds across metal-heteroatom bonds has been reported for two different classes of complexes early transition metal d° complexes with imido ligands and late transition metal complexes with amido, hydroxo, and aryloxo ligands (Scheme 11.36). These transformations are potentially related to o-bond metathesis reactions discussed above however, the presence of a lone pair on the heteroatom that receives the activated hydrogen may impart important differences. 

Hydroamination is an atom-economical process for the synthesis of industrially and pharmaceutically valuable amines. The hydroamination reaction has been studied intensively, including asymmetric reactions, and a variety of catalytic systems based on early and late transition metals as well as main-group metals have been developed." However, Group 5 metal-catalysed hydroaminations of alkenes had not been reported until Hultzsch s work in 2011. Hultzsch discovered that 3,3 -silylated binaphtho-late niobium complex 69 was an efficient catalyst for the enantioselective hydroaminoalkylation of iV-methyl amine derivatives 70 with simple alkenes 71, giving enantioselectivities up to 80% (Scheme 9.30). Enantiomerically pure (l )-binaphtholate niobium amido complex 69 was readily prepared at room temperature in 5 min via rapid amine elimination reactions between Nb(NMe2)5 and l,l-binaphthyl-2-ol possessing bullqr 3,3 -silyl substituents. Since the complex prepared in situ showed reactivity and selectivity identical... 

Overview of Metal-Amido Complexes of the Late Transition Metals... 

The reaction chemistry of late-transition-metal-amido complexes resembles that of organometallic complexes more than that of early-transition-metal amides. Thus, the chemistry of this class of amido complex is presented first. Several reviews of the chemistry of late-metal amido complexes have been published. -... 

The properties of late-transition-metal-amido complexes result, in part, from the mismatch of a hard ligand with a soft metal. This pairing of ligand and metal leads to a... 

As noted again later in the section on late-transition-metal-alkoxo complexes, the role of hard/soft and dir-pir interactions in controlling the properties of these compounds has been debated. In some cases, the "ir-repulsion has been cited as a factor that leads to the instability of these complexes, but in other cases arguments have been made that the reactivity of these complexes can be explained without invoking "ir-donation. In other cases, ar-donation from an amido group into an unoccupied orbital of a 16-electron late transition metal complex has been proposed to account for the structures of such species. ... 

Late-transition-metal-amido complexes have been prepared by metathetical substitution reactions, or-bonded ligand exchange, deprotonation of amine complexes, and oxidative addition of N-H bonds. Metathetical substitution is the most common route to late-metal-alkylamido complexes, whereas metathetical substitution and a-bonded ligand exchange have both been used commonly to prepare arylamido compounds. 

Like late-transition-metal-amido complexes, pir-dir interactions between the electron pair on oxygen and the filled d-orbitals on the metal can affect the thermod)mamic stability and the reactivity of aUcoxo complexes. Naturally, this effect in metal-aUcoxo complexes is less pronounced than in metal-amido complexes because of tihe lower basicity of an alkox-ide. At the same time, the presence of two electron pairs on oxygen causes this effect in aUcoxo complexes to depend less on geometry than in amido complexes. Sudh Tr-interactions have been studied in detail by Caulton, and have been used to rationalize the geometries, nucleophilicity, and basicity of late metal alkoxides and amides (Figure 4.16). ... 

However, the most common route to metal-imido compounds is some type of fv-elimination. For example, imido complexes have been prepared by the addition of amine or alkali metal amides to a metal halide (Equations 13.54 and 13.55). This reaction most likely occurs through an a-elimination from an amido halide intermediate. In addition, the first low-valent, late-transition-metal-imido complex was prepared by the simple reaction of [Cp rCyj with hindered lithium amides (Equation 13.56). - ... 

The elimination of an alkane from an amido alkyl complex has also formed imido complexes in many cases (Equations 13.57 and 13.58), ° and reactions of a metal polyha-Ude with a silylamine has generated metal-imido complexes. Deprotonation of a cationic amido complex has been used as a route to a monomeric, late transition metal terminal... 

Dimetallic elimination reactions leading to metal-metal bond formation are the amine or alkane eliminations that result from the condensation of a late transition metal hydrido complex with an early transition metal amido or alkyl complex, respectively. Examples of this method are Selegue s synthesis of the first Ti-Fe and Ti-Ru complexes 8a,b [8, 9] and the reaction of [Zr(CH2Ph)4) with [CoH(CO)4], although only spectroscopic evidence was provided for compound 18 (Scheme 4.3) [20]. 

Early transition metal, lanthanide and actinide alkoxy and amido complexes are common, and they often are stable because of the interaction between the filled p orbital of the O or N atom of the ligand and an empty d metal orbital. The alkoxy and aryloxy ligands play a crucial role in the catalytic properties of group 5-7 metal-alkylidene and metal-alkylidyne complexes for the metathesis of simple, double and triple bonds. - On the other hand, the behavior of late transition-metal alkoxy and amido complexes is less known. Many of them are stable, however, in spite of the possible repulsion between the filled d orbital and the p orbital of the heteroatom. The metal-heteroatom bonds are robust, and the main characteristic of these is that they are strongly polar and possess a significant ionic character. They exhibit nucleophilic reactivity and sometimes form strong bonds to proton donors (they even deprotonate relatively weak acids). 

Amine activatitMi pathway has been well studied in catalysis by lanthanides, early transition metals, and alkali metals. In metal amide chemistry of late transition metals, there are mainly two pathways to synthesize metal amide complexes applicable under hydroamination conditions [54], One is oxidative addition of amines to produce a metal amide species bearing hydride (Scheme 8a). The other gives a metal amide species by deprotonation of an amine metal intermediate derived from the coordination of amines to metal center, and it often occurs as ammonium salt elimination by the second amine molecule (Scheme 8b). Although the latter type of amido metal species is rather limited in hydroamination by late transition metals, it is often proposed in the mechanism of palladium-catalyzed oxidative amination reaction, which terminates the catalytic cycle by p-hydride elimination [26]. Hydroamination through aminometallation with metal amide species demands at least two coordination sites on metal, one for amine coordination and another for C-C multiple bond coordination. Accordingly, there is a marked difference between the hydroamination via C-C multiple bond activation, which demands one coordination site on metal, and via amine activation. 

The design of the PNP amidodiphosphine ligand was based on the idea that the combination of both hard and soft donors should allow access to a variety of oxidation states for different transition metal complexes. For the late metals, the soft phosphine donors would serve to stabilise the hard amido-ligand bond while for the early transition elements, the amido donor would anchor the soft phos-... 

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