Late transition metal amide

The thermodynamics of the oxidative addition process tends to be favored by increased electron density at the metal centre, hence the focus on later transition metal derivatives. Furthermore, as discussed above, it is believed that M—N n-bonds to the later transition metals are of significance only if the transition metal complex is unsaturated. Saturated late transition metal amides (parent or substimted) often exhibit the so-called n-conflict (see above) so that the nitrogen centre displays no n-bonding to the metal and retains its lone pair character and basicity. 

H.E. Bryndza u. W. Tam, Chem. Rev. 88, 1163-1188 (1988) Monomeric Metal. .Hydroxides, Alkoxides, and Amides of the Late Transition Metals Synthesis, Reactions, and Thermochemistry". 

In general, amino-containing compounds are poor partners in CM due to their tendency to coordinate to the metal catalyst, even when late-transition metal systems are employed. To preserve catalyst activity, amino groups are therefore typically masked as cyanides, carbamates, amides, or phthalimides. In substrates where iV-coordination to the catalyst is less favored, such as with hindered /V-aryl-/V-allylamines, no protecting group is required. 

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 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. -... 

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). ... 

As described in Chapter 2, phosphines have sufficiently large barriers to inversion that phosphines with three different substituents can be resolved. In contrast, the inversion of configuration at phosphorus in metal-phosphido complexes tends to occur near room temperature. In imsaturated transition metal complexes, a vacant acceptor orbital stabilizes the planar transition state. This effect is shown by the interconversion between pyramidal structures through a planar transition state, like the one shown in Equation 4.101 and formed during inversion of related compounds. - In saturated middle and late transition metal complexes, inductive effects explained below destabilize the groimd state and lead to lower barriers to inversion. The presence of an ancillary planar phosphide or amide ligand can contribute to a low barrier by accepting the lone pair from a p)Tamidal phosphide, as in Equation 4.101. 

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). - ... 

Few examples of intramolecular additions of amines to alkenes catalyzed by late transition metals have been published more examples of the additions of amides, carbamates, and tosylamides to alkenes catalyzed by this type of complex have been reported. Addition of a secondary amine across a tethered olefin catalyzed by a simple platinum-halide complex is shown in Equation 16.65a. A more recent catalyst based on [Rh(COD)JBF and a biaryldialkylphosphine leads to cyclizations of aminoalkenes with greater scope (Equation 16.65b). These reactions occur to form five- and six-membered rings, with or without groups that bias the system toward cyclization. They also occur with both internal and terminal olefins and with both primary and secondary amines. 

The hydroamination of olefins has been shown to occur by the sequence of oxidative addition, migratory insertion, and reductive elimination in only one case. Because amines are nucleophilic, pathways are available for the additions of amines to olefins and alkynes that are unavailable for the additions of HCN, silanes, and boranes. For example, hydroaminations catalyzed by late transition metals are thought to occur in many cases by nucleophilic attack on coordinated alkenes and alkynes or by nucleophilic attack on ir-allyl, iT-benzyl, or TT-arene complexes. Hydroaminations catalyzed by lanthanide and actinide complexes occur by insertion of an olefin into a metal-amide bond. Finally, hydroamination catalyzed by dP group 4 metals have been shown to occur through imido complexes. In this case, a [2+2] cycloaddition forms the C-N bond, and protonolysis of the resulting metallacycle releases the organic product. 

Scheme 15.18 General mechanism of late-transition-metal-catalyzed hydroamination with insertion into metal-amide or metal-hydride bonds.

While the formation of C—O bond in Fenton and Gif chemistry is well documented, the formation of C—N bonds through the activation of unactivated sp C—H bonds has been challenging. The primary nitrogen source in most reported procedures involve the use of nitrenes or their derivatives. " Much of this chemistry anploys the expensive late transition metals. It would be beneficial to employ inexpensive first row transition metals as catalysts to generate C—bonds. An early example that demonstrates this is an efficient, inexpensive, and air-stable catalyst/an oxidant (FeCl2 and A-bromosuccinimide or NBS) system that promotes amidation of ben-zylic sp C—H bonds in ethyl acetate under mild conditions. ... 

Bryndza HE, Tam W (1988) Monomeric metal hydroxides, alkoxides, and amides of the late transition metals synthesis, reactions, and thermochemistry. Chem Rev 88 1163-1185... 

M[N(SiR3)2] can then be purified—for the main group derivatives (for application in the synthesis of alkoxides, see Zn (Goel, 1990), Cd (Boulmaaz, 1992), Pb (Matched, 1990 Papiemik, 1989), Bi (Massiani, 1990 Goel, 1990))—by sublimation direct from the reaction mixture, after removal of the Et20 in vacuum, and—for the early transition metal con tounds (Cr(II), Mn(ll) (Horvath, 1979)), after the removal of ether—by the extraction from the residue with pentane or hexanes, separating LiCl by decantation. It should be mentioned that this approach is hardly practically applicable for the synthesis of the derivatives of late transition metals such as Co, Ni or Cu because of poor stability of their amide derivatives (Bryndza, 1988). 

In comparison to late transition metals (LTMs), reports of early transition metal (ETM) complexes with phosphane ligands are scarce. On the contrary, amides are more common ligands for ETMs than for LTMs. This can be explained by the electronic demands of the hard Lewis acidic ETMs, which... 

Examples of this behavior at late transition metal centers are also known, when the amide is stabilized, for example, by coordination to a ring. Thus, the dicarbonyl complex, [Re(CO)2(PPh3)2 S2CN(Me)CNSC2H2 ], has been prepared in low yield from the thermolysis of [Re(CO)2(PPh3)2(mat)] (mat = 2-methylaminothiazole) with excess carbon disulfide (Eq. 20) (194). 

Activation of an amine by a coordinatively unsaturated late transition metal, which leads to metal amide species, is also proposed as a potential pathway (Scheme 8). 

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