Hydroamination transition metal amides

Summaries of results of hydroamination mediated with Rh(I) amide complexes584 and comprehensive reviews giving detailed information of the field are available.585-587 Therefore, only the more important relatively new findings are presented here. In most of the transformations reported transition metals are applied as catalysts. The feasibility of the use of tcrt-BuOK was demonstrated in the base-catalyzed amination of styrenes with aniline.588... 

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.

Early transition metals or main group metals other than the rare earth elements have been scarcely used in intramolecular diene hydroamination. However, the lithium amide-catalyzed cyclization of aminodienes was recently reported [134, 135] in the context of asymmetric hydroamination and will be discussed in Sect. 6.2. 

The inteimolecular hydroamination of allenes is readily catalyzed by early transition metal complexes to yield imines. An addition of aromatic and ahphatic amines to aUene requires high reaction temperatures (90-135°C) and long reaction times (1-6 days) when mediated by zirconocene- [41] and tantalum-imido [178] catalysts. The more efficient titanium half-sandwich imido-amide complex 42 operates under significantly milder reaction conditions (27) [179], Because the metal-imido species are prone to dimerization, sterically more hindered aliphatic and aromatic amines are more reactive. Simple, sterically unencumbered aliphatic amines add to aUenes in the presence of the bis(amidate) titanium complex 43 (28), although higher reaction temperatures are required [180]. 

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. 

An alternative mechanism starts from the coordination of an amine, and the successive deprotonation gives a metal amide species (Scheme 8b). Coordination of a C-C multiple bond to this metal center is followed by migratory insertion into the M-N bond. The newly formed M-C bond is cleaved by protonolysis to regenerate the active metal species. The advantage of this pathway is that it does not require the change of oxidation number of metal, and it looks similar in mechanism to hydroamination of other group metals (for group 4 metals, metathet-ical reaction takes place at the step of C-N bond formation) and partially similar in mechanism to oxidative amination of late transition metals. However, so far, most hydroamination reactions catalyzed by late transition metals can be explained by the mechanisms discussed in Sects. 3.1 and 3.2.2. If the activation of the C-C... 

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