Olefin complexes metal amides

Cationic transition metal amide complexes have been investigated in part because of their potential in catalysis p irticularly for olefin polymerization. Much of this work has concerned polydentate amido, linked cyclopentadienyl-amido or delocalized nitrogen centred bidentate ligands (see later). However, the structures of a small number of cationic complexes containing monodentate amido ligands have been determined. These include... 

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. 

Non-metallocene complexes, such as aryloxide 31 and amide 138, have also been utilized as catalyst systems for the polymerization of a-olefins. Moreover, the homogeneous olefin polymerization catalysts have been extended to metals other than those in Group 4, as described in Sect. 7. Complexes such as mono(cyclopentadienyl)mono(diene) are in isoelectronic relationship with Group 4 metallocenes and they have been found to initiate the living polymerization of ethylene. These studies will being further progress to the chemistry of homogeneous polymerization catalysts. 

First, solvent molecules, referred to as S in the catalyst precursor, are displaced by the olefinic substrate to form a chelated Rh complex in which the olefinic bond and the amide carbonyl oxygen interact with the Rh(I) center (rate constant k ). Hydrogen then oxidatively adds to the metal, forming the Rh(III) dihydride intermediate (rate constant kj). This is the rate-limiting step under normal conditions. One hydride on the metal is then transferred to the coordinated olefinic bond to form a five-membered chelated alkyl-Rh(III) intermediate (rate constant k3). Finally, reductive elimination of the product from the complex (rate constant k4) completes the catalytic cycle. 

The Ir(III) metal centres in the products, which are bound to a terminal hydride and a bridging —NH2 group, represented the first X-ray stnictural authentication of a transition metal species with both amide and hydride bound to a metal centre. The reactivity of the complexes is low, however, and appears to be dominated by the stability of the lr(p-NH2)2lr bridging unit. More recent work has shown that olefin-iridium(l) complexes, such as the propene species [ HC(CH2CH2PBu 2)2 Ir(CH2CHMe)], react diiectly with ammonia at room temperature as shown in Equation (6.12)." ... 

After successful application of the silver catalyst shown in olefin aziridination (Section 6.1.1), He and coworkers showed that intramolecular amidation was possible with both hydrocarbon-tethered carbamates and sulfamate esters.24 They found that only the Bu3tpy silver complex could catalyze efficient intramolecular amidation, while other pyridine ligands gave either dramatically lower yields or complicated product mixtures. In an interesting control study, both copper and gold were also tested in this reaction. Both the copper and gold Bu tpy complexes can mediate olefin aziridination, but only silver can catalyze intramolecular C-H amidation, indicating that the silver catalyst forms a more reactive metal nitrene intermediate. 

When the biphosphine is chiral, but posesses a symmetry axis, there are only two possible diastereomeric modes of binding in an enamide complex in which olefin and amide groups are coordinated to the metal. R - Phenylbis (diphenylphosphino) ethane complexes are disymmetric and consequently there are four possible enamide complexes - either face of the olefin may be cis or trans to PI. Complexation of methyl a-acetamidoacrylate gives only one of these but complexation of acetamidocinnamic acid or its esters leads to two diastereomeric complexes (21) (Figure 2). A combination of labelling experiments demonstrates that these are regioisomers (same olefin face coordinated to the metal) rather than stereoisomers. 

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