Metal-amide bonding

The possibility of insertion of C02 into metal-amide bonds via a mechanism not unlike S02 insertion into metal-alkyl bonds remains a viable possibility in C02 chemistry although it was not realized in these studies by Chisholm and co-workers. In this mechanism, C02 acting as an electrophile, attacks an amide nitrogen, with the intermediate then rearranging to the metal carbamate, (68). 

Certain metal hydrides that are acidic are also known to react with metal-amide bonds liberating amine and forming metal-metal bonds (equations 74220,221 and 7 5222). 

In conclusion, we have demonstrated that the reactions of metal amide bonds, M-NR2, with acidic hydrogen atoms of wVfo-carboranes provide an attractive route to the synthesis of metallacarboranes of the early/middle transition elements. The structural preferences of the amide ligands in the products provide us with valuable lessons on the relationship between electronic and molecular structures. Early transition metal... 

The reaction of 16-electron carbonyl complexes of Pd and Pt with alcohols and amines may presumably proceed without metal-alkoxy or metal-amide bond formatiom (cf. Scheme 8.5). The reaction of alkoxymetal complexes with carbon monoxide offers an interesting mechanistic diversity (Scheme 8.13). 

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. 

H2L435 and diamines. Attempts to carry out non-template macrocyclisation under conditions of high dilution were also unsuccessful only starting materials and small amounts of insoluble polymeric materials were isolated from the reaction mixture. It proved to be impossible to demetallate the oxamide macrocyclic nickel(II) complexes using cyanide because of the high stability of the metal-amide bonds 382]. 

The strength of the metal-amide bond in yttrium complex 57 (Scheme 6.6) allowed Arnold et al. to monitor the reactivity of the NHC-metal bond via Jyc coupling in NMR. Triphenylphosphane or trimethylamine A-oxide did not displace the carbene, showing again that the NHC moiety has stronger donating properties than most other donors. Only tmeda and triphenylphosphane oxide successfully dissociated the carbene to form the corresponding metal complexes with pendant NHCs. 

The hydroamination/bicyclization of diaUcenylamines, dialkynylamines and alken-ylalkynylamines opens a straightforward route to a family of bicyclic amines in a tandem C-N and C-C bond-forming process. An important prerequisite for the success of this reaction sequence is a sufficient lifetime of the metal alkyl intermediate formed in the initial insertion process of the alkene/alkyne in the metal-amide bond in order to permit the carbocyclization step. Close proximity of the unsaturation to the metal-amide moiety allows facile bicyclization over protonolysis leading to the normal hydroamination product. Lanthanocene catalysts have been found applicable for this transformation (Scheme 13) [23,139]. 

The last isomerization is remarkable in that the triple bond can shift through a long carbon chain to the terminus, where it is fixed as the (kinetically) stable acetylide. The reagent is a solution of potassium diami no-propyl amide in 1,3-di-aminopropane. In some cases alkali metal amides in liquid ammonia car also bring about "contra-thermodynamic" isomerizations the reactions are successful only if the triple bond is in the 2-position. 

Polymers, metals, ceramics, and glasses may be utilized as biomaterials. Polymers (see Ppolymerprocessing), an important class of biomaterials, vary gready in stmcture and properties. The fundamental stmcture may be one of a carbon chain, eg, in polyethylene or Tedon, or one having ester, ether, sulfide, or amide bond linkages. PolysiHcones, having a —Si—O—Si— backbone, may contain no carbon. 

Coordinating properties of the amide bond. Stability and structure of metal ion complexes of peptides and related ligands. H. Sigel and R. B. Martin, Chem. Rev., 1982, 82, 385-426 (409). 

Metal-Nitrogen Bond Lengths and Torsion Angles between the Metal and Nitrogen Coordination Planes for Three-Coordinate Aluminum, Gallium, Indium, and Thallium Amides... 

The insertion of unsaturated molecules into metal-carbon bonds is a critically important step in many transition-metal catalyzed organic transformations. The difference in insertion propensity of carbon-carbon and carbon-nitrogen multiple bonds can be attributed to the coordination characteristics of the respective molecules. The difficulty in achieving a to it isomerization may be the reason for the paucity of imine insertions. The synthesis of amides by the insertion of imines into palladium(II)-acyl bonds is the first direct observation of the insertion of imines into bonds between transition metals and carbon (see Scheme 7). The alternating copolymerization of imines with carbon monoxide (in which the insertion of the imine into palladium-acyl bonds would be the key step in the chain growth sequence), if successful, should constitute a new procedure for the synthesis of polypeptides (see Scheme 7).348... 

For the synthesis of 2-519, the amines 2-516 were first treated with AlMe3 in benzene at r.t. and after addition of the enol acetates 2-515, easily accessible from 2-513 and 2-514, heated under reflux. Mechanistic investigations using on-line NMR spectroscopy, reveal that a metalated amide 2-517 is formed first. This then leads to a N-acyliminium ion 2-518 which undergoes an electrophilic substitution. Overall, three new bonds are formed selectively in the domino process, and the alkaloid scaffolds 2-519 are provided in very good yields of 79-89%. Interestingly, use of the keto esters 2-513 instead of 2-515 did not lead to the desired products 2-519. 

A ring opening reaction of (1-lactams promoted by methoxide generated nitrogen nucleophiles in situ that subsequently added to proximal allenes producing trisubstituted pyrroles <06CC2616>. In the event, treatment of (3-lactam 3 with MeONa led to pyrrole-2-acetic ester 4 after cleavage of the amide bond, 5-exo-dig cyclization, and loss of methanol. The sequence was notable as no metal catalyst was required. 

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