Amides Amido complexes

Scheme 3 summarizes the reductive elimination chemistry of arylpalladium amides. Arylpalladium amido complexes containing PPh3 as the dative ligand were stable enough to isolate, and... 

Two equiv. of 6,6-di(cyclopropyl)fulvene react at 60 °C over a period of a week with Ca[N(SiMe3)2]2-(THF)2 bis in THF to yield the metallocene 170. The heteroleptic amido complex 171 is detected as an intermediate with 111 and 13C 1H NMR spectroscopy. A 1 1 reaction of the calcium amide and 170 also produces 171 in solution, an equilibrium involving these three derivatives exists (Equation (30)). The calcocene 170 crystallizes at — 20 °C from THF as colorless cuboids. The metal center is surrounded by the four ligands in a distorted tetrahedral manner, and the cyclopentadienyl group and the propylidene fragment are coplanar with each other.393... 

Lactams Lactams represent a special type of C=N system due to the tautomerization between the lactam (keto amine) and lactim (hydroxyimine) forms. The lactim form is much more favored for cyclic than for non-cyclic amides of carbocyclic acids. In the reaction of complex 2b with N-methyl-e-caprolactam, a simple ligand exchange reaction occurs and complex 87 can be isolated. With P-propiolactam, the alkenyl-amido complex 88 is formed, which indicates an agostic interaction. The reaction of complex 1 with e-caprolactam gives, after elimination of the alkyne and of molecular hydrogen, complex 89 with a deproto-nated lactam in a r]2-amidate bonding fashion [47]. 

A transition metal complex such as bpyNi(COD), generalized as LjNi, reacts with NCA in a complex reaction sequence that generates a propagating species XLV whose active center is a 5-membered amido-amidate metallacyclic complex. Propagation involves a nucleophilic attack by the amido nitrogen of the amido-amidate at the C-5 carbonyl of NCA. The... 

Magnesium amides can form a wide range of mixed metal amido complexes with alkali metal ions (M2[Mg(NH2)4]) (M = K, Rb or Cs).These contain tetrahedral Mg-centered [Mg(NH2)4] ions connected in three-dimensional networks by coordination of the amido groups to the group 1 metal ions. The most common hetero metal is lithium and lithium amido magnesiates are readily accessible by the addition of a lithium amide to a magnesium amide. 

A key objective on our part was to complement material that had already been reviewed as well as to provide an overview of the key developments. Several reviews and commen-taries have appeared since the 1980 book and almost half of these have been published since 2000. These have dealt with, either fully or in part, derivatives of specific types of amido and related ligands, the applications of amido substituted complexes in chemical transformations, and the use of amido complexes as precursors for electronic materials or catalysis. The increasing interest in the use of multidentate amido and similar ligands of various types, which had been a notable development of mainstream amide chemistry since 1980, has resulted in the largest numbers of reviews. These cover ligands such as... 

Two other routes to transition metal amides were not generally discussed in the 1980 book. The first of these involves the deprotonation of aminometal complexes as shown in Equation (6.3), which has afforded several new amido complexes. 

R = R = Cy R = 1-adamantyl, R = CfiH 5. Te2-3,5 5 ). The vanadium analogue of the latter [V N(l-Ad)(C6H3Me2-3,5) 3] " 4 has also been characterized. Furthermore, a more efficient route to [Cr N(SiMe3)2 3] and a new crystal structure determination has been described. Three-coordinate metal amides have been treated in a general review that covers three-coordinate transition metal species with hard ligands. The electronic structure and bonding in tricoordinate amido complexes of transition metals have also been detailed... 

Lewis Base Complexes, Chelated Metal Amides and Heteroleptic Amido Complexes... 

The chemistry of indium complexes of aU types in metal oxidation states lower than +3 has been comprehensively reviewed. Few lower oxidation state mononuclear amido complexes of indium are well characterized, however, and no structure has been reported for an In(I) amide. The compound In N(SiMe3)2 n. which is unstable, " has been characterized NMR spectroscopy but its structure is unknown. The structures of several In(I) complexes, related to amides but outside our current scope, have been described. Like its aluminium and gallium counterparts, the p-diketuninate derivative [ In N(Dipp)C(Me) 2CH] has been characterized, as has the closely related species [ In N(Dipp)C(CF3) 2CH]. ° These feature V-shaped, two-coordination at the metal. The less bulky [(In N(Mes)C(Me) 2-CH)2] ° and 15-2.6-.Vlc,)( (Me) i are dimeric with long In In bonds of... 

Table XIII (189-199) gives details of solid-state lithium amide monomeric complexes (69)—(87). These include just three [(79), (80), and (87)] solvent-separated ion pairs. The remainder are contact-ion pairs, each with an (amido)N—Li bond. Association to dimers or higher oligomers is prevented sterically. The size of the R and/or R group in the RR N- anions can lead to monomers even when Li+ is complexed only by a single bidentate (e.g., TMEDA) or by two monodentate (e.g., THF or Et20) ligands. In such cases [(69), (71), (72), (75)-(78), and (81)—(83) ], the lithium centers are only three coordinate. Electronic factors in the anion [notably, B N multiple bonding in (75)—(78) ] also may reduce the charge density at N, and lower the ability to bridge two...

In the case of complexes containing macrocyclic amines, it is often possible to isolate the intermediate deprotonated amido complexes. In the next section, we will consider some very important aspects of co-ordination chemistry, in which reactions of co-ordinated amine and/or amide play a crucial role. 

The extensive chemistry of amido complexes, and, more particularly, of alkylamido complexes, reveals that the planar form is almost invariably found, along with bridging amides (221). Much attention has been paid to the synthesis of metal amido complexes of early transition metals, lanthanides and actinides. The amido group, particularly where it is bulky, confers unusual low coordination numbers on the metals and can also produce materials with considerable kinetic stability toward attack by nucleophiles (42, 67). However, the relevance of this extensive and fascinating chemistry to nitrogen fixation is somewhat problematic. 

In the original process using tin amides, transmetallation formed the amido intermediate. However, this synthetic method is outdated and the transfer of amides from tin to palladium will not be discussed. In the tin-free processes, reaction of palladium aryl halide complexes with amine and base generates palladium amide intermediates. One pathway for generation of the amido complex from amine and base would be reaction of the metal complex with the small concentration of amide that is present in the reaction mixtures. This pathway seems unlikely considering the two directly observed alternative pathways discussed below and the absence of benzyne and radical nucleophilic aromatic substitution products that would be generated from the reaction of alkali amide with aryl halides. 

An alternative pathway when soluble alkoxide or silylamido bases are used, involves reaction of a palladium amido aryl complex with the alkoxide or silylamide to form an intermediate alkoxide or amide. These complexes can react with amines to form the required amido aryl intermediate. This pathway seems to occur for aryl halide animations catalyzed by complexes with chelating ligands. The inorganic... 

The reaction of (Pd(PPh3)(Ph)(p-OH) 2 with primary alkylamines to generate palladium amido complexes and water (Eq. (47)) [56,207] was an initial indication that the conversion of an alkoxide to an amide could be occurring during the catalytic cycle. These reactions are reversible, but the equilibrium favors the amido complex. 

The amination chemistry depends on the absence of irreversible P-hydrogen elimination from the amido complexes before reductive elimination of amine. At the early stages of the development of the amination chemistry, it was remarkable that the unknown reductive elimination of arylamines could be faster than the presumed rapid [57,58] P-hydrogen elimination from late metal amides. In fact, directly-observed P-hydrogen elimination from late metal amido complexes was rare, and no examples were observed to occur irreversibly from a simple monomeric amido species [69], At this point, it is clear that C-N bond-forming reductive elimination of amines and ethers can be rapid, and that P-hydrogen elimination can be slow. 

The transition metal catalyzed synthesis of arylamines by the reaction of aryl halides or tri-flates with primary or secondary amines has become a valuable synthetic tool for many applications. This process forms monoalkyl or dialkyl anilines, mixed diarylamines or mixed triarylamines, as well as N-arylimines, carbamates, hydrazones, amides, and tosylamides. The mechanism of the process involves several new organometallic reactions. For example, the C-N bond is formed by reductive elimination of amine, and the metal amido complexes that undergo reductive elimination are formed in the catalytic cycle in some cases by N-H activation. Side products are formed by / -hydrogen elimination from amides, examples of which have recently been observed directly. An overview that covers the development of synthetic methods to form arylamines by this palladium-catalyzed chemistry is presented. In addition to the synthetic information, a description of the pertinent mechanistic data on the overall catalytic cycle, on each elementary reaction that comprises the catalytic cycle, and on competing side reactions is presented. The review covers manuscripts that appeared in press before June 1, 2001. This chapter is based on a review covering the literature up to September 1, 1999. However, roughly one-hundred papers on this topic have appeared since that time, requiring an updated review. 

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]. 

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