Amides, coordinated

The crystal structure of a CODH/ACS enzyme was reported only in 2002.43,44 It reveals a trio of Fe, Ni, and Cu at the active site (6). The Cu is linked to the Ni atom through two cysteine-S, the Ni being square planar with two terminal amide ligands. Planarity and amide coordination bear some resemblance to the Ni porphinoid in MCR. A two-metal ion mechanism is likely for acetyl CoA synthesis, in which a Ni-bound methyl group attacks an adjacent Cu—CO fragment with formation of a Cu-acyl intermediate. A methylnickel species in CODH/ACS has been identified by resonance Raman spectroscopy.45... 

Another important N-donor group is the amide group. Contrary to the basic amino groups, the more acidic amide functions tend to be deprotonated in the complex and therefore operate as a monoanionic donor. Alkaline conditions promote the deprotonation and subsequent complex formation. The amide group is a very useful component of mixed donor sets, such as N2S2 or N3S, as discussed in the next chapter. Whether a pure amide coordination may occur in M(V) complexes has not yet been proved. Tetrapeptides do form Tc(V) complexes [68], apparently without involvement of the carboxyl group. The N-donor atom provided by Schiflf bases plays only a role in mixed donor sets and will be discussed below. 

Striking examples of this phenomenon are presented for allyl and homoallyl alcohols in Eqs. (5) to (7). The stereodirection in Eq. (5) is improved by a chiral (+)-binap catalyst and decreased by using the antipodal catalyst [60]. In contrast, in Eq. (6) both antipode catalysts induced almost the same stereodirection, indicating that the effect of catalyst-control is negligible when compared with the directivity exerted by the substrate [59]. In Eq. (7), the sense of asymmetric induction was in-versed by using the antipode catalysts, where the directivity by chiral catalyst overrides the directivity of substrate [52]. In the case of chiral dehydroamino acids, where both double bond and amide coordinate to the metal, the effect of the stereogenic center of the substrate is negligibly small and diastereoface discrimination is unsuccessful with an achiral rhodium catalyst (see Table 21.1, entries 9 and 10) [9]. 

Dendrimers with Multiple Amide Coordinating Units... 

Isomerism of Amides Coordinated to Platinum—X-Ray Crystal Structure of Dichloro-Bisacetamide-Platinum(II) (CM et al., 1996)... 

A range of binary and ternary complexes of nicotinic add and nicotinamide have been described499,500 and crystal structural analyses of [CdL2(H20)2(HC02)2] (L = nicotinamide)501 and [CdL2(H20)2(OAc)2] (L = A, A-diethylnicotinamide)502 have been reported. Both complexes are octahedral, with the amides coordinated to the metal through nitrogen (Cd—OAc 2.281 A Cd—N 2.376 A Cd—OH 2.299 A). 

Steric hindrance controls a three-way divergence of regioselectivity in the metallation of 530.456 n-BuLi lithiates ortho to OMe at the site where coordination to the secondary amide can also be achieved. r-BuLi prefers to lithiate at the less hindered benzylic site, still presumably benefitting from amide coordination. The superbase, on the other hand, cares nothing for coordination to the amide and metallates at the less hindered site ortho to OMe. 

A. Dendrimers with Multiple Coordination Sites A.I. Amide coordinating units... 

Fujii et al. [144] studied the complexation of copper(II) ion with some amide solvents such as N-methylformamide (NMF), formamide (FA), NJ4-dimethylacetamide (DMA) and N-mediylacetamide (NMA) by titration calorimetry in acetonitrile containing (C2H5)4C104 as an ionic medium at 298 K. The reported results demonstrated that these amides coordinate to the metal ion to form a series of mononuclear complexes, and their formation constants, enthalpies and entropies have been obtained. 

The reasons for the effectiveness of silylated amide bases might consist in an internal protection of the reactive functionalities, either via the formation of tightly associated lithium ion pairs following the deprotonation, or via the formation of silylated alcohols or silylated amides operating as protecting groups. Both pathways would prevent the alkoxide or amide coordination to palladium resulting in catalyst deactivation. 

The few cases of trigonal bip5 amidal coordination are mainly represented by the adducts of the amides with slender nitriles, Ln[(N(SiMe3)2]s(NCMe)2, and by a few or-bonded organometallics, e.g., [ScPh3(thf)2]... 

Studies on OCH(CF3)2, OCMe(CF3)2, OCMc2(CF3), and 0C(CF3)3 derivatives of the alkali metds, alkaline earth metals, transition metals, and the lanthanide elements are reviewed, with emphasis on work reported since 1988. Alkali and alkaline earth fluoroalkoxides are generally made from reaction between the alcohol and the metal, its hydride, or organometallics. Most syntheses of transition metal derivatives involve reaction between metal halides and alkali or alkaline earth salts, or the alcoholysis of metal alkyls, alkoxides and amides. Coordination between organic fluorine and electropositive metals (ie. Na, Ba, Tl, Pr) is often observed in the crystal structures of these fluoroalkoxides and may be related to their use as chemical vapor dqx)sition precursors for metal fluorides. 

IPr [Ar W -bis-(2,6-diisopropylphenyl)imidazolyl] carbine ligand, and norbornene (4equiv.) [156]. In a typical example, the treatment of an amide that has an alkene moiety with the aforementioned Ir catalyst gave cyclization of the alkene-amide moiety through coupling of the sp C-H bonds. The reaction is proposed to proceed by the formation of an amide-coordinated iridium-IPr-Cl complex as the key intermediate, which reacts by C-H bond activation of the double bond followed by C-C formation to give the cyclized product (Scheme 11.9). 

N-acetyl-L-cysteine and N-acetyl-D-penicillamine are the simplest ligands containing the thiolate and amide donor functions in the same molecule. For understanding the coordination chemistry of L-cysteine and its peptide derivatives, it is important to take into account that the thiol group can form a five-membered chelate with the amide-N and a six-membered one with the carboxylate-O donor atoms. All studies rule out a cadmium(ll)-induced amide coordination in these complexes, therefore the formation of the (S,0) chelate is the governing factor during complex formation with N-acetyl-L-cysteine. The equilibrium data have... 

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