Metals, activated deprotonation

R = Ph) shows that the C—H bond is activated by interaction with the metal center. Deprotonation of the benzylidene ligand was demonstrated with moderately strong bases such as pyrrolidinocyclopentene to generate the labile dichlorotungsten carbyne complex 120 [Eq. (103)]. Substitution of one chloride ligand in 120 occurs under very mild conditions [Eq. (103)]. 

Figure 9.18 The activation strategies for three classes of proteases. The peptide carbonyl group is attacked by (A) a histidine-activated cysteine in the cysteine proteases, [B] an aspartate-activated water moiecule in the aspartyi proteases, and (C) a metal-activated water molecule in the metalloproteases. For the metalloproteases, the letter B represents a base (often glutamate) that helps deprotonate the metal-bound water...

Of the non-substituted alkali amides, lithium and sodium amide are commercially available. In many cases, however, the bases are prepared from the alkali metals in liquid ammonia, which is often also used as the solvent for the subsequent reaction [6, 7]. A small amount of ferric nitrate or chloride is necessary for the conversion of the metals into the amides. The actual catalyst is zero-valent iron, which is formed as a black or grey colloidal solution. In contrast to the other alkali amides, potassium amide is soluble in liquid ammonia and therefore it is kinetically more active. Deprotonations of compounds having pK values near that of ammonia are more complete with potassium amide than with the insoluble sodium or lithium amide [26]. If desired, lithium or sodium amide can be isolated in a dry ... 

Brpnsted bases have been explored in cooperative catalysis to deprotonate substrates that subsequently add to metal-activated electrophiles. Regarding the use of these organocatalysts in cooperative catalysis, the combination of quinine and quinine-derived thiourea organocatalysts with Cu(II)-PhBox and Cu(OTf) has been employed in aza-Henry [23] and... 

The inverse of the Kenzyme activity as a function of added metal ion concentration were plotted as a function of pH. Bell-shaped profiles were obtained for the Co(II), Zn(II) and Mn(II) binding and a sigmoidal curve for Cd(II) binding to the p-site indicating that for the first three metals two deprotonation events influence the... 

Later it turned out that activation of enamine components could not only be achieved by deprotonation of the nitrogen atom but also by connecting it with certain metals, e.g. Ni(II), Pd(II), or Co(II), and subsequent treatment with base. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

Due to mechanistic requirements, most of these enzymes are quite specific for the nucleophilic component, which most often is dihydroxyacetone phosphate (DHAP, 3-hydroxy-2-ox-opropyl phosphate) or pyruvate (2-oxopropanoate), while they allow a reasonable variation of the electrophile, which usually is an aldehyde. Activation of the donor substrate by stereospecific deprotonation is either achieved via imine/enamine formation (type 1 aldolases) or via transition metal ion induced enolization (type 2 aldolases mostly Zn2 )2. The approach of the aldol acceptor occurs stereospecifically following an overall retention mechanism, while facial differentiation of the aldehyde is responsible for the relative stereoselectivity. 

Oxalamidinate anions represent the most simple type of bis(amidinate) ligands in which two amidinate units are directly connected via a central C-C bond. Oxalamidinate complexes of d-transition metals have recently received increasing attention for their efficient catalytic activity in olefin polymerization reactions. Almost all the oxalamidinate ligands have been synthesized by deprotonation of the corresponding oxalic amidines [pathway (a) in Scheme 190]. More recently, it was found that carbodiimides, RN = C=NR, can be reductively coupled with metallic lithium into the oxalamidinate dianions [(RN)2C-C(NR)2] [route (c)J which are clearly useful for the preparation of dinuclear oxalamidinate complexes. The lithium complex obtained this way from N,N -di(p-tolyl)carbodiimide was crystallized from pyridine/pentane and... 

Continuing his studies on the metallation of tetrahydro-2-benzazepine formamidines, Meyers has now shown that the previously unsuccessful deprotonation of 1-alkyl derivatives can be achieved with sec-butyllithium at -40 °C <96H(42)475>. In this way 1,1-dialkylated derivatives are now accessible. The preparation of 3//-benzazepines by chemical oxidation of 2,5- and 2,3-dihydro-l/f-l-benzazepines has been reported <96T4423>. 3Af-Diazepines are also formed by rearrangement of the 5//-tautomers which had been previously reported to be the products of electrochemical oxidation of 2,5-dihydro-lAf-l-benzazepine <95T9611>. The synthesis and radical trapping activities of a number of benzazepine derived nitrones have been reported <96T6519, 96JBC3097>. 

The conjugate base of the latter is the actual nucleophile. Thus the increase of activity up to pH 7.3 accounts for the deprotonation of this water molecule while the decrease in activity above this pH is related to the reduced ability of a phosphate to compete with a hydroxide for coordination to the metal center. This is a common feature of many catalysts with similar mechanisms. 

Complex (868) is a unique example where urea forms a single-atom bridge between the proximate metal ions through its carbonyl-O atom.2080 An N,0-bridging coordination of (deprotonated) urea occurs in (869), 4 which forms in an equilibrium reaction between the active H302-bridged species... 

---