Metal-substrate complex

This account summarizes our own results and the reports of other authors regarding the photochemical reactions between transition metal complexes and gases at high pressures. The reactions usually take place in a liquid solvent between dissolved substrates, metal complexes, and dissolved gases which are in equilibrium with a gas phase reservoir. 

Recent studies on Ru catalysts bearing an arene and a secondary diamine or ethanolamine as ancillary ligands, in the presence of a base such as KOH as co-catalyst, seem, however, to contradict statement (a), suggesting a mechanism where substrate/metal complexation is not essential for alcohol —> C = X (X = O and N) hydrogen transfer to occur.30 115... 

Transformed Substrate — Metal] A °-/lv> Transformed Substrate + Metal Complex... 

In spite of the many known silicon-transition metal complexes (15,16), little systematic work has appeared on the reaction mechanisms of silyl metal complexes. This state of affairs is in marked contrast to the current work on cr-alkyl transition metal complexes, where much emphasis is placed on determining detailed decomposition mechanisms (75-76). The reason for the interest in decomposition mechanisms is that the products of a transition metal-catalyzed reaction are released by the decomposition of the product-metal complex. Thus, to understand a catalytic process, one must have knowledge not only of the interaction of the reactants with the metal (leading to a substrate-metal complex), but also of the mechanisms whereby substrates are transformed on the metal and the manner in which the products are released. 

The reaction below shows a rate law that is second order in substrate metal complex and first order in phosphine. The A.S is negative, and an intermediate was isolated and shown to be (PPh3)(CO)4Mn(NO). Propose a mechanism for the reaction. What is the nature of the bonding of the NO ligand at each stage of the reaction ... 

As ATP is often the substrate in the case of enzyme-substrate-metal complexes, most metals are active for they mostly bind to the triphosphate. Copper (II), mercury (II), and other very strong Lewis acceptors are inhibitors as they bind to the ring nitrogens of ATP and in enzymes they could also block essential sulfhydryls. 

Free bases of the picket-fence type, a,a,a,a isomer, porphyrins have at most four convergent interaction sites above the plane and the metal ion may add a fifth effective interaction site for substrate. Metal complexes of the a,a,a,a isomer allow the substrate to access the metal through both functional group space and open space as shown in Figure 3. In order to simplify the system, stepwise syntheses of multifunctional porphyrins enable us to prepare porphyrins having more than three functional groups and equivalent interaction sites above and below the plane (see compound 7). - ... 

Metal cofactors do not always bind to the enzyme but rather bind to the primary substrate. The resulting substrate-metal complex binds to the enzyme and facilitates its activity. Creatine kinase catalyses the transfer of phosphoryl groups from adenosine triphosphate (ATP), which is broken down to adenosine diphosphate (ADP). The reaction requires the presence of magnesium ions. These, however, do not bind to the enzyme but bind to ATP, forming an ATP Mg complex. It is this complex that binds to the enzyme and allows transfer of the phosphoryl group ... 

The hydroformylation of 2,3-dihydrofuran and 3,4-dihydro-2//-pyran can be accompanied by partial isomerization to the isomeric cyclic allyl ethers. In general, dihydrofurans as substrates require conditions milder than those used for dihydropyrans, which has been explained by the more planar structure of the five-membered ring that facilitates the formation of substrate-metal complexes [17]. 

Formation of a substrate-metal binding, that is important for activation of substrate, has been suggested from the formation of stable model substrate-metal complexes. Recently, however, the substrate-bound iron enzymes have been demonstrated by X-ray crystallography in the cases of intradiol [15] and extradiol [9] catechol dioxygenases It is possible that these species isolated is not the direct intermediate involved in the catalytic cycle but those stabilized in the isolation process, but crystallographic data provide strong supports for the substrate-metal species. 

Mordant Dyes. MetaUizable azo dyes are appHed to wool by the method used for acid dyes and then treated with metal salts such as sodium chromate [7775-11-5] sodium dichromate [10588-01-9] and chromium fluoride [1488-42-5] to form the metal complex in situ. This treatment usually produces a bathochromic shift ia shade, decreases the solubUity of the coloring matter, and yields dyeiags with improved fastness properties. The chromium salts can be appHed to the substrate before dyeiag (chrome-mordant or chrome-bottom method), together with the dye ia a single bath procedure (metachrome process), or as a treatment after dyeiag (afterchrome process). 

Phase-transfer catalysis succeeds for two reasons. First, it provides a mechanism for introducing an anion into the medium that contains the reactive substrate. More important, the anion is introduced in a weakly solvated, highly reactive state. You ve already seen phase-transfer catalysis in another fonn in Section 16.4, where the metal-complexing properties of crown ethers were described. Crown ethers pennit metal salts to dissolve in nonpolai solvents by sunounding the cation with a lipophilic cloak, leaving the anion free to react without the encumbrance of strong solvation forces. 

Different main-group-, transition- and lanthanide-metal complexes can catalyze the cycloaddition reaction of activated aldehydes with activated and non-activated dienes. The chiral metal complexes which can catalyze these reactions include complexes which enable substrates to coordinate in a mono- or bidentate fashion. 

The enantioselective inverse electron-demand 1,3-dipolar cycloaddition reactions of nitrones with alkenes described so far were catalyzed by metal complexes that favor a monodentate coordination of the nitrone, such as boron and aluminum complexes. However, the glyoxylate-derived nitrone 36 favors a bidentate coordination to the catalyst. This nitrone is a very interesting substrate, since the products that are obtained from the reaction with alkenes are masked a-amino acids. One of the characteristics of nitrones such as 36, having an ester moiety in the a position, is the swift E/Z equilibrium at room temperature (Scheme 6.28). In the crystalline form nitrone 36 exists as the pure Z isomer, however, in solution nitrone 36 have been shown to exists as a mixture of the E and Z isomers. This equilibrium could however be shifted to the Z isomer in the presence of a Lewis acid [74]. 

As one would expect, in those cases in which the ionic liquid acts as a co-catalyst, the nature of the ionic liquid becomes very important for the reactivity of the transition metal complex. The opportunity to optimize the ionic medium used, by variation of the halide salt, the Lewis acid, and the ratio of the two components forming the ionic liquid, opens up enormous potential for optimization. However, the choice of these parameters may be restricted by some possible incompatibilities with the feedstock used. Undesired side reactions caused by the Lewis acidity of the ionic liquid or by strong interaction between the Lewis acidic ionic liquid and, for example, some oxygen functionalities in the substrate have to be considered. 

In comparison with catalytic reactions in compressed CO2 alone, many transition metal complexes are much more soluble in ionic liquids without the need for special ligands. Moreover, the ionic liquid catalyst phase provides the potential to activate and tune the organometallic catalyst. Furthermore, product separation from the catalyst is now possible without exposure of the catalyst to changes of temperature, pressure, or substrate concentration. 

In a catalytic asymmetric reaction, a small amount of an enantio-merically pure catalyst, either an enzyme or a synthetic, soluble transition metal complex, is used to produce large quantities of an optically active compound from a precursor that may be chiral or achiral. In recent years, synthetic chemists have developed numerous catalytic asymmetric reaction processes that transform prochiral substrates into chiral products with impressive margins of enantio-selectivity, feats that were once the exclusive domain of enzymes.56 These developments have had an enormous impact on academic and industrial organic synthesis. In the pharmaceutical industry, where there is a great emphasis on the production of enantiomeri-cally pure compounds, effective catalytic asymmetric reactions are particularly valuable because one molecule of an enantiomerically pure catalyst can, in principle, direct the stereoselective formation of millions of chiral product molecules. Such reactions are thus highly productive and economical, and, when applicable, they make the wasteful practice of racemate resolution obsolete. 

Despite the fact that transition metal complexes have found wide application in the synthesis of carbo- and heterocycles, [3+3] cyclisation reactions mediated or assisted by transition metals remain almost unexplored [3, 86]. However, a few examples involving Fischer carbene complexes have been reported. In all cases, this complex is a,/J-unsaturated in order to act as a C3-synthon and it reacts with different types of substrates acting as C3-synthons as well. 

Transition metal complexes that are easy to handle and store are usually used for the reaction. The catalytically active species such as Pd(0) and Ni(0) can be generated in situ to enter the reaction cycle. The oxidative addition of aryl-alkenyl halides can occur to these species to generate Pd(II) or Ni(II) complexes. The relative reactivity for aryl-alkenyl halides is RI > ROTf > RBr > RC1 (R = aryl-alkenyl group). Electron-deficient substrates undergo oxidative addition more readily than those electron-rich ones because this step involves the oxidation of the metal and reduction of the organic aryl-alkenyl halides. Usually... 

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