Complex-mediated reactions

Thus far, we have discussed the transition metal complex-catalyzed hydrogenation of C=C, C=0, and C N bonds. In this section, another type of transition metal complex-mediated reaction, namely, the hydroformylation of olefins, is presented. 

Not all ionization methods rely on such strictly unimolecular conditions as El does. Chemical ionization (Cl, Chap. 7), for example, makes use of reactive collisions between ions generated from a reactant gas and the neutral analyte to achieve its ionization by some bimolecular process such as proton transfer. The question which reactant ion can protonate a given analyte can be answered from gas phase basicity (GB) or proton affinity (PA) data. Furthermore, proton transfer, and thus the relative proton affinities of the reactants, play an important role in many ion-neutral complex-mediated reactions (Chap. 6.12). 

Traeger, J.C. Hudson, C.E. McAdoo, D.J. Isomeric Ion-Neutral Complexes Generated From Ionized 2-Methyl-propanol and n-Butanol the Effect of the Polarity of the Neutral Partner on Complex-Mediated Reactions. J. Am. Chem. Soc. Mass Spectrom. 1991, 3,409-416. 

The bleeding potential is similar among the agents. However, thrombocytopenia, particularly profound thrombocytopenia (platelet count <50,000 mrrT3) occurs with a two-to four-fold higher frequency with abciximab (0.4— 1.0%) compared with eptifibatide (0-0.2%) or tirofiban (0,1 —0.3%) (6), The exact mechanism of this difference is not clear, However, immune complex-mediated reaction (due to an anamnestic response to the humanized chimeric antibody) may contribute to rapid precipitation of thrombocytopenia with abciximab (6), Platelet counts should, therefore, be measured early (within the first one to four hours) after administration of these agents and followed for the duration of therapy. Platelet transfusion should be considered for profound thrombocytopenia with or without serious bleeding (6). 

Normally, organocatalytic reactions proceed either by a much tighter or a much looser transition structure than chiral metal complex-mediated reactions. The... 

Considerable effort has been devoted to the development of enantiocatalytic MBH reactions, either with purely organic catalysts, or with metal complexes. Paradoxically, metal complex-mediated reactions were usually found to be more efficient in terms of enantioselectivity, reaction rates and scope of the substrates, than their organocatalytic counterparts [36, 56]. However, this picture is actually changing, and during the past few years the considerable advances made in organocatalytic MBH reactions have allowed the use of viable alternatives to the metal complex-mediated reactions. Today, most of the organocatalysts developed are bifunctional catalysts in which the chiral N- and P-based Lewis base is tethered with a Bronsted acid, such as (thio)urea and phenol derivatives. Alternatively, these acid co-catalysts can be used as additives with the nucleophile base. 

Asymmetric organocatalytic Morita-Baylis-Hillman reactions offer synthetically viable alternatives to metal-complex-mediated reactions. The reaction is best mediated with a combination of nucleophilic tertiary amine/phosphine catalysts, and mild Bronsted acid co-catalysts usually, bifunctional chiral catalysts having both nucleophilic Lewis base and Bronsted acid site were seen to be the most efficient. Although many important factors governing the reactions were identified, our present understanding of the basic factors, and the control of reactivity and selectivity remains incomplete. Whilst substrate dependency is still considered to be an important issue, an increasing number of transformations are reaching the standards of current asymmetric reactions. 

C-C Bond Formation by Metal or Metal Complex Mediated Reactions. 317... 

Metal or metal complex-mediated reactions have been widely applied to C-glycoside synthesis [41]. Accordingly, various metal or metal complex catalysts such as rhodium, indium, samarium, palladium, and so forth have also been developed for the preparation of C-branched-chain sugars. 

Complex-mediated reaction An elementary bimolecu-lar reaction proceeding via a long-lived collision complex having lifetimes ranging from several vibrational periods (100 fs) to many rotational periods (>10 ps). If complex lifetimes exceed several rotational periods, product angular distributions from crossed beam reactions exhibit forward-backward symmetry in the center-of-mass frame of reference. 

Innnunologic Dextran 70 has been used as stabilizer in a measles-mumps-rubella (MMR) vaccine product named Morupar. This vaccine was associated with dextran-driven hypersensitivity reactions with high concentrations of dextran-spedfic IgG [43"]. The most probable mechanism is immune complex-mediated reactions caused by naturally occurring dextran-specific antibodies. Morupar was withdrawn from the market. 

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