Self-selected catalysts

Further exploration of the higher activity of the Ni complexes compared to Pd analogs led to the discovery of a novel nano-sized catalytic system with superior performance for hydrothiolation and hydroselenation reactions of alkynes [ 152,153]. Furthermore, it was found that with a simple catalyst precursor - Ni(acac)2 - the reaction was carried out with excellent yields and excellent selectivity, even at room temperature. Both terminal and internal alkynes were successfully involved in the addition reaction. This catalytic system was tolerant to various functional groups in alkynes and was easily scaleable for the synthesis of 1-50 g of product (Scheme 3.85) [152, 153]. The proposed mechanism of the catalytic reaction involved (i) catalyst self organization with nano-sized particles formation, (ii) alkyne insertion into the Ni—Z bond and (iii) protonolysis with RZH (Scheme 3.86). 

DCC is typically associated with the amplification of the fittest. In fact, nearly all studies discussed in the previous section rely on the self-selection of high-affinity binders from a dynamic library pool. Nonetheless, many processes in chemistry are guided by weak differences in energy. Examples are the interaction of a catalyst with the transition state of a chemical reaction, protein folding, as well as the self-assembly of nanoarchitectures. In all these cases, a few kilojoules of energy can make the difference between success and failure. This section deals with the application of DCC in the realm of weak interactions, using a methodology referred to as dynamic covalent capture The essential... 

Prins, Scrimin, et at. have illustrated the potential of dynamic covalent capture for catalyst development (Figure 9.7a) [10]. Inspired by enzymes that operate according to a covalent catalysis mechanism, they focused on the self-selection of functional groups that can assist intramolecularly in the cleavage of a flanking ester bond [39]. 

Albahily K, Fomitcheva V, Gambarotta S, et al Preparation and characterization of a reduced chromium complex via vinyl oxidative coupling formation of a self-activating catalyst for selective ethylene trimerization, J Am Chem Soc 133(16) 6380-6387, 2011a. 

Because lactic acid has both hydroxyl and carboxyl functional groups, it undergoes iatramolecular or self-esterificatioa and forms linear polyesters, lactoyUactic acid (4) and higher poly(lactic acid)s, or the cycUc dimer 3,6-dimethyl-/)-dioxane-2,5-dione [95-96-5] (dilactide) (5). Whereas the linear polyesters, lactoyUactic acid and poly(lactic acid)s, are produced under typical condensation conditions such as by removal of water ia the preseace of acidic catalysts, the formation of dilactide with high yield and selectivity requires the use of special catalysts which are primarily weakly basic. The use of tin and ziac oxides and organostaimates and -titanates has been reported (6,21,22). 

Steady state measurements of NO decomposition in the absence of CO under potentiostatic conditions gave the expected result, namely rapid self-poisoning of the system by chemisorbed oxygen addition of CO resulted immediately in a finite reaction rate which varied reversibly and reproducibly with changes in catalyst potential (Vwr) and reactant partial pressures. Figure 1 shows steady state (potentiostatic) rate data for CO2, N2 and N2O production as a function of Vwr at 621 K for a constant inlet pressures (P no, P co) of NO and CO of 0.75 k Pa. Also shown is the Vwr dependence of N2 selectivity where the latter quantity is defined as... 

Gonzalez-Arellano, C., Corma, A., Iglesias, M. and Sanchez, F. (2005) Homogeneous and heterogenized Au(III) Schiff base-complexes as selective and general catalysts for self-coupling of aryl boronic acids. Chemical Communications, (15), 1990. 

Selective Hydroformylation Catalysts Through Self-Assembly. 157... 

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