Active sites alkynes

These reactions illustrate the importance of ruthenium vinylidene species, as activated forms of terminal alkynes, in catalysis, because they favor the addition of O-nudeophiles (carbamic and carboxylic acids, alcohols, water) to terminal alkynes and completely reverse the expected regioselectivity of the addition. These examples also show that the activation processes are very sensitive to the nature of the nucleophiles, and the success of the awtt-Markovnikov addition to terminal alkynes is highly dependent on both the electron richness and steric hindrance of the ancillary ligands coordinated to the active site. 

The adsorbed metallocene can be prereacted with additional MAO, and the prereaction time plays a role. Scavengers can be used, which react with poisons such as oxygen, alkynes, and sulfur compounds. During the prereaction, the active site is formed faster by higher concentrations of the compounds. The supported catalyst is washed to remove the excess MAD. 

The nature of the solvent in liquid-phase alkyne hydrogenations and the extent to which it can influence the competitive adsorption factors needed to attain selectivity should also be considered. The semihydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts of varying metal load gave 1-octene with a selectivity of 100% over a wide range of metal loads when the reaction was run in heptane.38 n-propanol, however, reaction selectivity increased with decreasing metal load. Apparently the alcohol interacted with the catalyst to modify the active sites and influenced the relative adsorption characteristics of the acetylenic and olefinic species. This can affect reaction selectivity particularly if reactant diffusion assumes some importance in the reaction. 

The use of secondary modifiers, e. g. quinoline, and the choice of solvent also play important roles in directing semi-hydrogenation selectivity. For example, in the hydrogenation of 1-octyne over a series of Pd/Nylon-66 catalysts metal loading had no effect on selectivity when the reaction was performed in n-heptane as solvent. When the same experiment was conducted in n-propanol, however, an inverse relationship between selectivity and catalyst metal loading was observed [56], This effect has been interpreted as a polar solvent-induced modification of the Pd active sites, which alters the relative adsorption behavior of the alkyne and alkene species [57], Modification by addition of quinoline is reported to benefit the selective production of a cij-vitamin D precursor from the related disubstituted alkyne [58] ... 

These results have been used to define laboratory testing conditions enabling the correct simulation of engine bench results. In addition they open the way to new kinetic and surface studies dedicated to a better description of active site inhibition by alkynes. 

Nitrile, Isonitrile, and Alkyne Analogs. Exploration of N2ase-catalyzed reductions of selected nitrile, isonitrile, and alkyne analogs provides examples of novel chemical reactions and yields information about the topography of the active sites of Noase. 

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