A-Hydrogen, activation

The product of simple addition across the double bond is only weakly acidic whereas the final product has a hydrogen activated by two carbethoxy groups and is removed from the equilibrium by conversion to the enolate salt. The stability of the final salt serves to drag the reaction over the barrier that the cyclobutane intermediate must represent. 

FIGURE 4.3 Enantiomerization of activated AP-substituted /V -bcnz.yloxycarbonyl-i,-histidine by enolization (A) promoted by the basic Ji-nitrogen atom of the imidazole ring that is in proximity to the a-hydrogen. Activation of the substituted derivative (B) proceeds without enolization. [Jorgenson, 1970 Weber, 1975 Jones et al.,1982]... 

It is now suggested that in the absence of Mo 2 as a hydrogen activating phase the thermal coke formation is substantial. A very limited amount of Mo is already sufficient to dissociate hydrogen which leads to termination of radicals, thus preventing their condensation to coke. This explains the strong drop of the coke selectivity going from zero to 0,2% Mo,... 

Mills et al. IS) proposed formal reaction schemes in which conversion proceeds through olefinic reaction intermediates, such as the one illustrated by Fig. 10 for the Ce-hydrocarbons. They introduced the concept of separate catalytic functions, in terms of an isomerization activity—operative on the olefinic intermediate—associated with the acidic oxide base and a hydrogenation activity associated with platinum. Ciapetta and Hunter... 

A. Iron, cobalt, and nickel are the catalysts for the hydrogenation of carbon monoxide to hydrocarbons. Cobalt has the greatest tendency to produce aliphatic hydrocarbons with more than one carbon atom per molecule. Iron was less active and nickel showed too high a hydrogenation activity favoring the formation of methane. 

Relative toluene hydrogenation activity per Mo atom, as well as relative cyclohexane isomerization per catalyst volume unit, are reported on Figure 4 as a function of the amount of TMSi grafted on the support. No clear trend is seen for isomerization, as may be expected from such a low silicium loading [15]. On the contrary, an almost linear increase is observed for toluene hydrogenation. Indeed, when taking catalyst prepared on alumina as a reference (100 %), CoMo catalyst prepared on 0.9-TMSi-alumina has a hydrogenation activity of 123 %. 

No explanation is offered at present for the mechanism of a-hydrogen activation, for the lack of symmetry with respect to the behavior of GL... 

The pH measurement and reference electrodes each have an internal galvanic half-cell. The measurement and reference electrode half cells typically both consist of a silver wire w ith a silver chloride coating at its end immersed in a solution with chloride ions as shown in Figures l-4a and l-4c in Chapter 1. The reference electrode fill is usually a concentrated potassium chloride solution for aqueous processes while the measurement electrode fill is usually a chloride buffer with a hydrogen activity equivalent to 7 pH. 

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