Half-hydrogenated

The commonly accepted mechanism of heterogeneously catalyzed hydrogenation involves activation of both the hydrogen and the C—C multiple bond adsorbed on the metal surface. First one hydrogen atom is transferred to the least hindered position of the multiple bond to give a half-hydrogenated adsorbed species. This reaction is fully reversible and ac-... 

Catalysts have a profound effect on the extent of double-bond migration. The influence is a property of the metal itself and its structure and is little altered by the support(7 7,7 ). It is related to the relative tendencies of the half-hydrogenated states to reform an unadsorbed olefin. A decreasing ordering of metals for double-bond migration (46) is Pd > Ni Rh Ru Os > Ir - Pt. 

Palladium is good for hydrogenations of most unsaturations except benzenes. It is frequently used by synthetic organic chemists for hydrogenolyzing off protecting groups. It is especially useful for the half-hydrogenation of acetylenes. 

FIGURE 1.10 Various possible surface species on a Pt or Pd (111) surface. A and B represent possible locations of 1,2-di-a-Cj 2-cyclohexane, and C, D, and E represent possible locations of Jt-complexed Jt-C -cyclohexene. Full complements of hydrogens are assumed at each angle and terminal that is not either a- or Jt-bonded to a surface site as indicated by a small circle. Half-hydrogenated states, which are mono-a-C -adsorbed species (where n is the number of the carbon attached to the surface), would be represented by one small circle at the carbon bonded to a surface site. F, G, and I represent possible locations of Jt-C -cyclohexene. F shows the three carbons of the Jt-allyl moiety adsorbed in three adjacent three-point hollow sites and G shows it over one three-point hollow site, whereas I shows adsorption over the approximate tops of three adjacent atoms. (Note Label H is not used to avoid confusion with hydrogen, which is not shown.)... 

Before we examine the hydrogenation of each type of unsaturation, let us first take a look at the basic mechanism assumed to be operating on metal catalytic surfaces. This mechanism is variously referred to as the classic mechanism, the Horiuti-Polanyi mechanism, or the half-hydrogenated state mechanism. It certainly fits the classic definition, since it was first proposed by Horiuti and Polanyi in 193412 and is still used today. Its important surface species is a half-hydrogenated state. This mechanism was shown in Chapter 1 (Scheme 1.2) as an example of how surface reactions are sometimes written. It is shown in slightly different form in Fig. 2.1. Basically, an unsaturated molecule is pictured as adsorbing with its Tt-bond parallel to the plane of the surface atoms of the catalyst. In the original Horiuti-Polanyi formulation, the 7t-bond ruptures... 

FIGURE 2.1 Classical Horiuti-Polanyi half-hydrogenated state mechanism for hydrogenation, double bond migration, cis-trans isomerization, and deuterium exchange. 

Half-hydrogenations of alkynes are usually accomplished over poisoned Pd catalysts, but other catalysts also may also be used.13 Unpoisoned Pd sometimes does not give high stereoselectivity.14 One of the most mentioned Pd cat-... 

Other poisons (modifiers) used to create such selective Pd catalysts may be metals 23 Zn, Cd, Zr, Ru, Au, Cu, Fe, Hg, Ag, Pb, Sb, and Sn or solvents (organic modifiers) 24 pyridine, quinoline, piperidine, aniline, diethylamine, other amines, chlorobenzene, and sulfur compounds. Hydroxides have also been used to increase the half-hydrogenation selectivity of Pd. 

The mechanism of half-hydrogenation of alkynes is not fully understood, but some details are recognized. For example, it has long been recognized that alkynes adsorb more strongly than alkenes. During the half-hydrogenation of... 

FIGURE 2.2 Mechanism of half-hydrogenation of alkynes showing two possible adsorption sites. 

The kind of surface site catalyzing alkyne half-hydrogenation has been the subject of some speculation. Two possibilities are shown in Fig. 2.2. Certainly such a site should fit the evidence Burwell and associates obtained from the hydrogenation of di-tert-butyl acetylene (2,2,5,5-tetramethyl-3-hexyne). 31 They proposed that the molecule dislocated a surface atom, pulling it up out of the plane of the surface (Fig. 2.3). That work concluded that hydrogenation... 

For small-scale synthetic organic chemistry, the half-hydrogenation of dienes is sometimes useful. For example, at —10°C the half-hdrogenation by... 

After half-hydrogenation, migration of the remaining double bond (sometimes mistaken for 1,4-addition) can be inhibited by poisoning with CO.53... 

Cis-trans isomerization occurs either by formation of a half-hydrogenated state (Horiuti-Polanyi mechanism) followed by rotation around the newly formed single bond and abstraction of an appropriate hydrogen onto the surface or by double bond migration (either Horiuti-Polanyi or Jt-allyl) from a cis (trans) position to an adjacent trans (cis) position (deuterium exchange studies favor the rotation mechanism). 

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