Hydrogenation Horiuti-Polanyi mechanism

The hydrogenation and isomerization of alkenes can usually be described by the classical Horiuti-Polanyi mechanism. According to that mechanism, in a deuterium atmosphere, double bond migration incorporates deuterium into the allylic position. 

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... 

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). 

For all catalysts, the hydrogenation mechanism performed under our conditions (liquid phase and low hydrogen pressure) is consistent with an Horiuti-Polanyi mechanism. We found that the phenylacetylene (PhAc) hydrogenation reaction was zero order with respect to hydrocarbon reactants up to 80% of conversion, in agreement with several other studies devoted to the hydrogenation of polyunsaturated compounds." ... 

Following a variation of the well-known Horiuti-Polanyi mechanism, we consider the following steps as possible for the system -butane- -butenes-hydrogen over chromia alumina catalyst ... 

The relative contribution of the two mechanisms to the actual isomerization process depends on the metals and the experimental conditions. Comprehensive studies of the isomerization of n-butenes on Group VIII metals demonstrated179-181 that the Horiuti-Polanyi mechanism, the dissociative mechanism with the involvement of Jt-allyl intermediates, and direct intramolecular hydrogen shift may all contribute to double-bond migration. The Horiuti-Polanyi mechanism and a direct 1,3 sigma-tropic shift without deuterium incorporation may be operative in cis-trans isomerization. 

According to the Horiuti-Polanyi mechanism, isomerization requires the participation of hydrogen. The first addition step, formation of the half-hydrogenated state [Eq. (11.3)], cannot take place without hydrogen. Numerous investigations have supported the role of hydrogen in these so-called hydroisomerizations. 

In accordance with this observation, the fraction of the cis isomer increases with increasing hydrogen pressure. Since an increase in the hydrogen partial pressure affects step 3 [Eq. (11.3)] in the Horiuti-Polanyi mechanism by shifting the equilibrium to the formation of the half-hydrogenated state, isomerization is suppressed. Palladium, in turn, which exhibits the highest tendency to isomerization among platinum metals, may yield the trans isomer as the major product under certain conditions. 

Fig. 39. Reaction scheme for the hydrogenation of 1,2-dimethylcyclohexene by a Horiuti—Polanyi mechanism as proposed by Siegel [221].

The cis stereochemistry is consistent with the Horiuti-Polanyi mechanism with the Langmuir-Hinshelwood pathway. The stepwise addition of the two hydrogen atoms from... 

Metal-catalyzed hydrogenation of olefins proceeds via the Horiuti-Polanyi mechanism (24), generally accepted since the 1930s (Scheme 1). 

Fig. 1. Turnover frequencies for isobutylene production at 773 K and 0.016 atm isobutane pressure (a) and 0.099 atm hydrogen pressure (b) for reaction catalyzed by Pt/Sn/SiC>2 (O), Pt/Sn/K/SiC>2 ( ), and Pt/Sn/K-L (O). Adapted from (40). Solid lines represent the fit for each set of data using the Horiuti-Polanyi mechanism.

Scheme 1. Classic Horiuti-Polanyi mechanism for olefin hydrogenation (2).

As already mentioned, the stereochemistry of simple olefin hydrogenation can usually be understood by utilizing the classic Horiuti-Polanyi mechanism (1,2). A number of different mechanistic rationales have been put forth, however, to account for the stereochemical data obtained on hydrogenation of a, /3-unsaturated ketones in different media. Actually, no single explanation can be used to account for all of the stereochemical observations, but it is possible to blend the various proposals to give a mechanistic framework from which it is possible by extrapolation to obtain the desired stereochemical information. 

None of these mechanistic proposals is sufficiently general to use to rationalize all of the stereochemical data observed on the hydrogenation of a,[3-unsaturated ketones. By a judicious combination of segments of each of these proposals along with the Horiuti-Polanyi mechanism (2), it is possible, however, to develop a uniform mechanistic rationale that can be useful in determining the effect of solvent on product stereochemistry. In addition, the influence of hydrogen availability, the type and quantity of catalyst, and the nature of other substituents on the reacting molecule on the product isomer distribution can also be more readily understood. 

The classic Horiuti-Polanyi mechanism proposed in 1934 for hydrogenation of ethylene on Ni is shown in Scheme 1. Since then isotopic tracer studies,... 

A valuable indirect method of probing the Horiuti-Polanyi mechanism is the study and comparison of competitive rates of hydrogenation of olefins using both homogeneous and heterogeneous catalysts. Comparisons of individual rates... 

Hussey et al. interpret the relative competitive rates on platinum catalysts as measures of competitive rates of alkene adsorption. They note that very little isomerization accompanies hydrogenation, which suggests that desorption of the alkene is slow. A rapid interconversion of adsorbed alkene and the alkyl intermediate of the Horiuti-Polanyi mechanism is indicated by the distribution of deuterium in the deut-erated alkane formed when D2 is used in place of H2 yet little or no deuterium appears in the recovered alkene. ... 

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