Horiuti-Polanyi mechanism

A (either a transition state or a free energy minimum) reacts to form B. In hydrogenation, R in C is an alkyl group formed by reaction of H with adsorbed olefin and the mechanism is not Horiuti-Polanyi-like. Rather reaction via both A and C are among the few relatively clear examples of Rideal-Eley processes. 

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. 

Scheme 1 Horiuti-Polanyi (classical) mechanism for double bond migration.

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. 

Double bond migration occurs either by the Jt-allyl mechanism (abstraction-addition) or by the Horiuti-Polanyi mechanism (addition-abstraction). Pd is thought to favor Jt-allyl and Pt Horiuti-Polanyi mechanisms. 

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." ... 

Fig. 6. Reduction and isomerization of 1,2-dimethylcyclopentene according to the Horiuti-Polanyi mechanism.

The Horiuti-Polanyi mechanism can account for the change in cisjtrans ratio because the product-controlling step in the given sequence can he altered by varying the pressure, and different product-controlling steps could lead to different ratios of saturated stereoisomeric products, the difference depending upon the nature of the required geometries of the respective transition states. 

Two other limiting conditions can be defined by the use of a formalized argument 64) based upon a simple steady state analysis of the Horiuti-Polanyi mechanism. 

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 majority of observations concerning the isomerization of alkenes are in harmony with the Horiuti-Polanyi associative mechanism,170 171 which involves the reversible formation of the 24 half-hydrogenated state (Scheme 4.11) (see in more detail in Section 11.1.2). 

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. 

A unique mechanism was suggested to interpret the difference observed in the isomerization and hydrogenation of 1-butene and ds-2-butene over a stepped Pt(775) surface.360 It was observed that the hydrogenation rates were insensitive to surface structure for both 1-butene and ds-2-butene. The isomerization rates of cis-2-butene to give only trans-2-butene on the stepped Pt(775) surface, however, was double that of 1-butene to yield both cis- and trans-2-butenes. The Horiuti-Polanyi associative mechanism, that is, the involvement of the 2-butyl intermediate (see Section 4.3.2), cannot explain this difference. However, a facile dehydrogenation of ds-2-butene to 2-butyne followed by a rehydrogenation is consistent with the experimental observations ... 

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). 

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