The Horiuti-Polanyi Mechanism

The following basic facts have been established from exchange with deuterium of numerous alkanes and polycycloalkanes especially on Pd catalysts. 

The aP process propagates readily along a chain of carbon atoms, and, in acyclic paraffins with rapid rotation about C—C bonds, every H atom is readily replaced, as evidenced by the very large quantities of the perdeutero isomer observed by Gault and Kemball (2) in initial products from exchange of n-hexane on Pd films. 

Propagation of the exchange reaction is blocked if the chain of carbon atoms contains a quaternary center or a bridgehead such as that in bicyclo [2,2,1 ] -heptane. 

Typical examples of compounds possessing isolated paiis of vicinal hydrogen atoms (4,5). 

However, the exchange data also afforded details that gave rise to two major problems  

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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... 

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. 

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. 

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. 

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.

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. 

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... 

The Horiuti-Polanyi mechanism for olefin hydrogenation as discussed in Section 7.4 involves 4 steps ... 

Scheme 1 The Horiuti-Polanyi mechanism of alkene hydrogenaiion...

The mechanisms of isomerization which have been considered fall into two categories associative , first proposed by Horiuti and Polanyi, and dissociative , advanced by Farkas et al The associative mechanism is a consequence of the reversibility of the formation of the alkyl intermediate shown in Scheme 1 and in equation (10), while the dissociative mechanism, in its current form, involves allylic species (5)-(7) shown in Scheme 2. The Horiuti-Polanyi mechanism implies that double bond isomerization and the addition of H2 proceed through a common intermediate, whereas the dissociative mechanism represents an independent path. ... 

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. ... 

The observed phenomena may be adequately explained on grounds of the mechanism of gradual addition of hydrogen species to the double bond, the basic form of which is called the Horiuti-Polanyi mechanism (49) ... 

Table 1 provides data on the hydrogenation of a range of cycloalkenes. Curiously, there is no obvious trend in the hydrogenation yields of the olefins, cyclopentene and cyclooctene being reduced faster than cyclohexene and cycloheptene. These variations are, therefore, not on account of any size selectivity of the catalyst. The heats of hydrogenation (AH) of the cycloalkenes (Cs-Cg) are different from each other [12]. However, the difference in the AH supports the variation we have observed in the hydrogenation yields of cycloalkenes. The orientation of the substrate towards the catalyst could be interpreted on the basis of the Horiuti-Polanyi mechanism [13]. 

An inevitable consequence is the subsequent formation of more extensively exchanged ethene and of ethanes containing from zero to six deuterium atoms, but the relative rates of the exchange and addition vary widely with the metal and with operating conditions (Section 7.31). These observations are neatly explained by the Horiuti-Polanyi mechanism, which proposed a reversible stepwise addition of deuterium (or hydrogen) atoms, e.g. 

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