Anderson-Avery mechanism

Three explanations have been suggested for bond shift. The Anderson-Avery mechanism (Scheme VIIIA) assumes surface hydrocarbon species multiply bonded to the metal (7, 86). 

Fig. 2. Mechanisms of bond-shift isomerization on metal catalysts, (a) Anderson-Avery mechanism (63). (b) Muller-Gault mechanism for isomerization of neopentane on Pt (64). (c) McKervey-Rooney-Samman mechanism (69). (d) Muller-Gault mechanism of isomerization of isobutane on Pd (64). (e) Clarke-Rooney mechanism (46). (f) Garin-Gault mechanism (65).

FIG. 11. The Anderson-Avery mechanism for bond-shift isomerization on Pt (47). 

We believe that concern over feasibility of the reverse reaction is unnecessary in the arguments. By the principle of microscopic reversibility, the forward path can be reversed by the same elementary steps. The incomplete nature of the Anderson-Avery mechanism (Fig. 11) has apparently caused some confusion. 

Of the four basic mechanisms, two, the Anderson-Avery mechanism (Scheme 19) and the Garin-Gault mechanism (Scheme 25), should be associated with isomerization and hydrogenolysis two, the Muller-Gault mechanism (Scheme 20) and the Rooney-Samman or the Clarke-Rooney mechanism (Schemes 22 and 28) should promote only isomerization. 

In contrast, the Anderson-Avery mechanism does not discriminate among the various isomerization reactions. Moreover, for hydrogenolysis, it forecasts the rupture, in the 1,1,3-triadsorbed precursor, of the C2-C3 bond next to the single metal-carbon bond. However, in a recent investigation of the hydrogenolysis of a number of hydrocarbons on platinum catalysts, Leclercq, Leclercq, and Maurel (82) showed that the C-C bonds in the P position to a tertiary carbon atom are preferentially ruptured. As pointed out by these... 

One involving an o,a,P triadsorbed precursor, Anderson-Avery mechanism [5], and the Garin-Gault mechanism involving a metaUacyclobutane intermediate [26], Scheme 17.1. 

The mechanism of bond-shift isomerization of alkanes is still a subject of controversy. Several suggested mechanisms involved an intermediacy of multiply bonded species [Anderson-Avery (63), Muller-Gault (64), and Garin-Gault (65) mechanisms] (Fig. 2). More recent data (51, 52, 66-68) seem to support the 1,2 bond-shift isomerization involving monoadsorbed alkyl species (69). Figure 2 shows these mechanistic ideas. 

Anderson and Avery 24, 128) have proposed a bond shift mechanism based upon a 1-3 adsorbed species which, when formed from neopentane, is... 

Anderson and Avery s bond shift mechanism has the consequence of predicting that a quaternary carbon atom cannot be generated in the hydrocarbon product. In fact, Anderson and Avery (24) showed that in the isomerization of isopentane over platinum films, only a very small amount (<1%) of neopentane was produced (although the equilibrium constant for isopentane <= neopentane is 0.16 at 278°C). Furthermore,... 

Noncarbonium-ion-type 1-2-methyl shifts have been described by Barron et al. (11), and by Anderson and Avery (34). The reaction proceeds through a-y-diasorbed intermediates over platinum on neutral supports and does not involve carbonium-ion intermediates. According to this mechanism, the n-butylbenzene isobutylbenzene reaction involves the following steps (surface sites are represented by ) ... 

In order to explain the isomerization of neopentane to isopentane on platinum films, Anderson and Avery [34) proposed a mechanism involving, as precursor, an a,a,y-triadsorbed species, and, in the transition state, a 71 complex of the Dewar type, attached to the surface by two carbon-metal bonds. By simplified Hiickel molecular orbital (MO) calculations, they showed that hyperconjugative effect and partial charge transfer to the metal could account for the relative isomerization rates of the various molecules studied (neopentane > isobutane > n-butane) (Scheme 19). 

Nevertheless, the metal-catalyzed isomerization reaction is of interest from the point of view of understanding the nature of hydrocarbon transformations on metal surfaces. It has been suggested that carbonium-ion-like intermediates are involved in alkane isomerization reactions on platinum (23), and a specific mechanism has been proposed by Anderson and Avery (24). 

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