Bonding Characteristics and Isomerism

Skeletal ring contraction steps of primary C7 and Cg rings are more probable than bicyclic intermediates (132b). Aromatization of methylcyclo-pentane indicated no carbonium mechanism with a nonacidic catalyst. Instead, Pines and Chen (132b) proposed a mechanism similar to that defined later as bond shift. This is a methyl shift. Two additional isomerization pathways characteristic of chromia have also been demonstrated vinyl shift (94) and isomerization via C3 and C4 cyclic intermediates (90a). These were discussed in Section III. 1,1-Dimethylcyclohexane and 4,4-dimethyl-cyclohexene gave mainly toluene over various chromia catalysts. Thus, both skeletal isomerization and demethylation activities of chromia have been verified. The presence of an acidic almnina support enhances isomerization dual function effects are thus also possible. 

Dienes undergo isomerization due to shifts of the double bonds. The reversible isomerization of allenes to acetylenes is catalyzed characteristically by basic reagents (see Section 4.2.2). Nonconjugated alkadienes tend to isomerize to conjugated alkadienes the conversion is usually accompanied by polymerization. Among other catalysts, activated alumina and chromia-alumina may be used to catalyze the formation of conjugated dienes.89,106-108... 

In most cases, the two types of mechanisms, the bond shift and cyclic mechanisms, are not exclusive but parallel pathways. With increasing molecular weight, the contribution of the cyclic mechanism increases and may become dominant. The pure selective mechanism on iridium is a unique exception. Hydrogenolysis, however, is the characteristic transformation on this metal. The nature of possible surface intermediates in metal-catalyzed alkane reactions, the role of electronic and geometric effects in their formation, and the relation of isomerization and hydrogenolysis have been reviewed.163... 

Tin oxide-based materials are potent oxidation and isomerization catalysts. Their bulk and surface properties, as well as their presumed mechanism in oxidation catalysis, have been reviewed (53j. Considerable uncertainty remains concerning the phase compositions, solid-solution range, and the redox behavior (Sn / Sn" vs. Sb WSb ) of these materials. Structural investigations have so far concentrated on the use of " Sn and Sb Mossbauer spectroscopy. Surprisingly, no " Sn solid-state NMR studies have appeared to date on this system, although it was recently demonstrated that isotropic " Sn chemical shifts and chemical shift anisotropies give characteristic fingerprints of the various tin coordination environments in Sn(IV) oxide compounds [54]. In situ C NMR has been used to study the double bond shift of 1-butene to t /.s-2-butene, and the subsequent cis-trans isomerization over tin antimony oxide catalysts [55 j. 

The first step in the sequence involves the dissociative loss of a neutral ligand from 19 to form 20 with no change in formal oxidation number of the metal. The second, and especially characteristic stage, involves the isomerization of 20 to 21. This reaction may be regarded as an oxidative addition of phenyl and H to the metal with a formal increase in oxidation number from 4-2 to 4-4 it may also be viewed as metal insertion into a C—H sigma bond. Such an isomerization is analogous to an earher established example 47) of this type of reaction ... 

There are two major reasons for the astonishing number of organic compounds the bonding characteristics of carbon atoms, and the isomerism of carbon-containing molecules. As a group IVA(14) element, a carbon atom has four valence electrons. Two of these outermost-sheU electrons are in an s orbital, and two are inp orbitals ... 

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