Isomerization, olefin atom shifts

The absence of free isomerized olefins, the Constance of isomeric composition of the products throughout the whole reaction in hydro-formylation experiments of 1-pentene and 4-methyl-1-pentene under high carbon monoxide pressure, the distribution of deuterium in the hydro-formylation products of 3-methyl-l-hexene-3-di and 3-(methyl-d3)-l-butene-4-d3, and the results of carbonylation of olefins containing a quaternary carbon atom indicate initial formation of an olefin-cobaltcarbonyl complex. Isomerization of this complex, resulting in 1,2 hydrogen shifts in its organic moiety, can produce the necessary precursors of the various aldehydes that are formed. 

Isomerization the carbonium ion intermediates can undergo rearrangement by hydrogen or carbon atom shifts, leading to, e.g. a double-bond isomerization of an olefin. Other important isomerization reactions are methyl group shift and isomerization of saturated hydrocarbons. 

Allylic alcohols are isomerized via direct interaction of the ruthenium atom with alcohol. /3-Elimination of ruthenium hydride from metal alkoxide yields a ruthe-nium-enone species C which undergoes insertion of the olefinic moiety into the Ru-H to form an oxyallylic intermediate D. As a result, the hydrogen atom shifts from the a- to y-position of the allylalcohol. Protonolysis of the oxyallylic species leads to a saturated carbonyl compound and cationic unsaturated species, [CpRu(PPh3)2] A. 

However, cis-trans isomerization cannot be so explained, since free rotation is not possible in a 7r-allyl-adsorbed olefin as it is in an alkyl radical. This can only occur if a double-bond shift first occurs, as shown in Fig. 3. The mechanisms of olefin isomerization may be expected to be clarified by the use of deuterium, when it is possible to study the manner of the appearance of deuterium atoms in the isomerized olefin (32). 

Isomerization of the Olefin. The formation of isomeric olefins by double bond shifts and as a consequence the formation of aldehydes other than those expected on the basis of attachment of the formyl group to one of the two carbon atoms of the original double-bond is most pronounced with cobalt catalysts. Unmodified rhodium catalyst is also effective in olefin isomerization. 

Olefins react secondarily for isomerization and hydrogenation (on cobalt sites that are not active for chain growth lower scheme in Figure 9.15). There is a first reversible H-addition (at the alpha- or beta-C-atom of the double bond) to form an alkyl species, and a slow irreversible second H-addition to form the paraffin (lower scheme in Figure 9.15). Thus, double-bond shift and double-bond hydrogenation are interrelated by a common intermediate to produce olefins with internal double bonds or paraffins from the primary FT alpha-olefins. Experimental results1018 are presented in Figures 9.16 and 9.17. 

Once the carbonium ions are formed, the modes of interaction constitute an important means by which product formation occurs during catalytic cracking. For example, isomerization either by hydride ion shift or by methyl group shift, both of which occur readily. The trend is for stabilization of the carbonium ion by movement of the charged carbon atom toward the center of the molecule, which accounts for the isomerization of a-olefins to internal olefins when carbonium ions are produced. Cyclization can occur by internal addition of a carbonium ion to a double bond which, by continuation of the sequence, can result in aromatization of the cyclic carbonium ion. 

Vinyl mercaptan may arise via the isomerization of a short-lived excited singlet biradical (28c), or by the direct insertion of S( Z>) atoms into the vinylic C—H bond (28d). To distinguish between these alternatives is extremely difficult. Nevertheless, the presently available data appear to be more consistent with step (28b) that is with the initial formation of a singlet biradical, followed by rapid isomerization through shift of an H atom. This question will be discussed in somewhat more detail later in this chapter, in light of the information obtained from the studies with higher olefins. 

The aldehydes and alcohols produced are a mixture of normal and iso-compounds. This is due not only to the orientation of the hydrogen with respect to the C-CO bonds in the initial reaction complex but also to the isomerization of the olefin under the process conditions. It may be significant that nickel carbonyl does not readily shift the olefin double bond under the 0X0 process conditions, and nickel compoimds are very poor catalysts for the process. From isooctene 32% n-nonyl alcohol, and from propylene 50% n-butyl alcohol are obtained, the remainder of the products being isoalcohols. In general, using a-olefins as raw material, one obtains about 60% isoalcohols. The synthesis will not occur unless a labile hydrogen atom is available in the olefin reactant. With diolefins the reaction takes place at only one double bond. 

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