Bond Isomerization in Olefins

Because of its ready availability and its importance in the commercial 0X0 syntheses, and because substantial experimental and theoretical work on its properties and reactions has been reported in the literature, our discussion will begin with a consideration of isomerizations in which HCo(CO)4 is the catalyst. 

After the experimental demonstration by Orchin and associates (13) that under oxo conditions the catalytic form of the active catalyst is cobalt hydrotetracarbonyl [systematic name, hydrogen tetracarbonyl-cobaltate( —I)], HCo(CO)4, considerable effort has been devoted to the However, see below. 

When hexane solutions of the octacarbonyl are treated with an excess of dimethylformamide (DMF) and the resulting mixture is slowly acidified at 0° with 12 N HCl, according to the procedure of Kirch and Orchin (15), the liberated HCo(CO)4 goes into the hexane layer. The bottom aqueous-DMP layer containing CoClg is syringed away from the top layer the top hexane layer is then washed and dried and provides a dilute solution of hydrocarbonyl of known concentration. 

In studies of the isomerization of olefins by HCo(CO)4, it must be borne in mind that the catalyst HCo(CO)4 is consumed stoichiometrically via the hydroformylation reaction with the formation of aldehydes and dicobalt octacarbonyl, as shown by Kirch and Orchin (16)  

Since under these conditions the olefin is not isomerized by Cojj(CO)8, any isomerization that occurs must proceed at a rate about equal to or greater than the rate of hydroformylation. 

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We know that the rotation about a double bond is impossible without disturbing the n bond, which requires a large amount of energy (about 60 K cals/mole). This restricted rotation gives rise to cis-trans isomerism in olefines and their derivatives. 

While double bond migration in olefins might arise from base (31) as well as acid catalysis (32), the occurrence of skeletal isomerization under these conditions can be ascribed to acid catalysts. This presumption would attribute acidic properties to the alumina. 

An ab initio method has been employed to study the mechanism of the thermal isomerization of buta-1,2-diene to buta-1,3-diene. The results of the study have indicated619 that the transformation proceeds in a stepwise manner via a radical intermediate. Experimental free energies of activation for the bond shift in halocyclooctatetraenes have been reported and analyzed by using ab initio MO calculations.620 The isomerization of hexene using a dihydridorhodium complex in dimethyl sulfoxide has been reported,621 and it has been suggested622 that the Pd(II)-catalysed homogeneous isomerization of hexenes proceeds by way of zr-allylic intermediates. A study has been made623 of alkene isomerization catalysed by the rhodium /-phosphine-tin dichloride dimeric complex, and the double-bond isomerization of olefinic amines over potassium amide loaded on alumina has been described.624... 

Reactions studied include dehydrations of alcohols, double bond shifts in olefins, isomerization of hydrocarbons, racemization of optically active compounds, etc.. In the literature a rather rigid separation is made between a Brested acid, which is actually a proton donor, and a Lewis acid, which works as a hydride abstractor. We may illustrate this difference by using the double bond shift in olefins as the model reaction. 

Basic zeolites are able to catalyze double bond isomerization of olefins [133]. Although this can also be achieved with acidic zeolites, the lower reactivity of basic zeolites towards hydrocarbons (i.e., the complete absence of skeletal isomerization) leads to higher yields [134]. A good example for this is the double bond isomerization of 1-octene over potassium loaded NaY. It is claimed that high yields can be achieved in that way and that the impregnation of the zeolite with an excess of alkali cations is important to obtain a good catalyst [135]. 

In contrast to solid acid catalysts (including solid superacids) described earlier, solid base catalysts (including superbases) have received much less attention. A common basic catalyst used in organic synthesis is Ba(OH)2. The use of alumina-supported sodium in the double bond isomerization of olefins, first studied by Pines et al. (1955), is also widely known. More recent developments include the use of K, Na/K, MgO, Zr02, Th02, and K/KOH/AI2O3 zeolites containing different metal ions to control their acid-base properties, solid superbases, and... 

Zeolite MCM-22 in its Br0nsted-acid form has been described in the hterature as a useful catalyst for a variety of acid-catalyzed reactions, such as iso-alkane/olefin alkylation [e.g.40,41],skeletal and double-bond isomerization of olefins [42] and ethylbenzene synthesis via alkylation of benzene with ethylene [43], to name merely a few. Moreover, due to its very large intracrystalline cavities, zeolite MCM-22 has also been demonstrated to be a suitable host material for a variety of catalytically active guests, e.g. transition metal complexes which are useful in selective oxidation [44] or hydrogenation [45] reactions. Due to these interesting properties it seems worthwhile to focus on the synthesis features of MCM-22 (see below). 

However, the true particular sites of aluminas for most catalytic reactions are very likely the anion-cation couples, which have very high activity and work syner-gistically. Alcohol adsorption experiments [127,164] allow the characterization of such sites where dissociative adsorption occurs. Mechanistic studies suggest that such cation-anion couples are likely those active in alcohol dehydration [165], in alkylchloride dehydrochlorination [152,166], and in double bond isomerization of olefins [167] over y-Al203. 

Chemical Properties. Higher a-olefins are exceedingly reactive because their double bond provides the reactive site for catalytic activation as well as numerous radical and ionic reactions. These olefins also participate in additional reactions, such as oxidations, hydrogenation, double-bond isomerization, complex formation with transition-metal derivatives, polymerization, and copolymerization with other olefins in the presence of Ziegler-Natta, metallocene, and cationic catalysts. All olefins readily form peroxides by exposure to air. 

This process accounts for most of the observations relating to product stereochemistry, double bond isomerism, deuterium exchange and other features encountered in the hydrogenation and deuteration of olefins. 140-142,144 addition of hydrogen to the double bond proceeds in... 

The double bond migration in steroid hydrocarbons catalyzed by acids or noble metals (see, for example, ref. 185) will not be discussed here. A general review of nonsteroid olefin isomerization has recently been published. Iron carbonyl has been used to isomerize steroidal dienes. 

Hydrogenation of aromatics under mild conditions gives mainly the all-cis isomer as if hydrogen addition takes place from only one side of the molecule (23,24). Reductions under more vigorous conditions may give other isomers by isomerization of the initially formed all-cis product. Under mild conditions, other isomers are accounted for by desorption and readsorption in a new orientation of intermediate olefins, as well as by double-bond migration in the... 

Not only the linear Cl0-Cl8 a-olefins but also the linear C10-Cl8 olefins with internal double bonds, the so-called -v /-olefins, are of great importance in surfactant chemistry, n-a-Olefins and n-y-olefins have the same suitability for the manufacture of linear alkylbenzenes, the most important synthetic anionic surfactants, by alkylation of benzene. Nowadays medium molecular weight n- /-olefins are industrially produced by two processes the catalytic dehydrogenation of the corresponding n-alkanes [4,28] and the cometathesis of low and high molecular weight n-v /-olefins, obtained by double-bond isomerization of the isomeric n-a-olefins [29]. 

If cobalt carbonylpyridine catalyst systems are used, the formation of unbranched carboxylic acids is strongly favored not only by reaction of a-olefins but also by reaction of olefins with internal double bonds ( contrathermo-dynamic double-bond isomerization) [59]. The cobalt carbonylpyridine catalyst of the hydrocarboxylation reaction resembles the cobalt carbonyl-terf-phos-phine catalysts of the hydroformylation reaction. The reactivity of the cobalt-pyridine system in the hydrocarboxylation reaction is remarkable higher than the cobalt-phosphine system in the hydroformylation reaction, especially in the case of olefins with internal double bonds. This reaction had not found an industrial application until now. 

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