Alkene isomerization, homogeneous

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

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... 

The alkene isomerization B — iso-B also may have another stereochemical consequence (part II of Figure 17.75) the destruction of the configurational homogeneity of a stereocenter C HR5R6 in the allylic position of the substrate. In that case, the hydrogenation results in a mixture containing the stereoisomers A and iso-A. 

In the previous chapters we discussed alkene-based homogeneous catalytic reactions such as hydrocarboxylation, hydroformylation, and polymerization. In this chapter we discuss a number of other homogeneous catalytic reactions where an alkene is one of the basic raw materials. The reactions that fall under this category are many. Some of the industrially important ones are isomerization, hydrogenation, di-, tri-, and oligomerization, metathesis, hydrocyana-tion, hydrosilylation, C-C coupling, and cyclopropanation. We have encountered most of the basic mechanistic steps involved in these reactions before. Insertions, carbenes, metallocycles, and p -allyl complexes are especially important for some of the reactions that we are about to discuss. 

In any homogeneous catalytic reaction involving an alkene, isomerization of the alkene is always a possibility. The insertion of alkene into the M-H bond can occur in a Markovnikov or anti-Markovnikov manner (see Section 5.2.2). Alkene isomerization involves Markovnikov addition, which is followed by a /3-hydride elimination. A simplified catalytic cycle is shown in Fig. 7.1. 

Zr and Hf hydrides have been proposed as intermediates or by-products in heterogeneous and homogeneous catalysis of alkene isomerization, polymerization, and trimerization reactions.388 657-659 For example, heating the well-characterized silica-bound Zr(neopentyl)ra (n 1, 2) with dihydrogen afforded silica supported Zr-hydrides as indicated by IR spectroscopy. 58 Interestingly, this silica-supported Zr hydride exchanged via cr-bond metathesis with alkanes to produce the corresponding Zr alkyl derivatives.388,660,661... 

Alkene isomerizations involving homogeneous, catalyzed hydrogen migrations... 

This section concerns alkene isomerizations in which hydrogen, rather than an anion or uncharged hydroxylic molecule, is the migrating entity. Only isomerizations involving migration of carbon-carbon double bonds under the influence of acidic, basic, or other catalysts in homogeneous systems are considered. 

The mechanistic and stereochemical options available to allylic carbanions are fully as complex as those available to allylic carbonium ions. Much that is known concerning the mechanisms of base-catalyzed alkene isomerizations was obtained from stereochemical, isotope exchange, and product composition studies. The present discussion is limited to alkene isomerizations whose kinetics have been studied in homogeneous systems. Isomerizations involving allylic carbanion intermediates have been reviewed by Cram . ... 

Several kinetic studies of alkene isomerizations catalyzed by transition-metal coordination complexes in homogeneous solutions have been reported. 

Hydroformylation is a metal-catalyzed reaction in which an olefin, CO, and H react to produce an aldehyde. The reaction was discovered at BASF by Otto Roelen, and was called hydroformylation by Adkins. This transformation is also sometimes referred to as the "oxo" process. In a formal sense, the elements of formaldehyde are added across a C=C bond. Common side reactions include aUcene hydrogenation, aldehyde hydrogenation, and alkene isomerization. Hydroformylation is one of the largest volume reactions conducted with homogeneous catalysts in the chemical industry. It is used to produce over 14 billion pounds of aldehydes per year (two pounds per year for every person on Earth ). The aldehydes are converted to alcohols, acids, and other materials as useful end products. One large-volume use of hydroformylation is the conversion of propene to a mixture of -butyraldehyde and /-butyraldehyde (Equation 17.2). Since the desired product is n-butyraldehyde, a great deal of effort has been expended to maximize the n i (noimal to iso, or often also called l/b for linear to branched) ratio of aldehydes and to understand the factors that control it. 

Two reviews deal with the relation between homogeneous and heterogeneous catalysis. One of these reviews is of a general physical and theoretical nature, while the other is concerned with specific classes of compounds — alkenes, alkynes, and fats — and concentrates on their hydrogenation and isomerization. Homogeneous catalysis by ruthenium complexes has been reviewed. The application of molecular orbital symmetry rules, to organic as well as to organometallic reaction mechanisms, has been discussed, and a set of rules similar to, but simpler to apply than, the Woodward-Hoffmann rules has been described. ... 

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