Metal catalyzed olefin isomerization

Scheme 5.2 Two mechanisms of metal-catalyzed olefin isomerization.

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

Conventionally, organometallic chemistry and transition-metal catalysis are carried out under an inert gas atmosphere and the exclusion of moisture has been essential. In contrast, the catalytic actions of transition metals under ambient conditions of air and water have played a key role in various enzymatic reactions, which is in sharp contrast to most transition-metal-catalyzed reactions commonly used in the laboratory. Quasi-nature catalysis has now been developed using late transition metals in air and water, for instance copper-, palladium- and rhodium-catalyzed C-C bond formation, and ruthenium-catalyzed olefin isomerization, metathesis and C-H activation. Even a Grignard-type reaction could be realized in water using a bimetallic ruthenium-indium catalytic system [67]. 

In hydrocarboxylations, as in the 0x0 process, selectivity of linear versus branched products is an important issue, because (in general) mixtures of isomeric carboxylic acids are obtained, owing not only the occurrence of both Markovni-kov and anti-Markovnikov addition of the alkene to the metal hydride, but also to metal-catalyzed alkene isomerization (eq. (2)). In the case of higher olefins, Co2(CO)g as catalyst leads to a number of different carboxylic acid isomers due to the isomerization activity of the catalyst. 

Transition metal complexes can readily catalyze olefin isomerization. This can occur without cocatalysts via 7i-allyl complexes. Formulate such a reaction between the co-ordinatively unsaturated complex D-ML and the olefin RCH2CH=CH2. 

Although many transition metal complexes catalyze olefin isomerization, early studies have shown that the presence of silane markedly affects such reactions that occur concurrently with hydrosilylation. Double-bond migration, which is a characteristic feature of most coordination catalytic reactions, can be illustrated schematically in (Scheme 6) as a side reaction occurring during hydrosilylation (3,10,47). 

The transition-metal-catalyzed olefin metathesis is a still recent, but already well-established, development, which has evolved toward a highly useful synthetic tool [27]. The ring-closing variant of this reaction ]ring-closing metathesis (RCM)] has proved to be very suitable for the preparation of carbo- and heterocycles of any ring size, except for those that are strained ]28, 29]. In the case of 5,6-dihydropyran-2-ones, RCM of homoaUyl acrylates has very often been used for the direct creation of this heterocyclic system, most particularly in the past decade (Figure 2.4). Allyl esters of fi,y-unsaturated acids may also be subjected to RCM with the formation of P,y-unsaturated 8-lactones, which can be easily isomerized by acid or base catalysis to their a,fi-unsaturated counterparts ]6, 30-39]. 

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. 

We have explored rare earth oxide-modified amorphous silica-aluminas as "permanent" intermediate strength acids used as supports for bifunctional catalysts. The addition of well dispersed weakly basic rare earth oxides "titrates" the stronger acid sites of amorphous silica-alumina and lowers the acid strength to the level shown by halided aluminas. Physical and chemical probes, as well as model olefin and paraffin isomerization reactions show that acid strength can be adjusted close to that of chlorided and fluorided aluminas. Metal activity is inhibited relative to halided alumina catalysts, which limits the direct metal-catalyzed dehydrocyclization reactions during paraffin reforming but does not interfere with hydroisomerization reactions. 

In the presence of transition-metal complexes, organic compounds that are unsaturated or strained often rearrange themselves. One synthetically useful transition-metal catalyzed isomerization is the olefin migration reaction. Two general mechanisms have been proposed for olefin migrations, depending on the type of catalyst employed (A and B) (Scheme 3.8).137... 

The transition metal-catalyzed reaction of diazoalkanes with acceptor-substituted alkenes is far more intricate than reaction with simple alkenes. With acceptor-substituted alkenes the diazoalkane can undergo (transition metal-catalyzed) 1,3-dipolar cycloaddition to the olefin [651-654]. The resulting 3//-pyrazolines can either be stable or can isomerize to l//-pyrazolines. 3//-Pyrazolines can also eliminate nitrogen and collapse to cyclopropanes, even at low temperatures. Despite these potential side-reactions, several examples of catalyzed cyclopropanations of acceptor-substituted alkenes with diazoalkanes have been reported [648,655]. Substituted 2-cyclohexenones or cinnamates [642,656] have been cyclopropanated in excellent yields by treatment with diazomethane/palladium(II) acetate. Maleates, fumarates, or acrylates [642,657], on the other hand, cannot, however, be cyclopropanated under these conditions. 

There have been a number of isolated studies of metal-ion catalyzed nucleophilic reactions of other groupings. Particularly interesting is the induced nucleophilic attack on olefins. Hydration is normally very sluggish. Enzymes can speed up such reactions. Aconitase, an iron-containing enzyme, catalyzes the isomerization of citric acid to isocitric acid, through the intermediacy of cis-aconitic acid. A possible mechanism has been suggested based on the following Co(III) model chemistry. Rapid cyclization of the maleate ester produces Ai and AS chelated malate half ester ... 

Since the basic or carbanion intermediate can continue to go to product by Steps 2 and 3, we have a chain reaction which is consistent with the rapid isomerizations which may be obtained using these catalysts. This mechanistic interpretation was proposed in one of the first papers published on this subject (5) it and similar interpretations have been very helpful in bringing about an understanding of base-catalyzed reactions. The chain-reaction sequence may be terminated by reaction with a formation of a material which is not basic enough to metallate the olefin. Such compounds may be polyunsaturated hydrocarbons which may be formed by elimination of hydride ions from a carbanion. 

In the early 1970s it was known that palladium films isomerize n-butane and isobutane (63, 70) but not neopentane (63). Similarly, only pure cracking of neopentane was observed on Pd/Si02 catalysts (71). The observed inability of Pd to catalyze neopentane isomerization was confirmed in the case of more complicated molecules having a quaternary carbon atom (64). On the other hand, platinum seemed to be the unique metal in isomerizing neopentane. Comparing the catalytic behavior of both metals, Gault came to the conclusion that palladium, due to its well-established propensity toward formation of 7r-olefinic and ir-allylic adspecies, can... 

On the other hand, the transition metal-catalyzed isomerization of terminal olefins into internal olefins has been extensively studied, and in general a mixture of 1-alkenes and E- and Z-2-alkenes, reflecting the thermodynamic equilibrium, is obtained [67]. Some low-valent titanocene derivatives are highly effective and stereoselective in favor of the E-2 isomer [68]. When non-conjugated dienes such as 92, containing one or two substituted vinyl groups, are treated with the zirconocene 21, a regioisomerization of the less-substituted... 

X = H, OOCR, Cl, OR). A similar mechanism has been suggested for metal carbonyl-catalyzed thermal isomerization of olefins 185). 

Olefin isomerization has been widely studied, mainly because it is a convenient tool for unravelling basic mechanisms involved in the interaction of olefins with metal atoms (10). The reaction is catalyzed by cobalt hydrocarbonyl, iron pentacarbonyl, rhodium chloride, palladium chloride, the platinum-tin complex, and by several phosphine complexes a review of this field has recently been published (12). Two types of mechanism have been visualized for this reaction. The first involves the preformation of a metal-hydrogen bond into which the olefin (probably already coordinated) inserts itself with the formation of a (j-bonded alkyl radical. On abstraction of a hydrogen atom from a diflFerent carbon atom, an isomerized olefin results. 

Free radical promoted hydrosilation of olefins typically gives poorer conversions than are produed by hydrosilation mediated by metal catalysts. However, they are not subject to isomerization of the olefin, a side reaction that can be a nuisance in metal-catalyzed hydrosilations. This is due to the extremely efficient hydrogen atom transfer reaction from Si-H to carbon radicals and the resulting very short lifetimes of the latter. The generally accepted mechanism for these processes is illustrated in Scheme 1. 

The review of the experimental results, which now follows, will concentrate on assessing the extents to which the transition metals catalyze the processes of olefin exchange and isomerization and the mechanisms of these processes in the light of the subject matter of this section. 

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