Double-bond migration metal-catalyzed

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. 

Fio. 13. Geometrical relationships which affect the mechanism of the metal-catalyzed double bond migration in steroids. 

When the transfer reaction competes successfully with further insertion, as in the case of nickel, dimerization becomes the dominant transformation. When metal hydride elimination, in turn, is slow relative to insertion, polymeric macromolecules are formed. Ligand modification, the oxidation state of the metal, and reaction conditions affect the probability of the two reactions. Since nickel hydride, like other metal hydrides, catalyzes double-bond migration, isomeric alkenes are usually isolated. 

Hydrocarbons. The reaction of isoprene with toluene, ethylbenzene, or isopropylbenzene is catalyzed by sodium or potassium (72). The reactions are carried out at 125°C in a pressure autoclave by adding the isoprene slowly to the alkylarene in which the alkali metal is dispersed along with a trace quantity of 0-chlorotoluene which is used as a chain initiator. The products are chiefly monopentenylated in the side chain, and no information can be obtained on whether the addition is 1,4- or 1,2- since under these conditions the double bond migrates. The alkene products subsequently are reduced to alkanes by hydrogenation using 5% palladium on charcoal as catalyst. 

The Alder-ene reaction is an atom-economic reaction which forms a new carbon carbon-bond from two double bond systems (alkenes, carbonyl groups, etc.) with double bond migration [5]. This reaction follows the Woodward-Hoffmann rules if the reaction is performed under thermal conditions. However, when transition metal catalysts are involved, thermally forbidden Alder-ene reactions can also be realized (Scheme 9.1). Examples of such processes are the formal [4 + 4]-Alder-ene reaction catalyzed by low-valent iron catalysts. 

Although it is now almost fifty years since Speier and his colleagues first announced the chloroplatinic acid-catalyzed hydrosilation of olefins, we are still far from complete control of the chemistry. A particular problem is the suppression of double bond migration. A solution of this problem will require a more detailed understanding of the factors affecting the relative rates of P-hydride elimination from an alkyl group and of the reductive elimination of Si-H from a platinum silyl hydride complex. Another factor which is poorly understood is suppression of the irreversible reduction of the platinum catalyst to Pt° metal. Both of these problems can greatly increase costs of production of certain products. 

Among the most significant developments in the field of catalysis in recent years have been the discovery and elucidation of various new, and often novel, catalytic reactions of transition metal ions and coordination compounds 13, 34). Examples of such reactions are the hydrogenation of olefins catalyzed by complexes of ruthenium (36), rhodium (61), cobalt (52), platinum (3, 26, 81), and other metals the hydroformylation of olefins catalyzed by complexes of cobalt or rhodium (Oxo process) (6, 46, 62) the dimerization of ethylene (i, 23) and polymerization of dienes (15, 64, 65) catalyzed by complexes of rhodium double-bond migration in olefins catalyzed by complexes of rhodium (24,42), palladium (42), cobalt (67), platinum (3, 5, 26, 81), and other metals (27) the oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process) (47, 48, 49,... 

Complexes of many transition metals including cobalt, rhodium, iridium, iron, nickel, palladium, and platinum have been found to catalyze double-bond migration in terminal olefins. Evidence for a mechanism of the following type, which is probably also applicable to some of the other catalysts, has been obtained by Cramer 24, 27) for the rhodium chloride-catalyzed reaction (Reaction 37). 

Subsequently, it has been demonstrated that, under aprotic reaction conditions, various metal chlorides can catalyze not only double bond migration but also C-3 epimerization [189]. 

The double bond migration or cis-trans isomerization of linear pentenes catalyzed by a variety of transition metal complexes (Fe(CO)s, Fe3(CO)i2, Ru3(CO)i2) in the presence of irradiation illustrates the operation of case 1.3 [20, 21] (Scheme 3). Case 1.4, which covers photoinduced electron transfer... 

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). 

A wider variety of reactions has been reported if the metal-catalyzed process is the first, followed by the organocatalytic reaction. Recent sequential metal catalysis/organocatalytic 1,4 additions involve allylic oxidation [102] and double bond migration [103] metal-catalyzed processes (Scheme 26.20). 

Formal hydration of the double bond appeared by the hydroboration-oxidation sequence. Desymmetrization reactions with catalytic asymmetric hydroboration are not restricted to norbornene or nonfunctionalized substrates and can be successfully applied to meso bicyclic hydrazines. In the case of 157, hydroxy derivative 158 is formed with only moderate enantioselectivity both using Rh or Ir precatalysts. Interestingly, a reversal of enantioselectivity is observed for the catalytic desymmetrization reaction by exchanging these two transition metals. Rh-catalyzed hydroboration involves a metal-H insertion, and a boryl migration is involved when using an Ir precatalyst (Equation 17) <2002JA12098, 2002JOC3522>. 

Although various transition-metal complexes have reportedly been active catalysts for the migration of inner double bonds to terminal ones in functionalized allylic systems (Eq. 3.2) [5], prochiral allylic compounds with a multisubstituted olefin (Rl, R2 H in eq 2) are not always susceptible to catalysis or they show only a low reactivity [Id]. Choosing allylamines 1 and 2 as the substrates for enantioselective isomerization has its merits (1) optically pure citronellal, which is an important starting material for optically active terpenoids such as (-)-menthol, cannot be obtained directly from natural sources [6], and (2) both ( )-allylamine 1 and (Z)-allylamine 2 can be prepared in reasonable yields from myrcene or isoprene, respectively, The ( )-allylamine 1 is obtained from the reaction of myrcene and diethylamine in the presence of lithium diethylamide under Ar in an almost quantitative yield (Eq. 3.3) [7], The (Z)-allylamine 2 can also be prepared with high selectivity (-90%) by Li-catalyzed telomerization of isoprene using diethylamine as a telomer (Eq. 3.4) [8], Thus, natural or petroleum resources can be selected. 

Chiral rhodium(II) oxazolidinones 5-7 were not as effective as Rh2(MEPY)4 for enantioseleetive intramolecular cyclopropanation, even though the sterie bulk of their chiral ligand attachments (COOMe versus /-Pr or C Ph) are similar. Significantly lower yields and lower enantiomeric excesses resulted from the decomposition of 11 catalyzed by either Rh2(4S-IPOX)4, Rh2(4S-BNOX)4, or Rh2(4R-BNOX)4 (Table 3). In addition, butenolide 12, the product from carbenium ion addition of the rhodium-stabilized carbenoid to the double bond followed by 1,2-hydrogen migration and dissociation of RI12L4 (Scheme II), was of considerable importance in reactions performed with 5-7 but was only a minor constituent ( 1%) from reactions catalyzed by Rh2(5S-MEPY)4. This difference can be attributed to the ability of the carboxylate substituents to stabilize the earboeation form of the intermediate metal carbene. 

The pyrolytic decomposition of the sodium salts of various fluorinated carboxylic acids to give isomeric unsaturated compounds has also been reported. The products were identified as alkenes with the C = C bond inside the carbon chain, mainly alk-2-enes. This isomerization may be catalyzed by the coal-like products formed during the pyrolytic decarboxylation of the salts, but the metal fluoride formed in the reaction may also be responsible for the isomerization. When potassium perfluoro(5-chloropentanoate) is heated in a rocking autoclave at 300 C for 2 hours, perfluorobut-2-ene (2b) is isolated in 82% yield.This is only possible by migration of the double bond away from the terminal position after carbon dioxide elimination and halogen exchange to form potassium chloride. ... 

In both schemes cis insertion is the most hypothetical step. Cossee has assumed 164) that similar processes take place during olefin polymerization in the presence of Ziegler-Natta catalysts, and in some other reactions catalyzed by the transition metal compounds. This author came to the conclusion 165) that the d orbitals of the metal combine with the migrating group to facilitate such processes. In the course of 7r-allyl transfer the palladium orbitals overlap with the antibonding orbitals of double bond of the <7-bonded allyl group so as to favor an insertion reaction. 

It has long been recognized that certain transition metal complexes can catalyze the migration of carbon-carbon double bonds. When the catalyst is a transition metal hydride, the mechanism involves initial reversible addition of the metal... 

Isomerization of olefins which takes place as a result of the transfer of hydrogen atoms with concomitant migration of the double bond is catalyzed by transition metal complexes which may react with olefins to form organometallic compounds. Often, it is necessary that cocatalysts be present as sources of hydrogen. Most commonly utilized cocatalysts are acids, water, alcohol, hydrogen, silanes, etc. The formation of hydrido complexes during isomerization reactions is crucial. The following mechanisms of olefin isomerization reactions are known ... 

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