Metal-catalyzed isomerization

The allyl group was used to protect the nitrogen in a /3-lactam synthesis, but was removed in a four-step sequence. Whether a transition-metal-catalyzed isomerization to the enamide followed by hydrolysis is an effective cleavage procedure remains to be tested and warrants further study. ... 

Scheme 53 Transition metal catalyzed isomerization of allylic alcohols...

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... 

Allylic C/H insertion accompanied by an allylic rearrangement has been observed for carbenoid reactions of ethyl diazoacetate with allylamines (Scheme 23)1S1). Apparently, metal-catalyzed isomerization 117 118 proceeds the C/H insertion process. Although mechanistic details have not yet been unraveled, T)3-allyl complexes... 

Various transition metal complexes, in particular of late transition metals, were reported to be effective catalysts for such double bond isomerization. Because organic synthesis is the focus of this volume, this section will cover the transition metal-catalyzed isomerization of alkenes, which has the significant synthetic and industrial utilities. This chapter will also include the synthetic application, asymmetric reactions,4-6 and isomerization of alkynes, in particular, that of propargylic alcohols. 

Metal-catalyzed isomerization of allylamides is slower than that of allylamines. The isomerization was examined at higher temterature (>100°C) using [Rh((+)-BINAP) (cod)]+. Although the enantioselecdvity was high, the yield of the desired enamide was low due to the formation of dienamide (Equation (5)).9... 

Metal-catalyzed C-H bond formation through isomerization, especially asymmetric variant of that, is highly useful in organic synthesis. The most successful example is no doubt the enantioselective isomerization of allylamines catalyzed by Rh(i)/TolBINAP complex, which was applied to the industrial synthesis of (—)-menthol. A highly enantioselective isomerization of allylic alcohols was also developed using Rh(l)/phosphaferrocene complex. Despite these successful examples, an enantioselective isomerization of unfunctionalized alkenes and metal-catalyzed isomerization of acetylenic triple bonds has not been extensively studied. Future developments of new catalysts and ligands for these reactions will enhance the synthetic utility of the metal-catalyzed isomerization reaction. 

The first examples of transition metal-catalyzed [5 + 2]-cycloadditions between vinylcyclopropanes (VCPs) and 7r-systems were reported in 1995 by Wender and co-workers.10 This [5 + 2]-reaction was based conceptually on the Diels-Alder reaction, replacing the four-carbon, four-7r-electron diene with a five-carbon, four-electron VCP (Scheme 1). Although the [5 + 2]-reaction of VCPs and 7r-systems can be thought of as a homolog of the Diels-Alder [4 +21-reaction, the kinetic stability of VCPs (activation barrier for the thermal isomerization of VCP to cyclopentene has been reported as 51.7 kcal mol-1)11 makes the thermal [5 + 2]-reactions involving VCPs and 7r-systems very difficult to achieve. A report of a thermal [5 + 2]-cycloaddition between maleic anhydride and a VCP has been published,12 but this reaction has not been reproduced by others.13 14 Based on the metal-catalyzed isomerization of VCPs to cyclopentenes and dienes,15-20 Wender and co-workers hypothesized that a metal might be used to convert a VCP to a metallocyclohexene which in turn might be trapped by a 7r-system to produce a [5 + 2]-cycloadduct. Based on its previous effectiveness in catalyzed [4 + 2]-21 and [4 + 4]-cycloadditions (Section 10.13.2.4), nickel(0) was initially selected to explore the potential of VCPs as four-electron, five-carbon components in [5 + 2]-cycloadditions. 

Hydrogenative ring opening of cycloalkanes is also a well-studied area.16 252 253 289-292 Mainly cyclopropanes and cyclopentanes were studied, since three- and five-membered adsorbed carbocyclic species are believed to be intermediates in metal-catalyzed isomerization of alkanes (see Section 4.3.1). Ring-opening reactivity of different ring systems decreases in the order cyclopropane > cyclobutane > cyclopentane > cyclohexane.251 Cyclopropane and its substituted derivatives usually react below 100°C. 

Transition metal catalyzed isomerization reactions are closely related to asymmetric hydrogenations, especially because similar catalysts are used. A number of these reactions are used at scale (Chapters 12 and 31). 

This reaction based on the petrochemical crude material isobutylene makes the synthetic route to P-ionone (36) substantially shorter and cheaper, especially since the isomeric double bond proves to be advantageous in the subsequent reactions. In addition, i-methylheptenone (37 a) can be converted into methylheptenone (37) by noble metal-catalyzed isomerization. The reaction steps ethynylation (C2 addition), Carroll reaction (C3 addition), ethynylation and partial hydrogenation (C2 addition) lead from methylheptenone (37) via dehydrolinalool (42), pseudoionone (43) and p-ionone (36) to the C15 alcohol p-vinylionol (44). With triphenylphosphine (15), the desired C15 phosphonium salt (13), which is the second important synthetic building block for vitamin A and carotenoids16), is obtained directly from p-vinylionol, by allyl rearrangement. 

Metal-catalyzed isomerization of unsymmetrical diallyl ethers is a unique route to synthesize allyl vinyl ethers for the Claisen rearrangement. In 1977, Reuter and Salomon reported that heating the diallyl ether 23 in the presence of a catalytic amount of tris(triphenylphosphine)ruthenium(II) dichloride resulted in the... 

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... 

Metal enolates have played a Umited role in the metal-catalyzed isomerization of al-kenes . As illustrated in a comprehensive review by Bouwman and coworkers, ruthenium complex Ru(acac)3 (51) has been used to isomerize a wide range of substituted double bonds, including aUylic alcohols (131), to the corresponding ketones (132) (equation 38) . The isomerization of aUylic alcohols affords products that have useful applications in natural product synthesis and in bulk chemical processes. An elegant review by Fogg and dos Santos shows how these complexes can be used in tandem catalysis, where an alkene is subjected to an initial isomerization followed by a hydroformylation reaction ... 

The two established pathways for transition metal-catalyzed alkene isomerization are the jr-allyl metal hydride and the metal hydride addition-elimination mechanisms. The metal hydride addition-elimination mechanism is the more common pathway for transition metal-catalyzed isomerization. In this mechanism, free alkene coordinates to a metal hydride species. Subsequent insertion into the metal-hydride bond yields a metal alkyl. Formation of a secondary metal alkyl followed by y3-elimination yields isomerized alkene and regenerates the metal hydride. The jr-allylhydride mechanism is the less commonly found pathway for alkene isomerization. Oxidative addition of an activated allylic C-H bond to the metal yields a jr-allyl metal hydride. Transfer of the coordinated hydride to the opposite end of the allyl group yields isomerized alkene. 

The arene oxides 76, 77, 86, and 88-94 have all been formed by direct thermal isomerization of the corresponding oxaquadricyclanes. Transition-metal-catalyzed isomerization of oxaquadricyclanes, however, yields arene oxides with alternative substituent patterns for example, 95. 

Along with typical acid catalyzed reactions such as cracking, the metal catalyzed reactions such as hydrogenolysis and isomerization could also take place over Pt-HZSM-5 catalysts [4, 5]. An earlier study of us [11] showed, however, that metal catalyzed isomerization over a Pt catalyst on a strong acidic support can manifest themselves under specific experimental conditions at low temperatures (<573 K) and with higher Ho/nH ratios only. While n-hexane underwent aromatization also on HZSM catalyst without Pt [11], the presence of Pt was indispensable for skeletal isomerization. The appearance of methylcyclopentane (MCP) - as a possible intermediate [12] - was crucial to assume that the so-called C5-cyclic metal catalyzed isomerization took place. 

The cyclopropene — indene rearrangement is a well known reaction which can be affected by acid87), heat 88,65d) or irradiation 20). The first transition metal catalyzed isomerization 89) has been published in 1968 (Eq. 30). 

Metal-catalyzed isomerization of oxygen-substituted cyclopropanes generally gives allyl ethers and/or enol ethers as the ring-opened products. 

Although the transition metal catalyzed isomerization of cyclopropanes to alkenes has received considerable attention for the past two decades/° that of donor-substituted cyclopropanes... 

Investigations of the isomerization of alkanes in recent years have provided evidence that the reaction can occur on certain metals, notably platinum, in the absence of a separate acidic component in the catalyst (20-22). While it has been shown that a purely metal-catalyzed isomerization process can occur, the findings do not challenge the commonly accepted mode of action of bifunctional reforming catalysts in which separate metal and acidic sites participate in the reaction. The available data at conditions commonly employed with commercial reforming catalysts indicate that a purely metal-catalyzed process does not contribute appreciably to the overall isomerization reaction on a bifunctional catalyst. 

Nevertheless, the metal-catalyzed isomerization reaction is of interest from the point of view of understanding the nature of hydrocarbon transformations on metal surfaces. It has been suggested that carbonium-ion-like intermediates are involved in alkane isomerization reactions on platinum (23), and a specific mechanism has been proposed by Anderson and Avery (24). 

When the Pt(0) complex 239, prepared from isomerically pure olefin 176, was recrystallized from refluxing ether, a 7 3 mixture of 239 and 240 could be obtained. The relevance of this and complementary experiments to the transition metal catalyzed isomerization of olefins has been discussed (178a). Although smaller bridgehead alkenes could be generated and trapped at low temperature, attempts to stabilize bicyclo[2.2.1]hept-l-ene (68) by complex-ation with transition metals have hitherto failed (179). The Pt(II) complex of bicyclo[3.3.1]non-l-ene (69) has been prepared by displacement of ethylene in 241 (180). Whereas 69 dimerizes at room temperature with a half-life time of about 3 days, the complex 242 is stable for several months. In reactions of 69 with Pd(II)-acetate or (CH3CN)2PdCl2, only bicyclo[3.3.1]non-2-ene rather than appropriate complexes could be detected. 

Unstable metallacycloalkane intermediates play an important role in a variety of transition-metal-catalyzed isomerizations or rearrangements of strained carbocyclic systems and olefin metathesis reactions ... 

---