Allylic species formation, isomerization

Allylic intermediate, 27 185-187 ii-Allylic nickel intermediates, 33 15-18, 22 Allylic oxidation, see Oxidation, allylic Allylic species, 30 21 formation, isomerization, 30 18-19 free allyl radicals, 30 149 a-hydrogen abstraction, 30 147 Allyl methacrylate oxidation, 41 305 Allyls... 

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 mechanisms of isomerization which have been considered fall into two categories associative , first proposed by Horiuti and Polanyi, and dissociative , advanced by Farkas et al The associative mechanism is a consequence of the reversibility of the formation of the alkyl intermediate shown in Scheme 1 and in equation (10), while the dissociative mechanism, in its current form, involves allylic species (5)-(7) shown in Scheme 2. The Horiuti-Polanyi mechanism implies that double bond isomerization and the addition of H2 proceed through a common intermediate, whereas the dissociative mechanism represents an independent path. ... 

This mechanism, which appears to be well-established in certain systems (iO), has close analogies with suspected mechanisms for heterogeneous hydrogenation and isomerization of olefins (6). The second suggested mechanism involves hydrogen abstraction from the olefin with the reversible formation of a 7r-allylic species. 

For hydrogenations over metals which show little stereoselectivity (and where isomerization was not a factor), Meyer and Burwell suggested the intermediate formation of 7r-allylic species (27). Rooney and Webb inferred that stereoselective hydrogenations resulted from the reaction of a (T-bonded species (42). While our results with the pentacyanoco-baltate(II) system indicate that either tt- or a-bonded allylic species may lead to a high degree of stereoselectivity, in general, they support the above observations. 

The extent of double bond isomerization also varies with the nature of the catalyst. The degree of isomerization over metal catalysts usually decreases in the order Pd > Ni > Rh > Ru > Os =Ir =Pt.5.6 The extensive double bond isomerization observed with palladium and, to some extent, with nickel catalysts can be attributed to the formation of the adsorbed 7t-allyl species with these catalysts. While double bond isomerization may not be important in a routine alkene hydrogenation, it may influence a selective hydrogenation because the isomerized olefin can have different adsorption characteristics from those of the... 

An interesting feature of Scheme 24, devised for palladium, is that it also provides a straightforward explanation for the bond shift isomerization on platinum. An adsorbed metallocyclobutane complex is similar to the trimethylene di-tr-complex of platinum, which is readily formed from hexa-chloroplatinic acid and cyclopropane (75). Also, platinum is known to promote easily a-y exchange of some hydrocarbons with deuterium (57, 34, 76). Therefore, in the case of platinum, the direct formation of metal-locyclobutanes, without intermediacy of 7t-allylic species, would explain the remarkable ability of this metal to promote the isomerization of neopentane to isopentane (Scheme 25). 

This scenario can explain, at least in part, the lower than expected catalyst activation by low hydrogen levels observed by some authors hydrogenolysis reactivates both Zr-secondary growing chain and Zr(allyl) species with formation of Zr—H initiating species, but at the same time the activation is limited by the formation of secondary, slower Zr(i-Pr) initiating species, which require either isomerization to n-propyl or a new hydrogenolysis to be converted in the faster centers. 

In the palladium-catalyzed carbonylation process, allyl formate, prepared by the reaction of allyl alcohol with formic acid, oxidatively adds to Pd(0) species with the C-0 bond cleavage to give allyl palladium formate. The CO insertion into the allylpalladium bond produces butenoyl palladium formate, which reductively ehminates butenoic formic anhydride with regeneration of the catalytically active Pd(0) species. Spontaneous decarbonylation of the mixed anhydride yields 3-butenoic acid, which isomerizes to 2-butenoic acid [61]. The process to give the butenoic acid proceeds only under CO pressure, suggesting that the CO insertion into the allyl-Pd bond is favored under CO pressure. When the reaction is carried out under normal pressure of CO, decarboxylation of the formate to give palladium hydride takes place. Reductive elimination of the allylpalladium hydride yields hydrogenation product of the allyl moiety [62]. 

The insertions of the monomers are believed to occur in two steps [268, 269]. In the first one, the incoming monomer coordinates with the transition metal. This results in formation of a short-lived a-allylic species. In second one, the metal-carbon bond is transferred to the cocrdinated mmiomer with formation of a 71-butenyl bond. Coordination of the diene can take place through both double bonds, depending upon the transition metal [270] and the structure of the diene. When flie mmiomer coordinates as amonodentate ligand, then asyn complex forms. If however, it coordinates as abidentate ligand, then an anti complex results [271]. In the syn complex, carbons one and four have the same chirality while in the anti complex they have opposite chiralities [268]. Due to lower thermodynamic stability the anti complex isomerizes to a syn complex [268]. If the aUyUc system does not have a substituent at the second carbmi, then the isomerization of anti to syn usually occurs spontaneously even at room temperature [268]. 

It appears that with the nonsymmetrical 3-methyl-1,2-butadiene, the formation of the N-C bond is under kinetic control. Usually the most abundant regioisomer formed is the one having the most substituted carbon atom attached to N, a feature frequently encountered in related allylic derivatives. This latter species may isomerize in solution in the presence of catalytic amounts of Pd(0) to the thermodynamically more stable isomer that has a CH2 group bound to N. This reaction offers a large range... 

The allylic resonance may give rise to formation of a mixture of isomeric allylic bromides, e.g. 6 and 8 from but-l-ene. The product ratio depends on the relative stability of the two possible allylic radical species 5 and 7 ... 

Complex a is readily converted into a Fe-y-H agnostic complex b within an early picosecond timescale and then the 7i-allyl hydride complex c is generated by hydride abstraction. The energy level of the 2-alkene isomer d, which is calculated by DPT experiments, is similar to that of the 1-alkene complex b. In the next step, Fe (CO)3(t -l-alkene)(ri -2-alkene) f, which is generated via intramolecular isomerization of the coordinated 1-alkene to 2-alkene and the coordination of another 1-alkene, is a thermodynamically favored product rather than formation of a Fe(CO)3(ri -l-alkene)2 e. Subsequently, release of the 2-aIkene from f regenerates the active species b to complete the catalytic cycle. 

At 210° C the equilibrium constant is 1-3. This isomerization may take place by a concerted mechanism or via the intermediate formation of an allylically stabilized species as shown below ... 

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