Catalytic allylic alkylation mechanism

The mechanism of the reaction is a combination of the simple steps we have seen before. The phosphine ligands reversibly dissociate and associate, and an oxidative addition occurs. The new step that occurs in catalytic allylic alkylation is nucleophilic attack on an allyl ligand coordinated to a metal. 

Catalytic Allylic Alkylation (Section 24.4) Catalytic allylic alkylation commonly takes allyl acetate species and substitutes the acetate with a nucleophile. Some of the most useful nucleophiles are enolates derived from methylenes that are flanked by two electron-withdrawing groups. The mechanism of the reaction involves the oxidative addition of the allyl acetate to palladium and results in the intermediacy of Tj -allyl complexes that give inversion of configuration at the carbon with the... 

Years earlier, Nicholas and Ladoulis had found another example of reactions catalyzed by Fe2(CO)9 127. They had shown that Fe2(CO)9 127 can be used as a catalyst for allylic alkylation of allylic acetates 129 by various malonate nucleophiles [109]. Although the regioselectivites were only moderately temperature-, solvent-, and substrate-dependent, further investigations concerned with the reaction mechanism and the catalytic species were undertaken [110]. Comparing stoichiometric reactions of cationic (ri -allyl)Fe(CO)4 and neutral (rj -crotyl ace-tate)Fe(CO)4 with different types of sodium malonates and the results of the Fe2(CO)9 127-catalyzed allylation they could show that these complexes are likely no reaction intermediates, because regioselectivites between stoichiometric and catalytic reactions differed. Examining the interaction of sodium dimethylmalonate 75 and Fe2(CO)9 127 they found some evidence for the involvement of a coordinated malonate species in the catalytic reactions. With an excess of malonate they... 

The same catalytic system as described for the CDC of amines and ni-troalkanes, complemented with C0CI2 as a co-catalyst, also proved efficient for the allylic alkylation via cross-dehydrogenative coupling between various cycloalkenes and diketones (Eq. 9). Again, the exact mechanism or role of the organic peroxide are not known to date, but the formation of water probably provides the thermodynamic driving force for these reactions [121,122]. 

Although heterobimetallic complexes with alkylated rare-earth metal centers were proposed to promote 1,3-diene polymerization via an allyl insertion mechanism, details of the polymerization mechanism and of the structure of the catalytically active center(s) are rare [58,83,118-125]. Moreover, until now, the interaction of the cationizing chloride-donating reagent with alkylated rare-earth metal centers is not well-understood. Lanthanide carboxylate complexes, which are used in the industrial-scale polymerization of butadiene and isoprene, are generally derived from octanoic, versatic, and... 

Mechanistic implications of a general cross-metathesis of vinylsilicon with allyl-substituted heteroorganic compounds have been studied in detail for the reaction with allyl alkyl ethers [13]. The detailed NMR study of the stoichiometric reaction of Grubbs catalyst with allyl-n-butyl ether has provided information on individual steps of the catalytic cycle. A general mechanism of the cross-metathesis of vinyltri(alkoxy, siloxy)silanes (as well as octavinylsilsesquioxane) with 3-heteroatom-containing 1-alkenes in the presence of ruthenium carbene is shown in Scheme 5. 

Alternatively, additives can affect the rate of interconversions of diastereomeric intermediates, relative to the rate of formation of product from these intermediates, This affect of additives has been exploited to achieve Curtin-Hammett conditions in allylic alkylation. Halide ions are known to catalyze the ir-u-ir isomerization by the mechanism in Equation 14.15. Thus, the concentration of additives can affect enantio-selectivity by shifting the enantioselectivity-determining step from formation of the allyl intermediate to reaction of the allyl intermediates with the nucleophile. In fact, this affect of additives on the identity of the enantioselectivity-determining step can even cause reactions in the presence of additives to form the enantiomer that is the opposite of the one formed in the absence of additives. Even variation in temperature can cause a change in the enantioselectivity-determining step of a catalytic process and formation of opposite product enantiomers at low and high temperature. This effect has been observed in the asymmetric hydroformylation processes described in Chapter 17. ... 

Ga203 activated above about 800 K behaves like AI2O3 for olefin isomerization a ff-allyl intermediate is formed on basic sites. However, when it is activated at 573 K, it shows a broad IR band at 2940 cm assigned to OH stretching of 0(-GaO(OH) and a band at 3650 cm due to surface OH, and exhibits peculiar catalytic activity for isotopic exchange between D2 and hydrocarbon, which cannot be explained by mechanisms such as alkyl reversal and T-allyl intermediate mechanisms. For example, direct cis-trans isomerization of n-alkenes was totally selective below 433 K. Novel mechanisms involving o-bonded alkyls and vinyls adsorbed via bonding to oxide ions on the surface have been proposed. 

Alkyl diazoacetates undergo little or no allylic C/H insertion when decomposed catalytically in the presence of appropriate olefins 6,13,I4). In contrast, such insertions occur with diazomalonates or ot-diazoketones. From the available facts, the conclusion can be drawn that different pathways may lead to what finally looks like the direct or rearranged allylic insertion product, but convincing evidence for one or the other mechanism is available only in a few cases. As Scheme 22 shows, the C/H insertion products 98-100 may arise from one of three major sources ... 

Brown and Suzuki have shown that treatment of trialkylboranes with ethenyl-(Scheme 42, Eq. 42a) and ethynyloxiranes (Scheme 42, Eq. 42b) in the presence of a catalytic amount of oxygen, affords the corresponding allylic or allenic alcohols. The mechanism may involve the addition of alkyl radicals to the unsaturated system leading to l-(oxiranyl)alkyl and l-(oxiranyl)alkenyl radicals followed by rapid fragmentation to give alkoxyl radicals that finally complete the chain process by reacting with the trialkylborane [104-106]. 

The catalytic isomerization reation of olefins is caused by either an associative or a dissociative mechanism. The associative mechanism involves either dissociative mechanism involves allylic intermediates, as described in Scheme 2, where M—H and M represent active sites. 

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