Allylic ligand oxidation

Scheme 6 Proposed mechanism for allylic ligand oxidation...

Table IV presents the results of the determination of polyethylene radioactivity after the decomposition of the active bonds in one-component catalysts by methanol, labeled in different positions. In the case of TiCU (169) and the catalyst Cr -CjHsU/SiCU (8, 140) in the initial state the insertion of tritium of the alcohol hydroxyl group into the polymer corresponds to the expected polarization of the metal-carbon bond determined by the difference in electronegativity of these elements. The decomposition of active bonds in this case seems to follow the scheme (25) (see Section V). But in the case of the chromium oxide catalyst and the catalyst obtained by hydrogen reduction of the supported chromium ir-allyl complexes (ir-allyl ligands being removed from the active center) (140) C14 of the...

The most plausible mechanism proceeds through oxidative addition of the aldehyde to an active Ru(0) species to form (acyl)(hydrido)ruthenium(ll) complex 155. Insertion of the less-substituted double bond of the 1,3-diene into the Ru-H bond occurs to generate an (acyl)( 73-allyl)ruthenmm(ll) intermediate of type 156. Successive regioselective reductive eliminations between the acyl and the 73-allyl ligands provide the desired product with regeneration of the... 

The proposed mechanism for allyhc acetoxylation of cyclohexene is illustrated in Scheme 15. Pd -mediated activation of the allyhc C - H bond generates a Jt-allyl Pd intermediate. Coordination of BQ to the Pd center promotes nucleophilic attack by acetate on the coordinated allyl ligand, which yields cyclohexenyl acetate and a Pd -BQ complex. The latter species reacts with two equivalents of acetic acid to complete the cycle, forming Pd(OAc)2 and hydroquinone. The HQ product can be recycled to BQ if a suitable CO catalyst and/or stoichiometric oxidant are present in the reaction. This mechanism reveals that BQ is more than a reoxidant for the Pd catalyst. Mechanistic studies reveal that BQ is required to promote nucleophilic attack on the Jt-allyl fragment [25,204-206]. 

Transition metal-catalyzed allylic alkylation is generally considered to involve mechanistically four fundamental steps as shown in Scheme 1 coordination, oxidative addition, ligand exchange, and reductive elimination. A key step of the catalytic cycle is an initial formation of a (7r-allyl)metal complex and its reactivity. The soft carbon-centered nucleophiles, defined as those derived from conjugate acids whose pAj, < 25, usually attack the allyl ligand from the opposite side... 

Tin enolates add to rr-allylpalladium complexes directly on the allyl ligand (inversion).106 383 Therefore, in tandem with the inversion of configuration incurred in the oxidative addition of a Pd° catalyst into an allyl acetate, a net overall retention is observed (equation 155). 

The mechanism of the catalytic cycle is outlined in Scheme 1.37 [11]. It involves the formation of a reactive 16-electron tricarbonyliron species by coordination of allyl alcohol to pentacarbonyliron and sequential loss of two carbon monoxide ligands. Oxidative addition to a Jt-allyl hydride complex with iron in the oxidation state +2, followed by reductive elimination, affords an alkene-tricarbonyliron complex. As a result of the [1, 3]-hydride shift the allyl alcohol has been converted to an enol, which is released and the catalytically active tricarbonyliron species is regenerated. This example demonstrates that oxidation and reduction steps can be merged to a one-pot procedure by transferring them into oxidative addition and reductive elimination using the transition metal as a reversible switch. Recently, this reaction has been integrated into a tandem isomerization-aldolization reaction which was applied to the synthesis of indanones and indenones [81] and for the transformation of vinylic furanoses into cydopentenones [82]. 

In order to permit complete conversion to one product enantiomer under the influence of a chiral catalyst, substrates for palladium-catalyzed allylic substitution either have to possess a meso structure (equation 1) or else give rise to complexes with 7t-allyl ligands as depicted in equations 2 and 3. Whereas oxidative addition of the substrate to the palladium(O) species constitutes the enantioselective step for meso compounds (equation 1), nucleophilic attack determines the absolute configuration of the product for reactive intermediates with a meso tt-allyl ligand (equation 2) or a zr-allyl unit that undergoes rapid epimerization by the n-a-n mechanism10-59 relative to substitution (equation 3). 

An X-ray structure determination was carried out on (C5Me5)RuBr2(Jt-C3H5), and showed a pseudo-piano-stool structure with two Br atoms and two terminal carbons for the endo-Jt-allyl ligand located at the basal positions. A crystal mirror plane bisects the pentamethylcyclopentadienyl and jt-allyl ligands. The oxidative addition of allylic halides to (C5R5)Ru(CO)2X is reversible, since the reductive elimination of... 

Elimination of alkyl or 7t-allyl ligands as hydrocarbons on reaction with acidic OH (or SH) groups may graft metal ensembles to inorganic surfaces such as oxides. Some of these reactions occur via initial oxidative addition of OH (or SH) to the metal followed by reductive elimination of the hydrocarbon, leaving the metal-support M—O bonds ... 

It is clear that there is an exception to every rule even in AD there are a few cases known where other ligands gave improved stereoselectivities compared with PHAL, PYR, or IND. Thus, allylic phosphine oxides undergo AD to yield diols which could be used for the synthesis of optically active allylic alcohols [43]. 

A wide variety of nucleophiles add to an -rf-allyl ligand. Desirable nucleophiles typically include stabilized carbanions such as CH(COOR)2 or 1° and II0 amines. Unstabilized nucleophiles such as MeMgBr or MeLi often attack the metal first and then combine with the n-allyl by reductive elimination. The Tsuji-Trost reaction, which is typified by the addition of stabilized carbanions to T 3—allyl ligands complexed to palladium followed by loss of the resulting substituted alk-ene, comprises an extremely useful method of constructing new C-C bonds, and many applications of this reaction have appeared in the literature.61 Equation 8.43 illustrates an example of a Pd-catalyzed addition of a stabilized enolate to an allyl acetate.62 The initial step in the catalytic cycle is oxidative addition of the allyl acetate to the Pd(0) complex, followed by nq1 to nq3—allyl isomerization, and then attack by the nucleophile to a terminal position of the T 3—allyl ligand. We will discuss the Tsuji-Trost reaction, especially in regard to its utility in chiral synthesis,63 more extensively in Chapter 12. 

The experimental results that both branched and linear allylic ethers are obtained in the palladium-catalyzed decarboxylation of branched allylic carbonate indicate occurrence of direct oxidative addition involving the C-0 bond cleavage followed by the nucleophilic attack of the alkoxide liberated on either the substituted or non-substituted terminus of the allylic ligand (Scheme 4) [1]. 

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