Insertion—reduction mechanism

Key words ONIOM, hydrogenation, enantioselectivity, asymmetric catalysis, DFT, reaction mechanism, chiral phosphine, ab initio, valence bond, oxidative addition, migratory insertion, reductive elimination. 

PdCl2-promoted stoichiometric dichlorocarbonylation of acetylene (Eq. 20) is the first example of oxidative carbonylation of an alkyne that appeared in the literature [69,70], and presumably occurs through the mechanism shown in Scheme 13, involving addition of PdC to the triple bond followed by CO insertion, reductive elimination, oxidative addition to the C - Cl bond, further CO insertion and reductive elimination (Scheme 13, path a). 

Concomitant with continued olefin insertion into the metal-carbon bond of the titanium-aluminum complex, alkyl exchange and hydrogen-transfer reactions are observed. Whereas the normal reduction mechanism for transition-metal-organic complexes is initiated by release of olefins with formation of hydride followed by hydride transfer (184, 185) to an alkyl group, in the case of some titanium and zirconium compounds a reverse reaction takes place. By the release of ethane, a dimetalloalkane is formed. In a second step, ethylene from the dimetalloalkane is evolved, and two reduced metal atoms remain (119). 

In addition to isolation and characterization of the ruthenacycle complexes 18 or 32, the detailed reaction mechanism of the [2 + 2 + 2] cyclotrimerization of acetylene was analyzed by means of density functional calculations with the Becke s three-parameter hybrid density functional method (B3LYP) [25, 33]. As shown in Scheme 4.12, the acetylene cyclotrimerization is expected to proceed with formal insertion/reductive elimination mechanism. The acetylene insertion starts with the formal [2 + 2] cycloaddition of the ruthenacycle 35 and acetylene via 36 with almost no activation barrier, leading to the bicydic intermediate 37. The subsequent ring-... 

The currently accepted mechanism of the DBR is shown above. The rate-determining step is thought to be loss of a carbon monoxide ligand to form a coordinatively unsaturated intermediate II. This process can be facilitated thermally or photolytically. An alkyne can then coordinate to form 12. The alkyne inserts into the carbene heteroatom bond to give a new chromium carbene 13. At this point there are at least two possible pathways. In the first pathway, carbon monoxide can insert to provide chromium complexed ketene 14, which undergoes electrocyclization to give the hexadienone 15. Tautomerization completes the reaction to provide the phenol 2. Alternatively, metallacycle 16 can form prior to carbon monoxide insertion. Reductive elimination before carbon monoxide insertion leads to pentadiene 5, a commonly observed by-products of the DBR. Cyclopentanones 6, cyclobutenones 7, and indenes have also been observed as by-products in the... 

The mechanism by which a Li-S battery is discharged is an unusual one, because it does not involve insertion reactions, conversion reactions or alloy formation. Unconventionally, the active material passes successively between the solid state and the dissolved state depending on the SOC of the battery. Most of the reduction mechanism occius in solution. It is therefore possible to speak of a cathol rie (= cathode dissolved in the electrolyte). 

The [3S+1C] cycloaddition reaction with Fischer carbene complexes is a very unusual reaction pathway. In fact, only one example has been reported. This process involves the insertion of alkyl-derived chromium carbene complexes into the carbon-carbon a-bond of diphenylcyclopropenone to generate cyclobutenone derivatives [41] (Scheme 13). The mechanism of this transformation involves a CO dissociation followed by oxidative addition into the cyclopropenone carbon-carbon a-bond, affording a metalacyclopentenone derivative which undergoes reductive elimination to produce the final cyclobutenone derivatives. 

The reaction of methyl acrylate and acrylonitrile with pentacarbonyl[(iV,iV -di-methylamino)methylene] chromium generates trisubstituted cyclopentanes through a formal [2S+2S+1C] cycloaddition reaction, where two molecules of the olefin and one molecule of the carbene complex have been incorporated into the structure of the cyclopentane [17b] (Scheme 73). The mechanism of this reaction implies a double insertion of two molecules of the olefin into the carbene complex followed by a reductive elimination. 

The value of the EQCM is exemplified by the data shown in Fig. 17.177 The first reduction of the polypyrrole film was initially accompanied by a mass decrease, as expected for anion expulsion according to Eq. (1). However, after the reduction was ca. 75% complete, the mass began to increase, indicating a switch of the charge neutralization mechanism to cation insertion [Eq. (5)]. 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

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