Insertion/reductive elimination

One most important observation for the mechanistic discussion is the oxidative addition/insertion/reductive elimination processes of the iridium complex (31) (Scheme 1-10) [62]. The oxidative addition of catecholborane yields an octahedral iridium-boryl complex (32) which allows the anti-Markovnikov insertion of alkyne into the H-Ir bond giving a l-alkenyliridium(III) intermediate (34). The electron-... 

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). 

The overall transformation involves insertion of the C5=C6 and the C2=C1 k bonds into the Si7-H bond. An oxidative addition of Si-H to Y, insertion, insertion, reductive elimination sequence might occur. The problem with this is that the d° Y complex can t do oxidative addition. The alternative by which the Si-H... 

Another possible reason that ethylene glycol is not produced by this system could be that the hydroxymethyl complex of (51) and (52) may undergo preferential reductive elimination to methanol, (52), rather than CO insertion, (51). However, CO insertion appears to take place in the formation of methyl formate, (53), where a similar insertion-reductive elimination branch appears to be involved. Insertion of CO should be much more favorable for the hydroxymethyl complex than for the methoxy complex (67, 83). Further, ruthenium carbonyl complexes are known to hydro-formylate olefins under conditions similar to those used in these CO hydrogenation reactions (183, 184). Based on the studies of equilibrium (46) previously described, a mononuclear catalyst and ruthenium hydride alkyl intermediate analogous to the hydroxymethyl complex of (51) seem probable. In such reactions, hydroformylation is achieved by CO insertion, and olefin hydrogenation is the result of competitive reductive elimination. The results reported for these reactions show that olefin hydroformylation predominates over hydrogenation, indicating that the CO insertion process of (51) should be quite competitive with the reductive elimination reaction of (52). 

Cobaltacyclobutene 39 reacts with alkynes in a net insertion/reductive elimination process to yield stereoisomeric cobalt-77-cyclopentadiene complexes 55 (Equation 17) <1995JA8029>. 

Related complexes of group 10 metals are accessible by an oxidative addition/reductive cyclization protocol, exploiting the inverse electron demand (Scheme 27) (Pt <2005JA13494>, Ni <20030M3604>). The nickel complex is thermally unstable, proceeding to perylene via a bimolecular reductive elimination or, in the presence of alkynes, delivering acenaphthylene derivatives by an insertion/reductive elimination pathway. 

Palladium(0)-catalysed coupling reactions of haloarenes with alkenes, leading to carbon-carbon bond formation between unsaturated species containing sp2-hybridised carbon atoms, follow a similar mechanistic scheme as already stated, the general features of the catalytic cycle involve an oxidative addition-alkene insertion-reductive elimination sequence. The reaction is initiated by the oxidative addition of electrophile to the zero-valent metal [86], The most widely used are diverse Pd(0) complexes, usually with weak donor ligands such as tertiary phosphines. A coordinatively unsaturated Pd(0) complex with a formally d° 14-electron structure has meanwhile been proven to be a catalytically active species. This complex is most often generated in situ [87-91],... 

M. J. Wax, J. M. Stryker, J. M. Buchanan, C. A. Kovac, and R. G. Bergman, Reversible C—H Insertion/Reductive Elimination in ( 5-Pentamethylcyclopentadienyl)(trimethyl-phosphine)iridium Complexes. Use in Determining Relative Metal-Carbon Bond Energies and Thermally Activating Methane, J. Am. Chem. Soc. 106, 1121-1122 (1984). 

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-... 

An insertion-reductive elimination sequence is involved in the rhodium-catalyzed, intramolecular hydroacylation of 4-alkenals to form cyclopentanes ... 

In comparison, group 9 metals can generate either the linear or the branched isomer, depending on the nature of the ancillary ligands and are thought to proceed by oxidative addition/syn-insertion/reductive elimination. Reactions of alkane thiols were achieved for the first time with Rh and later with Pd and actinides. Depending on the metal and ligand choice, either the branched or linear product can... 

This Pd-catalyzed transformation uses two imines an acid chloride and CO. Mechanistically, this reaction is proposed to proceed via Pd addition to the activated imine, followed by CO insertion. Reductive elimination of Pd with HCl provides the ketene, which undergoes formation of the Munchnone. Miinchnones are known to undergo cycloaddition reactions with electron-deficient imines to yield cycloaddition adducts, which lose CO2 and TsH to yield imidazole. 

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... 

Mechanistically, the decarbonylation is initiated by the oxidative cleavage of the C-C bond in the square planar Rh(I) complex, with the assistance of the pyridine group to form the octahedral acyl/Rh(III) species 3 or 4. After the reverse migratory insertion, reductive elimination afforded the desired alkylarene and the Rh(I) catalyst that reentered the catalytic cycle (Scheme 22.6b). 

Scheme 1.19 Competition between migratory insertion-reductive elimination and double bond dissociation in the dihydride intermediate 64. (From Gridnev, I. D. and Imamoto, T. Izv. Akad. Nauk, Ser. Khim., 1514-1534,2016. With permission.)...

It is readily apparent that C-H insertion, reductive elimination, and displacement by ancillary ligands are some of the main pathways for decomposition of NHC ligands. In this section, we will address ligand design elements that can be employed to minimise or eliminate such decomposition reactions. 

---