Bonding reductive elimination

Ca.ta.lysis, Iridium compounds do not have industrial appHcations as catalysts. However, these compounds have been studied to model fundamental catalytic steps (174), such as substrate binding of unsaturated molecules and dioxygen oxidative addition of hydrogen, alkyl haHdes, and the carbon—hydrogen bond reductive elimination and important metal-centered transformations such as carbonylation, -elimination, CO reduction, and... 

Key words hydrogenation of carbon dioxide, insertion of carbon dioxide into the metal-hydride bond, reductive elimination of formic acid, C-bond metathesis... 

Similarly, a double functionalization can be reached when an activating group is present in close vicinity to the triple bond. Tsuji et al. have discovered that with a diphosphine palladium(O) complex, a carbonate function in the a-position of the alkyne provides by decarboxylation a palladium methoxy species on which the alkyne moiety can be isomerized into an al-lenyl a -bonded group. CO insertion in the Pd - C bond, reductive elimination with the methoxy group and further cyclization with incorporation of a second CO molecule give rise to the corresponding cyclopentenone as shown in Scheme 21 [127]. 

Conversion of a Co2(CO)6-alkyne complex into a cyclopentenone is the Pauson-Khand reaction. It proceeds by loss of CO from one Co to make a 16-electron complex, coordination and insertion of the C6=C7 K bond into the C2-Co bond to make the C2-C6 bond and a C7-Co bond, migratory insertion of CO into the C7-Co bond to make the C7-C8 bond, reductive elimination of the C1-C8 bond from Co, and decomplexation of the other Co from the C1=C2 k bond. The mechanism is discussed in the text (Section B.l.f). 

Hydrogen insertion into the Co-C bond (reductive elimination)... 

Presumably, the use of a bidentate ligand such as ( )-BINAP or DPPF results in the occupation of a vacant coordination site, preventing -hydride elimination of the Pd (II) amide intermediate [53]. Dissociation of the imine and C-H bond reductive elimination results in formation of the reduced aryl bromide. If this -hydride elimination is rapid relative to reductive elimination and reversible, then significant erosion of the enantiomeric excess of optically active a-substituted amines maybe observed during the reaction (Scheme 3). 

The compound R2Bi-O BiR2 has been found as an intermediate in the oxidation of trialkylbismuth compounds. Unlike BiPhs, the compound Bi(Rf)3 [Rf = 2,4,6-(Cp3)3C6H2] is air sensitive and slowly evolves RfORf and Rf OH upon standing in air. These findings are consistent with the intermediacy of an 0=Bi(Rf)3 complex followed by the insertion of O into a Bi C bond. Reductive elimination from such an intermediate would generate the ether, while hydrolysis would produce the phenol. 

Scheme 10 Ligand-centered redox events triggering C—C bond reductive elimination...

Synthetic applications that consist of metal catalyzed addition of X-X to carbon-carbon unsaturated substrates, generally alkynes, have been described for X = X = SR, SeR, TeR and X = SeR, X = PR2 as well as X = SR, X = BR2, SiR3 [201], These transformations generally involve oxidative addition of the X-X reactant to the metal center followed by insertion of the alkyne, into the M-X bond. Reductive elimination leads to the final disubstituted alkene, which shows cis stereochemistry. 

After realizing that our hypotheses about oxidative cross-coupling reactions were not as unique as assumed, we quickly turned our attentirai to intermolecular oxidative amination reactions. In the carbazole example, regioselectivity was coti-trolled by the presence of a Lewis base that was attached near the C—H bmid that would be cleaved, resulting in a metallacyle intermediate. For die development of an intramolecular reaction, we chose to take advantage of the selectivity that is often observed in the selective metalation of electron-rich heteroarenes. At the time, the palladation of indoles was presumed to operate by an electrophilic aromatic substitution mechanism. (This has since been demonstrated to be incorrect, vide infra.) We hypothesized that regioselective palladation of an indole substrate could be followed by a subsequent C—N bond reductive elimination. At the time, the exact mechanism by which the intermediate containing Pd—C and Pd—N bonds could be formed was not clear, nor was the order of the two metalation steps, but the overall process seemed plausible. 

The couplings of di5mes with a third unsaturated component are described in several subsections throughout Section 1.1.3. However, several other processes merit discussion here that do not fall into the other categories described. The hydrosilylation of diynes provides an excellent route to cyclic dienylsilanes. A mechanism was proposed that involves Si—H oxidative addition, alkyne sUylmetalation, allgme insertion, and C—H bond reductive elimination to give 59 (Scheme A related process ensues when diynes are treat-... 

Kochi also showed that EtjFefbpy) reductively eliminates butane after oxidation to the corresponding dication (Scheme 8.4). In contrast, thermolysis of the neutral species leads to products from p-hydrogen elimination, and reaction of the monocation leads to products from homolysis of the Fe-C bond. Reductive eliminations to form carbon-carbon bonds from cobalt complexes bonded to sp -hybridized carbons were shown in Scheme 8.1."°... 

Another type of ring formation is the so-called palladium-ene cyclizations - developed by Oppolzed (Scheme 55). The presence of an olefin near the cationic 17 -allylpalladium complex facilitates its insertion into the C—Pd bond. Reductive elimination and proton elimination result in the formation of cyclic dienes. Similar reactions have been reported by others. " p i... 

B.i.b. Alkynes Containing Proximate 1,3-Dicarbonyl Groups. 2-Propargyl-1,3-dicarbonyl compounds react with alkenyl triflates or alkenyl/aryl/heteroaryl halides to give 2,3,5-trisubstituted-furans (Scheme 4). The process probably proceeds through an oxypalladation step that involves a nucleophilic attack of a stabilized enolate across the activated carbon-carbon triple bond, reductive elimination of a palladium(0) species from the resultant oxypalladation adduct, and isomerization of the initially formed alkylidene derivative. 

---