Ligand Exchange and Reductive Elimination

Thus, geometrical isomerization, phosphine ligand exchange and reductive elimination of H2 all proceed by independent kinetic processes. ... 

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... 

Aryl-A3-iodane oxidation of amines to imines also involves a combination of ligand exchange and successive reductive -elimination. Oxidation of pyrrolidine with iodosylbenzene 18 affords quantitatively an equilibrium mixture of 1-pyrroline and its trimer [72]. When oxidation of piperidine with 18 (2 equiv) was carried out in water, 2-piperidone was produced [73]. In the latter reaction, a sequence of ligand exchange and reductive -elimination was repeated two times [Eq. (38)]. 

Reaction of hydrazide 45 with bis(trifluoroacetoxy)-A3-iodane 12 gives the cyclic hydrazide 47, which on zinc dust reduction affords 6-lactam [74]. The formation of 47 involves an intramolecular ene reaction of azodicarbonyl intermediate 46, produced from a sequence of ligand exchange on iodine(III) and reductive -elimination. 

The efficiency of /-elements in catalysis originates from unconventional electrophilic pathways. In contrast to rf-elements oxidative addition/reductive elimination sequences are not accessible. Instead, substrate adduct formation, ligand exchange and insertion reactions rule the mechanistic scenarios. Therefore, the main emphasis is put on the fine-tuning of the spectator ligand of the precatalyst. 

Organometallic complexes of PdIV were almost unknown until a few years ago. They are however accessible by the oxidative addition of Mel or PhCH2Br to palladium dialkyl complexes stabilized by nitrogen ligands such as bipy. Most are thermally sensitive and reductively eliminate R—Me at room temperature.49 The first PdIV silyl complex was obtained in a clean methyl exchange reaction between (dmpe)PdMe2 and l -QH SiFI the compound is thermally remarkably stable.50... 

The Mechanism of the cross coupling reaction can be accommodated by an oxidative addition of 1-bromopropene to iron(l) followed by exchange with ethylmagnesium bromide and reductive elimination. Scheme 3 is intended to form a basis for discussion and further study of the catalytic mechanism. In order to maintain the stereospecificity, the oxidative addition of bromo-propene in step a should occur with retention. Similar stereochemistry has been observed in oxidative additions of platinum(O) and nickel(O) complexes.(32)(33) The metathesis of the iron(lll) intermediate in step b is ixp icted to be rapid in analogy with other alkylations.(34) The formation of a new carbon-carbon bond by the redilcTive elimination of a pair of carbon-centered ligands in step c has been demonstrated to occur... 

The only problem with the oxidative addition-reductive elimination mechanism is that Cu(III) is a relatively high energy species. The mechanism would be much more reasonable if the Cu cycled between Cu(0) and Cu(II) instead of Cu(I) and Cu(III). Such a mechanism can be proposed if an initial electron transfer from CuCN to CuCN to give [N=C-Cu(0)] is supposed. Oxidative addition of Ar-Br to Cu(0) and reductive elimination of Ar-CN from Cu(II) affords the organic product and [Br-Cu(O)]. Finally, ligand exchange with another equivalent of N=C-Cu(I) regenerates [N=C-Cu(0). ... 

The mechanism of the coupling was not explored in detail, but likely involves ligand exchange at the rhodium center and coordination by the pyridine substrate, directed cyclometalation, transmetalation of the organotin reagent, and reductive elimination to afford the ortho arylated products (Scheme 28). The nature of the reoxidation process is more equivocal. Trichloroethylene is generated during the reaction when 1,1,2,2-tetrachloroethane is chosen as solvent, which may be... 

Similarly to organometaUic compounds, ligand exchange and ligand coupling (or reductive elimination) reactions are typical for hypervalent compounds. 

It is driven by a metal-centred redox process, beginning with oxidative addition of the halide substrate to the metal, ligand exchange and then reductive elimination leading to product formation as shown below ... 

A Ni(II)/Ni(III) catalytic cycle is proposed (Scheme 29). Catalysis is initiated by the coordination of 46 to Ni(II) followed by a ligand exchange and the cleavage of C-H bonds gives the cyclometalated Ni(II) complex 50, which is oxidized to the Ni (III) species 65 by TEMPO. Reductive elimination gives the desired product 60 with the generation of a Ni(I) species that is oxidized to Ni(II) species by TEMPO. 

The formation of cyclopropanes from o(,P-unsaturated esters occurs under conditions at least as severe as those employed in the phosphine substitution and CO exchange reactions of carbene complexes. Coordinatively unsaturated carbene complexes are therefore reasonable intermediates for these reactions. Complexation of an alkene to the metal complex provides a means of bringing the carbene and alkene ligands into close proximity. Formation of a metallocyclobutane and reductive elimination of a cyclopropane complete our suggested mechanism (see Scheme 8). 

A simplified mechanism of the reaction is described in Scheme 13.23. This includes the oxidative addition of RX to Pd(0) forming a R-Pd(II)-X complex 93. Then, ligand exchange and Pd complexation to the alkyne yields a Pd acetylide 95. In the final step, the product is released through reductive elimination with regeneration of active Pd(0). Notably, the copper acetylide is formed in situ during the catalytic cycle. 

The mechanism of action of the cyanation reaction is considered to progress as follows an oxidative addition reaction occurs between the aryl halide and a palladium(O) species to form an arylpalladium halide complex which then undergoes a ligand exchange reaction with CuCN thus transforming to an arylpalladium cyanide. Reductive elimination of the arylpalladium cyanide then gives the aryl cyanide. 

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