Metal-promoted reductive elimination

Ligand-promoted reductive elimination of ketones is directly observed with alkyl-acyl rhodium complexes formed via oxidative addition of either Wilkinson s catalyst or [ethylene insertion into the metal hydride bond33-37. 

Metal-promoted reductive p-elimination reactions in 1,2-difunctionalized substrates have been one of the most important means of generating C=KZ bonds [9]. In the past few years, a number of reductive elimination reactions of 1,2-difunctionalized compounds have been reported under various organometallic reagents, such as Sml2 [10], Zn [11], or In [12], In addition, those low-valent organometallic species exhibit excellent properties for sequential organic reactions and several examples have been described in which a metal-promoted ehmination reaction occurs as a step of a sequential process [13],... 

The formation of vinylboranes and vinylboronate esters during some metal-promoted hydroboration of alkenes has led to the suggestion of an alternative mechanistic pathway. Insertion of the alkene into the metal-boron bond occurs in preference to insertion into the metal-hydride bond.44,51,52 In a competing side-reaction to reductive elimination, f3-H elimination from the resulting borylalkyl intermediate furnishes the vinylborane byproduct.52 There remains however a substantial body of evidence, both experimental53 and theoretical,54 that supports the idea that transfer of hydride to the coordinated alkene precedes transfer of the boryl fragment. 

Reductive eliminations can be promoted by stabilisation of the low-valent state of the product. This means ligands that are good K-acceptors, bulky ligands, and ligands preferring bite angles more suited for tetrahedral than for square-planar complexes, when we deal with group 10 metals. 

Knochel and co-workers developed the Ni(acac)3-catalyzed cross-coupling reaction between polyfunctional primary iodoalkanes and a variety of primary diorganozinc compounds in the presence of w-trifluoromethylstyrene as a promoter. " The addition of this unsaturated promoter is required in order to coordinate to the nickel center and remove electron density from the metal atom, to facilitate the reductive elimination step. " " The scope of the reaction is extended, when Ni(acac)2 is used in the presence of BU4NI and fluorostyrene (Scheme With these... 

None of these difficulties arise when hydrosilylation is promoted by metal catalysts. The mechanism of the addition of silicon-hydrogen bond across carbon-carbon multiple bonds proposed by Chalk and Harrod408,409 includes two basic steps the oxidative addition of hydrosilane to the metal center and the cis insertion of the metal-bound alkene into the metal-hydrogen bond to form an alkylmetal complex (Scheme 6.7). Interaction with another alkene molecule induces the formation of the carbon-silicon bond (route a). This rate-determining reductive elimination completes the catalytic cycle. The addition proceeds with retention of configuration.410 An alternative mechanism, the insertion of alkene into the metal-silicon bond (route b), was later suggested to account for some side reactions (alkene reduction, vinyl substitution).411-414... 

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