Mechanism of cross-coupling

Because of their frequent use, some late transition metal catalyzed carbon-carbon bond forming reactions evolved into name reactions. The most prominent examples are cross-coupling reactions, where distinction is usually made on the basis of the transmetalating agent used. The common mechanism of cross-coupling reactions and its name variants are discussed in Chapter 2.1. 

Scheme 62 Proposed mechanism of cross-coupling in a nickel-catalyzed Arbuzov reaction of aryl halides...

Scheme 1.7. Mechanism of cross-coupling of an aryl halide with an aUcyne.

The reactions of aryl halides with carbon monoxide and either alcohols or amines to form esters or amides occur by two related yet distinct mechanistic pathways. These pathways are related to the mechanism of cross coupling presented in Chapter 19, and the reader might find it helpful to refer to the discussion of mechanisms of cross coupling in that chapter. Scheme 17.29 outlines the pathways for the carbonylation of aryl halides to form esters and amides. 

In the quest for coordinatively unsaturated palladium catalysts, the more radical approach uses ligandless conditions [165, 166] following the work pioneered by Beletskaya [38b, 167]. However, the mechanism of cross-coupling reactions under these conditions is not known [168]. 

It should be emphasized that the results of these kinetic studies do not represent only the TBAF-promoted coupling of alkenylsilanols. Analyzed in the context of the spectroscopic studies and reported observations - which reveal that sile-tanes, silanols, and fluorosilanes, silyl hydrides, and heterocyclic silanes all form related species when mixed with TBAF - the mechanism deduced by the kinetic studies likely represents the mechanism of cross-coupling of all of these species. These mechanistic results are therefore relevant to a significant body of work, encompassing everything from the early fiuorosilane cross-coupling systems to the more recently developed TBAF-promoted coupling of pyridyl-, thienyl-, and benzylsilanes. 

The mechanism of cross-coupling of alkenylsilanolates with 2-iodothiophene has been elucidated by kinetic measurements and involves interesting key intermediates (Scheme 12). After formation of a siloxypalladium(ii)aryl complex 45 by transmetallation with [PdX(Ar)L ], a second silanolate acts as a nucleophilic activator and generates a pentacoordinate siliconate 46, from which transmetallation to Pd occurs. This generates the alkenylpalladium(ii) complex that reductively eliminates the product. ... 

Scheme 2.16 General Mechanism of Cross-coupling Reaction Using Homogeneous Pd Catalyst.

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