Cross coupling mechanisms oxidative addition

It proceeds by the standard mechanism for cross-coupling reactions oxidative addition of Pd(0) to the C-I bond, transmetallation to give the C-Pd(II)-C compound, and reductive elimination. 

In a generahzed and simpHfied mechanism, the reaction usually follows the standard catalytic cycle of metal-catalyzed cross-coupling reactions oxidative addition of the C(sp ) -X bond to paUadium(O), followed by coordination of the amine to the resulting palladium complex, occurring with extrusion of HX that is captured by the base. Finally, reductive elimination yields the couphng product, regenerating the catalyticaUy active paUadium(O) species. 

Cross-coupling to form carbon heteroatom bonds occurs by oxidative addition of an organic halide, generation of an aryl- or vinylpalladium amido, alkoxo, tholato, phosphido, silyl, stannyl, germyl, or boryl complex, and reductive elimination (Scheme 2). The relative rates and thermodynamics of the individual steps and the precise structure of the intermediates depend on the substrate and catalyst. A full discussion of the mechanism for each type of substrate and each catalyst is beyond the scope of this review. However, a series of reviews and primary literature has begun to provide information on the overall catalytic process.18,19,22,23,77,186... 

A new C-C bond is formed between a nucleophilic C-Sn and an electrophilic C-Br. This Stille coupling proceeds through the standard oxidative addition, transmetallation, reductive elimination process characteristic of Pd-catalyzed cross-couplings. The mechanism was discussed in the text (Section 6.3.4). 

The overall mechanism is closely related to that of the other cross-coupling methods. The aryl halide or triflate reacts with the Pd(0) catalyst by oxidative addition. The organoboron compound serves as the source of the second organic group by transmetala-tion. The disubstituted Pd(II) intermediate then undergoes reductive elimination. It appears that either the oxidative addition or the transmetalation can be rate-determining, depending on reaction conditions.134 With boronic acids as reactants, base catalysis is normally required and is believed to involve the formation of the more reactive boronate anion.135... 

It is assumed that the mechanism of the palladium-catalyzed cross-coupling reactions of iodonium salts involves the initial oxidative addition step, followed by ligand coupling at the iodine and then at the palladium centers analogously to the mechanism shown in Scheme 31 [63,66]. 

The mechanism of the Sonogashira reaction has not yet been established clearly. This statement, made in a 2004 publication by Amatore, Jutand and co-workers, certainly holds much truth [10], Nonetheless, the general outline of the mechanism is known, and involves a sequence of oxidative addition, transmetalation, and reductive elimination, which are common to palladium-catalyzed cross-coupling reactions [6b]. In-depth knowledge of the mechanism, however, is not yet available and, in particular, the precise role of the copper co-catalyst and the structure of the catalytically active species remain uncertain [11, 12], The mechanism displayed in Scheme 2 includes the catalytic cycle itself, the preactivation step and the copper mediated transfer of acetylide to the Pd complex and is based on proposals already made in the early publications of Sonogashira [6b]. 

The reductive elimination to form C-C and C-H bonds [45] is a crucial step in the cross-coupling processes, as well as many other transition metal-catalyzed reactions. Reductive elimination reactions comprise an early chapter in any organometallic text. Many examples of these reactions have been studied, and a great deal is known about the mechanisms of these processes. Similarly, the cleavage of C-H bonds by oxidative addition, including the C-H bond in methane, is now known [46]. Again, questions remain about how these reactions occur, but a variety of mechanistic studies have revealed key features of these reactions. Even some remarkably mild C-C cleavage reactions have now been observed with soluble transition metal complexes [47,48]. 

Both amido and pinacol derivatives of B-Si compounds 125 and 126 added to terminal and internal alkynes in the presence of a palladium244-246 or platinum(O) catalyst247 by a mechanism involving an oxidative addition-insertion process (Equation (39)).248 On the other hand, phosphine-free nickel(O) catalyst resulted in the dimerization of alkynes giving a Z,Z-isomer of l-silyl-4-borylbutadiene derivatives.249 Since the palladium-catalyzed cross-coupling at the C-B bond is faster than the G-Si bond of 137, a silylboration-cross-coupling sequence provided a method for the synthesis of 1-alkenylsilanes.246... 

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 mechanism of the palladium-catalyzed head-to-head cross-coupling reactions described above are poorly understood. However, our recent investigation suggests that the head-to-he reaction can be accomodated by an oxidative addition of an organic halide R X to Pd(0) followed by an exchange with sodium alkoxide, trans-metallation with 1-alkenylborates and reductive elimination, as depicted in Scheme 1... 

Mechanism The reaction proceeds first by the oxidative addition of organohalide to the Pd(0) complex to give a palladium(II) intermediate as in the case of Stille coupling. The Pd(II) complex then undergoes transmetallation with the base-activated boronic acid to give complex B. This is followed by reductive elimination to form the active Pd(0) species, HX and the cross-coupled product (Scheme 5.17). 

The mechanism for this palladium-catalyzed cross-coupling reaction comprises the initial oxidative addition of the electrophile 37 to the palladium(O) catalyst followed by transmetallation of triethylsilane to yield the corresponding bis(organo)palladium(II) complex 39, which then undergoes reductive elimination to form the alkene 40 and to regenerate the palladium(O) catalyst. 

The palladium-catalyzed cross-coupling reaction of a vinyl or aryl stannane with an arylhalogenide or -triflate is known as a Stille reaction. The mechanism of this Stille reaction is outlined below The palladium precatalyst loses two ligands and forms the catalytic species 36. The catalytic cycle starts with the oxidative addition of the catalytic species 36 into the carbon-triflate bond of 23 forming complex 41, which, however, does not undergo the required transmetallation step with stannane 22. Therefore, the triflate ion is... 

---