Cross coupling mechanisms transmetallation

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. 

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... 

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. 

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 following mechanism is suggested for the cross-coupling of alkenylsilanes. Nucleophilic attack of a fluoride ion to the silicon atom of alkenylsilanes is first assumed to afford a pentacoordinate silicate and enhance both nucleophilicity of the silicon-substituted carbon and Lewis acidity of silicon to assist transmetalation effectively through a four-centered transition state (Scheme 2). Lewis acidity on silicon is critical as evidenced by the fact that hexacoordinate pentafluorosilicates that are fully coordinated and thus should have sufficient nucleophilicity are much less effective for the cross-coupling reaction (Eq. 2, vide supra). 

The cross-coupling reaction of alkenyl(fluoro)silanes with aryl halides sometimes produces, in addition to the desired ipso-cowpled products, small amounts of cmc-coupled products [14]. The czne-coupling is often striking in the reaction with organotin compounds. The isomer ratio of products produced by the reaction of l-fluoro(dimethyl)silyl-l-phenylethene with aryl iodides is found to depend on the electronic nature of a substituent on aryl iodides (Eq. 11) an electron-withdrawing group like trifluoromethyl and acetyl favors the formation of the ipso-coupled product. To explain the substituent effect, the mechanism depicted in Scheme 3 is proposed for the transmetalation of alkenylsilanes with palladium(ll) complexes. It is considered that an electron-donating substituent on Ar enhances... 

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). 

If the mechanism in Eq. (d) were the only path coordinatively saturated stable organometallics would not undergo transmetallation. However, certain coordinatively saturated ate complexes may also transmetallate e.g., the reaction of aryl iodides with alkynylaluminate complexes in the presence of Pd(PPh3)4 gives the corresponding cross-coupled products ... 

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... 

The mechanism of the Sonogashira cross-coupling follows the expected oxidative addition-reductive elimination pathway. However, the structure of the catalytically active species and the precise role of the Cul catalyst is unknown. The reaction commences with the generation of a coordinatively unsaturated Pd species from a Pd " complex by reduction with the alkyne substrate or with an added phosphine ligand. The Pd " then undergoes oxidative addition with the aryl or vinyl halide followed by transmetallation by the copper(l)-acetylide. Reductive elimination affords the coupled product and the regeneration of the catalyst completes the catalytic cycle. 

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