Cross-coupling reactions transmetallation. palladium complexes

An acyl-palladium complex might undergo a series of follow up reactions. Subsequent transmetalation and reductive elimination lead to the formation of a carbonyl compound. This process is also coined carbonylative coupling, referring to the cross-coupling reaction, which would take place in the absence of carbon monoxide under similar conditions (for more details see Chapter 2.4.). 

Early findings by Suzuki and co-workers [109] showed that the palladium-catalyzed iminocarbonylative cross-coupling reaction between 9-alkyl-9-BBN derivatives, t-butylisocyanide, and arylhalides gives access to alkyl aryl ketones 132 after hydrolysis of the corresponding ketimine intermediates 131. Presumably, the concentration of free isocyanide is kept to a minimum by its coordination with the borane. Formation of an iminoacylpalladium(II) halide 130 by insertion of isocyanide to the newly formed arylpalladium complex followed by a transmetallation step afford the ketimine intermediates 131 (Scheme 8.52). 

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 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... 

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... 

It is known that the halogen ligand on an organopalladium(II) halide is readily displaced by an alkoxy, hydroxy, or acetoxy anion to provide the reactive Pd-OR complexes 20 [58], which have been postulated as reaction intermediates [59] or isolated [55, 56] from the reaction of organopalladium(II) halides with sodium hydroxide or methoxide. It is not yet obvious, in many reactions, which process shown in Scheme 2-18 or 2-23 is predominant however, the formation of alkoxo-, hydroxo- or acetoxopalladium(II) intermediates should be considered to be one of the crucial transmetallation processes in base/palladium-induced cross-coupling reactions. 

The catalytic cycle for the Suzuki cross-coupling reaction involves an oxidative addition (to form RPd(II)X)-transmetalation-reductive elimination sequence. The transmeta-lation between the RPd(lI)X intermediate and the organoboron reagent does not occur readily until a base, such as sodium or potassium carbonate, hydroxide or alkoxide, is present in the reaction mixture. The role of the base can be rationalized by its coordination with the boron to form the corresponding ate-complex A, thereby enhancing the nucleophilicity of the organic group, which facilitates its transfer to palladium. Also, the base R O may activate the palladium by formation of R-Pd-OR from R-Pd-X. 

Miyaura, N. Cross-coupling reaction of organoboron compounds via base-assisted transmetalation to palladium " complexes. J. 

Additional examples of Au(I)/Pd(II) transmetallation have been reported [322], and palladium has been found essential to catalyze couplings involving organogold(I) compounds [314]. Complexes [AuR(PPh3)] also react with aryl and allyl electrophiles in cross-coupling reactions in the presence of palladium [323, 324] or nickel catalyst [325]. 

---