Transmetallation 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 mechanism was proposed to involve the formation of a nickel metallacycle by the oxidative cyclization of Ni(0) with the aldehyde and alkyne, followed by conversion of the metallacycle to product by a transmetalation/reductive elimination sequence. If R possesses a P-hydrogen, then P-hydride elimination after the transmetalation step generates the product with R = H in some instances. The mechanism was shown to be ligand dependent, and the mechanism depicted below is undoubtedly oversimplified. ... 

Electrophilic haloarene function and nucleophilic areneboronic acid function undergo carbon-carbon coupling only in the presence of Pd(0) catalytic species. The general catalytic cycle for this aryl-aryl coupling, involving an oxidative addition-transmetallation-reductive elimination sequence, follows the mechanistic scheme shown in Figure 8.4 for aryl-alkyl coupling. 

The reactions may also be carried out under an atmosphere of carbon monoxide, CO (Scheme 10.22), when the usual catalytic cycle occurs. CO inserts easily into the palladium complex Ar-Pd -X. The aryl ligand migrates on to the carbonyl group to form a metal-acyl species, X-Pd - C(0)Ar. A transmetallation-reductive elimination sequence follows, forming the ketone and regenerating the Pd catalyst. 

A general catalytic cycle for the cross-coupling reaction of organometallics involves an oxidative addition-transmetallation-reductive elimination sequence, as depicted in Scheme... 

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. 

The reaction of 17 (or 25) to give 19 (or 31) can not be explained by the oxidative addition-transmetallation-reductive elimination mechanism. In the reaction of 25, carbopalladation to form 26 and 27 is the first step. Desilylpalladation of 26 affords the expected ipso product 28. On the other hand, the intermediate 27 undergoes syn dehydropalladation to give 29, to which syn addition of H-PdX occurs to generate 30. Then anti desilylpalladation provides the cine product 31. This reaction is not completely fluoride-free, because the Bp4 anion is present. 

The arylation of aromatic ketones with arylboronates proceeds by an oriho-ruthenation with RuH2(CO)(PPh3)3 as the catalyst [137]. In this transformation, the final C—C bond is formed in a transmetallation-reductive elimination process. In a different reaction, ortHo-arylated compounds are obtained from o-aminoaryl ketones and arylboronates through substitution of the amino function catalyzed by the same Ru(II) complex [138], A nitrogen-directed homocouphng of aromatic compounds takes place with Ru(II) catalysts in the presence of aUylic chlorides or acetates by a mechanism that presumably involves Ru(IV) intermediates [139],... 

The authors proposed a catalytic cycle that involves oxidative addition into the acyl C-C bond to form the five-membered Rh(III) metallacycle H (Scheme 14). Transmetalation with boronic acid generates intermediate I that undergoes phosphine-promoted reductive elimination. Oxidation of the resulting Rh(I) chelate J with O2 in the presence of Cul gives an Rh(III) species K that participates in a second transmetalation. Reductive elimination of complex L yields product 71. 

Organometals, enolates, and metal hydrides used throughout this Handbook, especially in cross-coupling and related reactions (Part HI) and the Tsuji-Trost reaction (Part V), can, in general, readily reduce Pd(II) complexes via transmetallation-reductive elimination, as shown in Scheme 7. 

With commercially available boronate esters as aryl coupling partners, the first Pd(ll)-catalyzed P-C(sp )-arylation of simple carboxylic acids was reported (Scheme 1.2) [5]. The reaction presumably relied on the binding of carboxylate directing group to Pd(ll) center, triggering a C-H activation/transmetalation/ reductive elimination sequence. The P-arylated carboxylic acids can be obtained in satisfactory yields. It represents an important step forward in arylation of C(sp )-H bonds. 

The mechanistic features of Pd(0) catalyzed coupling reactions of organic electrophiles and terminal alkynes are controversial. In gener, it is not regarded as involving an insertion mechanism similar to the Heck reaction (73), The terminal acetylene proton is readily deprotonated by base, therefore an oxidative addition, transmetallation, reductive elimination sequence (vide infra) is expected. A brief summary of the features of the most frequently proposed mechanism of Pd(0)/Cu(I) acetylene coupling reactions is contained in reference 81. However, an insertion mechanism was postulated for equation 27 (79). 

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