Arylpalladium alkoxides

On the other hand, the arylative ring opening takes place in the presence of a Pd(0) catalyst, an aryl halide, and a base (Scheme 3, reaction b) [32-35]. Oxidative addition of aryl halides toward Pd(0) gives ArPdX species, which can readily interact with the alcohols affording arylpalladium alkoxide intermediates. Then, p-carbon elimination and subsequent reductive elimination occur to give y-arylated ketones and regenarate Pd(0) species. An example is shown in Eq. 15. 

The electronic effects on the reductive elimination of aryl ethers from BlNAP-ligated palladium complexes (Eq. 6) was studied by Widenhoefer and Buchwald. Because the reductive elimination of acyclic ethers required activated palladium-bound aiyl groups, the scope of the arylpalladium alkoxide complexes was limited. An example of elimination from an oxametallacycle that contained an unactivated aryl group demonstrated that the formation of cychc ethers may become more general. Nonetheless, studies on these systems showed that the factors that control elimination of ethers are similar to those that control elimination of amines. For example, alkoxide complexes that are more electron rich at the alkoxide oxygen underwent faster reductive elimination than those that are more electron poor. Elimination of diaryl ethers from arylpalladium phenoxides did not occur. 

In chemistry related to the reactions of alkali metal alkoxides with late metal halide complexes, late metal alkoxides have been formed by the reaction of alcohols with metal halides in the presence of a base. This process has been used to form arylpalladium alkoxides during the coupling of aryl halides with alcohols discussed in Chapter 19 (cross-coupling). In addition, Milstein used an intramolecular version of this process to prepare the metallaoxetane in Equation 4.68. ... 

During the cross-couplings to form C—N, C—O, C—S, and C—P bonds, the arylpalladium halide complexes are converted to arylpalladium amide, alkoxide, thiolate, and phosphide complexes. Examples of each type of complex have now been isolated, and the reductive elimination of the organic products has been studied. Although the reductive elimination to form carbon-hydrogen and carbon-carbon bonds is common, reductive elimination to form carbon-heteroatom bonds has been studied only recently. This reductive elimination chemistry has been reviewed.23... 

An alternative pathway when soluble alkoxide or silylamido bases are used involves reaction of an arylpalladium halide complex with the alkoxide or silylamide to form an intermediate alkoxide or amide. These complexes can react with amines to form the required amido aryl intermediates. The reaction of Pd(PPh3)(Ph)(//-OH) 2 with primary alkylamines to generate palladium amido complexes and water (Eq. (49)) [70, 280] gave an early indication that the conversion of an alkoxide to an amide could be occurring during the catalytic cycle. These reactions are reversible, but the equilibrium favors the amido complex. 

Three general mechanisms can be envisioned for the formation of antido complexes from arylpalladium halides direct substitution of halide by alkali antide generated by simple deprotonation of free amine by free base, coordination of antine to the metal center followed by deprotonation of the more acidic coordinated amine, or formation of a palladium alkoxide... 

The final pathway, which involves alkoxide intermediates, has been shown to occur when reactions employ NaO-t-Bu as base and DPPF as ligand (Eq. 52). DPPF-ligated arylpalladium t-butoxide complexes have been isolated. Reactions of these complexes with arylamines led to rapid formation of the amido complex and subsequent reductive elimination of diarylamine. Reactions with secondary alkylamines led to formation of di-alkylanilines, presumably through the intermediacy of an arylpalladium dialkylamido intermediate. Reaction of the DPPF-ligated, arylpalladium halide complexes with alkoxide and dialkylamine together first formed the alkoxide complex. This complex subsequently reacted with amine and reductively eliminated the dialkylaniline. The observation of the alkoxide complex in this reaction strongly suggests the intermediacy of the alkoxide complex in the catalytic reactions. ... 

The negatively charged base reacts with the arylpalladium(II) halide to give the arylpalladium hydroxide or alkoxide complex, which is able to form the dimeric palladium-boron complex XXIII what is crucial for the transmetallation process [2-6]. It is apparent that the metal cation (from the base) accelerates the formation of the latter, as clearly showed by Zhang and coworkers [15]. They have developed the SM coupling procedure for sterically bulky arylboronic acids when the clear influence of the anion basicity and the cation effect were discovered. The cationic radius is presumably an important parameter which influences the formation of dimeric... 

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