Cross-coupling silver® carbonate

Palladium-catalyzed dimerization of 2-arylpyridines by employing oxone as the terminal oxidant has been reported [75], The reaction was shown to proceed via a Pd(II)-Pd(TV) pathway. A method for cross-coupling of simple arenes with 2-arylpyridine derivatives by using catalytic palladium acetate in the presence of two equivalents of silver carbonate requires use of a large excess of the less reactive arene [76],... 

Fused thiophene-cyclopentanes 39 can be synthesized by intramolecular dual C-H activation of 2-arylthiophenes 38 (Scheme 17, Table 10) [67]. The cross-coupling proceeds moderately well using palladium(ll) acetate as catalyst and silver (1) carbonate as oxidant. When the thiophene moiety is not substituted at positimi 2, homocoupling occurs easily. However, in contrast to this direct route, a two-step sequence consisting of a prior bromination and a subsequent palladium-catalyzed arylation is much more effective forming the fused thiophene-cyclopentane 39 in a yield of 83%. 

We reasoned that such a decarboxylation step could also be employed in a redox-neutral cross-coupling reaction with carbon electrophiles. On this basis, we drew up a catalytic cycle that starts with an oxidative addition of aryl halides or pseudohalides to a coordinatively unsaturated palladium(O) species f (Scheme 5). The more weakly coordinating the leaving group X, the easier should be its subsequent replacement by a carboxylate. At least for X = OTf, the palladium(ll) carboxylate h should form quantitatively, whereas for X = halide, it should be possible to enforce this step by employing silver or thallium salts as species g. The ensuing thermal decarboxylation of the palladium(ll) intermediate i represents the most critical step. Myers results indicated that certain palladium(ll) carboxylates liberate carbon dioxide on heating. However, it remained unclear whether arylpalladium (II) carboxylate complexes such as i would display a similar reactivity. If this were to be the case, they would form Ar-Pd-Ar intermediates k, which in turn are... 

Silver(I) carbonate functioned as an oxidant in combination with TBAI to provide optimal yields. Pivalic acid was superior to pyridine as an additive. Thiazole, pyrazole, thiophene, and pyrrole substrates could be cross-coupled however, heteroarenes bearing electron-donating substituents afforded better yields compared with electron-withdrawing groups. The reactions proceeded in high regioselectivity at the C2/C5 position. 

Crabtree prepared rhodium, iridium, ruthenium, and palladium complexes e.g. 30) bearing NHCs by heating the carboxylate compound and an appropriate metal precursor in acetonitrile [eqn (2.7)]. It was also shown that the carboxylate of l,3-dimethylimidazol-2-ylidene (IMe) could be prepared from the direct reaction of 1-methylimidazole with dimethyl carbonate. Subsequently, Delaude showed that these species can be used in lieu of free NHCs for the synthesis of alkene metathesis pre-catalysts and to form palladium cross-coupling catalysts in situ while Olszewski has used these to prepare silver(i) and eopper(i) complexes. Falvey has shown that solvent choice is important, beeause the rate of decarboxylation is higher in less polar solvents. ... 

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