Nickel catalysts Stille coupling

The synthesis of thioethers has been developed using various nickel-NHC complexes. The first report by Ying and coworkers [57] highlighted the abilitjr of a nickel catalyst to couple thiophenol with a variety of aryl bromides and iodides, producing high yields of diaryl thioethers. A recyclable nickel-NHC catalyst was also used in the formation of carbon-sulfur bonds, but was still restricted to the synthesis of diaryl thioethers [58]. Well-defined allyl nickel complexes were shown to be efficient for both the formation of carbon-sulfur and carbon-... 

In 1971, Kochi reported that a catalytic silver salt induced Grignard coupling reaction with organic halides to form hydrocarbons [428,429 Eqs. (185), (186) and (187) 22,428]. However, the pair- [see Eq. (185)] and the stereoselectivity [see Eq. (187)] were not satisfactory hence, similar reactions with copper, nickel, or palladium catalysts are far more common. If a coupling reaction to be carried out involves RMgX and RX having the same R group, a silver catalyst still finds a use [Eq. (188) 430]. 

The catalyst used is often palladium (0) (like Pd(PPh3)4, Pd2(dba)3), or a source of palladium (II) (like Pd(OAc)2, BnPdCl(PPh3)2 to name a few that gets reduced to the active species palladium(O) in situ. Methods using other metals like manganese, copper, and nickel have been reported the latter has been applied for instance in the successful Stille coupling of unreactive aryl chlorides as well as in the coupling of unactivated primary and secondary alkyl halides. ... 

Other transition metals, like manganese and nickel, have been used in the Stille coupling (as additive or as replacement of palladium). As we will see later, nickel proved to be very powerful in replacing palladium as catalyst and is very promising in terms of expanding the scope of the reaction. 

A key was the realization that Ni, the neighbour of Pd in the groupVIII, was superior in such coupling. Nickel catalysts have already been used in the Stille coupling for the generation of C(sp )-C(sp ) sigma carbon-carbon bonds. For instance Shirakawa and Hiyama had shown that Ni(0) catalyst... 

The use of nickel catalysts in the Stille coupling reactions has been sporadically explored. In general, the yields of the reactions are not satisfactory. Dimerization products are occasionally side products [78,79]. 

Search for more ubiquitous metal catalysts for the transformation Although nickel catalysts have been used quite successfully depending on the structure of the H-P(O) compounds, the coverage is still rather hmited. Also, nickel is stiU near the only one non-noble metal that is active for the transformation. The successful progress in cross-coupling chemistry provides a good example, which shows the direction of the research toward the use of more ubiquitous metals. There is no reason why H-P(O) bond addition caimot be a next example. 

To conclude the details on zero-valent nickel carboxylation catalysts, some recent synthetic approaches worthy of note showed that this area of research still has a rich chemistry. For example, Louie and coworkers reported on the use of N-heterocyclic carbenes (diaryl-imidazolylidene) as new efficient ligands in the Ni-catalyzed coupling of various symmetrical di-ynes with C02 (Scheme 5.19) [60a]. 

There were a few isolated examples of coupling reactions that form C—and C—S bonds.f Tunney and Stille had reported the Pd-catalyzed formation of the C—P bonds in aromatic phosphines, and several groups had reported the formation of C—S bonds in sulfides using catalysts based on both palladium and nickel. In addition there were suggestions that simple procedures for C—bond formation could be developed and that palladium complexes could act as catalysts for C—N bond-forming cross-coupling processes. For example, Boger and co-workers used stoichiometric amounts of palladium to form aromatic C—N bonds in a /3-carbolide natural product by an intramolecular process (Eq. 1). Perhaps more closely related to the work discussed in detail here. 

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