Phosphines, Buchwald-Hartwig amination

Although monoligated Pd° is the dominant paradigm in NHC-Pd catalyst development for Buchwald-Hartwig amination, other approaches were investigated with some success. - Shi reported the use of a (ferrocenyl)phosphine-functionalised NHC, but its activity was low, giving credit to the monodentate approach. ... 

In papers published back-to-back in 1996, Hartwig and Buchwald reported amination chemistry with palladium complexes of DPPF and BINAP as catalysts [50,86]. These palladium complexes provided animations of aryl bromides and iodides with primary alkyl amines, with cyclic secondary amines, and with anilines. It is ironic that the amination chemistry was first discovered by using a particularly labile phosphine, but was dramatically improved by the use of chelating ligands. 

An amine can be coupled with an aryl bromide, iodide or triflate in the presence of a palladium catalyst, a base, typically KOBu or CSCO3, and a ligand such as the bidentate phosphine BINAP. These reactions are known as Buchwald or Buchwald-Hartwig reactions (Scheme 10.24). A catalytic cycle is again involved, with the amine displacing X from Ar-Pd -X to form Ar-Pd -NHR2. Abstraction of a proton by the base produces Ar-Pd -NRj, which undergoes a reductive elimination. 

Buchwald s XPhos ligand (L7) works well to catalyze the amination of 5-bromopyrimidine with anilines and secondary cyclic amines.67 Buchwald also found that ligands L20 and L21 were the best ligand for the palladium-catalyzed amination of benzothiazoles with secondary amines and anilines. Hartwig reports that (r-Bu)3P was excellent for the catalytic amination of 2-chloroheteroaryl compounds with secondary amines and anilines.35,68 As with other examples using bulky mono-dentate phosphines, primary aliphatic amines were not reported. 

Besides the Hartwig-Buchwald reaction, amination can be achieved by a Pd-catalyzed variant of the Ullmann reaction in the presence of copper salts. Though the scope of this reaction is much narrower, it does not require expensive phosphine ligands and thus is more economical. The arylation of diphenylamine with water-insoluble aryl iodides can be achieved in aqueous microemulsions in the presence of phosphine-free palladium catalyst and copper(I) iodide (Scheme 58). 

Transition-metal-catalyzed synthesis of poly(arylene)s via carbon-carbon coupling reactions was started by Yamamoto et al. three decades ago [52,53] since then various carbon-carbon bond formation processes with transition-metal catalysts have been applied to polycondensation [54-57]. In recent years, Buchwald et al. and Hartwig et al. developed Pd-catalyzed amination and etherification of aromatic halides by using bulky, electron-rich phosphine ligands [58-60], and this chemistry has been applied to polycondensation for... 

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