Electron-rich ligands, Buchwald-Hartwig

The Buchwald-Hartwig amination is an exceedingly general method for generating any type of aromatic amine from an aryl halide or aryl sulfonates.1,2 The key feature of this methodology is the use of catalytic palladium modulated by various electron-rich ligands. Strong bases, such as sodium ferf-butoxide, are essential for catalyst turnover. 

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... 

Pd-catalyzed aryl etherifications to yield dihydrobenzofurans have been achieved by Hartwig and co-workers using bulky ferrocenylphosphine ligands (Equation 135) <2000JA10718> and by Buchwald and co-workers using bulky electron-rich o-biarylphosphine ligands <2000JA12907>. 

Recently, the limitations for the intermolecular coupling have been significantly mitigated. Kawatsura and Hartwig [14] and Buchwald et al. [15] have demonstrated that aryl bromides and even aryl chlorides react with malonates (Eq. 3) and 1,3-diketones (Eq. 4) by using sterically bulky and electron-rich phosphines, such as P(f-Bu)3, D BPF, and 2-(di-ferf-butylphosphino)-2 -methyl-biphenyl, as ligands. 

Asymmetric a-arylations of ketones to form products containing quartemary benzyhc stereogenic centers with high enantioselectivities were also reported by Hartwig et al. and Buchwald et al. Initial studies employed the traditional Pd/BINAP or Tol-BINAP complexes, which in many cases resulted in modest enantioselectivity. Later, other combinations of Pd or Ni precatalysts with ligands such as electron-rich monodentate or bidentate phosphines, l,l -bis-substi-tuted ferrocenyl phosphines, or A(-heterocyclic carbenes/-phosphines expanded the scope of this reaction. 

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