Introduction A family of biphenyl ligands for transition-metal-catalyzed cross-coupling reactions is becoming well documented in the literature.
[1-5 ]
Buchwald and colleagues invented and developed ligands 1 and 2 , which have found utility in C-C,2 C-N
[2a ]
[3a ]
[3c ]
and C-O
[3b ]
bond formation.
Figure 1 Biphenyl-based phosphine ligands
Palladium complexes supported by ligands 1 and 2 are efficient catalysts for C-C, and C-heteroatom (C-N and C-O) bond formation of aryl halides,
[2 ]
[3 ]
triflates
[3a ]
and sulfonates.
[4 ]
Catalysts supported by the ligands shown in Figure 1 are reactive, efficient and operationally simple to generate. Ligand 1 is effective for hindered substrates at very low catalyst loadings (0.000001 mol%)
[2 ]
compared with ligand 2 . The efficiency of catalysts derived from ligands 1 and 2 is most likely due to a combination of several factors such as steric and electronic properties, basicity of the phosphorus and their abilities to form palladacycles.
[2 ]
Preparation of the ligands 1 and 2
Ligands 1 and 2 were prepared in high yields in a one-pot transformation (Scheme 1) and are also now commercially available.
[2b ]
They are generally air-stable, white, crystalline solids that require no special handling. These ligands interact in unusual and interesting manners with the metal and thereby modulate the reactivity of the metal for various transformations. There is also a balance between steric and electronic properties among these compounds.
[2b ]
Scheme 1