Transition Metal-Catalyzed Formation of C–O and C–S Bonds - New Trends in Cross-Coupling: Theory and Applications - page 261

O

N CO 2 H

N N N N N N N

26 27 28 29 30

N

O
OH

NH 2
NH 2

N

N N

N

N N

OH

N N

31 32 33 34 35

OH

HO

OH

O O

O

P OH

OH

H

OPh

tBu tBu

HO

36 37 38

39

Figure 6.2 Representative ligands for copper catalysis.

proposed mechanism for copper-catalyzed etherifcations proceeds through
either initial oxidative addition of the aryl halide C-X bond or through a
radical anion intermediate. Many ligated copper complexes have provided
carbon-oxygen bond formations with product yields similar to those with
palladium catalysts; however, palladium-catalyzed etherification reactions
maintain a broader scope than those using copper and therefore continue to
dominate the field. Another major difference between these two catalyst
systems is the identity of the ligands employed. In palladium catalysis,
phosphines predominate, whereas in copper catalysis, oxygen- and nitrogen-
containing ligands predominate. A number of representative ligands for
copper are highlighted in Figure 6.2 (26, 49 27, 50,51 28, 52,53 29, 54 30, 55 31, 56
32, 57 33, 58 34, 59 35, 60,61 36, 62 37, 63 38, 64 39 65 ).

6.3.1 Copper-Catalyzed Formation of Diaryl Ethers
Diaryl ethers are formed from aryl bromides or aryl iodides and the cor-
responding phenol derivatives with copper catalysts in good yields
(Scheme 6.9). 54 Many sterically and electronically diverse phenols can be
employed; however, the use of aryl chlorides remains limited owing to
generally poor conversion. One of the drawbacks with copper catalysis is the
ineffectiveness of copper in activating aryl chlorides, hence most catalytic
reactions with copper are limited to aryl bromides and aryl iodides.

6.3.2 Copper-Catalyzed Cross-Coupling Reactions of Vinyl
Halides With Phenols
The use of copper catalysts to promote the coupling of vinyl halides with
phenols has also been successful (Scheme 6.10). 59 The optimal catalyst
system is formed in situ from copper iodide and an equimolar amount of
benzimidazole ligand 34. Alkene geometry may be retained or scrambled in