Ligand or Base Effects

The ligand effect appears to consist of two opposing components. As described earlier, a large excess of triethylamine reportedly slows the reaction by occupying the free coordination sites at the copper center. However, in the absence of triethylamine, the substrate reacts with arylboronic acid to form the arylboronic acid monoamide adduct, which is less active or inactive [28a]. 

As discussed in Section 5.3.2, one can suppress protiodeboronation side reactions with the addition of 5 equiv. of DMAP [21]. For N-arylation, the substrates can sometimes serve as base/ligand. Thus no external base/ligand needs to be added in many cases [26]. Conversely, for 0-arylation, base/ligand must be used, except for the case of trifluoroborate salts [7, 8, 47]. 

Copper(iii) species are known [52] and are postulated to be intermediate in the Ullmann reaction. The copper(I) formed can then be easily oxidized by oxygen to cop-per(ii) to complete the catalytic cycle. 

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