Electron-rich triarylamines

Aminations of five-membered heterocyclic halides, such as furans and thiophenes, are limited. These substrates are particularly electron-rich. As a result, oxidative addition of the heteroaryl halide and reductive elimination of the amine are slower than for simple aryl halides (see Sections 4.7.1 and 4.7.3). In addition, the amine products can be air-sensitive and require special conditions for their isolation. Nevertheless, Watanabe has reported examples of successful couplings between diarylamines and bromothiophenes [126]. Triaryl-amines are important for materials applications because of their redox properties, and these particular triarylamines should be especially susceptible to electrochemical oxidation. Chart 1 shows the products formed from the amination of bromothiophenes and the associated yields. As can be seen, 3-bromothiophene reacted in higher yields than 2-bromothiophene, but the yields were more variable with substituted bromothiophenes. In some cases, acceptable yields for double additions to dibromothiophenes were achieved. These reactions all employed a third-generation catalyst (vide infra), containing a combination of Pd(OAc)2 and P(tBu)3. The yields for reactions of these substrates were much higher in the presence of this catalyst than they were in the presence of arylphosphine ligands. 

Due to poor reactivity, aryl amines normally require higher reaction temperatures than aliphatic amines to ensure good conversion. In early studies, phenathroline and its Cu(I)-complex were used in the arylation of aryl amine [10, 11], but they were only applicable to the synthesis of triarylamine from secondary aryl amines. L-Proline (LI) promoted Cul-catalyzed arylation of primary aryl amines took place at 90°C (Table 9.1, entry 1) [3]. However, only electron-rich anilines gave complete conversion, while electron-deficient anilines provided low yields. Fu found that this drawback could be overcome by heating at 110°C and using pipecolinic acid (L5) as a ligand (entry 2) [12]. Similar studies were reported by Liu and coworkers in which DMEDA (Lll) was found to be a better ligand (entry 3) [13], Recently, Buchwald reported that pyrrole-2-carboxylic acid (L6) [14] is an efficient promoter for the synthesis of diarylamines (entry 4) (Table 9.3). 

The use of P(f-Bu)3 was an important discovery, because this phosphine has scarcely been used as a ligand of active transition metal catalysts. It was believed that P(r-Bu)3 is too bulky for stable coordination. It is known that only two molecules of P(r-Bu)3 can coordinate to Pd to form the highly coordinatively unsaturated, but stable bis(tri-t-butyl)phoshinepalladium complex [12]. This complex is commercially available as a good precursor of active Pd catalysts. Inspired by the discovery that P(r-Bu)3 is an important ligand, several researchers have synthesized a number of electron-rich and bulky phosphine ligands. For example, the asymmetric triarylamine 15 was prepared from 3-methoxyaniline (14) using the biphenylylphosphine IV-1 in a one-pot reaction. Similarly, the arylamine 17 was prepared by one-pot, step-wise arylation of m-diaminobenzene (16) [13]. 

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