Copper aryl ether formation

FIGURE 1.6 Copper-catalyzed aryl ether formation... 

Copper-catalyzed Aryl Ether Formation. Traditional methods for formation of aryl ethers involve stoichiometric Ullmann couplings. A new system was developed exploiting catalytic CuCl and NMI as a ligand. The title compound was chosen due to its similarities to histidine, a common amino acid for binding metals. This methodology was applied to the synthesis of the insecticide diafenthiuron that produced the desired aryl ether in greater than 98% yield compared to 39% reported in current patents (eq 32). ... 

The aryl ether formation was, until the late 1990s, the domain of copper-catalyzed processes, namely the Ullmann reaction. Around 15 years ago, Hartwig and Buchwald independently discovered the palladium-catalyzed alkoxylation of aryl halides with phenols. Later, these reactions were extended to aliphatic alcohols and to hydroxide as nucleophiles (vide infra. Scheme 5-159). The mechanism of the reductive elimination has been elucidated on isolated complexes. ... 

Aryl/Alkenyl Ether Formation via Cross-Coupling Reactions 10.14.2.1 Copper-mediated Cross-coupling Reactions... 

Another phenoxide activating approach published by Buchwald et al. [18] is based on the reaction of cesium phenoxides with aryl bromides or iodides in the presence of catalytic amounts of copper(I) triflate and ethyl acetate in refluxing toluene (Scheme 3b). In certain cases equimolar amounts of 1-naphthoic acid have been added in order to increase the reactivity of the phenoxide. The authors assume the formation of a cuprate-like intermediate of the structure [(ArO)2Cu] Cs+ as the reactive species. In addition, diaryl ether formation between phenols and aryl halides has been achieved using a phosphazene base forming naked phenoxide in the presence of copper bromide in refluxing toluene or 1,4-dioxane [19]. 

Aryl ethers. Pd-mediated C-O bond formation ameliorates the undesirable feature of the Ullmann method employing stoichiometric quantities of copper salts. Electron-deficient ArBr and electron-rich ArONa are suitable reaction partners. Electron-poor phosphine ligands improve the yields of the displacement reaction between aryl bromides and sodium phenoxides. 

The catalytic effect of copper salts on the 0-arylation of alcohols has been observed, but its synthetic importance remains limited. 7 jhe formation of 2-iodo-2 -acetoxybiphenyl (30) by reaction of the biphenyleneiodonium sulfate (29) with sodium acetate in boiling glacial acetic acid was quantitative when the reaction was catalysed by copper salts.(see 5.1.4.2) In the case of phenols, the (9-arylation by diaryliodonium salts is preferentially performed in a weakly basic medium (generally triethylamine) in the presence of copper bronze in dichloromethane or methanol at room temperature or at mild temperatures (around 50 C). Under these conditions, the 0-aryl ethers are obtained easily in good yields. l ... 

Table 9.9 Copper/ligand catalyzed diaryl ether formation from aryl halides and phenols.

Copper-catalyzed C-O, C-N, and C-S Coupling. While there is an extensive variety of palladium catalysts for C(aryl)-X bond formation (X = 0, N, and S), copper corrqtlexes have recently gained renewed popularity in these coupling processes. Use of the (CuOTf)2. benzene complex allows the formation of diaryl ethers from aryl bromides or iodides and phenols in very good yields (76-93%) (eq 121). The reaction occurs in toluene in the presence of cesium carbonate as the base and a catal)4ic quantity of ethyl acetate whose role is probably to increase the solubility of the copper species. In the case of less reactive phenols, yields can be increased by the addition of a stoichiometric amount of carboxylic acid. A slight modification of these conditions has been used in the key diaryl ether formation in the synthesis of verbenachalcone. ... 

The copper-catalyzed coupling of glycolic acids with aryl iodides has been accomplished using common copper catalysts (Scheme 2.54) [79], This was a chemoselective transformation that resulted in the preferential formation of the alkyl aryl ether instead of the ester. The carboxylic acid was retained in the product and was able to be further transformed in subsequent reactions. The electronic composition of the aryl iodide did not significantly influence the outcome of the reaction, and a range of electron-rich and electron-poor arenes were well tolerated by the chemistry. 

SCHEME 2.54 Chemoselective copper-catalyzed formation of alkyl aryl ethers [79]. 

Cross-coupling reactions 5-alkenylboron boron compounds, 9, 208 with alkenylpalladium(II) complexes, 8, 280 5-alkylboron boron, 9, 206 in alkyne C-H activations, 10, 157 5-alkynylboron compounds, 9, 212 5-allylboron compounds, 9, 212 allystannanes, 3, 840 for aryl and alkenyl ethers via copper catalysts, 10, 650 via palladium catalysts, 10, 654 5-arylboron boron compounds, 9, 208 with bis(alkoxide)titanium alkyne complexes, 4, 276 carbonyls and imines, 11, 66 in catalytic C-F activation, 1, 737, 1, 748 for C-C bond formation Cadiot-Chodkiewicz reaction, 11, 19 Hiyama reaction, 11, 23 Kumada-Tamao-Corriu reaction, 11, 20 via Migita-Kosugi-Stille reaction, 11, 12 Negishi coupling, 11, 27 overview, 11, 1-37 via Suzuki-Miyaura reaction, 11, 2 terminal alkyne reactions, 11, 15 for C-H activation, 10, 116-117 for C-N bonds via amination, 10, 706 diborons, 9, 167... 

Among common carbon-carbon bond formation reactions involving carbanionic species, the nucleophilic substitution of alkyl halides with active methylene compounds in the presence of a base, e. g., malonic and acetoacetic ester syntheses, is one of the most well documented important methods in organic synthesis. Ketone enolates and protected ones such as vinyl silyl ethers are also versatile nucleophiles for the reaction with various electrophiles including alkyl halides. On the other hand, for the reaction of aryl halides with such nucleophiles to proceed, photostimulation or addition of transition metal catalysts or promoters is usually required, unless the halides are activated by strong electron-withdrawing substituents [7]. Of the metal species, palladium has proved to be especially useful, while copper may also be used in some reactions [81. Thus, aryl halides can react with a variety of substrates having acidic C-H bonds under palladium catalysis. 

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