Biaryl cobalt

Electroreduction of the cobalt(II) salt in a mixture of either dimethylform-amide-pyridine or acetonitrile-pyridine as solvent, often in the presence of bipyridine, produces a catalytically active cobalt(I) complex which is believed to be cobalt(I) bromide with attached bipyridine ligands (or pyridine moieties in the absence of bipyridine). As quickly as it is electrogenerated, the active catalyst reduces an aryl halide, after which the resulting aryl radical can undergo coupling with an acrylate ester [141], a different aryl halide (to form a biaryl compound) [142], an activated olefin [143], an allylic carbonate [144], an allylic acetate [144, 145], or a... 

Biaryls (8, 478-479). The details for oxidative dimerization of arcnes containing electron-donating groups to biaryls with Tl FA have been published. Lead tetraacetate and cobalt(III) fluoride are equally effective reagents.1... 

The possibihty of chiral modiflcation of NaBH4 for reduction of ketones has been scrutinized. Some success is reported by adding the inexpensive cychc borinate 183 derived from tartaric acid and PhB(OH)2 to the reaction medium. Atropo-enantioselective reduction of biaryl lactones occurs on exposure to NaBH in the presence of the cobalt chelate 184. ... 

By modification of the reaction condition, biaryl lactones could be efficiently reduced to the corresponding biaryl products by this cobalt-catalyzed system. Various axially chiral biaryl compounds were obtained with high ee values (80-93% ee) by the atmpo-enantioselective borohydride reduction with the dynamic kinetic resolution of biaryl lactones in the presence of EtOH and l-(2-pyridinyl)ethanol (eq 39). 

Cobalt chloride (ous) Diethyl pyrocarbonate Lithium citrate BHT substitute, polyolefins Octadecyl 3,5-di-t-butyM-hydroxyhydrocinnamate Bi detection Dimethyl glyoxime biaryl formation Trifluoromethane sulfonic acid bilirubin testing, urine 5-Sulfosalicylic acid 5-Sulfosalicylic acid dihydrate binder... 

The transition metal-catalyzed [2+2+2] cycloaddition has been applied to the atroposelective biaryl synthesis. The first example is the chiral cobalt(I) complex-catalyzed [2+2+2] cycloaddition of aryl-diynes with nitriles to produce axially chiral arylpyridines [19a]. Subsequently, the enantioselective synthesis of axially chiral biaryl phosphine oxides has also been achieved (Scheme 21.16) [19b]. 

Furthermore, besides activated nickel, cobalt, and copper (see Chapter 2), indium has been proved as an effective active metal for the coupling of aryl iodides. Thus by heating an equimolar amount of indium and aryl iodides in refluxing DMF, symmetrical biaryls were furnished with high to excellent yields [78]. The indium-promoted homo-coupling reaction tolerates the presence of free hydroxy and carboxylic acid groups well. For example, 2-iodobenzoic acid (104) was converted to diphenic acid (57) in 75% yield. Scheme 22. 

The [2 -I- 2 -I- 2] cycloaddition reaction can give rise to chiral compounds, especially biaryls [3q]. Control of the enantioselectivity in such transformations is of prime importance, notably because biaryls can be used as ligands in asymmetric catalysis. This topic is covered in detail in Chapter 9. Nowadays, cobalt still looks like a poor relation in this field, which is largely dominated by rhodium. Nevertheless, a report from Heller et al. shows for the first time that phosphorus-bearing axially chiral biaryls 9 can be formed by enantioselective benzene formation using the neomenthyl-indenyl cobalt complex II as a catalyst (Scheme 1.3) [7]. Good yields... 

In 2004, Gutnov et al. reported the atrop-selective biaryl synthesis by cobalt-catalyzed [2-I-2-I-2] cycloaddition [6a]. They developed two types of chiral cyclopentadi-enyl cobalt(I) complex-catalyzed reactions of alkynes with nitriles, which realize the enantioselective synthesis of axially chiral 2-arylpyridines 7 and 10 (Schemes... 

Heller et al. applied the methodology above to the enantioselective synthesis of axially chiral biaryl monophosphine ligands. The reactions of alkynylphosphine oxides 11 with acetylene in the presence of the chiral cyclopentadienyl cobalt(I) catalyst... 

Cobalt(I)/chiral cyclopentadiene complexes are able to catalyze the enan-tioselective synthesis of axially chiral arylpyridines and biaryl phosphoms... 

Metal-catalysed biaryl formation continues to be of interest, and there has been a theoretical study of reactivity and regioselectivity in biaryl formation involving the cleavage of carbon-hydrogen bonds by a concerted metalation-deprotonation pathway. Various combinations of metal/ligand/base have been evaluated for the arylation of benzene with aryl bromides at high temperatures and pressures. The combination of cobalt(III) acetylacetonate and lithium bis(trimethylsilyl)amide proved to be effective. ... 

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