Biaryl stoichiometric

The possible mechanism for the reactions involving stoichiometric amount of preformed Ni(0) complexes is shown in Fig. 9.8. The first step of the mechanism involves the oxidative addition of aryl halides to Ni(0) to form aryl Ni(II) halides. Disproportion of two aryl Ni(II) species leads to a diaryl Ni(II) species and a Ni(II) halide. This diaryl Ni(II) species undergoes rapid reductive elimination to form the biaryl product. The generated Ni(0) species can reenter the catalytic cycle. 

The homocoupling of aryl halide to diaryl compounds, known as Ull-mann coupling, is a synthetically useful reaction and has wide applications in material research. Such couplings have been studied in aqueous conditions. In 1970, arylsulfinic acids were coupled with Pd(II) in aqueous solvents to biaryls (Eq. 6.25).53 However, the reaction required the use of a stoichiometric amount of palladium. In the presence of hydrogen gas, aryl halides homocoupled to give biaryl compounds in moderate yields (30-50%) in an aqueous/organic microemulsion (Eq. 6.26).54... 

Symmetrical biaryls are important intermediates for synthesising agrochemicals, pharmaceuticals and natural products (1). One of the simplest protocols to make them is the Ullmann reaction (2), the thermal homocoupling of aryl chlorides in the presence of copper iodide. This reaction, though over a century old, it still used today. It has two main disadvantages, however First, it uses stoichiometric amounts of copper and generates stoichiometric amounts of CuL waste (Figure 1, left). Second, it only works with aryl iodides. This is a problem because chemicals react by their molarity, but are quantified by their mass. One tonne of iodobenzene, for example, contains 620 kg of iodo and only 380 kg of benzene . 

Iron-catalysed homo-coupling of aryl Grignard reagents has been successfully developed.96 A variety of aryl Grignard reagents have been efficiently converted into the corresponding symmetrical biaryls in the presence of FeCl3 and a stoichiometric amount of 1,2-dichloroethane as oxidant (Scheme 18). 

The enantiomeric atropoisomers of 1,1 -binaphthyl-2,2 -diol (BINOL) and bis-diphenylphosphonate derivatives (BINAP) are completely synthetic molecules that have been developed to exploit the axial dissymmetry induced by the restricted rotation about the biaryl bond (Scheme 1.8) [64]. During the past 15 years, these compounds have become the most widely used ligands for both stoichiometric and catalytic asymmetric reactions, with many analogues and derivatives having been developed recently. 

During their work on the arylation of aromatic compounds by substitution, Fujiwara, et al. observed biaryl formation when aromatic compounds were placed in the presence of olefin-palladium complexes and silver nitrate.80 Developing this reaction as a method for biphenyl synthesis, these authors showed that the more stable the olefin-palladium complex was, the lower the yield. Ethylene dichloropalladium proved to be the best choice, when used with silver nitrate. However, the reaction required stoichiometric amounts of both catalysts (Scheme 10.47). Benzene derivatives substituted by electron-donating or -withdrawing groups reacted as well, but a mixture of regioisomers was produced, except for nitrobenzene, which only gave m,m -dinitrobiphenyl. 

D. The use of chiral oxazaborolidines as enantioselective catalysts for the reduction of prochiral ketones, imines, and oximes, the reduction of 2-pyranones to afford chiral biaryls, the addition of diethylzinc to aldehydes, the asymmetric hydroboration, the Diels-Alder reaction, and the aldol reaction has recently been reviewed.15b d The yield and enantioselectivity of reductions using stoichiometric or catalytic amounts of the oxazaborolidine-borane complex are equal to or greater than those obtained using the free oxazaborolidine.13 The above procedure demonstrates the catalytic use of the oxazaborolidine-borane complex for the enantioselective reduction of 1-indanone. The enantiomeric purity of the crude product is 97.8%. A... 

Previously Davidson and Triggs have reported that arylboronic acids react with sodium palladate to give the dimeric biaryls. The synthetic utility of this dimerization reaction is, however, limited owing to stoichiometric requirement of the palladium compound. On the other hand, the palladium-catalyzed crosscoupling reaction between arylboronic acids and haloarenes in the presence of bases provides a clean synthesis of biaryls (Eq. 106) In this case, sodium carbonate has been proven to be most effective base. 

The reaction of Ni(0) complexes with aryl halides is a standard method for the synthesis of biaryls. The reactions require stoichiometric amounts ofNi, although they can be made catalytic in Ni by using Zn as a reducing agent. The first stage is oxidative addition of ArX to Ni(0), and with excess aryl halide the other final products are the arylphosphonium salt and NiX2 (or NiXj ) ... 

The nickel-catalyzed reductive homocoupling of haloarenes is an excellent method for the synthesis of symmetrical biaryls (Eq. 22). The remarkable ability of [Ni(cod)2] to promote the stoichiometric formation of biaryls from aryl iodides and bromides was originally reported by Semmelhack, Helquist, and Jones [25a]. Unlike the Ullmann reaction [3], the Ni-promoted homocoupling of... 

Importantly, aryl chlorides, bromides, and iodides led to the desired biaryl products. In particular, aryl bromides gave rise to the most efficient catalysis, whereas aryl iodides required AgOTf as stoichiometric additive to obtain satisfactory yields. 

The pioneering studies by Moritani and co-workers set the stage for further applications of palladium-catalyzed C—H bond functionalizations to oxidative C (sp2)-C(sp2) bond-forming processes. Thus, catalyzed oxidative arylations for biaryl syntheses could be accomplished with either stoichiometric [120] or catalytic [121] amounts of palladium complexes. 

The Ullmann coupling [30] involves treatment of aryl halides with stoichiometric copper at high temperatures to yield biaryl compounds. Considerable effort has been made to lower the temperature necessary for initiation of the reaction, as well as decrease the required amount of copper salts. These advances served as the basis for extending this method for the synthesis of aryl heteroatom bonds, known as the Ullmann Condensation Reaction [22, 31, 32]. After several reports for C-O and C-N bond formation, methods for C-S bond formation began to emerge. 

Dienes biaryls. Vinylmercuric chlorides can be dimerized to 1,3-dienes by stoichiometric amounts of palladium(II) chloride and lithium chloride (Lia-PdCU) . More recently, this reaction has been improved by use of this rhodium(I) complex in combination with lithium chloride. This system can function in catalytic amounts. RhCls and LiCl ate somewhat less effective. An example is the synthesis of /ranj,tranj-5,7-dodecadiene (equation I). ... 

Early observations of benzylic acetoxylation were made in the study of arene acetoxylation and biaryl coupling when toluene was used as a substrate. In 1966, the reaction between stoichiometric Pd(OAc)2 and toluene to give benzyl acetate as the major product was disclosed [72]. Two years later, acetoxylation of toluene with catalytic Pd salts was reported by Union Carbide by using phosphines or a combination of Sn(OAc)2, charcoal, and air as oxidant to give 96TONs [73]. Additional metal acetates such as KOAc are beneficial for the reaction [74]. These acetoxylation methods were further applied to other arenes [75] (e.g., benzene, cyclohexene) and the synthesis of benzyl diacetate [76] (a precursor to benzalde-hyde). 

Uses of bases of this type, such as DBU and DBN have been reviewed [15] and more recently they have been used as stoichiometric bases in the synthesis of 2H-isoindoles [ 16],biaryl thioethers [17],phthalocyanines [18] and the related macrocyclic compounds [18], 2,4-dioxo-l,2,3,4-tetrahydroquinazolines [19], dihydro alanine-containing peptides [20] and [[pyrazolylmethyl)amino]-propyl azepinones and -pyrrolidinones [21]. Chiral DBU/DBN-related molecules have also been synthesized which were fovmd to be useful as catalysts in an asymmetric Michael reaction [22]. 

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