Arenes with arylboronic acids

Table 3. Synthesis of biaryls by arylation of arenes with arylboronic acid under an influence of manganese(III) acetate [9]...

Cross-coupling. Copper(II) trifluoroacetate has been demonstrated to mediate the C-H bond arylation of arenes with arylboronic acids. Various arylboronic acids reacted with electron-rich benzene derivatives to give the corresponding biaryls in moderate to good yields (eq 1). In all cases, the cross-coupling product was solely obtained. 

Shi has reported a method for /V-alkylanilidc arylation by simple arenes [53], The reaction conditions include heating the anilide with excess arene in propionic acid in the presence of catalytic palladium acetate and copper triflate under oxygen atmosphere. Use of monosubstituted arenes leads to the formation of isomer mixtures however, from the point of atom economy C-H/C-H couplings are the most efficient way for formation of C-C bonds if oxygen is used as the terminal oxidant. Shi has also reported that anilides can be coupled with arylboronic acids and trialkoxyaryl-silanes [54, 55], Silver and copper salts are used as terminal oxidants. 

Various unsymmetrically substituted diaryliodonium triflates 269 can be synthesized by the reaction of iodosylbenzene [380] or (diacetoxyiodo)arenes [381] with arenes in trifluoromethanesulfonic acid (Scheme 2.76). This simple procedure affords diaryliodonium triflates in relatively high yields, but it is limited to aromatic substrates that are not sensitive to strong acids. In a milder, more selective variation of this procedure (diacetoxyiodo)benzene is reacted with arylboronic acids in the presence of triflic acid at -30 °C to afford aryl(phenyl)iodonium triflates in 74-97% yields [377]. 

NiCl2(PCy3)2 associated with PCyj promotes the selective cross-coupling of arylto-sylates with arylboronic acids under relatively mild reaction conditions, and various functional groups are tolerated in both arene reactants. This is one of the simplest, most efficient experimental procedures for coupling arylboronic acids with aryl tosy-lates reported to date (Equation 71) [94],... 

It is proposed that the cleaved methyl or phenyl group in 21 is coupled with arylboronic acid via intermediate 24, and is released as a methyl- or phenyl-arene, respectively. The resultant acylrhodium 25 is then oxidized to 26 by the action of Cul and O2 to undergo additional transmetalation from arylboronic acid. In accordance with the proposed mechanism, the reaction did not proceed at all under a nitrogen atmosphere. 

The first preparations of diaryliodonium salts have been reported in the 19th century, but refinements and improvements keep appearing to date. In most cases an iodoaryl species containing iodine(III) is coupled with an arene or a derivative of it in a typical electrophilic aromatic substitution. Lithiated, stannylated or silylated aryls and arylboronic acids or borates have been introduced recently in order to avoid harsh conditions and to improve yields. The iodoaryl species may be also formed in situ from arenes and iodine(III) reagents. 

Examples for o-phenylene scaffolds for bis-carbene ligands come from the research groups of Peris [344,345] and Herrmann [346]. Synthesis of the bis-imidazolium salt is achieved by reaction of a,a -xylene dichloride and the N-substituted imidazole. The rhodium(l) and iridinm(I) complexes can then be made by addition of the imidazolium salt to a solution of [M(cod)Cl]2 (M = Rh, Ir) in ethanol or acetonitrile (with NEtj as auxiliary base) (see Figure 3.108). The rhodium complexes were used successfully in the hydrosi-lylation of styrene [344] whereas both the rhodium and iridium complexes were used for the direct borylation of arenes making functionalised arylboronic acid esters accessible by a simple one-pot reaction [346]. 

Vogler T, Studer A (2008) Oxidative coupling of arylboronic acids with arenes via Rh-catalyzed direct C-H arylation. Org. Lett. 10 129-131... 

Shi and co-workers have developed various types of C—H bond functionalization and C—C bond formation. Recently, they reported a novel transformation to realize ortho-arylation of acetanilides with trialkoxyarylsilanes through direct C—H functionalization (Equation 11.34) [72]. Furthermore, they also demonstrated a novel method for the direct construction of biaryl C—C bonds via Pd(II)-catalyzed cross-coupling of (hetero)arenes and various arylboronic acids [73]. Various aromatic rings show good selectivity, even without directing groups, under mild conditions. 

Numerous methods for the preparation of symmetrical and unsymmetrical diaryl- and hetaryliodo-nium organosulfonates have been developed. A common synthetic approach to unsymmetric diaryl-and hetaryl(aryl)iodonium tosylates (e.g 262, 264, 266 and 268) is based on the reactions of [hydroxy(tosyloxy)iodo]arenes with aryltrimethylsilanes 261 [198], aryltributylstannanes 263 [376], aryl-boronic acids 265 [377], or the appropriate heteroaromatic precursors 267 (Scheme 2.75) [378,379]. The reaction of HTIB with arylstannanes proceeds under milder conditions compared to arylsilanes and is applicable to a wide range of arenes with electron-withdrawing substituents. Arylboronic acids in general have some advantage over arylstannanes in the case of the electron-rich heterocyclic precursors [377]. 

Free-radical arylations of arenes with arylhydrazines and arylboronic acids... 

Additonal improvement has been introduced by using the arylboronic acids as aryl radical precursors. Arylboronic acids are reacted with anhydrous manganese(III) acetate (3 eq.) in refluxing liquid arene, e.g. benzene, thiophene, or furan, to give... 

The aryllead(IV) tricarboxylates can be prepared, among several possible routes, the most conveniently by the reaction of aryltri-n-butylstannanes [66], or arylboronic acids [67] with lead(IV) acetate in the presence of mercury(II) trifluoroacetate as the catalyst, or, in the case of electron-rich arenes such as polyalkoxybenzenes, by direct plumbylation with Pb(OAc)4 or its derivatives. Scheme 32. 

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