Aryl transfer reagents

In 1989, two groups working independently prepared pentafluoro-phenylxenon borates by nucleophilic displacement of fluorine in XeFa using BiCeFs), as an aryl-transfer reagent. The resulting colorless solid, which has a stable xenon-carbon bond, was characterized in solution by Xe and F NMR [e.g., Eq. (22)] (165, 166) ... 

Polystyrene-supported diaryliodonium salts 33 can be prepared by the reaction of diacetate 4 with arenes in the presence of sulfuric acid (Scheme 5.19) [55,56]. Polymer-supported aryliodonium salts are useful aryl transfer reagents in Pd(II)-catalyzed cross-coupling reactions with salicylaldehydes [56] and aromatic hydrocarbons [55]. 

Since this first report, a large variety of catalysts (e.g., Pd(OAc)2, RhChPPhjij, RuCljlq -C H ), Ni(cod)j), directing groups (heterocycles, carboxylate based, phenols), and aryl transfer reagents (Ar-Hal, Ar-OR, Ar-B(OR)3, Ar-H) has been employed to achieve direct arylations [47-51]. Many of these reactions have been reviewed elsewhere [52-55], and only few representative examples win be discussed in detail in this chapter. 

In an effort to move away from precious metal catalysts, various reports in recent years have focused on the use of first-row metal catalysts for direct arylations [57-60]. As a representative example of these new developments, we illustrate in Scheme 23.15 the chelate-assisted ortho-C-H arylation of arenes with Fe catalysts [61]. With iron being cheap, nontoxic, and ubiquitous, this protocol is highly attractive for pharmaceutical syntheses. Using the catalyst precursor Fe(acac)j in conjunction with bidentate pyridine ligands, Zn-aryl reagents as aryl transfer reagents and 1,2-dichloroisobutane as the oxidant, excellent yields of the arylated product were obtained. An interesting feature of this reaction is the hydrolysis of the imine moiety after work-up. The reaction conditions tolerate additional functionalities such as cyanides, chlorides, triflates, tosylates, and thiophenes. 

A number of approaches have been tried for modified halo-de-diazoniations using l-aryl-3,3-dialkyltriazenes, which form diazonium ions in an acid-catalyzed hydrolysis (see Sec. 13.4). Treatment of such triazenes with trimethylsilyl halides in acetonitrile at 60 °C resulted in the rapid evolution of nitrogen and in the formation of aryl halides (Ku and Barrio, 1981) without an electron transfer reagent or another catalyst. Yields with silyl bromide and with silyl iodide were 60-95%. The authors explain the reaction as shown in (Scheme 10-30). The formation of the intermediate is indicated by higher yields if electron-withdrawing substituents (X = CN, COCH3) are present. In the opinion of the present author, it is likely that the dissociation of this intermediate is not a concerted reaction, but that the dissociation of the A-aryl bond to form an aryl cation is followed by the addition of the halide. The reaction is therefore mechanistically not related to the homolytic halo-de-diazoniations. 

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

The proximity of the reaction centre to the second phenyl ring makes the aryl cation, formed by heterolytic dediazoniation, a serious competitor to the aryl radical. This is evident in Table 10-6 from various examples where the yield obtained in aqueous mineral acid (varying from 0.1 m to 50% H2S04) is higher than in the presence of an electron-transfer reagent. This competition was studied in three types of product analyses by Cohen s group (Lewin and Cohen, 1967 Cohen et al., 1977), by Huisgen and Zahler (1963 a, 1963 b), and by Bolton et al. (1986). 

Aryl zinc reagents are considerably more reactive than alkylzinc reagents in these catalyzed additions to aldehydes.151 Within the same computational framework, phenyl transfer is found to have about a lOkcal/mol advantage over ethyl transfer.152 This is attributed to participation of the tt orbital of the phenyl ring and to the greater electronegativity of the phenyl ring, which enhances the Lewis acid character of the catalytic zinc. 

Biaryl synthesisThis reagent promotes coupling of aryl Grignard reagents to symmetrical biaryls in 70-95% yield with formation of allene and 3-aryl-2-chloropropene as co-products. The reaction is retarded by galvinoxyl and evidently involves an electron-transfer from the Grignard reagent to the dichloropropene. 

Biphenyl synthesis. In the presence of l,4-dichloro-2-butene, aryl Grignard reagents couple to form biphenyls. The coupling involves electron transfer to the... 

In a related report [16] it has been further demonstrated that aryl triflates could be readily synthesised using 4-nitrophenyl triflate 33 as the transfer reagent (Scheme 8). Deprotonation of the phenol 32 with PTBD 26 in acetonitrile at an elevated temperature (80 °C), followed by the addition of 4-nitrophenyl triflate 33 gave the required aryl triflate 35 after simple filtration. Any unreacted phenolate still present was removed as the ionic polymer 34 through filtration (Scheme 8). 

The reaction can be performed at room temperature with various heteroaryl halides (Equation 73) <20050L697>. It was found that (2-pyridyl)allyldimethylsilanes are pyridyl-transfer reagents in palladium-catalyzed coupling reactions of aryl iodides in the presence of silver oxide as an activator <20060L729>. 

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