Direct arylations tetrafluoroborate

Arylation of Quinoline N-Oxides and Pyridines N-Oxides. An efficient protocol to directly arylate quinoline A-oxides was developed using palladium acetate as the catalyst and potassium carbonate as the hase. A survey of different phosphines demonstrated that the sterically less encumbered di-tert-butyl(methyl)phosphonium tetrafluoroborate provides higher yields relative to tri-iert-hutylphosphine. Using this protocol, 15 quinohne A-oxides were arylated in good to excellent yields (eq 1). ... 

Palladium-catalyzed Direct Arylation of Indoles and Thiophenes. Five-membered ring heterocycles possessing only one heteroatom and no Af-oxide function can also be arylated using palladium(II) complexes, a phosphine ligand, and an inorganic base. In one example, a tandem palladium-catalyzed Heck coupling reaction and direct intramolecular C2 arylation reaction on a )V-(2-chlorobenzyl)-5-bromoindole was reported (eq 30). The procedure, which is catalyzed by palladium acetate, uses tn-tert-butylphosphonium tetrafluoroborate as a ligand and ferf-butyl acrylate as the alkene for the Heck reaction (eq 30). ... 

Diazonium salts are another useful source of free radicals, and the formation of the reactive species can be achieved by reductive electrolysis or direct treatment with diazonium tetrafluoroborate salts [39]. By this route, several aryl derivatives could be introduced onto the nanotube sidewalls [40]. Aryl groups bearing halogen or alkyne functionalities are particularly interesting as they can be further reacted in Pd-catalyzed coupling reactions (Suzuki, Heck) or in click chemistry reactions to create products with great potential in materials science [41]. 

Ethylene reacted with iodonium salts in the presence of a palladium catalyst and a base to afford directly 1,2-bis arylated products (stilbenes). Styrene underwent arylation under similar conditions [44], Allylic cyclic carbonates were efficiently phenylated by diphenyliodonium tetrafluoroborate because of the mild conditions, no ring opening occurred, as was the case when iodobenzene was used. 

Interest has been restricted this year to studies of photoelimination of nitrogen from arenediazonium salts and attempts to characterize the resulting aryl cations. Nanosecond laser photolysis techniques have been used to examine the photodecomposition of diazonium tetrafluoroborates. Aryl cations could not directly be detected, but evidence from bleaching experiments indicates that, in water, the phenyl cation has a lifetime of about 500 ps. The isolation of the ether (131) as the sole product of photodecomposition of the diazonium salt (132) in methanol demonstrates that rearrangement of the intermediate aryl cation does not occur. Both heterolytic and homolytic pathways have been described for photodecompositions of arenediazonium salts complexed with crown ethers. A useful synthetic application of this conversion is the formation in 53% yield of ethyl 2,4-difluoroimidazole-5-carboxylate (133) on irradiation of the diazonium fluoroborate (134). ... 

The bicyclic and tricyclic tetrazoles ((1), (9), and (10)) could also be arylated <85M1227> directly by diphenyliodonium tetrafluoroborate with a similar regioselectivity as was shown in the alkylation reactions. Thus, l-phenyltetrazolo[l,5-u]pyridinium fluoroborate (17 R = Ph, X = Bp4) and 2-phenyltetrazolo[5,l-u]isoquinolinium fluoroborate (19 R = Ph, X = BF4) were prepared. 

Redox potentials of the halide ions explain that direct electron release to the benzenediazonium ion takes place only with iodide (and astatide, At ). This corresponds well with experience in organic synthesis iodo-de-diazoniations are possible without catalysts, light or other special procedures. For bromo- and chloro-de-diazoniations, catalysis by cuprous salts (Sandmeyer reaction) is necessary. For fluorination, the Balz-Schiemann reaction of arenediazonium tetrafluoroborates in the solid state (thermolysis) or in special solvents must be chosen, i.e. a heterolytic dediazoniation without electron transfer. GaUi demonstrated that in chloro-de-diazoniations the yield is strongly dependent on the redox potential of electron transfer catalysts (highest yields with Cu" and Sn +), but that the rate of electron transfer influences the yield also. Electron transfer is likely to be the rate-limiting step of aryl radical formation in dediazoniations catalyzed by transition metal salts. 

Conversion of diazonium compounds to aryl chlorides, bromides, or cyanides is usually accomplished using cuprous salts, and is known as the Sandmeyer reaction. Since a CN group is easily converted to a CO2H group (eq. 10.13), this provides another route to aromatic carboxylic acids. The reaction with KI gives aryl iodides, usually not easily accessible by direct electrophilic iodination. Similarly, direct aromatic fluorination is difficult, but aromatic fluorides can be prepared from diazonium compounds and tetrafluoroboric acid, HBF4. 

Stereoselective carbon-carbon bond formations with hypervalent iodine reagents are also prominently described in the literature. Direct asynunetric a-arylation reactions are not easy to perform. Ochiai et al. synthesized chiral diaryliodonium salts such as [l,l -binaphthalen]-2-yl(phenyl)iodonium tetrafluoroborate derivatives 21 via a BFs-catalyzed tin-X -iodane exchange reaction and developed the direct asymmetric a-phenylation of enolate anions derived from cyclic p-ketoesters (Scheme 7) [37]. A beautiful example of direct asymmetric a-arylation of cyclohexanones in the course of a natural product synthesis was presented through the desymmetrization of 4-substituted cyclohexanones using Simpkin s base, followed by coupling with diaryliodonium salts [38]. Other binaphthyl iodonium salts related to 21 have also been reported [39]. 

Arylation of Thiazoles and Oxazoles. The protocol that was previously developed for the C-H activation of azine and diazine (V-oxides with aryl triflates was used to effect the arylation of flve-membered ring heterocycles, such as oxazoles and thiazoles. In contrast to another protocol that was previously reported by the same group, the transformation did not require an V-oxide function. However, in order to direct the arylation at the C4-position, to prevent the formation of a mixture of regioisomers, and to minimize the generation of diarylated products, a C5-chloride was used as a blocking group. The procedure, which is promoted by palladium acetate and di-tert-butyl(methyl)phosphonium tetrafluoroborate, uses an aryl bromide as the electrophile. 

Arylation of Other Heterocycles. Y)i-tert-hutyl(methyl)phosphonium tetrafluoroborate has also been used for the C-H arylation reaction of other heterocycles. For exart5)le, the direct C5-arylation of imidazo[l,5-a]pyrazines was disclosed using this ligand, in combination with palladium acetate as the catalyst (eq 24). ... 

The Ar-H functionalization approach has also been used in the preparation of polymers. For instance, a diketopyrrolopyrrole-based polymer was prepared via a palladium-catalyzed direct C-H (hetero)arylation reaction between a di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-l,4-dione and 4,7-dibromo-2,l,3-benzothiadiazole (eq31). The reaction was conducted using palladium acetate as the catalyst and pivalic acid as an additive. A survey of different phosphines demonstrated that molecular weights of the same order and similar polydispersity indexes (PDI) were obtained using tri-terf-butylphosphonium and tri-ferf-cyclohexyl tetrafluoroborate. ... 

We have found that SWNTs can be functionalized by electrochemical reduction of aryl diazonium salts in their presence. Using this method, about 1 in 20 carbon atoms of the nanotube framework are reacted. We have also found that the SWNTs can be functionalized by direct treatment with aryl diazonium tetrafluoroborate salts in solution, or by in situ generation of the diazonium moiety using an alkyl nitrite reagent. These functional groups give us handles with which we can direct further, more selective derivatization (Figure 5.2) and we recently reviewed the area of covalent sidewall derivatization of SWNTs. 

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