Arylboronic acids phenols

Polystyrene-derived phenylboronic acids have been used for the attachment of diols (carbohydrates) as boronic esters [667]. Cleavage was effected by treatment with acetone/water or THF/water. This high lability towards water and alcohols severely limits the range of reactions that can be performed without premature cleavage of this linker. Arylboronic acids esterified with resin-bound diols can be oxidatively cleaved to yield phenols (Entry 8, Table 3.36). Alcohols have also been prepared by nucleophilic allylation of aldehydes with polystyrene-bound, enantiomerically enriched allyl-silanes [668], as well as by Pummerer reaction followed by reduction of resin-bound sulfoxides [669]. 

Aryl halides of many different types, including simple unsubstituted halides, may be conveniently converted into phenols by an indirect route involving the preparation of an arylboronic acid and its subsequent oxidation with hydrogen peroxide. The arylboronic acid (3) is normally prepared by reaction of the corresponding arylmagnesium halide with a borate ester (typically tributyl borate) at between —60 and — 80 °C, to yield the dialkyl boronate ester (2) which is then hydrolysed to the arylboronic acid (3). The latter may be isolated, purified and then oxidised with hydrogen peroxide as described in the preparation of m-cresol (Expt 6.101). Alternatively the crude reaction mixture from the preparation of (3) may be treated directly with hydrogen peroxide.36... 

This reaction allows aryl carbon-heteroatom bond formation via an oxidative coupling of arylboronic acids, stannanes or siloxanes with N-H or O-H containing compounds in air. Substrates include phenols, amines, anilines, amides, imides, ureas, carbamates, and sulfonamides. The reaction is induced by a stoichiometric amount of copper(II) or a catalytic amount of copper catalyst which is reoxidized by atmospheric oxygen. 

By using this mild and versatile methodology, symmetrical diaryl ethers have been synthesized in a one-pot, two-step procedure starting from arylboronic acids and their partial conversion to the corresponding phenols by oxidation with hydrogen peroxide and a subsequent coupling of the formed phenols with the remaining arylboronic acids upon addition of copper(II) acetate, molecular sieves and triethyl amine (Scheme 7) [22],... 

The arylation of N-H and 0-H containing compounds, such as amines, amidos, imines, and phenols, with arylboronic acids is promoted by copper(ll) acetate (1 equivalent) and tertiary amine (2-5 equivalents) at room temperature [244-248]. The mild reaction conditions at room temperature permit the synthesis of phenolic amino acids without racemization, methodology that has been applied to an efficient synthesis of (S,S)-isodityrosine from two natural amino acids [248] (Scheme 45). 

Many aromatic boron compounds are readily oxidized to phenols, as illustrated in equation (11) for the oxidation of aryldihydroxyboranes (arylboronic acids). This is a useful synthetic pathway as arylboronic acids can resist both alkaline permanganate and nitric acid, so that many derivatives are available. ... 

Evans, D. A., Katz, J. L., West, T. R. Synthesis of diaryl ethers through the copper-promoted arylation of phenols with arylboronic acids. An expedient synthesis of thyroxine. Tetrahedron Lett. 1998, 39, 2937-2940. 

Arylations. Copper(II) acetate catalyzes the reaction of arylboronic acids with phenols and amines (including imidazole and amides) to provide diaryl ethers (e.g., for synthesis of thyroxine) and arylamines, respectively. The reactions are quite erratic, although some preparatively useful cases have been found. 

An interesting alternative which combines both boron and thallium chemistry has been developed. The arylthallium compound is treated with diborane to provide the arylboronic acid which, by oxidation under standard conditions, yields the phenolic compound in good yield (equations 58 and 59)". ... 

Arylbismuth 671, 673 Arylboronic acid 671, 673 Arylbutenes, formation of 613 Aryl-2-cyclohexenones 653 Aryl ethers—see also AUyl aryl ethers. Diaryl ethers. Phenyl ethers, Propargyl aryl ethers formation from calixarenes 1387 Aryl haUdes, as phenol precursors 396, 397 Ai-Arylhydroxylamines, isomerization of 801-805 oxidation of 419 3-Arylindoles, synthesis of 1236 Aryl ketones, oxidation of 424, 425 Aryloxylium cations 179 Asatone, synthesis of 1178, 1179 Ash, from incineration of municipal waste, phenoUc compounds in 938 Aspersitin, synthesis of 1327, 1328 Aspirin 10, 11... 

Substitution reactions. Benzylation of phenols by benzyl methyl carbonates with Pd catalysis proceeds via transesterification and decarbonylation. Triarylmethanes are obtained from a reaction of benzhydryl carbonates with arylboronic acids. ... 

Phenols arylboronic acids. Treatment of arylthallium bistrifluoroacetates with diborane in THF leads to an intermediate, possibly ArBHj, that is converted into arylboronic acids on treatment with water or into a phenol on treatment with alkaline hydrogen peroxide. Treatment of the intermediates with silver nitrate fails to produce biaryls. ... 

It has been speculated that the mechanism of copper-mediated (stoichiometric in copper) arylations of phenols involves the following elemental steps (i) transmetal-lation of Cu(II) with the arylboronic acid (ii) coordination of the phenol nucleophile to copper(II) and (iii) reductive elimination, slowly via the Cu(II) species or via air oxidation to the Cu(III) species which can be expected to undergo reductive eUmination more rapidly, thereby regenerating a potentially catalyticaUy active copper(I) species (Scheme 4.2). A plausible catalytic mechanism (not illustrating the potential role of substrates as copper ligands) is also shown. 

J. Simon, S. Salzbrunn, G. K. S. Prakash, N. A. Petasis, G. A. Olah, Regiose-lective conversion of arylboronic acids to phenols and subsequent coupling to symmetrical diaryl ethers, Journal of Organic Chemistry 2001, 66, 633. 

Collman and Zhong reported the first catalytic, oxidative C-N cross coupling between an arylboronic acid and a substrate containing an N-H bond. " In the presence of 10 mol % of commercially available [Cu(OH)TMEDA]jClj in dichloromethane, imidazole coupled with phenylboronic acid at room temperature. Lam and co-workers and Antilla and Buchwald expanded the scope of this type of catalytic coupling to encompass the reactions of amines. Lam and co-workers reported the first catalytic version of the reactions of aiylboronic acids with phenols to yield aryl ethers in good yields. The highest yields were obtained when Oj was used as co-oxidant. After these early discoveries, several reports on... 

In general, these compounds were synthesized from appropriately substituted and selectively protected phenolic esters. A reaction sequence that consisted of selective deprotection, oxidation to the quinoid system, and addition of an amino acid gave vinylogous esters. Linkage to the trans-decalin building blocks with an exocyclic double bond was achieved with a Suzuki coupling of substituted arylboronic acids to the corresponding decalin-derived vinyl bromides. Reduction of... 

Phenols 26 substitute chloride in 2-chloro-4,6-dimethoxy-l,3,5-tri-azine 27 to give a series of corresponding 2-aryloxy-4,6-dimethoxy-l, 3,5-triazines 28 in high yields. This quite trivial reaction deserves special attention because the aryl—O bond in 28 can be selectively cleaved with inexpensive, air-stable catalyst NiCl2(dppf) in the Suzuki-Miyaura coupling of 28 with arylboronic acids.The reaction proceeds more easily in comparison with other aryl ethers or esters yielding different diaryls 29 in 50—90% yields (13JOC5078). 

The treatment of arylboronic acids and esters with alkaline hydrogen peroxide to produce the corresponding phenols was first reported more than 75 years ago [324]. The oxidation of alkyl- and alkenyl- boronic acid derivatives leads to alkanols [40] and alde-hydes/ketones, respectively [85, 257, 279, 316]. With a-chiral alkylboronates, the re-action proceeds by retention of configuration (Equation 53, Figure 1.32) [359, 121]. In fact, the oxidation of boronic acids and esters is a synthetically useful process, mainly in the preparation of chiral aliphatic alcohols via asymmetric hydroboration reactions [300, 302], or from Matteson homologation chemistry [322]. Paradoxically, the... 

The ready availability of arylboronates by an aromatic C-H borylation provides a synthetic link to the well-established palladium-catalyzed cross-coupling reactions, rhodium-catalyzed 1,4-addition to a,p-unsaturated carbonyl compounds, and other bond forming reactions using arylboronic esters (Scheme 2.12). Borylation of 1,3-dichlorobenzene with pinacolborane is followed directly by a cross-coupling reaction with methyl p-bromobenzoate for the synthesis of a biaryl product in 91% yield [60]. Pinacol esters of arylboronic acids react much slower than the free acids [62], but both derivatives achieve high isolated yields and comparable enantioselectivities (91% ee) in asymmetric 1,4-addition to N-benzyl crotonamides [63]. Borylation of arenes followed by oxidation of the C-B bond is synthetically equivalent to an aromatic C-H oxidation to phenols [64]. Oxidation of the resulting arylboronates with Oxone in a 1 1 acetone-water solution is completed within 10 min at room temperature. 

Scheme 5.1 Arylation of 3,5-di(t-butyl)phenol with arylboronic acids.

Lam et al. [23] have studied the competition between O- and N-arylation. With 3,5-di-t-butylphenol and 4-t-butylanUine. N-arylation is nine times faster than O-arylation (Scheme 5.22). Since N-arylation is a faster reaction, the side reaction of arylboronic acid converting into the phenol is probably not as pronounced, and thus the use of molecular sieves may not be necessary (vide infra). 

In general, the reaction requires excess arylboronic acid (1.5-2.0 equiv,) since it undergoes side-reactions of protiodeboronation and/or conversion into phenol [8, 51a], Lam and co-workers found that in the absence of substrates, 4-biphenylboronic acid has a half-life of 30 min (Scheme 5.34) [23a]. For the origin of phenol formation, there are two possibilities. Evans and co-workers [8] have speculated that the 0-aryla-tion of adventitious water (formed in the arylboronic acid/triarylboroxine equilibrium) is the source of phenol side-product. However, arylboronic acid might be oxidized to phenol via the copper(iii) species or the hydrogen peroxide formed as a result of the transformation of oxygen into water. To differentiate between these two mechanisms, a study was performed with labeled O2 and HjO in the absence of substrates (Scheme 5.35) [51a]. No incorporation was observed in the isolated phenol... 

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