Arylboronic acid esters oxidations

Fig. 14.41. H202/H0Ac oxidation of an arylboronic acid ester (for the preparation of this compound, see Figure 5.45).

Rearrangements also are involved in the oxidations of trialkylboranes with H202/ NaOH (Figure 11.36) and of arylboronic acid esters with H202/H0Ac (Figure 11.37). 

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... 

The metal-catalyzed addition reaction of organoboron compounds have not yet been well developed, but die reaction of NaBPh4 or arylboronic acids with enones in the presence of Pd(OAc)2 and NaOAc or SbCL, was recently reported by Uemura and his coworkers.2 The reaction was proposed to proceed through the oxidative addition of the C-B bond to the Pd(0) species however, another probable process, the transmetalation to transition metals, may allow a similar catalytic transformation by the use of organoboronic acids. We report here the 1,4-addition reaction of organoboronic acids to oc, J-unsaturated ketones or esters and 1,2-addition to aldehydes or imines catalyzed by a (acac)Rh(CHj=CH2)2/phosphine complex, which may involve the B-Rh transmetalation as the key step. 

Biaryls. Various arylmetals, arylboronic acids, and arylfluorosilanes undergo Pd(II)-catalyzed homo-coupling using 2,3-dibromoalkanoic esters as oxidants. 

Directed lithiations. Arylboronic acids containing an o-carboxyl group are prepared from the corresponding neopentyl esters via directed lithiation and boration. Chiral o-hydroxyaryl phosphine oxides which are potential chiral ligands are obtained from a... 

Tsuji and co-workcrs have shown that propargylic carbonate 22 oxidatively adds to the palladium(O) complex to provide an (alkoxo)palladium intermediate 23 with elimination of carbon dioxide [57]. Thus, the reaction of 22 with alkylboranes, 1-alkenyl-, 1-alkynyl-, and arylboronic acids or their esters gives 24 in high yields under neutral conditions (Scheme 2-22) [50]. 

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... 

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. 

One-pot Synthesis of Arylboronic Acids and Aryltrifluorobo-rates. Hartwig also reported the synthesis of arylboronic acids and aryltrifluoroborates in a one-pot sequence by Ir-catalyzed borylation of arenes followed by the oxidative cleavage of the boronate ester with NaI04 and the displacement of pinacol by KHF2, respectively (eq 4). These two-step sequences give products that are more reactive and selective in the subsequent chemistry, such as Suzuki coupling, than the initially formed pinacol-boronates. 

Arylboronic esters (Figure 5.48) can be oxidized with H202 in acetic acid to give aryl borates, which can then be hydrolyzed to provide phenols. The mechanism for this oxidation is similar to that for the OOH oxidation of trialkylboranes and is discussed in Section 14.4.3. In combination with the frequently simple synthesis of arylboronic esters one can thus achieve the overall conversions Ar—H —> Ar—OH or Ar—Br —> Ar-OH in two or three steps. 

A nonaqueous workup procedure has been reproted for the preparation of arylboronic esters [ArBCOR a)]-" Uncontrollable polymerization or oxidation of much of the boronic acid occurred during the final stages of the isolation procedure, but could be avoided by in situ conversion to the dibutyl ester by adding the crude product to 1-butanol. The samarium(lll)-catalyzed hydroboration of olefins with catecholborane is a good synthesis of boronate esters." ... 

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