Arylboronic acid esters

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). 

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]. 

Table 7. Aryl Fluorides by Reaction of Arylboronic Acid Esters with Cesium Fluoroxysulfate...

Entry Arylboronic Acid Ester 17 R Aryl Fluoride 16 Yield (%) Ref... 

Figurel.18 Common methods for the synthesis of arylboronic acids (esters).

As shown in Fig. 2.19, starting from o-cresol 2.2.16, with NBS ortho-bro-mination and then MOM protection of phenolic hydroxyl group, compound 2.2.17 was obtained in two steps with 55 % overall yield. Reacted in butyl lithium and then quenched with trimethyl borate, arylboronic acid ester was obtained. The boric acid ester was in situ hydrolyzed to boric acid compound 2.2.18 with diluted HCl to give 45 % yield. Compound 2.2.18 was reacted with lead (IV) acetate/ mercury (II) acetate to convert the boric acid into lead reagent 2.2.19. Without... 

Recently, the Chan-Lam reaction was also adapted for the synthesis of aniline derivatives. Indeed, in 2009, Fu et al. described the base- and Ugand-free copper-catalyzed coupling of arylboronic acids with aqueous ammonia (Scheme 30) [234]. This very simple system used 10% of CuzO under air at room temperature and affords the corresponding aniline derivatives in very good yields. The reaction can also be carried out from arylboronic acid esters. A related reaction, performed in water with CUSO4.5H2O as catalyst, was more recently reported by Wu [235]. Fu et al. also reported a quite similar system [236]. 

As first described by Krizan and Martin,6 the in situ trapping protocol, i.e., having the base and electrophile present in solution simultaneously, makes it possible to lithiate substrates that are not applicable in classical ortho-lithiation reactions.7 Later, Caron and Hawkins utilized the compatibility of lithium diisopropylamide and triisopropyl borate to synthesize arylboronic acid derivatives of bulky, electron deficient neopentyl benzoic acid esters.8 As this preparation illustrates, the use of lithium tetramethylpiperidide instead of lithium diisopropylamide broadens the scope of the reaction, and makes it possible to functionalize a simple alkyl benzoate.2... 

The conversion of arylboronic acids to the corresponding neopentyl glycol arylboronic esters has several advantages The esters are readily soluble in organic solvents, shelf stable, non-hygroscopic and easily characterized as a single entity.9 Furthermore, boronic esters can be utilized in many of the transformations where arylboronic acids usually are employed, making them an attractive alternative from a practical point of view. 

Soluble polymers have also been used as support. These exploit the combined advantage of homogeneous with those of soHd-phase chemistry [36]. PEG linked 5-bromothiophene-2-carboxyUc acid was cross-coupled with several arylboronic acids under microwave irradiation (constant power of 75 W) using water as the solvent (Scheme 17). Interestingly, microwave irradiation gave less ester cleavage than classical heating (70 °C). The polymeric support remained stable under both conditions. 

Salicylideneamino alcohols are easily prepared ligands that react readily with arylboronic acids to form the corresponding esters, which may be monomeric 45-49 or dimeric 50-59 (Fig. 16) [87-94]. 

Boro nophenyl) alanine (BPA) is a practical boron compound which is clinically used for the treatment of not only malignant melanoma but also of brain tumors, in neutron capture therapy (Scheme 1-40) [105, 152, 153]. Although (pinacolato)di-boron (82) is an excellent reagent to afford the corresponding boronate in 88% yield, it strongly resists the hydrolysis to arylboronic acids. Alternatively, the 1,3-diphenyl-propanediol ester (85) is more convenient to deprotect the diol moiety by catalytic hydrogenolysis [105]. 

Not only arylboronic acids but also 1-alkenylboronic acids or esters add to aldehydes to give the corresponding allylic alcohols (Equation (39)).395 Isomerization of the allylic alcohols to saturated ketones occurs in less protic media and at higher temperatures. 

Simiraly, alkynones undergo arylative cyclization with arylboronic acids in the presence of a rhodium catalyst (Equation (49)).400 When acetylenic /3-keto esters are employed as shown in Equation (50), arylative cyclization (formation of cyclobutanols) and subsequent, facile acid-catalyzed bond cleavage take place to give <5-keto esters.401 Ring expansions of cyclic [3-keto esters are also possible according to this reaction. 

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