Arylboronic acids anhydrides

Complete characterization of arylboronic acids is often difficult because they are readily transformed into stable cyclic anhydrides called boroxines5 and other polymeric species. Arylboronic acids are also known to be hygroscopic. Thus, arylboronic acids are often prepared and used directly as a mixture of different entities. Commercial arylboronic acids will very often contain varying amount of anhydrides. 

Cyclotriboroxanes (RBO)3 (also known as boroxines) are formally anhydrides of the corresponding boronic acids RB(OH)2 only mild heating is required to convert these dibasic acids into a wide range of cyclic trimeric anhydrides [eqn (9.18)]. This dehydration process can be adapted for the synthesis of B3O3 rings with different aryl groups on the boron atoms by using two or more arylboronic acids in the appropriate stoichiometric ratio. 

A convenient purification procedure for an arylboronic acid is to convert it into the trimeric anhydride (4) by removal of water as a benzene azeotrope (see Expt 6.101, Note (3)). 

It is not possible to obtain an analytically pure sample of arylboronic acids since, on drying, partial conversion to the trimeric anhydride occurs. A quantitative conversion to the anhydride is achieved in the case of m-tolylboronic acid by heating with forty times its weight of benzene in a flask fitted with a Dean and Stark water separator (Fig. 2.31(a)). When no further water droplets separate the benzene solution... 

Reactions between a representative range of alkyl- and aryl-amines and of aliphatic and aromatic acids showed that the direct formation of amides from primary amines and carboxylic acids without catalyst occurs under relatively low-temperature conditions (Scheme 1). The best result obtained was a 60% yield of N-bcnzyl-4-phenylbutan-amide from benzylamine and 4-phenylbutanoic acid. For all these reactions, an anhydride intermediate was proposed. Boric and boronic acid-based catalysts improved the reaction, especially for the less reactive aromatic acids, and initial results indicated that bifunctional catalysts showed even greater potential. Again, anhydride intermediates were proposed, in these cases mixed anhydrides of carboxylic acids and arylboronic acids, e.g. (I).1... 

The mechanism we have proposed to explain boronic acid-catalyzed amidation is depicted in Sch. 2. Arylboronic acid usually contains different amounts of cyclic tri-meric anhydrides (boroxines). The rate-determining step is the generation of ArB (OCOR)(OH). 

The acylpalladium 141 is formed by oxidative addition of acid anhydride to Pd(0), and can be used for synthetic purposes [61]. The mixed anhydride 143 was prepared in situ by the reaction of benzoic acid with dimethyl dicarbonate (142), and the benzoylpalladium methoxide 144 is generated by decarboxylative oxidative addition of 143, and is used for Suzuki coupling with arylboronic acid to yield the diaryl ketone 145 [62]. 

Bryce et have investigated nine arylboronic acids, seven arylboronic catechol cyclic esters, and two trimeric arylboronic anhydrides (Fig. 17) by solid state B and NMR, IR, X-ray techniques, and theoretical... 

The two key catalytic intermediates have been observed by electrospray mass spectrometry [394]. Although the exact role and influence of the base remains unclear [395], the transmetallation is thought to be facilitated by base-mediated formation of the tetracoordinate boronate anion [396], which is more electrophilic than the free boronic acid (Sections 1.5.1 and 1.5.2). A useful carbonylative variant has also been developed to access benzophenones (Equation 70) [397], which can also be produced from the coupling of acid chlorides [398] or anhydrides [399], A variant of this method allows the preparation of a, 3-unsaturated esters from alkenylboronic esters [243]. In all of these reactions, one dreaded limitation with some ortho-substituted and electron-poor arylboronic acids is the possible occurrence of a competitive protolytic de-boronation, which is exacerbated by the basic conditions and the use of a transition metal catalyst (Section 1.5.1). Methods to minimize this side reaction were developed in particular the use of milder alternative bases [400] such as fluoride salts [401], and... 

Scheme 10.2 depicts the proposed mechanism. In general, arylboronic acids contain varying amounts of cyclic trimeric anhydrides (boroxines). 

A related theoretical study on palladium-catalyzed borylation of iodobenzene with diboron was reported by Sakaki et al. [73]. Specifically, the mechanism for the transmetalation step between the Pd complexes Pd(X)(Ph)(PH3)2 (X = OH, F) and diboron B2(eg)2 (eg = -OCH2CH2O-) was investigated. Other theoretical studies, but now within the context of organic acids as electrophiles, were reported by Goo/ en et al. [74, 75]. The palladium catalyzed Suzuki reaction of carboxylic anhydrides with arylboronic acids was analyzed, showing that in those cases monophosphine complexes were also involved in the transmetalation process. 

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