Electrophilic substitution arylboronic acids

The first preparations of diaryliodonium salts have been reported in the 19th century, but refinements and improvements keep appearing to date. In most cases an iodoaryl species containing iodine(III) is coupled with an arene or a derivative of it in a typical electrophilic aromatic substitution. Lithiated, stannylated or silylated aryls and arylboronic acids or borates have been introduced recently in order to avoid harsh conditions and to improve yields. The iodoaryl species may be also formed in situ from arenes and iodine(III) reagents. 

The last electrophilic substitution reaction of a Grignard compound we want to consider is a transmetalation, namely one that leads to an arylboronic ester (Figure 5.45). Arylboronic esters or their hydrolysis products, the arylboronic acids, are valuable reagents in modem aro-... 

Electrophilic Substitutions of Arylboronic Acids and Arylboronic Esters... 

Arylboronic acids and their esters are nucleophiles in which the —B(OH)2- or the —B(OR)2 group can be substituted by many electrophiles. Representative examples are shown in Figures 5.48 and 5.49. 

The carbamate substituted DHP derivative 647 has been deprotonated with f-BuLi in THF at — 78 °C to give the vinyllithium 648950 (Scheme 169). This reagent has been functionalized with a variety of electrophiles in good yields. The iodinated derivative 649 (X = I) underwent Suzuki-Miyaura couplings with arylboronic acids to afford the corresponding a-arylated DHPs. 

B(0H)2 group (Op = 0.12) directs electrophilic substitution at the meta position. Amino derivatives are prepared by a sequence of nitration and catalytic hydrogenation of arylboronic acids. ... 

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... 

There has been a wide-ranging review of transmetalation reactions of arenes concentrating on the use of boron-containing compounds. The reaction of arylboronate esters and related compounds with alkyl halides is catalysed by copper(I) formation of an aryl—copper intermediate followed by an 5N2-type reaction with the alkyl electrophile is likely. Palladium complexed with a diimine is an excellent catalyst in the phenylation reaction of Michael acceptors with phenylboronic acid so as to yield products such as (16). The nickel-catalysed reaction of phenylboronic acid with styryl epoxides has been shown to yield a-substituted alcohols such as (17). 

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