Arylboronic ester

For the synthesis of a suitable arylboron compound, usually an aryl halide is converted to an aryllithium or aryl Grignard derivative, and then reacted with a trialkoxyborane to yield an arylboronic ester, e.g. the phenylboronic acid diisopropyl ester 13 from bromobenzene 11 ... 

SYNTHESIS OF ORTHO SUBSTITUTED ARYLBORONIC ESTERS BY IN SITU TRAPPING OF UNSTABLE LITHIO INTERMEDIATES 2-(5,5-DIMETHYL-l,3,2-DIOXABORINAN-2-YL)BENZOIC ACID ETHYL ESTER... 

The direct isolation of the well defined and stable neopentyl glycol arylboronic esters, without the need for an intermediate aqueous work-up. 

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. 

Regarding the use of other metals for this transformation, Shirai and co-workers reported that a system constituted by palladium(II) complex [Pd(p-Cl)(r -aUyl)]2 and thioether-imidazolium chloride 19 achieved the arylation of aldehydes with boronic acids [33] and potassium trifluoroborates in good to excellent yields (Scheme 7.5) [34], More recently, Buffard and Itami showed that a NKcod) / IPr-HCl system could catalyse the reaction of arylboronate esters and inactivated aldehydes and ketones (Scheme 7.5) [35]. 

The direct preparation of arylboronic esters from aryl halides or triflates now allows a one-pot, two-step procedure for the synthesis of unsymmetrical biaryls (Scheme 1-41) [147]. The synthesis of biaryls is readily carried out in the same flask when the first coupling of the triflate with diboron 82 is followed by the next reaction with another triflate. The synthesis of naturally occurring biflavanoids and the couphng of N-(phenylfluorenyl)amino carbonyl compounds to polymeric supports are reported [154]. 

For the domino transition metal-catalyzed synthesis of macrocycles, conditions must be found for two distinct cross-coupling reactions, of which one is inter- and the other intramolecular. For this purpose, Zhu s group [115] has developed a process of a Miyura arylboronic ester formation followed by an intramolecular Suzuki reaction to give model compounds of the biphenomycin structure 6/1-232 containing an endo-aryl-aryl bond. 

Recently, a rhodium-catalyzed tandem cyclization has been reported with an arylboronic ester bearing a pendant Michael-type acceptor olefin and acetylenic65 or olefinic66 derivatives. This transformation proceeds in a water-containing medium as solvent and proton source. This catalyst system is optimized with electron-rich and bulky ligands to stabilize the rhodium intermediate and decrease the protonolysis of boron derivatives in a protic solvent. 

Water added at this point hydrolyzes the arylboronic ester extremely rapidly to 2-methoxynaphthalene. 

Ishiyama, T. Murata, M. Miyaura, N. Palladium (O)-Catalysed Cross-Coupling Reaction of Alkoxydiboron with Haloarenes A Direct Procedure for Arylboronic Esters, J. Org. Chem. 1995, 60, 7508-7510. 

A. Base-free oxidative homocoupling of arylboronic esters. Tetrahedron Lett. 2003, 44, 1541-1544. 

The first synthesis of arylboronic esters 209-215 via the coupling of bis(pinacolato)diborane(4) with easily prepared aryldiazonium tetra-fluoroborate salts was reported. The palladium-catalyzed borylation reaction proceeds efficiently under mild reaction conditions in the absence of a base to afford various functionalized arylboronic esters in moderate to high yields (Scheme 34).121... 

As for the biaryl ether containing macrocycles, an array of bioactive macrocycles with an endo aryl-aryl bond exist in nature. A new palladium catalyzed reaction has been recently developed in which bis(pinacolato)diborane(4) mediated the process to reach such a structural motif. The reaction consists of a domino process involving a Miyaura s arylboronic ester synthesis and an intramolecular Suzuki-coupling. Synthesis of a bicyclic A-B-O-C ring system of RP-66453 273, a neurotensine receptor antagonist, with an endo aryl-aryl and an endo aryl-aryl ether bond was described (Scheme 53).141... 

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... 

Fig. 5.48. Reactions of an arylboronic ester (for preparation, see Figure 5.45) with selected electrophiles.

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. 

Arylboronic esters (Figure 5.48) and arylboronic acids (Figure 5.49) can also react with unsaturated electrophiles, which cannot be introduced in one step into metal-free aromatic... 

Arylboronate esters are made by a two-step process First, we convert the aryl halide to the aryllithium compound (Chapter 10). Addition of a trialkyl borate (often trimethyl borate) allows the organolithium compound to form a carbon-boron bond and expel an alkoxide group. 

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