Aromatic boronic acids

Enantioselective D-A reactions of acrolein are also catalyzed by 3-(2-hydroxyphenyl) derivatives of BINOL in the presence of an aromatic boronic acid. The optimum boronic acid is 3,5-di-(trifluoromethyl)benzeneboronic acid, with which more than 95% e.e. can be achieved. The TS is believed to involve Lewis acid complexation of the boronic acid at the carbonyl oxygen and hydrogen bonding with the hydroxy substituent. In this TS tt-tt interactions between the dienophile and the hydroxybiphenyl substituent can also help to align the dienophile.114... 

Boronic acids (69 and 70) (Fig. 45) with more than one boronic acid functionality are known to form a polymer system on thermolysis through the elimination of water.93 Specifically, they form a boroxine (a boron ring system) glass that could lead to high char formation on burning. Tour and co-workers have reported the synthesis of several aromatic boronic acids and the preparation of their blends with acrylonitrile-butadiene-styrene (ABS) and polycarbonate (PC) resins. When the materials were tested for bum resistance using the UL-94 flame test, the bum times for the ABS samples were found to exceed 5 minutes, thereby showing unusual resistance to consumption by fire.94... 

Nakazawa, 1. Suda, S. Masuda, M. Asai, M. Shimizu, T. pH-dependent reversible polymers formed from cyclic sugar- and aromatic boronic acid-based bolaamphiphiles. Chem. Commun. 2000, 881-882. 

The imidoyl chloride moiety of 5-chloro-l-alkyl-1,4-benzodiazepin-2-ones participates in Pd-catalyzed, Suzuki crosscoupling reactions, reacting with a range of functionalized aromatic boronic acids to provide an efficient and versatile approach to 5-aryl and 5-heteroaryl compounds (Scheme 19) <2003JOC2844>. This chemistry readily extends to 3-amino-substituted compounds that are orally bioavailable inhibitors of the aspartyl protease 7-secretase <2003BML4143>. 

Poole, C.F., Singhawangcha, S., Zlatkis, A., and Morgan, E.D., Polynuclear aromatic boronic acids as selective fluorescent reagents for HPTLC and HPLC, J. High Resolut. Chromatogr. Chromatogr. Commun., 1, 96, 1978. 

Morgan, A.B., Jurs, J.L., and Tour, J.M. 2000. Synthesis, flame retardancy testing, and preliminary mechanism studies of nonhalogenated aromatic boronic acids. J. Appl. Polym. Sci. 76, 1257. 

The B-G bond of allyl-, alkynyl-, aryl-, and benzylboranes is more easily cleaved than that of alkylboranes. The C-B bond of aromatic boronic acids is stable to water, but polyfluorinated phenylboronic acids are exceptionally sensitive to Me0H-H20-K0H and Me0H-H20-pyridine.488 Allylic organoboranes were hydrolyzed with water at room temperature with complete allylic rearrangement (Equation (102)).489,490 Addition of water resulted in the rapid hydrolysis of benzylic G-B bond of 329 due to the electron-withdrawing effect of Cr(CO)3 group (Equation (103)).491... 

The fluorination of A-methyldiethanolaminc esters 1 of aromatic boronic acids with cesium fluoroxysulfate in acetonitrile at room temperature results in the formation of fluoroaromatic compounds 2 in low to fair yield. 

Subtilisin [11] and cholesterol esterase [12, 13] are also inhibited by a range of aliphatic and aromatic boronic acids. Work on inhibiting a-CHT and cell replication with a variety of boronic acids concluded that the a-CHT activity was associated with chromatin in normal and tumorous tissue of mice [14]. Certain boronic acids were demonstrated to inhibit protease activity in rat liver chromatin [15]. It was concluded that good boronate inhibitors of CHT, like PEBA, inhibit cell replication and that this effect is expected to be higher in rapidly proliferating cancer cells than in normal tissue [14]. 

M.G. Finn and co-workers developed a procedure for the preparation of 2/-/-chromene derivatives that includes a Petasis three-component reaction between salicylaldehyde, vinylic- and aromatic boronic acids, and dibenzylamine. The hydroxyl group of the salicylic aldehyde is essential for the activation of the boronic acid. The initially formed allylic amine undergoes a cyclization upon ejecting the dibenzylamine, thus rendering the process catalytic in the amine. 

Fig. 6 Synthesis of aromatic boronic acid flame retardants, showing the dehydration process that can take place in formulated products when subjected to heat.

A factor hindering the expansion of PAn chemistry to date has been the lack of a generic route to variously substituted derivatives. An exciting development in this respect is the recent synthesis of the novel poly(aniline boronic acid).138 Aromatic boronic acids are versatile chemical precursors, undergoing a wide range of transformations and this provides a facile route to a wide range of substituted PAn s that are difficult to synthesize directly from their respective monomers. This approach has been successfully demonstrated for the synthesis of poly(hydroxyaniline) and for poly(halogenoaniline).138... 

Scheme 16 Sequential alkylation of an aromatic boronic acid by norbomene under rhodium migration conditions...

Liu studied the activity of cyclopalladated imines in the Suzuki reaction and found them to be extremely active (Fig. 10.13) [129]. He was able to show that reaction of his palladacycle with phenylboronic acid in the presence of K2CO3 and ethanol led to the formation of palladium nanoparticles with a diameter range of 50-60nm (TEM). Upon addition of p-bromoanisole to this solutionp-methoxybi-phenyl was formed quantitatively. Reaction of the same complex with ethanol produced acetaldehyde and also led to the formation of nanoparticles, however, in this case palladium black was soon formed. It thus would appear that aromatic boronic acids are capable of stabUising the palladium nanoparticles. 

A round-bottom flask containing a magnetic stir bar was charged with an aromatic boronic acid (1 mmol), methanol (2 mL), 25% aqueous ammonia (5 mmol), and CU2O (0.1 mmol, 15 mg). The flask was not sealed, and the mixture was allowed to stir under an atmosphere of air at RT until complete (as monitored by TLC). The mixture was then filtered, and the solvent of the filtrate was removed via rotary evaporation. The residue was purified by column chromatography on silica gel to provide the desired product. 

Highly efficient rhodium-catalyzed direct arylations were accomplished through the use of 2,2, 6,6 -tetramethylpiperidine-N-oxyl (TEMPO) as terminal oxidant [17]. Thereby, a variety of pyridine-substituted arenes was regioselectively functionalized with aromatic boronic acids (Scheme 9.5). However, in order for efficient catalysis to proceed, 4equiv. of TEMPO were required. The use of molecular oxygen as terminal oxidant yielded, unfortunately, only unsatisfactory results under otherwise identical reaction conditions. However, a variety of easily available boronic acids could be employed as arylating reagents. 

Similarly, self-doped PABA can be prepared using excess of saccharide and one equivalent of fluoride to monomer. Complexation between saccharides and aromatic boronic acids is highly pH dependent, presumably due to the tetrahedral intermediate involved in complexation [25]. Because the pKa of 3-aminophenylboronic acid is 8.75, complexation requires pH values above 8.6. This pH range is not compatible with the electrochemical synthesis of polyaniline, which is typically carried out near a pH value of 0. However, Smith et al. have shown that the addition of fluoride can stabilize the complexation of molecules containing vicinal diols with aromatic boronic acids [23]. Based on this work, it was postulated that the electrochemical polymerization of a saccharide complex with 3-aminophenylboronic acid in the presence of one molar equivalent of fluoride at pH values lower than 8 is possible if a self-doped polymer is produced in the process. 

In 1998, James published the first fluorescent sensors with a selectivity for fluoride. " Fluorescence quenching of a series of simple aromatic boronic acids (72-74)... 

Hydrophobie interaction is one main secondary interaction that influences the selectivity of boronate affinity. Boronate ligands used in affinity materials are usually aromatic boronic acids. In addition, some support materials contain hydrophobic groups. These factors can give rise to hydrophobic interactions or aromatic n-n interactions, which can cause the nonspecific adsorption of analytes like proteins. Hydrophobic interaction can be reduced or eliminated by ehoosing hydrophilic support materials and/or hydrophilic... 

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