Benzeneboronates

Phenylboric acid (benzeneboronic acid) [98-80-6] M 121.9, m -43", 215-216" (anhydride), 217-220", pKj 8.83. It recrystallises from H2O, but can convert spontaneously to benzeneboronic anhydride or phenylboroxide on standing in dry air. Possible impurity is dibenzeneborinic acid which can be removed by washing with pet ether. Heating in an oven at 110°/760mm Ih converts it to the anhydride m 214-216°. Its solubility in H2O is 1.1% at 0° and 2.5% at 25° and in EtOH it is 10% (w/v). [Gilman and Moore J Am Chem Soc 80 3609 1958.] If the acid is required, not the anhydride, the acid (from recrystallisation in H2O) is dried in a slow stream of air saturated with H2O. The anhydride is converted to the acid by recrystallisation from H2O. The acid gradually dehydrates to the anhydride if left in air at room temperature with 30-40% relative humidity. The melting point is usually that of the anhydride because the acid dehydrates before it melts [Washburn et al. Org Synth Coll Vol IV 68 7965]. 

The kinetics of protodeboronation of a range of substituted benzeneboronic acids in aqueous sulphuric acid mixtures were also examined. Good first-order kinetics were obtained in all cases except for the 3-fluoro compound (due to the sulphonation side reaction) and the 3-trifluoromethyl compound, which hydrolysed, hydrogen fluoride being produced the rates with this latter compound were only followed to 30 % reaction. The kinetic details are summarised in Table 190, from... 

Extension of these studies to formic acid media (containing 4 vol. % ethylene glycol and 1.3 vol. % water) showed that for protodeboronation of 4-methoxy-benzeneboronic acid at 25 °C) rates were invariant of a tenfold variation in acidity produced by adding sodium formate (0.05-0.20 M) to the medium (Table 194), and in this range the concentration of molecular formic acid is essentially constant. This was, therefore, assumed to be the reactive species. At higher acidities the rate increased, which was attributed to the increase in concentration of hydronium ions and protonated formic acid ions which bring about reaction more readily625. 

A third mechanism of protodeboronation has been detected in the reaction of benzeneboronic acids with water at pH 2-6.7625. In addition to the acid-catalysed reaction described above, a reaction whose rate depended specifically on the concentration of hydroxide ion was found. In a preliminary investigation with aqueous malonate buffers (pH 6.7) at 90 °C, 2-, 4-, and 2,6-di-methoxybenzeneboronic acids underwent deboronation and followed first-order kinetics. A secondary reaction produced an impurity which catalysed the deboronation, but this was unimportant during the initial portions of the kinetic runs. 

Temp. (°C) Benzeneboronic acid 8.5 MHClOt H0 = -4.69 Thiophene-2-boronic acid Thiophene-3-boronic acid 6.0 M HClOx Ho 2.83... 

Only one kinetic study of mercurideboronation has been carried out726, this being the reaction between benzeneboronic acid and basic phenylmercuric perchlorate... 

Bronlc acids containing electron-capturing subsitituents were developed by Poole and co-workers. Table 8.19 (451,535,536). In terms of volatility, stability of derivatives, and response to the electron-capture detector the 3,5-bis(trifluoromethyl)benzeneboronic acid, 2,4-dichlorobenzeneboronic acid, and 4-bromo-benzeneboronic acid were recommended for general applications. In particular, the 3,5-bis(trifluoromethyl)benzeneboronate derivatives are remarkably volatile, more so than the benzeneboronates, and are suitable for the analysis of bifunctional compounds of low volatility. All the benzeneboronate derivatives are susceptible to solvolysis which is the primary limitation to their general use for trace analysis. 

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... 

H NMR (DMSO-d6) is consistent with the desired structure, but does indicate the presence of low levels of benzeneboronic acid and other aromatic impurities. 

Bromobenzaldehyde and triphenylphosphine were purchased from Aldrich Chemical Company, Inc. Benzeneboronic acid and palladium acetate were obtained from Lancaster Synthesis. Sodium carbonate was purchased from EM Science and deionized water was used to prepare the 2 M solution. Reagent grade 1-propanol is available from Mallinkrodt Inc. All reagents and solvents are used without purification or degassing. There was no need to dry the glassware rigorously. 

R)-3-Phenylcyclohexanone Cyclohexanone, 3-phenyl-, (R)- (9) (34993-51-6) Phenylboronic acid Benzeneboronic acid (8) Boronic acid, phenyl- (9) (98-80-6)... 

This extension of the Smith analysis, involving methylation of the polyalcohol, has been studied in detail by Bose, both on model systems and on the Ti fructan.84 When a ffuctan is used, this method may give l-hydroxy-3-methoxy-2-propanone and l,3-dimethoxy-2-propa-none, together with methylated glycerols. These compounds may react further under methanolysis conditions to yield 2,5-dimethoxy-2,5-bis(methoxymethyl)-l,4-dioxane and l,3-dimethoxy-2-propanone dimethyl acetal, respectively. Several of these compounds may be obtained from sucrose by a model reaction. In view of the wide variety of hydroxy compounds obtained in this study,84 their separation and identification as benzeneboronates was investigated, and certain of the latter derivatives were purified by gas-liquid chromatography. The characterization of 1,2- and 1,3-diols by the mass spectra of their cyclic benzeneboronates has also been described.634... 

Metalated ferrocenes have served as valuable intermediates for the synthesis of a number of other derivatives. Treatment of lithiated ferrocenes with tributyl borate followed by hydrolysis leads to ferroceneboronic acid (XXXIII) as well as the diboronic acid (73). Ferroceneboronic acid, like benzeneboronic acid, is readily cleaved by cupric bromide or cupric chloride to form the corresponding halo derivatives (XXXIV). Ferrocene-l,l -diboronic acid reacts in the same manner, and either one or two carbon-boron bonds can be cleaved. Further reactions of this type have led to a variety of mixed dihaloferrocenes (73, 75). 

Phenyl boric acid (benzeneboronic acid) see Chapter 4. 

Farrall and Frechet recognized the possibility of forming polymer-bound boronic acids in 1976.66 Supported pnra-benzeneboronic acid groups were generated by direct lithiation of polystyrene, giving a para-lithio intermediate that could be used to generate a host of other resins also (Scheme 27). Conversion to the boronic acid was one of the more successful transformations. The purpose of this transformation was to allow the attachment of sugars to the solid phase via the boronate. 

Caution Benzeneboronic acid and its anhydride are toxic substances and may irritate mucous tissues such as those of the eyes. 1 n case of contact, carefully wash exposed parts of the body with soap and water (Note 1). 

While stirring is continued, the aqueous distilland is cooled in an ice bath (Note 10). The benzeneboronic acid, which separates as small white crystals, is collected on a Buchner funnel and washed with petroleum ether. The petroleum ether removes traces of dibenzeneborinic acid, which are seen in the hot mother liquor as globules of brown oil and which may color the product. The acid is dehydrated to benzeneboronic anhydride by heating it in an oven at 110° and atmospheric pressure for 6 hours (Note 11). Benzeneboronic anhydride is obtained as a colorless solid, weight 240-247 g. (77-79%) (Note 12), m.p. 214-216°. 

A summary of the physiological activity of benzeneboronic acid may be found in reference 4a. 

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