Benzeneboronic anhydride

Submitted by Robert M. Washburn, Ernest Levens, Charles F. Albright, and Franklin A. Billig.1 

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). 

Three hundred thirty-six milliliters (312 g., 3.00 moles) of methyl borate is distilled directly into the 500-ml. dropping funnel shortly before starting the reaction (Note 4). One liter (511 g., 3.0 moles) of a 34/ ethereal solution of phenylmagnesium bromide is pressure-transferred with dry nitrogen into the 1-1. 

The stirred mixture, maintained at or below 0°, is hydrolyzed by the addition of 200 ml. of distilled water during 5 minutes. It is then neutralized by addition of a solution of 84 ml. of concentrated sulfuric acid in 1.7 1. of distilled water during 15 minutes. The mixture is transferred to a 5-1. separatory funnel, the ether layer is separated, and the aqueous layer is extracted with three 250-ml. portions of ether. 

The combined ether layer and extracts are transferred to a 5-1. round-bottomed flask equipped with a Hershberg stirrer,3 a dropping funnel, a Claisen head with a water-cooled condenser, an electric heating mantle, and an ice-cooled receiver (Note 8). After approximately one-half of the ether has been removed by distillation from the stirred mixture, 1.5 1. of distilled water is added slowly while the distillation is continued until a head temperature of 100° is reached (Note 9). 

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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]. 

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°. 

Benzene, (cHLORO-teri-BuTYL)-, 32, 90 2-chloro-1, 3-dinitro-, 32, 23 ETHYNYL-, 30, 72 HEXAMETHYL-, 35, 73 1,2,3-trimethyl-, 34, 56 Benzeneboronic ACID, 39, 4 Benzeneboronic anhydride 39, 3 Benzenediazonium chloride, 32, 84 Benzenesulfenyl chloride, 36, 101 Benzenesulfonyl chloride, 31, 83 9,10-o-Benzenoanthracene, 9,10-dihydro-, 39, 75 Benzhydrol, 33, 11 Benzhydryl chloride, 39, 74 Benzidine, 36, 21... 

We found that cyclic benzeneboronic anhydride (BBA), diphenyl carbonate (DPC) (4), diphenyl terephthalate (DPT) (5), diphenyl oxalate (DPO) (5), diphenyl malonate (DPM) (5), tetraphenyl orthocarbonate (POC) (6), hexaphenyl orthoterephthalate (POT) (7) are suitable compounds as chain extenders. 

Elimination of Water. Effect of benzeneboronic anhydride is shown by Reactions 3, 4, and 5. The reaction products, cyclic ethylenebenzene-boronate and water, diffused more readily through the PET polycondensation system than did ethylene glycol. 

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