ARYLBORONIC ACIDS - A MEDIUM-SIZE RING CONTAINING BORONIC ESTER GROUPS

Dehydration of hydroxyethylboronophthalide (2) is shown to occur by a bimolecular route giving a dimeric boronic ester (4). This ester is not easily hydrolyzed, and under mild conditions it does not react with such reagents as 2-aminoethanol and o-phenylenediamine. Such remarkable stability is attributed to an intramolecular coordination of oxygen atoms to electron-deficient boron atoms. Conditions of aromatic nitration cause aromatic substitution along with solvolysis of the borate bond to form 5-nitroboronophthalidylethyl nitrate. The nitration and esterification of boronophthalidylacetic acid and the esterification of 5-nitroboronophthalidylacetic acid are also studied. Ir and nmr spectra of α-substituted boronophthalides are discussed. © 1969, American Chemical Society. All rights reserved.