Autoaccelerative diazo coupling with calix[4]arene: unusual co-operativity of the calixarene hydroxy groups

New hydrogen bond assisted synthesis of calixarene quinone-hydrazones

“…In a typical experiment, octahydroxycalix [8]arene (obtained from the reaction of p-tertbutyl(octahydroxy)calix [8]arene with AlCl 3 ) 5 was coupled in THF-pyridine (1:5), with diazotized solutions of 2-aminobenzoic acid and 4-nitrophenyl-2-aminobenzoate (NaNO 2 /HCl, 0-5°C), respectively, to yield red solids (Ia,b), which were purified by recrystalization from THF-petroleum ether (1: 1). The NMR, IR and UV of the resultant compounds were indicative of the assigned gross structures given in Scheme 1.…”

“…This is confirmed by the fact that azo calixarenes lacking carboxyl groups in the ortho positions are usually present as their azo tautomers. [6][7][8][9] In the present case, when ortho carboxyl group was esterified, the resultant compound was again observed to exhibit a hypsochromic shift on addition of a base.…”

“…The open chain analog III presumably exists mainly in its azo form (IV) probably because of an increase in electron density at 2 N, which gets protonated by the ortho carboxylic proton. Such protonation of 2 N in the calixarene series seems to be inhibited because of extensive flip-flop hydrogen bonding of lower rim hydroxyls which do not allow significant enhancement in basicity of 2 N. 5,8,9 Such hydrogen bond assisted stabilization of calixarene quinone-hydrazone might also be dependent upon the size and shape of calixarene cavity and this is a subject for further investigation in our laboratories.…”

“…Procedure for coupling octahydroxycalix [8]arene with diazotized 2-aminobenzoic acid. A solution of 2-aminobenzoic acid (0.719 g, 5.24 mmol), sodium nitrite (excess) and concentrated HCl (5 ml) in water (10 ml) was slowly added (over a period of about 15 minutes) into an ice cold solution of 49,50,51,52,53,54,55,56-octahydroxycalix [8]arene (0.214 g, 0.25 mmol) dissolved in tetrahydrofuran : pyridine (25 ml) (5 : 2) with constant stirring to give a red suspension which was stirred for 15 minutes at the same temperature and then gradually allowed to attain room temperature.…”

“…A solution of 2-aminobenzoic acid (0.719 g, 5.24 mmol), sodium nitrite (excess) and concentrated HCl (5 ml) in water (10 ml) was slowly added (over a period of about 15 minutes) into an ice cold solution of 49,50,51,52,53,54,55,56-octahydroxycalix [8]arene (0.214 g, 0.25 mmol) dissolved in tetrahydrofuran : pyridine (25 ml) (5 : 2) with constant stirring to give a red suspension which was stirred for 15 minutes at the same temperature and then gradually allowed to attain room temperature. The reaction mixture was kept for 10 minutes at room temperature and then poured into water (500 ml) and acidified with concentrated HCl to give a red precipitate, which was filtered and subsequently washed with water.…”