Synthesis of Borophosphonate Cage Compounds: Influence of Substituent and Concentration Effects on Product Distribution in Condensation Reactions of Aryl Phosphonic Acids and Boronic Acids

Abstract

Aryl borophosphonate cage compounds [ArPO3BAr′]n with n = 4 or 6 were synthesized by condensation reactions of ArP(O)(OH)2 and Ar′B(OH)2. (3,5-tBu2-Ph)P(O)(OH)2 (1) reacts with arylboronic acids that contain electron-withdrawing substituents to form borophosphonate tetramers [Ar1PO3BAr2]4 (Ar1 = 3,5-tBu2-Ph; Ar2 = o-Br-Ph, o-CF3-Ph, p-CF3-Ph, p-CHO-Ph; 3a–d) and hexamers [Ar1PO3BAr2]6 (Ar2 = p-CF3-Ph, p-CHO-Ph; 4c–d) in 80–93% NMR yield. For Ar2 = p-CF3-Ph and p-CHO-Ph, both products were observed, with the tetramer being favored under dilute reaction conditions and the hexamer favored under concentrated reaction conditions. Interconversion between tetramer (3c or 3d) and hexamer (4c or 4d) was observed at room temperature and above in toluene. The phosphine phosphonic acid (2-PPh2-Ph)P(O)(OH)2 (6) reacts with arylboronic acids that contain electron-withdrawing substituents to form tetramers [Ar1PO3BAr2]4 (Ar1 = 2-PPh2-Ph; Ar2 = p-CF3-Ph, p-CHO-Ph; 7c–d) in 70–75% NMR yield. The reactions of 1 or 6 with (p-tolyl)B(OH)2 (2f), and the reaction of 6 with (o-CF3-Ph)B(OH)2 (2b), yield only trace amounts of borophosphonate cage compounds and instead afford the corresponding [ArBO]3 boroxines and condensation products with unknown structures.