Synthesis and reactivity of novel Schiff bases containing boronate esters

Condensation of 2-aminophenol with boronate ester derivatives of benzaldehyde afforded the corresponding boron-containing Schiff bases, 2-HOC6H4N=C(H)C6H4R (1a: R = 2-Bpin; 1b: R = 3-Bpin; 1c: R = 4-Bpin; pin = 1,2-O2C2Me4). Crystals of 1b were triclinic, space group P (1) over bar, a = 11.9420(6), b = 13.0871(7), and c = 13.2720(7) Angstrom, alpha = 70.983(1), beta = 67.793(1), and gamma = 78.380(1)degrees, Z = 2. Reaction of 2-aminophenol with 2-HC(O)C6H4B(OH)(2) in EtOH, however, gave a macrocyclic dimer 2 with a OBOBO structural unit. The molecular structure of this dimer has been confirmed by an X-ray diffraction study. Crystals of 2 were monoclinic, space group P2(1)/c, a = 10.0447(8), b = 21.0894(15), and c = 12.6214(9) Angstrom, beta = 105.301(2)degrees, Z = 4. Further reaction of these Schiff bases with manganese triacetate in toluene afforded 2-arylbenzoxazoles 3a-c via an oxidative cyclization pathway. The molecular structure of the 4-Bpin derivative (3c) was characterized by an X-ray diffraction study. Crystals of 3c were monoclinic, space group P2(1)/n, a = 6.5392(3), b = 16.3330(8), and c = 16.1942(8) Angstrom, beta = 97.9620(10)degrees, Z = 4.