Boronic, diboronic and boric acid esters of 1,8-naphthalenediol – synthesis, structure and formation of boronium salts

Abstract

The 1,8-naphthalenediolate [1,8-O₂C₁₀H₈] supported boronic and boric acid esters of general formula X-B(1,8-O₂C₁₀H₈), where X = C₆H₅ (1a), C₆F₅ (2a), 3,4,5-F₃-C₆H₂ (3a), 2,4,6-F₃-C₆H₂ (4a), 2,6-F₂-C₆H₃ (5a), 2,6-Cl₂-C₆H₃ (6a), 2,4,6-Me₃-C₆H₂ (7a), 2,6-(MeO)₃-C₆H₃ (8a), Buⁿ (9a), MeO (10a), OH (11a) and Cl (13a), were synthesized, NMR spectroscopically characterized, and the solid-state structures of 1a–5a, 8a and 10a determined by X-ray crystallography. The acceptor numbers of 1a–7a and 13a were determined and found to be similar to their catecholate analogues, R-Bcat, indicating similar Lewis acidities of these two classes of boronic acid esters. The reaction of B₂(NMe₂)₄ with 1,8-naphthalenediol, followed by addition of HCl furnished the diboronic acid ester B₂(1,8-O₂C₁₀H₈)₄ (16a) in ca. 70% yield. Cl-B(1,8-O₂C₁₀H₈) (13a) was shown to react with OPEt₃, DMAP, 1,10-phenanthroline and 2,2′-bipyridine, resp., to give the boronium salts [(Et₃PO)₂B(1,8-O₂C₁₀H₈)]Cl (18a), [(DMAP)₂B(1,8-O₂C₁₀H₈)]Cl (22a), [(2,2′-bipyridine)B(1,8-O₂C₁₀H₈)]Cl (23a) and [(1,10-phenanthroline)B(1,8-O₂C₁₀H₈)]Cl (24a), which were characterized by NMR spectroscopy and X-ray crystallography.