New Spiro-Borane and Spiro-Borate Derived from Dipyrromethane and Their H/D Exchange Properties

The reaction between diethyldipyrromethane Et2C(C4H4N)(2) and BH3 center dot THF or the treatment of the lithium salt of dipyrromethane with BF3 center dot OEt2 in THF gave a new four coordinate boron(III) compound 1 having the spiro structure formed by one dianionic and one monoanionic dipyrromethane units. Compound 1 contains one 2H-pyrrole ring and is acidic. Hence, it readily reacts with bases such as n-Bu4NOH, NaH, DMAP or Et3N to give the spiroborate complexes 2 containing two dianionic dipyrromethane moieties. The deprotonation of 1 gives back aromaticity to the pyrrole ring. Compound 1 is rigid, a rather well resolved H-1 NMR spectrum was obtained in aromatic solvents compared to CDCl3 and peaks were assigned using 2D NOESY spectrum. Compound 1 showed fast hydrogen-deuterium exchanges at the pyrrolic positions in CDCl3/D2O or CD3CN/D2O mixture catalyzed by HBF4. However, the H/D exchange occurs in step wise fashion as shown by H-1 NMR analysis. The structures of compound 1 and 2 were confirmed by spectroscopic and X-ray diffraction methods. (C) 2020 Elsevier Ltd. All rights reserved.