Abstract
The reaction between diethyldipyrromethane Et2C(C4H4N)2 and BH3·THF or the treatment of the lithium salt of dipyrromethane with BF3·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 1H 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 1H NMR analysis. The structures of compound 1 and 2 were confirmed by spectroscopic and X-ray diffraction methods.
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