1,8-Bis(tetramethylguanidino)naphtalene (TMGN): a new superbasic and kinetically active „proton sponge

Abstract

1,8-Bis(tetramethylguanidino)naphthalene (TMGN, 1) is a new, readily accessible, and stable proton sponge with an experimental pKBH+ value of 25.1 in MeCN, which is nearly seven orders of magnitude higher in basicity than the classical proton sponge 1,8-bis(dimethylamino)-naphthalene (DMAN). Because of the sterically less crowded character of the proton-accepting sp2-nitrogen atoms, TMGN also has a higher kinetic basicity than DMAN, which is shown by time-resolved proton self-exchange reactions. TMGN is more resistant to hydrolysis and is a weaker nucleophile towards the alkylating agent EtI in comparison to the commercially available guanidine 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD). Crystal structures of the free base, of the mono- and bisprotonated base were determined. The dynamic behavior of all three species in solution was investigated by variable-temperature 1H NMR experiments. G values obtained by spectra simulation reveal a concerted mechanism of rotation about the C-N bonds of the protonated forms of TMGN.