1,8‐Diazabicyclo[6.6.6]eicosane, 1,8‐Diazabicyclo[6.6.5]nonadecane and 1,8‐Diazabicyclo[6.6.4]octadecane and Their Diprotonated Forms

The preparation of the title compounds 9−11 was achieved by catalytic hydrogenation of 1,8‐diazabicyclo[6.6.6]eicosa‐4,11‐diyne ( 13 ), 1,8‐diazabicyclo[6.6.5]nonadeca‐4,11‐diyne ( 14 ), and 1,8‐diazabicyclo[6.6.4]octadeca‐4,11‐diyne ( 15 ), respectively. As catalyst we used Pd(OH) 2 on carbon. NMR studies on 9−11 revealed an in / in conformation at the bridgehead positions. Treatment of 9−11 with an excess of trifluoroacetic acid yielded, in a kinetically controlled reaction, mixtures of the out / out and in / out diprotonated species [ 9· 2H] 2+ , [ 10· 2H] 2+ and [ 11· 2H] 2+ . The thermodynamically controlled species, the corresponding in / in conformers, are formed at higher temperatures and longer reaction times. The experimental observations are supported by AM1 calculations. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)