Quinuclidine bridgehead nitrogen atom

Quinuclidine (1) is a saturated bicyclic system with a bridgehead nitrogen atom. It has, in contrast to tertiary aliphatic amines and -substituted piperidines, a rigid structure. The atoms forming the quinuclidine ring are incapable of changing their relative positions by rotation around bond axes. These bond axes are included in the bicyclic system with each ring in the boat form. 

The quinuclidine molecule is a saturated bicyclic system having a bridgehead nitrogen atom [1-3]. In contrast to tertiary aliphatic amines and AT-sub-... 

Consideration of the chemical properties of quinuclidine derivatives led to the conclusion that this type of compound was characterized both by the presence of unshielded free lone pair electrons attached to the bridgehead nitrogen atom and by the specific conformation of the molecule. In contrast to aliphatic tertiary amines and iV-substituted piperidines quinuclidine has a rigidly fixed structure, each of the two rings in the structure having a bath form. 

This apparent characteristic enhancement in the basicity has been used quite frequently for the determination of the position of a double bond with respect to the nitrogen atom in unsaturated amines. The cases such as neostrychnine (134) and dehydroquinuclidine (139) in which the protonation at the 8-carbon atom cannot occur due to the lack of overlap between the electron pair on the nitrogen atom and the tt electrons of the double bond, since this would involve the formation of a double bond at the bridgehead— a violation of Bredt s rule—show a decrease in basicity. For instance the basicities of quinuclidine (140) and dehydroquinuclidine (139) have been shown by Grob et al. (82), to differ by 1.13 pK units in aqueous solution at 25. This decrease in basicity has been attributed to the electron-withdrawing inductive effect of the double bond. 

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