Hindered proton transfer from molecular cavities

5 Hindered proton transfer from molecular cavities 

In Section 4 data were described that show that removal of the intramolecularly hydrogen-bonded proton by a general base from protonated 

The most dramatic rate retardations of proton transfers have been observed when the acidic or basic site is contained within a molecular cavity. The hrst kinetic and equilibrium studies of the protonation of such a basic site were made with large ring bicyclic diamines [72] (Simmons and Park, 1968 Park and Simmons, 1968a). It was also observed (Park and Simmons, 1968b) that chloride ion could be trapped inside the diprotonated amines. The binding of metal ions and small molecules by macrocyclic compounds is now a well-known phenomenon (Pedersen, 1967, 1978 Lehn, 1978). In the first studies of proton encapsulation, equilibrium and kinetic measurements were made with several macrobicyclic diamines [72] using an nmr technique. 

The diamines and their mono- and di-protonated ions can exist in various conformations, in which the nitrogen lone pairs and the protons on nitrogen are directed in (/ or / ) or directed out (o or o ) from the molecular cavity. Diprotonated l,10-diazabicyclo[8.8.8]hexacosane, for example, may exist in either of the three forms in equation (79). When the out-out isomer of l,10-diazabicyclo[8.8.8]hexacosane bis hydrochloride (o o ) is dissolved in aqueous acidic solution, isomerisation to the in-in isomer (/ / ) occurs so 

Medium-ring bicyclic diamines have now been prepared and their proton transfer behaviour has been investigated (Alder, 1983). The diamine 1,6-diaza-bicyclo[4.4.4)tetradccane [73] is reluctant to undergo outside-protonation, presumably because outward pyramidalisation of the nitrogen atoms intro duces strain the diamine exists in the in-in conformation. The strain involved 

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Hindered proton transfer from molecular cavities. 184... 

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