Quinuclidines proton affinities

However, in more complicated amines, this straight correlation is violated. The bicyclic tertiary amine l-azabicyclo[4.4.4]tetradecane (22) and the acyclic tertiary amine n-Bu3N have nearly the same first IP (7.84 and 7.90 eV, respectively), but the proton affinity of the bicyclic amine is 20 kcal mol 1 lower than that of the acyclic52. On the other hand, for other bridge-head tertiary amines like l-azabicyclo[2.2.2]octane (quinuclidine, 20) and l-azabicyclo[3.3.3]undecane (manxine, 21) the expected relation between proton affinities and IP values is observed. The extraordinary properties of l-azabicyclo[4.4.4]tetradecane (22) are caused by its unusual conformation the nitrogen lone-pair is directed inward into the bicycle where protonation is not possible. In the protonated form, the strained out-conformation is adopted. This makes it the least basic known tertiary amine with purely saturated alkyl substituents. Its pKa, measured in ethanol/water, is only +0.693. Strain effects on amine basicities have been reviewed by Alder88. 

Heilbronner and coworkers94 have studied several 2-, 3- and 4-substituted quinuclidines (23-25) by PES and ICR spectroscopy. A linear correlation of the gas-phase basicities and the ionization energies—relative to the unsubstituted parent molecule—was established. Comparison of the solution pKa values with gas-phase basicities revealed that 2-substituted quinuclidines (23) exhibit sizeable solvent-induced proximity effects, i.e. that the corresponding quinuclidinium ions are more acidic in solution than expected on the basis of proton affinities. 

A comparison of calculated and measured proton affinities (basicities) of nitrogen bases relative to the proton affinity of ammonia as a standard is provided in Table 6-17. The calculations correspond to the usual theoretical models, and the experimental data derive from equilibrium measurements in the gas phase. The data span a large range the proton affinity of the strongest base examined, quinuclidine, is some 27 kcal/mol greater than that of the weakest base, ammonia. 

The possibility of using 2,6-disubstituted pyridines and 2,6,7-trisubstituted quinuclidines, where the substituents feature remote atoms with lone pairs to stabilize the hydrogen upon protonation, are proposed snperbases that have been explored by computational approaches. There is interest in synthesizing macrocyclic proton chelaters as catalytically active organic snperbases,and a new strnctnral motif for snperbases featuring caged secondary amines has been reported. The alkali metal hydroxides, of eqnal basicity in aqueous solution, have proton affinities in the order LiOH (1000 kJ/mol) < NaOH < KOH < CsOH (1118 kJ/mol). This order matches the increasing ionic character of the alkah metal-hydroxide bonds. 

The proton affinities of quinuclidine and triethylamine are nearly identical, 983.3 and 981.8 kJ/mol (Table 6.6). When mixed with trimethylboron, whose methyl groups are large enough to interfere with the ethyl groups of triethylamine, the quinuclidine reaction is twice as exothermic as that of triethylamine (-84 vs. -42 kJ/mol). Whether the... 

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