Abstract
Proton affinities of several efficient organocatalysts METHOX, QUINOX, ANETOX, KOTOX, FUREOX, and FUROOX bearing a pyridine N-oxide or 2,2'-bipyridyl N,N'-dioxide moiety were de- termined by using extended kinetic method and density functional theory calculations. Proton affinities are in the range of 1030 - 1060 kJ mol -1 . Using isodesmic reactions, the effect of combining two pyridine N-oxide units in the neutral and the protonated molecule was studied: The combination of an unfavorable interaction in the former case and a favorable interaction in the latter accounts for the superbasic proper- ties of 2,2'-bipyridyl N,N'-dioxides. Last but not least, the theoretically predicted pKa in ethanol are 0.1, -2.7, 0.9, 1.8, 1.9, and 2.3 for the METHOX, QUINOX, ANETOX, FUROOX, FUREOX, and KOTOX, respectively.
Highlights
Investigation of strong organic bases represents an interesting academic topic.[1,2,3,4,5] Development of new superbases is usually based on two approaches
The replacement of the phenyl substituents with tetrahydrofuryl groups leads to a decrease of proton affinity by ca 10 – 20 kJ mol–1 (FUROOX and FUREOX), both according to the calculations and experiment
When we analyze the magnitude of the effects at the B3LYP level of theory, the protonated forms are by ~5 kJ mol–1 more stabilized by the second N-oxide function than by the methoxy substituent
Summary
Investigation of strong organic bases (superbases) represents an interesting academic topic.[1,2,3,4,5] Development of new superbases is usually based on two approaches. We have shown that molecules derived from 2,2′-bipyridyl N,N′-dioxide have very large proton affinities (1050 – 1070 kJ mol–1) in the gas phase and can be denoted as superbases.[21] The catalysts examined here can be divided into two different classes according to their structure.
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