Metal 4‐Alkylidene‐4 H ‐pyridin‐1‐ides and 2 H ‐Imidazol‐4‐ones from Novel Highly N ‐(Pyridin‐4‐yl)methyl‐Substituted Azomethines

Novel azomethines 3 synthesized from 4‐(aminomethyl)pyridine ( 1 ) and various ketones 2 exhibit a higher acidity of the Cα‐protons compared with other 4‐alkylpyridines like 4‐benzylpyridine or 4‐(tosylmethyl)pyridine. Therefore, they can easily be metalated to give a variety of specific anions 3(−) , which deserve interest for further synthetic applications. Important properties of compounds 3 and their lithiated derivatives 4 (lithium 4 H ‐pyridin‐1‐ides, synthesized by deprotonation of the α‐CH 2 group in 3 ) can be controlled by the carbonyl reactant 2 . These carbanions 3(−) are remarkable intermediates regarding their NMR properties. Comparing the NMR spectra of the lithium 4 H ‐pyridin‐1‐ides 4 with their parent azomethines 3 reveal very distinct changes in the chemical shifts from which important structural and electronic properties of the carbanions can be deduced. The observation of E / Z isomers in the case of 3c and their mutual conversion at higher temperature inspire DFT calculations on the B3LYP/6‐311++G(d,p) level of theory to get insight into the energetic demand of such isomerization processes. Moreover, calculations on the same level of theory were carried out on the model azomethine anion 5 and different monomeric lithium coordination isomers 6a − d . Experimentally, two selected examples will exemplify the reactivity of 4 H ‐pyridin‐1‐ide anions towards heterocumulenes. The 2‐(pyridin‐4‐yl)acetate 7 obtained from the reaction of 4e with carbon dioxide appears to be a CO 2 storage molecule as it easily releases CO 2 under protolysis conditions comparable with the vitamin B 6 reaction mode. Interesting five‐membered heterocycles were isolated after reaction of 4 with isocyanates followed by hydrolysis and oxidation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)