Erste röntgenographische Charakterisierung N‐lithiierter 4‐Alkyliden‐1,4‐dihydropyridine des Typs [{4‐(CH3CR)C5H4N}Li(TMEDA)]2: Vergleich von Kristall‐ und MNDO‐Strukturdaten sowie Untersuchungen zur Lithium‐Koordination in Lösung

Abstract

The First X‐ray Structures of N‐Lithiated 4‐Alkylidene‐1,4‐dihydropyridines [{4‐(CH3CR)C5H4N}Li(TMEDA)]2: Comparison of the X‐ray and MNDO Geometries and Investigation of Lithium Coordination in SolutionThe 4‐ethyl‐ and ‐isopropylpyridines 5a and b were transformed into the lithiated compounds [{4‐(CH3CH)C5H4N}‐Li(TMEDA)]2 (6a) and [{4‐(CH3CCH3)C5H4N}Li(TMEDA)]2 (6b) by reaction with LDA and in the presence of TMEDA. These compounds were recrystallized from a toluene/hexane solution to obtain suitable crystals for X‐ray investigations. Both structures are dimeric containing nonstoichiometric amounts of toluene in the crystals. The structural parameters of the heterocyclic moieties of 6a or b are typical of N‐lithiated 4‐alkylidene‐1,4‐dihydropyridines. MNDO geometries compare quite well with the X‐ray structure of 6a. The lithium atom of 6a in solution prefers the same type of coordination as in the crystal: Two‐dimensional 6Li,1H‐HOESY showed Li contacts only to ortho‐hydrogen atoms (Figure 3). 6a and b react differently with chlorotrimethylsilane (8). The variations of the alkylidene moieties of 6a and b, although seemingly small, are decisive: While the α,α‐dimethyl compound 6b reacts exclusively as an N nucleophile, the α‐methyl compound 6a functions also exclusively as a Cα nucleophile.