Developing lithium chemistry of 1,2-dihydropyridines: from kinetic intermediates to isolable characterized compounds.

Generally considered kinetic intermediates in addition reactions of alkyllithiums to pyridine, 1-lithio-2-alkyl-1,2-dihydropyridines have been rarely isolated or characterized. This study develops their "isolated" chemistry. By a unique stoichiometric (that is, 1:1, alkyllithium/pyridine ratios) synthetic approach using tridentate donors we show it is possible to stabilize and hence crystallize monomeric complexes where alkyl is tert-butyl. Theoretical calculations probing the donor-free parent tert-butyl species reveal 12 energetically similar stereoisomers in two distinct cyclotrimeric (LiN)3 conformations. NMR spectroscopy studies (including DOSY spectra) and thermal volatility analysis compare new sec-butyl and iso-butyl isomers showing the former is a hexane soluble efficient hydrolithiation agent converting benzophenone to lithium diphenylmethoxide. Emphasizing the criticalness of stoichiometry, reaction of nBuLi/Me6 TREN with two equivalents of pyridine results in non-alkylated 1-lithio-1,4-dihydropyridine⋅Me6 TREN and 2-n-butylpyridine, implying mechanistically the kinetic 1,2-n-butyl intermediate hydrolithiates the second pyridine.