On the Structure and Mechanism of Formation of the Lansbury Reagent, Lithium Tetrakis(N-dihydropyridyl)aluminate

Article abstract of DOI:10.1021/jo00059a041

The reaction of lithium aluminium hydride (LAH) and pyridine yields five lithium tetrakis(N-dihydropyridyl)aluminate (LDPA) isomers.The LDPA isomers are formed reversibly and contain both 1,2- and 1,4-dihydropyridyl ligands.The 1,2-dihydropyridyl ligands are incorporated as the products of kinetic control while the 1,4-dihydropyridyl ligands are formed as the thermodynamic products.When LDPA is synthesized using lithium aluminium deuteride and the deuterated LDPA is placed in pyridine solvent, the ligands exchange with the pyridine in the solvent pool and form pyridine which is deuterated mainly in the 2- and 4-position.A small amount of 3-deuterated pyridine is also detected.The formation of 3-deuteriopyridine suggests that the pyridine radical anion is an intermediate present during the reaction of LAH with pyridine.In support of this suggestion, when LAH and pyridine are mixed, the EPR spectrum of the lithium salt of the pyridyl radical anion is observed.The stepwise addition of ligands to form LDPA is observed (NMR).Five aluminate species are detectable (27Al NMR): LAH , mono-, di-, and -trisubstituted aluminium hydride, and LDPA.The hydrolysis of LDPA in solvent pyridine-d5 yields a mixture of 1,4-, 1,2-, and 2,5-dihydropyridines.The dihydropyridines are stable in the absence of oxygen.