(E)/(Z) EQUILIBRIA .13. MOLECULAR-STRUCTURES OF A LITHIATED SCHIFF-BASE AND A RELATED DIALKYL KETONE IMINE

The ether-solvated lithium derivative of pinacolone anil (2a) is shown to be a centrosymmetric dimer (1) by X-ray diffraction analysis. Each lithium atom is bound to two nitrogen atoms, to the oxygen atom of diethyl ether, and weakly to two carbon atoms of a phenyl ring, but not to the vinyl moiety of the enamide anion. Despite conformational deconjugation, 1 is methylated at the carbon atom by methyl iodide. H-1-NMR shifts of 1 in solution are solvent-dependent. The results are discussed with respect to the ''syn effect'' in 1-azaallyl anions. The molecular structure of the dianil 3 of 2,2,7,7-tetramethyl-3,6-octanedione shows that a CH proton may approach the pi-face of an aromatic carbon atom down to a nonbonding distance of 2.74 angstrom without causing much strain.