Thermodynamically Favored Anion Rearrangements in Li and Na Complexes of (S)-N-α-(Methylbenzyl)allylamine

Metalation of the chiral amine (S)-N-alpha-(methylbenzyl)allylamine with BuM (M = Li, Na) leads to the formation of complexes with three distinct isomeric anion forms: allylamide [(PhC(H)CH3NCH2CH=CH2)](-), 1-aza-allyl [(PhC(H)CH3NC(H)CHCH3)](-), and aza-enolate [(PhC(=CH2)N(CH2CH2CH3)](-). The anionic form is dependent on the metal, the Lewis donor, and the thermal history of the complex. From their use in asymmetric syntheses and a previously described hmpa (hexamethylphosphoramide) complex, the allylamide form predominates with Li at room temperature in the presence of monodentate donors. The use of the bidentate donor tmeda (N,N,N',N'-tetramethylethylenediamine) results in the crystallization and characterization of the 1-aza-allylic complex (S)-{[(PhC(H)CH3NC(H)CHCH3)Li}(2)center dot tmeda](infinity), in which the allylamide moiety has undergone a 1,3-sigmatropic rearrangement. In the crystal structure, the tmeda molecules bridge rather than chelate the Li cations. The tridentate donor pmdta (N,N,N',N',N ''-pentamethyldiethylenetriamine) gives crystals of [(PhC(=CH2)N(CH2CH2CH3)M center dot pmdta] (M = Li, Na) in which a further rearrangement occurs to an aza-enolate form, resulting in a loss of chirality. Theoretical calculations on the various anion forms and the role of the metal and Lewis donors supported the solid-state observations, revealing a -75 kJ mol(-1) energy gain on rearrangement of the anion from allylamide to either the aza-allylic or the aza-enolate form.