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

Metalation of the chiral amine (S)-N-α-(methylbenzyl)allylamine with nBuM (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·tmeda]∞, 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·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.