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

Abstract

Metalation of the chiral amine (S)-N-R-(methylbenzyl)allylamine with n BuM (M ) Li, Na) leads to the formation of complexes with three distinct isomeric anion forms: allylamide [(PhC(H)CH 3 NCH 2 CHd CH 2 )] -, 1-aza-allyl [(PhC(H)CH 3 NC(H)CHCH 3 )] -, and aza-enolate [(PhC(dCH 2 )N(CH 2 CH 2 CH 3 )] -. 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)CH 3 NC(H)CHCH 3 )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(dCH 2 )N(CH 2 CH 2 CH 3 )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 solidstate 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.

Loss of Chirality through Facile Lewis Base Mediated Aza-enolate Formation in Na and K (S)-N-(α-Methylbenzyl)methallylamides

“…15−17 Heating the HMPA complex in solution to 90 °C prompts a 1,3-sigmatropic rearrangement of the anion to the 1-aza-allyl form, a rearrangement that is observed at room temperature when employing the bidentate donor TMEDA (N,N,N′,N′-tetramethylethylenediamine), with either lithium or sodium. 18,19 The use of the tridentate donor PMDETA (N,N,N′,N″,N″pentamethyldiethylenetriamine) prompts a further internal anion rearrangement to the aza-enolate, with an accompanying loss of chirality within the complex. 18 Highlighting the influence of the metal on anion isomerization, metalation of (R)-N-α-mba with potassium in the presence of THF alone leads to the aza-enolate form of the anion.…”

Synthesis, Structure, and Solution Studies of Lithiated Allylic Phosphines and Phosphine Oxides

“…As established in previous reported studies, nitrogen systems are well-known to form desirable aza-allyl species. − Since two isomeric forms of complex 3 were isolated in the solid state, we sought to determine the transition between 3a and 3b . Density functional theory (DFT) calculations were employed using the Gaussian 16 suite of software to help understand the cocrystallization of two isomeric forms of [Ph 2 PCHC­(Me)­CH 2 Li­(PMDETA)] ( 3a and 3b ) observed when using PMDETA as a ligand (Figure ).…”

Homo- and heteroanionic alkali metal aza-enolate aggregates derived from o-methylvalerolactim ether

“…The addition of Lewis base donors of higher denticity, such as pmdta, tends to maximise deaggregation, with a large number of organolithium monomers isolated and characterised as pmdta solvates. [16][17][18][19][20] On this occasion one equivalent of pmdta was added at 0 1C and the solution left to stir at ambient temperature for 30 min. Removal of all the volatile components in vacuo again resulted only in a viscous orange oil, which was solubilised in hexane.…”