Synthesis, structure and anion rearrangements of alkali metallated amines and imines

Abstract

Reactions of amines and imines with alkali metals of the source nBuM and MNR₂ (where M= Li, Na, K and R= ⁱPr, SiMe₃) with Lewis donors such as PMDETA, TMEDA and THF were undertaken to observe the structure and anionic rearrangements of these systems. The unexpected aza-enolate propyl addition complex [(PhC=CH₂)(CH=C(CH₂CH₂CH₃)CH₃)NNa.THF]∞ 32 was isolated when (S)-N-α-(methylbenzyl)allylamine reacted with nBuNa. Analytical studies revealed decomposition of the sodium 1-aza-allylamide to a sodium enamide and propene, confirmed by solution and GC-headspace studies, respectively. The aza-enolate structure [(PhC=CH₂)(CH₂CH₂CH₃)NNa]∞ 33 was isolated and identified as an intermediate in the reaction pathway. A mechanism was proposed where propene adds to the carbon tautomer of the sodium 1-aza-allylamide followed by anionic rearrangements to later form the aza-enolate propyl addition complex. The formation of the aza-allyl complex [{Ph(Me)C…N…C(H)Ph}K.TMEDA]n 62 when (S)-N-α-(methylbenzyl)benzylamine ((S)-α-mbba) reacts with nBuK in the presence of TMEDA via β-hydride elimination was isolated and characterised by solution studies. An unexpected co-secondary product, where C-C bond cleavage has occurred eliminating methane, was produced when tridentate or pseudo-tridentate donor ligands were used in the affording the unexpected 1,3-diphenyl-1-aza-allyl complexes [{Ph(H)C…N…C(H)Ph}M.L] (M= Na, L= (THF)₃ 66 or TMEDA/THF 68, M= K, L= PMDETA 64). Formation of methane elimination was confirmed via GC-headspace analysis experiments. DFT calculations on model systems support the experimental findings that thermodynamic gas elimination pathways are favoured by the heavier alkali metals with the elimination of H₂ gas kinetically favoured over CH₄. The decomposition reaction of bis-α-(methylbenzyl)amine (B-α-mba ) with nBuNa in the presence of PMDETA to give the sodium enamide [{PhC(=CH₂)N(H)}Na.PMDETA]n 61 was investigated. Analytical studies revealed a decomposition of the sodium amide to the sodium enamide with the by-product identified as styrene by GC-MS experiments. Reactions of B-α-mba with nBuK in the presence of PMDETA and TMEDA lead to complex reaction mixtures with solution studies revealing the presence of multiple species. A large library of aryl and heterocyclic chiral and achiral imines were generated efficiently through the use of microwave irradiation with a small amount of molecular sieve. The reactions are complete in a matter of minutes and quantitative, significantly reducing the time and amount of solvents used in compound isolation and purification. From the library synthesised, (S)-N-(4-methoxybenzylidene)-α-methylbenzylamine 75 and (S)-N-(2-methoxybenzylidene)-α-methylbenzylamine 76 were reacted with nBuM (M= Na, K) in the presence of PMDETA to give the 2-aza-allyl complexes [{Ph(Me)C…N…C(H)Ph(p-OMe)}Na.PMDETA] 109, [{Ph(Me)C…N…C(H)Ph(o-OMe)}Na.PMDETA] 110, and [{Ph(Me)C…N…C(H)Ph(p-OMe)}K.PMDETA]₂ 111. A third imine, N-[[4-methoxyphenyl]methylene]-benzenamine 78, was reacted with nBuNa to give the unprecedented 1,2 addition species [{PhNCH(CH₂CH₂CH₂CH₃)Ph(p-OMe)}Na.THF]∞ 112. When reacted with NaHMDS the deprotonated species [{PhNCPh(4-OMe)]Na}₂{[PhN=CHPh(4-OMe)}] 114 was isolated. DFT calculations suggest the potential use for the formation of a new type of acyclic single amino carbene species. A one pot cascade sequence of ortho-lithiation, 1,2 addition and intramolecular 1,2-addition/cyclisation was achieved when imine 78 was reacted with LDA in the presence of THF. The complex [{5-(OMe)3-Ph(4-OMe)2-(Ph)isoindol-1-ine}(Ph)NLi.THF] 115 was isolated which revealed the formation of a 2,3,4,5-substituted isoindoline.