Unexpected and intriguing reactivity of α-imino esters and iminium salts

An intriguing umpolung of the reactivity of imines possessing electron-withdrawing groups was observed. For example, N-alkylation-coupling reaction of the imines derived from glyoxylates was conducted with dialkylaluminum chloride in acetonitrile to give N-monoalkylated 1,2-diamines in good yields. On treatment of various a-imino esters with organoaluminum reagents and allyltributyltin in the presence of benzoyl peroxide (BPO), the tandem reaction proceeded to give the N-alkylation/C-allylation products in good yields. Diethyl 2-[N-(p-methoxyphenyl)imino]-malonate underwent amination reactions with alkyl Grignard reagents to give N-alkylation products in good yields. The obtained N-alkylation products were readily converted into N-alkyl-p-anisidines by the oxidative removal of the malonate moiety. In order to enhance the reactivity of imines, an iminium salt was prepared using the oxidation of amino ketene silyl acetal with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), and the subsequent nucleophilic addition to this iminium species proceeded to afford alpha-amino esters in good yields.