RuO4-mediated oxidation of N-benzylated tertiary amines. Are amine N-oxides and iminium cations reactions intermediates?

Abstract

N-Benzylmorpholine, -piperidine, and -pyrrolidine (1A-C, resp.) are oxidised by RuO4 (generated in situ) at both endocyclic and exocyclic (benzylic) N-α-{methylene positions to afford lactams (and dioxo-derivatives) and benzaldehyde (and benzoyl derivatives), respectively. The N-oxides of 1A-C, formed by a minor side reaction, are not involved as intermediates. Control experiments showed the transient formation of endo- and exocyclic iminium cations trapped with NaCN as the corresponding nitriles. The proposed course of the RuO4-mediated oxidation of 1A-C involves the consecutive steps 1 ⇒ iminium cations + cyclic enamine ⇒ oxidation products. The endocyclic/exocyclic regioselectivity of the oxidation reaction lies between 0.8 (for 1A) and 2.1 (for 1B). The amine cation radical and the N-α-C· carbon-centered radical seem not to be involved. © Central European Science Journals. All rights reserved.