α- vs Ortho-Lithiation of N-Alkylarylaziridines: Probing the Role of the Nitrogen Inversion Process
journal contributionposted on 05.12.2008, 00:00 by Francesco Affortunato, Saverio Florio, Renzo Luisi, Biagia Musio
The lithiation reaction of monophenyl- and diphenylaziridines has been investigated in detail in an effort to understand why the former undergo exclusively or mainly ortho-lithiation while the latter are lithiated exclusively at the α-position. Evidence is reported that ruled out the possibility that the α-lithiation, observed for the diphenylaziridines, is the result of an ortho- to α-translocation phenomenon, thus substantiating a direct α-deprotonation process. The role of the aziridine nitrogen lone-pair has been considered: dynamics at the aziridine nitrogen as well as complex-induced proximity effects seem to be responsible for the observed regioselectivity in both monophenyl and diphenylaziridines. It turns out that, by tuning the reaction conditions for the lithiation of trans-1-alkyl-2-methyl-3-phenylaziridines, it is possible to generate with high regioselectivity α- and/or ortho-lithiated aziridines, which can be stereoselectively functionalized by electrophilic trapping. A regioselective ortho-functionalization of diphenylaziridines is made possible by halogen− or tin−lithium exchange and by deprotonation of bis-deuterated aziridines.