Catalytic Carbocation Generation Enabled by the Mesolytic Cleavage of Alkoxyamine Radical Cations

Abstract

We describe a new catalytic method for accessing carbocation intermediates via the mesolytic cleavage of alkoxyamine radical cations. In this process, electron transfer between an excited state oxidant and a TEMPO-derived alkoxyamine substrate gives rise to a radical cation with a remarkably weak C-O bond. Spontaneous scission results in the formation of the stable nitroxyl radical TEMPO• as well as a reactive carbocation intermediate that can be intercepted by a wide range of nucleophiles. Notably, this process occurs under neutral conditions and at comparatively mild potentials, enabling catalytic cation generation in the presence of both acid sensitive and easily oxidized nucleophilic partners.

Blue Light Photocatalytic Glycosylation without Electrophilic Additives

“…3,10–14 Consequently, we were attracted by recent work on the photocatalytic generation of simple tertiary alkyl, allyl and benzyl cations from sterically hindered O -alkyl hydroxylamines that proceeds in the absence of stoichiometric additives in which the photocatalytic cycle is closed by reduction of the expelled nitroxyl radical (Scheme 2, R = t -Alkyl). 15 …”

“…Adapting the method of Knowles, 15 we photolyzed 0.1 M nitromethane solutions of the various Tempol glycosides with blue LEDs through Pyrex at room temperature in the presence of powdered 4 Å MS, 5 mol % of iridium photocatalyst 10 , 15 and the model acceptor alcohols 11 – 13 , monitoring with TLC or ESI MS.…”

“…Catalyst 10 was selected for this study as the optimum system in the Knowles work 15 on tertiary cation generation. Similarly, nitromethane was chosen as solvent by extrapolation from that study; 15 indeed, a brief investigation of acetonitrile as solvent revealed it to be far inferior.…”

“…Similarly, nitromethane was chosen as solvent by extrapolation from that study; 15 indeed, a brief investigation of acetonitrile as solvent revealed it to be far inferior. This preliminary investigation was limited to the use of Tempol derivatives following Knowles, 15 but it is recognized that other hydroxylamine derivatives may eventually prove advantageous. Blank reactions conducted in the dark or by photolysis in the absence of 10 did not proceed and indicated the need for 10 and its photolytic activation.…”

Late‐Stage Functionalization of Arylacetic Acids by Photoredox‐Catalyzed Decarboxylative Carbon–Heteroatom Bond Formation

“…Decarboxylation of B then occurs to give D . In parallel, Ru III oxidizes imide 2 a to provide imidyl radical E . The resulting proton might react with butoxide ( n BuO − ) or 2‐iodobenzoate ( C ).…”