A complementary approach to conjugated N-acyliminium formation through photoredox-catalyzed intermolecular radical addition to allenamides and allencarbamates

Abstract

An intermolecular radical addition, using photoredox catalysis, to allenamides and allencarbamates is reported. This transformation synthesizes N-acyl-N’-aryl-N,N’-allylaminals, and proceeds by a conjugated N-acyliminium intermediate that previously has principally been generated by electrophilic activation methods. The radical adds to the central carbon of the allene giving a conjugated N-acyliminium that undergoes nucleophilic addition by arylamines and alcohols.

Allenamides Playing Domino: A Redox‐Neutral Photocatalytic Synthesis of Functionalized 2‐Aminofurans

“…IV/III = + 0.77 V vs. SCE), [17] restoring the Ir(III)-complex and generating the allylic carbocation C, further stabilized by the adjacent nitrogen atom. Unlike what was reported by Kimber, [16] that is an intermolecular nucleophilic addition (Scheme 2b), in our case a nucleophilic carbonyl function is already present in the molecule. Therefore, the intermediate carbocation C undergoes a polar cyclization providing the cyclic cation D, which stabilizes by losing a proton and generating the dihydrofuran E. This species is characterized by a conjugated electron-rich exocyclic double bond, prone to be attacked by a second molecule of the electrophilic radical A.…”

“…This intermediate regioselectively adds to the most electron-rich carbon of allenamide 2 (Cβ), providing the α-amino radical B, that is readily oxidized (E 1/2 = À 1.03 V vs. SCE) [26] by Ir(IV) species (E 1/2 IV/III = + 0.77 V vs. SCE), [17] restoring the Ir(III)-complex and generating the allylic carbocation C, further stabilized by the adjacent nitrogen atom. Unlike what was reported by Kimber, [16] that is an intermolecular nucleophilic addition (Scheme 2b), in our case a nucleophilic carbonyl function is already present in the molecule. Therefore, the intermediate carbocation C undergoes a polar cyclization providing the cyclic cation D, which stabilizes by losing a proton and generating the dihydrofuran E. This species is characterized by a conjugated electron-rich exocyclic double bond, prone to be attacked by a second molecule of the electrophilic radical A.…”

“…[14] The use of allenamides in radical transformations is extremely rare [15] and there is only a very recent example in the literature reporting on the application of these substrates in a photoredox catalytic process (Scheme 2b). [16] In this paper Kimber et al presented the intermolecular addition of a photocatalytically generated radical to some allenamides. The obtained radical intermediate was oxidized to conjugated N-acyliminium which underwent an intermolecular nucleophilic addition.…”

“…In the last decades several radical reactions involving allenes have been proposed, [13] but only a few recent examples explore the reactivity between allenes and radical species generated by photocatalysts excited by visible‐light [14] . The use of allenamides in radical transformations is extremely rare [15] and there is only a very recent example in the literature reporting on the application of these substrates in a photoredox catalytic process (Scheme 2b) [16] . In this paper Kimber et al .…”

Synthesis of 1,2-diselenides via potassium persulfate-mediated diselenation of allenamides with diselenides

“…On the other hand, the limited examples of metallo- and organo-photoredox-catalyzed alkyl radical additions on allenamides are governed in the majority by toxic and expensive transition metals. 6 Therefore, the development of new methods for the radical reaction of allenamides is still highly desired.…”