Visible‐Light‐Mediated Generation of Nitrogen‐Centered Radicals: Metal‐Free Hydroimination and Iminohydroxylation Cyclization Reactions

Abstract

The formation and use of iminyl radicals in novel and divergent hydroimination and iminohydroxylation cyclization reactions has been accomplished through the design of a new class of reactive O-aryl oximes. Owing to their low reduction potentials, the inexpensive organic dye eosin Y could be used as the photocatalyst of the organocatalytic hydroimination reaction. Furthermore, reaction conditions for a unique iminohydroxylation were identified; visible-light-mediated electron transfer from novel electron donor–acceptor complexes of the oximes and Et3N was proposed as a key step of this process.

Organic Photoredox Catalysis

“…232 Nitrogen-centered iminyl radicals are rarely encountered but useful intermediates, as demonstrated by Leonori and coworkers in their recent report of an intramolecular alkene hydroimination enabled by photoredox catalysis (Scheme 144). 473 The divergent pathway is proposed to originate from addition of alkyl radical to dinitrophenolate. The catalyst-free transformation was found to be similarly general in scope with comparable yields to the photoredox-catalyzed hydroimination reaction.…”

Visible-Light-Mediated Synthesis of Amidyl Radicals: Transition-Metal-Free Hydroamination and N-Arylation Reactions

“…11 We decided to evaluate whether this activation mode could be applied to the generation of amidyl radicals and use it as a general platform for the synthesis of N-containing molecules. We reasoned that electron poor aryloxy-amides 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 would be ideal amidyl radical precursors owing to the aryloxy motif serving as a (π*+σ*)-electrophore 7b that should facilitate a photoredox SET reductionfragmentation cascade en route to the amidyl radical.…”

“…1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 We started the optimization of our proposed process using the di-nitro-substituted amide 4a due to its synergistic (i) ease of synthesis, (ii) ease of SET reduction and (iii) very favorable N-O bond fragmentation (Scheme 4). 11 Our optimized reaction conditions required the use of eosin Y as the PC, 1,4-CHD as the H-atom source, and K 2 CO 3 as the base. Acetone was found to be the best reaction medium and in terms of the light source, we found that green LEDs (λ = 530 nm) gave the best results.…”

Organic Reactions Promoted by Metal-Free Organic Dyes Under Visible Light Irradiation

“…Eosin Y (EY) is the typical organic dye to induce the synthetically useful photoredox transformations [14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. EY that absorbed visible light populates in the lowest excited singlet state.…”

“…EY could be used as the photocatalyst for the 5-exo-trig cyclization of iminyl radicals generated from O-aryl oximes [24]. Among several aryl oximes evaluated, 2,4-dinitro-substituted aryl oxime 14 has the excellent reactivity due to its low reduction potential ( Figure 6).…”