Expedient access to saturated nitrogen heterocycles by photoredox cyclization of imino-tethered dihydropyridines

Bissonnette, Noah B.; Ellis, J. Michael; Hamann, Lawrence G.; Romanov‐Michailidis, Fedor

doi:10.1039/c9sc03429c

Cited by 22 publications

(16 citation statements)

References 55 publications

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“… 380 In 2019, Romanov-Michailidis and co-workers reported a concerted PCET-driven method for radical generation from readily accessible 4-alkyl-DHP reagents ( Scheme 131 ). 381 The group prepared a series of redox-active imino-tethered DHP reagents through a one-pot condensation reaction of an amine substrate with an aldehyde, which upon visible-light irradiation in the presence of Ir photocatalyst [Ir(dF(CF 3 )ppy) 2 (bpy)]PF 6 ([ Ir-7 ]PF 6 ) and Li + ( n -BuO) 2 P(O)O – Brønsted base co-catalyst, cyclized to give the saturated heterocycle products. This mode of heterocycle assembly via radical cyclization is conceptually related to the organotin (SnAP) 382 , 383 and organosilicon (SLAP) 384 reagents introduced by the Bode group, but avoids problems associated with the handling of organotin compounds in the former, and a requirement for a neighboring sulfide moiety in substrates in the latter.…”

Section: N -Centered Radical Generation From N–h Bonds Through Photochemical and Electrochemical Pcet Processesmentioning

confidence: 99%

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

et al. 2021

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We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner’s guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N–H, O–H, S–H, and C–H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X=Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

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Section: N -Centered Radical Generation From N–h Bonds Through Photochemical and Electrochemical Pcet Processesmentioning

confidence: 99%

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

et al. 2021

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“…Even for the deracemization of ureas, PCET is involved . Further coupling reactions were recently designed including an aldehyde–olefin coupling via reductive ketyl radical formation, the generation of pyridine based heterocycles, and the copper-based photocatalytic coupling of ketones . Moreover, the PCET concept was employed in visible light mediated selective C–C bond cleavage of lignines and the C–H functionalization of cyclic ethers …”

Section: Introductionmentioning

confidence: 99%

Extended Hydrogen Bond Networks for Effective Proton-Coupled Electron Transfer (PCET) Reactions: The Unexpected Role of Thiophenol and Its Acidic Channel in Photocatalytic Hydroamidations

et al. 2021

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Preorganization and aggregation in photoredox catalysis can significantly affect reactivities or selectivities but are often neglected in synthetic and mechanistic studies, since the averaging effect of flexible ensembles can effectively hide the key activation signatures. In addition, aggregation effects are often overlooked due to highly diluted samples used in many UV studies. One prominent example is Knowles's acceleration effect of thiophenol in proton-coupled electron transfer mediated hydroamidations, for which mainly radical properties were discussed. Here, cooperative reactivity enhancements of thiophenol/disulfide mixtures reveal the importance of H-bond networks. For the first time an in-depth NMR spectroscopic aggregation and H-bond analysis of donor and acceptor combined with MD simulations was performed revealing that thiophenol acts also as an acid. The formed phosphate-H + -phosphate dimers provide an extended H-bond network with amides allowing a productive regeneration of the photocatalyst to become effective. The radical and acidic properties of PhSH were substituted by Ph 2 S 2 and phosphoric acid. This provides a handle for optimization of radical and ionic channels and yields accelerations up to 1 order of magnitude under synthetic conditions. Reaction profiles with different light intensities unveil photogenerated amidyl radical reservoirs lasting over minutes, substantiating the positive effect of the H-bond network prior to radical cyclization. We expect the presented concepts of effective activation via Hbond networks and the reactivity improvement via the separation of ionic and radical channels to be generally applicable in photoredox catalysis. In addition, this study shows that control of aggregates and ensembles will be a key to future photocatalysis.

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“…59). 208 This reaction proceeds via a proton coupled electron transfer (PCET) mechanism, as shown in Fig. 60, with the excited PC being reductively quenched by the imino-DHP, facilitated by the presence of the Brønsted-basic phosphate anion.…”

Section: Alternative Radical Precursors In Forming C-c or C-x Bondsmentioning

confidence: 99%

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

2021

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Expedient access to saturated nitrogen heterocycles by photoredox cyclization of imino-tethered dihydropyridines

Abstract: This work describes an expedient access to sp3-rich nitrogen heterocycles via mild photoredox cleavage of 4-alkyl-1,4-dihydropyridines followed by cyclization of the resultant carbon-centered radicals with tethered imines.

Cited by 22 publications

References 55 publications

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis

Extended Hydrogen Bond Networks for Effective Proton-Coupled Electron Transfer (PCET) Reactions: The Unexpected Role of Thiophenol and Its Acidic Channel in Photocatalytic Hydroamidations

Organic thermally activated delayed fluorescence (TADF) compounds used in photocatalysis

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