Light-Driven Palladium-Radical Hybrid Species: Mechanistic Aspects and Recent Examples

Abstract

Visible light-induced Pd mediated reactions have become a novel and promising field in organic synthesis. This photocatalytic arsenal presents complementary features towards traditional Pd-chemistry, allowing the achievement of new and unique reactivities by generation of versatile Pd-radical hybrid species. These putative intermediates can be produced by direct excitation of Pd complexes, together with organic radical precursors. This review aims at describing recent advances regarding the combination of Pd-based photocatalytic manifolds and radical generation in the functionalization of important motifs in synthetic chemistry, encompassing detailed mechanism descriptions and relevant examples.

Recent Advances in Visible Light Induced Palladium Catalysis

“…[10] In contrast to the traditional ground-state Pd catalysis [11] relying on 2-electron redox events, [12] contemporary excited-state Pd chemistry usually involves single electron transfer (SET) processes, leading to novel radical transformations via the intermediacy of hybrid Pd radicals. [2][3][4][5][6][7][8] A majority of them involve oxidative quench of the photoexcited Pd 0 with an electrophile (Figure 1A). In this mode, 'oxidative addition' is facilitated via an innersphere SET, which has low or even no activation barrier according to the computational studies by Rueping, [13] Shang, [14] and Koenigs.…”

“…Commonly used Pd sources, ligands, solvents, and bases are also briefly summarized (Figure 1E). This topic has been reviewed by several research groups; [2][3][4][5][6][7][8] this minireview will therefore focus on the recent advances in this emerging area accompanied with mechanistic discussion. The UV-induced transformations are not discussed herein.…”

“…Since the first report by the Gevorgyan group in 2016, a tremendous progress has been made in the field of visible light‐induced Pd catalysis [1] . In this new paradigm, [2–9] Pd catalyst plays a dual role by harvesting photon energy as a photocatalyst, as well as by catalyzing the bond forming/breaking events, thus, obviating the use of an exogeneous photosensitizer [10] . In contrast to the traditional ground‐state Pd catalysis [11] relying on 2‐electron redox events, [12] contemporary excited‐state Pd chemistry usually involves single electron transfer (SET) processes, leading to novel radical transformations via the intermediacy of hybrid Pd radicals [2–8] .…”

Radical‐Dearomative Generation of Cyclohexadienyl Pd(II) toward the 3D Transformation of Nonactivated Phenyl Rings

“…Recent reports on excited‐palladium catalysis have developed novel and powerful reaction patterns beyond ground‐state palladium. [ 61 , 62 , 63 , 64 , 65 ] One of these patterns is the radical generation of π‐allyl Pd(II) including the single‐electron transfer (SET) release of Pd(I) and the recombination of the allylic radical with the Pd(I) species. [ 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 74 , 75 , 76 , 77 ] To date, this strategy has been successful with various 1,3‐dienes [ 66 , 67 , 68 , 69 , 70 , 71 , 72 , 73 , 75 , 76 , 77 ] as the precursor for the π‐allyl Pd(II) via radical addition (Scheme 1b ).…”

Recent Advances in Visible Light Induced Palladium Catalysis

“…In this new paradigm, [2–9] Pd catalyst plays a dual role by harvesting photon energy as a photocatalyst, as well as by catalyzing the bond forming/breaking events, thus, obviating the use of an exogeneous photosensitizer [10] . In contrast to the traditional ground‐state Pd catalysis [11] relying on 2‐electron redox events, [12] contemporary excited‐state Pd chemistry usually involves single electron transfer (SET) processes, leading to novel radical transformations via the intermediacy of hybrid Pd radicals [2–8] . A majority of them involve oxidative quench of the photoexcited Pd 0 with an electrophile (Figure 1A).…”