Energy Transfer Photocatalytic Radical Rearrangement in N-Indolyl Carbonates

Abstract

A new type of sp3-like N-centered radical has been generated by selective energy transfer catalysis. Upon photoexcitation, homolytic N–O bond cleavage of N-indolyl carbonate in the presence of an Ir complex produced N- and O-centered radicals. The high spin density at the C3 position of indole led to radical recombination with the O-centered radical, affording valuable 3-oxyindole derivatives without decarboxylation. Transformations of the desired products into various molecules were also demonstrated.

N–O Bond Activation by Energy Transfer Photocatalysis

“…68 Recently we envisaged the skeletal rearrangement of N-indolyl carbonates 13 through homolytic N−O bond cleavage followed by [3,3]-radical recombination in the presence of [Ir−F] (Scheme 7). 62 The redox potentials of 13a…”

“…Careful selection of the substrate and photocatalyst in terms of compatible triplet energies and mismatched redox potentials is a prerequisite for the success of the EnT process. Figure shows substrates and photocatalysts used in N–O bond activation by selective EnT, including oximes (aryl oximes, − oxime esters, ,− oxime carbonates, , and oxime (gly)­oxalates), 1,2,4-oxadiazolines, , phthalimide enolates, N -indolyl carbonates, N -benzoyloxy carbamates, and N -enoxybenzotriazoles, with a disclosure of their triplet excited state energies. Our laboratory (often in collaboration with the You and Shin groups) utilized a diverse range of substrates such as 1,2,4-oxadiazolines, oxime esters, N -indolyl carbonates, and N -enoxybenzotriazoles.…”

“…Recently we envisaged the skeletal rearrangement of N -indolyl carbonates 13 through homolytic N–O bond cleavage followed by [3,3]-radical recombination in the presence of [Ir–F] (Scheme ). The redox potentials of 13a ( E red = −1.70 V vs Ag/AgCl; E ox = 1.23 V vs Ag/AgCl) and [Ir–F]* ( E ([Ir–F IV ]/[Ir–F III ]*= −0.84 V vs Ag/AgCl; E ([Ir–F III ]*/[Ir–F II ]) = 1.16 V vs Ag/AgCl) in TFE solvent suggested an absence of electron transfer (Scheme ). On the contrary, EnT from [Ir–F]* to 13a was favored due to the higher triplet state energy of [Ir–F]* (60.8 kcal/mol) compared with 13a (58.7 kcal/mol).…”

Hydrogen-Borrowing Reduction/Dehydrogenative Aromatization of Nitroarenes through Visible-Light-Induced Energy Transfer: An Entry to Pyrimidoindazoles and Carbazoles

“…The reduction potential ( E red ) of 1a (−1.54 V vs SCE in CH 3 CN) and 3a (−1.56 V vs SCE in CH 3 CN) were lower than the excited state oxidation potential ( E * ox ) of the [Ir III ]-catalyst (−1.04 V vs SCE, CH 3 CN), and the oxidation potentials ( E ox ) of 1a (1.69 V vs SCE in CH 3 CN) and 3a (2.16 V vs SCE in CH 3 CN) were more positive than the excited state reduction potential ( E * red ) of the [Ir III ]-catalyst (1.39 V vs SCE, CH 3 CN) (see Figure S8 in the SI) . These results indicated that both reductive and oxidative electron transfer between photoexcited [Ir III ]-catalyst and substrates ( 1a and 3a ) were thermodynamically forbidden.…”

[3,3]-Rearrangements of N-Oxyindoles

“…6 The Shin and our groups have recently disclosed photosensitized energy transfer (EnT) processes of N-indolyl carbonates (Scheme 1a(v)). 7,8 The homolytic scission of N−O bond via EnT generated both N-and O-centered radicals, 9 which were further involved in radical-radical recombination to afford formal [3,3]-shifted 3-oxyindole products. 7,10 Building on these findings, we have developed a new approach using Ag-catalysis to achieve the concerted [3,3]-rearrangement of N-oxyindoles to produce 3-oxyindole derivatives with a broader substrate scope, including ester derivatives which were not compatible with our previous photocatalytic process (Scheme 1b).…”