Importance of Singlet Oxygen in Photocatalytic Reactions of 2-Aryl-1,2,3,4-tetrahydroisoquinolines Using Chalcogenorosamine Photocatalysts

Abstract

Aerobic oxidation of 2-aryl-1,2,3,4-tetrahydroisoquinolines was achieved photocatalytically using chalcogenorosamine photocatalysts and LED irradiation. The photocatalytic aza-Henry reaction between these substrates and nitromethane was more efficient with selenorosamine and tellurorosamine photocatalysts than with thiorosamine and rosamine photocatalysts, corresponding to the propensity of the photocatalysts to generate singlet oxygen (1O2). Appropriately, yields for the photocatalytic aza-Henry reaction were greatly reduced when the reactions were conducted under a nitrogen atmosphere. The 2-aryl-1,2,3,4-tetrahydroisoquinolines were oxidized to the corresponding 2-aryl-3,4-dihydroisoquinolones 13a–13c with selenorosamine and tellurorosamine photocatalysts in 2% aqueous acetonitrile. Di-2-aryl-1,2,3,4-tetrahydroisoquinolin-1-yl peroxides 14a and 14b were shown to be intermediates in this reaction. Thiorosamine photocatalysts, which do generate 1O2 upon irradiation, did not give 2-aryl-3,4-dihydroisoquinolones. These results suggested that the exciplex between 1O2 and the chalcogen atom of the chalcogenorosamines (the corresponding pertelluoxide, perselenoxide, or persulfoxide) and/or the hydrated perchalcogenoxide [hydroxy (perhydroxy)­tellurane, -selenane, or -thiane] might be an active oxidant in the formation of 13a–13c. Computational methods were employed to provide support for the observed photocatalytic reactivity of the tellurorhodamine and selenorhodamine chromophores compared to the thiorosamine chromophores. ΔG values were determined for the oxidation and hydration of 10-Te, 10-Se, and 10-S for formation of perchalcogenoxides and hydroxyl­(perhydroxy)­chalcogenanes, respectively. Calculations indicate formation of the pertelluroxide perselenoxide, and persulfoxide exciplex intermediates are energetically favorable. Hydration of the exciplexes of 10-Te and 10-Se have similarly small ΔG of −3.49 and 4.51 kcal/mol, respectively. However, a significantly higher ΔG value of +22.4 kcal/mol is observed for the hydration of 10-S, which suggests that this reactive intermediate is not readily formed.

Ligand electronic fine-tuning and its repercussion on the photocatalytic activity and mechanistic pathways of the copper-photocatalysed aza-Henry reaction

“…More recently, Detty et al reported that singlet oxygen can activate the quinoline substrate 6 while using a chalcogenorosamine photocatalyst. 69 Complicating the analysis even further, oxygen can be activated to singlet oxygen by photo-induced energy transfer (PET) from the Cu(I) photocatalysts, and 1 O 2 can then oxidize 6. 43 Thus, there are three possible operative quenching pathways of the photocatalyst in this reaction: reductive (A) or oxidative (B) quenching via single-electron transfer and oxygen photosensitization (C) via energy transfer (Fig.…”

Phenazine Radical Cations as Efficient Homogeneous and Heterogeneous Catalysts for the Cross‐Dehydrogenative Aza‐Henry Reaction

“…Apart from the expected aza‐ Henry coupling product ( 4 ), almost in every case the corresponding 3,4‐dihydroisoquinolone ( 5 ) was obtained, in some cases, to our surprise, as the major product (the identity of the isoquinolone product was confirmed by single crystal X‐ray diffractometry; Figures S3 and S4 ). These amides are common minor side products in oxidation of benzylic amines and isoquinolines and its selective formation can be achieved under the right reaction conditions [63,92,93] . What came as a surprise was the strong selectivity dependence on the electronic properties of the substrate.…”

“…These amides are common minor side products in oxidation of benzylic amines and isoquinolines and its selective formation can be achieved under the right reaction conditions. [63,92,93] What came as a surprise was the strong selectivity dependence on the electronic properties of the substrate. For instance, when the molecular catalyst [1b]BF 4 was used with the electron rich p-methoxy tetrahydroisoquinoline 3c, the almost exclusive formation of the isoquinolone 5c was achieved (95 %, Table 2, entry 5).…”

High‐performance photocatalysts for the selective oxidation of alcohols to carbonyl compounds

“…However, for the aza ‐Henry reactions of 2‐aryl‐1,2,3,4‐tetrahydroisoquinolines using chalcogenorosamine photocatalysts, it was observed that 1 O 2 plays a vital role in the photocatalytic reaction system as 1 O 2 accepts electrons from the excited substrate to be converted to O 2 − • in‐situ along with the radical carbocation of the reactant (Figure 4). [ 37 ]…”

“…Proposed mechanism for aza‐Henry reaction involving 1 O 2 as a key ROS intermediate. Redrawn from Clark et al [ 37 ] …”