Cooperative Photocatalysis Research Articles

2D sp2Carbon‐Conjugated Porphyrin Covalent Organic Framework for Cooperative Photocatalysis with TEMPO

Abstract2D covalent organic frameworks (COFs) are receiving ongoing attention in semiconductor photocatalysis. Herein, we present a photocatalytic selective chemical transformation by combining sp2carbon‐conjugated porphyrin‐based covalent organic framework (Por‐sp2c‐COF) photocatalysis with TEMPO catalysis illuminated by 623 nm red light‐emitting diodes (LEDs). Highly selective conversion of amines into imines was swiftly afforded in minutes. Specifically, the π‐conjugation of porphyrin linker leads to extensive absorption of red light; the sp2−C=C− double bonds linkage ensures the stability of Por‐sp2c‐COF under high concentrations of amine. Most importantly, we found that crystalline framework of Por‐sp2c‐COF is pivotal for cooperative photocatalysis with (2,2,6,6‐tetramethylpiperidin‐1‐yl)oxyl (TEMPO). This work foreshadows that the outstanding hallmarks of COFs, particularly crystallinity, could be exploited to address energy and environmental challenges by cooperative photocatalysis.

2D sp2 Carbon-Conjugated Porphyrin Covalent Organic Framework for Cooperative Photocatalysis with TEMPO.

2D covalent organic frameworks (COFs) are receiving ongoing attention in semiconductor photocatalysis. Herein, we present a photocatalytic selective chemical transformation by combining sp2 carbon-conjugated porphyrin-based covalent organic framework (Por-sp2 c-COF) photocatalysis with TEMPO catalysis illuminated by 623 nm red light-emitting diodes (LEDs). Highly selective conversion of amines into imines was swiftly afforded in minutes. Specifically, the π-conjugation of porphyrin linker leads to extensive absorption of red light; the sp2 -C=C- double bonds linkage ensures the stability of Por-sp2 c-COF under high concentrations of amine. Most importantly, we found that crystalline framework of Por-sp2 c-COF is pivotal for cooperative photocatalysis with (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). This work foreshadows that the outstanding hallmarks of COFs, particularly crystallinity, could be exploited to address energy and environmental challenges by cooperative photocatalysis.

Polyimide-TiO2 hybrid photocatalysis: Visible light-promoted selective aerobic oxidation of amines

Polyimides are a class of high-performance polymers with good thermal and chemical stability. One specific polyimide can be formed via thermal polymerization of melamine (MA) and pyromellitic dianhydride (PMDA) precursors. Herein, polyimide-TiO2 hybrids were straightforwardly synthesized with a mixture of MA, PMDA and TiO2 by annealing at 325 °C. The crystal phase of anatase TiO2 can be preserved and polyimide can be formed on the surface of TiO2 to enable visible light absorption and high specific surface areas for the polyimide-TiO2 hybrid. Next, this polyimide-TiO2 hybrid as a photocatalyst were successfully applied for visible light-promoted selective aerobic oxidation of amines into imines under irradiation of 460 nm blue light-emitting diodes (LEDs). With 4 mol% of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) as the co-catalyst, polyimide-TiO2 hybrid photocatalyst reached 8 times of that of pristine TiO2 for the selective aerobic oxidation of benzylamine. Overall, the cooperative photocatalysis of polyimide-TiO2 hybrid and TEMPO was demonstrably efficient for visible light-promoted selective oxidation of amines with aerial O2 as the oxidant. Moreover, by means of reactive oxygen species (ROS) quenching experiments and in-situ electron paramagnetic resonance (EPR) spectroscopy, superoxide anion radical (O2−) was certified as the crucial ROS in coupling polyimide-TiO2 hybrid photocatalysis and TEMPO catalysis. This work suggests that hybridization of metal oxide semiconductor with polymer like polyimides is viable to forge visible light photocatalysts.

Copper-Catalyzed Radical Relay for Asymmetric Radical Transformations.

The direct transformation of C-H bonds into diverse functional groups represents one of the most atom- and step-economical strategies for organic synthesis and has received substantial attention over the last few decades. Despite recent advances, asymmetric C-H bond functionalizations are less developed, especially asymmetric oxidations of sp3 C-H bonds. Inspired by enzyme (e.g., P450) catalysis, chemists have made great efforts to develop non-enzymatic systems for enantioselective oxidations of sp3 C-H bonds. However, the involvement of highly reactive radical intermediates makes enantioselective transformations extremely challenging. In this Account, we present our recent studies on the enantioselective induction of prochiral benzylic radicals using a chiral bisoxazoline (Box)/Cu catalytic system. This reaction system was developed on the basis of our extensive studies of copper-catalyzed intermolecular alkene difunctionalizations, such as azidotrifluoromethylations, trifluoromethylcyanations, and trifluoromethylarylations. In these reactions, the proposed catalytic cycle starts from the oxidation of the Cu(I) species by the activated Togni-I reagent (via a Lewis acid/base interaction with a silyl reagent or arylboronic acid) through a single electron transfer process. The generated CF3 radical can efficiently add to the alkene, and the resultant carbon-centered radical is subsequently trapped by an active Cu(II) species bearing a nucleophile (e.g., an N3, CN, or Ar moiety) to form a new C-heteroatom or C-C bond and regenerate the Cu(I) catalyst. Kinetic studies of the trifluoromethylarylation of alkenes support a Cu(I/II/III) catalytic cycle in which the carbon radical reacts with the Cu(II) species to form a highly reactive Cu(III) intermediate and its reductive elimination contributes to the final bond formation. This assumption inspired us to explore asymmetric radical transformations by introducing chiral ligands. Enantioselective cyanations and arylations of benzylic radicals have been demonstrated in the presence of chiral Box/Cu(I) catalysts, and a series of asymmetric difunctionalizations of styrenes have been successfully achieved. In addition, by means of the same benzylic radical trapping process, enantioselective decarboxylative cyanations have been demonstrated using a cooperative photocatalysis and copper catalysis system. Compared with radical addition and decarboxylative processes, hydrogen atom abstraction (HAA) provides direct and facile access to benzylic radicals. By using bisbenzenesulfonimidyl radical for HAA, our group has developed an enantioselective cyanation of benzylic C-H bonds via a copper-catalyzed radical relay, and excellent reactivity and enantioselectivity were achieved in the presence of chiral Box/Cu(I) catalysts. In addition, a regioselective benzylic C-H bond arylation proceeding through a similar process was also developed, providing simple access to 1,1-diarylalkanes. Notably, alkyl arenes were used as the limiting reagent in these reactions, which allowed the late-stage functionalization of sp3 C-H bonds in complex molecules, including natural products, pharmaceuticals, and agrochemicals.

Visible light photocatalysis of dye-sensitized TiO2: The selective aerobic oxidation of amines to imines

Herein, we report an efficient protocol for the selective oxidation of amines with the sole involvement of conduction band electrons (ecb−) of TiO2 for both the activation of O2 and the ensuing oxidation reaction. Visible light was harvested by dye molecules on the surface of TiO2, which injects electrons into its conduction band, generating dye radical cations. Charge transfers from dye radical cations to amines are relayed by the catalytic cycle of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) to TEMPO+, which is an oxidant that can conduct the direct two-electron oxidation of amines to imines. TEMPO was regenerated by the oxidation of TEMPOH with superoxide generated by the interaction of ecb− and O2. This work suggests that one can exploit the strategy of cooperative photocatalysis with each component performing in its optimal role: TiO2 for O2 activation, dye for visible light harvesting, and TEMPO for oxidative transformations.

Visible-light-enhanced Suzuki-Miyaura coupling reaction by cooperative photocatalysis with an Ru-Pd bimetallic complex.

A photo-assisted Suzuki-Miyaura coupling reaction has been realized by a molecule Pd(II) complex containing a Ru(II)-polypyridyl derivative as a visible light photosensitizing unit, where an electron transfer from excited Ru species to Pd forms an active electron rich Pd(0) and facilitates the oxidative addition step with aryl halides.