Photoactive Metal-Organic Frameworks for the Selective Synthesis of Thioethers: Coupled with Phosphine to Modulate Thiyl Radical Generation

Reticular framework materials for photocatalytic organic reactions

Photocatalytic organic reactions, harvesting solar energy to produce high value-added organic chemicals, have attracted increasing attention as a sustainable approach to address the global energy crisis and environmental issues. Reticular framework materials, including metal-organic frameworks (MOFs) and covalent organic frameworks (COFs), are widely considered as promising candidates for photocatalysis owing to their high crystallinity, tailorable pore environment and extensive structural diversity. Although the design and synthesis of MOFs and COFs have been intensively developed in the last 20 years, their applications in photocatalytic organic transformations are still in the preliminary stage, making their systematic summary necessary. Thus, this review aims to provide a comprehensive understanding and useful guidelines for the exploration of suitable MOF and COF photocatalysts towards appropriate photocatalytic organic reactions. The commonly used reactions are categorized to facilitate the identification of suitable reaction types. From a practical viewpoint, the fundamentals of experimental design, including active species, performance evaluation and external reaction conditions, are discussed in detail for easy experimentation. Furthermore, the latest advances in photocatalytic organic reactions of MOFs and COFs, including their composites, are comprehensively summarized according to the actual active sites, together with the discussion of their structure-property relationship. We believe that this study will be helpful for researchers to design novel reticular framework photocatalysts for various organic synthetic applications.

Tuning Selectivity in the Visible-Light-Promoted Coupling of Thiols with Alkenes by EDA vs TOCO Complex Formation

The visible-light-promoted catalyst-free condition has been demonstrated for self- and cross-coupling reactions of thiols in an ambient atmosphere. Further, synthesis of β-hydroxysulfides is accomplished under very mild conditions involving the formation of an electron donor-acceptor (EDA) complex between a disulfide and an alkene. However, the direct reaction of thiol with alkene via the formation of a thiol-oxygen co-oxidation (TOCO) complex failed to produce the desired compounds in high yields. The protocol was successful with several aryl and alkyl thiols for the formation of disulfides. However, the formation of β-hydroxysulfides required an aromatic unit on the disulfide fragment, which supports the formation of the EDA complex during the course of the reaction. The approaches presented in this paper for the coupling reaction of thiols and the synthesis of β-hydroxysulfides are unique and do not require toxic organic or metal catalysts.

Anthraquinone-Based Metal-Organic Frameworks as a Bifunctional Photocatalyst for C-H Activation

Metal-organic frameworks (MOFs) have gained attention as multifunctional catalytic platforms, allowing us to gain important insights into synergistically activating both C-H bonds and oxygen for improving oxidation. Herein, by ingenious incorporation of anthraquinone, we report an anthraquinone-based MOF as a bifunctional heterogeneous photocatalytic platform to simultaneously activate inert C(sp³)-H bonds and oxygen for C-H bond oxidation. Making use of the rigid framework with the fixation and isolation effect, both a great chemical stability and bifunctional synergistic photocatalytic effects were obtained through the immobilization of anthraquinone into a MOF. Importantly, while decorating two carboxyl groups on anthraquinone, the carbonyl groups of anthraquinone photosensitizers were not involved in coordinating the self-assembly and orderly arranged on the wall of channels that were constructed through a π-π interaction between the anthraquinone moieties in the adjacent layers, which was beneficial to form and stabilize the excited-state radical intermediates in the molecule-fenced channels, and the close proximity between the catalytic sites and the substrates to abstract a hydrogen atom from the substrate through the hydrogen atom transfer process aimed at activating the inertness of C-H bonds. Moreover, high-density-distributed anthraquinone dyes in the confined channels would activate oxygen to form singlet oxygen (¹O₂) through an energy transfer pathway, further promoting inert C(sp³)-H bond oxidation efficiency. Under visible light irradiation, this anthraquinone-based MOF was successfully applied to explore activation and oxidation of a series of substrates containing benzylic C(sp³)-H bonds in the presence of air or oxygen to produce the corresponding carbonyl products. This bifunctional photocatalytic platform based on a heterogeneous MOF provides an available catalytic avenue to develop a scalable and sustainable synthetic strategy using green and sustainable oxygen as the potent oxidant.

Post-Synthetic Modification of Metal-Organic Frameworks Bearing Phenazine Radical Cations for aza-Diels-Alder Reactions

Metal-organic frameworks (MOFs) consisting of organic radicals are of great interest because they have exhibited unique and intriguing optical, electronic, magnetic, and chemo-catalytic properties, and thus have demonstrated great potential applications in optical, electronic, and magnetic devices, and as catalysts. However, the preparation of MOFs bearing stable organic radicals is very challenging because most organic radicals are highly reactive and difficult to incorporate into the framework of MOFs. Herein we reported a post-synthetic modification strategy to prepare a novel MOF containing phenazine radical cations, which was used as heterogeneous catalyst for aza-Diels-Alder reaction. The zinc-based metal-organic framework Zn₂ (PHZ)₂ (dabco) (N) was successfully synthesized from 5,10-di(4-benzoic acid)-5,10-dihydrophenazine (PHZ), triethylene diamine (dabco) with Zn(NO₃ )₂  ⋅ 6H₂ O by solvothermal method. The as-synthesized MOF N was partially oxidized by AgSbF₆ to form MOF R containing ∼10% phenazine radical cation species. The resultant MOF R was found to keep the original crystal type of N and very persistent under ambient conditions. Consequently, MOF R was successfully employed in radical cation-catalyzed aza-Diels-Alder reactions with various imine substrates at room temperature with high reaction conversion. Moreover, heterogeneous catalyst MOF R was reusable up to five times without much loss of catalytic activity, demonstrating its excellent stability and recyclability. Therefore, the post-synthetic modification developed in this work is expected to become a versatile strategy to prepare radical-based MOFs for the application of heterogeneous catalysts in organic synthesis.

Synthesis of vinyl sulfides and thioethers via a hydrothiolation reaction of 4-hydroxydithiocoumarins and arylacetylenes/styrenes

The synthesis of vinyl sulfides (3a-m) and thioethers (7a-k), exclusive Markovnikov products, is reported by a copper(I) iodide catalyzed regioselective hydrothiolation reaction of terminal alkynes/alkenes and 4-hydroxydithiocoumarins. However, anti-Markovnikov hydrothiolation products (5a-f) were obtained in the case of 2-ethynylpyridine, with exclusive Z selectivity in good yields. The important aspects of this protocol are the absence of expensive metal complexes and additives to act as ligands, mild reaction conditions, high regioselectivity, good yields, and shorter reaction times.