Aluminum(III) Salen Complexes as Active Photoredox Catalysts

Biomimetic Chemical Reactions with Natural Products Using Metalloporphyrins and Salen Complexes as Catalysts: a Brief Review

The cytochrome P450 is a superfamily of hemoproteins mainly present in the liver and are versatile biocatalysts. They participate in the primary metabolism and biosynthesis of various secondary metabolites. Chemical catalysts are utilized to replicate the activities of enzymes. Metalloporphyrins and Salen complexes can contribute to the products' characterization and elucidate biotransformation processes, which are investigated during pre-clinical trials. These catalysts also help discover biologically active compounds and get better yields of products of industrial interest. This review aims to investigate which natural product classes are being investigated by biomimetic chemical models and the functionalities applied in the use of these catalysts. A limited number of studies were observed, with terpenes and alkaloids being the most investigated natural product classes. The research also revealed that Metalloporphyrins are still the most popular in the studies, and the identity and yield of the products obtained depend on the reaction system conditions.

Visible-Light Activation of Diorganyl Bis(pyridylimino) Isoindolide Aluminum(III) Complexes and Their Organometallic Radical Reactivity

We report on the synthesis and characterization of a series of (mostly) air-stable diorganyl bis(pyridylimino) isoindolide (BPI) aluminum complexes and their chemistry upon visible-light excitation. The redox non-innocent BPI pincer ligand allows for efficient charge transfer homolytic processes of the title compounds. This makes them a universal platform for the generation of carbon-centered radicals. The photo-induced homolytic cleavage of the Al-C bonds was investigated by means of stationary and transient UV/Vis spectroscopy, spin trapping experiments, as well as EPR and NMR spectroscopy. The experimental findings were supported by quantum chemical calculations. Reactivity studies enabled the utilization of the aluminum complexes as reactants in tin-free Giese-type reactions and carbonyl alkylations under ambient conditions, which both indicated radical-polar crossover behavior. A deeper understanding of the physical fundamentals and photochemical process was provided, furnishing in turn a new strategy to control the reactivity of bench-stable aluminum organometallics.

Light-enabled deracemization of cyclopropanes by Al-salen photocatalysis

Privileged chiral catalysts-those that share common structural features and are enantioselective across a range of reactions-continue to transform the chemical-research landscape1. In recent years, new reactivity modes have been achieved through excited-state catalysis, processes activated by light, but it is unclear if the selectivity of ground-state privileged catalysts can be matched. Although the interception of photogenerated intermediates by ground-state cycles has partially addressed this challenge2, single, chiral photocatalysts that simultaneously regulate reactivity and selectivity are conspicuously scarce3. So far, precision donor-acceptor recognition motifs remain crucial in enantioselective photocatalyst design4. Here we show that chiral Al-salen complexes, which have well-defined photophysical properties, can be used for the efficient photochemical deracemization5 of cyclopropyl ketones (up to 98:2 enantiomeric ratio (e.r.)). Irradiation at λ = 400 nm (violet light) augments the reactivity of the commercial catalyst to enable reactivity and enantioselectivity to be regulated simultaneously. This circumvents the need for tailored catalyst-substrate recognition motifs. It is predicted that this study will stimulate a re-evaluation of many venerable (ground-state) chiral catalysts in excited-state processes, ultimately leading to the identification of candidates that may be considered 'privileged' in both reactivity models.

Group 13 salphen compounds (In, Ga and Al): a comparison of their structural features and activities as catalysts for cyclic carbonate synthesis

Many complexes based on group 13 elements have been successfully applied as catalysts for the synthesis of cyclic carbonates from epoxides and CO₂ and to date these have provided some of the most active catalysts developed. It is notable that most reports have focused on the use of aluminium-based compounds likely because of the well-established Lewis acidity of this element and its cost. In comparison, relatively little attention has been paid to the development of catalysts based on the heavier group 13 elements, despite their known Lewis acidic properties. This study describes the synthesis of aluminium, gallium and indium compounds supported by a readily prepared salphen ligand and explores both their comparative structures and also their potential as catalysts for the synthesis of cyclic carbonates. In addition, the halide ligand which forms a key part of the compound has been systematically varied and the effect of this change on the structure and catalytic activity is also discussed. It is demonstrated that the indium compounds are actually, and unexpectedly, the most active for cyclic carbonate synthesis, despite their lower Lewis acidity when compared to their aluminium congeners. The experimental observations from this work are fully supported by a Density Functional Theory (DFT) study, which provides important insights into the reasons as to why the indium catalyst with bromide, [InBr(salphen)], is most active.

Catalytic photochemical enantioselective α-alkylation with pyridinium salts

We have developed a chiral amine catalyzed enantioselective α-alkylation of aldehydes with amino acid derived pyridinium salts as alkylating reagents. The reaction proceeds in the presence of visible light and in the absence of a photocatalyst via a light activated charge-transfer complex. We apply this photochemical stereoconvergent process to the total synthesis of the lignan natural products (-)-enterolactone and (-)-enterodiol. Mechanistic studies support the ground-state complexation of the reactive components followed by divergent charge-transfer processes involving catalyst-controlled radical chain and in-cage radical combination steps.

Homoleptic Al(III) Photosensitizers for Durable CO2 Photoreduction

Exploiting noble-metal-free systems for high-performance photocatalytic CO₂ reduction still presents a key challenge, partially due to the long-standing difficulties in developing potent and durable earth-abundant photosensitizers. Therefore, based on the very cheap aluminum metal, we have deployed a systematic series of homoleptic Al(III) photosensitizers featuring 2-pyridylpyrrolide ligands for CO₂ photoreduction. The combined studies of steady-state and time-resolved spectroscopy as well as quantum chemical calculations demonstrate that in anerobic CH₃CN solutions at room temperature, visible-light excitation of the Al(III) photosensitizers leads to an efficient population of singlet excited states with nanosecond-scale lifetimes and notable emission quantum yields (10-40%). The results of transient absorption spectroscopy further identified the presence of emissive singlet and unexpectedly nonemissive triplet excited states. More importantly, the introduction of methyl groups at the pyrrolide rings can greatly improve the visible-light absorption, reducing power, and durability of the Al(III) photosensitizers. With triethanolamine, BIH (1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole), and an Fe(II)-quaterpyridine catalyst, the most methylated Al(III) photosensitizer achieves an apparent quantum efficiency of 2.8% at 450 nm for selective (>99%) CO₂-to-CO conversion, which is nearly 28 times that of the unmethylated one (0.1%) under identical conditions. The optimal system realizes a maximum turnover number of 10250 and higher robustness than the systems with Ru(II) and Cu(I) benchmark photosensitizers. Quenching experiments using fluorescence spectroscopy elucidate that the photoinduced electron transfer in the Al(III)-sensitized system follows a reductive quenching pathway. The remarkable tunability and cost efficiency of these Al(III) photosensitizers should allow them as promising components in noble-metal-free systems for solar fuel conversion.

Mechanosynthesis and photophysics of colour-tunable photoluminescent group 13 metal complexes with sterically demanding salen and salophen ligands

A series of four photoluminescent Al and In complexes were synthesised using an environmentally-benign mechanosynthesis strategy. Sterically crowded 3,5-di-tert-butyl functionalised salophen and salen ligands and their respective complexes have been synthesised in the solid-state and fully characterised. Subsequent photophysics and electrochemistry studies of the resulting complexes suggest that these new group 13 complexes can be viable alternatives to traditional photoluminescent complexes based on expensive and low abundant noble metals. The herein-reported strategy avoids the use of organic solvents and provides a process with low environmental impact and enhanced energy efficiency.

Synergistic Strategies in Aminocatalysis

Synergistic catalysis offers the unique possibility of simultaneous activation of both the nucleophile and the electrophile in a reaction. A requirement for this strategy is the stability of the active species towards the reaction conditions and the two concerted catalytic cycles. Since the beginning of the century, aminocatalysis has been established as a platform for the stereoselective activation of carbonyl compounds through HOMO‐raising or LUMO‐lowering. The burgeoning era of aminocatalysis has been driven by a deep understanding of these activation and stereoinduction modes, thanks to the introduction of versatile and privileged chiral amines. The aim of this review is to cover recent developments in synergistic strategies involving aminocatalysis in combination with organo‐, metal‐, photo‐, and electro‐catalysis, focusing on the evolution of privileged aminocatalysts architectures.

Cooperative Heterometallic Catalysts for Lactide Ring-Opening Polymerization: Combining Aluminum with Divalent Metals

While homometallic (salen)Al catalysts display excellent performance in lactide ring-opening polymerization (ROP), heterometallic (salen)Al complexes have yet to be reported. Herein, we describe four heterobimetallic (salen)Al catalysts and show that the choice of the heterometal is key. Cooperative Al/Mg and Al/Zn combinations improved the catalyst activity by a factor of up to 11 compared to the mono-Al analogue, whereas the mono-Mg and mono-Zn analogues were completely inactive. In contrast, Al/Li and Al/Ca heterocombinations stunted the polymerization rate. Kinetic and computational studies suggest that Al/Mg and Al/Zn cooperativity arises from the close intermetallic proximity facilitating chloride bridging (thus enhancing initiation), which promotes a rigid square pyramidal geometry around the Al center and further increases the available monomer coordination sites. This work also translates the use of ab initio molecular dynamics calculations to ROP, introducing a useful method of investigating catalyst flexibility and revealing that ligand strain and molecular rigidity can enhance heterometallic catalyst performance.

Metallaphotoredox catalysis with organic dyes

Here…comes the fun…Combination of metals and organic photocatalysts allows the practical invention of new methodologies!

Syntheses of new chiral chimeric photo-organocatalysts

A new family of chiral chimeric photo-organocatalysts derived from phosphoric acid were synthesized and their spectroscopic and electrochemical properties were investigated. Then, the ability of these photo-activable molecules to catalyse an asymmetric tandem electrophilic β-amination of enecarbamates was evaluated.

A new family of chimeric chiral photocatalysts in which a BINOL derived phosphoric acid embeds one or two photosensitizer dyes was prepared. We have demonstrated their ability to catalyse an enantioselective electrophilic amination reaction.

A supramolecular bifunctional iridium photoaminocatalyst for the enantioselective alkylation of aldehydes

The construction of a hybrid metal-organo-photoredox catalyst based on the conjugation of an imidazolidinone organocatalyst and Ir(ppy)2(bipy) (ppy = 2-phenylpyridine, bipy = bipyridine) is described. The introduction of the desired organocatalyst into the bipyridine moiety is quite modular, allowing the preparation of different hybrid photocatalysts, and is realized though a simple click reaction. The hybrid photocatalysts obtained were employed in the benchmark photoredox alkylation of aldehydes. Remarkably, the conjugation of a first-generation MacMillan catalyst produces an active and stereoselective hybrid photoredox catalyst.

Sulphonic acid functionalized porphyrin anchored with a meso-substituted triazolium ionic liquid moiety: a heterogeneous photo-catalyst for metal/base free C–C cross-coupling and C–N/C–H activation using aryl chloride under visible light irradiation

We report an easy process to synthesize sulphonic acid functionalized porphyrin with meso-substituted triazolium ionic liquid moiety for metal/base free C–C cross-coupling and C–N/C–H activation using aryl chloride under visible light irradiation.