Jonathan P. Knowles

Trifluoromethyl thianthrenium salts represent valuable reagents in photocatalysis; however, their activation traditionally necessitates external one-electron donors, such as photocatalysts, electron-donor-acceptor complexes. Direct photoexcitation can proceed without exogenous one-electron donors, thereby circumventing the stringent redox potential constraints inherent in the ground and excited states of conventional photocatalytic systems. Such direct excitation facilitates a transition of substrates to an electronically excited state, promoting homolytic cleavage to release the CF ₃ ^• radical. Nevertheless, this process typically requires near-ultraviolet excitation (390 nm) to attain the high-energy excited singlet state of the substrate. In this study, we report a pioneering strategy involving the design of an acid-base ternary ion-pair complex utilising trifluoromethyl thianthrenium salts. In contrast to conventional ion-pair complex catalysis, this methodology eliminates the requirement for a photocatalyst and bypasses the single electron transfer process. This conceptual advancement lowers the activation energy to the excited singlet state of TT-CF ₃ OTf, enabling the generation of CF ₃ ^• species via homolytic cleavage under industrially preferred 450 nm excitation. As a proof of concept, the ion-pair complex was utilised to facilitate the trifluoromethylative difunctionalisation of diverse alkenes. This method yields pharmaceutically significant compounds and natural products with moderate to high yields. Both DFT calculations and experimental results demonstrate the formation of an ion-pairing complex, with binding constants K _a of 4.36 in acetonitrile and 2.28 in chloroform, the latter increasing to 4.20 upon the addition of KOAc. This methodology exhibits substantial potential for future applications within biological and biochemical contexts.

Cinchona alkaloids have played an important part in the history of fluorescence studies and the recent discovery of their long-lived emission makes them a useful tool for exploring new applications in labelling and tagging. Furthermore, they are excellent model systems for exploring the fundamental nature of charge transfer due to the through-space charge transfer between the nitrogen atom on the cage and the quinoline aryl group. However, the synthetic routes and pathways to modification are cumbersome and new synthetic routes are required. Here we demonstrate new synthetic pathways to create a wide range of unnatural quinine analogues giving control over the chromophore point of attachment and N-aryl distance. Furthermore the molecules demonstrate long-lived emission confirming that we are capturing some of the novel photophysical properties of their natural analogues.

Macrocycles and medium-sized rings have myriad uses in various applied fields. Effective and scalable methods for their synthesis are therefore of high value. Ring expansion reactions are important in this context, as they allow larger rings to be “grown” from more easily accessible smaller ring systems, while avoiding inefficient end-to-end cyclization reactions. This review is focused on successive ring expansion strategies, in which two (or more) ring expansion steps have been performed in sequence to enable the synthesis of functionalized macrocycles and medium-sized rings.

The photophysical properties of cinchona alkaloids, particularly their long-lived emission characteristics, make them valuable compounds for photoluminescent applications. However, their synthetic modification remains challenging. This work presents a modular synthetic approach to unnatural quinoline alkaloid analogues, using fluorine substitution of the quinolyl moiety to tune emissive properties while minimising steric effects. Six fluorinated quinoline derivatives were synthesized and photophysical studies across solvents of varying polarity revealed that fluorine substitution enables rational tuning of emission wavelength, as well as strength of charge transfer and photoluminescence quantum yields. Unlike methoxy-substituted analogues, fluorinated compounds exhibited enhanced charge transfer even in apolar media. These findings demonstrate fluorine's utility as a tuning element for optimising photophysical properties while limiting steric demand in conformationally flexible donor-acceptor systems. The results have wide implications for developing luminescent probes and tags.

Cinchona alkaloids have played an important part in the history of fluorescence studies and the recent discovery of their long-lived emission makes them a useful tool for exploring new applications in labelling and tagging. Furthermore, they are excellent model systems for exploring the fundamental nature of charge transfer due to the through-space charge transfer between the nitrogen atom on the cage and the quinoline aryl group. However, the synthetic routes and pathways to modification are cumbersome and new synthetic routes are required. Here we demonstrate new synthetic pathways to create a wide range of unnatural quinine analogues giving control over the chromophore point of attachment and N-aryl distance. Furthermore the molecules demonstrate long-lived emission confirming that we are capturing some of the novel photophysical properties of their natural analogues.

We have shown that palladium-catalyzed cascade processes provide modular access to rigid quinoline-containing tetracyclic amines. This modular approach enables fine-tuning of the through-space charge transfer (TSCT) state formation between the lone pair localized on the nitrogen atom in the cage moiety and the quinoline moiety by variation of both the intramolecular N-aryl distance and quinoline substitution. Decreasing this N-aryl distance enhances the formation of the TSCT species, giving control over the emission color and photoluminescence quantum yield. Methoxylation of the quinoline unit decreases the propensity of TSCT formation. The development of this structure-activity relationship provides great insight for TSCT formation with an impact on further understanding dimeric, excimeric, and exciplex species. This understanding is crucial for the work underpinning their use in biosensor applications, and the conclusions are of relevance to the broader field of photoluminescence.

Sequential oxidative cleavage and double-Mannich reactions enable the stereoselective conversion of simple norbornenes into complex alkaloid-like structures. The products undergo a wide range of derivatization reactions, including regioselective enol triflate formation/cross-coupling sequences and highly efficient conversion to an unusual tricyclic 8,5,5-fused lactam. Overall, the process represents a formal one-atom aza-ring expansion with concomitant bridging annulation, making it of interest for the broader derivatization of alkene feedstocks.

Sequential oxidative cleavage and double-Mannich reactions enable the stereoselective conversion of simple norbornenes into complex alkaloid-like structures. The products undergo a wide range of derivatization reactions, including a regioselec-tive enol triflate formation/cross-coupling sequences and highly efficient conversion to an unusual tricyclic-8,5,5 fused lac-tam. Overall, the process represents a formal 1-atom aza-ring expansion with concomitant bridging annulation, making it of interest for the broader derivatization of alkene feedstocks.

We have shown that Pd-catalysed cascade processes provide molecular access to rigid quinoline-containing tetracylic amines. This enables fine-tuning of the through-space charge transfer (TSCT) state formation by variation of both N-aryl distance and quinoline substitution. It was observed that decreasing the N-aryl distance enhanced the formation of the TSCT species giving control over emission colour and photoluminescence quantum yield. Methoxylation of the quinoline unit decreases the propensity of TSCT formation. This systematic study provides great insight for TSCT formation with impact on further understanding dimeric, excimeric and exciplex species.

Palladium-catalysed allylic amine rearrangements enable the ring expansion of 5- and 6-membered cyclic amines to their azepane and azocane homologues. Reaction conditions are mild and tolerate a range of functionality, including aryl halides.