A systematic, user‐friendly assessment tool that delivers a clear overview of the sensitivity of reactions to key parameters is highly desirable. Herein, the development of such a method is ...described. The intuitive, standardized presentation of the results in a radar diagram enables the sensitivity of a protocol to be rapidly assessed. This method was applied to five different visible‐light‐mediated photochemical reactions, and the results were correlated to the underlying mechanism. Ultimately, we believe that this assessment will help to increase the uptake of new synthetic methods and their reproducibility.
Goldilocks and the sensitivity screen: This work introduces a standardized, systematic, and user‐friendly tool to gain valuable information on the sensitivity of a reaction, with the aim of enhancing reproducibility and supporting troubleshooting.
The concept of reductive radical-polar crossover (RRPCO) reactions has recently emerged as a valuable and powerful tool to overcome limitations of both radical and traditional polar chemistry. ...Especially in case of additions to carbonyl compounds, the synergy of radical and polar pathways is of great advantage since it enables the use of traditional carbonyl electrophiles in radical reactions. The most recent and synthetically important transformations following this line are summarised in the first part of this review. The second part deals with transformations, in which the concept of RRPCO promotes the usage of alkyl halides as electrophiles in radical reactions.
The concept of reductive radical-polar crossover (RRPCO) reactions has recently emerged as a valuable and powerful tool to overcome limitations of both radical and traditional polar chemistry.
Herein, we present a novel strategy for the utilization of simple carbonyl compounds, aldehydes and ketones, as intermolecular radical acceptors. The reaction is enabled by visible light photoredox ...initiated hole catalysis and the in situ Brønsted acid activation of the carbonyl compound. This regioselective alkyl radical addition reaction does not require metals, ligands or additives and proceeds with a high degree of atom economy under mild conditions. The proposed mechanism is supported by both experimental and theoretical studies.
Described herein is a new and straightforward decarboxylative di‐ and trifluoromethylthiolation of alkyl carboxylic acids promoted by visible light. This approach enables the synthesis of ...biologically relevant alkyl SCF2H and SCF3 compounds from cheap and abundant carboxylic acids. The method is operationally simple, using irradiation from household light sources, and its mild reaction conditions make it tolerant of a range of functional groups. The strategy employs electrophilic phthalimide‐derived di‐ and trifluoromethylthiolation reagents and exploits the ability of the imidyl radical to carry a radical chain.
Visible light‐promoted decarboxylative di‐ and trifluoromethylthiolation of 1°, 2°, and 3° alkyl carboxylic acids has been developed. The strategy employs electrophilic phthalimide‐derived di‐ and trifluoromethylthiolation reagents, with mechanistic studies suggesting the reaction involves a photoinitiated radical chain. The method features mild conditions, broad substrate scope and does not require additional oxidants.
A visible‐light‐mediated approach to carbonyl–olefin cross‐metathesis is described. Photoinduced hole catalysis was used to promote the formation of 1,3‐diols from aldehydes and styrenes, which were ...then readily fragmented under acidic conditions to form the cross‐metathesis products. The use of 1,3‐diols as intermediates, rather than the energetically more demanding oxetanes, provides a new, orthogonal mechanistic strategy for carbonyl–olefin cross‐metathesis. Furthermore, this approach does not require any metals, ligands, or additives, and provides the products with high levels of E selectivity. A mechanistic rationale is provided and supported by both theoretical calculations and experiments. Additionally, a practical synthesis of a new acridinium‐based photocatalyst, including full characterization, is presented.
XOXO: A carbonyl–olefin cross‐metathesis (COCM) is described. The method proceeds through a visible‐light‐induced hole‐catalyzed 1,3‐diol formation and a subsequent acid‐induced Grob‐type 1,3‐diol fragmentation. This strategy is orthogonal to existing COCM methods since, in contrast to difficult‐to‐access oxetanes, the key intermediates, 1,3‐diols, are readily formed.
Versatile substituted electron-deficient trichloromethylarenes can easily be synthesized and combined with a Safranine O/triarylalkylborate salt to form a highly efficient three-component ...photo-initiation system that starts free radical polymerization to finally form holographic gratings with a single-pulsed laser. The mechanism of this photo-initiation most likely relies on an electron transfer from the borate salt into the semi-occupied HOMO of the excited dye molecule Safranine O, which after fragmentation generates an initiating alkyl radical and longer-lived dye radical species. This dye radical is most probably oxidized by the newly introduced trichloromethylarene derivative as an electron acceptor. The two generated radicals from one absorbed photon initiate the photopolymerization and form index gratings in a suitable holographic recording material. This process is purely photonic and does not require further non-photonic post treatments.
Harnessing visible light to access excited (triplet) states of organic compounds can enable impressive reactivity modes. This tutorial review covers the photophysical fundamentals and most ...significant advances in the field of visible-light-mediated energy transfer catalysis within the last decade. Methods to determine excited triplet state energies and to characterize the underlying Dexter energy transfer are discussed. Synthetic applications of this field, divided into four main categories (cyclization reactions, double bond isomerizations, bond dissociations and sensitization of metal complexes), are also examined.
Herein, we describe a photoinitiated and regioselective synthesis of 2-substituted indoles under mild reaction conditions. This biologically privileged scaffold was accessed in good yields from ...N-aroylbenzotriazoles, a quencher class previously identified using our mechanism-based luminescence screening, and terminal alkynes in the presence of a photocatalyst and blue light irradiation. This straightforward protocol displays a broad substrate scope and functional group tolerance. Furthermore, the mildness and robustness of the reaction were assessed by the application of an additive-based robustness screen. The determination of the reaction quantum yield and Stern–Volmer studies support the proposed photoinitiated radical chain mechanism.
A new highly reactive photoinitiating system based on a penta-methine cyanine dye and a triarylalkylborate salt for radical photopolymerization reactions using near infra-red (NIR) light was ...investigated. The remarkably high reactivity is partly ascribed not only to the photochemical properties of the dye such as its very high absorption coefficients but also to the combination of the dye with the borate salt. Interestingly, the correlation between the borate salt concentration and the rate of polymerization can be displayed in a saturation graph. This effect is presumably caused by the formation of an ion-pair complex consisting of the cationic dye and anionic borate molecule in a medium of moderate polarity. It is shown that the association constant of such ion-pairs is quite high and, due to the short lifetime of the excited state of the dye, it is the only possible route for the generation of initiating radicals after excitation. Finally, it is found that the rate of polymerization correlates with the calculated rate of initiation, giving confidence to the proposed initiation mechanism. Advantageously, the high sensitivity of the new photoinitiating system allows for the use of a low LED light power of 40 mW cm
−2
and thus supports a truly photonic mechanism of initiation instead of a thermal one.
A highly sensitive NIR photoinitiating system based on dye/borate associated pairs.
Three new visible‐light‐promoted functionalizations of benzotriazole substrates were discovered using a mechanism‐based screening method. ortho‐Thiolated, borylated, and alkylated N‐arylbenzamide ...products were obtained under mild reaction conditions in a new denitrogenative synthetic approach to functionalized aniline derivatives. The functional group tolerance of the borylation reaction was further analyzed in the first application of an additive‐based robustness screen in a photocatalytic transformation. All the functionalizations proceed via photocatalytically initiated chain mechanisms as indicated by determination of the reaction quantum yields and Stern–Volmer analyses.
Trinity! Three visible‐light‐promoted functionalization reactions of benzotriazole substrates were discovered using a mechanism‐based screening method and provided ortho‐thiolated, ‐borylated, and ‐alkylated N‐arylbenzamide products. The functional group tolerance of the borylation reaction was further analyzed by the first application of an additive‐based robustness screen in a photocatalytic transformation.
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