Terpene cyclases catalyze one of the most powerful transformations with respect to efficiency and selectivity in natural product (bio)synthesis. In such polyene cyclizations, structurally highly complex carbon scaffolds are built by the controlled ring closure of linear polyenes. Thereby, multiple C,C bonds and stereocenters are simultaneously created with high precision. Structural preorganization of the substrate carbon chain inside the active center of the enzyme is responsible for the product- and stereoselectivity of this cyclization. Here, we show that in-situ formed fluorinated-alcohol-amine supramolecular clusters serve as artificial cyclases by triggering enzyme-like reactivity and selectivity by controlling substrate conformation in solution. Because of the dynamic nature of these supramolecular assemblies, a broad range of terpenes can be produced diastereoselectively. Mechanistic studies reveal a finely balanced interplay of fluorinated solvent, catalyst, and substrate as key to establishing nature’s concept of a shape-selective polyene cyclization in organic synthesis.

Project Area C

Upon irradiation in the presence of a chiral benzophenone catalyst (5 mol %), a racemic mixture of a given chiral imidazolidine-2,4-dione (hydantoin) can be converted almost quantitatively into the same compound with high enantiomeric excess (80–99% ee). The mechanism of this photochemical deracemization reaction was elucidated by a suite of mechanistic experiments. It was corroborated by nuclear magnetic resonance titration that the catalyst binds the two enantiomers by two-point hydrogen bonding. In one of the diastereomeric complexes, the hydrogen atom at the stereogenic carbon atom is ideally positioned for hydrogen atom transfer (HAT) to the photoexcited benzophenone. Detection of the protonated ketyl radical by transient absorption revealed hydrogen abstraction to occur from only one but not from the other hydantoin enantiomer. Quantum chemical calculations allowed us to visualize the HAT within this complex and, more importantly, showed that the back HAT does not occur to the carbon atom of the hydantoin radical but to its oxygen atom. The achiral enol formed in this process could be directly monitored by its characteristic transient absorption signal at λ ≅ 330 nm. Subsequent tautomerization leads to both hydantoin enantiomers, but only one of them returns to the catalytic cycle, thus leading to an enrichment of the other enantiomer. The data are fully consistent with deuterium labeling experiments and deliver a detailed picture of a synthetically useful photochemical deracemization reaction.

Project Area A & B

Ligand-to-metal charge transfer (LMCT) photocatalysis allows the activation and synthetic utilization of halides and other heteroatoms in metal complexes. Many metals are known to undergo LMCT but so far remain underutilized in the field of catalysis. A screening assay identifying LMCT activity helped us to expand this catalysis concept to the application of bismuth LMCT in organic radical coupling reactions. We demonstrate its application for the generation of two different radicals (chlorine and carboxyl) in net-oxidative as well as redox-neutral photochemical reactions. Detailed investigation of the model Giese-type coupling revealed BiCl₄^– and BiCl₅^2– as catalytically active bismuth species under 385 nm irradiation. Combined cyclic voltammetry and UV–vis studies gave insight into the reactivity of the highly reactive bismuth(II) catalyst fragment.

Project Area A

Salicylimines are versatile compounds in which an excited-state intramolecular proton transfer and torsional motions may set in upon photoexcitation. Here, we study N-(α-phenylethyl)salicylimine (PESA) to elucidate how the photochemical reaction pathways depend on the excitation wavelength and to what extent the relative photoproduct distribution can be steered towards a desired species. DFT structure and potential energy calculations disclose that the most stable ground-state conformer is an enol species and that the photodynamics may proceed differently depending on the excited state that is reached. With matrix isolation infrared spectroscopy, the predominance of the enol conformer of PESA is confirmed. Illumination of the cryogenic sample with different wavelengths shifts the ratio of enol and keto products, and by sequential irradiation a selective re- and depopulation is possible. Femtosecond transient absorption spectroscopy further reveals that also at room temperature, the outcome of the photoreaction depends on excitation wavelength, and in combination with the calculations, it can be rationalized that the decisive step occurs within the first hundred femtoseconds. Since the ultrafast dynamics mostly match those of similar salicylimines, our findings might also apply to those systems and provide additional insight into their reported sensitivity on excitation energy.

Project Area A

Bicyclo[2.1.1]hexanes have become increasingly popular building blocks in medicinal chemistry as bridged scaffolds that provide unexplored chemical space. We herein report a visible light-driven approach to these compounds that relies on an intramolecular crossed [2 + 2] photocycloaddition of styrene derivatives enabled by triplet energy transfer. Bicyclo[2.1.1]hexanes were obtained in good to high yields (19 examples, 61%-quantitative yield) and allowed for further functionalizations by consecutive reactions, thereby opening different pathways to decorate the aliphatic core structure.

Project Area B

Of the methods for direct fluorination of unactivated C(sp³)–H bonds, photosensitization of SelectFluor is a promising approach. Although many substrates can be activated with photosensitizing catalysts, issues remain that hamper fluorination of complex molecules. Alcohol- or amine-containing functional groups are not tolerated, fluorination regioselectivity follows factors endogenous to the substrate and cannot be influenced by the catalyst, and reactions are highly air-sensitive. We report that benzoyl groups serve as highly efficient photosensitizers which, in combination with SelectFluor, enable visible light-powered direct fluorination of unactivated C(sp³)–H bonds. Compared to previous photosensitizer architectures, the benzoyls have versatility to function both (i) as a photosensitizing catalyst for simple substrate fluorinations and (ii) as photosensitizing auxiliaries for complex molecule fluorinations that are easily installed and removed without compromising yield. Our auxiliary approach (i) substantially decreases the reaction's induction period, (ii) enables C(sp³)–H fluorination of many substrates that fail under catalytic conditions, (iii) increases kinetic reproducibility, and (iv) promotes reactions to higher yields, in shorter times, on multigram scales, and even under air. Observations and mechanistic studies suggest an intimate ‘assembly’ of auxiliary and SelectFluor prior/after photoexcitation. The auxiliary allows other E_nT photochemistry under air. Examples show how auxiliary placement proximally directs regioselectivity, where previous methods are substrate-directed.

A two-step protocol allowing the C−H amination of cyclic ethers with iminoiodinanes, followed by the reduction of the resulting intermediate has been developed for the preparation of amino alcohols. The initial C−H functionalization is accelerated by visible light, improving the reactivity compared to the thermal process performed in the dark. The effect of different substituents on the photochemical reactivity of iminoiodinanes has been studied both experimentally and computationally. Photophysical measurements and DFT calculations were performed to better understand the observed reactivities and corroborate the proposed mechanistic proposal.

Project Area C

A photo-induced Büchner-Curtius-Schlotterbeck type reaction for carbonyl homologation is described. The protocol allows the use of carbonyl compounds as safe and readily available diazo precursors through direct photoexcitation of corresponding N-tosylhydrazone anions. Functionalized aliphatic aldehydes and ketones are prepared in a practical and iterative manner.

Project Area C

In concert with carbonyl compounds, Lewis acids have been identified as a versatile class of photocatalysts. Thus far, research has focused on activation of the substrate, either by changing its photophysical properties or by modifying its photochemistry. In this work, we expand the established mode of action by demonstrating that UV photoexcitation of a Lewis acid–base complex can lead to homolytic cleavage of a covalent bond in the Lewis acid. In a study on the complex of benzaldehyde and the Lewis acid BCl₃, we found evidence for homolytic B–Cl bond cleavage leading to formation of a borylated ketyl radical and a free chlorine atom only hundreds of femtoseconds after excitation. Both time-dependent density functional theory and transient absorption experiments identify a benzaldehyde-BCl₂ cation as the dominant species formed on the nanosecond time scale. The experimentally validated B–Cl bond homolysis was synthetically exploited for a BCl₃-mediated hydroalkylation reaction of aromatic aldehydes (19 examples, 42–76% yield). It was found that hydrocarbons undergo addition to the C═O double bond via a radical pathway. The photogenerated chlorine radical abstracts a hydrogen atom from the alkane, and the resulting carbon-centered radical either recombines with the borylated ketyl radical or adds to the ground-state aldehyde-BCl₃ complex, releasing a chlorine atom. The existence of a radical chain was corroborated by quantum yield measurements and by theory. The photolytic mechanism described here is based on electron transfer between a bound chlorine and an aromatic π-system on the substrate. Thereby, it avoids the use of redox-active transition metals.

Project Area B & C

A dual photocatalytic protocol was developed to generate acyl radicals from readily available aldehydes via hydrogen atom transfer (HAT). Synergistic cooperation, being supported by DFT studies, between earth-abundant iron(III)chloride and 9,10-diphenylanthracene (DPA) to activate the aldehyde for a HAT step proved to be an efficient, economic, and green route for the hydroacylation of electron-deficient alkenes under UV-light irradiation with broad functional group compatibility. This methodology can be conveniently scaled up and applied to produce valuable materials from renewable feedstock chemicals.

Project Area A

The controllable divergent reactivity of 1,3-dicarbonyls is described, which enables the efficient hydro- and oxoalkylation of vinyl arenes. Both reaction pathways are initiated through the formation of polarity-reversed C-centered-radical intermediates at the active methylene center of 1,3-dicarbonyls via direct photocatalytic C–H bond transformations. The oxoalkylation of alkenes is achieved under aerobic conditions via a Cu(II)-photomediated rebound mechanism, while the corresponding hydroalkylation becomes possible under a nitrogen atmosphere by the combination of 4CzIPN and a Brønsted base. The breadth of these divergent protocols is demonstrated in the late-stage modification of drugs and natural products and by the transformation of the products to a variety of heterocycles such as pyridines, pyrroles, or furans. Moreover, the two catalytic modes can be combined synergistically for the stereoselective construction of cyclopentanol derivatives in a formal [4+1]-annulation process.

Project Area A

Chemical bond activations mediated by H-bond interactions involving highly electronegative elements such as nitrogen and oxygen are powerful tactics in modern catalysis research. On the contrary, kindred catalytic regimes in which heavier, less electronegative elements such as selenium engage in H-bond interactions to co-activate C−Se σ-bonds under oxidative conditions are elusive. Traditional strategies to enhance the nucleofugality of selenium residues predicate on the oxidative addition of electrophiles onto Se^II-centers, which entails the elimination of the resulting Se^IV moieties. Catalytic procedures in which Se^IV nucleofuges are substituted rather than eliminated are very rare and, so far, not applicable to carbon-carbon bond formations. In this study, we introduce an unprecedented combination of O−H⋅⋅⋅Se H-bond interactions and single electron oxidation to catalytically generate Se^III nucleofuges that allow for the formation of new C−C σ-bonds by means of a type I semipinacol process in high yields and excellent selectivity.

Project Area A & B

Cascade (domino) reactions facilitate the formation of complex molecules from simple starting materials in a single operation. It was found that 1-naphthaldehyde derivatives can be converted to enantioenriched (82–96% ee) polycyclic benzoisochromenes via a cascade of ortho photocycloaddition and ensuing acid-catalysed rearrangement reactions. The cascade was initiated by irradiation with visible light (λ = 457 nm) and catalysed by a chiral AlBr₃-activated 1,3,2-oxazaborolidine (14 examples, 65–93% yield). The absolute configuration of the products was elucidated by single crystal X-ray crystallography. Mechanistic experiments suggest that the ortho photocycloaddition occurs on the triplet hypersurface and that the chiral catalyst induces in this step the observed enantioselectivity.

Project Area B

Endergonic photocatalysis is the use of light to perform catalytic reactions that are thermodynamically unfavourable. While photocatalysis has become a powerful tool in facilitating chemical transformations, the light-energy efficiency of these processes has not gathered much attention. Exergonic photocatalysis does not take full advantage of the light energy input, producing low-energy products and heat, whereas endergonic photocatalysis incorporates a portion of the photon energy into the reaction, yielding products that are higher in free energy than the reactants. Such processes can enable catalytic, atom-economic syntheses of reactive compounds from bench-stable materials. With respect to environmental friendliness and carbon neutrality, endergonic photocatalysis is also of interest to large-scale industrial manufacturing, where better energy efficiency, less waste and value addition are highly sought. We therefore assess here the thermochemistry of several classes of reported photocatalytic transformations to showcase current advances in endergonic photocatalysis and point to their industrial potential.

Project Area C

We report the iridium–nickel dual photocatalytic acceptorless and redox neutral dehydrogenation of aliphatic heterocycles yielding cyclic alkenes without overoxidation at room temperature. Excitation of the iridium photocatalyst initiates the formation of a nickel hydride intermediate that yields alkenes and H₂ via β-hydride elimination. The reaction proceeds regioselectively and the scope was demonstrated by the synthesis of 12 biologically relevant molecules and drugs. In addition, commercially and easily available N-heterocyclic alkane starting materials were converted into functionalized alkenes of high synthetic and commercial value using the method.

Project Area C

Visible light excitation of a hypervalent iodine (III)-BF₃ complex enables the formation of carbocations from C(sp³)–H bonds. The complex is generated catalytically from a iodoarene, a carboxylate ligand, an oxidizing agent (Selectfluor) and the Lewis acid BF₃. This modular catalytic system allowed us to conveniently screen performance of different in situ formed complexes. Their excitation leads to hydrogen abstraction from the C(sp³)–H precursor which is followed by a rapid oxidation of the formed carbon radical to give carbocationic intermediates. These were reacted in a Ritter-type amination to give access to synthetically valuable amine derivatives. Moreover, the method is practically simple and based on readily available chemicals.

Project Area C

Reduced density matrix functional theory (RDMFT), a promising direction in the problem of describing strongly correlated systems, is currently limited by its explicit dependence on natural orbitals and, by extension, the costly need to construct two-electron integrals in the molecular orbital basis. While a resolution-of-the-identity approach can reduce the asymptotic scaling behavior from O(N⁵) to O(N⁴), this is still prohibitively expensive for large systems, especially considering the usually slow convergence and the resulting high number of orbital optimization steps. In this work, efficient integral-direct methods are derived and benchmarked for various approximate functionals. Furthermore, we show how these integral-direct methods can be integrated into existing self-consistent energy minimization frameworks in an efficient manner, including improved methods for calculating diagonal elements of the two-electron integral tensor as required in self-interaction-corrected functionals and second derivatives of the energy with respect to the occupation numbers. In combination, these methods provide speedups of up to several orders of magnitude while greatly diminishing memory requirements, enabling the application of RDMFT to large molecular systems of general chemical interest, such as the challenging triplet–quintet gap of the iron(II) porphyrin complex.

Project Area C

Cross coupling: A practical dual Ni/photoredox catalyzed cross-electrophile coupling of organic halides by employing readily available α-amino radicals as halogen atom transfer agents is reported. Both coupling partners can be flexibly chosen as limiting reactant to forge 6 different types of C−C bond for a broad scope of applications (>70 examples) including late-stage functionalizations of pharmaceutical compounds. The reaction is scalable in batch and continuous flow.

Cyclohept-2-enone undergoes isomerization upon irradiation with UV light to its (E)-isomer, which exhibits significant ring strain. XMS-CASPT2 calculations revealed that this process can proceed on both the singlet and triplet hypersurface. (E)-Cyclohept-2-enone can undergo a Diels-Alder reaction with different dienes at room temperature, yielding trans-fused six-membered rings (up to 98% yield). In case of cyclic dienes, the exo-product was the major isomer in most cases. The Diels-Alder reaction with furan was studied in greater detail. Experimental and theoretical outcome via DLPNO-CCSD(T) calculations matched and the relative configuration of the diastereomers was corroborated via X-ray analysis. Other cyclic enones were also successfully employed. In the end, the method was applied to the total synthesis of racemic trans-α-himachalene.

Project Area B & C

Flavoenzymes are involved in a multitude of chemical reactions and the cofactor can act as the catalytically active species in different oxidation states. While (photo)oxidation reactions with molecular flavins are established, no report of reduced, molecular flavins in the conversion of organic substrates are available. We report a catalytic method using reduced, molecular flavins as photoreductants and γ-terpinene as sacrificial reductant. A design for air-stable, reduced flavins using a conformational bias strategy is presented and circumvents rapid reduction of O₂ from air. Using our catalytic strategy, we were able to perform a 5-exo-trig cyclization of barbituric acid derivatives, effectively replacing SmI₂ as a reductant for this reaction. Such flavin catalyzed reductions are anticipated to be beneficial for other transformations and other reactivities are highlighted in this publication.

Project Area B

Copper-catalyzed [3+2] cycloadditions of N-tosylcyclopropylamine with alkynes and alkenes have been accomplished under visible light irradiation. The developed approach is compatible with a range of functionalities and allows the synthesis of diversified aminated cyclopentene and cyclopentane derivatives being relevant for drug synthesis. The protocol is operationally simple and economically affordable as it does not require any ligand, base, or additives. As the key step, the one-electron oxidation of the N-tosyl moiety by visible light-induced homolysis of a transient Cu(II)-tosylamide complex is proposed, providing a facile entry for N-centered radicals.

Project Area A

Atom transfer radical addition (ATRA) reactions are linchpin transformations in synthetic chemistry enabling the atom-economic difunctionalization of alkenes. Thereby a rich chemical space can be accessed through smart combinations of simple starting materials. Originally, these reactions required toxic and hazardous radical initiators or harsh thermal activation and thus, the recent resurgence and dramatic evolution of photocatalysis appeared as an attractive complement to catalyze such transformations in a mild and energy-efficient manner. Initially, this technique relied primarily on complexes of precious metals, such as ruthenium or iridium, to absorb the visible light. Hence, copper photocatalysis rapidly developed into a powerful alternative, not just from an economic point of view. Originally considered to be disadvantageous as a pathway for deactivation by quenching their excited state, the dynamic nature of Cu-complexes enables them to undergo facile ligand exchange and thus opens up special opportunities for transformations utilizing their inner-coordination sphere. Moreover, the ability of Cu(II), representing a persistent radical, to capture incipient radicals offers the possibility to access heretofore elusive two-component, but also three-component, ATRA reactions, not feasible with ruthenium or iridium catalysts. In this regard, the idea of using Cu(I)-substrate assemblies as active photocatalysts is an emerging field to achieve such 3-component coupling reactions even under enantioselective control, which is reflected by an increasing number of reports being covered in this review.

Project Area A

Showcasing the concept of light-induced homolysis for the generation of radicals, the Cu^II-photocatalyzed decarboxylative oxygenation of carboxylic acids with molecular oxygen as the terminal oxidant is described. Two Cu^II-carboxylate complexes with different coordination geometries were synthesized and characterized by X-ray analysis, correlating their structure with their ability to initiate light-induced decarboxylations.

Project Area A

The inherent aromaticity of aromatic compounds, especially unactivated aromatic compounds, render them challengeable to undergo dearomatization process. We report here the dearomatizative coupling of benzylmalonates and alkynes to spiroanellated 1,4-cyclohexadienes, which proceeds via activation with visible light with perfect atom economy in high yields (up to 98%). In this protocol, no preactivation of the substrates or employment of stoichiometric high energy reagents, being the traditional approach to drive thermoneutral transformations, is necessary. Calculations suggest that the overall process is approximately thermoneutral, showcasing the special opportunities of light-driven processes to develop sustainable transformations that defy thermodynamic requirements.

Project Area A