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Summary C–H functionalization reactions constitute a key pillar in streamlining synthesis methods. In this context, the use of visible light to mediate metal-catalyzed C–H functionalization reactions is currently emerging as an important strategy to facilitate such transformations and enables mild C–H functionalization reactions at ambient conditions without the need for external photosensitizers or reactive reagents. Herein, we report on a photochemical approach that allows for C(sp3)–H functionalization reactions using simple [Ru(cymene)Cl2]2 and a phosphoric acid diester as catalysts. This strategy enables formal C(sp3)–C(sp3) coupling reactions to access valuable, sterically demanding amines and expands photochemical C–H functionalization reactions toward reaction pathways exclusive to photochemistry. Detailed mechanistic experiments and density functional theory calculations suggest the formation of a cyclometalated ruthenium complex that is stabilized by the phosphoric acid diester anion and serves as a catalyst for C–H functionalization reactions and single-electron transfer to access C–H functionalization reactions via a deprotonation-radical addition mechanism.

The C-H functionalization of indole heterocycles constitutes a key strategy to leverage the synthesis of endogenous signaling molecules such as tryptamine or tryptophol. Herein, we report on the photocatalytic reaction of ethyl diazoacetate with indole, which shows an unusual solvent dependency. While C2-functionalization occurs under protic conditions, the use of aprotic solvents leads to a complete reversal of selectivity and exclusive C3-functionalization occurs. To rationalize for this unexpected reactivity switch, we have conducted detailed theoretical and experimental studies, which suggest the participation of a triplet carbene intermediate that undergoes initial C2-functionalization. A distinct cationic [1,2]-alkyl radical migration then leads to formation of C3-functionalized indole. We conclude with the application of this photocatalytic reaction to access oxidized tryptophol derivatives including gram-scale synthesis and derivatization reactions. (46 examples, up to 93% yield).

The amination with monovalent, nitrogen-based intermediates constitutes an important reaction for the construction of valuable amines. The high basicity of reagents, reaction intermediates or products however poses significant challenges to metal-catalyzed amination through coordination and blocking of catalytically active sites and hampering their efficiency. In this context, high-yielding intermolecular amination reaction of allenes remain an unsolved challenge in organic synthesis and general methods are not available. Herein, we describe a photochemical approach towards the intermolecular amination of allenes via free nitrene radical anions as the key reactive intermediate. This reaction proceeds without the participation of catalyst-bound nitrogen species and can thus overcome current limitations. We report on the application in the amination of allenes to give azetidine and cyclopropyl amines with a broad and general substrate scope. Experimental and theoretical studies were performed to provide an understanding of the reaction mechanism and rationalize the high efficiency of this photocatalytic approach.

Chemistry - a European journal 28(15), e202104397 (2022). doi:10.1002/chem.202104397, Published by Wiley-VCH, Weinheim

In this combined computational and experimental study, the C-H functionalization of 2-phenyl pyridine with diazoalkanes was investigated. Initial evaluation by computational methods allowed the evaluation of different metal catalysts and diazoalkanes and their compatibility in this C-H functionalization reaction. With these findings, suitable reaction conditions for the C-H methylation reactions were quickly identified by using highly reactive TMS diazomethane and C-H alkylation reactions with donor/acceptor diazoalkanes, which is applied to a broad scope on alkylation reactions of 2-aryl pyridines with TMS diazomethane and donor/acceptor diazoalkane (51 examples, up to 98 % yield).

The C–H functionalization reaction of N-heterocycles with an unprotected N–H group is one of the most step-economic strategies to introduce functional groups without the need for installation and r...

The photolysis of diazoalkanes is a timely strategy to conduct carbene‐transfer reactions under mild and metal‐free reaction conditions, and has developed as an important alternative to conventional metal‐catalyzed carbene‐transfer reactions. One of the major limitations lies within the rapidly occurring side reaction of the carbene intermediate with remaining diazoalkane molecules that result in the use of an excess of the reaction partner and thus impacts on the reaction efficiency. Herein, we describe a protocol that takes advantage of the in situ generation of donor–acceptor diazoalkanes by Bamford–Stevens reaction. Following this strategy, the concentration of the diazoalkane reaction partner can be minimized to reduce unwanted side reactions and to now conduct photochemical carbene transfer reactions under stoichiometric reaction conditions. We have explored this approach in the C−H and N−H functionalization and cyclopropanation reaction of N‐heterocycles and could demonstrate the applicability of this method in 51 examples., Cyclopropanation: Bamford–Stevens reactions in the presence of blue light open up a pathway to overcome typical limitations of photochemical carbene‐transfer reactions. Following this methodology, the C−H functionalization of indole heterocycles can be performed in high efficiency under stoichiometric conditions (see scheme).

The reaction mechanism of oxygen and sulfur ylide mediated rearrangements is even today a matter of debate. In this report, we describe ring expansion reactions of oxetane and thietane heterocycles that allow probing the underlying reaction mechanism under metal-free, photochemical conditions. This ring expansion proves highly efficient and allows the synthesis of tetrahydrofuran and thiolane heterocycles under mild and operationally simple reaction conditions. These studies reveal marked differences in the stereoselectivity of the ring expansion of oxygen or sulfur ylides, which were further investigated computationally. DFT calculations show that carbenes react under ylide formation and that the corresponding ring expansion reactions proceed via a diradical pathway. The different bond lengths in free oxygen or sulfur ylide intermediates cause the distinctive stereochemical outcome.

Herein we describe a multiple C−H functionalization reaction of carbazole heterocycles with diazoalkanes. We show that gold catalysts play a distinct role in enabling a multiple C−H functionalization reaction to introduce up to six carbene fragments onto molecules containing multiple carbazole units or to link multiple carbazole units into a single molecule. A one‐pot stepwise approach enables the introduction of two different carbene fragments to allow orthogonal deprotection and straightforward derivatization., Paved with gold: We describe a multiple C−H functionalization reaction of carbazole heterocycles with diazoalkanes. We show that gold catalysts play a distinct role in enabling a multiple C−H functionalization reaction to introduce up to six carbene fragments onto molecules containing multiple carbazole units or to link multiple carbazole units into a single molecule. A one‐pot stepwise approach enables the introduction of two different carbene fragments to allow orthogonal deprotection and straightforward derivatization.

The photochemical carbene transfer reaction is a timely strategy in organic synthesis and generally proceeds via singlet carbene intermediates. In this combined experimental and computational studies, we show that the photolysis reaction of diaryl diazoalkanes gives access to both singlet and triplet diarylcarbene intermediates. The electronic properties of substituents of the aryl substituents show a strong influence on the electronic properties of the carbene intermediate and result in significantly reduced singlet triplet energy gaps. Depending on the spin state and electronic properties of the carbene intermediate, the reaction with alkynes provides access towards chemoselective cyclopropenation, cascade, or C-H functionalization reactions.

Sigmatropic rearrangements are an important fundamental toolbox in organic synthesis to access complex molecular fragments. Yet, the rearrangement reactions of onium ylides via gold catalyzed carbene transfer reactions are relatively unexplored. Herein, we describe a gold-catalyzed sigmatropic rearrangement of sulfonium and selenium ylides (39 examples, up to 99% yield). Furthermore, we report on the limitations of sigmatropic rearrangement reactions of aryl allyl anilines, which deliver exclusively C-H functionalized products.

The C-H functionalization reaction of N-heterocycles with unprotected N-H group is one of the most step-economic strategies to introduce functional groups without the need of installation and removal of protecting groups. Despite recent significant advances in C-H functionalization chemistry, this strategy remains unsatisfactorily developed. In this report, we disclose a simple and straightforward protocol to allow for the selective C-H functionalization of unprotected double benzannellated N-heterocycles via gold catalyzed carbene transfer reactions (29 examples, up to 86% yield). The scope of the reaction can also be expanded to the corresponding protected heterocycles (37 examples, up to 98% yield), further demonstrating the generality of this method. Mechanistic studies by DFT calculations underpin the importance of the gold catalyst and reveal that the selectivity of this reaction is driven by trace amounts of water present in the reaction mixture.

The C-H functionalization reaction of N-heterocycles with unprotected N-H group is one of the most step-economic strategies to introduce functional groups without the need of installation and removal of protecting groups. Despite recent significant advances in C-H functionalization chemistry, this strategy remains unsatisfactorily developed. In this report, we disclose a simple and straightforward protocol to allow for the selective C-H functionalization of unprotected double benzannellated N-heterocycles via gold catalyzed carbene transfer reactions (29 examples, up to 86% yield). The scope of the reaction can also be expanded to the corresponding protected heterocycles (37 examples, up to 98% yield), further demonstrating the generality of this method. Mechanistic studies by DFT calculations underpin the importance of the gold catalyst and reveal that the selectivity of this reaction is driven by trace amounts of water present in the reaction mixture.

Molecules 25(4), 880 (2020). doi:10.3390/molecules25040880, Published by MDPI22525, Basel

Among the available methods to increase the molecular complexity, sigmatropic rearrangements occupy a distinct position in organic synthesis. Despite being known for over a century sigmatropic rearrangement reactions of ylides via carbene transfer reaction have only recently come of age. Most of the ylide mediated rearrangement processes involve rupture of a σ‐bond and formation of a new bond between π‐bond and negatively charged atom followed by simultaneous redistribution of π‐electrons. This minireview describes the advances in this research area made in recent years, which now opens up metal‐catalyzed enantioselective sigmatropic rearrangement reactions, metal‐free photochemical rearrangement reactions and novel reaction pathways that can be accessed via ylide intermediates., The development of sigmatropic rearrangement reactions via ylide intermediates has witnessed significant advances in recent years. In this minireview, we summarize these advances that have contributed to the fundamental understanding of sigmatropic rearrangement reactions and that opened up new avenues in organic synthesis methodology beyond classic rearrangements.

Advanced synthesis & catalysis 362(21), 4716-4722 (2020). doi:10.1002/adsc.202000818, Published by Wiley-VCH, Weinheim

Hexafluoroisopropanol is typically considered as an unreactive solvent and not as a reagent in organic synthesis. Herein, we report on a mild and efficient photochemical reaction of aryl diazoacetates with hexafluoroisopropanol that enables, under stoichiometric reaction conditions, the synthesis of fluorinated ethers in excellent yield. We support these findings with mechanistic studies, which indicate a preorganization of hexafluoroisopropanol and the diazoalkane as an unreactive Lewis Acid-Base adduct. Only upon photoexcitation, this adduct undergoes a protonation-substitution reaction to the reaction product. We conclude with investigations on the applicability of this photochemical transformation and could show that a broad variety of acidic alcohols can be subjected to this transformation and thus demonstrate the feasibility of this concept in O-H functionalization reactions (52 examples, up to 98% yield).