This chapter covers substitution reactions of oxygen, sulfur, selenium, and tellurium functions. Since the publication of COFGT (1995), many works have dealt with radical reactions. Many efforts have also been made in the field of cross-couplings and their asymmetric catalysis. As in other domains of organic chemistry, some of these reactions were carried out on solid phase with the aim of applying them to combinatorial chemistry. Some papers deal with efforts to perform the reaction in a sustainable way (green chemistry). In this context ionic liquids are frequently used as a reaction medium. Rearrangements implying substitution of chalcogen are particularly studied in order to control their stereoselectivity.

Photochemical reactions of levoglucosenone (LGO) considerably extend its potential as a synthon in organic synthesis. At the photochemical excited state, LGO undergoes a reaction that resembles Norrish type I reactions. Ketene intermediates are formed and trapped with alcohols or amines. A variety of alkenes with interesting substitution patterns are thus obtained. The reaction mechanism has been determined and compared to the classical mechanism of the Norrish type I reaction of Cyrene. The triplet energies of both compounds have also been determined by phosphorescence at low temperature. In the case of photochemically excited LGO, electron transfer from tertiary aromatic amines is possible. Such a photochemical electron transfer can also be induced by sensitization involving triplet energy transfer from a sensitizer to LGO. These processes are competitive with the efficient Norrish type I reactions. In such reactions, the bicyclic structure of LGO is conserved, and aniline derivatives are stereospecifically added.

Photochemical reactions play a key role in organic synthesis. Many compounds or compound families are only or more easily accessible with these reactions. They are easily optimized for large scale transformations or for industrial processes. Photocycloadditions, photochemically induced electrocyclic or radical reactions are thus applied in fine chemistry. More recently, photoredox catalytic reactions with visible light are intensively investigated. Photooxygenations involving singlet oxygen are highly selective. They are carried out under mild conditions, also at the industrial scale.

The cyclodextrin (CD) complexes of several butenyloxybenzonitriles were synthesized and irradiated as aqueous suspensions. An enantioselective intramolecular [2 + 2] or ortho photocycloaddition in such inclusion complexes is described. α-Cyclodextrin and β-cyclodextrin induce chirality in the opposite direction. At the reaction temperature (<7 °C), the enantiomeric excess (ee) is significantly increased (34% for α-CD, -10% for β-CD). The product yield in the cyclodextrin complex (35%) was close to that one of the racemic reaction in solution (42%). A topological analysis describes C2-symmetric chiral induction in a C6-symmetric host structure (α-Cyclodextrin). In a computational study, the energies of the inclusion complex were calculated for different conformations of compound 3a.

Red-light-activated photocatalysis has become a powerful approach for achieving sustainable chemical transformations, combining high efficiency with energy-saving, mild conditions. By harnessing the deeper penetration and selectivity of red and near-infrared light, this method minimizes the side reactions typical of higher-energy sources, making it particularly suited for large-scale applications. Recent advances highlight the unique advantages of both metal-based and metal-free catalysts under red-light irradiation, broadening the range of possible reactions, from selective oxidations to complex polymerizations. In biological contexts, red-light photocatalysis enables innovative applications in phototherapy and controlled drug release, exploiting its tissue penetration and low cytotoxicity. Together, these developments underscore the versatility and impact of red-light photocatalysis, positioning it as a cornerstone of green organic chemistry with significant potential in synthetic and biomedical fields.

Over the past decade, the circular economy has become a key component of sustainability due to environmental issues and more stringent environmental regulations. In this connection, the valuation of bio-based monomers possessing innovative structures remains an important challenge. Furthermore, several goals must be achieved: 1) the homopolymerization of such bio-based monomers by free radical photopolymerization which requires small energy inputs with limited emission of volatile organic compounds (COV) and 2) their copolymerization with Elium® thermoplastic resins to increase biogenic carbon in these novel low-viscosity methacrylic resins developed by Arkema. The challenge will be, in this context, to maximize the biomass carbon content while maintaining or even surpassing the properties exhibited by their petrochemical counterpart. In this work, a series of alkoxyfuranones derived from furfural have been synthesized. The four alkoxyfuranones (AF1, AF2, AF3 and AF4) varying by the functionality of the side chains (but-3-en-1-yl, acrylate, isopropyl and ethyl) attached via acetalization of the 5-hydroxy-2(5H)-furanone were studied as potential monomers or co-monomers for free radical polymerization. The synthesis of AF1 and AF2 has never been reported in the literature. The homopolymerization of two of them (AF1 and AF2) was successfully achieved, but only one led to a high-glass transition temperature (Tg) bio-based polymer. On the other hand, copolymerization with Elium® thermoplastic resins was obtained with different ratios of AF2 as a co-monomer. As a result, the expected gain value, such as faster photopolymerization was successfully obtained. Nevertheless, the copolymerization doesn’t lead to obtaining high-Tg polymers.

Platform chemicals obtained from biomass will play an important role in chemical industry. Already existing compounds or not yet established chemicals are produced from this renewable feedstock. Using photochemical reactions as sustainable method for the conversion of matter furthermore permits to develop processes that are interesting from the ecological and economical point of view. Furans or levoglucosenone are thus obtained from carbohydrate containing biomass. Photochemical rearrangements, photooxygenation reactions or photocatalytic radical reactions can be carried out with such compounds. Also, sugars such pentoses or hexoses can be more easily transformed into heterocyclic target compounds when such photochemical reactions are used. Lignin is an important source for aromatic compounds such as vanillin. Photocycloaddition of these compounds with alkenes or the use light supported multicomponent reactions yield interesting target molecules. Dyes, surfactants or compounds possessing a high degree of molecular diversity and complexity have been synthesized with photochemical key steps. Alkenes as platform chemicals are also produced by fermentation processes, for example, with cyanobacteria using biological photosynthesis. Such alkenes as well as terpenes may further be transformed in photochemical reactions yielding, for example, precursors of jet fuels.

Diastereoselective photochemical reactions play an important role for asymmetric synthesis, for example for the synthesis of natural or biologically active compounds. Chirality is either induced by a chiral auxiliary or by a chiral center already present in the substrates. In the first case, the source of chirality is removed during the ongoing multistep synthesis. In the second case, the chirality source is conserved in the target compound. In the present chapter examples of the [2 + 2] photocycloaddition of carbonyl compounds with alkenes (Paternò-Büchi reaction), between α,β-unsaturated carbonyl compounds and alkenes as well as between two alkenes are discussed. Typical diastereoselective photocycloaddition reactions with aromatic compounds are reported. Currently, photoinduced radical reactions are intensively investigated and photoredox or more generally photocatalysis play a key role. In such reactions neutral radicals and radical ions are typical intermediates. Under photochemical conditions, tautomeric equilibria significantly change. Due to this fact, interesting diastereoselective reactions have been reported. Electrocyclization has often been applied to the synthesis of heterocyclic compounds. Photochemical rearrangements provide convenient accesses to complex polycyclic compounds. The diastereoselectivity is significantly increased when the corresponding reactions are carried out under confinement conditions, for example with inclusion complexes. Photooxygenation with singlet oxygen is a very convenient oxidation method. Generally, such reactions are highly regio- stereo- and chemoselective.

Attractive for applications to organic synthesis, photocycloadditions of electronically excited benzene derivatives with alkenes represent an interesting example of photochemical reactions that cause complete changes in the reaction. Starting from simple compounds, in only one step, the photochemical reaction, a high degree of molecular complexity and diversity is generated without using a chemical reagent. Another important aspect is the use of benzene derivatives and alkenes to build up complex three dimensional structures by the transformation of sp2 into sp3 hybridized carbons. These reactions can find important applications in pharmaceutical chemistry and are the basis of this review.

Biomass and biomass-derived compounds have become an important alternative feedstock for chemical industry. They may replace fossil feedstocks such as mineral oil and related platform chemicals. These compounds may also be transformed conveniently into new innovative products for the medicinal or the agrochemical domain. The production of cosmetics or surfactants as well as materials for different applications are examples for other domains where new platform chemicals obtained from biomass can be used. Photochemical and especially photocatalytic reactions have recently been recognized as being important tools of organic chemistry as they make compounds or compound families available that cannot be or are difficultly synthesized with conventional methods of organic synthesis. The present review gives a short overview with selected examples on photocatalytic reactions of biopolymers, carbohydrates, fatty acids and some biomass-derived platform chemicals such as furans or levoglucosenone. In this article, the focus is on application to organic synthesis.

Photocycloadditions of benzene derivatives with alkenes play an important role as key steps in organic synthesis. Intramolecular reactions have been most frequently studied in this context. Often, meta or [2 + 3] photocycloadditions take place in competition with ortho or [2 + 2] additions. The influence of the substitution pattern and the spin multiplicity of the excited state on the outcome of these reactions is discussed. A topological analysis permitting a systematic application of the [2 + 3] photocycloadditions to the total synthesis of natural products is presented and a selection of corresponding syntheses is discussed. More recently the [2 + 2] photocycloaddition and consecutive rearrangements on organic synthesis have been published. Some approaches in the context of asymmetric synthesis have also been reported.

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Heterocyclic Compounds in Enantioselective Photochemical Reactions Norbert Hoffmann

Photochemically induced intramolecular hydrogen atom transfer in oxazolones is reported. An acetal or thioacetal function at the side chain acts as a hydrogen donor while the photochemical exited oxazolone is the acceptor. A one-step process─the electron and the proton are simultaneously transferred─is productive, while electron transfer followed by proton transfer is inefficient. Radical combination then takes place, leading to the formation of a C-C or C-N bond. The regioselectivity of the reaction is explained by the diradical/zwitterion dichotomy of radical intermediates at the singlet state. In the present case, the zwitterion structure plays a central role, and intramolecular electron transfer favors spin-orbit coupling and thus the intersystem crossing to the singlet state. The reaction of corresponding thioacetal derivatives is less efficient. In this case, photochemical electron transfer is competitive. The photoproducts resulting from C-C bond formation easily undergo stepwise thermal decarboxylation in which zwitterionic and polar transition states are involved. A computational study of this step has also been performed.

The Front Cover shows the stereo and regio selective addition of radical species such as alkyl or formamidyl radicals to levoglucosenone. Levoglucosenone is a biobased platform chemical obtained from celluloses. The radical intermediates are generated by hydrogen atom transfer involving photocatalysis with tetra-n-butylammonium decatungstate (TBADT). Cover picture created with the help of Tony Leclet. More information can be found in the Article by N. Hoffmann et al.

Polymethine dyes are prepared using a convenient synthesis and characterized by physicochemical and computational methods.

Various methods are discussed leading directly to enantiopure heterocyclic compounds. Asymmetric catalysis with chiral templates involving hydrogen bonds or metal complexes provides efficient access to enantiopure heterocycles. Enzyme catalysis can be simplified when one or more enzyme activities are replaced by a simple photochemical step, especially photoredox catalysis. Chiral induction in photochemical reactions performed with chiral crystals is highly efficient. Such reactions can also be carried out with crystalline inclusion complexes. Inclusion of a photochemical substrates and an enantiopure compound in zeolites also leads to enantioselective compounds. The general strategy of all these methods is to reduce or control conformational mobility. Memory of chirality is a particular case in which a chiral information is temporally lost but the rigid conformations stabilize the molecular structure, which may also lead to the formation of enantiopure compounds.

The Paternò–Büchi reaction is a useful approach in organic synthesis to prepare oxetanes. Understanding the mechanism of this reaction is required for the prediction and control of the diastereoselectivity of the products.

Using photocatalysis with tetra‐n‐butylammonium decatungstate (TBADT), alkanes, cyclic acetals, cyclic ethers, formamide and aldehydes were added in a stereoselective way to levoglucosenone (LGO). A hydrogen atom is transferred from the donor compound to the photochemically excited TBADT, and the resulting radicals add onto LGO in a stereoselective way. In the case of the addition of adamantane, two regio‐isomers were obtained which form a crystalline solid solution. Cyrene™, obtained by hydrogenation of LGO, was added under the same conditions. In this case, only two of 32 possible isomers of the resulting Cyrene™ dimer were formed. The regio‐selectivity of the HAT step is discussed in detail. For this purpose, bond dissociation energies and partial charges have been calculated. Transition state calculations of the radical addition to LGO explain the stereospecificity of this reaction step.

Different methods for the direct enantioselective photochemical synthesis of heterocycles are presented. Currently, asymmetric catalysis with templates involving hydrogen bonds or metal complexes is intensively investigated. Enzyme catalysis can be simplified under photochemical conditions. For example, in multi enzyme systems, one or more enzyme catalytic steps can be replaced by simple photochemical reactions. Chiral induction in photochemical reactions performed with homochiral crystals is highly efficient. Such reactions can also be carried out with crystalline inclusion complexes. Inclusion of a photochemical substrate and an enantiopure compound in zeolites also leads to enantioselective compounds. In all these methods, the conformational mobility of the photochemical substrates is reduced or controlled. Memory of chirality is a particular case in which a chiral information is temporally lost but the rigid conformations stabilize the molecular structure which leads to the formation of enantiopure compounds. Such studies allows a profound understanding on how particular conformations determine the configuration of the final products. Graphical abstract