Papers by Norbert Hoffmann
European Journal of Organic Chemistry, Feb 7, 2017
Proton-coupled electron transfer (PCET) is studied in different research domains of chemistry. Ma... more Proton-coupled electron transfer (PCET) is studied in different research domains of chemistry. Many reports involve biochemical processes. Although related phenomena have often been investigated in photochemical electron-and hydrogen-transfer processes, only recently have a larger variety of such steps come to be discussed under the comprehensive concept of PCET. In photoredox catalytic reactions applied to [a]
HAL (Le Centre pour la Communication Scientifique Directe), Nov 21, 2022
The Paternò-Büchi reaction is a useful approach in organic synthesis to prepare oxetanes. Underst... more 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. Photochemical reactions are an important tool in organic synthesis and in the chemical and pharmaceutical industry, these reactions are particularly useful in the search for biologically active compounds. The reason for this success in different applications is that upon electronic excitation by light absorption, or by energy transfer in the case of photochemical sensitization, the electronic configuration of a compound changes and thus the chemical reactivity changes. This means that compounds or compound families that are difficult to, or cannot be, synthesized by conventional methods become available under photochemical conditions. Traditionally, organic photochemistry is closely linked to physical chemistry. 2 This is because techniques such as time resolved spectroscopy are often used for mechanistic studies, for example, to understand electron transfer processes or to detect and characterize intermediates in these reactions. In recent years, the technology of photochemical reactors has been developed to increase the efficiency of photochemical reactions, such as by reducing the optical trajectory to increase the concentration of the chromophore as well as through the development of continuous flow to further optimize the reactions.
Catalysts, Jul 26, 2019
Photopolymerization processes, and especially those carried out under visible light, are more and... more Photopolymerization processes, and especially those carried out under visible light, are more and more widespread for their multiple advantages compared to thermal processes. In the present paper, new 1,8-naphthalimide derivatives are proposed as photoinitiators for free-radical polymerization upon visible light exposure using light-emitting diodes (LEDs) at 395, 405, and 470 nm. These photoinitiators are used in combination with both iodonium salts and phosphine. The synthesis of these compounds as well as their excellent polymerization initiation ability for methacrylate monomers are presented in this article. A full picture of the involved chemical mechanisms is also provided thanks to photolysis, radical characterization, and redox measurements.
Organic and Biomolecular Chemistry, 2016
Traditionally, organic photochemistry when applied to synthesis strongly interacts with physical ... more Traditionally, organic photochemistry when applied to synthesis strongly interacts with physical chemistry. The aim of this review is to illustrate this very fruitful interdisciplinary approach and cooperation. A profound understanding of the photochemical reactivity and reaction mechanisms is particularly helpful for optimization and application of these reactions. Some typical reactions and particular aspects are reported such as the Norrish-Type II reaction and the Yang cyclization and related transformations, the [2 + 2] photocycloadditions, particularly the Paternò-Büchi reaction, photochemical electron transfer induced transformations, different kinds of catalytic reactions such as photoredox catalysis for organic synthesis and photooxygenation are discussed. Particular aspects such as the structure and reactivity of aryl cations, photochemical reactions in the crystalline state, chiral memory, different mechanisms of hydrogen transfer in photochemical reactions or fundamental aspects of stereoselectivity are discussed. Photochemical reactions are also investigated in the context of chemical engineering. Particularly, continuous flow reactors are of interest. Novel reactor systems are developed and modeling of photochemical transformations and different reactors play a key role in such studies. This research domain builds a bridge between fundamental studies of organic photochemical reactions and their industrial application.
HAL (Le Centre pour la Communication Scientifique Directe), Dec 1, 2022
HAL (Le Centre pour la Communication Scientifique Directe), Jan 4, 2021
Photochemical and Photobiological Sciences, Nov 25, 2021
Different methods for the direct enantioselective photochemical synthesis of heterocycles are pre... more 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.
Australian Journal of Chemistry
Photocycloadditions of benzene derivatives with alkenes play an important role as key steps in or... more 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.
Chemical Society Reviews, 2021
Photochemical reactions are a key method to generate radical intermediates. Often under these con... more Photochemical reactions are a key method to generate radical intermediates. Often under these conditions no toxic reagents are necessary. During recent years, photo-redox catalytic reactions considerably push this research domain. These reaction conditions are particularly mild and safe which enables the transformation of poly-functional substrates into complex products. The synthesis of heterocyclic compounds is particularly important since they play an important role in the research of biologically active products. In this review, photochemical radical cyclization reactions of imines and related compounds such as oximes, hydrazones and chloroimines are presented. Reaction mechanisms are discussed and the structural diversity and complexity of the products are presented. Radical intermediates are mainly generated in two ways: (1) Electronic excitation is achieved by light absorption of the substrates. (2) The application of photoredox catalysis is now systematical studied for these reactions. Recently, also excitation of charge transfer complexes have been studied in this context with many perspectives.
Diastereoselective photochemical reactions play an important role for asymmetric synthesis, for e... more 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 centers 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 reaction 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.
HAL (Le Centre pour la Communication Scientifique Directe), 2021
The Journal of Organic Chemistry, 2021
Intramolecular photocycloaddition reactions of 3,4-benzonitriles derived from vanillin with alken... more Intramolecular photocycloaddition reactions of 3,4-benzonitriles derived from vanillin with alkenes have been investigated. In contrast to previous reports on photochemical reactions with these compounds, mainly [2+3] cycloaddition has been observed. A competing [2+2] photocycloaddition plays a minor role. Most probably these additions occurs at the singlet state S1. In the case of a triplet reaction, a different regioselectivity of the [2+2] cycloaddition would be observed. A linear and an angular [2+3] cycloadduct are formed as major products. The first isomer is transformed in the second one by a photochemical vinyl-cyclopropane rearrangement which increases the selectivity of the reaction. The influence of the substitution pattern on the reactivity and the selectivity has also been investigated.
ChemInform, 2016
This reaction can be also conducted by using a photo‐flow setup which leads to significantly decr... more This reaction can be also conducted by using a photo‐flow setup which leads to significantly decreased reaction times.
Longtemps laissees au rang de processus d’activation ou de reactions de deprotection, les methode... more Longtemps laissees au rang de processus d’activation ou de reactions de deprotection, les methodes photo- chimiques et electrochimiques connaissent un veritable essor en synthese organique ces dernieres annees. Plus particulierement, les reactions dites de catalyse photoredox sont de plus en plus utilisees. Dans ces reactions, l’absorption de l’energie lumineuse par un catalyseur metallique ou organique permet d’exalter leurs proprietes redox : ce dernier devient un meilleur oxydant et un meilleur reducteur. Dans le cadre de ces reactions, un transfert d’electron permet d’amorcer le processus catalytique. Il est egalement possible d’injecter directement les electrons dans le milieu reactionnel par une source electrique, sans utiliser de lumiere ou de catalyseur pour declencher une reaction chimique : on parle alors d’electrochimie organique. Ces deux methodes, differentes du point de vue de la mise en œuvre, permettent neanmoins d’acceder a des structures moleculaires similaires imp...
The Journal of Organic Chemistry, 2011
Tables with two dimesional NMR Data …………………………………………. S3 Compound 10 …………………………………………. S3 Compoun... more Tables with two dimesional NMR Data …………………………………………. S3 Compound 10 …………………………………………. S3 Compound 19a …………………………………………. S4 Compound 19b …………………………………………. S5 Compound 20a …………………………………………. S6 Compound 20b …………………………………………. S7 Compound 21 …………………………………………. S8 Compound 26 …………………………………………. S9 NOE of compound 26 ………………………………………….
Industrial Crops and Products, Jul 1, 2014
ABSTRACT We studied the reactivity of wood based hemicelluloses (beechwood xylan and poplar chips... more ABSTRACT We studied the reactivity of wood based hemicelluloses (beechwood xylan and poplar chips) in acid-catalyzed transglycosidation in decanol. Wood based hemicelluloses revealed to be more recalcitrant to transglycosidation than hemicelluloses from cereal origin (wheat straw, Oat spelt xylan). The chemical composition of the hemicelluloses, such as ash content or the level of arabinose and uronic acids in the polysaccharides structures, seems to influence the reactivity. The use of dimethylsulfoxide as solvent in the transglycosidation improved pentosides and glucosides yields for beechwood xylan, the more recalcitrant material. The surfactant properties of glycosides’ compositions produced from poplar were studied and compared to those of glycosides’ compositions obtained from pure monomeric carbohydrates or from wheat straw. The cellulose rich residue from poplar was hydrolyzed by concentrated acid saccharification yielding glucose in yields up to 55%.
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Papers by Norbert Hoffmann