Ghambi

Two are better than one : By combining the input from two low energy photons, thermodynamically very challenging reactions can be driven. Empirical advances combined with insights from spectroscopy have made multi‐photon excitation processes amenable to preparative‐scale photoredox chemistry. A critical look at currently developed concepts and reactions identifies challenges, pitfalls and opportunities for future research.

Abstract

The energy of visible photons and the accessible redox potentials of common photocatalysts set thermodynamic limits to photochemical reactions that can be driven by traditional visible‐light irradiation. UV excitation can be damaging and induce side reactions, hence visible or even near‐IR light is usually preferable. Thus, photochemistry currently faces two divergent challenges, namely the desire to perform ever more thermodynamically demanding reactions with increasingly lower photon energies. The pooling of two low‐energy photons can address both challenges simultaneously, and whilst multi‐photon spectroscopy is well established, synthetic photoredox chemistry has only recently started to exploit multi‐photon processes on the preparative scale. Herein, we have a critical look at currently developed reactions and mechanistic concepts, discuss pertinent experimental methods, and provide an outlook into possible future developments of this rapidly emerging area.

Sun‐safe: Urea, an abundant and cheap substance, blocks access to a key site on the Methylene Blue backbone and thereby protects the dye against the deleterious photobleaching effects of sunlight.

Abstract

Methylene Blue has a long history as a photochemical reagent and is known to undergo photofading on exposure to visible light in aqueous solution. Under aerobic conditions, photobleaching occurs by way of a two‐step process involving intermediary formation of singlet oxygen. The first step is ascribed to regio‐selective addition of singlet oxygen within the precursor complex. This geminate reaction ultimately leads to formation of the leuco‐dye via a slower second step. Urea forms a weak ground‐state complex with Methylene Blue which affects the optical properties of the dye but is not evident by NMR spectroscopy. This complex is weakly fluorescent and undergoes intersystem crossing to the triplet manifold. The presence of urea decreases the rate of photobleaching of the dye and, at high concentrations of urea, the bleaching kinetics are consistent with an equilibrium mixture of complexed and free dye. The complexed dye does not bleach on the timescale of the experiment. Such protection might arise from urea blocking access to the site where geminate addition of O₂ takes place.

Divide et impera: A general strategy to access both aryl and alkyl radicals by photosensitized decarboxylation of the corresponding carboxylic esters is based on an energy‐transfer‐mediated homolysis of unsymmetric σ‐bonds. The independent aryl/alkyl radical generation step enables a series of key C−X and C−C bond‐forming reactions by simply changing the radical trapping agent.

Abstract

Despite significant progress in aliphatic decarboxylation, an efficient and general protocol for radical aromatic decarboxylation has lagged far behind. Herein, we describe a general strategy for rapid access to both aryl and alkyl radicals by photosensitized decarboxylation of the corresponding carboxylic acids esters followed by their successive use in divergent carbon–heteroatom and carbon–carbon bond‐forming reactions. Identification of a suitable activator for carboxylic acids is the key to bypass a competing single‐electron‐transfer mechanism and “switch on” an energy‐transfer‐mediated homolysis of unsymmetrical σ‐bonds for a concerted fragmentation/decarboxylation process.

Reinforcement learning (RL) has shown great success in increasingly complex single-agent environments and two-player turn-based games. However, the real world contains multiple agents, each learning and acting independently to cooperate and compete with other agents. We used a tournament-style evaluation to demonstrate that an agent can achieve human-level performance in a three-dimensional multiplayer first-person video game, Quake III Arena in Capture the Flag mode, using only pixels and game points scored as input. We used a two-tier optimization process in which a population of independent RL agents are trained concurrently from thousands of parallel matches on randomly generated environments. Each agent learns its own internal reward signal and rich representation of the world. These results indicate the great potential of multiagent reinforcement learning for artificial intelligence research.

A dearomatization process of 3‐nitroindoles via a chiral phosphine‐mediated [3+2] annulation reaction has been developed. This method makes use of readily available 3‐nitroindoles as an aromatic feedstock and quickly delivers a wide range of cyclopentaindoline alkaloid scaffolds in a highly enantioselective manner. Notably, this is the first time that phosphine‐triggered cyclization has been utilized in a dearomatization process.

Water lends a helping hand: The anti‐Markovnikov hydroazidation of alkenes has been accomplished under visible‐light irradiation by using [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆ as the photocatalyst and trimethylsilyl azide as the azidating agent. The reactions were significantly facilitated by water, the beneficial effect of which can be attributed to its participation in the reaction as the hydrogen donor.

Abstract

The anti‐Markovnikov hydroazidation of alkenes has been accomplished under visible‐light irradiation by using [Ir(dF(CF₃)ppy)₂(dtbbpy)]PF₆ as the photocatalyst and trimethylsilyl azide as the azidating agent. The reactions were greatly facilitated by water, the beneficial effect of which can be attributed to its participation in the reaction as the hydrogen donor, as indicated by deuterium isotope experiments. The reactions proceed under solvent free conditions in the presence of water. 4‐Dimethylaminopyridine also exhibited a beneficial effect on the reactions. The present method enabled hydroazidation of several types of unactivated alkenes with good yields and high regioselectivity.

Pyrimidopteridine N ‐oxide‐based heterocycles exhibit excellent excited state reduction potentials paired with a suitable ground state reduction potential allowing for catalyst turnover with mild oxidants. The structural similarity between pyrimidopteridines and recently uncovered flavine N ‐oxides may draw an important link between organic photoredox catalysis and chemical biology.

Abstract

Herein we report the photo‐ and electrochemical characterization of pyrimidopteridine N‐oxide‐based heterocycles. The potential of their application as organic photoredox catalysts is showcased in the photomediated contra‐thermodynamic E→Z isomerization of cinnamic acid derivatives and oxidative cyclization of 2‐phenyl benzoic acid to benzocoumarin using molecular oxygen as a mild oxidant. Furthermore, unprecedented intermolecular non‐covalent n–π‐hole interactions in solid state are discussed based on crystallographic and theoretical data.

Bimolecular nucleophilic substitution (S_N2) plays a central role in organic chemistry. In the conventionally accepted mechanism, the nucleophile displaces a carbon-bound leaving group X, often a halogen, by attacking the carbon face opposite the C–X bond. A less common variant, the halogenophilic S_N2X reaction, involves initial nucleophilic attack of the X group from the front and as such is less sensitive to backside steric hindrance. Herein, we report an enantioconvergent substitution reaction of activated tertiary bromides by thiocarboxylates or azides that, on the basis of experimental and computational mechanistic studies, appears to proceed via the unusual S_N2X pathway. The proposed electrophilic intermediates, benzoylsulfenyl bromide and bromine azide, were independently synthesized and shown to be effective.

A DMAP‐N‐oxide, featuring an α‐amino acid as the chiral source, was developed, synthesized and applied in asymmetric Steglich rearrangement. A series of O‐acylated azlactones afforded C‐acylated azlactones possessing a quaternary stereocenter in high yields (up to 97 % yield) and excellent enantioselectivities (up to 97 % ee).

Abstract

A DMAP‐N‐oxide, featuring an α‐amino acid as the chiral source, was developed, synthesized and applied in asymmetric Steglich rearrangement. A series of O‐acylated azlactones afforded C‐acylated azlactones possessing a quaternary stereocenter in high yields (up to 97 % yield) and excellent enantioselectivities (up to 97 % ee). Compared to the widespread use of pyridine nitrogen, which serves as the nucleophilic site in the asymmetric acyl transfer reaction, we discovered that chiral DMAP‐N‐oxides, in which the oxygen now acts as the nucleophilic site, are efficient acyl transfer catalysts. Our finding might open a new door for the development of chiral DMAP‐N‐oxides for asymmetric acyl transfer reactions.

Darwin proposed that mate choice might contribute to the evolution of cognitive abilities. An open question is whether observing the cognitive skills of an individual makes it more attractive as a mate. In this study, we demonstrated that initially less-preferred budgerigar males became preferred after females observed that these males, but not the initially preferred ones, were able to solve extractive foraging problems. This preference shift did not occur in control experiments in which females observed males with free access to food or in which females observed female demonstrators solving these extractive foraging problems. Our results suggest that direct observation of problem-solving skills increases male attractiveness and that this could contribute to the evolution of the cognitive abilities underlying such skills.