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Despite the recognized effectiveness of course-based undergraduate research experiences (CUREs), there are few examples of the development of a CURE-based course in a large-enrollment undergraduate organic chemistry laboratory. Herein, we describe the development and implementation of a series of undergraduate laboratory experiments centered around the synthesis and characterization of pyrylium salts in the context of photoredox catalysis. Pre- and post-survey data regarding the CURE features of collaboration, discovery, iteration, and student project ownership also highlight the strengths of the CURE implementation in this large-scale lab course.

Photoinduced electron transfer (PET) is a phenomenon whereby the absorption of light by a chemical species provides an energetic driving force for an electron-transfer reaction.sup.1-4. This mechanism is relevant in many areas of chemistry, including the study of natural and artificial photosynthesis, photovoltaics and photosensitive materials. In recent years, research in the area of photoredox catalysis has enabled the use of PET for the catalytic generation of both neutral and charged organic free-radical species. These technologies have enabled previously inaccessible chemical transformations and have been widely used in both academic and industrial settings. Such reactions are often catalysed by visible-light-absorbing organic molecules or transition-metal complexes of ruthenium, iridium, chromium or copper.sup.5,6. Although various closed-shell organic molecules have been shown to behave as competent electron-transfer catalysts in photoredox reactions, there are only limited reports of PET reactions involving neutral organic radicals as excited-state donors or acceptors. This is unsurprising because the lifetimes of doublet excited states of neutral organic radicals are typically several orders of magnitude shorter than the singlet lifetimes of known transition-metal photoredox catalysts.sup.7-11. Here we document the discovery, characterization and reactivity of a neutral acridine radical with a maximum excited-state oxidation potential of -3.36 volts versus a saturated calomel electrode, which is similarly reducing to elemental lithium, making this radical one of the most potent chemical reductants reported.sup.12. Spectroscopic, computational and chemical studies indicate that the formation of a twisted intramolecular charge-transfer species enables the population of higher-energy doublet excited states, leading to the observed potent photoreducing behaviour. We demonstrate that this catalytically generated PET catalyst facilitates several chemical reactions that typically require alkali metal reductants and can be used in other organic transformations that require dissolving metal reductants. Photoexcited acridine radical catalysts are found to have redox potentials more reducing than lithium, which is attributed to the population of higher-energy doublet excited states via a twisted intramolecular charge-transfer species., Author(s): Ian A. MacKenzie [sup.1] , Leifeng Wang [sup.1] , Nicholas P. R. Onuska [sup.1] , Olivia F. Williams [sup.1] , Khadiza Begam [sup.2] , Andrew M. Moran [sup.1] , [...]

Organic azides serve as synthetically useful surrogates for primary amines, a functional group which is ubiquitous in bioactive and medicinally relevant molecules. Historically, the formal hydroazidation of simple activated olefins and styrenes has proven difficult due to the inherent propensity of these compounds to oligomerize. Herein is disclosed a method for the anti-Markovnikov hydroazidation of activated olefins, catalyzed by an organic acridinium salt under irradiation from blue LEDs. This method is applicable to a variety of substituted and terminal styrenes and several vinyl ethers, yielding synthetically versatile hydroazidation products in moderate to excellent yield. [ABSTRACT FROM AUTHOR]

Expanding the toolbox of C-H functionalization reactions applicable to the late-stage modification of complex molecules is of interest in medicinal chemistry, wherein the preparation of structural variants of known pharmacophores is a key strategy for drug development. One manifold for the functionalization of aromatic molecules utilizes diazo compounds and a transition-metal catalyst to generate a metallocarbene species, which is capable of direct insertion into an aromatic C-H bond. However, these high-energy intermediates can often require directing groups or a large excess of substrate to achieve efficient and selective reactivity. Herein, we report that arene cation radicals generated by organic photoredox catalysis engage in formal C-H functionalization reactions with diazoacetate derivatives, furnishing sp 2 -sp 3 coupled products with moderate-to-good regioselectivity. In contrast to previous methods utilizing metallocarbene intermediates, this transformation does not proceed via a carbene intermediate, nor does it require the presence of a transition-metal catalyst., (© 2020 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)