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7,7,8,8-tetracyanoquinodimethane (TCNQ), a strong electron withdrawing olefin, has long been regarded as an ideal compound for the synthesis of semiconducting coordination polymers when coordinated to transition metals. Given that TCNQ is an extremely good electron acceptor, transition metals such as Cu, Ag, Mn, Fe, Co, Ni, Zn and Cd have been surveyed for the synthesis of a variety of metal-TCNQ (MTCNQ) compounds. While each of these metal-organic charge transfer complexes show unique physico-chemical properties, these materials have been primarily investigated for electronics applications. More recently, the applicability of TCNQ class of nanomaterials has emerged in other important areas including catalysis, sensing and biology. In this review, the emerging applicability of MTCNQ complexes for catalysis is discussed. In particular, the fabrication of MTCNQ-based materials where M = Cu or Ag is surveyed, followed by discussion on improving their electron-hole recombination properties through a simple galvanic replacement reaction. The mechanistic aspect during the fabrication of these materials with an emphasis on their applicability in catalysis is outlined.

Co(III)-catalyzed mild C–C couplings of arenes with strained rings such as 7-oxabenzonorbornadienes and 2-vinyloxirane have been realized. The transformation is proposed to undergo ortho C–H activation, olefin insertion, and subsequent β-oxygen elimination. A broad range of synthetically useful functional groups are compatible, thus providing a new entry to access diversely 2-functionalized indoles.

We report here that a cobalt catalyst generated from a cobalt(II) salt and a ligand using magnesium metal is capable of promoting hydroarylation reactions through chelation-assisted C−H activation. With the appropriate choice of the ligand and other reaction conditions, ketimine- or aldimine-directed branched selective hydroarylations of styrenes and ketimine-directed hydroarylations of vinylsilane have been achieved. The reported catalytic systems may serve as more convenient alternatives to previously developed catalytic systems employing Grignard reagents as reducing agents.

Several hitherto unknown acyclic 2-methyl-but-2-enyl nucleoside phosphonate analogs (ANPs) and their bis-(pivaloyloxymethyl) prodrug forms were prepared by a challenging ultrasonic-assisted olefin cross metathesis and further modifications under microwaves, giving rise to a small library of title compounds. These ANPs were evaluated against a wide spectrum of DNA/RNA viruses. Among them, the most active compound exhibited a micromolar anti-HIV-1 activity with an EC50 of 1.1 µM and an EC90 of 4.2 µM.

The enantioselective synthesis of isochroman motifs has been accomplished by palladium(II)-catalyzed allylic C−H oxidation from terminal olefin precursors. Critical to the success of this goal was the development and utilization of a novel chiral aryl sulfoxide-oxazoline (ArSOX) ligand. The allylic C−H oxidation reaction proceeds with the broadest scope and highest levels of asymmetric induction reported to date (avg. 92 % ee, 13 examples with greater than 90 % ee).

A novel palladium-catalyzed hydration–olefin insertion cascade assisted by internal nucleophiles was developed for the synthesis of biologically significant 2,3-dihydro-1H-inden-1-ones under mild conditions. A detailed mechanistic study revealed that the assistance of the internal nucleophiles is crucial to trigger the cascade reaction via nucleopalladation of the alkyne moiety. The overall reaction is equivalent to regioselective hydration of alkynes followed by intramolecular Michael addition. This highly efficient and 100% atom-economical domino sequence afforded cis-2,3-disubstituted 2,3-dihydro-1H-inden-1-ones in excellent yields (up to 99%) with complete diastereoselectivity.

Decarboxylative coupling of carboxylic acids with activated olefins has been accomplished using visible light photoredox catalysis. The strategic placement of a radical-stabilizing aromatic group at the β-position of the olefin component biases the regioselectivity of the addition, allowing reliable, facile access to anti-Michael-type products from readily available precursors. The scope of this methodology was demonstrated with a range of carboxylic acids and appropriately substituted olefins and was applied toward a two-step synthesis of the antiarrhythmic agent encainide.

N,N-Dibromo-p-toluene sulfonamide (TsNBr2) has been found to be an effective reagent for various aminofunctionalization reactions. This reagent behaves both as an electrophilic bromine source as well as amine to react with olefins under different conditions to yield aminoether, imidazoline, diamine and amino bromine. The reaction proceeds rapidly under mild conditions with high regioselectivity. Olefins react with TsNBr2 in moist THF to form δ-amino ether at room temperature. Treatment of TsNBr2 with olefin in MeCN at room temperature produced imidazoline in high yield. Further modification of the reaction condition resulted in the development of a one-step procedure for the synthesis of N-acetyl,N′-tosyl diamine derivatives directly from olefin. When the olefin was treated with 2.4 mol equiv of TsNBr2 in the presence of K2CO3, N,N′-ditosyl diamine derivative was obtained in moderate yield. Instantaneous formation of aminobromine was observed when an olefin was treated with the reagent in dry CH2Cl2 at ro...

ConspectusThe development of methods for anti-Markovnikov alkene hydrofunctionalization has been a focal point of catalysis research for several decades. The vast majority of work on the control of regioselectivity for this reaction class has hinged on transition metal catalyst activation of olefin substrates. While progress has been realized, there are significant limitations to this approach, and a general solution for catalysis of anti-Markovnikov hydrofunctionalization reactions of olefins does not presently exist.In the past several years, this research lab has focused on alkene activation by single electron oxidation using organic photoredox catalysts to facilitate anti-Markovnikov hydrofunctionalization. By accessing reactive cation radical intermediates, we have realized a truly general approach to anti-Markovnikov olefin hydrofunctionalization reactions. We have identified a dual organic catalyst system consisting of an acridinium photooxidant, first reported by Fukuzumi, and a redox-active hydro...

Starting from directing group-substituted arenes and electrophilic olefin coupling partners a strategy towards the preparation of bi or tricyclic products is developed.

Cofactor-mimetic aerobic oxidation has conceptually merged with catalysis of syngas reactions to form a wide range of Markovnikov-selective olefin radical hydrofunctionalizations. We cover the development of the field and review contributions to reaction invention, mechanism, and application to complex molecule synthesis. We also provide a mechanistic framework for understanding this compendium of radical reactions.

On the hypothesis that one carbon homologation of 4′-selenonucleosides might relieve the steric repulsion between cellular kinases and bulky selenium, 5′-homo-4′-Se-d4Ns, 3 a–e, as anti-HIV agents were designed and synthesized stereoselectively from d-gulonic γ-lactone, with the conversion of 2′,3′-diol into the olefin as the key step. The anti-HIV activity of all synthesized compounds, 5′-homo-4′-Se-d4Ns, was toxicity-dependent, unlike normal 4′-Se-d4Ns, which were inactive against HIV-1. This result indicates that 5′-homo-4′-Se-d4Ns might be phosphorylated by cellular kinases as per the hypothesis.

A highly enantioselective rhodium-catalyzed intramolecular addition of arylboronic acids to unactivated ketones has been developed. By employing a simple, chiral sulfur-based branched olefin as the ligand, the reaction occurs efficiently under mild conditions. A wide range of chiral 3-hydroxy-2,3-dihydrobenzofurans bearing a tetrasubstituted stereocenter at C-3 position can be obtained in high yields (up to 98%) with excellent enantiomeric excesses (up to 98% ee).

A concise stereoselective total synthesis of cryptomoscatone F1 has been accomplished by utilization of asymmetric acetate aldol reaction, Brown’s asymmetric allylation, and olefin cross-metathesis as key transformations.

Enantioenriched alcohols found in an array of bioactive natural products and pharmaceutical agents are often synthesized by asymmetric nucleophilic addition to carbonyls. However, this approach generally shows limited functional-group compatibility, requiring the use of preformed organometallic reagents in conjunction with a stoichiometric or substoichiometric amount of chiral controller to deliver optically active alcohols. Herein we report a copper-catalyzed strategy for the stereoselective nucleophilic addition of propargylic and other alkyl groups to ketones, using easily accessible (poly)unsaturated hydrocarbons as latent carbanion equivalents. Our method features the catalytic generation of highly enantioenriched organocopper intermediates and their subsequent diastereoselective addition to ketones, allowing for the effective construction of highly substituted stereochemical dyads with excellent stereocontrol. Moreover, this process is general, scalable, and occurs at ambient temperature.

An efficient and practical method for the chemoselective ipso-hydroxylation of arylboronic acids is demonstrated using urea-hydrogen peroxide under catalyst free condition at room temperature. Remarkably, oxidation sensitive functional groups such as olefin, aldehyde, alcohol, ketone, and sulfide as well as heterocycles such as pyridine and thiophene were tolerated under the standard reaction condition. In addition to the solution phase, a solid phase ipso-hydroxylation of arylboronic acids has been investigated with urea hydrogen peroxide. The scope and limitations of the solid phase protocol is discussed.

An unusual diastereoselective Grignard reaction is explored, where the Grignard reagents are derived from 1,n-dihaloalkanes. A steric bias due to the presence of a quaternary centre adjacent to the acetonide ester at the benzylic position is responsible for the formation of an intramolecularly reduced product in almost quantitative yield. This steric hindrance is responsible for the diastereoselectivity observed with a variety of aromatic as well as aliphatic esters. The unusual Grignard reaction furnishes long chain secondary alcohols possessing a terminal olefin, which are synthetically important intermediates. As an application of this method, the diastereoselective synthesis of styryl lactones viz. 7-epi-(+)-goniodiol (29) and 8-epi-(−)-goniodiol (30) has been achieved.

The rhodium(III)-catalyzed direct cross-coupling reaction of electron-deficient acrylamides with maleimides is described. This protocol displays broad functional group tolerance and high efficiency, which offers a new opportunity to access highly substituted succinimides. Dependent on the substituent positions of acrylamides and reaction conditions, olefin migrated products were obtained with high regio- and stereoselectivity.

The developed ligands allow for an efficient, highly enantioselective transformation of cyclic and acyclic α,β-unsaturated carbonyl compounds according to the title reaction.

New C 2 -symmetric chiral sulfurous diamides were synthesized from (1 R ,2 R )-(+)-1,2-diphenylethylenediamine and used as efficient chiral sulfur–olefin hybrid ligands for enantioselective rhodium-catalyzed asymmetric 1,4-addition reactions. High yields (up to 99%) and excellent enantioselectivities (up to 96%) were achieved for both cyclohexenone and cyclopentenone substrates.

A novel Pd(0)-catalyzed intermolecular carbocyclization of phenol-derived biaryls with 1,3-dienes has been implemented through a sequence of oxidative addition to the C-I bond, regioselective olefin insertion, and allylative dearomatization. This method provides a broad range of attractive spirocyclic compounds bearing two contiguous tertiary/quaternary carbon centers in good yields with excellent chemoselectivity and regioselectivity. Moreover, preliminary results indicate that asymmetric control of this process is feasible with chiral ligands.

The aryl to vinyl palladium 1,4-migration was realized for the first time. The generated alkenyl palladium species was trapped by diboron reagents under Miyaura borylation conditions, providing a new method to synthesize β,β-disubstituted vinylboronates. The excellent regioselectivity and broad substrate scope were observed for this novel transformation.

In the field of olefin polymerization, catalyst modification remains the prevalent strategy to control the polymerization and copolymerization processes. The development of alternative strategies is, nevertheless, highly fascinating. Here, the concept of redox-controlled polymerization and its applications in the ring-opening polymerization of cyclic ester type monomers is presented. Early work on the introduction of the redox-control concept into the field of olefin polymerization is described, and our approach to address the early issues and realize redox controlled olefin polymerization and copolymerization is demonstrated. Finally, some conclusions and perspectives are discussed. 1 Introduction 2 The Concept of Redox Control in Polymerization 3 Early Work on Redox Control in Olefin Polymerization 4 Our Approach to Realize Redox-Controlled Olefin (Co)Polymerization 5 Conclusions and Perspectives

A tandem one pot reaction involving gem-dichlorination, hydrolysis and nitrile oxide generation from 7-alkoxy-4-chloro-2,2-dimethyl-2H-chromene-6-carbaldehyde oxime followed by olefin/alkyne cycloaddition has been carried out in good yield at ambient temperature using open flask chemistry. The synthesized 4-chromanone derivatives show conformational isomerism as determined by VT 1H-NMR, confirmed by X-ray crystallography and explained by DFT calculations. This can be a general method for C-3 electrophilic chlorination of benzopyran derivatives.

A chiral phosphoric acid and a ruthenium complex were found to catalyze an olefin cross-metathesis–asymmetric intramolecular Friedel–Crafts alkylation of N-tethered olefin pyrroles and conjugated enones to provide a variety of chiral tetrahydroindolizine derivatives in moderate to good yields and enantioselectivity (up to 93% ee).

A suitable procedure for the title reaction is evaluated which differs in the proton source (MeOH or AcOH, variation in equivalents).

Herein, we report the successful transformation of a 1(st) generation Grubbs metathesis catalyst into an asymmetric transfer hydrogenation (ATH) catalyst. Upon addition of a chiral amine ligand, an alcohol and a base, the 1(st) generation Hoveyda-Grubbs catalyst (HG-I) was found to promote the enantioselective reduction of acetophenone to 1-phenylethanol. After optimizing the order of addition and the reaction conditions, the substrate scope was assessed leading to enantiomeric excesses up to 97% ee. NMR experiments were run in order to get information about the in situ-generated ATH catalyst. Furthermore, the possibility to perform olefin metathesis and ketone transfer hydrogenation sequentially in one pot was demonstrated, and the first tandem olefin metathesis-ketone asymmetric transfer hydrogenation was carried out.

The employed catalyst, an earth-abundant metal cobalt(II) complex based on a pincer-type PNP ligand, shows good functional group and water tolerance to the presented olefin transfer hydrogenation reactions of various aliphatic and aromatic alkenes.

The intramolecular cycloaddition of o-quinone methides (o-QMs) with a carbonyl group has been envisaged and executed successfully in the context of constructing the complex and rare [6,6,6,6]-tetracyclic core found in the integrastatins, epicoccolide A, and epicocconigrone A. These transient o-QMs were generated easily from the oxidative dearomatization of the corresponding C2-(aryl)benzofuran by employing Oxone in acetone–water at rt. The subsequent cycloaddition with the carbonyl (or conjugated olefin) present on the C2-aryl group was spontaneous.

The first kinetically controlled, highly trans-selective (>98%) olefin cross-metathesis reaction is demonstrated using Ru-based catalysts. Reactions with either trans or cis olefins afford products with highly trans or cis stereochemistry, respectively. This E-selective olefin cross-metathesis is shown to occur between two trans olefins and between a trans olefin and a terminal olefin. Additionally, new stereoretentive catalysts have been synthesized for improved reactivity.

The enantioselective bromocyclization of allylic amides catalyzed by phosphorus-containing Lewis bases was examined in detail. A series of control experiments and NMR studies showed that a partially oxidized bis-phosphine generated in situ serves as the actual enantioselective catalyst. The reaction mechanism involves distinct roles of two Lewis basic sites, P and P=O, with P+ Br serving as a fine-tuning element for substrate fixation in the chiral environment, and P+ OBr as the Br+ transfer agent to the olefin. Catalyst loading could be reduced to as little as 1 mol %, and the reaction affords enantioenriched oxazolines with up to >99.5 % ee.

Substituted oxocene derivatives have been synthesized by Lewis acid catalyzed reactions of e-hydroxyalkene and substituted aromatic aldehydes. The Cu(OTf)2-bis-phosphine catalyzed reaction typically provides substituted dihydropyran derivatives through an olefin migration followed by a Prins cyclization. The corresponding reaction catalyzed by TMSOTf or BF3·OEt2 provided eight-membered cyclic ethers (oxocenes), selectively. This methodology provides convenient access to a variety of 2,4,8-trisubstituted oxocenes in good yields and excellent diastereoselectivities.

A facile synthesis based on the addition of ascorbic acid to a mixture of Na2 PdCl4, K2 PtCl6, and Pluronic P123 results in highly branched core-shell nanoparticles (NPs) with a micro-mesoporous dandelion-like morphology comprising Pd core and Pt shell. The slow reduction kinetics associated with the use of ascorbic acid as a weak reductant and suitable Pd/Pt atomic ratio (1:1) play a principal role in the formation mechanism of such branched Pd@Pt core-shell NPs, which differs from the traditional seed-mediated growth. The catalyst efficiently achieves the reduction of a variety of olefins in good to excellent yields. Importantly, higher catalytic efficiency of dandelion-like Pd@Pt core-shell NPs was observed for the olefin reduction than commercially available Pt black, Pd NPs, and physically admixed Pt black and Pd NPs. This superior catalytic behavior is not only due to larger surface area and synergistic effects but also to the unique micro-mesoporous structure with significant contribution of mesopores with sizes of several tens of nanometers.

Palladium(0)-catalyzed carbocyclization of 1,7-enynes mediated by (chlorodimethylsilyl)pinacolborane proceeds with 1,8-addition of the silicon and boron functions to give functionalized cyclohexane derivatives with boron attached to the exocyclic olefin. A variety of chromane dervatives are accessible by this method. In contrast to the analogous reactions with 1,6-enynes, the configuration of the newly formed stereogenic center is controlled by a stereogenic center present in the substrate.

Phosphine ligands have been successfully used along with Cu(I) for several catalytic reactions, nevertheless these ligands were less explored relatively for carbene involved reactions owing to the formation of carbene-phosphine ylides. In this report we successfully used three different phosphine stabilized Cu(I) complexes (1–3) as catalysts for chemoselective carbene insertion into the N–H bond of different aromatic amines over the formation of olefin (carbene dimerized product). In order to understand the substrate scope, different α-diazo esters have been reacted with large number of amines and all the reactions produced reasonably good yields under normal experimental conditions (38 examples). All the carbene inserted products have been isolated by column chromatography and fully characterized using standard spectroscopic techniques without any ambiguities. Several control reactions have been conducted in order to understand the importance of the type of phosphine ligands used in the catalysts 1–3 and found that without these catalysts we observed less selectivity (more of olefin as the product over N–H inserted product) and low yield. From this present study, it can be noted that the rigid framework phosphine ligands would be the better choice for carbene chemistry. The results obtained from the current studies would inspire chemists to develop more novel Cu(I)-phosphine catalysts for carbene related reactions including asymmetric versions in the near future.

We report highly chemo- and regioselective reductive transpositions of methyl carbonates to furnish olefin products with complementary regioselectivity to that of established Pd-catalysis. These Rh- and Ir-catalysed transformations proceed under mild conditions and enable selective deoxygenation in the presence of functional groups that are susceptible to reduction by metal hydrides.

A variety of structurally novel phosphoalkoxy ester derivatives were prepared via highly regio- and diastereoselective four-component reaction involving olefin, cyclic ether, halogen reagent and organic phosphate. All components can be varied flexibly and moderate to excellent yields were obtained at room temperature. Several of these ester derivatives showed strong anticancer activity against human lung adenocarcinoma cells.

Oxidative generation of synthetically important amidyl radicals from N-H amides is an appealing and yet challenging task. Previous methods require a stoichiometric amount of a strong oxidant and/or a costly noble-metal catalyst. We report herein the first electrocatalytic method that employs ferrocene (Fc), a cheap organometallic reagent, as the redox catalyst to produce amidyl radicals from N-aryl amides. Based on this radical-generating method, an efficient intramolecular olefin hydroamidation reaction has been developed.

A new method was developed to synthesize polyfunctionalized dihydrofuran and tetrahydrofuran derivatives from the three-component [2 + 2 + 1] cycloaddition of the diazoesters with aryl/alkenyl aldehydes and alkyne/olefin dipolarophiles by using a Ag(I) N-heterocyclic carbene complex as the catalyst. A carbonyl ylide intermediate was generated, which undertook an endo-type 1,3-dipolar cycloaddition to provide the desired dihydro-/tetrahydrofurans in high regio- and diastereoselectivities by using α-aryl or α-alkenyl diazoesters.

This paper reports a novel approach for the direct and facile synthesis of 1,2-oxyamino moieties via an intermolecular copper-catalyzed oxyamination of olefins. This strategy utilizes O-benzoylhydroxylamines as an electrophilic amine source and carboxylic acids as a nucleophilic oxygen source to achieve a modular difunctionalization of olefins. The reaction proceeded in a regioselective manner with moderate to good yields, exhibiting a broad scope of carboxylic acid, amine, and olefin substrates.

A novel Pd(II)-catalyzed approach for the direct synthesis of highly functionalized carbazoles from unprotected styrylindoles has been developed. The reaction features a variety of olefin substrates, which are readily switchable by subtle tuning of the reaction conditions. Investigations of the mechanism suggest that the C-H activation proceeds via enamine formation.

The synthesis of vinyl boronates and vinyl silanes was achieved by employing a Ru-catalyzed alkene-alkyne coupling reaction of allyl boronates or allyl silanes with various alkynes. The double bond geometry in the generated vinyl boronates can be remotely controlled by the juxtaposing boron- and silicon groups on the alkyne substrate. The synthetic utility of the coupling products has been demonstrated in a variety of synthetic transformations, including iterative cross-coupling reactions, and a Chan-Lam-type allyloxylation followed by a Claisen rearrangement. A sequential one-pot alkene-alkyne-coupling/allylation-sequence with an aldehyde to deliver a highly complex α-silyl-β-hydroxy olefin with a handle for further functionalization was also realized.

N-Alkyl-N-(2-vinylbenzyl)prop-2-en-1-amine derivatives undergo a one-pot olefin isomerization/aliphatic enamine ring-closing metathesis (RCM)/oxidation/1,3-dipolar cycloaddition sequence with the ruthenium complex, Ru(CO)HCl(PPh3)3, a second generation Hoveyda–Grubbs catalyst, and a 1,3-dipolarophile. Overall, in a single operation the reaction sequence converts simple benzylamine derivatives into isoindolo[1,2-a]isoquinolines with a π-conjugated four-ring system, through three unique ruthenium-catalyzed transformations.

Here we report a ternary catalyst system for the intramolecular hydroamidation of unactivated olefins using simple N-aryl amide derivatives. Amide activation in these reactions occurs via concerted proton-coupled electron transfer (PCET) mediated by an excited state iridium complex and weak phosphate base to furnish a reactive amidyl radical that readily adds to pendant alkenes. A series of H-atom, electron, and proton transfer events with a thiophenol cocatalyst furnish the product and regenerate the active forms of the photocatalyst and base. Mechanistic studies indicate that the amide substrate can be selectively homolyzed via PCET in the presence of the thiophenol, despite a large difference in bond dissociation free energies between these functional groups.

A catalyst-free and efficient method for simultaneous olefin hydrogenation and oxidation of arylboronate esters to phenols with hydrazine hydrate and molecular oxygen is presented. The process is based on the utilization of a readily available Lewis acidic arylboron compound, which evades common problems associated with the catalyst-free aerobic hydrogenation of olefins with diimide. Using an operationally simple procedure, the protocol smoothly delivers phenol derivatives and various alkanes in excellent yields with remarkable functional group compatibility. The method allows the reaction to be scaled up to 1 g of the starting materials.

The complex ABC-tricyclic structure of crotophorbolone, a derivative of the tigliane diterpenoids, was assembled by coupling of simple fragments. The six-membered C-ring fragment, having five contiguous stereocenters, was stereoselectively constructed from (R)-carvone. After attachment of the five-membered A-ring through the π-allyl Stille coupling reaction, the α-alkoxy bridgehead radical reaction effected the endo-cyclization of the seven-membered B-ring by forming the sterically congested bond at C9 and C10 stereospecifically and stereoselectively, respectively. Finally, the functional groups on the 5/7/6-membered ring system were manipulated by rhodium-catalyzed C2 olefin isomerization, C13 decarboxylative oxidation, and C4 hydroxylation, thus completing the first total synthesis of crotophorbolone.

The direct chlorination and bromination of (E)-enamines and (Z)-enamides to the corresponding (Z)-configurated α-chloroenamines, α-bromoenamines, and α-chloroenamides have been realized using NiCl2·6H2O or tetrabutyl ammonium bromide as a halide source and (1,1-diacetoxyiodo)benzene as an oxidant. The high stereoselective reactions which produce products with only (Z)-configurations can be attributed to the structure of the intermediates, the conformations of which are controlled by the electrostatic attractions between the positively charged nitrogen atoms and the oxygen atoms of the carbonyl group. This type of electrostatic effect has never been reported in olefin halogenations. For this reason, the three-membered bromonium ion is only a minor intermediate in the enamine bromination pathway. These methods open pathways to prepare α-chloroenamines and α-chloroenamides, which are not accessible via the currently used methods.

Herein, Nature’s flavin-mediated activation of complex (poly)enes has been translated to a small molecule paradigm culminating in a highly (Z)-selective, catalytic isomerization of activated olefins using (−)-riboflavin (up to 99:1 Z/E). In contrast to the prominent Z → E isomerization of the natural system, it was possible to invert the directionality of the isomerization (E → Z) by simultaneously truncating the retinal scaffold, and introducing a third olefin substituent to augment A1,3-strain upon isomerization. Consequently, conjugation is reduced in the product chromophore leading to a substrate/product combination with discrete photophysical signatures. The operationally simple isomerization protocol has been applied to a variety of enone-derived substrates and showcased in the preparation of the medically relevant 4-substituted coumarin scaffold. A correlation of sensitizer triplet energy (ET) and reaction efficiency, together with the study of additive effects and mechanistic probes, is consistent...