Photocascade chemoselective controlling of ambident thio(seleno)cyanates with alkenes via catalyst modulation.

Controlling the ambident reactivity of thiocyanates in reaction manifolds has been a long-standing and formidable challenge. We report herein a photoredox strategy for installing thiocyanates and isothiocyanates in a controlled chemoselective fashion by manipulating the ambident-SCN through catalyst modulation. The methodology allows redox-, and pot-economical 'on-demand' direct access to both hydrothiophene and pyrrolidine heterocycles from the same feedstock alkenes and bifunctional thiocyanomalonates in a photocascade sequence. Its excellent chemoselectivity profile was further expanded to access Se- and N-heterocycles by harnessing selenonitriles. Redox capability of the catalysts, which dictates the substrates to participate in a single or cascade catalytic cycle, was proposed as the key to the present chemodivergency of this process. In addition, detailed mechanistic insights are provided by a conjugation of extensive control experiments and dispersion-corrected density functional theory (DFT) calculations.

A two carbon homologation of Friedel-Crafts alkylation enabled by photochemical alkene stitching: modular assembly of cyclolignans.

Herein we report an efficient two-carbon homologated variant of Friedel-Crafts alkylation via photochemical radical alkene stitching. Readily available feedstock alkenes are used as bridges between photogenerated alkyl radicals and arenes, opening a route to γ-aryl-carbonyls for chemo-divergent access to aryltetralone and γ-lactones, a gateway to 2,7'-cyclolignans.

Organo-photoredox-Catalyzed Selective Mono- and Bis-C-H Alkylation of Electron-Rich (Hetero)Arenes.

Herein, we disclose a simple strategy for the C-H alkylation of electron-rich (hetero)arenes with alkyl bromides employing visible-light-mediated organo-photocatalytic SET processes. The generality of this method has been evidenced by the inclusion of a variety of alkyl radicals (α-alkyl-carbonyl, benzyl, cyanomethyl) as well as diverse biologically active electron-rich arenes and (hetero)arenes under mild conditions. The extent of alkylation with alkyl bromides was found to be controlled by introducing Zn(OAc)2 as a bromide scavenger, ensuring the blocking of potential bromo-arene byproduct formation under photoredox conditions. In addition, a sequential C-H alkylation strategy for selective bis-alkylation has also been developed via chronological incorporation of different alkyl radical precursors in one pot quite efficiently.

Recent Developments on Photochemical Synthesis of 1,n-Dicarbonyls.

1,n-dicarbonyls are one of the most fascinating chemical feedstocks finding abundant usage in the field of pharmaceuticals. Besides, they are utilized in a plethora of synthesis in general synthetic organic chemistry. A number of 'conventional' methods are available for their synthesis, such as the Stetter reaction, Baker-Venkatraman rearrangement, oxidation of vicinal diols, and oxidation of deoxybenzoins, synonymous with unfriendly reagents and conditions. In the last 15 years or so, photocatalysis has taken the world of synthetic organic chemistry by a remarkable renaissance. It is fair to say now that everybody loves the light and photoredox chemistry has opened a new gateway to organic chemists towards milder, more simpler alternatives to the previously available methods, allowing access to many sensitive reactions and products. In this review, we present the readers with the photochemical synthesis of a variety of 1,n-dicarbonyls. Diverse photocatalytic pathways to these fascinating molecules have been discussed, placing special emphasis on the mechanisms, giving the reader an opportunity to find all these significant developments in one place.

Photocatalytic sequential C-H functionalization expediting acetoxymalonylation of imidazo heterocycles.

The importance of functionalized imidazo heterocycles has often been featured in several impactful research both from academia and industry. Herein, we report a direct C-3 acetoxymalonylation of imidazo heterocycles using relay C-H functionalization enabled by organophotocatalysis starring zinc acetate in the triple role of an activator, ion scavenger as well as an acetylating reagent. The mechanistic investigation revealed a sequential sp2 and sp3 C-H activation, followed by functionalization driven by zinc acetate coupled with the photocatalyst PTH. A variety of imidazo[1,2-a]pyridines and related heterocycles were explored as substrates along with several active methylene reagents, all generating the products with excellent yields and regioselectivity, thus confirming excellent functional group tolerability.

Zinc acetate-promoted blocking of the ATRA process with alkyl halides enabling photochemical alkylamination of olefins.

Organic photoredox-catalyzed alkylamination of olefins is performed with alkyl halides and nitrile solvent by blocking the traditional photoredox-ATRA process with Zn(OAc)2. A range of carbon-centered radicals (α-alkylcarbonyl, benzyl, cyanomethyl) are effectively participating in this strategy giving rise to versatile carboamination products with high synthetic value.

Organic Dye-Catalyzed Intermolecular Radical Coupling of α-Bromocarbonyls with Olefins: Photocatalytic Synthesis of 1,4-Ketocarbonyls Using Air as an Oxidant.

An organic dye-catalyzed visible light-promoted ketocarbonylation protocol of vinyl arenes has been disclosed with the help of α-bromocarbonyls where aerial oxygen played a role of an oxidant to install the keto-oxygen functionality. This unique process is compatible with both internal and terminal olefins and tolerates a diverse array of functional groups (ketone, ester, amide, diketones, ketoester, and malonate). This process is mild and environmentally friendly and deals with greener oxidants like oxygen, affording 1,4-ketocarbonyls as a value-added end product.

Photoredox-Catalyzed Intermolecular Radical Arylthiocyanation/Arylselenocyanation of Alkenes: Access to Aryl-Substituted Alkylthiocyanates/Alkylselenocyanates.

An efficient and highly selective approach for intermolecular arylthiocyanation/arylselenocyanation of alkenes has been reported under mild conditions. Using diazonium salts as the arylating agent and ammonium thiocyanate as the thiocyanate source, chemoselective difunctionalization of alkenes has been done under irradiation of visible light. Both styrenes and acrylates work well to deliver various aryl-substituted alkylthiocyanates in good to excellent yields. In addition, hitherto unknown ß-aryl alkylselenocyanates were also synthesized using the developed protocol with potassium selenocyanate.

TBAI/TBHP-Promoted Generation of Malonyl Radicals: Oxidative Coupling with Styrenes Leads to γ-Keto Diesters.

A metal-free protocol for oxidative coupling of malonic esters with styrenes to form γ-keto diesters has been developed. Key to the success of this process is the generation of malonyl radicals from unfunctionalized malonic esters under organo-catalysis conditions with TBAI and TBHP. This process tolerates both terminal and internal olefins with diverse malonic esters. It provides a new green metal-free alternative to traditional metal mediated process for generation of malonyl radicals and there by γ-keto diesters.

Visible light mediated intermolecular [3 + 2] annulation of cyclopropylanilines with alkynes.

Intermolecular [3 + 2] annulation of cyclopropylanilines with alkynes is realized using visible light photoredox catalysis, yielding a variety of cyclic allylic amines in fair to good yields. This method exhibits significant group tolerance particularly with heterocycles. It can also be used to prepare complex heterocycles such as fused indolines.

Synthetic studies on schisandra nortriterpenoids. Stereocontrolled synthesis of enantiopure C-5-epi ABC ring systems of micrandilactone A and lancifodilactone G using RCM.

A stereocontrolled approach for the construction of ABC ring systems of micrandilactone A and lancifodilactone G has been developed. The synthesis involves construction of an enantiopure functionalized cycloheptene derivative 17 through RCM of the dienol 14 prepared from the known D-mannitol-derived unsaturated ester 12. A remarkable regioselectivity during hydroboration of the cycloheptene derivative 17 was observed during its transformation to the cycloheptanone 20. RCM of the diene 24 prepared stereoselectively from 20 gave the spiro-dihydrofuran 25. The ketal unit in 25 was then converted into the carbinols 28 and 36. A bromonium ion initiated highly stereocontrolled intramolecular etherification in 28 and 37 led to the tricyclic ethers 29 and 38, respectively. Reductive removal of bromine from 29 and 38 followed by RuO(4) oxidation led to the furo-furanone derivatives 31 and 40, the C-5-epi ABC ring systems of the schisandra nortriterpenoids 1 and 2.