Synlett

A facile one-step synthetic approach to dihydrofuro-[3,2-c]chromenones and furo[3,2-c]chromenones by the reaction of methyl enol ethers with 4-hydroxycoumarins under metal-free conditions is presented. Dihydrofuro[3,2-c]chromenones and furo[3,2-c]-chromenones were selectively obtained by controlling the stoichiometry of boron trifluoride diethyl etherate. An unexpected aryl-group migration followed by aromatization of the furan moiety, leading to a variety of furo[3,2-c]chromenone derivatives in good yields, is reported.

A photocatalytic synthesis of aryl allyl sulfones by a radical cascade reaction from an aryl diazonium salt, DABCO·(SO2)2 and 3-bromoprop-1-ene has been developed. This reaction employed ­DABCO·(SO2)2 as the SO2 source and a polyaniline–graphitic carbon ­nitride–titanium dioxide composite as the photocatalyst. A series of substrates were tolerated, providing the corresponding products in good yields. Moreover, the photocatalyst could be readily recovered and reused several times with only a slight decrease in its catalytic activity.

Free-radical-promoted tandem trifluoromethylthiolation of alkenes with AgSCF3, which provides an efficient protocol for the construction of SCF3-modified indolo[2,1-a]isoquinolines with moderate to good yields. A series of scaled-up experiments, other substrate transformations, and radical inhibition experiments were operated to verify the merit of this reaction system.

The application of bifunctional organocatalysts in organic chemistry has advanced remarkably owing to their high stability to moisture and air, relatively low toxicity, ease of handling, and recoverability. Among chiral bifunctional organocatalysts, squaramides have emerged as a privileged catalyst in recent years. This review article presents a comprehensive report on fine-tunable bifunctional chiral squaramide-catalyzed sulfa-Michael addition, which is based upon synchronous interplay of synergistic ‘electrophilic–nucleophilic’ dual activation strategies via multiple H-bonding interactions for the construction of both common and complex molecular entities bearing multiple stereocenters. Mechanistic discussions are kept brief, but significant understandings have been recorded. The contribution of squaramide catalyst to the construction of C–S bonds via sulfa-Michael addition has been applied in medicinal, natural, and industrial chemistry. Attention is focused on summarizing the progress made in chiral squaramide-catalyzed asymmetric sulfa-Michael addition and subsequent cascade/domino reaction sequences between 2011 and 2022.1 Introduction2 Quinine-Squaramide Organocatalysis3 Iminophosphorane-Squaramide Organocatalysis4 Chinchona-Squaramide Organocatalysis5 trans-1,2-Diaminocyclohexane-Squaramide Organocatalysis6 Conclusion

This review is devoted to underpinning the contributions of Indian researchers towards asymmetric organocatalysis. More specifically, a comprehensive compilation of reactions mediated by a wide range of non-covalent catalysis is illustrated. A detailed overview of vividly catalogued asymmetric organic transformations promoted by hydrogen bonding and Brønsted acid catalysis, alongside an assortment of catalysts is provided. Although asymmetric organocatalysis has etched itself in history, we aim to showcase the scientific metamorphosis of Indian research from baby steps to large strides within this field. 1 Introduction2 Non-Covalent Catalysis and Its Various Activation Modes3 Hydrogen-Bonding Catalysis3.1 Urea- and Thiourea-Derived Organocatalysts3.1.1 Thiourea-Derived Organocatalysts3.1.2 Urea-Derived Organocatalysts3.2 Squaramide-Derived Organocatalysts3.2.1 Michael Reactions3.2.2 C-Alkylation Reactions3.2.3 Mannich Reactions3.2.4 [3+2] Cycloaddition Reactions3.3 Cinchona-Alkaloid-Derived Organocatalysts3.3.1 Michael Reactions3.3.2 Aldol Reactions3.3.3 Friedel–Crafts Reactions3.3.4 Vinylogous Alkylation of 4-Methylcoumarins3.3.5 C-Sulfenylation Reactions3.3.6 Peroxyhemiacetalisation of Isochromans3.3.7 Diels–Alder Reactions3.3.8 Cycloaddition Reactions3.3.9 Morita–Baylis–Hilman Reactions4 Brønsted Acid Derived Organocatalysts4.1 Chiral Phosphoric Acid Catalysis4.1.1 Diels–Alder Reactions4.1.2 Addition of Ketimines4.1.3 Annulation of Acyclic Enecarbamates5 Conclusion

The industrial fragrance compound dehydromuscone was synthesized in five linear steps and 19% overall yield. The synthesis features a highly efficient nondiluted ring-closing metathesis macrocyclization reaction as a key step that proceeds at a 0.2 M concentration in the presence of 0.1 mol% Nitro-Grela catalyst. The synthesis employs commercially available linear starting materials and is shorter by at least two steps than the current industrial synthesis route.

Electrophilic and nucleophilic substitutions of aromatic substrates share common mechanistic pathways. In both scenarios reacting species attack rings at the unsubstituted (C–H) positions, giving cationic Wheland intermediates or anionic Meisenheimer complexes. However, the following step of rearomatization breaks the intrinsic symmetry, due to different leaving group ability of proton and hydride anion, respectively. In effect, electron-deficient arenes are prone to transformations unparalleled in electrophilic chemistry. In our article, we present transformations of anionic σH-adducts, formed between nitroarenes and carbanions of the Corey–Chaykovsky reagents. Depending on structure of the substrates and reaction conditions, the intermediates undergo cyclization to cyclopropanes (norcaradienes) or base-induced elimination to the alkylated products. Mechanistic studies reveal that order of the carbanions controls competition between the processes, due to steric hindrance developing at the β-elimination step.1 Introduction2 Cyclopropanation of Nitronaphthalenes3 Alkylation of Nitropyridines4 Mechanistic Studies5 Summary and Outlook

The synthesis of a series of planar chiral P,N-ferrocenylpyrrolidine-containing ligands, with varying substituents at the phosphorus donor atom, is described. The phosphorus donor atom was introduced via a diastereoselective ortho-directed metalation of N-methylpyrrolidinyl ferrocene followed by a quench with various chlorophosphines. These P,N systems are very active in Pd-catalyzed allylic alkylation of 1,3-diphenylpropenyl acetate with dimethylmalonate (conversions of up to 100%) and demonstrated good levels of enantioselectivity (up to 85% ees). Good selectivity for the (R)-enantiomer was observed and mechanistic studies, involving X-ray crystallography and NMR spectroscopic experiments, were employed to help rationalize the observed stereochemical outcome of the reaction.

A carbon–carbon (C–C) single bond longer than 1.7 Å shows unique bond flexibility, even though a C–C single bond is typically rigid and robust. We report here that the bond length of flexible C–C single bonds surrounded by bulky alkyl groups in novel hexaphenylethane-type hydrocarbons might be affected by weak noncovalent interactions, such as London dispersion. Thanks to London dispersion, an ultralong and flexible C–C single bond exhibits an obvious bond contraction. X-ray analyses and Raman spectroscopy provided direct information regarding the bond length and strength, whereas density functional theory calculations explained the bond contraction driven by London dispersion. The change in bond length of an extremely elongated flexible C–C bond would be a good probe for quantifying weak interactions that are usually difficult to detect.

In this joint theoretical and experimental study, an analysis of weak interligand noncovalent interactions within Co(IV) [Cp*Co(phpy)X]+ cobaltacycles (phpy = 2-phenylenepyridine, κ
C,N
) was carried out by using the independent gradient model/intrinsic bond strength index (IGM/IBSI) method to evaluate the dependency of the catalytically desired reductive elimination pathway (RE) on the nature of the X ligand. It is shown that the barrier for activation of the RE pathway correlates directly with the IBSI of the X-to-carbanionic chelate’s carbon. This correlation suggests that in silico prediction of which X ligand is more prone to operate an efficient Cp*Co-catalyzed directed X-functionalization of an aromatic C–H bond is attainable. A set of experiments involving various sources of X ligands supported the theoretical conclusions.