Eosin Y-catalyzed reductive homocoupling of para-quinone methides under visible-light.

In this manuscript, we demonstrate a visible-light driven dimerization of para-quinone methides using eosin Y catalyst via a reductive homocoupling process. This mild and operationally simple methodology was found to be compatible with a variety of differently substituted para-quinone methides and a broad range of tetra-arylethane derivatives were obtained in moderate to good yields (47%-87%).

Cavity Catalysis of an Enantioselective Reaction under Vibrational Strong Coupling.

Strong light-matter interaction is emerging as an exciting tool for controlling chemical reactions. Here, we demonstrate an L-proline-catalyzed direct asymmetric Aldol reaction under vibrational strong coupling. Both the reactants (4-nitrobenzaldehyde and acetone) carbonyl bands are coupled to an infrared photon and react in the presence of L-proline. The reaction mixture is eluted from the cavity, and the conversion yields and enantiomeric excess are quantified using NMR and chiral HPLC. The conversion yields increase by up to 90 % in ON-resonance conditions. Interestingly, a large increase in the conversion yield does not affect the enantiomeric excess. Further control experiments were carried out by varying the temperature, and we propose that the rate-limiting step may not be the deciding factor in enantioselectivity. Whereas the formation of the enamine intermediate is modified by cavity coupling experiments. For this class of enantioselective reactions, strong coupling does not change the enantiomeric excess, possibly due to the large energy difference in chiral transition states. Strong coupling can boost the formation of enamine intermediate, thereby favouring the product yield. This gives more hope to test polaritonic chemistry based on enantioselective reactions in which the branching ratios can be controlled.

Pd(II)-catalyzed annulation of terminal alkynes with 2-pyridinyl-substituted p-quinone methides: direct access to indolizines.

A Pd-catalyzed direct method has been developed to access 1,3-disubstituted indolizines. This reaction proceeds through a regiospecific annulation of terminal alkynes with 2-pyridinyl-substituted p-quinone methides and, in most of the cases, the desired 1,3-disubstituted indolizines were obtained in moderate to good isolated yields. The control experiments suggested that the reaction does proceed through a substrate-controlled regiospecific formal [3 + 2]-annulation pathway.

TfOH-Catalyzed Intramolecular Annulation of 2-(Aryl)-Phenyl-Substituted p-Quinone Methides under Continuous Flow: Total Syntheses of Selaginpulvilin I and Isoselagintamarlin A.

In this article, we describe a convenient method to access 9-aryl fluorene derivatives through a TfOH-catalyzed intramolecular 1,6-conjugate arylation of 2-(aryl)-phenyl-substituted p-quinone methides (QMs) under continuous flow using the microreaction technique. This method was found to be very effective for most of the p-QMs, and the corresponding 9-aryl fluorene derivatives were obtained in moderate to excellent yields. Moreover, this protocol was further elaborated to the first total syntheses of selaginpulvilin I and isoselagintamarlin A.

Construction of Oxygen- and Nitrogen-based Heterocycles from p-Quinone Methides.

In the last few years, there has been an explosive growth in the area of para-quinone methide (p-QM) chemistry. This boom is actually due to the unique reactivity pattern of p-QMs, and also their remarkable synthetic applications. In fact, p-QMs serve as synthons for unsymmetrical diaryl- and triarylmethanes, and also for the construction of diverse range of carbocycles and heterocycles. In the last few years, a wide range of structurally complex heterocyclic frameworks could be accessed through the synthetic transformations of structurally modified stable p-QMs. Therefore, the main focus of this review article is to cover the recent advancements in the transition-metal, Lewis acid and base-catalyzed/mediated synthetic transformations of the stable p-quinone methides (p-QMs) to oxygen- and nitrogen-containing heterocycles.

A Base-Mediated Approach Towards Dihydrofuro[2,3-b]Benzofurans from 2-Nitrobenzofurans and 1,3-Dicarbonyls.

A straight-forward approach for the synthesis of a dihydrofuro[2,3-b]benzofuran derivatives has been achieved through a base-mediated Michael addition of 1,3-dicarbonyls to 2-nitrobenzofurans followed by intramolecular cyclization. A variety of 1,3-dicarbonyls, including cyclic as well as trifluoromethylated ones, have been subjected to react with 2-nitrobenzofurans under optimal conditions, and the respective dihydrofuro[2,3-b]benzofurans could be accessed in moderate to excellent yields.

A One-Pot Approach to 2,3-Diarylbenzo[ b]furans through N-Heterocyclic Carbene-Catalyzed 1,6-Conjugate Addition Followed by Acid Mediated Dehydrative Annulation.

A one-pot protocol for the synthesis of 2,3-diarylbenzo[ b]furan derivatives through an N-heterocyclic carbene catalyzed 1,6-conjugate addition of aromatic aldehydes to 2-hydroxyphenyl-substituted para-quinone methides followed by acid-mediated dehydrative annulation has been developed. This protocol allowed us to access a wide range of 2,3-diarylbenzo[ b]furan derivatives in moderate to good yields.

1,6-Hydroolefination and Cascade Cyclization of p-Quinone Methides with Styrenes: Total Synthesis of (±)-Isopaucifloral F.

A Lewis acid-catalyzed intermolecular 1,6-hydroolefination of p-quinone methides with styrenes leading to vinyl diarylmethanes and indenes has been developed. This protocol was also elaborated to the total synthesis of (±)-isopaucifloral F. Besides, interestingly, the reaction between 2-alkynylated p-quinone methides and styrenes provided a straightforward access to dihydrobenzo[ a]fluorene derivatives in one pot with 100% atom-economy.

Tandem One-Pot Approach To Access 1,2,3-Triazole-fused Isoindolines through Cu-Catalyzed 1,6-Conjugate Addition of Me3SiN3 to p-Quinone Methides followed by Intramolecular Click Cycloaddition.

A Cu-catalyzed one-pot approach has been developed for the synthesis of 1,2,3-triazole-fused tricyclic heterocycles. This tandem approach actually involves the 1,6-conjugate addition of Me3SiN3 to o-alkynylated p-quinone methides followed by an intramolecular [3+2]-cycloaddition reaction. This protocol allowed us to access a wide range of 1,2,3-trazole-fused isoindoline derivatives in moderate to good yields.

Base-Catalyzed 1,6-Conjugate Addition of Nitroalkanes to p-Quinone Methides under Continuous Flow.

A mild base-catalyzed protocol for the synthesis of substituted nitroalkane derivatives has been developed under continuous flow using a microreaction technique. This transformation basically involves the 1,6-conjugate addition of nitroalkanes to p-quinone methides, leading to the substituted nitroalkanes in good to excellent yields.

1,6-Conjugate addition of zinc alkyls to para-quinone methides in a continuous-flow microreactor.

An efficient method for the synthesis of alkyl diarylmethanes through the 1,6-conjugate addition of dialkylzinc reagents to para-quinone methides (p-QMs) has been developed under continuous flow conditions using a microreactor. This protocol allows to access unsymmetrical alkyl diarylmethanes in moderate to excellent yields using a wide range of p-QMs and dialkylzinc reagents. Interestingly, this transformation worked well without the requirement of a catalyst.

Bis(amino)cyclopropenylidene-Catalyzed 1,6-Conjugate Addition of Aromatic Aldehydes to para-Quinone Methides: Expedient Access to α,α'-Diarylated Ketones.

A bis(amino)cyclopropenylidene-catalyzed direct method for the synthesis of α,α'-diarylated ketones from aromatic as well as heteroaromatic aldehydes has been developed. This unprecedented organocatalytic protocol offers access to a wide range of α,α'-diarylated ketones in moderate to excellent yields under mild conditions through umpolung of aldehydes followed by 1,6-conjugate addition with para-quinone methides.