Bis(amino)cyclopropenium Ion as a Hydrogen-Bond Donor Catalyst for 1,6-Conjugate Addition Reactions

Bis(aminoalkyl)cyclopropenylidene (BAC)-Catalyzed Intramolecular Annulation of 2-(2-Formylaryl)-aryl-Substituted p-Quinone Methides to 9-Phenanthrol Derivatives

This article describes a bis(aminoalkyl)cyclopropenylidene (BAC)-catalyzed intramolecular annulation of strategically designed 2-(2-formylaryl)-phenyl-substituted p-quinone methides (p-QMs) to access 9-phenanthrol derivatives and related carbocycles. In addition, the synthetic utility of this methodology has been demonstrated in the synthesis of the seven-membered carbocyclic core of resveratrol-based natural products (±)-shoreaphenol and (±)-malibatol A.

A HMPA-H2O mediated oxygenative carbocyclization of 2-alkynylphenyl-substituted p-quinone methides to indenones

Herein, we report a transition-metal and base-free protocol to access a wide range of functionalized indenone derivatives through a HMPA-H2O-mediated oxygenative annulation of 2-alkynylphenyl-substituted p-quinone methides. This method worked effectively for most of the p-QMs investigated and the corresponding indenone derivatives were obtained in moderate to good yields. This methodology was further extended to the formal synthesis of one of the resveratrol based natural products, (±)-isopaucifloral F.

Phosphine-Catalyzed γ'-Carbon 1,6-Conjugate Addition of α-Succinimide Substituted Allenoates with Para-Quinone Methides: Synthesis of 4-Diarylmethylated 3,4-Disubstituted Maleimides

In this paper, an interesting γ'-carbon 1,6-conjugate addition for phosphine-catalyzed α-succinimide substituted allenoates has been disclosed. A wide array of substrates was found to participate in the reaction, resulting in the production of diverse 4-diarylmethylated 3,4-disubstituted maleimides with satisfactory to outstanding yields. Furthermore, a plausible mechanism for the reaction was proposed by the investigators.

Aminocyclopropenium as a novel hydrogen bonding organocatalyst for cycloaddition of carbon disulfide and epoxide to prepare cyclic dithiocarbonate

The smallest Hückel aromatic ring cyclopropenium substituted by electron-donating C-amino groups produced a aminocyclopropenium electron-rich cation. A bifunctional aminocyclopropenium halide catalyst installed with bis-(hydroxyethyl) functions on the amino group was then designed. A typical (diethanolamino)cyclopropenium halide catalyst C5·I was screened optimally for the cycloaddition of carbon disulfide into an epoxide to produce cyclic dithiocarbonate with an excellent conversion (95%) and high selectivity (92%). The electrostatic enhancement of alkyl C-H HBD capability was implemented via vicinal positive charges on the cyclopropenium core, and the acidity of the terminal O-H hydrogen proton increased by intramolecular H-bonding between the two hydroxy groups on the diethanolamino group (O-H⋯O-H). Then, a hybrid H-bond donor comprising enhanced alkyl C-H and hydroxy O-H was formed. The hybrid HBD offered by aminocyclopropenium was vital in activating the epoxide and stabilizing the intermediate, resulting in reduced O/S scrambling. Moreover, weakly coordinated iodide anion served as a nucleophilic reagent to open the ring of the epoxide. The cooperative catalytic mechanism of the HBD cation and halide anion was supported by NMR titrations and control experiments. Eleven epoxides with various substituents were converted into the corresponding cyclic thiocarbonate with high conversion and selectivity under mild conditions (25 °C, 6 h) without a solvent. The cycloaddition of carbon disulfide with epoxides catalyzed by aminocyclopropenium developed a new working model for hydrogen bonding organocatalysis.

Synthesis of Functionalized Five-Membered Heterocycles from Epoxides: A Hydrogen-Bond Donor Catalytic Approach

The synthesis of highly functionalized five-membered oxa- and aza-heterocycles has been reported utilizing hydrogen-bond donor (HBD) catalysis. In this method, an epoxide was taken as a substrate and reacted with functionalized arylidene/alkylidene malononitrile derivatives in the presence of a newly designed HBD catalyst. In all the cases, the products 2,5-disubstituted tetrahydrofurans (2,5-THFs) were obtained in good to excellent yields (up to 86%) with high diastereoselectivity (dr up to 99:1) as a single regioisomer. The stereochemistry at the 2- and 5-positions of the five-membered ring has been confirmed by single-crystal X-ray analysis, and cis is found to be the major product. The same strategy has been further utilized to obtain substituted oxazolidines whenever the epoxide has been reacted with isocyanate as an electrophile. In order to induce enantioselectivity, a chiral epoxide has been reacted with both the electrophiles in the presence of the same catalyst system to afford the single stereoisomer of the final products. This synthetic methodology involves a low catalyst loading and ambient reaction condition and has been generalized with various substituents present in the starting electrophiles to produce the resultant products in acceptable yields and stereoselectivity.

Solvent-Mediated Tunable Regiodivergent C6- and N1-Alkylations of 2,3-Disubstituted Indoles with p-Quinone Methides

Indium-catalyzed, solvent-enabled regioselective C6- or N1-alkylations of 2,3-disubstituted indoles with para-quinone methides are developed under mild conditions. Notably, highly selective and switchable alkylations were selectively achieved by adjusting the reaction conditions. Moreover, scalability and further transformations of the alkylation products are demonstrated, and this operationally simple methodology is amenable to the late-stage C6-functionalization of the indomethacin drug. The reaction pathways were explained with the support of experimental and density functional theory studies.

Room-Temperature ZnBr2 -Catalyzed Regioselective 1,6-Hydroarylation of Electron-Rich Arenes to para-Quinone Methides: Synthesis of Unsymmetrical Triarylmethanes

A mild and efficient Zn(II)-catalyzed regioselective 1,6-hydroarylation of para-quinone methides (p-QMs) with electron-rich arenes protocol is reported. A variety of electron-rich arenes and para-quinone methides are well tolerated under mild conditions, delivering a broad range of triarylmethanes in good to excellent yields. The present method also works well for the hydroarylation of p-QMs with other nucleophiles, such as aniline, indole and phenol derivatives, offering the corresponding triarylmethanes with good yields under the standard conditions. The possible mechanism for the formation of C(sp³ )-C(sp² ) bonds in hydroarylation reactions has been explored by step-by-step control experiments, and the reaction may follow a second-order manner in a chemical kinetic study.

Metal-free, Phosphoric Acid-catalyzed Regioselective 1,6-Hydroarylation of para-Quinone Methides with Indoles in Water

An efficient, cheap and green protocol for the highly regioselective 1,6-hydroarylation of para -quinone methides ( p -QMs) with indoles at the C-3 position has been established by phosphoric acid catalysis in water under the transition-metal-free reaction conditions. A wide range of indole derivatives and para -quinone methides ( p -QMs) are compatible for the reaction, affording the corresponding 1,6-hydroarylation products with good to excellent yields. The possible mechanism of the reaction has been explored by step-by-step control experiments. The protocol is convenient for practical application, leading a safe, green and feasible way for the formation of C-3 diarylmethyl functionalized indole derivatives.

1,6-Addition of 1,2,3-NH triazoles to para-quinone methides: Facile access to highly selective N1 and N2 substituted triazoles

The regioselective syntheses of N¹ and N² substituted triazoles through a 1,6-addition reaction of 1,2,3-NH triazoles to p-quinone methide were achieved under mild reaction conditions. The present reactions showed superior results in terms of selectivity, mild reaction conditions, short reaction time and broad substrate scope with good functional-group compatibility. Considering the high synthetic value of N¹- and N²-substituted compounds and p-QM related research, the present strategy will greatly benefit researchers in various fields.

Tropylium Salt-Promoted Vinylogous Aza-Michael Addition of Carbamates to para-Quinone Methides: Elaboration to Diastereomerically Pure α,α'-Diarylmethyl Carbamates

Carbocation catalysis is emerging as an important subarea of Lewis acid catalysis. Some stable and isolable carbocations have been successfully utilized as Lewis acid catalysts and promoters in many synthetic transformations. In this manuscript, we report a tropylium cation-promoted vinylogous aza-Michael addition of carbamates to para-quinone methides (QMs) to access a wide range of unsymmetrical α,α'-diarylmethyl carbamates. This mild protocol was effective for the vinylogous conjugate addition of (-)-menthyl carbamate to p-QMs, and the respective diastereomerically pure α,α'-diarylmethyl carbamate derivatives could be obtained in excellent yields and diastereoselectivities (up to >20:1 de).

TEMPO-Mediated Synthesis of Indolyl/Imidazo[1,2-a]pyridinyl-Substituted para-Quinone Methides from Butylated Hydroxytoluene

A series of indolyl or imidazo[1,2-a]pyridinyl-substituted para-quinone methides (p-QMs) is prepared by a metal-free, TEMPO-mediated cross-dehydrogenative coupling of butylated hydroxytoluene (BHT) with indoles or imidazo[1,2-a]pyridines in good to high yields. Broad substrate scope with respect to indoles and imidazo[1,2-a]pyridines, good functional group tolerance, and acid/base-free conditions are advantageous feature of the developed protocol. The method was amenable for scale-up on the gram scale. Based on control experiments, a reaction mechanism is proposed to describe this transformation.

Stereoselective Synthesis of Tri- and Tetrasubstituted Olefins via 1,6-Additions of Diazo Compounds and Their Precursors to p-Quinone Methides

Reactions of para-quinone methides (p-QMs) with α-diazo-β-ketosulfones and their corresponding esters as well as simple β-dicarbonyl compounds and β-ketosulfones have been carried out under basic conditions. While the reaction of diazosulfone with p-QMs afforded trisubstituted olefins via deacylative 1,6-addition and elimination, α-diazo-β-ketoesters and various active methylene compounds such as 1,3-dicarbonyls and β-ketosulfones afforded tetrasubstituted olefins via 1,6-addition and aerial oxidation. These simple, environmentally benign, and mechanistically diverse protocols provided the products in moderate to excellent yields and selectivities.

Recent advances in the organocatalytic applications of cyclopropene- and cyclopropenium-based small molecules

The development of novel small molecule-based catalysts for organic transformations has increased noticeably in the last two decades. A very recent addition to this particular research area is cyclopropene- and cyclopropenium-based catalysts. At one point in time, particularly in the mid-20^th century, much attention was focused on the structural aspects and physical properties of cyclopropene-based compounds. However, a paradigm shift was observed in the late 20^th century, and the focus shifted to the synthetic utility of these compounds. In fact, a wide range of cyclopropene derivatives have been found serving as valuable synthons for the construction of carbocycles, heterocycles and other useful organic compounds. In the last few years, the catalytic applications of cyclopropene/cyclopropenium-based compounds have been uncovered and many synthetic protocols have been developed using cyclopropene-based compounds as organocatalysts. Therefore, the main objective of this review is to highlight recent developments in the catalytic applications of cyclopropene-based small molecules in different areas of organocatalysis such as phase-transfer catalysis (PTC), Brønsted base catalysis, hydrogen-bond donor catalysis, nucleophilic carbene catalysis, and electrophotocatalysis developed within the past two decades.

Silver(i)-catalyzed novel ipso-cycloaddition and retro-Friedel–Crafts reaction of ortho-hydroxyphenyl-substituted para-quinone methides

The Lewis acid AgPF6 was found to catalyze novel [2 + 2] ipso-cycloaddition and retro-Friedel–Crafts reaction of o-hydroxyphenyl-substituted p-quinone methides in the presence of H2O, affording the polysubstituted triarylmethanes in up to 97% yields.