Phosphine-Catalyzed [3 + 3]-Domino Cycloaddition of Ynones and Azomethine Imines To Construct Functionalized Hydropyridazine Derivatives

Conceptual advances in nucleophilic organophosphine-promoted transformations

Catalysis by trivalent nucleophilic organophosphines has emerged as an essential tool in organic synthesis. Several new organic transformations promoted by phosphines substantiate and complement the existing synthetic chemistry tools. Mere design of the substrate and reagent combinations has introduced new modes of reactivity patterns, which are otherwise difficult to achieve. These design considerations have led to the rapid build-up of complex molecular entities and laid a solid foundation to synthesise bioactive natural products and pharmaceuticals. This article presents an overview of some of the conceptual advances, including our contributions to nucleophilic organophosphine chemistry. The scope, limitations, mechanistic insights, and applications of these metal-free transformations are discussed elaborately.

Oxidative (3+3) Cycloaddition/Ring-Opening Reactions of Simple Urea Derivatives and N,N'-Cyclic Azomethine Imines to Construct 1,2,3,5-Tetrazine-4(1H)-one Derivatives

A novel oxidative (3 + 3) cycloaddition/ring-opening reaction of N,N'-cyclic azomethine imines with the in situ generated diaza-oxylallyl cations from simple urea derivatives in the presence of base and PhI(OAc)₂ has been developed. This transformation performs well over a broad substrate scope, which provides facile and rapid access to 1,2,3,5-tetrazine-4(1H)-one derivatives in good yields.

Diastereodivergent Synthesis of Pentacyclic Spiroindolines via a Magnesium(II)-Catalyzed Cascade Reaction of N,N'-Cyclic Azomethine Imines with Indolyl-Substituted Isocyanides

Magnesium(II)-catalyzed cascade reactions of N,N'-cyclic azomethine imines with indolyl-substituted isocyanides are reported herein. The method exhibited a high functional group tolerance and broad substrate scope. A series of anti-pentacyclic spiroindolines containing N,N'-fused heterocycle skeletons were obtained in up to 82% yield with 8.5:1 dr under mild reaction conditions. Intriguingly, a sequential HOAc-mediated protonation results in a diastereoenriched epimerization, which gives rise to the syn-pentacyclic spiroindolines as the sole isomers.

Microwave-assisted decarboxylative reactions: advanced strategies for sustainable organic synthesis

Recent advances in the microwave-assisted decarboxylative reactions of carboxylic acids and their derivatives, including transition-metal-catalyzed and metal-free approaches, are summarized.

Recent Advances in Lewis Base-Catalysed Chemo-, Diastereo- and Enantiodivergent Reactions of Electron-deficient Olefins and Alkynes

Lewis base catalysis provides powerful synthetic strategies for the selective construction of carbon-carbon and carbon-heteroatom bonds. Thus continuous efforts have been deployed to develop effective methodologies involving Lewis base catalysis. The nucleophilicity and steric hindrance of Lewis base catalyst often plays a major role in catalytic reactivity and selectivity in the reaction. In the past decades, tremendous progress has been made in the divergent construction of valuable motifs under Lewis base catalysis. In this review, we provide a comprehensive and updated summary of Lewis base-catalysed chemo-, diastereo- and enantiodivergent reaction, as well as the related mechanism will be highlighted in detail.

Phosphine-Catalyzed Annulations Based on [3+3] and [3+2] Trapping of Ketene Intermediates with Thioamides

With the aim of developing novel annulations via ketene intermediates, allenyl imide and alkynoates bearing good leaving groups are used for their function in a tandem conjugate addition-elimination reaction (SN2' type) promoted by nucleophilic phosphine catalysts. By utilizing thioamides as 1S,3N-bis-nucleophiles, [3+3] and [3+2] annulations have been established to allow rapid access to 1,3-thiazin-4-ones and 5-alkenyl thiazolones in high yields, respectively. Furthermore, the possible reaction mechanisms are proposed on the basis of deuterium labeling experiments and density functional theory calculations.

Phosphine-Catalyzed Synthesis of 3-Allyl-4-pyrones by the Tandem Reaction of Diynones and Allylic Alcohols

A simple and effective tandem reaction of diynones and allylic alcohols was developed to afford functionalized 3-allyl-4-pyrones in moderate to excellent yields. This protocol underwent a Michael addition─Claisen rearrangement─O-cyclization process, which exhibited broad substrate tolerance, high regioselectivity, and atom economy under a metal-free condition. Moreover, functional transformation of the products was also further studied.

Lewis Acid Mediated [3+2] and [3+3] Annulations of an Azomethine Imine with Donor–Acceptor Cyclopropanes

Two different Lewis acids were used for developing [3+2] and [3+3] regioselective cycloaddition reactions of an azomethine imine with activated cyclopropanes. Scandium(III) triflate catalyzes a [3+2] cycloaddition reaction of the azomethine imine with cyclopropanes to form tetrahydropyrazolone derivatives and tricyclic tetrahydrofuran derivatives in moderate yields. Complementary to this, a novel [3+3] cycloaddition reaction of the azomethine imine with activated cyclopropanes was developed by using EtAlCl2 as a Lewis acid to form hexahydropyridazinone derivatives in high regioselectivity.

Synthesis of 2-chromanone-fused [3.2.0] bicycles through a phosphine-mediated tandem [3 + 2] cyclization/intramolecular Wittig reaction

A phosphine-mediated tandem [3 + 2] cyclization/intramolecular Wittig reaction of alkynone is described. 2-Chromanone-fused bicyclo[3.2.0]heptenones were synthesized in moderate to high yields with remarkably high regio- and diastereoselectivities.

An overview of the reactions involving azomethine imines over half a decade

Azomethine imines constitute a versatile class of 1,3-dipoles which was used extensively for biologically relevant N-heterocycle synthesis – a five-year recap.

Synthesis of spiro(indoline-2,3′-hydropyridazine) via an “on-water” [4 + 2] annulation reaction

An on-water [4 + 2] annulation reaction between 2-methyl-3H-indolium salt and α-bromo N-acyl hydrazone has been developed. The environmentally friendly strategy provides the first facile access to spiro(indoline-2,3'-hydropyridazine) scaffolds.

Phosphine-Promoted [4 + 3] Annulation of Allenoate with Aziridines for Synthesis of Tetrahydroazepines: Phosphine-Dependent [3 + 3] and [4 + 3] Pathways

In this manuscript, phosphine-dependent [3 + 3] and [4 + 3] annulation reactions of allenoate with aziridines were disclosed. The alkyldiphenylphosphine-promoted [4 + 3] annulation of allenoate with aziridines has been achieved under mild conditions, providing biologically interesting functionalized tetrahydroazepines in moderate to excellent yield with moderate to excellent regioselectivity and diastereoselectivity.

Phosphine-dependent [3 + 3] and [4 + 3] annulation reactions of allenoate with aziridines were disclosed. The alkyldiphenylphosphine-promoted [4 + 3] annulation produced tetrahydroazepines.

DABCO-mediated [3+3] cycloaddition of azomethine imines with in situ generated nitrile oxides from hydroximoyl chlorides

A novel cross 1,3-dipolar cycloaddition between azomethine imines with in situ generated nitrile oxides has been developed. This is the first example of employing a reaction partner containing two heteroatoms in the [3+3] cycloaddition involving azomethine imines. This strategy not only provides structurally diverse N,O-heterocycles but also greatly enriches the chemistry of azomethine imines and nitrile oxides.

Phosphine-Catalyzed α-Umpolung-Aldol Reaction for the Synthesis of Benzo[ b]azapin-3-ones

A novel phosphine-catalyzed intermolecular cyclization between 2-sulfonamidobenzaldehyes and ynones is reported. This methodology serves as a conduit for the construction of benzo[ b]azepin-3-ones in good to excellent yields under mild conditions. The resulting 2-benzylidene moieties are formed exclusively in the E-configuration. Mechanistically, this unusual annulation occurs through a phosphine-catalyzed α-umpolung addition, followed by an aldol reaction. One of the benzo[ b]azepin-3-one products was converted to the core structure of 3-amino-[ a]benzazepin-2-one-1-alkanoic acids, many of which function as angiotensin-converting enzyme inhibitors.

Solvent incorporated sequential [3 + 2] annulation/substitution reaction of azomethine imines and propargyl sulfur ylide

A novel solvent incorporated sequential [3 + 2] cycloaddition/substitution reaction of azomethine imines with propargyl sulfur ylide was developed. In the actual three-component reaction, propargyl sulfur ylide acts as a dipole reagent to furnish the annulation with azomethine imines, followed by the protic solvents acting as nucleophiles. The simple, mild, catalyst-free and practical protocol allows for the formation of N,N-bicyclic pyrazolidinones in moderate to excellent yields. Further transformation and gram-scale operations could also be achieved efficiently.

Mechanisms of phosphine-catalyzed [3+3] cycloaddition of ynones and azomethine imines: a DFT study

Mechanisms of PPh₃-catalyzed [3+3] cycloaddition between a ynone and an azomethine imine.

Facile synthesis of 9H-pyrrolo[1,2-a]indoles via Brønsted acid catalyzed cascade reactions

A Brønsted acid system promotes the Friedel–Crafts alkenylation/1,6-addition/condensation cascade reaction of ynones with indoles to access 9H-pyrrolo[1,2-a]indoles.

Iridium(iii)-catalysed annulation of pyrazolidinones with propiolates: a facile route to pyrazolo[1,2-a] indazoles

Iridium(iii)-catalysed C–H bond activation/subsequent [4+1] cyclization for the synthesis of pyrazolo[1,2-a] indazoles has been developed.

Phosphine Organocatalysis

The hallmark of nucleophilic phosphine catalysis is the initial nucleophilic addition of a phosphine to an electrophilic starting material, producing a reactive zwitterionic intermediate, generally under mild conditions. In this Review, we classify nucleophilic phosphine catalysis reactions in terms of their electrophilic components. In the majority of cases, these electrophiles possess carbon-carbon multiple bonds: alkenes (section 2), allenes (section 3), alkynes (section 4), and Morita-Baylis-Hillman (MBH) alcohol derivatives (MBHADs; section 5). Within each of these sections, the reactions are compiled based on the nature of the second starting material-nucleophiles, dinucleophiles, electrophiles, and electrophile-nucleophiles. Nucleophilic phosphine catalysis reactions that occur via the initial addition to starting materials that do not possess carbon-carbon multiple bonds are collated in section 6. Although not catalytic in the phosphine, the formation of ylides through the nucleophilic addition of phosphines to carbon-carbon multiple bond-containing compounds is intimately related to the catalysis and is discussed in section 7. Finally, section 8 compiles miscellaneous topics, including annulations of the Hüisgen zwitterion, phosphine-mediated reductions, iminophosphorane organocatalysis, and catalytic variants of classical phosphine oxide-generating reactions.

Organocatalytic umpolung annulative dimerization of ynones for the synthesis of 5-alkylidene-2-cyclopentenones

A novel phosphine-catalyzed umpolung [3 + 2]-annulative dimerization of ynones was developed to furnish functionally rich 5-alkylidene-2-cyclopentenones. In this protocol, ynone acts as both C2 and C3 synthons, which undergo [3 + 2]-annulative dimerization.

Rh(iii)-Catalyzed synthesis of pyrazolo[1,2-a]cinnolines from pyrazolidinones and diazo compounds

Pyrazolo[1,2-a]cinnolines were achieved via a rhodium(iii)-catalyzed redox-neutral annulation procedure, exhibiting good regioselectivity and high functional group tolerability.

Phosphine catalyzed δ-carbon addition and isomerization of alkynones to ketimines: the preparation of 1,3-diene substituted dihydroquinazolinones and 3-aminooxindoles

A novel strategy of the phosphine catalyzed addition of alkynones to ketimines for the synthesis of α-amino diene derivatives has been developed for the first time, thus providing diene incorporating α-amino products in moderate to excellent yields.

Phosphine-mediated partial reduction of alkynes to form both (E)- and (Z)-alkenes

A mild, tunable method to form both (E)- and (Z)-alkenes from alkynes is reported using PPh₃ as a reductant.

Lewis base-catalyzed diastereoselective [3 + 2] cycloaddition reaction of nitrones with electron-deficient alkenes: an access to isoxazolidine derivatives

A Lewis base-catalyzed [3 + 2] cycloaddition reaction of nitrones with electron-deficient alkenes has been achieved under mild reaction conditions, affording functionalized isoxazolidines as single diastereomers in moderate to excellent yields.

Catalyst-free [3 + 2] cyclization of imines and Morita–Baylis–Hillman carbonates: a general route to tetrahydropyrrolo[2,1-a]isoquinolines and dihydropyrrolo[2,1-a]isoquinolines

A mild and efficient catalyst-free [3 + 2] cyclization of isoquinoline imines and Morita–Baylis–Hillman carbonates has been developed.

Phosphine-catalyzed [3 + 2] and [4 + 2] annulation reactions of ynones with barbiturate-derived alkenes

The phosphine-catalyzed [3 + 2] annulation reaction of ynones and barbiturate-derived alkenes has been developed with the assistance of a weak acid, giving functionalized spirobarbiturate-cyclopentanones.

Tunable regiodivergent phosphine-catalyzed [3 + 2] cycloaddition of alkynones and trifluoroacetyl phenylamides

The regiodivergent phosphine-catalyzed [3 + 2] cycloaddition of 2-(2,2,2-trifluoroacetyl)phenylamides and alkynones have been achieved through adjusting the acidities of substrates, changing reaction temperature and addition of protic additive H₂O. The detailed mechanism was systematically studied.

A stereoselective thiocyanate conjugate addition to electron deficient alkynes and concomitant cyclization to N,S-heterocycles

A regio- and stereoselective thiocyanate addition to ynones and the concomitant cyclizations of resultant vinyl thiocyanate to access thio-aza heterocycles are achieved using KSCN in AcOH without the need of any metal catalyst.

Catalyst-free synthesis of novel dimeric β-carboline derivatives via an unexpected [2 + 2 + 2] annulation

An unexpected catalyst-free [2 + 2 + 2] annulation reaction has been developed, allowing access to novel complex dimeric β-carboline derivatives in a single step.