Phosphine-Catalyzed Domino β/γ-Additions of Benzofuranones with Allenoates: A Method for Unsymmetrical 3,3-Disubstituted Benzofuranones

Synthesis of 3,4-Disubstituted Maleimide Derivatives via Phosphine-Catalyzed Isomerization of α-Succinimide-Substituted Allenoates Cascade γ'-Addition with Aryl Imines

3,4-disubstituted maleimides find wide applications in various pharmacologically active compounds. This study presents a highly effective approach for synthesizing derivatives of 3,4-disubstituted maleimides through the direct isomerization of α-succinimide-substituted allenoates, followed by a cascade γ'-addition and aryl imines using PR3 as a catalyst. The resulting series of 3,4-disubstituted maleimides exhibited excellent stereoselectivities, achieving yields of up to 86%. To our knowledge, the phosphine-mediated γ'-addition reaction of allenoates is seldom reported.

Phosphine-Catalyzed Allylic Alkylation of (Hetero)Aryl Alkynes with Pronucleophiles: Concise Total Synthesis of (±)-Esermethole

Allylic alkylations are valuable in the construction of versatile carbon-carbon bonds, which are mostly catalyzed by noble transition metals with additional waste byproduct generation. Here, we present the first organophosphine-catalyzed allylic alkylation of (hetero)aryl alkynes with various carbo-nucleophiles. The methodology is highly atom economical and compatible with a wide substrate scope (more than 38 examples). Moreover, the reaction could be easily scaled up, and deuterium labeling experiments have been conducted to elucidate the plausible mechanism. Finally, the protocol has been utilized to achieve the concise total synthesis of natural product (±)-esermethole.

Diastereoselective synthesis of chroman bearing spirobenzofuranone scaffolds via oxa-Michael/1,6-conjugated addition of para-quinone methides with benzofuranone-type olefins

A simple and convenient cyclization of ortho-hydroxyphenyl-substituted para-quinone methides with benzofuran-2-one type active olefins via oxa-Michael/1,6-conjugated addition has been developed, which afforded an easy access to enriched functionalized chroman-spirobenzofuran-2-one scaffolds with good to excellent yields (up to 90%) and diastereoselectivities (up to >19 : 1 dr). This reaction provided an efficient method for constructing desired spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one.

Highly diastereoselective synthesis of spirocyclic compounds combining both well-known heterocyclic pharmacophores chroman and benzofuran-2-one.

Carbene-Catalyzed Enantioselective Aldol Reaction: Post-Aldol Stereochemistry Control and Formation of Quaternary Stereogenic Centers

The dominated approaches for asymmetric aldol reactions have primarily focused on the aldol carbon-carbon bond-forming events. Here we postulate and develop a new catalytic strategy that seeks to modulate the reaction thermodynamics and control the product enantioselectivities via post-aldol processes. Specifically, an NHC catalyst is used to activate a masked enolate substrate (vinyl carbonate) to promote the aldol reaction in a non-enantioselective manner. This reversible aldol event is subsequently followed by an enantioselective acylative kinetic resolution that is mediated by the same (chiral) NHC catalyst without introducing any additional substance. This post-aldol process takes care of the enantioselectivity issues and drives the otherwise reversible aldol reaction toward a complete conversion. The acylated aldol products bearing quaternary/tetrasubstituted carbon stereogenic centers are formed in good yields and high optical purities.

Vinylogous Elimination/C-H Functionalization/Allylation Cascade Reaction of Allenoate Adducts: Synthesis of Ring-Fused Dihydropyridinones

A palladium-catalyzed cascade reaction of β'-allenoate adducts with aryl/heteroaryl carboxamides through a vinylogous elimination/C-H functionalization/intramolecular allylation reaction sequence has been developed with high Z stereoselectivity. Various ring-fused dihydropyridinones bearing an α,β-unsaturated ester substituent are obtained. It is the first example of application of the allenoate adducts to C-H functionalization annulations as practical precursors of hard-to-get functionalized electron-deficient 1,3-butadienes. Using air as the terminal oxidant also shows a great advantage in environmental friendliness.

Substrate-controlled, PBu3-catalyzed annulation of phenacylmalononitriles with allenoates enables tunable access to cyclopentenes

Divergence in the PBu3-catalyzed [3+2] annulation of phenacylmalononitriles with allenoates, controlled by the γ-substitution on allenoates, offers a tunable synthesis of multifunctionalized cyclopentene carboxamides and cyclopentenols. An unprecedented formation of cyclopentene carboxamide was observed when allenic esters bearing a substitution at the γ-position were employed, while unsubstituted allenoates produced cyclopentenols. The former reaction likely involves a Michael/aldol/nucleophilic cyclization sequence in a domino manner.

Phosphine-Catalyzed (4+1) Annulation: Rearrangement of Allenylic Carbamates to 3-Pyrrolines through Phosphonium Diene Intermediates

We have developed a phosphine-catalyzed (4+1) annulative rearrangement for the preparation of 3-pyrrolines from allenylic carbamates via phosphonium diene intermediates. We employed this methodology to synthesize an array of 1,3-disubstituted- and 1,2,3-trisubstituted-3-pyrrolines, including the often difficult to prepare 2-alkyl variants. A mechanistic investigation employing allenylic acetates and mononucleophiles unexpectedly unveiled that a phosphine-catalyzed (4+1) reaction for the construction of cyclopentene products, previously reported by Tong, might not occur through a phosphonium diene, as had been proposed, but rather through multiple mechanisms working in concert. Consequently, our phosphine-catalyzed rearrangement is most likely the first transformation to involve the unequivocal formation of a phosphonium diene intermediate along the reaction pathway. To demonstrate the synthetic utility of this newly developed reaction, we have completed concise formal syntheses of the pyrrolizidine alkaloids (±)-trachelanthamidine and (±)-supinidine.

Design, synthesis and application of a new type of bifunctional in highly enantioselective γ-addition reactions of N-centered nucleophiles to allenoates

A novel class of cyclic phosphine derived bifunctional catalysts (Le-Phos) is reported, which can be readily prepared from inexpensive and commercially available starting materials and exhibit good performances in enantioselective γ-addition reactions of N-centered nucleophiles and allenoates under mild conditions. The salient features of this reaction include high product yields, good enantioselectivity, mild reaction conditions, and broad substrate scope and gram-scale scalability.

Phosphine-catalyzed (3+2)/(2+3) sequential annulation involving a triple nucleophilic addition reaction of γ-vinyl allenoates

A phosphine-catalyzed (3+2)/(2+3) sequential annulation involving a triple nucleophilic addition reaction of γ-vinyl allenoates was successfully developed. The reaction provided efficient and more practical access to functionalized hydropyrroloimidazolones with good to excellent yields under mild reaction conditions. Notably, γ-vinyl allenoate served as a triple-electrophilic intermediate in this protocol.

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.

An elimination/Heck coupling/allylation cascade reaction: synthesis of 2,3-dihydrobenzofurans from allenoate adducts

A highly regio- and stereoselective Pd-catalyzed cascade reaction of allenoate adducts has been developed. Various allenoate adducts of phosphine-catalyzed addition of allenoates are found to be efficient substrates for the preparation of 2,3-dihydrobenzofuran. It is the first example of allenoate adducts used as ideal and practical precursors of hard-to-get functionalized 1,3-butadienes for heterocycle synthesis.

Synthesis of functionalized 2,5-dihydropyrrole derivatives via a convenient [3 + 2] annulation of azomethine ylides with allenoates

A convenient [3 + 2] annulation of azomethine ylides with allenoates to synthesize highly functionalized 2,5-dihydropyrrole derivatives was developed.

Phosphine-catalyzed [4 + 2] annulation of γ-benzyl allenoates: facile synthesis of benzothieno[3,2-b]pyran derivatives

An efficient tris(4-methoxyphenyl)phosphane catalyzed domino reaction between γ-benzyl allenoate and ethyl (Z)-2-(3-oxobenzo[b]thiophen-2(3H)-ylidene)acetate has been developed, which produces a series of 2H-benzo[4,5]thieno[3,2-b]pyran derivatives in high yields.

Phosphine-catalysed α-umpolung addition of nucleophiles to δ-acetoxy allenoates: stereoselective synthesis of 2,4-dienoates

The first example of phosphine-catalyzed α-umpolung addition of nucleophiles to allenoates is described, which features the use of δ-acetoxy allenoate to generate a 3-phosphonium-2,4-dienoate intermediate, thus facilitating the α-umpolung addition of nucleophiles.

Bisphosphine catalyzed sequential [3 + 2] cycloaddition and Michael addition of ynones with benzylidenepyrazolones via dual α′,α′-C(sp3)–H bifunctionalization to construct cyclopentanone-fused spiro-pyrazolones

A sequential [3 + 2] cycloaddition and Michael addition reaction of ynones with benzylidenepyrazolones afforded cyclopentanone-fused spiro-pyrazolones catalyzed by DPPB via dual α′,α′-C(sp³)–H bifunctionalization.