Catalyst-Controlled Annulations of Strained Cyclic Allenes with π-Allylpalladium Complexes

Palladium-Catalyzed (4 + 1) Annulation of 4-Vinylbenzodioxinones with Sulfur Ylides: Diastereoselective Synthesis of Dihydrobenzofuran Derivatives

Palladium-catalyzed (4 + 1) annulation of 4-vinylbenzodioxinones with sulfur ylides has been developed to afford various dihydrobenzofuran derivatives in moderate to high yields with excellent diastereoselectivities. The scale-up reaction and further derivations of the product worked well, demonstrating the application potential of the current reaction in organic synthesis.

Palladium-Catalyzed Cycloaddition Reactions of π-Allylpalladium 1,4-Dipoles with 1,3,5-Triazinanes: Access to Hexahydropyrimidines, 1,3-Oxazinanes, and 1,5-Diazocanes

Palladium-catalyzed decarboxylation of 5-methylene-1,3-oxazinan-2-ones and 5-methylene-1,3-dioxan-2-ones to generate aza-π-allylpalladium and oxa-π-allylpalladium 1,4-dipoles for [4 + 2] cycloaddition reaction with 1,3,5-triazinanes was developed, affording a wide range of hexahydropyrimidine and 1,3-oxazinane derivatives in good to excellent yields (up to 99%). The acyclic sulfonamido-substituted allylic carbonates as aza-π-allylpalladium 1,4-dipole precursors also apply to the developed synthesized strategy, achieving the synthesis of hexahydropyrimidines. Moreover, the in situ-generated aza-π-allylpalladium 1,4-dipoles undergoing dimeric [4 + 4] cycloaddition were also demonstrated by the construction of 1,5-diazocane derivatives.

Access to Complex Scaffolds Through [2 + 2] Cycloadditions of Strained Cyclic Allenes

We report the strain-induced [2 + 2] cycloadditions of cyclic allenes for the assembly of highly substituted cyclobutanes. By judicious choice of trapping agent, complex scaffolds bearing heteroatoms, fused rings, contiguous stereocenters, spirocycles, and quaternary centers are ultimately accessible. Moreover, we show that the resulting cycloadducts can undergo thermal isomerization. This study provides an alternative strategy to photochemical [2 + 2] cycloadditions for accessing highly functionalized cyclobutanes, while validating the use of underexplored strained intermediates for the assembly of complex architectures.

Isolable Spirocyclic Silylone: π-Delocalized Spiro[3.3]heptasila-2,6-diylidone

Strained cyclic tetrylones are important synthons due to various synthetic applications. Connecting two cyclic tetrylone rings through a single shared quaternary group 14 element atom to form a spirocyclic molecule has been unexplored both theoretically and experimentally. The formation of a spirocyclic motif has been a synthetic challenge. In contrast, the reaction of amidinato disilicon(I) 1, (Me3P)2SiCl4, and KC8 afforded π-delocalized spiro[3.3]heptasila-2,6-diylidone2 and tetrasilacyclobutadiene byproducts 3 and 4. Compound 2 is the smallest spirocyclic tetrylone derivative, which is composed of a σ-type lone pair and delocalized π bond in each all-silicon spirocyclic ring. The electronic property is supported by its coordination with a W(CO)5 moiety.

Palladium-Catalyzed Enantioselective [4+2] Cycloaddition of 4-Vinylbenzodioxinones with Barbiturate-Derived Alkenes: Con-struction of Chiral Spirobarbiturate-Chromanes

In this paper, Pd-catalyzed [4+2] decarboxylative cycloaddition of 4-vinylbenzodioxinones with barbiturate-derived alkenes has been developed, leading to various spirobarbiturate-chromane derivatives in high yields with excellent diastereo- and enantioselectivities. The scale-up reaction and further derivation of the product were demonstrated. A plausible reaction mechanism was also proposed.

Generation and reactivity of unsymmetrical strained heterocyclic allenes

Strained cyclic allenes are short-lived intermediates that confine a functional group with a preferred linear geometry, an allene, into a small ring, inducing strain-driven reactivity. Nitrogen-containing variants, or azacyclic allenes, have proved valuable for the assembly of complex nitrogen-containing compounds. Whereas 3,4-azacyclic allenes, which bear a symmetrical core, have been the focus of multiple studies, their unsymmetrical 2,3-azacyclic counterparts have remained underexplored. In the present study, we report density functional theory studies investigating the structure of such unsymmetrical azacyclic allenes and experimental efforts to access and engage them in strain-promoted cycloadditions under mild conditions. Control experiments support either concerted or stepwise diradical mechanisms for these reactions, depending on the type of cycloaddition examined. Moreover, we generate the corresponding 2,3-oxacyclic allene and demonstrate its reactivity in cycloadditions and a metal-catalysed process. Given the scaffolds accessed, coupled with the observed selectivity trends, these results are expected to encourage the application of unsymmetrical heterocyclic allenes for the synthesis of heterocycles that bear a high fraction of sp3-hybridized atoms.

Functional-Group-Directed Regiodivergent (3 + 2) Annulations of Electronically Distinct 1,3-Dienes and 2-Formyl Arylboronic Acids

Presented herein is a palladium-catalyzed asymmetric (3 + 2) annulation reaction between 1,3-dienes and 2-formylarylboronic acids, proceeding in a cascade vinylogous addition and Suzuki coupling process. Both electron-neutral and electron-deficient 1,3-dienes are compatible under similar catalytic conditions, and distinct regioselectivity is observed via functional-group control of 1,3-diene substrates. A collection of 1-indanols with dense functionalities is constructed stereoselectively.

4-Vinylbenzodioxinones as a new type of precursor for palladium-catalyzed (4+3) cycloaddition of azomethine imines

In this paper, benzo-fused cyclic carbonates were designed and synthesized as a new type of precursor of π-allylpalladium zwitterionic intermediates, and were applied in Pd-catalyzed diastereo- and enantioselective (4+3) cycloaddition with C,N-cyclic azomethine imines, leading to various biologically important 1,3,4-benzoxadiazepine derivatives in 43-99% yields with 6 : 1 to >20 : 1 dr and up to 95% ee.

Recent advances in ligand-enabled palladium-catalyzed divergent synthesis

Developing efficient and straightforward strategies to rapidly construct structurally distinct and diverse organic molecules is one of the most fundamental tasks in organic synthesis, drug discovery and materials science. In recent years, divergent synthesis of organic functional molecules from the same starting materials has attracted significant attention and has been recognized as an efficient and powerful strategy. To achieve this objective, the proper adjustment of reaction conditions, such as catalysts, solvents, ligands, etc., is required. In this review, we summarized the recent efforts in chemo-, regio- and stereodivergent reactions involving acyclic and cyclic systems catalyzed by palladium complexes. Meanwhile, the reaction types, including carbonylative reactions, coupling reactions and cycloaddition reactions, as well as the probable mechanism have also been highlighted in detail.

Nickel-catalyzed acylzincation of allenes with organozincs and CO

Transition metal-catalyzed carbonylative reaction with CO gas are among the central task in organic synthesis, enabling the construction of highly valuable carbonyl compound. Here, we show an earth-abundant nickel-catalyzed three-component tandem acylzincation/cyclization sequence of allene and alkylzinc reagent with 1 atm of CO under mild conditions. This protocol is featured by broad functional group tolerance with high reaction selectivity, providing a rapid and convenient synthetic method for the construction of diverse fully substituted benzotropone derivatives. Mechanistic studies reveal that the installation of a cyano group tethered to allene moiety enables the high regio- and stereoselectivity of this acylzincation of allene, allowing the selective formation of three consecutive C-C bonds in a highly efficient manner.

Asymmetric cascades of the π-allyl complex: a journey from transition-metal catalysis to metallaphotocatalysis

The enantioselective catalytic cascade involving Tsuji-Trost allylation has provided a viable strategy for the construction of multiple asymmetric C-C and C-X centres and numerous methods have been developed around it for the synthesis of various vital scaffolds. The synthetic utility of this strategy was enhanced by replacing the customary allyl acetates with ethylene diacetates/dicarbonates, vinyl epoxides, vinyl oxetanes, vinyl ethylene carbonates, vinyl cyclopropanes, enynes, and dienes using transition-metal catalysis. One more milestone was achieved when metallaphotocatalysis provided the necessary platform for these cascades by using a cheaper metal. This review will provide a summary of these enantioselective catalytic cascades from 2015.

Cyclic Allene Approach to the Manzamine Alkaloid Keramaphidin B

We report an approach to the core of the manzamine alkaloid keramaphidin B that relies on the strain-promoted cycloaddition of an azacyclic allene with a pyrone trapping partner. The cycloaddition is tolerant of nitrile and primary amide functional groups and can be complemented with a subsequent retro-Diels-Alder step. These efforts demonstrate that strained cyclic allenes can be used to build significant structural complexity and should encourage further studies of these fleeting intermediates.