Rhodium(II)-catalyzed olefin cyclopropanation with the phenyliodonium ylide derived from Meldrum's acid

Reactions of Thioamides with Iminoiodinanes: An Approach toward 1,2-Thiazines, Thiophenes, and Sulfonylisothiazolines

Transition metal-catalyzed and noncatalytic reactions of acrylic thioamides with N-sulfonyliminoiodinanes enabling the selective synthesis under mild conditions of a series of fully decorated S- and N,S-containing heterocycles are described. Fine-tuning of the thioamide structure allows selective preparation in good yields of ortho-fused thiophenes, 1,2-thiazines, and spiro-fused isothiazolines. A one-pot thermally induced rearrangement of sulfonylisothiazolines was discovered as a complementary route to thiophenes.

Rh(II)-Catalyzed Enantioselective S-Alkylation of Sulfenamides with Acceptor-Acceptor Diazo Compounds Enables the Synthesis of Sulfoximines Displaying Diverse Functionality

The Rh(II)-catalyzed enantioselective S-alkylation of sulfenamides with α-amide diazoacetates at 1 mol % catalyst loading to obtain sulfilimines in high yields and enantiomeric ratios of up to 99:1 is reported. The enantioenriched sulfilimine products incorporate versatile amide functionality poised for further elaboration to diverse sulfoximines with multiple stereogenic centers, including by highly diastereoselective sulfilimine and sulfoximine α-alkylation with alkylating agents and epoxides and by interconversion of the amide to N-tert-butanesulfinyl aldimines, followed by diastereoselective additions.

Dirhodium-Catalyzed Transannulation of N-Sulfonyl-1,2,3-triazoles to 2,3-Dehydropiperazines

The dirhodium(II)-catalyzed synthesis of a range of C2-substituted 2,3-dehydropiperazines using 1-mesyl-1,2,3-triazoles and β-haloalkylcarbamates is reported. The reaction is proposed to proceed through an α-imino rhodium carbene 1,3-insertion into N-H followed by a base-mediated cyclization. C-Substituted dehydropiperazines can also be conducted directly from terminal alkynes in a three-step, one-pot operation, forming the triazole in situ. This methodology has also been expanded to afford several 2,5-disubstituted 2,3-dehydropiperazines as well as a larger 4,5,6,7-tetrahydro-1H-1,4-diazepine derivative.

Asymmetric Cyclopropanation with 4-Aryloxy-1-sulfonyl-1,2,3-triazoles: Expanding the Range of Rhodium-Stabilized Donor/Acceptor Carbenes to Systems with an Oxygen Donor Group

Rhodium-catalyzed enantioselective synthesis of 1-phenoxycyclopropane-1-carbaldehydes by intermolecular cyclopropanation of terminal alkenes followed by imine hydrolysis is described. This methodology utilizes 4-aryloxy-1-sulfonyl-1,2,3-triazoles as the carbene precursors and the chiral dirhodium(II) tetracarboxylates Rh2(S-NTTL)4 or Rh2(S-DPCP)4 as the catalysts. These reactions are considered to proceed via rhodium-stabilized donor/acceptor carbene intermediates, and these studies demonstrate that a heteroatom donor group is compatible with an enantioselective transformation.

Catalytic Enantioselective Sulfur Alkylation of Sulfenamides for the Asymmetric Synthesis of Sulfoximines

Sulfoximines are increasingly incorporated in agrochemicals and pharmaceuticals, with the two enantiomers of chiral sulfoximines often having profoundly different binding interactions with biomolecules. Therefore, their application to drug discovery and development requires the challenging preparation of single enantiomers rather than racemic mixtures. Here, we report a general and fundamentally new asymmetric synthesis of sulfoximines. The first S-alkylation of sulfenamides, which are readily accessible sulfur compounds with one carbon and one nitrogen substituent, represents the key step. A broad scope for S-alkylation was achieved by rhodium-catalyzed coupling with diazo compounds under mild conditions. When a chiral rhodium catalyst was utilized with loadings as low as 0.1 mol %, the S-alkylation products were obtained in high yields and with enantiomeric ratios up to 98:2 at the newly generated chiral sulfur center. The S-alkylation products were efficiently converted to a variety of sulfoximines with complete retention of stereochemistry. The utility of this approach was further demonstrated by the asymmetric synthesis of a complex sulfoximine agrochemical.

Recent progress in the application of iodonium ylides in organic synthesis

This review summarizes the recent advances in the synthetic application of iodonium ylides covering 2017 to 2022.

Synthesis, Structure, and Dynamics of Chiral Eight-Membered Cyclic Molecules with Thienylene and Cyclopropylene Units Alternately Connected

A rhodium(II)-catalyzed asymmetric cyclooligomerization of bifunctional monomers possessing triazolyl and vinyl groups at 2,3- and 3,4-positions on the thiophene ring is studied. Structurally interesting cyclic dimers in which thienylene and cyclopropylene units are alternately connected are obtained as the major components. The eight-membered rings in the center are non-planar and adopt a tub-shaped conformation. We also observe the phenomenon of racemization caused by a tub-to-tub ring-flipping, the activation energy of which is determined as 108 kJ mol -1 by electronic circular dichroism spectra measurement.

Chiral Macrocycles Having C3 Symmetry Resulting from Orientation of Thiophene Rings

An chiral Rh^II -catalyzed cyclooligomerization reaction of thiophenes having triazolyl and vinyl substituents at the 2- and 4-positions was studied. Structurally interesting cyclic trimers, having chirality that is ascribed only to the orientation of the 2,4-disubstituted thiophene rings, are obtained. The 2,4-disubstitution of the starting thiophene monomer allows production of each of the enantiomers. The observed electronic circular-dichroism spectra are in accord with those simulated by density-functional theory calculations.

Synthesis of chiral pyrimidine-substituted diester D-A cyclopropanes via asymmetric cyclopropanation of phenyliodonium ylides

A highly enantioselective cyclopropanation to synthesize pyrimidine-substituted diester D-A cyclopropanes is reported. Various N1-vinylpyrimidines react well with phenyliodonium ylides, delivering chiral cyclopropanes in up to 97% yield with up to 99% ee. Through simple [3+2] annulation with benzaldehyde or ethyl glyoxylate, different chiral pyrimidine nucleoside analogues with a sugar ring could be obtained.

Scalable Synthesis of Esp and Rhodium(II) Carboxylates from Acetylacetone and RhCl3·xH2O

Rhodium(II) carboxylates are privileged catalysts for the most challenging carbene-, nitrene-, and oxo-transfer reactions. In this work, we address the strategic challenges of current organic and inorganic synthesis methods to access these rhodium(II) complexes through an oxidative rearrangement strategy and a reductive ligation reaction. These studies illustrate the multiple benefits of oxidative rearrangement in the process-scale synthesis of congested carboxylates over nitrile anion alkylation reactions, and the impressive effect of inorganic additives in the reductive ligation of rhodium(III) salts.

Synthesis of 2-alkenylfurans via a Ag(i)-catalyzed tandem cyclization/cross-coupling reaction of enynones with iodonium ylides

A silver(i)-catalyzed tandem cyclization/cross-coupling reaction of enynones with iodonium ylides to construct carbon-carbon double bonds has been developed. The strategy provides a novel method for the synthesis of 2-alkenylfurans. This is the first cross-coupling reaction between metal-carbene complexes and iodonium ylides.

Synthesis of amino acids and peptides with bulky side chains via ligand-enabled carboxylate-directed γ-C(sp3)-H arylation

Amino acids and peptides with bulky side chains are of significant importance in organic synthesis and modern medicinal chemistry. The efficient synthesis of these molecules with full enantiocontrol and high diversity remains challenging. Herein we report a Pd-catalyzed ligand-enabled γ-C(sp3)-H arylation of tert-leucine and its derived peptides without using an external directing group (DG) via a less favored six-membered palladacycle. Structurally diverse bulky side chain amino acids and peptides were accessed in a step-economic fashion and the reaction could be conducted on a gram scale with retention of chirality. The resulting amino acids can be used as chiral ligands in Co(iii)-catalyzed enantioselective C(sp3)-H amidation. It is worth noting that the weakly coordinating carboxylate DG outcompetes the strongly coordinating bidentate DG of the peptide backbone, providing the products of γ-C(sp3)-H arylation of Tle residue exclusively. This protocol represents the first example of late stage C(sp3)-H functionalization of peptides using a weakly coordinating directing group.

Late-stage C-H amination of abietane diterpenoids

This study aims at highlighting the synthetic versatility of the rhodium-catalyzed C-H amination reactions using iodine(iii) oxidants for the late-stage functionalization of natural products. Inter- and intramolecular nitrene insertions have been performed from various abietane diterpenoids, leading to the amination of the C-3, C-6, C-7, C-11 and C-15 positions. Ca. 20 aminated compounds have been isolated with yields of up to 86% and high levels of regio-, chemo- and stereoselectivities.

Blue LED Irradiation of Iodonium Ylides Gives Diradical Intermediates for Efficient Metal-free Cyclopropanation with Alkenes

A facile and highly chemoselective synthesis of doubly activated cyclopropanes is reported where mixtures of alkenes and β-dicarbonyl-derived iodonium ylides are irradiated with light from blue LEDs. This metal-free synthesis gives cyclopropanes in yields up to 96 %, is operative with cyclic and acyclic ylides, and proceeds with a variety of electronically-diverse alkenes. Computational analysis explains the high selectivity observed, which derives from exclusive HOMO to LUMO excitation, instead of free carbene generation. The procedure is operationally simple, uses no photocatalyst, and provides access in one step to important building blocks for complex molecule synthesis.

Asymmetric Synthesis and Stereochemical Assignment of 12C/13C Isotopomers

A synthesis of chiral hydrocarbons having C₁ axis and C₃ symmetry, which owe their chirality due to asymmetrical distribution of ¹²C/¹³C isotopes, is reported. Their absolute configurations assigned using the vibrational circular dichroism technique conform with those deduced from the absolute configurations of the parent α-formyl cyclopropanes.

Cyclopropanation of alkenes with metallocarbenes generated from monocarbonyl iodonium ylides

Reacting Wittig reagents and the hypervalent iodine reagent iodosotoluene, in the presence of 10 mol% Cu(tfacac)2 and 5 equiv. of alkene, results in a novel cyclopropanation reaction. The reagent combination is believed to generate a transient monocarbonyl iodonium ylide (MCIY) in situ, which can be intercepted by the copper catalyst to give a metallocarbene. Both ester and ketone derived phosphoranes can be used, as can styrenyl and non-styrenyl alkenes, which provides cyclopropanes in yields up to 81%.

Rhodium-Catalyzed B-H Bond Insertion Reactions of Unstabilized Diazo Compounds Generated in Situ from Tosylhydrazones

Although transition-metal-catalyzed B-H bond insertion of carbenes into stable borane adducts has emerged as a promising method for organoborane synthesis, all the diazo compounds used to date as carbene precursors have had an electron-withdrawing group to stabilize them. Herein, we report a protocol for rhodium-catalyzed B-H bond insertion reactions of unstabilized diazo compounds generated in situ from tosylhydrazones. In addition, by using chiral dirhodium catalysts, we also achieved an asymmetric version of the reaction with good to excellent enantioselectivities (up to 98:2 e.r.). This is the first enantioselective heteroatom-hydrogen bond insertion reaction to use unstabilized diazo compounds as carbene precursors. The protocol exhibited good functional group tolerance and could be carried out on a gram scale. It also enabled one-pot transformation of a carbonyl group to a boryl group enantioselectively. The B-H bond insertion products could be easily transformed into chiral alcohols and other widely used organoboron reagents with enantiomeric fidelity.

An unusual stereoretentive 1,3-quaternary carbon shift resulting in an enantioselective RhII-catalyzed formal [4+1]-cycloaddition between diazo compounds and vinyl ketenes

Enantioselective quaternary carbon construction in the assembly of cyclopentenones employing a RhII-catalyzed, formal [4+1]-cycloaddition is described. A Rh2(S-TCPTTL)4-catalyzed cyclopropanation of a vinyl ketene with a disubstituted diazo compound initiates a stereoretentive, accelerated ring expansion to provide the cycloadduct in good to excellent yields and enantioselectivity.

Rh(II)-Catalyzed Cyclopropanation of Furans and Its Application to the Total Synthesis of Natural Product Derivatives

Rh(II)-catalyzed enantioselective cyclopropanations of furans, providing access to synthetically useful building blocks, are reported. After screening of 10 Rh(II) catalysts, Rh₂(S-TCPTTL)₄ was identified as a highly efficient and selective catalyst (up to 98% ee, TON 88000, and TOF 24/s) for the cyclopropanation of furans. These cyclopropanes were successfully applied to the enantioselective synthesis of novel paraconic acid derivatives.

Synthesis of Penta-2,4-dien-1-imines and 1,2-Dihydropyridines by Rhodium-Catalyzed Reaction of N-Sulfonyl-1,2,3-triazoles with 2-(Siloxy)furans

A rhodium(II)-catalyzed reaction of N-sulfonyl-1,2,3-triazoles with 2-(siloxy)furans is reported. Either open-chain penta-2,4-dien-1-imines or cyclic 1,2-dihydropyridines are selectively obtained depending on the ligand on rhodium(II).

Facile and efficient approach for the synthesis of spirocyclopropylbarbiturates and 3-substituted-1,1,2,2-tetracyanocyclopropanes using N-halosulfonamides

The applications of [Formula: see text]-tetrabromobenzene-1,3-disulfonamide [TBBDA], poly(N-bromo-N-ethyl-benzene-1,3-disulfonamide) [PBBS], [Formula: see text]-tetrachlorobenzene-1,3-disulfonamide [TCBDA], and poly(N-chloro-N-ethyl-benzene-1,3-disulfonamide) [PCBS] as novel reagents for the preparation of spirocyclopropylbarbiturates from 2-arylidenemalononitriles and barbituric acids are described. In addition, an effective and simple domino procedure for the synthesis of 3-substituted-1,1,2,2-tetracyanocyclopropanes from carbonyl compounds and malononitrile in a one-pot manner is reported. These reactions involve Michael addition, halogenation, and intramolecular ring-closing (MHIRC) reaction sequences. We have developed an efficient procedure for the synthesis of spirocyclopropylbarbiturates and 3-substituted-1,1,2,2-tetracyanocyclopropanes using TBBDA, PBBS, TCBDA, and PCBS as new reagents.

Dinitrogen extrusion from enoldiazo compounds under thermal conditions: synthesis of donor-acceptor cyclopropenes

Donor-acceptor cyclopropenes are formed quantitatively or in high yield from enoldiazoacetates and enoldiazoacetamides under moderate thermal conditions. They are more versatile than their corresponding enoldiazocarbonyl compounds as carbene precursors.

Rh-Catalyzed Intermolecular Reactions of α-Alkyl-α-Diazo Carbonyl Compounds with Selectivity over β-Hydride Migration

Rh-carbenes derived from α-diazocarbonyl compounds have found broad utility across a remarkable range of reactivity, including cyclopropanation, cyclopropenation, C-H insertions, heteroatom-hydrogen insertions, and ylide forming reactions. However, in contrast to α-aryl or α-vinyl-α-diazocarbonyl compounds, the utility of α-alkyl-α-diazocarbonyl compounds had been moderated by the propensity of such compounds to undergo intramolecular β-hydride migration to give alkene products. Especially challenging had been intermolecular reactions involving α-alkyl-α-diazocarbonyl compounds. This Account discusses the historical context and prior limitations of Rh-catalyzed reactions involving α-alkyl-α-diazocarbonyl compounds. Early studies demonstrated that ligand and temperature effects could influence chemoselectivity over β-hydride migration. However, effects were modest and conflicting conclusions had been drawn about the influence of sterically demanding ligands on β-hydride migration. More recent advances have led to a more detailed understanding of the reaction conditions that can promote intermolecular reactivity in preference to β-hydride migration. In particular, the use of bulky carboxylate ligands and low reaction temperatures have been key to enabling intermolecular cyclopropenation, cyclopropanation, carbonyl ylide formation/dipolar cycloaddition, indole C-H functionalization, and intramolecular bicyclobutanation with high chemoselectivity over β-hydride migration. Cyclic α-diazocarbonyl compounds have been shown to be particularly resilient toward β-hydride migration and are the first class of compounds that can engage in intermolecular reactivity in the presence of tertiary β-hydrogens. DFT calculations were used to propose that for cyclic α-diazocarbonyl compounds, ring constraints relieve steric interaction for intermolecular reactions and thereby accelerate the rate of intermolecular reactivity relative to intramolecular β-hydride migration. Enantioselective reactions of α-alkyl-α-diazocarbonyl compounds have been developed using bimetallic N-imido-tert-leucinate-derived complexes. The most effective complexes were found by computation and X-ray crystallography to adopt a "chiral crown" conformation in which all of the imido groups are presented on one face of the paddlewheel complex in a chiral arrangement. Insight from computational studies guided the design and synthesis of a mixed ligand paddlewheel complex, Rh2(S-PTTL)3TPA, the structure of which bears similarity to the chiral crown complex Rh2(S-PTTL)4. Rh2(S-PTTL)3TPA engages substrate classes (aliphatic alkynes, silylacetylenes, α-olefins) that are especially challenging in intermolecular reactions of α-alkyl-α-diazoesters and catalyzes enantioselective cyclopropanation, cyclopropenation, and indole C-H functionalization with yields and enantioselectivities that are comparable or superior to Rh2(S-PTTL)4. The work detailed in this Account describes progress toward enabling a more general utility for α-alkyl-α-diazo compounds in Rh-catalyzed carbene reactions. Further studies on ligand design and synthesis will continue to broaden the scope of their selective reactions.

Nickel(ii)-catalyzed enantioselective cyclopropanation of 3-alkenyl-oxindoles with phenyliodonium ylide via free carbene

A chiral Lewis acid-promoted cyclopropanation using a phenyliodonium ylide as the carbene precursor was developed. An EPR spectroscopy study supported a stepwise biradical mechanism.

A chiral Lewis acid-promoted enantioselective cyclopropanation using phenyliodonium ylide as the carbene precursor was developed. A variety of spirocyclopropane-oxindoles with contiguous tertiary and all carbon quaternary centers were obtained in excellent outcomes (up to 99% yield, >19 : 1 d.r., up to 99% ee). EPR spectroscopy study supported a stepwise biradical mechanism.

Chiral dirhodium catalysts derived from l-serine, l-threonine and l-cysteine: design, synthesis and application

A series of dirhodium complexes were synthesized and found to be effective chiral catalysts for nitrene and carbene transfer reactions.

Stereoselective 1,3-insertions of rhodium(II) azavinyl carbenes

Rhodium(II) azavinyl carbenes, conveniently generated from 1-sulfonyl-1,2,3-triazoles, undergo a facile, mild, and convergent formal 1,3-insertion into N-H and O-H bonds of primary and secondary amides, various alcohols, and carboxylic acids to afford a wide range of vicinally bisfunctionalized (Z)-olefins with perfect regio- and stereoselectivity. Utilizing the distinctive functionality installed through these reactions, a number of subsequent rearrangements and cyclizations expand the repertoire of valuable organic building blocks constructed by reactions of transition-metal carbene complexes, including α-allenyl ketones and amino-substituted heterocycles.