Cu(I)/TF-BiphamPhos Catalyzed Reactions of Alkylidene Bisphosphates and Alkylidene Malonates with Azomethine Ylides: Michael Addition versus 1,3-Dipolar Cycloaddition

2,5-Di-tert-butyl-2,5-diethylpyrrolidine-1-oxyls: Where Is a Reasonable Limit of Sterical Loading for Higher Resistance to Reduction?

The pyrrolidine nitroxides with four bulky alkyl substituents adjacent to the N-O∙ group demonstrate very high resistance to reduction with biogenic antioxidants and enzymatic systems. This makes them valuable molecular tools for studying the structure and functions of biomolecules directly in a living cell and for functional EPR and NMR tomography in vivo. The first example of highly strained pyrrolidine nitroxides with both ethyl and tert-butyl groups at each of the α-carbon atoms of the nitroxide moiety with cis-configuration of the tert-butyl groups was prepared using a three-component domino reaction of tert-leucine and 2,2-dimethylpentan-3-one with dimethyl fumarate with subsequent conversion of the resulting strained pyrrolidine into 1-pyrroline-1-oxide and addition of EtLi. The nitroxide has demonstrated unexpectedly fast reduction with ascorbate, the rate constant k2 = (2.0 ± 0.1) × 10-3 M-1s-1. This effect was explained by destabilization of the planar nitroxide moiety due to repulsion with the two neighboring tert-butyl groups cis to each other.

Electron-rich benzofulvenes as effective dipolarophiles in copper(i)-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides

A series of benzofulvenes without any electron-withdrawing substituents were employed as 2π-type dipolarophiles for the first time to participate in Cu(i)-catalyzed asymmetric 1,3-dipolar cycloaddition (1,3-DC) reactions of azomethine ylides. An intrinsic non-benzenoid aromatic characteristic from benzofulvenes serves as a key driving force for activation of the electron-rich benzofulvenes. Utilizing the current methodology, a wide range of multi-substituted chiral spiro-pyrrolidine derivatives containing two contiguous all-carbon quaternary centers were formed in good yield with exclusive chemo-/regioselectivity and high to excellent stereoselectivity. Computational mechanistic studies elucidate the origin of the stereochemical outcome and the chemoselectivity, in which the thermostability of these cycloaddition products is the major factor.

Stereodivergent silver-catalyzed synthesis of pyroglutamic acid esters

We report here a silver-catalyzed method for the enantio- and diastereodivergent synthesis of chiral pyroglutamic acid esters with multiple stereocenters. This process proceeds through asymmetric conjugate addition of glycine imine esters to a broad range of β-substituted α,β-unsaturated perfluorophenyl esters followed by lactamization. By leveraging catalyst control and stereospecificity of the 1,4-addition process, all four product stereoisomers containing two adjacent stereocenters are accessible with high stereoselectivity.

Multi-Ion Bridged Pathway of N-Oxides to 1,3-Dipole Dilithium Oxide Complexes

Roussi's landmark work on the generation of 1,3-dipoles from tertiary amine N-oxides has not reached its full potential since its underlying mechanism is neither well explored nor understood. Two competing mechanisms were previously proposed to explain the transformation involving either an iminium ion or a diradical intermediate. Our investigation has revealed an alternative mechanistic pathway that explains experimental results and provides significant insights to guide the creation of new N-oxide reagents beyond tertiary alkylamines for direct synthetic transformations. Truhlar's M06-2x functional and Møller-Plesset second-order perturbation theory with Dunning's [jul,aug]-cc-pv[D,T]z basis sets and discrete-continuum solvation models were employed to determine activation enthalpies and structures. During these mechanistic explorations, we discovered a unique multi-ion bridged pathway resulting from the rate-determining step, which was energetically more favorable than other alternate mechanisms. This newly proposed mechanism contains no electrophilic intermediates, strengthening the reaction potential by broadening the reagent scope and limiting the possible side reactions. This thoroughly defined general mechanism supports a more direct route for improving the use of N-oxides in generating azomethine ylide-dilithium oxide complexes with expanded functional group tolerance and breadth of chemistry.

Catalytic Asymmetric Reactions with N-Metallated Azomethine Ylides

Optically active nitrogen-containing compounds have attracted substantial attention due to their ubiquity in the cores of natural products and bioactive molecules. Among the various synthetic approaches to nitrogenous frameworks, catalytic asymmetric 1,3-dipolar cycloadditions are one of the most attractive methods because of their powerful ability to rapidly construct various chiral N-heterocycles. In particular, N-metallated azomethine ylides, common and readily available 1,3-dipoles, have been extensively applied in dipolar cycloaddition reactions. Despite the fact that asymmetric transformations of azomethine ylides have been investigated for decades, most of the efforts have been directed toward the preparation of pyrrolidines using glycinate-derived α-unsubstituted aldimine esters as the precursors of the azomethine ylides. While α-substituted azomethine ylides derived from amino esters other than glycinate have seldom been harnessed, the construction of non-five-membered chiral N-heterocycles via 1,3-dipolar cycloadditions remains underexplored. In addition, the asymmetric α-functionalization of aldimine esters to prepare acyclic nitrogenous compounds such as α-amino acids, in which an in situ-generated N-metallated azomethine ylide serves as the nucleophile, has not been sufficiently described.In this Account, we mainly discuss the achievements we have made in the past decade toward broadening the applications of N-metallated azomethine ylides for the preparation of nitrogen-containing compounds. We began our investigation with the design and synthesis of a new type of chiral ligand, TF-BiphamPhos, which not only coordinates with Lewis acids to activate dipolar species but also serves as an H-bond donor to increase the reactivity of dipolarophiles with significantly enhanced stereochemical control. Using the Cu(I) or Ag(I)/TF-BiphamPhos complex as the catalyst, we achieved highly stereoselective (3+2) cycloadditions of glycinate and non-glycinate-derived azomethine ylides with diverse dipolarophiles, producing a variety of enantioenriched pyrrolidines with multiple stereocenters in a single step. To further expand the synthetic utility of N-metallated azomethine ylides, we successfully developed higher order cycloadditions with fulvenes, tropone, 2-acyl cycloheptatrienes, and pyrazolidinium ylides serving as the reaction partner, and this reaction provides straightforward access to enantioenriched fused piperidines, bridged azabicyclic frameworks, and triazines via (3+6)- and (3+3)-type cycloadditions. Using N-metallated azomethine ylides as the nucleophile, we realized Cu(I)-catalyzed asymmetric 1,4-Michael additions with α,β-unsaturated bisphosphates/Morita-Baylis-Hillman products, furnishing an array of structurally diverse unnatural α-amino acids. Based on the strategy of synergistic activation, we achieved highly efficient dual Cu/Pd and Cu/Ir catalysis for the α-functionalization of aldimine esters via the asymmetric allylic/allenylic alkylation of N-metallated azomethine ylides. Notably, Cu/Ir catalysis allowed the stereodivergent synthesis of α,α-disubstituted α-amino acids via a branched allylic alkylation reaction, in which the two distinct chiral metal catalysts independently have full stereochemical control over the corresponding nucleophile and electrophile. Furthermore, an expedient and stereodivergent preparation of biologically important tetrahydro-γ-carbolines was realized through a Cu/Ir-catalyzed cascade allylation/iso-Pictet-Spengler cyclization. In addition, when the steric congestion in the allylation intermediates was increased, the combined Cu/Ir catalysts provided an asymmetric cascade allylation/2-aza-Cope rearrangement, producing various optically active homoallylic amines with impressive results.

Kinetic Resolution of Alkylidene Norcamphors via a Ligand-Controlled Umpolung-Type 1,3-Dipolar Cycloaddition

Development of a general catalytic and highly efficient method utilizing readily available precursors for the regio- and stereoselective construction of bioactive natural-product-inspired spiro architectures remains a formidable challenge in chemical research. Transition metal-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides produces numerous N-heterocycles, but reaction control with the regioselectivity opposite to the conventional fashion has rarely been demonstrated. Herein, we report a unique ligand-controlled Cu(I)-catalyzed umpolung-type 1,3-dipolar cycloaddition of azomethine ylide to realize efficient kinetic resolution of racemic alkylidene norcamphors with the concomitant construction of previously inaccessible spiro N-heterocycles with high levels of regio- and stereoselectivity. The success of this methodology relies on the strategy of kinetic resolution, and the serendipitous discovery of a unique ligand-enabled regiospecific cycloaddition, which not only provides evidence for the existence of the minor zwitterionic resonance form in metallated azomethine ylide but also diversifies the existing chemistry of azomethine ylide-involved 1,3-dipolar cycloadditions with rare polarity inversion.

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Kinetic resolution of racemic alkylidene norcamphors

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Spiro architectures incorporating norbornane and pyrrolidine scaffolds

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Unique ligand-enabled umpolung-type 1,3-dipolar cycloaddition

Recent Advances in Metal-Catalyzed Asymmetric 1,4-Conjugate Addition (ACA) of Nonorganometallic Nucleophiles

The metal-catalyzed asymmetric conjugate addition (ACA) reaction has emerged as a general and powerful approach for the construction of optically active compounds and is among the most significant and useful reactions in synthetic organic chemistry. In recent years, great progress has been made in this area with the use of various chiral metal complexes based on different chiral ligands. This review provides comprehensive and critical information on the enantioselective 1,4-conjugate addition of nonorganometallic (soft) nucleophiles and their importance in synthetic applications. The literature is covered from the last 10 years, and a number of examples from before 2007 are included as background information. The review is divided into multiple parts according to the type of nucleophile involved in the reaction (such as C-, B-, O-, N-, S-, P-, and Si-centered nucleophiles) and metal catalyst systems used.

Copper(i)/TF-BiphamPhos catalyzed asymmetric nitroso Diels–Alder reaction

A highly efficient enantioselective nitroso Diels–Alder reaction of 6-methyl-2-nitroso pyridine with various 1,3-dienes was successfully developed using a copper(i)/(S)–TF-BiphamPhos complex as the catalyst. For most of the cyclic dienes, the cycloadducts were obtained in high yields with excellent regio-, and stereoselectivities. Acyclic 2-silyloxy-1,3-diene also worked well in the reaction.

Asymmetric Hydrogenation of Unprotected Indoles Catalyzed by η(6)-Arene/N-Me-sulfonyldiamine-Ru(II) Complexes

Protecting-group-free transformation is a challenging and important issue in atom-economical organic synthesis. The η(6)-arene/N-Me-sulfonyldiamine-Ru(II)-BF4 complex-catalyzed asymmetric hydrogenation of 2-substituted unprotected indoles in weakly acidic hexafluoroisopropanol gives optically active indoline compounds with up to >99% ee. Under mild reaction media, halogen atoms and synthetically important protecting groups (e.g., silyl ether, acetal, benzyl ether, and ester) on indoles are maintained, which is advantageous for the synthesis of further complex indoline molecules.

Ligand-Controlled Diastereoselective 1,3-Dipolar Cycloadditions of Azomethine Ylides with Methacrylonitrile

Copper-catalyzed reactions of glycine ester arylimines and methacrylonitrile provide selective access to either the endo or exo pyrrolidine cycloadducts. DFT calculations have elucidated the origins of ligand-controlled diastereoselectivity.

Cu(i)/TF–BiphamPhos-catalyzed asymmetric 1,3-dipolar cycloaddition of azomethine ylides with dimethyl itaconate and 2-methyleneglutarate

Catalytic asymmetric 1,3-dipolar cycloaddition of azomethine ylides with dimethyl itaconate and 2-methyleneglutarate was realized with Cu(i)/TF–BiphamPhos complex as the catalyst for the efficient construction of pyrrolidine derivatives bearing one unique all carbon-quaternary and two tertiary stereogenic centers.

The N-H functional group in organometallic catalysis

The organometallic approach is one of the most active topics in catalysis. The application of NH functionality in organometallic catalysis has become an important and attractive concept in catalyst design. NH moieties in the modifiers of organometallic catalysts have been shown to have various beneficial functions in catalysis by molecular recognition through hydrogen bonding to give catalyst-substrate, ligand-ligand, ligand-catalyst, and catalyst-catalyst interactions. This Review summarizes recent progress in the development of the organometallic catalysts based on the concept of cooperative catalysis by focusing on the NH moiety.