Gold(I)-Catalyzed Enantioselective Synthesis of Pyrazolidines, Isoxazolidines, and Tetrahydrooxazines

H-Bonded Counterion-Directed Catalysis: Enantioselective Gold(I)-Catalyzed Addition to 2-Alkynyl Enones as a Case Study

H-bonded counterion-directed catalysis (HCDC) is a strategy wherein a chiral anion that is hydrogen-bonded to the achiral ligand of a metal complex is responsible for enantioinduction. In this article we present the application of H-bonded counterion-directed catalysis to the Au(I)-catalyzed enantioselective tandem cycloisomerization-addition reaction of 2-alkynyl enones. Following the addition of C-, N- or O-centered nucleophiles, bicyclic furans were obtained in moderate to excellent yield and enantioselectivity (28 examples, 59-96 % yield, 62 : 38 to 95 : 5 er). The optimal catalytic system, comprising a phosphinosquaramide Au(I) chloride complex and a BINOL-derived phosphoramidate Ag(I) salt, was selected in a combinatorial fashion from a larger library with the help of high-throughput screening. An enantioselectivity switch of ca. 120 Δee% was observed upon addition of the achiral Au(I) component to the Ag(I) salt.

Stereodivergent Palladium- and Rhodium-Catalyzed Intramolecular Addition of Tosylureas to Allenes: Diastereoselective Synthesis of Tetrahydropyrimidinones

The intramolecular addition of tosylureas to allenes is highly syn-/anti-diastereoselective when employing a palladium or rhodium-based catalytic system and affords 1,3-cyclic ureas. Under palladium catalysis a range of thermodynamic anti-tetrahydropyrimidinones are accessible, while rhodium catalysis allows synthesis of the kinetic syn-tetrahydropyrimidinones. For a representative scope of substrates both cyclic ureas were obtained in excellent yields and diastereoselectivities. The obtained tetrahydropyrimidinones were shown to be easily deprotected and modified to demonstrate the synthetic value.

Cycloisomerization of π-Coupled Heteroatom Nucleophiles by Gold Catalysis: En Route to Regiochemically Defined Heterocycles

Since the dawn of millennium, catalytic gold chemistry is at the forefront to set off diverse organic reactions via unique activation of π-bonded molecules. Within this purview, cycloisomerization of heteroatom nucleophiles linked to a π-system is one of the well recognized chemistry for the construction of numerous heterocyclic cores. Though the rudimentary aspects of this transformation are reviewed by several groups in different timeline, a holistic view on regiochemistry of such reactions went largely overlooked. Hence, this account emphasizes the gold catalyzed regioselective cycloisomerization of structurally distinctive π-connected hetero-nucleophiles leading to different heterocycles documented in the last two decades. From an application perspective, this account also highlights those methodologies which find a role in the total synthesis of natural products. Wherever appropriate, mechanistic details and contributing factors for selectivity are also discussed.

Catalyst-Free Regioselective [3+2] Cycloadditions of α,β-unsaturated N-arylnitrones with Alkenes to Access Functionalized Isoxazolidines: A DFT Study

The catalyst-free regioselective [3+2]-cycloaddition of α,β-unsaturated N-arylnitrones with alkenes are developed. The series of synthetically important functionalized isoxazolidines are prepared in good to excellent yields by step economic pathway under ligand and transition-metal-free conditions. The regioselective cycloaddition pathway supported by control experiment and computational study.

Quantum chemical studies on the mechanistic aspects of tandem sequential cycloaddition reactions of cyclooctatetraene with ester and nitrones

The mechanisms of the tandem sequential [4 + 2]/[3 + 2] and [3 + 2]/[4 + 2] cycloaddition sequences involving an ester, cyclooctatetraene (COTE), and cyclic and acyclic nitrones for the formation of a diverse range of isoxazolidine derivatives and other synthetic precursors are reported. A thorough exploration of the PES has characterized several regio-, stereo- and enantio-selective mechanistic channels involved in these reactions. A perturbation molecular orbital (PMO) analysis been employed to rationalize the results. It has also been found that the initial electrocyclic ring closure of the COTE is the rate-determining step in the tandem sequential [4 + 2]/[3 + 2] addition sequence. The thermolytic breakdown of the tandem adducts to subsequent monocyclic, bicyclic and tricyclic adducts occurs generally with very high activation barriers making it an inconvenient synthetic approach. The different reactivity of all the three double bonds present in the dipolarophile is reported. Finally, the mechanistic possibilities of [3 + 2]/[4 + 2] addition sequences involving the same reaction components in the case of cyclic and acyclic nitrones are explored extensively. The results suggest a novel and convenient routes for obtaining products of high selectivity with less energetic requirements. In some instances, new cycloadducts hitherto unreported are obtained.

Rare-Earth-Metal-Catalyzed Kinetic Resolution of Chiral Aminoalkenes via Hydroamination: The Effect of the Silyl Substituent of the Binaphtholate Ligand on Resolution Efficiency

The kinetic resolution of α-substituted aminopentenes via intramolecular hydroamination was investigated using various 3,3'-silyl-substituted binaphtholate yttrium catalysts. High efficiencies in the kinetic resolution were observed for methyl-, benzyl-, and phenyl-substituted substrates utilizing the cyclohexyldiphenylsilyl-substituted catalyst 2c with resolution factors reaching as high as 90(5) for hex-5-en-2-amine (3a). Kinetic analysis of the enantioenriched substrates with the matching and mismatching catalyst revealed that the efficiency of catalyst 2c benefits significantly from a favorable Curtin-Hammett pre-equilibrium and by a large kfast/kslow ratio. Other binaphtholate catalysts were less efficient due to a less favorable Curtin-Hammett pre-equilibrium, which often favored the mismatching substrate-catalyst combination. Cyclization of the matched substrate proceeds generally with large trans-selectivity, whereas the trans/cis-ratio for mismatched substrates is significantly diminished, favoring the cis-cyclization product isomer in some instances.

Chiral Phosphate in Rhodium-Catalyzed Asymmetric [2+2+2] Cycloaddition: Ligand, Counterion, or Both?

Investigations based on NMR spectroscopy, mass spectrometry, and DFT calculations shed light on the metallic species generated in the rhodium-catalyzed asymmetric [2+2+2] cycloaddition reaction between diynes and isocyanates with the chiral phosphate TRIP. The catalytic mixture comprising [{Rh(cod)Cl}2 ], 1,4-diphenylphosphinobutane (dppb), and Ag(S)-TRIP actually gives rise to two species, both having an effect on the stereoselectivity. One is a rhodium(I) complex in which TRIP is a weakly coordinating counterion, whereas the other is a bimetallic Rh/Ag complex in which TRIP is a strongly coordinating X-type ligand.

Gold(I)-Catalyzed Enantioselective Desymmetrization of 1,3-Diols through Intramolecular Hydroalkoxylation of Allenes

A gold(I)-catalyzed enantioselective desymmetrization of 1,3-diols was achieved by intramolecular hydroalkoxylation of allenes. The catalyst system 3-F-dppe(AuCl)2/(R)-C8-TRIPAg proved to be specifically efficient to promote the desymmetrizing cyclization of 2-aryl-1,3-diols, which have proven challenging substrates in previous reports. Multisubstituted tetrahydrofurans were prepared in good yield with good enantioselectivity and diastereoselectivity by this method.

Palladium(II)/Brønsted Acid-Catalyzed Enantioselective Oxidative Carbocyclization-Borylation of Enallenes

An enantioselective oxidative carbocyclization-borylation of enallenes that is catalyzed by palladium(II) and a Brønsted acid was developed. Biphenol-type chiral phosphoric acids were superior co-catalysts for inducing the enantioselective cyclization. A number of chiral borylated carbocycles were synthesized in high enantiomeric excess.

Gold(I)-catalyzed asymmetric desymmetrization of meso-alkynyl diols and kinetic resolution of the corresponding DL-diols: effects of celite filtration and silver salts

The asymmetric desymmetrization of meso-2-alkynylbenzenediols through the use of a combination of axially chiral diphosphine(AuCl)2 precatalysts and silver salt co-catalysts gave optically active isochromene compounds with high enantioselectivities in good yields. The corresponding DL-diol isomers underwent efficient kinetic resolution to give the cyclized isochromenes and recovered diols with high enantioselectivities under similar conditions. The high reactivity and selectivity in the desymmetrization of the meso-diols is independent of the combination of axially chiral diphosphine(AuCl)2 precatalyst and silver salt co-catalyst, whereas the corresponding tricarbonylchromium complexes of alkynylbenzenediols were affected by the combination of the diphosphine(AuCl)2 and silver salt. The reactivity was largely dependent on the nature of the gold(I) species.

Room-temperature hydrohydrazination of terminal alkynes catalyzed by saturated abnormal N-heterocyclic carbene-gold(I) complexes

A number of saturated abnormal N-heterocyclic carbene (NHC) complexes of gold, in combination with KBAr(F) 4 as activator, were successfully applied in the chemoselective addition of hydrazine to alkynes. The reaction proceeds even at room temperature, which was not possible to date with gold catalysts. The reaction can be applied to a number of substituted arylalkynes. With alkylalkynes the yields are low. The saturated abnormal NHC ligands are resistant to isomerization to the saturated normal NHC coordination mode under basic reaction conditions. Under acidic conditions, a simple protonation at the nitrogen atom not neighboring the carbene center was observed and unambiguously characterized by an X-ray crystal-structure analysis. Computational studies confirm that such an isomerization would be highly exothermic, the observed kinetic stability probably results from the need to shift two protons in such a process.

Asymmetric ion-pairing catalysis

Charged intermediates and reagents are ubiquitous in organic transformations. The interaction of these ionic species with chiral neutral, anionic, or cationic small molecules has emerged as a powerful strategy for catalytic, enantioselective synthesis. This review describes developments in the burgeoning field of asymmetric ion-pairing catalysis with an emphasis on the insights that have been gleaned into the structural and mechanistic features that contribute to high asymmetric induction.