Asymmetric Direct Alkynylation Catalyzed by Chiral Ru−Bis(oxazolinyl)phenyl Complexes

Chiral Macrocyclic Catalysts for the Enantioselective Addition of Lithium Acetylides to Ketones

Alkynyl addition to carbonyl compounds is a valuable synthetic method for the preparation of versatile chiral alcohols that are widely found in pharmaceuticals and natural products. Although a variety of enantioselective variations have been reported, alkynyl addition to simple ketones remains an unmet challenge due to their low reactivity and difficult enantiofacial discrimination. Here, we report a method for the catalytic enantioselective addition of lithium acetylide to a variety of ketones using macrocyclic lithium binaphtholates as catalysts. These reactions generally suffer from facile aggregation of lithium species, which leads to less active and selective catalysts. The macrocyclic structure designed in this study prevents such aggregation, affording a monomeric and highly active catalyst that can furnish enantioenriched tertiary alcohols from a variety of ketones within 5-30 min. Moreover, the confined cavity and lipophilicity of the macrocycle confer substrate specificity on the system, demonstrating a multiselectivity similar to that of enzymatic reactions. Thus, these findings offer new insights into the rational design of small-molecule artificial enzymes that exhibit high levels of reactivity and multiselectivity.

Cu/SaBox-Catalyzed Photoinduced Coupling of Acylsilanes with Alkynes

The transition metal-catalyzed cross-coupling reaction with Fischer metal carbene intermediates bearing an electron-rich alkoxyl or siloxyl group remains a big challenge due to the lack of readily available corresponding carbene precursors. Herein, we report the coupling of alkynes with the Fischer-type copper carbene species bearing a α-siloxyl group, which could be in situ generated from acylsilanes catalytically under photoirradiation and redox-neutral conditions. The side-arm modified bisoxazoline (SaBox) ligands prove to be crucial for this coupling reaction, which provides the corresponding alkynyl alcohol in high yields with remarkable heterocycle tolerance and broad substrate scope.

Chiral-at-Ru Catalyst with Cyclometalated Imidazo[1,5-a]pyridin­ylidene for Enantioselective Intramolecular Cyclopropanations

A chiral ruthenium catalyst is introduced which contains a cyclometalated N-(3-nitrophenyl)-imidazo[1,5-a]pyridinylidene ligand in addition to a bidentate 4-mesityl-2-(pyridin-2-yl)thiazole and two acetonitriles to complement the octahedral coordination sphere of the monocationic complex. Tetrafluoroborate serves as the counterion. Since all coordinated ligands are achiral, the overall chirality is formally due to a stereogenic metal center generating either a left-handed (Λ) or right-handed (Δ) helical topology of this chiral-at-metal complex. Nonracemic Λ and Δ complexes were synthesized using (R)- and (S)-N-benzoyl-tert-butanesulfinamide as chiral auxiliary ligands, respectively. The position of the nitro group in the metalated phenyl moiety is of crucial importance for the generation of enantiomerically pure complexes. The catalytic activity of the cycloruthenated chiral-at-metal catalyst was demonstrated for the enantioselective intramolecular cyclopropanation of trans-cinnamyl diazoacetate and an alkenyl diazoketone to generate bicyclic cyclopropanes in high yields (96–97%) and with satisfactory enantioselectivity (93% ee).

Further Developments and Applications of Oxazoline-Containing Ligands in Asymmetric Catalysis

The chiral oxazoline motif is present in many ligands that have been extensively applied in a series of important metal-catalyzed enantioselective reactions. This Review aims to provide a comprehensive overview of the most significant applications of oxazoline-containing ligands reported in the literature starting from 2009 until the end of 2018. The ligands are classified not by the reaction to which their metal complexes have been applied but by the nature of the denticity, chirality, and donor atoms involved. As a result, the continued development of ligand architectural design from mono(oxazolines), to bis(oxazolines), to tris(oxazolines) and tetra(oxazolines) and variations thereof can be more easily monitored by the reader. In addition, the key transition states of selected asymmetric transformations will be given to illustrate the features that give rise to high levels of asymmetric induction. As a further aid to the reader, we summarize the majority of schemes with representative examples that highlight the variation in % yields and % ees for carefully selected substrates. This Review should be of particular interest to the experts in the field but also serve as a useful starting point to new researchers in this area. It is hoped that this Review will stimulate both the development/design of new ligands and their applications in novel metal-catalyzed asymmetric transformations.

Enantiocontrol by assembled attractive interactions in copper-catalyzed asymmetric direct alkynylation of α-ketoesters with terminal alkynes: OH···O/sp3-CH···O two-point hydrogen bonding combined with dispersive attractions

Copper-catalyzed asymmetric direct alkynylation of α-ketoesters with terminal alkynes with chiral prolinol-phosphine ligands, most preferably (αR,2S)-1-(2-dicyclohexylphosphinobenzyl)-α-neopentyl-2-pyrrolidinemethanol, afforded various enantioenriched chiral propargylic tertiary alcohols. Quantum-chemical calculations using the BP86 density functional including Grimme's empirical dispersion correction [DF-BP86-D3(BJ)-PCM(tBuOH)/TZVPP//DF-BP86-D3(BJ)/SVP] show the occurrence of OH···O/sp3-CH···O two-point hydrogen bonding between the chiral ligand and the carbonyl group of the ketoester in the stereo-determining transition states. Combined with the hydrogen-bonding interactions orienting the ketoester substrate, dispersive attractions between the chiral ligand (P-cyclohexyl groups) and the ketoester in the favored transition states, rather than steric repulsions in the disfavored transition state explain the enantioselectivity of the asymmetric copper catalysis.

A General Asymmetric Synthesis of (R)-Matsutakeol and Flavored Analogs

An efficient and practical synthetic route toward chiral matsutakeol and analogs was developed by asymmetric addition of terminal alkyne to aldehydes. (R)-matsutakeol and other flavored substances were feasibly synthesized from various alkylaldehydes in high yield (up to 49.5%, in three steps) and excellent enantiomeric excess (up to >99%). The protocols may serve as an alternative asymmetric synthetic method for active small-molecule library of natural fatty acid metabolites and analogs. These chiral allyl alcohols are prepared for food analysis and screening insect attractants.

Enantioselective additions of copper acetylides to cyclic iminium and oxocarbenium ions

The development of enantioselective, copper-catalyzed alkynylations of cyclic iminium and oxocarbenium ions is reviewed. The use of chiral copper-based catalysts has enabled high yields and enantioselectivites in the formation of nitrogen- and oxygen-containing heterocycles with α-stereogenic centers. This review highlights both the accomplishments and the future work needed in this important area.

The ruthenium-catalysed selective synthesis of mono-deuterated terminal alkynes

A highly efficient catalytic system is reported for chemoselective synthesis of mono-deuterated terminal alkynes using deuterium oxide in which the reaction proceeds via Ru–acetylide intermediates formed by selective activation of the sp-CH bond.

Enantioselective copper-catalyzed alkynylation of benzopyranyl oxocarbenium ions

We have developed highly enantioselective, copper-catalyzed alkynylations of benzopyranyl acetals. By using a copper(I) catalyst equipped with a chiral bis(oxazoline) ligand, high yields and enantioselectivities are achieved in the alkynylation of widely available, racemic isochroman and chromene acetals to deliver α-chiral oxygen heterocycles. This method demonstrates that chiral organometallic nucleophiles can be successfully used in enantioselective additions to oxocarbenium ions.

Tandem semi-hydrogenation/isomerization of propargyl alcohols to saturated carbonyl analogues by dodecanethiolate-capped palladium nanoparticle catalysts

The efficient one-pot conversion of propargyl alcohols to their saturated carbonyl analogues is carried out for the first time using metal nanoparticle catalysts, dodecanethiolate-capped Pd nanoparticles. Kinetic studies reveal that the reaction progresses through a semi-hydrogenation intermediate (allyl alcohols) followed by isomerization to carbonyls.

Controlling Enantioselectivity in Additions to Cyclic Oxocarbenium Ions via Transition Metal Catalysis

Controlling enantioselectivity in additions to oxocarbenium ions remains a challenge in asymmetric catalysis. By catalytically generating a chiral organometallic intermediate, a copper acetylide, we have developed a novel approach for additions of carbon nucleophiles to cyclic oxocarbenium ions in high enantioselectivities and yields.

Copper-catalyzed enantioselective additions to oxocarbenium ions: alkynylation of isochroman acetals

We have developed an enantioselective, copper(I)-catalyzed addition of terminal alkynes to racemic isochroman acetals. This method is one of the first transition-metal-catalyzed approaches to enantioselective additions to prochiral oxocarbenium ions. In this reaction, TMSOTf is used to form the oxocarbenium ion in situ under conditions compatible with simultaneous formation of the chiral copper acetylide. By using a bis(oxazoline) ligand, good yields and enantioselectivities are observed for a variety of enantioenriched 1-alkynyl isochromans.