Chiral Magnesium Research Articles

Chiral Magnesium Complex-Catalyzed Asymmetric Cyclization of Vinyl Diazo Compounds with Phenol Derivatives via Noncarbene Pathways

Chiral Magnesium Complex-Catalyzed Asymmetric Cyclization of Vinyl Diazo Compounds with Phenol Derivatives via Noncarbene Pathways

Magnesium‐Catalyzed Asymmetric Thia‐Michael Addition to α,β‐Unsaturated Ketones

AbstractWe demonstrate the application of chiral magnesium complexes in an asymmetric carbon‐sulfur bond‐forming reaction. Enantioselective and cost‐effective methodology under mild condition for the thia‐Michael addition, utilizing an in situ generated chiral dinuclear magnesium‐ProPhenol complex, has been developed. The versatility of this protocol is demonstrated with a broad range of thiol nucleophiles and a wide selection of enones. Enantioenriched β‐ketosulfides are obtained in good to excellent yields and moderate to excellent enantioselectivity. The presented catalytic system exhibits excellent tolerance for structurally different substrates while maintaining high enantioselectivity. This observation aligns with proposed mechanism, wherein the sulfur atom coordinates to the catalyst in close proximity to the reaction center.

Enantioselective Mukaiyama-Michael Reaction of β,γ-Unsaturated α-Keto Esters with Silyl Ketene Acetals Catalyzed by a Chiral Magnesium Phosphate.

We would like to describe an efficient and highly enantioselective Mukaiyama-Michael reaction of silyl ketene acetals with β,γ-unsaturated α-keto esters catalyzed by a chiral magnesium BINOL-derived phosphate. The resulting functionalized 1,5-dicarbonyl adducts are obtained in high yields (up to 96%) and with excellent enantioselectivities (up to 98%) under mild conditions. Two plausible mechanistic pathways were proposed, including a 1,4-addition and a hetero Diels-Alder [4 + 2] cycloaddition.

Enantioselective [2+2] Cycloaddition of Allenyl Imide with Mono‐ or Disubstituted Alkenes

AbstractAn efficient catalytic asymmetric [2+2] cycloaddition of allenyl imide and mono‐ or disubstituted alkenes is disclosed. The key feature of this method is the use of bidentate allenyl imide and weakly activated and less steric hindered alkene pair by utilizing chiral magnesium(II) complex of N,N′‐dioxide, which could provide through‐space dispersion interactions to orientate the arrangement of the alkene. This protocol allows the generation of a series of axially chiral cyclobutenes and four‐membered ring‐containing spirocycles (80 examples) in high yield (up to 99 %) with excellent enantioselectivity (up to >99 % ee), and the late‐stage modification of biologically active molecules as well. Experimental studies and DFT calculations revealed that this [2+2] cycloaddition proceeded via a stepwise mechanism involving a short‐lived zwitterionic intermediate. The π‐π interaction between the alkenes and the amide moiety in the ligand was crucial for the enantiocontrol.

Enantioselective [2+2] Cycloaddition of Allenyl Imide with Mono- or Disubstituted Alkenes.

An efficient catalytic asymmetric [2+2] cycloaddition of allenyl imide and mono- or disubstituted alkenes is disclosed. The key feature of this method is the use of bidentate allenyl imide and weakly activated and less steric hindered alkene pair by utilizing chiral magnesium(II) complex of N,N'-dioxide, which could provide through-space dispersion interactions to orientate the arrangement of the alkene. This protocol allows the generation of a series of axially chiral cyclobutenes and four-membered ring-containing spirocycles (80 examples) in high yield (up to 99 %) with excellent enantioselectivity (up to >99 % ee), and the late-stage modification of biologically active molecules as well. Experimental studies and DFT calculations revealed that this [2+2] cycloaddition proceeded via a stepwise mechanism involving a short-lived zwitterionic intermediate. The π-π interaction between the alkenes and the amide moiety in the ligand was crucial for the enantiocontrol.

Absence of Intermediates in the BINOL-Derived Mg(II)/Phosphate-Catalyzed Desymmetrizative Ring Expansion of 1-Vinylcyclobutanols.

The catalyzed desymmetrizative ring expansion of alkenylcyclobutanols promoted by halofunctionalization of the alkene moiety with N-bromosuccinimide has been experimentally and computationally studied. The reaction yields highly enantioenriched cyclopentanones bearing two all-carbon quaternary stereocenters, one of them being generated in the rearrangement of the cyclobutane ring and the other by enantioselective desymmetrization. The reaction is competitive with the formation of a spiroepoxide, but it turns completely selective toward the cyclopentanone when a chiral bisphosphonium magnesium salt is employed as a catalyst. Mechanistic studies support the formation of an ion pair leading to a complex with only a unit of phosphoric acid, which is the resting state of the catalytic cycle. Calculations reproduce in an excellent way the observed reactivity and predict the effect exerted by the substituents of the aromatic ring linked to the double bond. The computational studies also revealed the reaction as a highly asynchronous concerted process taking place as one kinetic step but in two stages: (i) halogenation of the double bond and (ii) rearrangement of the cyclobutane. No intermediates are present in the reaction as energy minima. The experimental scope of the reaction further confirms the predictions for the observed reactivity and selectivity.

Asymmetric Epoxidation of Enones Promoted by Dinuclear Magnesium Catalyst

AbstractAsymmetric synthesis with cheaper and non‐toxic alkaline earth metal catalysts is becoming an important and sustainable alternative to conventional catalytic methodologies mostly relying on precious metals. In spite of some sustainable methods for enantioselective epoxidation of enones, the development of a well‐defined and efficient catalyst based on magnesium complexes for these reactions is still a challenging task. In this perspective, we present the application of chiral dinuclear magnesium complexes for asymmetric epoxidation of a broad range of electron‐deficient enones. We demonstrate that the in situ generated magnesium‐ProPhenol complex affords enantioenriched oxiranes in high yields and with excellent enantioselectivities (up to 99% ee). Our extensive study verifies the literature data in this area and provides a step forward to better understand the factors controlling the oxygenation process. Elaborated catalyst offers mild reaction conditions and a truly wide substrate scope.magnified image

Catalytic Asymmetric C-7 Friedel-Crafts Alkylation/N-Hemiacetalization of 4-Aminoindoles.

A unique catalytic asymmetric C-7 Friedel-Crafts alkylation/N-hemiacetalization cascade reaction of 4-aminoindoles with β,γ-unsaturated α-keto esters has been described. Using a chiral magnesium H8-BINOL-derived bis(phosphate) complex as catalyst, the resulting functionalized 1,7-annulated indole scaffolds are obtained in high yields (up to 98%) and with good to excellent enantioselectivities (up to 99%) and diastereoselectivities (up to >20:1) under mild reaction conditions.

Desymmetrization Process by Mg(II)-Catalyzed Intramolecular Vinylogous Michael Reaction.

Chiral magnesium catalyzed intramolecular vinylogous Michael reaction of novel cyclohexadienones via a desymmetrization process is reported. (R)-BINOL derived ligand and an achiral amide were employed in the current in situ generated magnesium catalyst, giving the corresponding hydrogenated benzofuranone skeletons in good to excellent enantioselectivities with high yields. This simple and efficient strategy could be utilized for the synthesis of aromatized α,β-unsaturated ester and Br-substituted hydrogenated benzofuranone in good yields under mild conditions.

Catalytic Enantioselective Diels-Alder Reactions of Benzoquinones and Vinylindoles with Chiral Magnesium Phosphate Complexes.

Tetrahydrocarbazole and its derivatives have received much attention due to the prevalence of this scaffold in natural products and their use in organic synthesis. We have developed a Diels-Alder reaction of benzoquinones and 3-vinylindoles catalyzed by chiral magnesium phosphate complexes to provide tetrahydrocarbazole derivatives in excellent yields and enantioselectivities (up to >99% yield, 99% ee). This transformation features a wide substrate scope, excellent enantioselectivities, and mild conditions.

A Highly Enantio‐ and Diastereoselective Synthesis of Spirocyclic Dihydroquinolones via Domino Michael Addition‐Lactamization of ortho‐Quinone Methide Imines

Spirocyclic dihydroquinolones have been obtained with good yields and excellent diastereoselectivity (>20:1 d.r.), and enantioselectivity (up to 99:1 e.r.) from in situ generated ortho-quinone methide imines and cyclic β-oxo esters. This one-step domino Michael addition-lactamization process features mild reaction conditions, easily accessible starting materials, and products with two adjacent chiral centers one of which is quaternary. Mechanistic studies revealed the in situ generated chiral magnesium phosphate salt rather than the free phosphoric acid to be the more reactive catalyst for this reaction.

Highly Active Chiral Oxazolinyl Aminophenolate Magnesium Initiators for Isoselective Ring-Opening Polymerization of rac-Lactide: Dinuclearity Induced Enantiomorphic Site Control

The most important challenge in ring-opening polymerization of rac-lactide today is to obtain isotactic PLAs with high molecular weights. The goal of our work is to address this important challenge by developing novel catalysts that enable precise control over stereoselectivity and produce high molecular weight PLAs at the same time. Here we report some rare examples of isoselective magnesium complexes toward the ROP of rac-LA, which are supported by chiral oxazolinyl aminophenolate ligands. Preliminary kinetic investigations confirmed that isotactic PLAs could be achieved via an enantiomorphic site control mechanism by using these chiral magnesium complexes. A pair of racemic magnesium initiators proved to integrate hyperactivity (TOF up to 54 000 h–1 at 25 °C) with sufficient isoselectivity (Pm = 0.80) toward the ROP of rac-LA, leading to the formation of isotactic stereoblock PLA with high molecular weight (Mn = 461 kg mol–1) and a semicrystalline property (Tm = 164 °C). Detailed structural investigati...

Chiral Magnesium Bisphosphate-Catalyzed Asymmetric Double C(sp3)-H Bond Functionalization Based on Sequential Hydride Shift/Cyclization Process.

Described herein is a chiral magnesium bisphosphate-catalyzed asymmetric double C(sp3)-H bond functionalization triggered by a sequential hydride shift/cyclization process. This reaction consists of stereoselective domino C(sp3)-H bond functionalization: (1) a highly enantio- and diastereoselective C(sp3)-H bond functionalization by chiral magnesium bisphosphate (first [1,5]-hydride shift), and (2) a highly diastereoselective C(sp3)-H bond functionalization by an achiral catalyst (Yb(OTf)3, second [1,5]-hydride shift).

Chiral Magnesium(II) Complex‐Catalyzed Enantioselective Desymmetrization of meso‐Aziridines with Pyrazoles

AbstractA highly enantioselective catalytic protocol for the desymmetrization of meso‐aziridines via ring‐opening with pyrazoles is reported using an N,N′‐dioxide‐Mg(OTf)2 complex as the catalyst. The corresponding trans‐α‐pyrazole‐substituted amines were obtained in good yields and enantioselectivities (up to 99% yield and 94% ee) under mild reaction conditions. Moreover, a remarkably high asymmetric amplification was observed in the catalytic system.magnified image

Enantioselective Cycloaddition of Styrenes with Aldimines Catalyzed by a Chiral Magnesium Potassium Binaphthyldisulfonate Cluster as a Chiral Brønsted Acid Catalyst.

A chiral magnesium potassium binaphthyldisulfonate cluster, as a chiral Brønsted acid catalyst, was shown to catalyze an enantioselective cycloaddition of styrenes with aldimines for the first time. The strong Brønsted acidity of the catalyst precursors, which might dissolve drying agents and take up the leached Mg2+ and K+, serendipitously led to good enantioselectivity. Mechanistic aspects were supported by X-ray and ESI-MS analysis of the catalyst and a kinetics study of the reaction. Useful transformations to optically active 1,3-amino alcohols on a gram scale were also demonstrated.

Enantioselective magnesium-catalyzed transformations.

This review updates the major progress in the field of enantioselective transformations promoted by chiral magnesium catalysts, covering the literature since 2007, illustrating the power of these mild Lewis acid catalysts to provide a wide variety of novel asymmetric reactions.

A Modular Strategy for the Direct Catalytic Asymmetric α-Amination of Carbonyl Compounds

Summary Direct catalytic methods for the stereoselective introduction of various non-protected amines at the carbon atom adjacent to a carbonyl group remain elusive in synthetic chemistry, even though they offer the most straightforward entry to chiral α-aminocarbonyl compounds. Here, we demonstrate that the in situ activation of hydroxylamines with trichloroacetonitrile enables direct transfer of their amine moieties to the C-3 position of oxindoles with rigorous enantiocontrol under the catalysis of chiral 1,2,3-triazolium salts. This protocol provides a powerful and practical means of accessing optically active 3-aminooxindoles with a wide array of alkyl and aryl substituents on nitrogen. Other carbonyl compounds, such as β-ketoesters and α-cyanoesters, can also be directly transformed into the corresponding α-aminocarbonyls with high enantioselectivity.

Diastereoselective synthesis of chiral aminophenolate magnesium complexes and their application in the stereoselective polymerization of rac-lactide and rac-β-butyrolactone.

A series of magnesium silylamido complexes supported by chiral aminophenolate ligands were synthesized, and their catalytic behaviors toward the ring-opening polymerization of rac-lactide (LA) and rac-β-butyrolactone (BBL) were explored. The reactions of Mg[N(SiMe3)2]2 with chiral tridentate aminophenols [L = (S)-{[(1-R(4), 2-pyrrolidinyl)CH2N(R(3))-]}CH2-(6-R(1)-4-R(2))-C6H2O(-)] in a 1 : 1 molar ratio in toluene gave the chiral aminophenolate magnesium silylamido complexes L(1-10)MgN(SiMe3)2 as enantiopure compounds or as pairs of diastereomers. As determined by X-ray analysis, structures and of diastereomer mixtures adopt the configuration of SCSNRNRMg- and SCSNSNSMg-, respectively. It was found that most of these magnesium complexes served as highly active initiators for the ring-opening polymerization of rac-LA. Besides, the stereoselectivity of chiral magnesium complexes gradually changed from isoselective-biased (Pm = 0.67) to heteroselective-enriched (Pr = 0.81) via a stepwise adjustment of the substituents of the ligand framework. It was noted that, as the rare reported active magnesium initiator for the ROP of rac-BBL, complexes can efficiently initiate the ring-opening polymerization of rac-BBL in a controlled manner, giving moderate syndiotactic (Ps up to 0.73) PHBs.

Chiral amino-phosphine and amido-phosphine complexes of Ir and Mg. Catalytic applications in olefin hydroamination.

The reactions of rac- and (S,S)-trans-9,10-dihydro-9,10-ethanoanthracene-11,12-diamine (ANDEN) with PClPh2 in the presence of NEt3 yield the chiral amino-phosphine ligands rac-6 and (S,S)-6, respectively, on multi-gram scales. Both forms of 6 react quantitatively with MgPh2 to afford the C2-symmetric, N-bound Mg amidophosphine complexes rac-7 and (S,S)-7. The former crystallizes as a racemic conglomerate, which is a rare occurrence. Mixing (S,S)- or rac-6 with [IrCl(COE)2]2 leads in both cases to the homochiral dinuclear chloro-bridged P-ligated aminophosphine iridium complexes (S,S,S,S)-9 and rac-9 in excellent yields. X-ray quality single crystals only grow as the racemic compound (or 'true racemate') rac-9 thanks to its lowered solubility. In the coordinating solvent CH3CN, rac-9 transforms in high yield into mononuclear Ir-complex rac-10. The crystal structures of compounds rac-6, (S,S)-7, rac-9, and rac-10 reveal the ambidentate nature of the P-N function: amide-coordination in the Mg-complex (S,S)-7 and P-chelation of the softer Ir(i) centres in complexes rac-9 and rac-10. Furthermore, the crystal structures show flexible, symmetry lowering seven-membered P-chelate rings in the Ir complexes and a surprising amount of deformation within the ANDEN backbone. The simulation of this deformation by DFT and SCF calculations indicates low energy barriers. (S,S)-7 and (S,S,S,S)-9 catalyze the intra- and intermolecular hydroamination of alkenes, respectively: 5 mol% of (S,S)-7 affords 2-methyl-4,4'-diphenylcyclopentyl amine quantitatively (7% ee), and 2.5 mol% of (S,S,S,S)-9 in the presence of 5.0 mol% co-catalyst (LDA, PhLi, or MgPh2) gives exo-(2-arylamino)bornanes in up to 68% yield and up to 16% ee.

Magnesium plasmonics for UV applications and chiral sensing.

We demonstrate that chiral magnesium nanoparticles show remarkable plasmonic extinction- and chiroptical-effects in the ultraviolet region. The Mg nanohelices possess an enhanced local surface plasmon resonance (LSPR) sensitivity due to the strong dispersion of most substances in the UV region.