Isothiourea Catalyst Research Articles

Varying the Electronics on Isothiourea Catalysts: Basicity, Rate, and Selectivity

AbstractChiral and achiral substituted isothiourea catalysts were synthesized and employed in model silylation reactions to understand how changing the electronics on the catalyst core affected intermolecular interactions between catalyst intermediates and substrates, ultimately affecting selectivity and rate. Five different chiral catalysts were utilized in a silylation‐based kinetic resolution of 2‐(para substituted)phenylcyclohexanols and the rate of silylation of these same alcohols was investigated with three different achiral catalysts. Linear free energy relationships were examined, highlighting that rate and selectivity were highly dependent on the electronics of the catalyst and the substrate, and that both affected the intermolecular interactions that resulted.

Lewis Base-Catalyzed Dynamic Kinetic Asymmetric Transformation of Racemic Chlorosilanes en Route to Si-Stereogenic Silylethers.

Enantiopure Si-stereogenic organosilanes are highly valued in the fields of organic synthesis, development of advanced materials, and drug discovery. However, they are not naturally occurring, and their synthesis has been largely confined to resolution of racemic silanes or desymmetrization of symmetric silanes. In contrast, the dynamic kinetic asymmetric transformation (DYKAT) of racemic organosilanes offers a mechanistically distinct approach and would broaden the accessibility of Si-stereogenic silanes in an enantioconvergent manner. In this study, we report a Lewis base-catalyzed DYKAT of racemic chlorosilanes. The chiral isothiourea catalyst, (S)-benzotetramisole, facilitates silyletherification with phenols, yielding (R)-silylethers in good yields with high enantioselectivity (27 examples, up to 86% yield, up to 98:2 er). Kinetic analysis, control experiments, and DFT calculations suggest that a two-catalyst-bound pentacoordinate silicate is responsible for the Si-configurational epimerization of the ion-paired tetracoordinated silicon intermediates.

Stereoselective Synthesis of 3,4-Dihydrobenzofuro[3,2-b]pyridin-2(1H)-ones Enabled by Pd/Chiral Isothiourea Relay Catalysis

AbstractA highly enantioselective [4+2] cyclization of azadienes with ketene in situ generated from Pd-catalyzed carbonylation of benzyl bromides, is established through Pd/chiral isothiourea relay catalysis. The key in this transformation is the formation of a C1-ammonium enolate from the in situ generated ketene and a chiral isothiourea catalyst, which subsequently undergoes a formal [4+2] reaction, leading to 3,4-dihydrobenzofuro[3,2-b]pyridine derivatives in high yields and excellent levels of stereoselectivity.

Unveiling the impact of a CF2 motif in the isothiourea catalyst skeleton: Evaluating C(3)-F2-HBTM and its catalytic activity

Unveiling the impact of a CF2 motif in the isothiourea catalyst skeleton: Evaluating C(3)-F2-HBTM and its catalytic activity

Organocatalytic Enantio‐ and Diastereoselective Diels‐Alder Reaction between 2,4‐Dienals and α,β‐Unsaturated Esters

AbstractDespite advances in the development of catalytic asymmetric Diels‐Alder reactions, introducing α,β‐unsaturated esters as dienophiles remains challenging owing to their low reactivity. Herein, we report that a chiral aminocatalyst in conjunction with a chiral isothiourea catalyst or a Brønsted acid can promote enantio‐ and diastereoselective Diels‐Alder reactions between 2,4‐dienals and α,β‐unsaturated esters. This method proceeds through the formation of chiral trienamine intermediates for the exo‐cycloaddition with activated dienophiles, α,β‐unsaturated acylammonium species or protonated N‐heteroaryl‐substituted alkenes. Through synergistic activation of both dienes and ester dienophiles, densely functionalized cyclohexenes bearing multiple stereocenters can be afforded with up to >25:1 dr and >99% ee. Moreover, we show that variation of the configurations of the chiral catalysts and the olefin geometry allows the stereodivergent preparation of four stereoisomers of the enantiopure exo‐products.

Catalytic enantioselective Gilman-Speeter synthesis of β-lactams by nucleophilic activation of carboxylic acids

A novel Gilman-Speeter synthesis of β-lactams has been developed that was rendered highly enantioselective by virtue of isothiourea catalyst’s nucleophilicity. This process depends upon the HATU conversion of carboxylic acids to activated esters which then undergo attack by the homochiral catalyst and subsequently deprotonated by inorganic base to furnish catalytic quantities of the corresponding enolate ions that react with imines to provide the requisite β-lactams in good yields and enantiomeric excesses.

Scope, Limitations and Mechanistic Analysis of the HyperBTM‐Catalyzed Acylative Kinetic Resolution of Tertiary Heterocyclic Alcohols**

AbstractThe full scope and limitations of the catalytic acylative kinetic resolution of a range of tertiary heterocyclic alcohols (78 examples,sup to >200) is reported under operationally‐simple conditions, using low loadings of a commercially available Lewis basic isothiourea catalyst, HyperBTM (generally 1 mol %). The protocol is highly effective for the kinetic resolution of 3‐substituted 3‐hydroxyoxindole and α‐substituted α‐hydroxylactam derivatives bearing up to three potential recognition motifs at the stereogenic tertiary carbinol center. The full power of this methodology has been showcased through the synthesis of highly enantioenriched biologically‐active target compounds in both enantiomeric forms. To provide further insight into the reaction mechanism, a detailed kinetic analysis of this Lewis base‐catalyzed acylation of tertiary alcohols is reported using the variable time normalization analysis (VTNA) method.

Enantioselective α-Chlorination Reactions of in Situ Generated C1 Ammonium Enolates under Base-Free Conditions.

The asymmetric α-chlorination of activated aryl acetic acid esters can be carried out with high levels of enantioselectivities utilizing commercially available isothiourea catalysts under base-free conditions. The reaction, which proceeds via the in situ formation of chiral C1 ammonium enolates, is best carried out under cryogenic conditions combined with a direct trapping of the activated α-chlorinated ester derivative to prevent epimerization, thus allowing for enantioselectivities of up to e.r. 99:1.

2.2]Paracyclophane-Based Isothiourea-Catalyzed Highly Enantioselective α-Fluorination of Carboxylic Acids.

Planar chiral [2.2]paracyclophane-based isothiourea catalysts have been prepared over a few simple steps in high yields. In the presence of these catalysts, highly efficient catalytic enantioselective fluorination of carboxylic acids has been accomplished, providing a broad range of optically active α-fluoroesters in high yield and excellent enantioselectivity.

Mechanistic investigations of alcohol silylation with isothiourea catalysts.

The mechanism of the asymmetric silylation of alcohols with isothiourea catalysts was studied by employing reaction progress kinetic analysis. These reactions were developed by the Wiskur group, and use triphenyl silyl chloride and chiral isothiourea catalysts to silylate the alcohols. While the order of most reaction components was as expected (catalyst, amine base, alcohol), the silyl chloride was determined to be a higher order. This suggested a multistep mechanism between the catalyst and silyl chloride, with the second equivalent of silyl chloride assisting in the formation of the reactive intermediate leading to the rate-determining step. Through the addition of additives and investigating changes in the silyl chloride, an understanding of the catalyst equilibrium emerged for this reaction and provided pathways for further reaction development.

Catalytic enantioselective synthesis of 1,4-dihydropyridines via the addition of C(1)-ammonium enolates to pyridinium salts.

The regio- and stereoselective addition of C(1)-ammonium enolates – generated in situ from aryl esters and the isothiourea catalyst (R)-BTM – to pyridinium salts bearing an electron withdrawing substituent in the 3-position allows the synthesis of a range of enantioenriched 1,4-dihydropyridines. This represents the first organocatalytic approach to pyridine dearomatisation using pronucleophiles at the carboxylic acid oxidation level. Optimisation studies revealed a significant solvent dependency upon product enantioselectivity, with only toluene providing significant asymmetric induction. Using DABCO as a base also proved beneficial for product enantioselectivity, while investigations into the nature of the counterion showed that co-ordinating bromide or chloride substrates led to higher product er than the corresponding tetrafluoroborate or hexafluorophosphate. The scope and limitations of this process are developed, with enantioselective addition to 3-cyano- or 3-sulfonylpyridinium salts giving the corresponding 1,4-dihydropyridines (15 examples, up to 95 : 5 dr and 98 : 2 er).

Isothiourea-catalysed transfer hydrogenation of α,β-unsaturated para-nitrophenyl esters

A protocol for the isothiourea-catalysed transfer hydrogenation of α,β-unsaturated para -nitrophenyl esters using Hantzsch ester has been developed. Good to excellent yields are observed using α,β-unsaturated aryl esters bearing electron-withdrawing β-substituents. The aryl ester products can either be isolated directly in moderate to excellent yields (7 examples, 16–98%) or converted to the corresponding methyl esters (2 examples, 68–70% yield) or benzyl amides (2 examples, 44–88% yield) after in situ reaction of the hydrogenated ester with the appropriate nucleophile. Preliminary experiments showed that modest enantioinduction (76:24 er) is possible when a chiral isothiourea catalyst was used.

Isothiourea-Catalyzed Atroposelective N-Acylation of Sulfonamides.

We report herein an atroposelective N-acylation of sulfonamides using a commercially available isothiourea catalyst, (S)-HBTM, with a simple procedure. The N-sulfonyl anilide products can be obtained in good to high enantiopurity, which represents a new axially chiral scaffold. The application of the product as a chiral iodine catalyst is also demonstrated for the asymmetric α-oxytosylation of propiophenone.

Asymmetric Synthesis of Spirooxindole β‐lactams via Isothiourea‐catalyzed Mannich/lactamization Reaction of Aryl Acetic Acids with Isatin‐derived Ketimines

AbstractAn efficient [2+2] annulation strategy that isothiourea catalyst homobenzotetramisole (HBTM)‐catalyzed Mannich/lactamization cascade reaction of carboxylic acids with isatin‐derived ketimines has been disclosed. This protocol affords a range of structurally diverse spirooxindole β‐lactams bearing two vicinal stereogenic centers in good to high yields with good to excellent stereoselectivities.magnified image

Site-Selective Acylation of Carbohydrates Directed by Recyclable Polymer-Supported Isothiourea Catalysts

Site-Selective Acylation of Carbohydrates Directed by Recyclable Polymer-Supported Isothiourea Catalysts

Evaluating aryl esters as bench-stable C(1)-ammonium enolate precursors in catalytic, enantioselective Michael addition-lactonisations.

An evaluation of a range of aryl, alkyl and vinyl esters as prospective C(1)-ammonium enolate precursors in enantioselective Michael addition-lactonisation processes with (E)-trifluoromethylenones using isothiourea catalysis is reported. Electron deficient aryl esters are required for reactivity, with 2,4,6-trichlorophenyl esters providing optimal product yields. Catalyst screening showed that tetramisole was the most effective isothiourea catalyst, giving the desired dihydropyranone product in excellent yield and stereoselectivity (up to 90 : 10 dr and 98 : 2 er). The scope and limitations of this process have been evaluated, with a range of diester products being generated after ring-opening with MeOH to give stereodefined dihydropyranones with excellent stereocontrol (10 examples, typically ∼90 : 10 dr and >95 : 5 er).

Organocatalytic Enantioselective Synthesis of α-Fluoro-β-amino Acid Derivatives.

Asymmetric cyclocondensation of N-sulfonylimines with fluoroacetic acid promoted by isothiourea catalyst HBTM-2 generates 3-fluoro-β-lactams with high enantio- and diastereoselectivity. These reactive compounds are opened with alcohols or amines to produce the corresponding α-fluoro-β-amino acid derivatives in moderate yields.

Asymmetric Synthesis of Allenyl α-Amino Amides by an Isothiourea Catalyzed Enantioselective [2,3]-Sigmatropic Rearrangement.

Highly efficient catalytic asymmetric [2,3]-sigmatropic rearrangements of propargyl ammonium salts have been accomplished under mild reaction conditions. In the presence of the chiral isothiourea catalyst, a wide range of allenyl α-amino amide derivatives were obtained in generally good yields (up to 99%) with excellent enantioselectivities (up to 96% ee).

Direct Organocatalytic Enantioselective Functionalization of SiOx Surfaces

AbstractTraditional methods to prepare chiral surfaces involve either the adsorption of a chiral molecule onto an achiral surface, or adsorption of a species that forms a chiral template creating lattices with long range order. To date only limited alternative strategies to prepare chiral surfaces have been studied. In this manuscript a “bottom‐up” approach is developed that allows the preparation of chiral surfaces by direct enantioselective organocatalytic reactions on a functionalized silicon oxide supported self‐assembled monolayer (SAM). The efficient catalytic generation of enantiomerically enriched organic surfaces is achieved using a commercially available homogeneous isothiourea catalyst that promotes an enantioselective Michael‐lactonization process upon a silicon‐oxide supported SAM functionalized with a reactive trifluoroenone group. Chiral atomic force microscopy (χ‐AFM) is used to probe the enantiomeric enrichment of the organic films by measurement of the force distributions arising from interaction of d‐ or l‐cysteine‐modified AFM tips and the organic films.

Direct Organocatalytic Enantioselective Functionalization of SiOx Surfaces.

Traditional methods to prepare chiral surfaces involve either the adsorption of a chiral molecule onto an achiral surface, or adsorption of a species that forms a chiral template creating lattices with long range order. To date only limited alternative strategies to prepare chiral surfaces have been studied. In this manuscript a "bottom-up" approach is developed that allows the preparation of chiral surfaces by direct enantioselective organocatalytic reactions on a functionalized silicon oxide supported self-assembled monolayer (SAM). The efficient catalytic generation of enantiomerically enriched organic surfaces is achieved using a commercially available homogeneous isothiourea catalyst that promotes an enantioselective Michael-lactonization process upon a silicon-oxide supported SAM functionalized with a reactive trifluoroenone group. Chiral atomic force microscopy (χ-AFM) is used to probe the enantiomeric enrichment of the organic films by measurement of the force distributions arising from interaction of d- or l-cysteine-modified AFM tips and the organic films.