Organocatalytic Asymmetric Reduction of Fluorinated Alkynyl Ketimines

Predicting the stereoselectivity of chemical reactions by composite machine learning method

Stereoselective reactions have played a vital role in the emergence of life, evolution, human biology, and medicine. However, for a long time, most industrial and academic efforts followed a trial-and-error approach for asymmetric synthesis in stereoselective reactions. In addition, most previous studies have been qualitatively focused on the influence of steric and electronic effects on stereoselective reactions. Therefore, quantitatively understanding the stereoselectivity of a given chemical reaction is extremely difficult. As proof of principle, this paper develops a novel composite machine learning method for quantitatively predicting the enantioselectivity representing the degree to which one enantiomer is preferentially produced from the reactions. Specifically, machine learning methods that are widely used in data analytics, including Random Forest, Support Vector Regression, and LASSO, are utilized. In addition, the Bayesian optimization and permutation importance tests are provided for an in-depth understanding of reactions and accurate prediction. Finally, the proposed composite method approximates the key features of the available reactions by using Gaussian mixture models, which provide suitable machine learning methods for new reactions. The case studies using the real stereoselective reactions show that the proposed method is effective and provides a solid foundation for further application to other chemical reactions.

Recent Advancement in H8-BINOL Catalyzed Asymmetric Methodologies

H₈-BINOL, a partially reduced form of BINOL, is widely employed in a broad array of organocatalyzed asymmetric methodologies. Over the last 25 years, asymmetric organocatalysis has witnessed an incredible improvement, and an advancement still continues to get a single enantio-enriched product. The broad-spectrum applications of H₈-BINOL organocatalyst in C-C bond formation, C-heteroatom bond construction, name reactions, pericyclic reactions, and one pot and multicomponent reaction are attracting the attention of researchers. A diversified unique H₈-BINOL-based catalyst has been synthesized and screened for catalytic activity. In this Review we frame out the H₈-BINOL catalyzed novel discoveries from the last two decades.

Thermodynamics regulated organic hydride/acid pairs as novel organic hydrogen reductants

Organic hydride/acid pairs could realize transformation of N-substituted organic hydrides from hydride reductants to thermodynamics regulated hydrogen reductants on conveniently choosing suitable organic hydrides and acids with various acidities.

Enantioselective Peroxidation of C-Alkynyl Imines Enabled by Chiral BINOL Calcium Phosphate

Herein, we reported a catalytic enantioselective addition of C-alkynyl imines with hydroperoxides catalyzd by chiral BINOL calcium phosphate, affording a broad range of enantioenriched α-peroxy propargylamines in good yields (80-99%)...

Chiral Phosphoric Acids as Versatile Tools for Organocatalytic Asymmetric Transfer Hydrogenations

Herein, recent developments in the field of organocatalytic asymmetric transfer hydrogenation (ATH) of C=N, C=O and C=C double bonds using chiral phosphoric acid catalysis are reviewed. This still rapidly growing area of asymmetric catalysis relies on metal-free catalysts in combination with biomimetic hydrogen sources. Chiral phosphoric acids have proven to be extremely versatile tools in this area, providing highly active and enantioselective alternatives for the asymmetric reduction of α,β-unsaturated carbonyl compounds, imines and various heterocycles. Eventually, such transformations are more and more often used in multicomponent/cascade reactions, which undoubtedly shows their great synthetic potential and the bright future of organocatalytic asymmetric transfer hydrogenations.

Enantioselective decarboxylative Mannich reaction of β-keto acids with C-alkynyl N-Boc N,O-acetals: access to chiral β-keto propargylamines

The chiral keto-substituted propargylamines are an essential class of multifunctional compounds in the field of organic and pharmaceutical synthesis and have attracted considerable attention, but the related synthetic approaches remain limited. Therefore, a concise and efficient method for the enantioselective synthesis of β-keto propargylamines via chiral phosphoric acid-catalyzed asymmetric Mannich reaction between β-keto acids and C-alkynyl N-Boc N,O-acetals as easily available C-alkynyl imine precursors has been demonstrated here, affording a broad scope of β-keto N-Boc-propargylamines in high yields (up to 97%) with generally high enantioselectivities (up to 97 : 3 er).

Cu(I)-Catalyzed gem-Aminoalkynylation of Diazo Compounds: Synthesis of Fluorinated Propargylic Amines

The gem-aminoalkynylation of fluorinated diazo compounds catalyzed by a simple Cu(I) salt is described. This three-component reaction allows the synthesis of propargylic amines with broad functional group tolerance. Both electron-rich and electron-poor anilines can be used as nucleophiles and alkyl-, aryl-, and silyl-substituted EthynylBenziodoXoles (EBX) as electrophiles.

Catalytic Enantioselective Transfer Hydrogenation-Carboxylative Cyclization to 4-Fluoroalkyl 2-Oxazolidinone with CO2 as the C1 Synthon

We report a sequential catalytic asymmetric transfer hydrogenation-carboxylative cyclization for the facile construction of chiral 4-fluoroalkyl 2-oxazolidinones with high enantioselectivity. The resulting 2-oxazolidinones can be easily elaborated to synthetic useful chiral β-fluoroalkyl β-amino alcohols. This research also represents a rare example of catalytic asymmetric sequential reactions using CO₂ as a C1 synthon as well as carboxylative cyclization of α-fluoroalkyl propargylamines.

Asymmetric Mannich reactions of (S)-N-tert-butylsulfinyl-3,3,3-trifluoroacetaldimines with yne nucleophiles

In the present work, arylethynes were studied as new C-nucleophiles in the asymmetric Mannich addition reactions with (S)-N-tert-butylsulfinyl-3,3,3-trifluoroacetaldimine. The reactions were conducted under operationally convenient conditions affording the corresponding Mannich adducts with up to 87% yield and 70:30 diastereoselectivity. The isomeric products can be separated using regular column chromatography to afford diastereomerically pure compounds. The purified Mannich addition products were deprotected to give the target enantiomerically pure trifluoromethylpropargylamines. A mechanistic rationale for the observed stereochemical outcome is discussed.

Strategies for the Catalytic Enantioselective Synthesis of α-Trifluoromethyl Amines

The exploitation of the α-trifluoromethylamino group as an amide surrogate in peptidomimetics and drug candidates has been on the rise. In a large number of these cases, this moiety bears stereochemistry with the stereochemical identity having important consequences on numerous molecular properties, such as the potency of the compound. Yet, the majority of stereoselective syntheses of α-CF₃ amines rely on diastereoselective couplings with chiral reagents. Concurrent with the rapid expansion of fluorine into pharmaceuticals has been the development of catalytic enantioselective means of preparing α-trifluoromethyl amines. In this work, we outline the strategies that have been employed for accessing these enantioenriched amines, including normal polarity approaches and several recent developments in imine umpolung transformations.

Thermodynamic Network Cards of Hantzsch Ester, Benzothiazoline, and Dihydrophenanthridine Releasing Two Hydrogen Atoms or Ions on 20 Elementary Steps

In this work, thermodynamic driving forces on 20 possible elementary steps of Hantzsch ester (HEH₂), benzothiazoline (BTH₂), and dihydrophenanthridine (PDH₂) releasing two hydrogen atoms or ions were measured or derived from the related thermodynamic data using Hess' law in acetonitrile. Furthermore, thermodynamic network cards of HEH₂, BTH₂, and PDH₂ releasing two hydrogen atoms or ions on 20 elementary steps were first established. Based on the thermodynamic network cards, hydride-donating, hydrogen-atom-donating, and electron-donating abilities of XH₂ and XH^-, and two hydrogen-atom(ion)-donating abilities of XH₂ are discussed in detail. Obviously, the thermodynamic network cards of HEH₂, BTH₂, and PDH₂ not only offer rational data guidance for organic synthetic chemists to properly choose an appropriate reducer among the three reducing agents to hydrogenate various unsaturated compounds but also strongly promote elucidatation of the detailed hydrogenation mechanisms.

Enantioselective synthesis of chiral α-alkynylated thiazolidones by tandem S-addition/acetalization of alkynyl imines

A SPINOL-derived chiral phosphoric acid catalyzed asymmetric formal [2 + 3]-annulation of in situ generated alkynyl imines and 1,4-dithiane-2,5-diol has been developed to afford enantiopure α-alkynylated thiazolidones with up to 72% yield and 98.5 : 1.5 er. This tandem annulation involved a tandem S-addition of alkynyl imines/intramolecular acetalization, followed by PDC-mediated oxidation. The α-alkynylated thiazolidones could facilely afford the corresponding chiral α-alkynylated or α-alkenylated cyclic sulfoxides via further elaboration.

Synthesis of Chiral β-Fluoroalkyl β-Amino Acid Derivatives via Palladium-Catalyzed Hydrogenation

An enantioselective palladium-catalyzed hydrogenation of β-fluoroalkyl β-amino acrylic acid derivatives has been successfully developed, providing the corresponding chiral β-fluoroalkyl β-amino acid derivatives in good yields with excellent enantioselectivities. In addition, chiral γ-fluoroalkyl γ-amino alcohol could be synthesized by a simple reduction of the corresponding hydrogenated product. The mechanism of the reaction was explored by deuterium-labeling experiments.

Holistic prediction of enantioselectivity in asymmetric catalysis

When faced with unfamiliar reaction space, synthetic chemists typically apply reported conditions (reagents, catalyst, solvent, additives) from closely-related reactions to new substrate types. Unfortunately, this approach often fails due to subtle, albeit important, differences in reaction requirements. Consequently, a significant goal in synthetic chemistry is the ability to transfer chemical observations from one reaction to another, quantitatively. Here, we present such a platform by developing a holistic, data-driven workflow for deriving statistical models for one set of reactions that can be applied to predict out-of-sample examples. As a validating case study, published enantioselectivity data sets that employ BINOL-derived chiral phosphoric acids for a range of nucleophilic addition reactions to imines were combined and statistical models developed. These models reveal the general interactions imparting asymmetric induction and allow the quantitative transfer of this information to new reaction components. The disclosed techniques create opportunities for translating comprehensive reaction analysis to diverse chemical space, streamlining both catalyst and reaction development.

After the database of the reactions is constructed, the experimental output, enantiomeric ratios, were mathematically modelled through linear regression techniques to reveal which of the proposed parameters allow for the prediction of new outcomes. The detailed acquisition of parameters can be found in the Supplementary Information and the descriptor tables attached as an accompanying spreadsheet. The models produced were evaluated for their goodness of fit, R², and their robustness is demonstrated by external validations’ goodness of fit, _predR². The nearer the R² and slope values are to one (indicating a tight, one-to-one correlation between predicted and measured outcomes) and the nearer the intercept is to zero (indicating minimal systematic error), the more robust the model. Potential models were refined through number of parameters, because this allows for a mechanistically informative interrogation and cross-validation scores. Leave one reaction out (LORO) analysis was performed to probe general mechanistic principles, which provides the basis for mechanistic transfer of experimental observations and tested further by predicting out-of-sample.

BINOPtimal: a web tool for optimal chiral phosphoric acid catalyst selection

A catalyst selection program, BINOPtimal, has been developed.

Enantioselective synthesis of trifluoromethylated dihydroquinoxalinones via palladium-catalyzed hydrogenation

A highly enantioselective palladium-catalyzed asymmetric hydrogenation of 3-(trifluoromethyl)quinoxalinones has been successfully developed, providing a general and facile access to chiral 3-(trifluoromethyl)-3,4-dihydroquinoxalinones with up to 99% ee.