Linear Free-Energy Relationship and Rate Study on a Silylation-Based Kinetic Resolution: Mechanistic Insights

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.

Investigating Substituent Interactions with Cationic Catalysts

Rates of isothiourea catalyzed silylation and acylation reactions were measured for substrates with various electronic substituents at the aryl group. Through these measurements, the intermolecular interactions between cationic catalyst intermediates and different aryl groups were explored. These studies were performed to understand how changes in the catalyst structure affected electrostatic intermolecular interactions. Three different catalysts (N-methylimidazole and two isothioureas) were employed that varied in their ability to delocalize their cationic nature. The results show that more delocalization on the catalyst reduces the sensitivity to the electronics on the aryl group. Surprisingly, the isothiourea with a fused benzene ring provided additional points of interaction with groups that contained lone-pairs, significantly affecting the overall rate. This work helps explore the interactions that dominate in these types of catalytic systems, to aid in future organocatalysis development. Density functional theory (DFT) studies further confirmed isothiourea/aryl ring interaction with the alcohol substrate in the acylation process, which confirmed these hypotheses. Electron rich or lone-pair bearing functional groups stabilize the cationic catalyst core, thereby stabilizing the transition states and accelerating the reaction. It was also discovered that in one case, the formation of a stable substrate dimer was responsible for its lower reactivity.

Quantitative analysis of steric effects on the regioselectivity of the Larock heteroannulation reaction

Alkylphenylacetylene derivatives were synthesized and used as reactants in the Larock heteroannulation reaction to investigate the steric influence on regioselectivity. Large alkyl groups preferentially yielded 2-alkyl-3-phenylindole products, while smaller alkyl groups provided 3-alkyl-2-phenylindole as major products. The logarithm of regioisomeric product ratios exhibited good correlations with various steric parameters. Notably, the Charton values provided the best correlation when excluding the cyclopropyl group. In addition, the Boltzmann-weighted Sterimol parameter (wSterimol) was utilized to generate a good predictive model, indicating the B₁ wSterimol as the significant regiochemical determining parameter with no obvious deviation for the cyclopropyl group. Relative atomic distances within the DFT-optimized transition state structures revealed good correlations with the logarithm of regioisomeric ratios. Furthermore, the cyclopropyl adsorption complex indicated electronic contribution, explaining the peculiar behavior of this substituent in the experimental observation.

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.

Quantitative Structure-Selectivity Relationships in Enantioselective Catalysis: Past, Present, and Future

The dawn of the 21st century has brought with it a surge of research related to computer-guided approaches to catalyst design. In the past two decades, chemoinformatics, the application of informatics to solve problems in chemistry, has increasingly influenced prediction of activity and mechanistic investigations of organic reactions. The advent of advanced statistical and machine learning methods, as well as dramatic increases in computational speed and memory, has contributed to this emerging field of study. This review summarizes strategies to employ quantitative structure-selectivity relationships (QSSR) in asymmetric catalytic reactions. The coverage is structured by initially introducing the basic features of these methods. Subsequent topics are discussed according to increasing complexity of molecular representations. As the most applied subfield of QSSR in enantioselective catalysis, the application of local parametrization approaches and linear free energy relationships (LFERs) along with multivariate modeling techniques is described first. This section is followed by a description of global parametrization methods, the first of which is continuous chirality measures (CCM) because it is a single parameter derived from the global structure of a molecule. Chirality codes, global, multivariate descriptors, are then introduced followed by molecular interaction fields (MIFs), a global descriptor class that typically has the highest dimensionality. To highlight the current reach of QSSR in enantioselective transformations, a comprehensive collection of examples is presented. When combined with traditional experimental approaches, chemoinformatics holds great promise to predict new catalyst structures, rationalize mechanistic behavior, and profoundly change the way chemists discover and optimize reactions.

Origin of stereoselectivity in an isothiourea catalyzed Michael addition reaction of aryl ester with vinyl disulfone

The origin of stereoselectivity in an isothiourea-catalyzed addition reaction of aryl ester with vinyl disulfone was explored for the first time.

Polymer compositions on kinetic resolution of secondary alcohols using polymer-supported silyl chlorides

Modulating the polarity of the polymer backbone affects the selectivity of asymmetric silylation.

Investigation of Transfer Group, Tether Proximity, and Alkene Substitution for Intramolecular Silyloxypyrone-Based [5 + 2] Cycloadditions

Systematic investigation of intramolecular silyloxypyrone-based [5 + 2] cycloadditions revealed three significant factors impacting conversion to cycloadduct: (1) the silyl transfer group has a substantial influence on the rate of reaction, and the robust t-butyldiphenylsilyl group was found to be more effective overall than the conventional t-butyldimethylsilyl group; (2) α,β-unsaturated esters were generally more reactive than terminal olefins and afforded appreciable quantity of cycloadduct even at room temperature; and (3) the proximity of the tether to the silyl transfer group revealed a critical alignment trend between the pyrone and the alkene. Taken together, these investigations provided insight regarding the steric and electronic parameters that impact the scope and limitation of these reactions.

Making the Silylation of Alcohols Chiral: Asymmetric Protection of Hydroxy Groups

The non-enzymatic kinetic resolution of racemic mixtures of alcohols by silylation had been unknown before the turn of the century. This stands in stark contrast to established acylation techniques. The same applies to the related desymmetrization of diols. This might come as a surprise, given the significance of silyl ethers as protecting groups in multistep synthesis of complex molecules. The situation changed after a seminal report by Ishikawa nearly twenty years ago. Since then, enantioselective silylation of alcohols has matured and grown into an independent research field with organocatalytic and transition-metal-catalyzed approaches providing powerful solutions. This Minireview summarizes these recent advances with particular emphasis on the stereoselective dehydrogenative coupling of alcohols and hydrosilanes.

Size-dependent rate acceleration in the silylation of secondary alcohols: the bigger the faster

Relative rates for the reaction of secondary alcohols carrying large aromatic moieties with silyl chlorides carrying equally large substituents have been determined in organic solvents. Introducing thoroughly matching pairs of big dispersion energy donor (DED) groups enhanced rate constants up to four times, notably depending on the hydrogen bond donor ability of the solvent. A linear correlation between computed dispersion energy contributions to the stability of the silyl ether products and experimental relative rate constants was found. These results indicate a cooperation between solvophobic effects and DED-groups in the kinetic control of silylation reactions.

N-Heterocyclic carbene promoted enantioselective desymmetrization reaction of diarylalkane-bisphenols

An enantioselective NHC-catalyzed desymmetrization reaction of diarylalkane-bisphenols with aldehydes was reported under the guidance of linear free energy relationships (LFERs).

Broad-spectrum kinetic resolution of alcohols enabled by Cu-H-catalysed dehydrogenative coupling with hydrosilanes

The enantioselective silylation of racemic alcohols, where one enantiomer reacts faster than the other, is an alternative approach to established enzymatic and non-enzymatic acylation techniques. The existing art is either limited to structurally biased alcohols or requires elaborate catalysts. Simple substrates, such as benzylic and allylic alcohols, with no coordinating functionality in the proximity of the hydroxy group have been challenging in these kinetic resolutions. We report here the identification of a broadly applicable chiral catalyst for the enantioselective dehydrogenative coupling of alcohols and hydrosilanes with both the chiral ligand and the hydrosilane being commercially available. The efficiency of kinetic resolutions is characterized by the selectivity factor, that is, the ratio of the reaction rates of the fast-reacting over the slow-reacting enantiomer. The selectivity factors achieved with the new method are good for acyclic benzylic alcohols (≤170) and high for synthetically usefully cyclic benzylic (≤40.1) and allylic alcohols (≤159).

Kinetic resolution of alcohols by silylation is an attractive method to produce enantiopure compounds, but known systems often display limited substrate scope. Here the authors report a copper catalysed enantioselective dehydrogenative silylation of alcohols that have high selectivity across a broad range of substrates.

Model Building Using Linear Free Energy Relationship Parameters-Eliminating Calibration Curves for Optical Analysis of Enantiomeric Excess

Linear free energy relationship (LFER) parameters are routinely used to parametrize physicochemical effects while investigating reaction mechanisms. In this Communication, we describe an alternate application for LFERs: training sets for model building in an analytical application. In this study, the sterics, quantified by Charton parameters (Δv), of nine secondary chiral alcohol analytes were correlated to the circular dichroism output from a chiral alcohol optical sensor. To test the validity of the model, the correlative linear model was applied to determine the enantiomeric excess of samples of two alcohols without a priori knowledge of a calibration curve. The error in this method was comparable to those of previous experimental methods (<5%).

Mechanistic Studies Lead to Dramatically Improved Reaction Conditions for the Cu-Catalyzed Asymmetric Hydroamination of Olefins

Enantioselective copper(I) hydride (CuH)-catalyzed hydroamination has undergone significant development over the past several years. To gain a general understanding of the factors governing these reactions, kinetic and spectroscopic studies were performed on the CuH-catalyzed hydroamination of styrene. Reaction profile analysis, rate order assessment, and Hammett studies indicate that the turnover-limiting step is regeneration of the CuH catalyst by reaction with a silane, with a phosphine-ligated copper(I) benzoate as the catalyst resting state. Spectroscopic, electrospray ionization mass spectrometry, and nonlinear effect studies are consistent with a monomeric active catalyst. With this insight, targeted reagent optimization led to the development of an optimized protocol with an operationally simple setup (ligated copper(II) precatalyst, open to air) and short reaction times (<30 min). This improved protocol is amenable to a diverse range of alkene and alkyne substrate classes.