Homogeneous pd-catalyzed enantioselective decarboxylative protonation

Organocatalytic enantioselective decarboxylative protonation of α-alkyl-α-aryl malonate monoesters

In contrast to the well-established enzymatic enantioselective decarboxylative protonation (EDP), the corresponding chemocatalytic reactions of acyclic malonic acid derivatives remain challenging. Herein, we developed a biomimetic EDP of α-alkyl-α-aryl malonate monoesters using a chiral 1,2-trans-diaminocyclohexane-based N-sulfonamide as an organocatalyst. The method demonstrates excellent chemical yields, good enantioselectivity, mild reaction conditions, and the generation of only CO2 as waste.

Asymmetric α-benzylation of cyclic ketones enabled by concurrent chemical aldol condensation and biocatalytic reduction

Chemoenzymatic cascade catalysis has emerged as a revolutionary tool for streamlining traditional retrosynthetic disconnections, creating new possibilities for the asymmetric synthesis of valuable chiral compounds. Here we construct a one-pot concurrent chemoenzymatic cascade by integrating organobismuth-catalyzed aldol condensation with ene-reductase (ER)-catalyzed enantioselective reduction, enabling the formal asymmetric α-benzylation of cyclic ketones. To achieve this, we develop a pair of enantiocomplementary ERs capable of reducing α-arylidene cyclic ketones, lactams, and lactones. Our engineered mutants exhibit significantly higher activity, up to 37-fold, and broader substrate specificity compared to the parent enzyme. The key to success is due to the well-tuned hydride attack distance/angle and, more importantly, to the synergistic proton-delivery triade of Tyr28-Tyr69-Tyr169. Molecular docking and density functional theory (DFT) studies provide important insights into the bioreduction mechanisms. Furthermore, we demonstrate the synthetic utility of the best mutants in the asymmetric synthesis of several key chiral synthons.

Modular and diverse synthesis of amino acids via asymmetric decarboxylative protonation of aminomalonic acids

Stereoselective protonation is a challenge in asymmetric catalysis. The small size and high rate of transfer of protons mean that face-selective delivery to planar intermediates is hard to control, but it can unlock previously obscure asymmetric transformations. Particularly, when coupled with a preceding decarboxylation, enantioselective protonation can convert the abundant acid feedstocks into structurally diverse chiral molecules. Here an anchoring group strategy is demonstrated as a potential alternative and supplement to the conventional structural modification of catalysts by creating additional catalyst-substrate interactions. We show that a tailored benzamide group in aminomalonic acids can help build a coordinated network of non-covalent interactions, including hydrogen bonds, π-π interactions and dispersion forces, with a chiral acid catalyst. This allows enantioselective decarboxylative protonation to give α-amino acids. The malonate-based synthesis introduces side chains via a facile substitution of aminomalonic esters and thus can access structurally and functionally diverse amino acids.

Divergent Catalysis: Catalytic Asymmetric [4+2] Cycloaddition of Palladium Enolates

An asymmetric decarboxylative [4+2] cycloaddition from a catalytically generated chiral Pd enolate was developed, forging four contiguous stereocenters in a single transformation. This was achieved through a strategy termed divergent catalysis, wherein departure from a known catalytic cycle enables novel reactivity of a targeted intermediate prior to re-entry into the original cycle. Mechanistic studies including quantum mechanics calculations, Eyring analysis, and KIE studies offer insight into the reaction mechanism.

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.

Hydrodealkenylative C(sp3)-C(sp2) bond fragmentation

Chemical synthesis typically relies on reactions that generate complexity through elaboration of simple starting materials. Less common are deconstructive strategies toward complexity-particularly those involving carbon-carbon bond scission. Here, we introduce one such transformation: the hydrodealkenylative cleavage of C(sp³)-C(sp²) bonds, conducted below room temperature, using ozone, an iron salt, and a hydrogen atom donor. These reactions are performed in nonanhydrous solvents and open to the air; reach completion within 30 minutes; and deliver their products in high yields, even on decagram scales. We have used this broadly functionality tolerant transformation to produce desirable synthetic intermediates, many of which are optically active, from abundantly available terpenes and terpenoid-derived precursors. We have also applied it in the formal total syntheses of complex molecules.

Protodepalladation as a Strategic Elementary Step in Catalysis

Protodepalladation is the redox-neutral conversion of a C-Pd(II) bond to a C-H bond promoted by a Brønsted acid. It can be viewed as the microscopic reserves of Pd(II)-mediated C-H cleavage. In the context of catalytic reaction development, protodepalladation offers a means of converting organopalladium(II) intermediates to organic products without a change in oxidation state at the metal center. Hence, when integrated into catalytic cycles, it can be a uniquely enabling elementary step. The goal of this Review is to provide an overview of protodepalladation, including exploration of different reactions types, discussion of literature examples, and analysis of mechanistic features. Our hope is that this review will stimulate other researchers in the field to pursue new applications of this underexploited step in catalysis.

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

Enantioselective Construction of Acyclic Quaternary Carbon Stereocenters: Palladium-Catalyzed Decarboxylative Allylic Alkylation of Fully Substituted Amide Enolates

We report a divergent and modular protocol for the preparation of acyclic molecular frameworks containing newly created quaternary carbon stereocenters. Central to this approach is a sequence composed of a (1) regioselective and -retentive preparation of allyloxycarbonyl-trapped fully substituted stereodefined amide enolates and of a (2) enantioselective palladium-catalyzed decarboxylative allylic alkylation reaction using a novel bisphosphine ligand.

Advances in Stereoconvergent Catalysis from 2005 to 2015: Transition-Metal-Mediated Stereoablative Reactions, Dynamic Kinetic Resolutions, and Dynamic Kinetic Asymmetric Transformations

Stereoconvergent catalysis is an important subset of asymmetric synthesis that encompasses stereoablative transformations, dynamic kinetic resolutions, and dynamic kinetic asymmetric transformations. Initially, only enzymes were known to catalyze dynamic kinetic processes, but recently various synthetic catalysts have been developed. This Review summarizes major advances in nonenzymatic, transition-metal-promoted dynamic asymmetric transformations reported between 2005 and 2015.

NeoPHOX - a structurally tunable ligand system for asymmetric catalysis

A synthesis of new NeoPHOX ligands derived from serine or threonine has been developed. The central intermediate is a NeoPHOX derivative bearing a methoxycarbonyl group at the stereogenic center next to the oxazoline N atom. The addition of methylmagnesium chloride leads to a tertiary alcohol, which can be acylated or silylated to produce NeoPHOX ligands with different sterical demand. The new NeoPHOX ligands were tested in the iridium-catalyzed asymmetric hydrogenation and palladium-catalyzed allylic substitution. In both reactions high enantioselectivities were achieved, that were comparable to the enantioselectivities obtained with the up to now best NeoPHOX ligand derived from expensive tert-leucine.

Synthesis and Exploration of Electronically Modified (R)-5,5-Dimethyl-(p-CF3)3-i-PrPHOX in Palladium-Catalyzed Enantio- and Diastereoselective Allylic Alkylation: A Practical Alternative to (R)-(p-CF3)3-t-BuPHOX

The synthesis of the novel electronically modified phosphinooxazoline (PHOX) ligand, (R)-5,5-dimethyl-(p-CF₃)₃-i-PrPHOX, is described. The utility of this PHOX ligand is explored in both enantio- and diastereoselective palladium-catalyzed allylic alkylations. These investigations prove (R)-5,5-dimethyl-(p-CF₃)₃-i-PrPHOX to be an effective and cost-efficient alternative to electronically modified PHOX ligands derived from the prohibitively expensive (R)-t-leucine.

Sterically Demanding Unsymmetrical Diaryl-λ(3)-iodanes for Electrophilic Pentafluorophenylation and an Approach to α-Pentafluorophenyl Carbonyl Compounds with an All-Carbon Stereocenter

A sterically demanding unsymmetrical pentafluorophenyl-triisopropylphenyl-λ(3)-iodane was developed as an effective reagent for the electrophilic pentafluorophenylation of various β-keto esters and a β-keto amide. 17 examples of α-pentafluorophenylated 1,3-dicarbonyl compounds 3 having a quaternary carbon center are provided. The resulting compounds were nicely transformed into chiral α-pentafluorophenyl ketones with an all-carbon stereogenic center in high yields and high enantioselectivities using asymmetric organocatalysis (up to 98 % ee) or asymmetric metal catalysis (up to 82 % ee).

Palladium and organocatalysis: an excellent recipe for asymmetric synthesis

The dual activation of simple substrates by the combination of organocatalysis and palladium catalysis has been successfully applied in a variety of different asymmetric transformations. Thus, the asymmetric α-allylation of carbonyl compounds, α-fluorination of acyl derivatives, decarboxylative protonation of β-dicarbonyl compounds, cyclization reactions of alkynyl carbonyl compounds and β-functionalization of aldehydes have been efficiently achieved employing this double-catalytic methodology.

The reaction mechanism of the enantioselective Tsuji allylation: inner-sphere and outer-sphere pathways, internal rearrangements, and asymmetric C-C bond formation

We use first principles quantum mechanics (density functional theory) to report a detailed reaction mechanism of the asymmetric Tsuji allylation involving prochiral nucleophiles and nonprochiral allyl fragments, which is consistent with experimental findings. The observed enantioselectivity is best explained with an inner-sphere mechanism involving the formation of a 5-coordinate Pd species that undergoes a ligand rearrangement, which is selective with regard to the prochiral faces of the intermediate enolate. Subsequent reductive elimination generates the product and a Pd(0) complex. The reductive elimination occurs via an unconventional seven-centered transition state that contrasts dramatically with the standard three-centered C-C reductive elimination mechanism. Although limitations in the present theory prevent the conclusive identification of the enantioselective step, we note that three different computational schemes using different levels of theory all find that inner-sphere pathways are lower in energy than outer-sphere pathways. This result qualitatively contrasts with established allylation reaction mechanisms involving prochiral nucleophiles and prochiral allyl fragments. Energetic profiles of all reaction pathways are presented in detail.

Intercepted Decarboxylative Allylations of Nitroalkanoates

Using palladium-catalyzed decarboxylation, several cascade reactions of allyl and prenyl nitroalkanoates that lead to nitro-containing chemical building blocks are described. A nitronate Michael addition/Tsuji-Trost allylation cascade was developed, leading to functionally dense chemical building blocks. Likewise, a Tsuji-Trost/decarboxylative protonation sequence was developed for the synthesis of orthogonally functionalized 2° nitroalkanes. The latter method provides rapid access to the indolizidine core.

Palladium-Catalyzed Asymmetric Alkylation in the Synthesis of Cyclopentanoid and Cycloheptanoid Core Structures Bearing All-Carbon Quaternary Stereocenters

General catalytic asymmetric routes toward cyclopentanoid and cycloheptanoid core structures embedded in numerous natural products have been developed. The central stereoselective transformation in our divergent strategies is the enantioselective decarboxylative alkylation of seven-membered β-ketoesters to form α-quaternary vinylogous esters. Recognition of the unusual reactivity of β-hydroxyketones resulting from the addition of hydride or organometallic reagents enabled divergent access to γ-quaternary acylcyclopentenes through a ring contraction pathway or γ-quaternary cycloheptenones through a carbonyl transposition pathway. Synthetic applications of these compounds were explored through the preparation of mono-, bi-, and tricyclic derivatives that can serve as valuable intermediates for the total synthesis of complex natural products. This work complements our previous work with cyclohexanoid systems.

Enantioselective decarboxylative alkylation reactions: catalyst development, substrate scope, and mechanistic studies

α-Quaternary ketones are accessed through novel enantioselective alkylations of allyl and propargyl electrophiles by unstabilized prochiral enolate nucleophiles in the presence of palladium complexes with various phosphinooxazoline (PHOX) ligands. Excellent yields and high enantiomeric excesses are obtained from three classes of enolate precursor: enol carbonates, enol silanes, and racemic β-ketoesters. Each of these substrate classes functions with nearly identical efficiency in terms of yield and enantioselectivity. Catalyst discovery and development, the optimization of reaction conditions, the exploration of reaction scope, and applications in target-directed synthesis are reported. Experimental observations suggest that these alkylation reactions occur through an unusual inner-sphere mechanism involving binding of the prochiral enolate nucleophile directly to the palladium center.

Enantioselective protonation

Enantioselective protonation is a common process in biosynthetic sequences. The decarboxylase and esterase enzymes that effect this valuable transformation are able to control both the steric environment around the proton acceptor (typically an enolate) and the proton donor (typically a thiol). Recently, several chemical methods to achieve enantioselective protonation have been developed by exploiting various means of enantiocontrol in different mechanisms. These laboratory transformations have proven useful for the preparation of a number of valuable organic compounds.

A palladium-catalysed enolate alkylation cascade for the formation of adjacent quaternary and tertiary stereocentres

The catalytic enantioselective synthesis of densely functionalised organic molecules containing all-carbon quaternary stereocentres is a challenge to modern chemical methodology research. The catalytically controlled asymmetric α-alkylation of ketones represents another difficult task and has been of major interest to our and other research groups in the past. We now report a palladium-catalyzed enantioselective process that addresses both problems at once and allows the installation of vicinal all-carbon quaternary and tertiary stereocentres at the α-carbon of a ketone in a single step. This multiple bond forming process is carried out on readily available β-ketoester starting materials and proceeds via conjugate addition of an in situ-generated palladium enolate to activated Michael acceptors. In other words, the CO₂-moiety of the substrate is displaced by a C-C fragment in an asymmetric cut-and-paste reaction. The products are obtained in high yield, diastereomeric ratio, and enantiomeric excess.

Rapid synthesis of an electron-deficient t-BuPHOX ligand: cross-coupling of aryl bromides with secondary phosphine oxides

Herein an efficient and direct copper-catalyzed coupling of oxazoline-containing aryl bromides with electron-deficient secondary phosphine oxides is reported. The resulting tertiary phosphine oxides can be reduced to prepare a range of PHOX ligands. The presented strategy is a useful alternative to known methods for constructing PHOX derivatives.