Advancing the mechanistic understanding of an enantioselective palladium-catalyzed alkene difunctionalization reaction

Mechanistic study on the side arm effect in a palladium/Xu-Phos-catalyzed enantioselective alkoxyalkenylation of γ-hydroxyalkenes

Recently, the asymmetric bifunctionalization of alkenes has received much attention. However, the development of enantioselective alkoxyalkenylation has posed a considerable challenge and has lagged largely behind. Herein, we report a new palladium-catalyzed enantioselective alkoxyalkenylation reaction, using a range of primary, secondary, and tertiary γ-hydroxy-alkenes with alkenyl halides. By employing newly identified Xu-Phos (Xu8 and Xu9) with a suitable side-arm adjacent to the PCy2 motif, a series of allyl-substituted tetrahydrofurans were obtained in good yields with up to 95% ee. Besides (E)-alkenyl halides, (Z)-alkenyl halide was also examined and provided the corresponding (Z)-product as a single diastereomer, supporting a stereospecific oxidative addition and reductive elimination step. Moreover, deuterium labeling and VCD experiments were employed to determine a cis-oxypalladation mechanism. DFT calculations helped us gain deeper insight into the side-arm effect on the chiral ligand. Finally, the practicability of this method is further demonstrated through a gram-scale synthesis and versatile transformations of the products.

Copper-Catalyzed Enantioselective Aerobic Alkene Aminooxygenation and Dioxygenation: Access to 2-Formyl Saturated Heterocycles and Unnatural Proline Derivatives

Alkene aminooxygenation and dioxygenation reactions that result in carbonyl products are uncommon, and protocols that control absolute stereochemistry are rare. We report herein catalytic enantioselective alkene aminooxygenation and dioxygenation that directly provide enantioenriched 2-formyl saturated heterocycles under aerobic conditions. Cyclization of substituted 4-pentenylsulfonamides, catalyzed by readily available chiral copper complexes and employing molecular oxygen as both oxygen source and stoichiometric oxidant, directly provides chiral 2-formyl pyrrolidines efficiently. Reductive or oxidative workup of these aldehydes provides their respective amino alcohols or amino acids (unnatural prolines). Enantioselective synthesis of an indoline and isoquinolines is also demonstrated. Concurrently, cyclization of various alkenols under similar conditions provides 2-formyl tetrahydrofurans, phthalans, isochromans, and morpholines. The nature of the copper ligands, the concentration of molecular oxygen, and the reaction temperature all impact the product distribution. Chiral nitrogen and oxygen heterocycles are common components of bioactive small molecules, and these enabling technologies provide access to saturated heterocycles functionalized with ready-to-use carbonyl electrophiles.

Brønsted Acid Catalyzed Friedel-Crafts Alkylation of Naphthols with In Situ Generated Naphthol-Derived ortho-Quinone Methides: Synthesis of Chiral and Achiral Xanthene Derivatives

We disclose herein an enantioselective protocol for the Brønsted acid catalyzed addition of naphthols to in situ generated naphthol-derived ortho-quinone methides (o-QMs) followed by intramolecular cyclization, which delivers substituted chiral xanthene derivatives, in a one-pot reaction sequence under mild conditions. This process serves to convert naphthol-derived ortho-hydroxyl benzylic alcohols into reactive naphthol-derived o-QMs using a chiral phosphoric acid (CPA) catalyst. Moreover, it is helpful in controlling the enantioselectivity of the carbon-carbon bond-forming event via hydrogen-bonding followed by intramolecular cyclization. Additionally, for the first time, we observe a Brønsted acid catalyzed C(sp²)-C(sp³) bond cleavage of naphthol-derived ortho-hydroxyl benzylic alcohols for the synthesis of achiral xanthene (sigma plane containing) derivatives in good to excellent yields.

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.

Construction of azaheterocycles via Pd-catalyzed migratory cycloannulation reaction of unactivated alkenes

Azahetereocycles constitute important structural components in many biologically active natural compounds and marketed drugs, and represent the most promising scaffolds in drug discovery. Accordingly, the development of efficient and general synthetic methods for the construction of diverse azaheterocycles is the major goal in synthetic chemistry. Herein, we report the efficient construction of a wide range of azaheterocycles via a Pd-catalyzed migratory cycloannulation strategy with unactivated alkenes. This strategy enables the rapid synthesis of a series of 6-, 7- and 8-membered azaheterocycles in high efficiency, and features a broad substrate scope, excellent functional group tolerance under redox-neutral conditions. The significance of this finding is demonstrated by the efficient synthesis of drug-like molecules with high step-economy. Preliminary mechanistic investigations reveal that this reaction underwent a sequentially migratory insertion to alkenes, metal migration process, and the aza-Michael addition to a quinone methide intermediate.

Enantio- and Regioselective Palladium(II)-Catalyzed Dioxygenation of (Aza-)Alkenols

An oxidative Pd-catalyzed intra-intermolecular dioxygenation of (aza-)alkenols has been reported, with total regioselectivity. To study the stereoselectivity, different chiral ligands as well as different hypervalent-iodine compounds have been compared. In particular, by using a C-6 modified pyridinyl-oxazoline (Pyox) ligand and hypervalent iodine bearing an aromatic ring, an excellent enantio- and diastereoselectivity has been achieved.

High-throughput screening of α-chiral-primary amines to determine yield and enantiomeric excess

A novel screening protocol was developed using a combination of a fluorescent indicator displacement assay and a circular dichroism (CD) active Fe(II) complex to determine concentration and enantiomeric excess (ee) of α-chiral amines, respectively. The analyte concentration is quantified with a pre-formed non-fluorescent imine, where transimination with the chiral amine results in displacement of the fluorophore 2-naphthylamine. After discerning the concentration of amine via fluorescence in a wellplate reader, the analyte is then incorporated into a three-component octahedral Fe(II) assembly for ee determination using an EKKO CD plate-reader. With these two assays, both the ee and yield of asymmetric transformations of 192 samples could be determined with acceptable errors in under fifteen minutes (not counting the preparation time). This combined speed and accuracy provides an attractive solution to overcoming analytical bottlenecks when creating α-chiral amines.

Regio- and diastereoselective Pd-catalyzed aminochlorocyclization of allylic carbamates: scope, derivatization, and mechanism

The regio- and diastereoselective synthesis of oxazolidinones via a Pd-catalyzed vicinal C-N/C-Cl bond-forming reaction from internal alkenes of allylic carbamates is reported. The oxazolidinones are obtained in yields of 44 to 95% with high to excellent diastereoselectivities (from 6 : 1 to >20 : 1 dr) from readily available precursors. This process is scalable, and the products are suitable for the synthesis of useful amino alcohols. A detailed theoretical and experimental mechanistic study was carried out to describe that the reaction proceeds through an anti-aminopalladation of the alkene followed by an oxidative C-Pd(ii) cleavage with retention of the carbon stereochemistry to yield the major diastereomer. The role of Cu(ii) in a C-Cl bond-forming mechanism step has also been proposed.

Synthesis of 3-Carbonyl Trisubstituted Furans via Pd-Catalyzed Aerobic Cycloisomerization Reaction: Development and Mechanistic Studies

Herein, we report the synthesis of 3-carbonyl-trisubstituted furans via Pd-catalyzed oxidative cycloisomerization reactions of 2-alkenyl-1,3-dicarbonyl scaffolds, using molecular oxygen as the sole oxidant to regenerate active palladium catalytic species, featuring good functional tolerance and mild reaction conditions. Deep investigation of intermediates and transition states of the reaction mechanism were conducted via experimental and DFT studies, providing a detailed mechanistical profile. The new developed methodology presents a greener alternative to Wacker-type cycloisomerizations and avoids the use of stoichiometric amounts of oxidants and strong acid additives.

Silica gel-induced aryne generation from o-triazenylarylboronic acids as stable solid precursors

We developed o-triazenylarylboronic acids as stable solid aryne precursors, which generate arynes under mild conditions using silica gel as the sole reagent and undergo reactions with a range of arynophiles both in solution and in the solid-state.

Catalytic Asymmetric [4 + 2] Cycloaddition of ortho-Alkenyl Naphthols/Phenols with ortho-Quinone Methides: Highly Stereoselective Synthesis of Chiral 2,3,4-Trisubstituted Chromans

Chiral phosphoric acid-catalyzed biomimetic asymmetric [4 + 2] cycloaddition of ortho-alkenyl naphthols/phenols and ortho-quinone methides (o-QMs) has been demonstrated to afford various important 2,3,4-trisubstituted chromans in high yields with excellent enantio- and diastereoselectivities (up to 99% yield, 99% ee, >20:1 dr). Notably, this methodology not only enabled access to the trans-cis chiral trisubstituted chromans from 1-alkenyl 2-naphthols but also is compatible with 2-alkenyl 1-naphthols and phenols to deliver trans-trans chiral trisubstituted chromans.

Ligand-enabled gold-catalyzed 1,2-heteroarylation of alkenes

By adopting the interplay between ligand-enabled Au(i)/Au(iii) catalysis and the unique π-activation mode of gold complexes, a highly coveted 1,2-heteroarylation of alkenes has been accomplished.

Alkene Difunctionalization Using Hypervalent Iodine Reagents: Progress and Developments in the Past Ten Years

Hypervalent iodine reagents are of considerable relevance in organic chemistry as they can provide a complementary reaction strategy to the use of traditional transition metal chemistry. Over the past two decades, there have been an increasing number of applications including stoichiometric oxidation and catalytic asymmetric variations. This review outlines the main advances in the past 10 years in regard to alkene heterofunctionalization chemistry using achiral and chiral hypervalent iodine reagents and catalysts.

Brønsted Acid-Catalyzed, Diastereo- and Enantioselective, Intramolecular Oxa-Diels-Alder Reaction of ortho-Quinone Methides and Unactivated Dienophiles

A stereoselective, phosphoric acid-catalyzed synthesis of dihydrochromenochromenes has been developed using transient ortho-quinone methides ( o-QMs). Three contiguous stereogenic centers were formed with excellent yields, partially as single diastereomers and with moderate to excellent enantioselectivity. This intramolecular hetero-Diels-Alder reaction features unactivated dienophiles and o-QM precursors tethered by a simple phenoxy linker and furnishes cycloadducts with a prominent structural motif found in many natural products. Through an appropriate choice of dienophile configuration and backbone substitution either exo- or endo-stereoisomers were formed selectively with up to a 96:04 enantiomeric ratio.

FeCl3-catalyzed dimerization/elimination of 1,1-diarylalkenes: efficient synthesis of functionalized 4H-chromenes

An efficient synthesis of valuable poly-substituted 4H-chromenes was developed via a [4 + 2] cycloaddition followed by the elimination of 1,1-diarylalkenes. The inexpensive FeCl₃ salt has proven to be an efficient catalyst for this transformation. The commercial availability of the catalyst and reagents, together with a convenient procedure, makes this an attractive method for 4H-chromene synthesis.

An autonomous self-optimizing flow machine for the synthesis of pyridine–oxazoline (PyOX) ligands

A continuous flow synthesis of PyOX ligands has been developed using an autonomous self-optimizing flow reactor.

Metal-free Chlorothiolation of Alkenes using HCl and Sulfoxides

We report a novel method for the chlorothiolation of alkenes using HCl and sulfoxides to achieve the 1,2-difunctionalization of unactivated alkenes. The combination of our new HCl reagent (HCl/DMPU) with sulfoxides forms a unique chlorothiolation system. Both terminal and internal alkenes are suitable substrates. This method works at gram scale and is applicable in further synthetic elaborations.

Benzoate Cyclometalation Enables Oxidative Addition of Haloarenes at a Ru(II) Center

The first Ru(II)-catalyzed arylation of substrates without a directing group was recently developed. Remarkably, this process only worked in the presence of a benzoate additive, found to be crucial for the oxidative addition step at Ru(II). However, the exact mode of action of the benzoate was unknown. Herein, we disclose a mechanistic study that elucidates the key role of the benzoate salt in the C–H arylation of fluoroarenes with aryl halides. Through a combination of rationally designed stoichiometric experiments and DFT studies, we demonstrate that the aryl–Ru(II) species arising from initial C–H activation of the fluoroarene undergoes cyclometalation with the benzoate to generate an anionic Ru(II) intermediate. The enhanced lability of this intermediate, coupled with the electron-rich anionic Ru(II) metal center renders the oxidative addition of the aryl halide accessible. The role of an additional (NMe₄)OC(CF₃)₃ additive in facilitating the overall arylation process is also shown to be linked to a shift in the C–H pre-equilibrium associated with benzoate cyclometalation.

Asymmetric Aza-Wacker-Type Cyclization of N-Ts Hydrazine-Tethered Tetrasubstituted Olefins: Synthesis of Pyrazolines Bearing One Quaternary or Two Vicinal Stereocenters

We have developed an asymmetric aza-Wacker-type cyclization of N-Ts hydrazine-tethered tetrasubstituted olefins, affording optically active pyrazolines bearing chiral tetrasubstituted carbon stereocenters. This reaction is tolerant to a broad range of substrates under mild reaction conditions, giving the desired chiral products with high enantioselectivities. Generation of two vicinal stereocenters on the C═C double bonds was also achieved with high selectivities, a process which has been rarely studied for Wacker-type reactions. A mechanistic study revealed that this aza-Wacker-type cyclization undergoes a syn-aminopalladation process. It was also found that for substrates bearing two linear alkyl substituents on the outer carbon atom of the olefin, both of which are larger than a methyl group, the alkyl substituent that is cis to the intranucleophilic group participates more readily in β-hydride elimination. When one of the two alkyl substituents on the outer carbon atom of the olefin is a methyl group, β-hydride elimination proceeds selectively at the methylene side, thus both diastereomers can be prepared via switching the configuration of the olefin. Furthermore, the product can be converted to a pharmaceutical compound in high yields over three steps.

Second-Generation Palladium Catalyst System for Transannular C-H Functionalization of Azabicycloalkanes

This article describes the development of a second-generation catalyst system for the transannular C-H functionalization of alicyclic amines. Pyridine- and quinoline-carboxylate ligands are shown to be highly effective for increasing the reaction rate, yield, and scope of Pd-catalyzed transannular C-H arylation reactions of azabicyclo[3.1.0]hexane, azabicyclo[3.1.1]heptane, azabicyclo[3.2.1]octane, and piperidine derivatives. Mechanistic studies reveal that the pyridine/quinoline-carboxylates play a role in impeding both reversible and irreversible catalyst decomposition pathways. These ligands enable the first reported examples of the transannular C-H arylation of the ubiquitous tropane, 7-azanorbornane, and homotropane cores. Finally, the pyridine/quinoline-carboxylates are shown to promote both transannular C-H arylation and transannular C-H dehydrogenation on a homotropane substrate.

Renaissance of pyridine-oxazolines as chiral ligands for asymmetric catalysis

Oxazoline-containing ligands have been widely employed in numerous asymmetric catalytic reactions. Pyridine-oxazoline-type ligands, a class of hybrid ligands, were designed earlier than bisoxazoline and phosphine-oxazoline ligands; however, their unique properties have only been discovered recently. Pyridine-oxazoline-type chiral ligands are rapidly becoming popular for use in asymmetric catalysis, especially for several new and efficient asymmetric methodologies. Several types of challenging asymmetric reactions have been discovered recently using pyridine-oxazoline-type ligands showing their special properties and potential for future application in a wide range of new catalytic methodologies. This review provides an overview of this field, with the aim of highlighting both ligand design and synthetic methodology development.

Computational prediction of chemical reactions: current status and outlook

Over the past few decades, various computational methods have become increasingly important for discovering and developing novel drugs. Computational prediction of chemical reactions is a key part of an efficient drug discovery process. In this review, we discuss important parts of this field, with a focus on utilizing reaction data to build predictive models, the existing programs for synthesis prediction, and usage of quantum mechanics and molecular mechanics (QM/MM) to explore chemical reactions. We also outline potential future developments with an emphasis on pre-competitive collaboration opportunities.

Recent advances in the application of Diels–Alder reactions involving o-quinodimethanes, aza-o-quinone methides and o-quinone methides in natural product total synthesis

This review summarizes recent advances in Diels–Alder reactions involving o-QDMs, o-QMs and aza-o-QMs. The power and potential of this strategy in organic synthesis and natural product total synthesis is highlighted.

A facile 2H-chromene dimerization through an ortho-quinone methide intermediate catalyzed by a sulfonyl derived MIL-101 MOF

An efficient homo-dimerization of 2H-chromenes was achieved in high yield and diastereoselectivity using a MIL-101–SO₃H MOF catalyst.

Ligand-Promoted Palladium-Catalyzed Aerobic Oxidation Reactions

Palladium-catalyzed aerobic oxidation reactions have been the focus of industrial application and extensive research efforts for nearly 60 years. A significant transition occurred in this field approximately 20 years ago, with the introduction of catalysts supported by ancillary ligands. The ligands play crucial roles in the reactions, including promotion of direct oxidation of palladium(0) by O₂, bypassing the typical requirement for Cu salts or related redox cocatalysts to facilitate oxidation of the reduced Pd catalyst; facilitation of key bond-breaking and bond-forming steps during substrate oxidation; and modulation of chemo-, regio-, or stereoselectivity of a reaction. The use of ligands has contributed to significant expansion of the scope of accessible aerobic oxidation reactions. Increased understanding of the role of ancillary ligands should promote the development of new synthetic transformations, enable improved control over the reaction selectivity, and improve catalyst activity and stability. This review surveys the different ligands that have been used to support palladium-catalyzed aerobic oxidation reactions and, where possible, describes mechanistic insights into the role played by the ancillary ligand.