Enantioselective cyclization of 4-alkenoic acids via an oxidative allylic C-H esterification

Cu-Catalyzed Oxidative C(sp2)-H Cycloetherification of o-Alkenyl Arenols for the Preparation of Fused Furans

A practically efficient, copper-catalyzed approach for the synthesis of functionally embellished indeno-naphthofurans is developed from 1-(1H-inden-3-yl)naphthalen-2-ols. This intramolecular cycloetherification proceeds via C(sp2)-H oxygenation (C-H bond breaking and C-O bond forming), which enables the atom-economical synthesis of poly fused furans in high yields with large substrate diversity in the open air.

Bimetallic Palladium/Cobalt Catalysis for Enantioselective Allylic C-H Alkylation via a Transient Chiral Nucleophile Strategy

An asymmetric allylic C-H functionalization has been developed by making use of transient chiral nucleophiles, as well as bimetallic synergistic catalysis with an achiral Pd⁰ catalyst and a chiral N,N'-dioxide-Co^II complex. A variety of β-ketoesters and N-Boc oxindoles coupled with allylbenzenes and aliphatic terminal alkenes were well tolerated, furnishing the desired allylic alkylation products in high yields (up to 99 %) with excellent regioselectivities and enantioselectivities (up to 99 % ee).

Palladium-catalyzed regio- and enantioselective migratory allylic C(sp3)-H functionalization

Transition metal-catalyzed asymmetric allylic substitution with a suitably pre-stored leaving group in the substrate is widely used in organic synthesis. In contrast, the enantioselective allylic C(sp³)-H functionalization is more straightforward but far less explored. Here we report a catalytic protocol for the long-standing challenging enantioselective allylic C(sp³)-H functionalization. Through palladium hydride-catalyzed chain-walking and allylic substitution, allylic C-H functionalization of a wide range of acyclic nonconjugated dienes is achieved in high yields (up to 93% yield), high enantioselectivities (up to 98:2 er), and with 100% atom efficiency. Exploring the reactivity of substrates with varying pK_a values uncovers a reasonable scope of nucleophiles and potential factors controlling the reaction. A set of efficient downstream transformations to enantiopure skeletons showcase the practical value of the methodology. Mechanistic experiments corroborate the PdH-catalyzed asymmetric migratory allylic substitution process.

Alkene isomerizations and asymmetric C–H functionalizations have been independently studied, but their combination in one protocol is uncommon. Here the authors show a palladium-catalyzed method to iteratively “walk” a terminal alkene along a carbon chain to a position next to styrenes where a soft nucleophile is added asymmetrically.

[2 + 2 + 1] Cycloaddition of N-tosylhydrazones, tert-butyl nitrite and alkenes: a general and practical access to isoxazolines

N-Tosylhydrazones have proven to be versatile synthons over the past several decades. However, to our knowledge, the construction of isoxazolines based on N-tosylhydrazones has not been examined. Herein, we report the first demonstrations of [2 + 2 + 1] cycloaddition reactions that allow the facile synthesis of isoxazolines, employing N-tosylhydrazones, tert-butyl nitrite (TBN) and alkenes as reactants. This process represents a new type of cycloaddition reaction with a distinct mechanism that does not involve the participation of nitrile oxides. This approach is both general and practical and exhibits a wide substrate scope, nearly universal functional group compatibility, tolerance of moisture and air, the potential for functionalization of complex bioactive molecules and is readily scaled up. Both control experiments and theoretical calculations indicate that this transformation proceeds via the in situ generation of a nitronate from the coupling of N-tosylhydrazone and TBN, followed by cycloaddition with an alkene and subsequent elimination of a tert-butyloxy group to give the desired isoxazoline.

Acid-base-sensitive allylic oxidation of 2-allylbenzoic acids to form phthalides

Allylic oxidation of 2-allylbenzoic acids to phthalides, instead of Wacker-type isocoumarins, was achieved with 1,2-bis(phenylsulfinyl)ethane palladium(ii) acetate (White catalyst) and oxygen in DMSO. The selective formation of 3-ethylidenephthalides or 3-vinylphthalides was controlled by the addition of acids or bases, and the reaction conditions were applied to substituted 2-allylbenzoic acids to generate corresponding phthalides selectively. Mechanistic studies, including the corresponding reaction of (E)-2-(1-propenyl)benzoic acid to 3-methylisocoumarin, isomerization reaction of 3-vinylphthalide to 3-ethylidenephthalide, and the kinetic isotope effect using 2-(1,1-d2-allyl)benzoic acid, revealed the competition between Wacker-type oxidation and allylic C-H cleavage, which is the key step to generating phthalides. A natural product, 3-ethyl-6-hydroxyphthalide, was prepared by this method.

Nickel-catalyzed intermolecular oxidative Heck arylation driven by transfer hydrogenation

The conventional oxidative Heck reaction between aryl boronic acids and alkenes typically involved the PdII/Pd0/PdII catalytic cycle incorporating an external oxidant and often suffered C=C bond isomerization for internal alkyl-substituted alkenes via chain-walking. Herein, we demonstrate that the regioselectivity (γ-selectivity vs. δ-selectivity) and pathway selectivity (hydroarylation vs. oxidative Heck coupling) of a directed Ni-catalyzed alkene arylation can be controlled by judicious tuning of the coordination environment around the nickel catalyst via optimization of an appropriate phosphine ligand and directing group. In this way, the Ni(0)-catalyzed oxidative Heck arylation that relies on transfer hydrogenation of an acceptor olefin is developed with excellent E/Z selectivity and regioselectivity. Mechanistic investigations suggest that the addition of the acceptor is crucial for lowering the energy for carbometalation and for enabling catalytic turnover.

Metal-Free C-H Functionalization of Allenamides: An Access to Branched Allylic Esters

A regioselective acyloxylation with carboxylic acids at the proximal carbon of allenamides by an N-iodosuccinimide-mediated C-H functionalization is reported. The reaction proceeds rapidly, is scalable to a gram scale, and displays a broad substrate scope, providing an efficient and practical protocol for the synthesis of branched allylic esters. Notably, protected amino acids were tolerated under the reaction conditions and afforded allylic amino acid esters in moderate yields.

Access to Chiral Hydropyrimidines through Palladium-Catalyzed Asymmetric Allylic C-H Amination

A palladium-catalyzed asymmetric intramolecular allylic C-H amination controlled by a chiral phosphoramidite ligand was established for the preparation of various substituted chiral hydropyrimidinones, the precursors of hydropyrimidines, in high yields with high enantioselectivities. In particular, dienyl sodium N-sulfonyl amides bearing an arylethene-1-sulfonyl group underwent a sequential allylic C-H amination and intramolecular Diels-Alder (IMDA) reaction to produce chiral fused tricyclic tetrahydropyrimidinone frameworks in high yields and with high levels of stereoselectivity. Significantly, this method was used as the key step in an asymmetric synthesis of letermovir.

Palladium-Catalyzed Allylic C-H Oxidative Annulation for Assembly of Functionalized 2-Substituted Quinoline Derivatives

An efficient and practical palladium-catalyzed aerobic oxidative approach to afford functionalized 2-substituted quinolines in moderate to good yields from readily available allylbenzenes with aniline is developed. The present annulation process has high functional-group tolerance and high atom economy, making it a valuable and practical method in synthetic and medicinal chemistry. Moreover, this transformation is supposed to proceed through oxidation of allylic C-H functionalization to form C-C and C-N bonds in one pot.

Structural Features and Asymmetric Environment of i-Pr-SPRIX Ligand

Novel chiral diisopropyl spiro bis(isoxazoline) ligands, anti-i-Pr-SPRIX and syn-i-Pr-SPRIX, were designed and synthesized. Their catalytic utility, X-ray crystallographic analyses, and complexation studies demonstrated the structural features of tetraisopropyl spiro bis(isoxazoline) ligand, i-Pr-SPRIX, which is a prominent ligand in various enantioselective Pd catalytic processes: All i-Pr groups work in collaboration to create an effective asymmetric environment.

Pd-catalyzed enantioselective intramolecular α-arylation of α-substituted cyclic ketones: facile synthesis of functionalized chiral spirobicycles

Synthesis of chiral spirocyclic ketones was accomplishedviathe Pd-catalyzed intramolecular α-arylation of α-substituted cyclic ketones. The obtained spirocyclic ketone could be converted into an acid–base organocatalyst.

Palladium-catalyzed aerobic oxidative double allylic C–H oxygenation of alkenes: a novel and straightforward route to α,β-unsaturated esters

We reported a strategically distinct approach to synthesize α,β-unsaturated esters through palladium-catalyzed aerobic oxidative double allylic C–H oxygenation of alkenes.

Transition metal-catalyzed C–H bond functionalizations by the use of diverse directing groups

In this review, a summary of transition metal-catalyzed C–H activation by utilizing the functionalities as directing groups is presented.

Toward a more complete understanding of noncovalent interactions involving aromatic rings

Noncovalent interactions involving aromatic rings, which include π-stacking interactions, anion-π interactions, and XH-π interactions, among others, are ubiquitous in chemical and biochemical systems. Despite dramatic advances in our understanding of these interactions over the past decade, many aspects of these noncovalent interactions have only recently been uncovered, with many questions remaining. We summarize our computational studies aimed at understanding the impact of substituents and heteroatoms on these noncovalent interactions. In particular, we discuss our local, direct interaction model of substituent effects in π-stacking interactions. In this model, substituent effects are dominated by electrostatic interactions of the local dipoles associated with the substituents and the electric field of the other ring. The implications of the local nature of substituent effects on π-stacking interactions in larger systems are discussed, with examples given for complexes with carbon nanotubes and a small graphene model, as well as model stacked discotic systems. We also discuss related issues involving the interpretation of electrostatic potential (ESP) maps. Although ESP maps are widely used in discussions of noncovalent interactions, they are often misinterpreted. Next, we provide an alternative explanation for the origin of anion-π interactions involving substituted benzenes and N-heterocycles, and show that these interactions are well-described by simple models based solely on charge-dipole interactions. Finally, we summarize our recent work on the physical nature of substituent effects in XH-π interactions. Together, these results paint a more complete picture of noncovalent interactions involving aromatic rings and provide a firm conceptual foundation for the rational exploitation of these interactions in a myriad of chemical contexts.