Boron-Heck reaction of cyclic enaminones: regioselective direct arylation via oxidative palladium(II) catalysis

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

TfOH-promoted synthesis of indoles and benzofurans involving cyclative transposition of vinyl ketone

A metal-free approach to construct indole ring from vinylogous amides derived from o-alkynylanilines involving cyclization, retro-aza-Michael and amine trapping cascade is reported here. This atom-economical transformation has been extended to...

Transition-Metal-Catalyzed C–H Arylation Using Organoboron Reagents

Aryl rings are ubiquitous in the core of numerous natural product and industrially important molecules and thus their facile synthesis is of major interest in the scientific community and industry. Although multiple strategies enable access to these skeletons, metal-catalyzed C–H activation is promising due to its remarkable efficiency. Commercially available organoboron reagents, a prominent arylating partner in the cross-coupling domain, have also been utilized for direct arylation. Organoborons are bench-stable, inexpensive, and readily available coupling partners that promise regioselectivity, chemodivergence, cost-efficiency, and atom-economy without requiring harsh and forcing conditions. This critical, short review presents a summary of all major studies of arylation using organoborons in transition-metal catalysis since 2005.

1 Introduction

2 Arylation without Directing Group Assistance

2.1 Palladium Catalysis

2.2 Iron Catalysis

2.3 Gold Catalysis

3 Arylation with Directing Group Assistance

3.1 Palladium Catalysis

3.2 Ruthenium Catalysis

3.3 Rhodium Catalysis

3.4 Nickel Catalysis

3.5 Cobalt Catalysis

3.6 Copper Catalysis

4 Conclusion

Rapid Access to Azabicyclo[3.3.1]nonanes by a Tandem Diverted Tsuji-Trost Process

A three-step synthesis of the 2-azabicyclo[3.3.1]nonane ring system from simple pyrroles, employing a combined photochemical/palladium-catalysed approach is reported. Substrate scope is broad, allowing the incorporation of a wide range of functionality relevant to medicinal chemistry. Mechanistic studies demonstrate that the process occurs by acid-assisted C-N bond cleavage followed by β-hydride elimination to form a reactive diene, demonstrating that efficient control of what might be considered off-cycle reactions can result in productive tandem catalytic processes. This represents a short and versatile route to the biologically important morphan scaffold.

Fluoroalkenylation of boronic acids via an oxidative Heck reaction

A fluoroalkenylation of boronic acids with fluoroalkyl alkenes has been developed. The Pd-catalyzed oxidative Heck coupling reaction proceeds under an oxygen atmosphere at room temperature, in the absence of a base and/or a ligand, showing excellent practicality of the process. This simple transformation is highly stereoselective to provide only E-isomers. In addition to the general approach using alkenes with functionalized fluoroalkyl reagents, this method, by transferring an aromatic system to the electron-deficient fluoroalkyl alkene, provides an efficient alternative method to yield valuable organofluorines.

Palladium-Catalyzed Diastereoselective Synthesis of 3-Arylbutanoic Acid Derivatives

The first palladium-catalyzed diastereoselective conjugate addition of arylboronic acids to chiral imides is reported. The catalytic system employing 4-tert-butyloxazolidin-2-one as the chiral auxiliary in a mixed solvent system of MeOH/H₂O (1:3) under an air atmosphere provides the optically active 3-arylbutanoic acid derivatives in excellent yields with high diastereoselectivity.

Scalable synthesis of enaminones utilizing Gold's reagents

Several Gold's reagents were synthesized from cyanuric chloride and N,N-dialkylformamides. These synthetic equivalents of N,N-dimethylformamide dimethyl acetal were used in an optimized and scalable procedure for the regioselective synthesis of a variety of enaminones from ketone starting materials, whose utility was demonstrated by the stereoselective synthesis of Rawal-type dienes.

Efficient synthesis of polymethoxyselenoflavones via regioselective direct C-H arylation of selenochromones

Substantial research has suggested that the configuration and the total number of functional groups on flavones influence their bioactivity. To investigate the changes in the biological activities of selenoflavones in relationship to structural changes, the development of a generally applicable synthetic method was a key. Until now, an efficient pathway for palladium-catalyzed direct arylation with the selenocyclic enone systems is not known in the literature. We herein introduce a simple direct C-H arylation of two difficult coupling partners, selenochromones and electron-rich aryl bromide, affording diverse polymethoxyselenoflavones with great efficiency and high selectivity.

Cyclic enaminones. Part I: stereocontrolled synthesis using diastereoselective and catalytic asymmetric methods

Cyclic enaminones are versatile intermediates to construct a variety of azacyclic frameworks and have been widely used in alkaloid synthesis. Here, we summarize three approaches for stereoselective syntheses of cyclic enaminones and their functionalized derivatives. These include chiral substrates (chirons) as starting materials, syntheses employing non-catalytic (stoichiometric) reagents, and catalytic asymmetric methods.

Cyclic enaminones. Part II: applications as versatile intermediates in alkaloid synthesis

Among many other strategies, the enaminone approach is an important strategy to construct and diversify the azacyclic core in various alkaloids syntheses. In this brief review we discuss the application of cyclic enaminones as building blocks, as well as potential intermediates in the total synthesis of selected alkaloids.

Synthesis of (±)-Lasubine II Using N-Methoxyamines

The synthesis of (±)-lasubine II has been achieved through a three-component allylation capitalizing on the unique properties of N-methoxyamines. This reaction enabled the installation of all the carbon atoms of lasubine II in a single operation. The N-methoxy group was efficiently used for the subsequent nitrone formation. A single-step cyclization of isoxazolidines or N-methoxyamines to form functionalized piperidine rings was also developed.

Enantioselective oxidative boron Heck reactions

This review highlights the use of the oxidative boron Heck reaction in enantioselective Heck-type couplings.

A versatile approach to flavones via a one-pot Pd(ii)-catalyzed dehydrogenation/oxidative boron-Heck coupling sequence of chromanones

A variety of flavones were expediently synthesized from readily accessible chromanones via a one-pot sequence involving Pd(ii)-catalyzed dehydrogenation and oxidative boron-Heck coupling with arylboronic acid pinacol esters.

Synthesis of β,β-diaryl propiophenones via palladium-catalyzed domino arylboronation, elimination and enone hydroarylation of enaminones

The first example of a domino reaction involving enaminone C–N conversion-based arylation has been realized as an efficient new route toward the synthesis of β,β-diaryl propiophenones.

Direct access to functionalized benzotropones, azepanes, and piperidines by reductive cross-coupling of α-bromo enones with α-bromo enamides

High-yielding syntheses of functionalized azepenes and piperidines, bearing an α-benzotropone derivative, have been achieved through cobalt-catalysed reductive cross-coupling of α-bromo enamides with α-bromo enones.

Oxidative Heck desymmetrisation of 2,2-disubstituted cyclopentene-1,3-diones

Oxidative Heck couplings have been successfully developed for 2,2-disubstituted cyclopentene-1,3-diones. The direct coupling onto the 2,2-disubstituted cyclopentene-1,3-dione core provides a novel, expedient and useful way of desymmetrising all-carbon quaternary centres.