Direct Electrophilic C−H Alkynylation of Unprotected 2-Vinylanilines

Aryl-, Akynyl-, and Alkenylbenziodoxoles: Synthesis and Synthetic Applications

Hypervalent iodine reagents are in high current demand due to their exceptional reactivity in oxidative transformations, as well as in diverse umpolung functionalization reactions. Cyclic hypervalent iodine compounds, known under the general name of benziodoxoles, possess improved thermal stability and synthetic versatility in comparison with their acyclic analogs. Aryl-, alkenyl-, and alkynylbenziodoxoles have recently received wide synthetic applications as efficient reagents for direct arylation, alkenylation, and alkynylation under mild reaction conditions, including transition metal-free conditions as well as photoredox and transition metal catalysis. Using these reagents, a plethora of valuable, hard-to-reach, and structurally diverse complex products can be synthesized by convenient procedures. The review covers the main aspects of the chemistry of benziodoxole-based aryl-, alkynyl-, and alkenyl- transfer reagents, including preparation and synthetic applications.

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.

Remote ortho-C-H functionalization via medium-sized cyclopalladation

Compared to the tremendous progress made in directed ortho-C-H functionalization via five- or six-membered cyclopalladation, protocols with the ability to selectively activate more remote C-H bonds through the intermediacy of larger, less favorable, seven- or eight-membered metalacycles are particularly challenging and remain rare. However, such a strategy would provide new retrosynthetic opportunities for generating structural diversity and complexity. Intense recent research based on the use of either mono-anionic bidentate or monodentate directing groups is characterizing this approach as an increasingly viable tool for selective C-C and C-X bond-forming reactions. This short review provides an overview of these strategies with an emphasis on mechanistic details, synthetic applicability, limitations, and key challenges.

Selective para-C-H Alkynylation of Aniline Derivatives via Pd/S,O-Ligand Catalysis

Herein, we report a nondirected para -selective C-H alkynylation of aniline derivatives by a Pd/S,O-ligand-based catalyst. The reaction proceeds under mild conditions and is compatible with a variety of substituted anilines. The scalability and further derivatizations of the alkynylated products have been also demonstrated.

Tetrasubstituted 1,3-Enynes by Gold-Catalyzed Direct C(sp2)-H Alkynylation of Acceptor-Substituted Enamines

A gold-catalyzed synthesis of tetrasubstituted 1,3-enynes from hypervalent iodine(III) reagents and activated alkenes is reported. This reaction involves an in situ formed alkynyl Au(III) species and a subsequent direct C(sp²)-H functionalization of alkenes, offering 26 enynes in 62-92% yield with excellent functional group tolerance.

Synthesis, Characterization of Spirocyclic λ3 -Iodanes and Their Application to Prepare 4,1-Benzoxazepine-2,5-diones and 1,3-Diynes

Herein, a [3+2] cycloaddition of aza-oxyallylic cations with ethynylbenziodoxolones for synthesis of new λ³ -iodanes containing spirocyclic 4-oxazolidinone has been developed. This cyclic λ³ -iodanes display stability in air and excellent solubility in organic solvent. Using them as substrate, both the 4,1-benzoxazepine-2,5-diones and symmetrical 1,3-diynes derivatives were afforded in high yield under copper(I)-catalyzed conditions.

Indium-mediated annulation of 2-azidoaryl aldehydes with propargyl bromides to [1,2,3]triazolo[1,5-a]quinolines

An efficient indium-mediated cascade annulation reaction of 2-azidoaryl aldehydes with propargyl bromides is reported. The aromatic 5/6/6-fused heterocycles, [1,2,3]triazolo[1,5-a]quinoline derivatives, could be constructed in one pot in moderate yields with a broad substrate scope. Mechanistic studies indicated that the reaction proceeded through allenol formation, azide-allene [3 + 2] cycloaddition, and dehydration. The synthetic potential of the products including the denitrogenative functionalization and the Pd-catalyzed coupling reactions has also been explored.

RhIII -Catalyzed Double Dehydrogenative Coupling of Free 1-Naphthylamines with α,β-Unsaturated Esters

The Rh^III -catalyzed, consecutive double C-H oxidative coupling of free 1-naphthylamine and α,β-unsaturated esters through C-H/C-H and C-H/N-H bonds is reported. The one step reaction leads to the formation of biologically important alkylidene-1,2-dihydrobenzo[cd]indoles scaffolds. This efficient process is much more synthetically convenient and useful than others because the starting materials, such as 1-naphthylamine derivatives are readily available and the free amine serves as a directing group.

Copper-catalyzed cross-coupling and sequential allene-mediated cyclization for the synthesis of 1,2,3-triazolo[1,5-a]quinolines

In this paper, a tandem reaction involving copper-catalyzed cross-coupling and allene-mediated cyclization of 1-(2-ethynylaryl)-1,4-disubstituted-1,2,3-triazole with N-tosylhydrazone has been developed. This method features operational simplicity, excellent functional group compatibility, broad substrate scope, and easily available feedstock, providing an efficient and practical strategy for the synthesis of highly functionalized 1,2,3-triazolo[1,5-a]quinolines.

Stereocontrolled Synthesis of Halovinylbenziodoxoles by Hydro- and Iodochlorination of Ethynylbenziodoxoles

We report herein the synthesis of highly substituted and stereochemically well-defined vinylbenziodoxole (VBX) derivatives through hydrochlorination and iodochlorination of ethynylbenziodoxoles. The hydrochlorination is achieved using pyridine hydrochloride as an HCl source in an anti-fashion under mild, open-air conditions to afford a 2-chlorinated VBX product, which serves as a useful building block for the stereoselective synthesis of trisubstituted alkenes. Meanwhile, iodochlorination with iodine monochloride proceeds in an unusual syn-pathway, stereoselectively affording a tetrasubstituted VBX derivative.

Hypervalent iodine reactions utilized in carbon–carbon bond formations

This review covers recent developments of hypervalent iodine chemistry in dearomatizations, radicals, hypervalent iodine-guided electrophilic substitution, arylations, photoredox, and more.

Co(iii)-catalyzed Z-selective oxidative C–H/C–H cross-coupling of alkenes with triisopropylsilylacetylene

An inexpensive Co(iii)-catalyzed direct oxidative C–H/C–H cross-coupling reaction of acrylamides with triisopropylsilylacetylene to synthsize (Z)-1,3-enynes is presented.

Pseudo-tetrahedral Rhodium and Iridium Complexes: Catalytic Synthesis of E-Enynes

The reactions of the rhodium(I) and iridium(I) complexes [M(PhBP₃ )(C₂ H₄ )(NCMe)] (PhBP₃ =PhB(CH₂ PPh₂ )₃ ^- ) with alkynes have resulted in the synthesis of a new family of pseudo-tetrahedral complexes, [M(PhBP₃ )(RC≡CR')] (M=Rh, Ir), which contain the alkyne as a four-electron donor. The reactions of these unusual compounds with two-electron donors (L=PMe₃ , CNtBu) produced a change in the "donicity" of the alkyne from a 4e^- to a 2e^- donor to give five-coordinate complexes. These were the final products with the iridium complexes, whereas further reactions took place with the rhodium complexes. In particular, C(sp)-H bond activation of the alkyne occurred leading to hydrido alkynyl complexes. This process is essential for the further reactivity of the alkynes, and if the alkyne itself was used as reagent, E-enyne complexes were obtained. As a consequence of this chemistry, we show that the complex [Rh(PhBP₃ )(C₂ H₄ )(NCMe)] is a very efficient pre-catalyst for the regioselective di- and trimerization of terminal alkynes to E-enynes and benzene derivatives, respectively. Interestingly, acetonitrile significantly enhanced the catalytic activity by facilitating the C(sp)-H bond activation step. A hydrometalation mechanism to account for these experimental observations is proposed.

NH2-Directed C-H Alkenylation of 2-Vinylanilines with Vinylbenziodoxolones

The first directing-group-mediated C-H alkenylation with alkenyl-λ³-iodanes as electrophilic alkene-transfer reagents has been developed. The application of free aromatic amines as challenging but synthetically valuable directing groups in combination with an Ir^III catalyst enabled the synthesis of highly desirable 1,3-dienes in excellent yields of up to 98% with high to perfect (Z,E) stereoselectivity. A broad substrate scope and further synthetic modifications are demonstrated.