Transition-Metal-Free Formal Sonogashira Coupling and α-Carbonyl Arylation Reactions

Cross-Coupling Between Aryl Halides and Aryl Alkynes Catalyzed by an Odd Alternant Hydrocarbon

Catalytic cross-coupling between aryl halides and alkynes is considered an extremely important organic transformation (popularly known as the Sonogashira coupling) and it requires a transition metal-based catalyst. Accomplishing such transformation without any transition metal-based catalyst in the absence of any external stimuli such as heat, photoexcitation or cathodic current is highly challenging. This work reports transition-metal-free cross-coupling between aryl halides and alkynes synthesizing a rich library of internal alkynes without any external stimuli. A chemically double-reduced phenalenyl (PLY)-based molecule with the super-reducing property was employed for single electron transfer to activate aryl halides generating reactive aryl radicals, which subsequently react with alkyne. This protocol covers not only various types of aryl, heteroaryl and polyaryl halides but also applies to a large variety of aromatic alkynes at room temperature. With a versatile substrate scope successfully tested on more than 75 entries, this radical-mediated pathway has been explained by several control experiments. All the key reactive intermediates have been characterized with spectroscopic evidence. Detailed DFT calculations have been instrumental in portraying the mechanistic pathway. Furthermore, we have successfully extended this transition-metal-free catalytic strategy for the first time towards solvent-free cross-coupling between solid aryl halide and alkyne substrates.

Arylation of Terminal Alkynes: Transition-Metal-Free Sonogashira-Type Coupling for the Construction of C(sp)-C(sp2) Bonds

Alkynes are attractive synthons for organic chemistry. Despite the prevalence of transition-metal-catalyzed Sonogashira reactions, a transition-metal-free version of the arylation of terminal alkynes is elusive. Herein, we report an efficient transition-metal-free Sonogashira-type coupling reaction for the one-pot arylation of alkynes to construct C(sp)-C(sp²) bonds from a tetracoordinate boron intermediate with NIS as a mediator. With its high efficiency, wide substrate range, and good functional group tolerance, this method is further supported by the gram-scale synthesis and subsequent modification of complex molecules.

Regioselective synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds via transition metal-free C-C and C-N bond formation

Few methods are known for the synthesis of nitroindole derivatives. A simple and practical Cs₂CO₃-promoted method for the synthesis of 6-nitroindole derivatives from enaminones and nitroaromatic compounds has been developed. Two new C-C and C-N bonds were formed in a highly regioselective manner under transition metal-free conditions.

Facile one-pot synthesis of diarylacetylenes from arylaldehydes via an addition-double elimination process

A practical one-pot protocol has been developed to synthesize diarylacetylenes from arylaldehydes by treatment with 1-(arylmethyl)benzotriazoles and LiN(SiMe3)2. The reaction proceeded through imine formation, Mannich-type addition and double elimination to deliver products in up to 99% yields with broad substrate scope. In addition, gram-scale synthesis of 1-bromo-4-(phenylethynyl)benzene has been demonstrated.

Synthesis of spirocyclic Δ4-isoxazolines via [3 + 2] cycloaddition of indanone-derived ketonitrones with alkynes

A [3 + 2] cycloaddition of indanone-derived nitrones and alkynes under mild conditions is developed, allowing facile synthesis of spirocyclicindenyl isoxazolines with structural diversity. The sequential protocol of generated in situ ketonitrone from unsaturated ketones and N-alkylhydroxylamines is also achieved successfully, affording the desired products in considerable yield with moderate to good diastereoselectivity. Moreover, the spirocyclic product can be conveniently transformed into indenyl-based allylic alcohol and enamide.

A [3 + 2] cycloaddition of indanone-derived nitrones with alkynes under mild conditions has been developed. It is a highly efficient and straightforward method for the synthesis of diverse spirocyclicindenyl isoxazolines.

Trisubstituted Highly Activated Benzo[d]thiazol-2-yl-sulfone-Containing Olefins as Building Blocks in Organic Synthesis

In this paper, we report the formation of highly electrophilic 1,1-deactivated olefins, their use as novel synthetic building blocks, and their transformation to structurally diverse molecular scaffolds. Synthesis of 1,1-deactivated olefins substituted with a BT-sulfonyl group and a carbonyl or nitrile, respectively, consists of unusual Ti(OPrⁱ)₄-mediated Knoevenagel-type condensation and proceed in good to excellent yields. Generated olefins can be further transformed in a highly stereoselective manner and in good yields to various polyfunctionalized heterocycles and acyclic molecular scaffolds. Overall, the obtained structures are accessed in two to four steps starting from the (mostly) commercially available aldehydes. In addition, the presence of the BT-sulfonyl group in prepared structures allows for further chemoselective functionalization/post-synthetic transformations to provide structurally diverse final compounds.

Stereodivergent synthesis of alkenes by controllable syn-/anti-fragmentation of β-hydroxysulfonyl intermediates

Complementary fragmentation mechanisms of Hawkins and one-pot Julia olefinations were employed for the stereodivergent synthesis of alkenes.

A general and convenient method for the synthesis of 2,4-dinitrobenzyl ketones. Almost unlimited access to 2-substituted 6-nitroindoles from 2,4-dinitrotoluene and aldehydes

Starting from 2,4-dinitrotoluene and aldehydes, 2-substituted 6-nitroindoles with alkyl, cycloalkyl, aryl and heteroaryl (thienyl, furyl, pyridyl) substituents in the 2-position of the indole ring were synthesized. This methodology can be used for synthesis of indoles with non-racemic substituents.

Construction of Sulfonyl Oxabenzo[3.3.1]bicyclic Core via Cyclocondensation of β-Ketosulfones and o-Formyl Allylbenzenes

NH₄OAc mediated domino Knoevenagel/Diels-Alder cyclocondensation of β-ketosulfones 1 and o-formyl allylbenzenes 2 provides sulfonyl oxabenzo[3.3.1]bicyclic core 4 in a cosolvent of toluene and HOAc (v/v = 1/1) at reflux for 3 h. The intermediate 3 contains a chalcone motif. The uses of various ammonium salts and solvent systems are investigated for facile and efficient transformation. The plausible mechanisms have been proposed and the DFT calculations have been included.

Direct synthesis of nitroaryl acetylenes from acetylenes and nitroarenes via oxidative nucleophilic substitution of hydrogen

Acetylenic carbanions add to nitroarenes (dinitrobenzenes, nitropyridines, etc.) to form σ^H-adducts that are subsequently oxidized by DDQ according to the oxidative nucleophilic substitution of hydrogen (ONSH) pathway to give nitroaryl acetylenes.

Enantioselective Oxidative Rearrangements with Chiral Hypervalent Iodine Reagents

A stereoselective hypervalent iodine‐promoted oxidative rearrangement of 1,1‐disubstituted alkenes has been developed. This practically simple protocol provides access to enantioenriched α‐arylated ketones without the use of transition metals from readily accessible alkenes.

Double Smiles rearrangement of Passerini adducts towards benzoxazinones

A new straightforward access to benzoxazinones based on a three-component coupling is presented here.

To be or not to be metal-free: trends and advances in coupling chemistries

Coupling reactions have been part of several extensive studies in order to develop innovative and greener protocols that can generate a wide range of compounds with applications in pharmaceuticals, agrochemicals and biologically active compounds. Metal-free couplings are an important and increasingly trendy field that has attracted a significant deal of interest in recent years, generating a lot of controversy on the issue of whether metal free is really free. Aside from focusing on such a controversial topic itself, this contribution aims to provide a brief introduction on coupling chemistry to point out the transition of this technology from metal-catalyzed to metal-free. This is followed by a range of key selected synthetically useful metal-free processes and a brief commentary on the current debate of whether metal-free reactions are really metal-free and the required experiments for a full understanding of a metal-free coupling process.

Sulfur(IV)-mediated transformations: from ylide transfer to metal-free arylation of carbonyl compounds

The development of a direct ylide transfer to carbonyl derivatives and of a sulfoxide-mediated arylation is presented from a unified perspective. Mechanistic studies (including density functional calculations) support a common reaction pathway and showcase how subtle changes in reactant properties can lead to disparate and seemingly unrelated reaction outcomes.

Transition-metal-free α-arylation of β-keto amides via an interrupted insertion reaction of arynes

Direct α-arylation reactions of secondary β-keto amides with arynes, generated by fluoride-induced elimination of ortho-silyl aryltriflates, are described. The transformation proceeds via an interrupted insertion reaction of arynes and leads to densely functionalized aromatic compounds exhibiting a chiral 'all carbon' quaternary center under transition-metal-free conditions. An organocatalytic asymmetric version of the reaction also proved possible, affording the proof of concept that arynes can be involved in enantioselective transformations.

Transition-metal-free alkynylation of aryl chlorides

Two sets of conditions have been developed for a base-mediated, transition-metal-free alkynylation of aryl chlorides that proceeds via benzyne intermediates. The first set of conditions involves the use of TMPLi base in a pentane/THF mixture at 25 °C. The second set involves use of a metal alkoxide base in dioxane at elevated temperature. Reasonable functional group tolerance has been observed. Fluoro, trifluoromethyl, silyl, cyano, and alcohol functionalities are compatible with the reaction conditions.

Sulfoxide-mediated α-arylation of carbonyl compounds

A novel sulfoxide-mediated α-arylation of carbonyl compounds is reported. This reaction proceeds under very mild conditions at room temperature and does not require any transition-metal promoter or catalyst.