In recent years, remarkable progress has been made in dehydro or dehydrogenative Diels–Alder (D–A) reactions. This Minireview gives an overview of the major two strategies for dehydro(genative) Diels–Alder reactions, which differ in dehydrogenation and D–A cyclization sequence. Reactions in which D–A cycloaddition is followed by dehydrogenation are useful methods for the synthesis of various aromatic compounds, whereas advancements in dehydro genative procedures with oxidants or catalysts prior to D–A cycloaddition offer yet further new routes to functionalized cycloadducts. Recent leading findings are highlighted and the current state of the art, scope, and limitations of these processes are discussed in this Minireview.

Reported herein is an asymmetric [3+2] cycloaddition reaction of azomethine ylides with β-trifluoromethyl β,β-disubstituted enones, a reaction which is enabled by a Ming-Phos-derived copper(I) catalyst (Ming-Phos=chiral sulfinamide monophosphines, Figure 2). This method provides scalable and efficient access to the highly substituted pyrrolidines with a trifluoromethylated, all-carbon quaternary stereocenter in good yields with up to greater than 20:1 d.r. and 98 % ee. The reaction has a broad substrate scope and tolerates a wide range of functional groups.

Reported herein is an asymmetric [3+2] cycloaddition reaction of azomethine ylides with β-trifluoromethyl β,β-disubstituted enones, a reaction which is enabled by a Ming-Phos-derived copper(I) catalyst (Ming-Phos=chiral sulfinamide monophosphines, Figure 2). This method provides scalable and efficient access to the highly substituted pyrrolidines with a trifluoromethylated, all-carbon quaternary stereocenter in good yields with up to greater than 20:1 d.r. and 98 % ee. The reaction has a broad substrate scope and tolerates a wide range of functional groups.

A highly enantioselective gold(I)-catalyzed intermolecular annulation of 2-(1-alkynyl)-2-alken-1-ones with N-allenamides is presented. The present work represents the first example of a gold-catalyzed annulation with the proximal CC bond of an N-allenamide, and is distinctly different from the previously observed annulations at the distal CC bond. Interestingly, both enantiomers of the products could be obtained in good yields with high regio-, diastereo-, and enantioselectivity by using either diastereomer of a binol-derived phosphoramidite as a chiral ligand.

The first rhodium-catalyzed intramolecular hetero-[5+2] cycloaddition reaction of vinyl aziridines and alkenes was realized, wherein both internal and terminal alkenes were applicable. With this method, a variety of unique substituted chiral fused bicyclic azepines, bearing multiple contiguous stereogenic centers, were facilely accessed in a straightforward, high-yielding, and highly stereoselective manner under mild reaction conditions. Notably, the E/Z geometry of the CC bonds in the vinyl aziridine-alkene substrates impact the cis/trans stereochemistry of the cycloadducts and up to six stereoisomers could be delivered.

A novel class of chiral sulfinamide phosphine catalysts (Xiao-Phos) are reported, which can be easily prepared from inexpensive commercially available starting materials. The Xiao-Phos catalysts showed good performance in enantioselective intramolecular Rauhut–Currier reactions, generating α-methylene-γ-butyrolactones in high yields with up to 99 % ee under mild conditions. Moreover, kinetic resolution and parallel kinetic resolution were also observed with the use of two different substituted racemic precursors.

A highly enantioselective gold(I)-catalyzed intermolecular annulation of 2-(1-alkynyl)-2-alken-1-ones with N-allenamides is presented. The present work represents the first example of a gold-catalyzed annulation with the proximal CC bond of an N-allenamide, and is distinctly different from the previously observed annulations at the distal CC bond. Interestingly, both enantiomers of the products could be obtained in good yields with high regio-, diastereo-, and enantioselectivity by using either diastereomer of a binol-derived phosphoramidite as a chiral ligand.

The first rhodium-catalyzed intramolecular hetero-[5+2] cycloaddition reaction of vinyl aziridines and alkenes was realized, wherein both internal and terminal alkenes were applicable. With this method, a variety of unique substituted chiral fused bicyclic azepines, bearing multiple contiguous stereogenic centers, were facilely accessed in a straightforward, high-yielding, and highly stereoselective manner under mild reaction conditions. Notably, the E/Z geometry of the CC bonds in the vinyl aziridine-alkene substrates impact the cis/trans stereochemistry of the cycloadducts and up to six stereoisomers could be delivered.

An unprecedented strategy for in situ generation of indole-based ortho-quinodimethanes (oQDMs) from 2-methyl-3-alkylmethylindoles by either a metal-free DDQ- or BQ-mediated dehydrogenative process was developed. These oQDMs were trapped by electron-deficient dienophiles to provide a facile approach to synthetically valuable tetrahydrocarbazoles, carbazoles, and hetereoacenes. The salient features of this transformation include direct C(sp³)H bond functionalizations, readily available starting materials, metal-free conditions, high efficiency, operational simplicity, and ease of scale-up.

A novel class of chiral sulfinamide phosphine catalysts (Xiao-Phos) are reported, which can be easily prepared from inexpensive commercially available starting materials. The Xiao-Phos catalysts showed good performance in enantioselective intramolecular Rauhut–Currier reactions, generating α-methylene-γ-butyrolactones in high yields with up to 99 % ee under mild conditions. Moreover, kinetic resolution and parallel kinetic resolution were also observed with the use of two different substituted racemic precursors.

An unprecedented strategy for in situ generation of indole-based ortho-quinodimethanes (oQDMs) from 2-methyl-3-alkylmethylindoles by either a metal-free DDQ- or BQ-mediated dehydrogenative process was developed. These oQDMs were trapped by electron-deficient dienophiles to provide a facile approach to synthetically valuable tetrahydrocarbazoles, carbazoles, and hetereoacenes. The salient features of this transformation include direct C(sp³)H bond functionalizations, readily available starting materials, metal-free conditions, high efficiency, operational simplicity, and ease of scale-up.

Cyclobutenones have been explored as a new type of chiral 1,4-dipole four-carbon synthon, which readily undergoes organophosphine-mediated CC bond cleavage and asymmetric intermolecular 1,4-dipolar spiroannulation with isatylidenemalononitrile in the presence of amino acid-derived chiral phosphine catalyst to furnish enantioenriched 3-spirocyclohexenone 2-oxindoles in good yield with up to 87 % ee. To our knowledge, this is the first example of asymmetric transformation of cyclobutenones and the phosphine-catalyzed asymmetric 1,4-dipolar cycloaddition consisting of CC bond activation is unprecedented.

A highly enantioselective oxidative cyclopropanation of 1,6-enynes catalyzed by cationic Au^I/chiral phophoramidite complexes is presented. The new method provides convenient access to densely functionalized bicyclo[3.1.0]hexanes bearing three contiguous quaternary and tertiary stereogenic centers with high enantioselectivity (up to e.r. 98:2). Control experiments suggest that the quinoline moiety of the β-gold vinyloxyquinolinium intermediate in the reaction plays an important role in promoting good enantioselectivity through a transitional auxiliary effect in the transition state.

A simple, new type of chiral sulfinamide monophosphines, the so-called Ming-Phos ligands, is reported; these ligands could be easily prepared from inexpensive and commercially available starting materials. The Ming-Phos ligands performed well in the enantioselective gold-catalyzed cycloaddition reaction of 2-(1-alkynyl)-alk-2-en-1-ones with nitrones. Both enantiomers of the products could be obtained in good yields and with excellent diastereo- and enantioselectivity through transformations that were catalyzed by gold complexes derived from two diastereomers of Ming-Phos ligand M5 (Ar=1-naphthyl).

A strategy for in situ generation of furan-based ortho-quinodimethanes (o-QDMs) by the gold(I)-mediated dehydrogenative heterocyclization of 2-(1-alkynyl)-2-alken-1-ones in the presence of pyridine N-oxide under mild reaction conditions was developed. These in situ furan-based o-QDMs were trapped by electron-deficient olefins and alkynes, thus furnishing various 2,3-furan-fused carbocycles in good yields with high diastereo- and regioselectivities.

This account describes our recent efforts devoted to gold chemistry since 2009. Based on furyl–Au 1,3-dipole analogues and related gold carbene intermediates, a rich variety of gold-catalyzed cascade reactions have been developed, which provide facile access to a diverse range of novel carbo- and heterocycles. In these reactions, the selectivity can be well controlled by the catalyst (ligand and metal), substrate or reagent. In addition, we have also developed the corresponding enantioselective variants, which are guided by bis(phosphinegold) complexes derived from axially chiral scaffolds and asymmetric gold/chiral Brønsted acid relay catalysis.

A strategy for in situ generation of furan-based ortho-quinodimethanes (o-QDMs) by the gold(I)-mediated dehydrogenative heterocyclization of 2-(1-alkynyl)-2-alken-1-ones in the presence of pyridine N-oxide under mild reaction conditions was developed. These in situ furan-based o-QDMs were trapped by electron-deficient olefins and alkynes, thus furnishing various 2,3-furan-fused carbocycles in good yields with high diastereo- and regioselectivities.

A highly enantioselective oxidative cyclopropanation of 1,6-enynes catalyzed by cationic Au^I/chiral phophoramidite complexes is presented. The new method provides convenient access to densely functionalized bicyclo[3.1.0]hexanes bearing three contiguous quaternary and tertiary stereogenic centers with high enantioselectivity (up to e.r. 98:2). Control experiments suggest that the quinoline moiety of the β-gold vinyloxyquinolinium intermediate in the reaction plays an important role in promoting good enantioselectivity through a transitional auxiliary effect in the transition state.

A simple, new type of chiral sulfinamide monophosphines, the so-called Ming-Phos ligands, is reported; these ligands could be easily prepared from inexpensive and commercially available starting materials. The Ming-Phos ligands performed well in the enantioselective gold-catalyzed cycloaddition reaction of 2-(1-alkynyl)-alk-2-en-1-ones with nitrones. Both enantiomers of the products could be obtained in good yields and with excellent diastereo- and enantioselectivity through transformations that were catalyzed by gold complexes derived from two diastereomers of Ming-Phos ligand M5 (Ar=1-naphthyl).

Angularly fused carbocyclic frameworks and their heteroatom-substituted analogues exist in many natural products that display a broad and interesting range of biological activities. Preparation of polycyclic products by cycloaddition reactions have been the long-standing hot topic in the synthetic community. Dehydro-Diels–Alder (DDA) reactions are one class of dehydropericyclic reactions that are derived conceptually by systematic removal of hydrogen atom pairs. A base-promoted tandem Michael addition and DDA reaction of α,α-dicyanoolefins with electron-deficient 1,3-conjugated enynes was realized in which a DDA reaction takes place between the arylalkynes and electron-deficient tetrasubstituted olefin. The control experiments support the stepwise anionic reaction pathway rather than the concerted reaction pathway.

Nickel perchlorate catalyzed sequential Meinwald rearrangement of a terminal oxirane through C–O bond cleavage, and cycloaddition with a donor–acceptor oxirane through C–C bond cleavage was developed. The same catalyst mediates the ring opening of the oxirane moiety in two different ways. This method provides an alternative route to synthesize highly substituted 1,3-dioxolanes by using oxiranes instead of aldehydes.

Herein we describe the unexpected borylation of aryl iodides promoted by a Cs₂CO₃ and MeOH system. This formal nucleophilic boryl substitution could be applied to a wide range of functionalized aryl iodide compounds. The preliminary results indicate that this process is neither copper catalyzed nor radical mediated.

A formal gold(I)-catalyzed diastereoselective [4+3] cycloaddition of oxime ethers with imines, consisting of 1,2-alkyl migration under mild reaction conditions, has been developed; this provides facile access to a series of highly substituted trans-pyrrolo[3,4-c]azepines. This reaction is easily scaled up to the gram scale.

An approach to highly functionalized donor-acceptor aziridines by the aziridination of N-tosylimines and 2-bromomalonates in the presence of sodium hydride under mild conditions has been developed. The high-yielding reaction has a relatively broad scope and can be easily scaled up to the gram level.

An efficient approach for the synthesis of highly substituted pyrrolo[3,4-d][1,2]oxazepines has been achieved by gold(I)-catalyzed 1,3-dipolar cycloaddition reactions of 1-(1-alkynyl)cyclopropyl oximes with nitrones in good to excellent yields as a single diastereomer. A complete chirality transfer was observed in this transformation.

A series of 2-alkynyl carbonyl compounds that contain a cyclopentene ring or a heterocycle can be transformed into various fused dihydrobenzofurans and tetrahydrofuro[2,3-c]pyridines by means of a 1,2-alkyl migration process. Both of these reactions proceed with excellent regioselectivity and stereospecificity when using a cationic gold(I) catalyst. Treatment of 4-styrylcyclopent-1-enecarboxylates under different conditions affords a range of highly functionalized dihydrobenzofurans and dihydroisobenzofurans. A divergence in product selectivity, which depends on the anion of the silver salts used, was observed. Interestingly, ring-fused tetrahydroquinolines undergo only 1,2′-alkyl migration reaction by means of a CC cleavage/cyclization sequence to provide tetrahydroazepine derivatives. Mechanistic studies suggest that the gold complexes catalyze 1,2-alkyl migration reactions through a concerted reaction pathway and 1,2′-alkyl migration reactions through a stepwise reaction pathway.

A novel palladium(II)-catalyzed oxidative domino reaction sequence of diyne-enones and substituted indoles to afford polyheterocycles involving direct CH functionalization using air as oxidant has been developed.

Highly substituted 1,2-allenyl ketones can be easily and efficiently prepared from organometallic reagents and readily available 2-(1-alkynyl)-2-alken-1-ones. The synthetic application of 1,2-allenyl ketone products was also showcased by palladium-catalyzed further transformation.

This paper describes an efficient scandium triflate- and triflic acid-catalyzed benzannulation of 2-(2-alkynylarylidene)-1,3-dicarbonyl compounds to afford polysubstituted naphthalenes and benzo[a]fluorenols. The product selectivity could be tuned by subtle choice of the catalyst. An unprecedented process between alkynes and ketones is also explored.

A dramatic substituent effect was found in the silver(I)-catalyzed cyclization reaction of ortho-alkynylaryl aldehyde oxime derivatives. When R is an alkyl group, the Ag(I)-catalyzed reaction in dimethylacetamide at 110 °C (conditions A) affords isoquinolines in good to excellent yields, in contrast, isoquinolin-1(2 H)-ones were produced in moderate to high yields under conditions B (dimethylformamide, room temperature) when R is an acetyl group. A plausible mechanism was proposed for this product selectivity control reaction (PSCR) by subtle structure modification.

This paper describes a base-catalyzed domino reaction of electron-deficient enynes with malonate-derived α,β-unsaturated esters and ketones, which provides a rapid, stereoselective access to multi-functionalized cyclopentanes and inden-5(6 H)-ones in high yields. In this reaction, the alkynyl group of the enyne acts as an activating group rather than a reacting group.