The Aryne Ene Reaction

Strain-Release-Driven Alder-Ene Reaction of Bicyclo[1.1.0]butanes with β-Fluoroalkyl-α,β-Unsaturated Ketones

The Alder-ene reaction of the C-C bond in bicyclo[1.1.0]butanes would provide a unique and efficient synthesis route for cyclobutene frameworks. Herein, we report a regio- and diastereoselective Alder-ene reaction of bicyclo[1.1.0]butanes with β-fluoroalkyl-α,β-unsaturated ketones, giving a wide variety of cyclobutenes with two contiguous centers and diene products. The reaction features atoms and step economies and exhibits broad substrate scope. Several downstream transformations of these cyclobutenes were performed.

Catalytic asymmetric construction of 1,5-remote Si- and C-stereocenters via desymmetrizing ene reaction of bis(methallyl)silanes

The catalytic enantioselective synthesis of chiral silanes has long been a challenging pursuit. Achieving simultaneous construction of remote Si- and C-stereogenic centers in an acyclic molecule via desymmetrization is particularly difficult. Herein, we realized an example of a chiral nickel(ii) complex-catalyzed desymmetrizing carbonyl-ene reaction of bis(methallyl)silanes with α-keto aldehyde monohydrates, enabling the highly chemo-, diastereo- and enantioselective synthesis of chiral δ-hydroxy silanes featuring 1,5-remote Si- and C-stereocenters. This protocol demonstrated good functional group tolerance and a broad substrate scope. A bioactivity study revealed its potential applications in the synthesis of bioactive molecules.

Aryne Alder-Ene Reaction Enables Arylation of Conformationally Locked Styrenes

We report a transition metal-free aryne Alder-ene (AAE) reaction of conformationally locked styrenes like methylene indane (5,6-fusion) and methylene tetralin (6,6-fusion) to provide aryl-decorated indenes and dihydronaphthalenes. DFT studies compared the transition states between 5,6- and 6,6-fused systems. The synthetic utility of the 5,6- and 6,6-fused products was demonstrated.

Selectivity in the Formal [2 + 2 + 2] Cycloaromatization of Enyne-Allenes Generated by the Alder-ene Reaction from Triynes

1,3-Diynyl propiolates undergo the Alder-ene reaction to generate enyne-allenes, which participate in the Diels-Alder reaction to provide products of a formal [2 + 2 + 2] cycloaromatization of three alkynes. Without an external alkyne, enyne-allene reacts with one of the alkyne moieties of 1,3-diynyl propiolate, whereas external alkynes can be used to trap enyne-allene to provide various arene products. The substituents on the dienophilic alkynes have a profound impact on their reactivity. In this Diels-Alder reaction, 1,3-diynes display higher reactivity than monoynes; thus, an excess amount (4-5 equiv) of external monoynes needs to be employed to get good product selectivity.

3-Halo-5,6-dihydro-4H-1,2-oxazine N-oxides as synthetic equivalents of unsaturated nitrile oxides in the [3 + 2]-cycloaddition with arynes: synthesis of substituted 3-vinyl-1,2-benzisoxazoles

The reaction of 3-halo-5,6-dihydro-4H-1,2-oxazine N-oxides with arynes was studied. Arynes were generated from o-silylaryl triflates and underwent consecutive [3 + 2]-cycloaddition/[4 + 2]-cycloreversion with N-oxides leading to substituted 3-vinyl-benzisoxazoles in high yields. In the presented sequence, 1,2-oxazine N-oxides act as surrogates of rarely employed unsaturated nitrile oxides. A broad substrate scope was demonstrated. The influence of the substitution pattern of an aryne on the reaction outcome was determined. In the presence of bulky substituents, polycyclic 4,4a-dihydro-3H-benzofuro[3,2-c][1,2]oxazines were selectively formed. Mechanistic schemes for the observed reaction pathways were proposed. The synthetic utility of the products was demonstrated by their follow-up modifications.

Pseudocyclic Arylbenziodoxaboroles as Water-Triggered Aryne Precursors in Reactions with Organic Sulfides

Pseudocyclic arylbenziodoxaboroles are unique aryne precursors under neutral aqueous conditions that selectively react with organic sulfides, forming the corresponding sulfonium salts. This reaction is compatible with various substituents (alkyl, halogen, CN, NO2, CHO, and cyclopropyl) in the aromatic ring or alkyl group of the sulfide. Similar reactions of sulfoxides afford o-hydroxy-substituted sulfonium salts. The structures of key products were confirmed by X-ray analysis.

Stereoselective Alder-Ene Reactions of Bicyclo[1.1.0]butanes: Facile Synthesis of Cyclopropyl- and Aryl-Substituted Cyclobutenes

Bicyclo[1.1.0]butanes (BCBs), strained carbocycles comprising two fused cyclopropane rings, have become well-established building blocks in organic synthesis, medicinal chemistry, and chemical biology due to their diverse reactivity profile with radicals, nucleophiles, cations, and carbenes. The constraints of the bicyclic ring system confer high p-character on the interbridgehead C-C bond, leading to this broad reaction profile; however, the use of BCBs in pericyclic processes has to date been largely overlooked in favor of such stepwise, non-concerted additions. Here, we describe the use of BCBs as substrates for ene-like reactions with strained alkenes and alkynes, which give rise to cyclobutenes decorated with highly substituted cyclopropanes and arenes. The former products are obtained from highly stereoselective reactions with cyclopropenes, generated in situ from vinyl diazoacetates under blue light irradiation (440 nm). Cyclobutenes featuring a quaternary aryl-bearing carbon atom are prepared from equivalent reactions with arynes, which proceed in high yields under mild conditions. Mechanistic studies highlight the importance of electronic effects in this chemistry, while computational investigations support a concerted pathway and rationalize the excellent stereoselectivity of reactions with cyclopropenes.

Asymmetric reactions involving aryne intermediates

Although arynes are usually considered fleeting intermediates, they are highly valuable synthons because they enable the introduction of aromatic rings and the simultaneous formation of new bonds at two sites. Although catalytic reactions using transition metals are excellent method for constructing complex polycyclic aromatic molecules in a single step, the use of asymmetric catalysis for the capture of arynes remains a crucial goal for the progress of aryne chemistry. Catalytic asymmetric reactions of arenes are challenging, requiring sufficient interactions between the neutral and highly reactive short-lived aryne intermediates in a stereo-controlled fashion. In addition, spontaneous decomposition, as well as side reactions, has hindered their development and, until recently, highly enantioselective reactions using arynes had remained elusive. This Review highlights asymmetric reactions using arynes, featuring diastereoselective, enantioselective and catalytic enantioselective reactions.

Synthesis of o-Sulfinylanilines from N-Alkyl Sulfoximines and Arynes

N-Alkyl sulfoximines react with arynes generated in situ under mild conditions providing o-sulfinylanilines in good yields. The transformation is characterized by a broad substrate scope and a good functional group tolerance. The structure of a reaction product was confirmed by single-crystal X-ray diffraction.

Mechanism and the Origins of Periselectivity in Cycloaddition Reactions of Benzyne with Dienes

Density functional theory calculations have been used to explore the reaction mechanism of (4 + 2) and (2 + 2) cycloadditions of benzyne with classical dienes. The results indicate the following: (1) (4 + 2) products arise via concerted pathways, (2) (2 + 2) products arise via stepwise pathways with diradical intermediates, and (3) these diradical intermediates are formed via isomerization of carbene intermediates. The origins of periselectivity in these reactions are analyzed using distortion/interaction analysis for the key steps, and they indicate that the tiny distortion in the very early [4 + 2] transition structure, coupled with an entropic favorability, controls selective (4 + 2) cycloaddition.