Pengquan Chen

Transition-Metal-Free Cyclopropanation of 2-Aminoacrylates with N-Tosylhydrazones: A General Route to Cyclopropane α-Amino Acid with Contiguous Quaternary Carbon Centers

Cyclopropanation of 2-aminoacrylates with N-tosylhydrazones could proceed smoothly under transition-metal-free conditions via a [3 + 2] cycloaddition process. This robust protocol exhibits excellent generality, delivering a wide spectrum of cyclopropane α-amino acid esters bearing contiguous quaternary carbon centers in high yields and diastereoselectivities. With these readily available products, the steric convergence of cyclopropane α-amino acids could be readily obtained.

Transition-Metal-Free Diastereoselective Epoxidation of Trifluoromethylketones with N-Tosylhydrazones: Access to Tetrasubstituted Trifluoromethylated Oxiranes

The first example of the epoxidation reaction of trifluoromethylketones with N-tosylhydrazones under transition-metal free conditions is reported. This epoxidation reaction provided tetrasubstituted trifluoromethylated oxiranes with excellent yields and diastereoselectivities. The salient features of this reaction include readily available starting materials, mild conditions, broad substrate scope, high efficiency, and valuable further applications. Remarkably, this reaction proceeded through an unprecedented nucleophilic addition process, and the ammonium O-anion intermediate was detected and characterized by NMR and HRMS analysis.

Nucleo-Palladation-Triggering Alkene Functionalization: A Route to γ-Lactones

An unprecedented strategy for the highly effective synthesis of γ-lactones from homoallylic alcohols was achieved by palladium catalysis in one step. The protocol affords aryl, alkyl, and spiro γ-lactones directly from readily available homoallylic alcohols in good yields with excellent functional group tolerance and high chemoselectivity under mild conditions.

MnO2-Promoted Oxidative Radical Sulfonylation of Haloalkynes with Sulfonyl Hydrazides: C(sp)–S Bond Formation towards Alkynyl Sulfones

A catalyst-free oxidative radical sulfonylation of haloalkynes with sulfonyl hydrazides is reported. It represents an example of C(sp)-S bond formation using sulfonyl hydrazides as sulfonyl radical sources. Various alkynyl sulfones were synthesized in moderate to good yields. Having MnO2 as the oxidant is very critical for this transformation. Remarkably, the self-coupling reaction of haloalkynes through C(sp)-C(sp) bond formation is significantly inhibited under the standard reaction conditions.

Transition-Metal-Free [3+2] Cycloaddition of Dehydroaminophosphonates and N-Tosylhydrazones: Access to Aminocyclopropanephosphonates with Adjacent Quaternary-Tetrasubstituted Carbon Centers

A novel one-step strategy for the synthesis of aminocyclopropanephosphonates containing adjacent quaternary-tetrasubstituted carbon centers under transition-metal-free catalysis via [3+2] cycloaddition process has been developed. A series of aminocyclopropanephosphonates with adjacent quaternary-tetrasubstituted carbon centers including spirocyclopropyl adducts were obtained in moderate to excellent yields under mild reaction conditions. This protocol would find the potential applications in biochemistry and medicinal chemistry.

Conversion of Triple Bonds into Single Bonds in a Domino Carbopalladation with Norbornene

A domino carbopalladation reaction of haloalkynes is presented. Remarkably, the four-time carbopalladation process converts the carbon-carbon triple bonds of haloalkynes stepwise into carbon-carbon double bonds, and finally to carbon-carbon single bonds. Features of this reaction are that the carbon-carbon double bonds of stable vinyl palladium intermediates are transformed into carbon-carbon single bonds with the generation of unstable alkyl palladium intermediates. The subsequently formed π-allylpalladium species are independently trapped by N-tosylhydrazones, boronic acids, and B2 pin2 in a highly diastereoselective manner, delivering the corresponding polycyclic and twisted products with a bicyclo[3.2.1]oct-2-en-3-yl)tricyclo[3.2.1.02,4 ]octane core skeleton in moderate to good yields via C-C and C-B bond formations. Significantly, the dual roles of norbornenes, ring construction and ring expansion, and the identification of electron-rich tri(2-furyl)phosphine as the ligand are found to be critical for the success of these transformations.