Unusual Formal [1+4] Annulation through Tandem P(NMe2)3‐Mediated Cyclopropanation/Base‐Catalyzed Cyclopropane Rearrangement: Facile Syntheses of Cyclopentenimines and Cyclopentenones

An unusual formal [1+4] annulation of α‐dicarbonyl compounds with 1,1‐dicyano‐1,3‐dienes has been realized, leading to facile syntheses of cyclopentenimines and cyclopentenones in a unique manner. Mechanistic investigation implies that this reaction takes place through a P(NMe₂)₃‐mediated cyclopropanation followed by a base‐catalyzed cyclopropane rearrangement. It therefore represents an unprecedented [1+4] annulation mode involving Kukhtin–Ramirez adducts.     

Iron(0)-Mediated Stereoselective (3+2)-Cycloaddition of Thiochalcones via a Diradical Intermediate

Reactions of α,β-unsaturated aromatic thioketones 1 (thiochalcones) with Fe₃(CO)₁₂ leading to η⁴-1-thia-1,3-diene iron tricarbonyl complexes 2 , [FeFe] hydrogenase mimics 3 , and the thiopyrane adduct 4 are described. Obtained products have been characterized by X-ray crystallography and by computational methods. Completely regio- and diastereoselective formation of the five-membered ring system in products 3 , containing four stereogenic centers, can be explained by an unprecedented, stepwise (3+2)-cycloaddition of two thiochalcone molecules mediated by Fe₃(CO)₁₂. Quantum chemical calculations aimed at elucidation of the reaction mechanism, suggest that the formal (3+2)-cycloaddition proceeds via sequential intramolecular radical transfer events upon homolytic cleavage of one carbon-sulfur bond leading to a diradical intermediate.     

Formal Asymmetric (4+1) Annulation Reaction between Sulfur Ylides and 1,3‐Dienes

A highly enantioselective synthesis of functionalized cyclopentanoids by a formal asymmetric (4+1) annulation strategy was developed. The methodology consists of a stereoselective cyclopropanation reaction between chiral sulfur ylides and 1,3‐dienes followed by a, in situ, stereospecific MgI₂‐catalyzed rearrangement of vinylcyclopropanes. This method is distinguished by a remarkable compatibility with functional groups and a high stereocontrol.     

A Precision Ethylene‐Styrene Copolymer with High Styrene Content from Ring‐Opening Metathesis Polymerization of 4‐Phenylcyclopentene

Ring‐opening metathesis polymerization of 4‐phenylcyclopentene is investigated for the first time under various conditions. Thermodynamic analysis reveals a polymerization enthalpy and entropy sufficient for high molar mass and conversions at lower temperatures. In one example, neat polymerization using Hoveyda–Grubbs second generation catalyst at −15 °C yields 81% conversion to poly(4‐phenylcyclopentene) (P4PCP) with a number average molar mass of 151 kg mol⁻¹ and dispersity of 1.77. Quantitative homogeneous hydrogenation of P4PCP results in a precision ethylene‐styrene copolymer (H₂‐P4PCP) with a phenyl branch at every fifth carbon along the backbone. This equates to a perfectly alternating trimethylene‐styrene sequence with 71.2% w/w styrene content that is inaccessible through molecular catalyst copolymerization strategies. Differential scanning calorimetry confirms P4PCP and H₂‐P4PCP are amorphous materials with similar glass transition temperatures (T_g) of 17 ± 2 °C. Both materials present well‐defined styrenic analogs for application in specialty materials or composites where lower softening temperatures may be desired.

     

Intramolecular carbolithiation of 2,6-dilithio-1,6-heptadienes: an experimental and theoretical study

2,6-Dilithio-1,6-heptadienes 3 undergo intramolecular carbolithiation in Et(2)O/N,N,N',N'-tetramethylethylenediamine (TMEDA) at the lithiated double bonds to afford 1,2-bis(lithiomethyl)cyclopentenes 5. Reaction of these dianions with electrophiles affords a number of 1,2-difunctionalized cyclopentene derivatives 7-10. The ease of carbolithiation of 2,6-dilithio-1,6-heptadiene (3a) compared to that of 2-lithio-1,6-heptadiene (14) has been studied experimentally. A series of ab initio molecular-orbital calculations on the course of the reaction were carried out and the results were compared to those for the corresponding intramolecular carbolithiation of an isolated double bond. The Li-C interactions found in the transition state by this theoretical study support a carbolithiation pathway for the cyclization of 2,6-dilithio-1,6-heptadienes.     

Dynamic Kinetic Asymmetric Transformation (DYKAT) by Combined Amine‐ and Transition‐Metal‐Catalyzed Enantioselective Cycloisomerization

The first examples of one‐pot highly chemo‐ and enantioselective dynamic kinetic asymmetric transformations (DYKATs) involving α,β‐unsaturated aldehydes and propargylated carbon acids are presented. These DYKATs, which proceed by a combination of catalytic iminium activation, enamine activation, and Pd⁰‐catalyzed enyne cycloisomerization, give access to functionalized cyclopentenes with up to 99 % ee and can be used for the generation of all‐carbon quaternary stereocenters.     

Facile synthesis of alkylidene cyclopentenes via palladium catalyzed ring opening of fulvene derived bicyclic hydrazines

A facile synthesis of substituted alkylidene cyclopentenes through a palladium catalyzed ring opening of fulvene derived bicyclic hydrazines with various organometallic reagents is described. The products are versatile synthons with multiple points for functionalization and can be used in the synthesis of a number of biologically active molecules.