The [4+2], not [2+2], Mechanism Occurs in the Gold-Catalyzed Intramolecular Oxygen Transfer Reaction of 2-Alkynyl-1,5-Diketones

Force-reversible chemical reaction at ambient temperature for designing toughened dynamic covalent polymer networks

Force-reversible C-N bonds, resulting from the click chemistry reaction between triazolinedione (TAD) and indole derivatives, offer exciting opportunities for molecular-level engineering to design materials that respond to mechanical loads. Here, we displayed that TAD-indole adducts, acting as crosslink points in dry-state covalently crosslinked polymers, enable materials to display reversible stress-responsiveness in real time already at ambient temperature. Whereas the exergonic TAD-indole reaction results in the formation of bench-stable adducts, they were shown to dissociate at ambient temperature when embedded in a polymer network and subjected to a stretching force to recover the original products. Moreover, the nascent TAD moiety can spontaneously and immediately be recombined after dissociation with an indole reaction partners at ambient temperature, thus allowing for the adjustment of the polymer segment conformation and the maintenance of the network integrity by force-reversible behaviors. Overall, our strategy represents a general method to create toughened covalently crosslinked polymer materials with simultaneous enhancement of mechanical strength and ductility, which is quite challenging to achieve by conventional chemical methods.

Weak force-activated covalent bonds as crosslink points can increase mechanical strength and ductility in polymers but the bonds, once broken, cannot be reformed in real time under ambient conditions leading to irreversible damage. Here, the authors demonstrate that triazolinedione (TAD)-indole adducts acting as crosslink points enable materials to display already at ambient temperature reversible stress-responsiveness in real time.

Brønsted acid promoted substrate-dependent regiodivergent alkynylcyclopropane–cyclopentadiene rearrangement assisted by the internal carbonyl group

Facile Brønsted acid promoted alkynylcyclopropane–cyclopentadiene rearrangement, avoiding the five-membered cycloallene intermediate assisted by the internal carbonyl groups, has been developed.

Revealing the Mechanism and Origin of Reactivity of Au(I)-Catalyzed Functionalized Indenone Formation of Cyclic and Acyclic Acetals of Alkynylaldehydes

A density functional theory study is presented here to offer mechanistic insights and explications of experimentally intriguing observations in the Au(I)-catalyzed cyclization of cyclic and acyclic acetals of alkynylaldehydes that leads to indenone formation. The reactivity of catalytic cycles with and without methoxy migration is clearly defined when the alkyne terminus is phenylated. The reaction mechanism of indenone formation proceeds first with the coordination of Au(I) to alkyne to initiate the reaction with 1,5-H shift as a rate-determining step (RDS), and the fastest 1,5-H shift is achieved when one phenyl ring carries an electron-donating group and the other one is substituted with an electron-withdrawing group. Following the 1,5-H shift, the reaction undergoes feasible steps that are cyclization and 1,2-H shift before elimination to persist the iterative cycle, but the reactivity of both steps is highly affected by the existence of the phenyl group on the alkyne terminus. The unreactivity of the alkyne terminus not bearing a phenyl ring is because the cyclization is thermodynamically disfavorable, subsequently deactivating the 1,2-H shift kinetically and thermodynamically. The absence of a tether in the acetal unit considerably outpaces any 1,5-H shift and instead activates 1,5-methoxy migration, giving methoxy-migrated indenone, with the 1,2-OMe shift being an RDS.

(Radio)fluoroclick Reaction Enabled by a Hydrogen-Bonding Cluster

We have developed a widely applicable nucleophilic (radio)fluoroclick reaction of ynamides with readily available and easy-to-handle KF(18 F). The reactions exhibited high functional-group tolerance and needed only an ambient atmosphere. This 18 F addition to C-C unsaturated bonds proceeded with extraordinarily high radiochemical yields.

Gold-catalyzed tandem cycloisomerization/Petasis–Ferrier rearrangement: a direct route to 3-alkoxyindanones from enynals and alcohols

A method to prepare 3-alkoxyindanones by gold-catalyzed tandem cycloisomerization/Petasis–Ferrier rearrangement of ortho-ethynylarylaldehydes with alcohols is described. The reaction also offers a synthetic route to 3-alkoxylcyclopentenones.

O-Transfer-facilitated cyclizations of propargylamides with TMSN3: selective synthesis of tetrazoles and dihydroimidazoles

Herein, we disclose the O-transfer-facilitated cyclizations of propargylamides with TMSN₃ to selectively afford functionalized tetrazoles and diiodomethylated dihydroimidazoles.

1,2-boryl migration empowers regiodivergent synthesis of borylated furans

A regioselective transition metal-catalyzed cycloisomerization reaction of boron-containing alkynyl epoxides toward C2- and C3-borylated furans has been developed. It was found that the copper catalyst as well as the gold catalyst with more basic triflate counterion favor boryl migration toward C3-borylated furans, whereas employment of the cationic gold hexafluoroantimonate affords C2-borylated furan via a formal 1,2-hydrogen shift.

Nucleophile-dependent regio- and stereoselective ring opening of 1-azoniabicyclo[3.1.0]hexane tosylate

1-[(1R)-(1-Phenylethyl)]-1-azoniabicyclo[3.1.0]hexane tosylate was generated as a stable bicyclic aziridinium salt from the corresponding 2-(3-hydroxypropyl)aziridine upon reaction with p-toluenesulfonyl anhydride. This bicyclic aziridinium ion was then treated with various nucleophiles including halides, azide, acetate, and cyanide in CH3CN to afford either piperidines or pyrrolidines through regio- and stereoselective ring opening, mediated by the characteristics of the applied nucleophile. On the basis of DFT calculations, ring-opening reactions under thermodynamic control yield piperidines, whereas reactions under kinetic control can yield both piperidines and pyrrolidines depending on the activation energies for both pathways.

Ligand design in gold catalysis and chemistry of gold-oxonium intermediates

Gold catalysis is considered one of the most important breakthroughs in organic synthesis during the last decade. Many gold-catalyzed reactions suffer from high catalyst loading, which is a serious limitation on the application of gold catalysis in larger scale synthesis. Because ligands play a major role in the tuning of reactivity of transition metal catalysts, there has been an increased effort on rationally understanding ligand effects in gold catalysis and using a rational ligand design to achieve higher efficiency. In the first part of this chapter, selected examples of ligand design in gold catalysis are discussed. In the second part, the chemistry of gold-oxonium intermediate is examined. An oxonium intermediate is generated when a oxygen nucleophile (ketone, aldehyde, or ether) attacks a cationic gold-activated multiple bond. This oxonium intermediate, being a highly energetic species, has the potential to undergo further transformations such as electron transfer/rearrangement/protodeauration to form diverse products.

Cationic Ring-Opening Polymerization of 2-Propyl-2-oxazolines: Understanding Structural Effects on Polymerization Behavior Based on Molecular Modeling

The surprising difference in the cationic ring-opening polymerization rate of 2-cyclopropyl-2-oxazoline versus 2-n-propyl-2-oxazoline and 2-isopropyl-2-oxazoline was investigated both experimentally and theoretically. The polymerization kinetics of all three oxazolines were experimentally measured in acetonitrile at 140 °C, and the polymerization rate constant (k_p) was found to decrease in the order c-PropOx > n-PropOx > i-PropOx. Theoretical free energy calculations confirmed the trend for k_p, and a set of DFT-based reactivity descriptors, electrostatics, and frontier molecular orbitals were studied to detect the factors controlling this peculiar behavior. Our results show that the observed reactivity is dictated by electrostatic effects. More in particular, the charge on the nitrogen atom of the monomer, used to measure its nucleophilicity, was the most negative for c-PropOx. Furthermore, the electrophilicity of the cations does not change substantially, and thus, the nucleophilicity of the monomers is the driving factor for k_p.

Solvent-catalyzed ring-chain-ring tautomerization in axially chiral compounds

The mechanism of ring-chain-ring tautomerization and the prominent effect of the solvent environment have been computationally investigated in an effort to explain the enantiomeric interconversion observed in 2-oxazolidinone derivatives, heterocyclic analogues of biphenyl atropisomers, which were isolated as single stable enantiomers and have the potential to be used as axially chiral catalysts. This study has shed light on the identity of the intermediate species involved in the ring-chain-ring tautomerization process as well as the catalytic effect of polar protic solvents. These mechanistic details will prove very useful in predicting and understanding ring-chain tautomeric equilibria in similar heterocyclic systems and will further enable experimentalists to devise appropriate experimental conditions in which axially chiral catalysts remain stable as single enantiomers.

Construction of cyclic enones via gold-catalyzed oxygen transfer reactions

During the last decade, gold-catalyzed reactions have become a tour de force in organic synthesis. Recently, the gold-, Brønsted acid- or Lewis acid-catalyzed oxygen transfer from carbonyl to carbon–carbon triple bond, the so-called alkyne–carbonyl metathesis, has attracted much attention because this atom economical transformation generates α,β-unsaturated carbonyl derivatives which are of great interest in synthetic organic chemistry. This mini-review focuses on the most recent achievements on gold-catalyzed oxygen transfer reactions of tethered alkynones, diynes or alkynyl epoxides to cyclic enones. The corresponding mechanisms for the transformations are also discussed.