Stereoselective Rhodium-Catalysed [2+2+2] Cycloaddition of Linear Allene-Ene/Yne-Allene Substrates: Reactivity and Theoretical Mechanistic Studies

Rh(I) Complexes with Hemilabile Thioether-Functionalized NHC Ligands as Catalysts for [2 + 2 + 2] Cycloaddition of 1,5-Bisallenes and Alkynes

The [2 + 2 + 2] cycloaddition of 1,5-bisallenes and alkynes under the catalysis of Rh(I) with hemilabile thioether-functionalized N-heterocyclic carbene ligands is described. This protocol effectively provides an entry to different trans-5,6-fused bicyclic systems with two exocyclic double bonds in the cyclohexene ring. The process is totally chemoselective with the two internal double bonds of the 1,5-bisallenes being involved in the cycloaddition. The complete mechanism of this transformation as well as the preference for the trans-fusion over the cis-fusion has been rationalized by density functional theory calculations. The reaction follows a typical [2 + 2 + 2] cycloaddition mechanism. The oxidative addition takes place between the alkyne and one of the allenes and it is when the second allene is inserted into the rhodacyclopentene that the trans-fusion is generated. Remarkably, the hemilabile character of the sulfur atom in the N-heterocyclic carbene ligand modulates the electron density in key intermediates, facilitating the overall transformation.

Highly Selective Synthesis of Seven-Membered Azaspiro Compounds by a Rh(I)-Catalyzed Cycloisomerization/Diels-Alder Cascade of 1,5-Bisallenes

The synthesis of spiro compounds featuring seven- and six-membered rings in the spirobicyclic motif is successfully achieved through a cascade process encompassing a rhodium(I)-catalyzed cycloisomerization followed by a highly selective Diels–Alder homodimerization. The scope of the reaction is analyzed based on a series of synthetic substrates, and control experiments and DFT calculations led us to justify the exquisite degree of selectivity observed.

Intramolecular rhodium-catalysed [2 + 2 + 2] cycloaddition of linear chiral N-bridged triynes: straightforward access to fused tetrahydroisoquinoline core

A route for the preparation of merged symmetrical tetrahydroisoquinolines with central chirality through a rhodium-catalyzed intramolecular [2 + 2 + 2] cycloaddition involving enantiopure triynes as substrates is described. The results show that linear triynes lacking a 3-atom tether can undergo efficient cyclisation. The N-tethered 1,7,13-triynes used in our approach were easily prepared from readily accessible chiral homopropargyl amides, the basic building blocks in our approach, which were efficiently obtained by diastereoselective addition of propargyl magnesium bromide to Ellman imines. Additional substitution at the benzene rings could be attained when substituted triynes at the terminal triple bonds were employed, giving access to more complex tetrahydroisoquinolines after the rhodium-catalyzed intramolecular [2 + 2 + 2] cycloaddition. Among the different transition-metal catalysts, the Wilkinson complex (RhCl(PPh₃)₃) afforded higher yields in the cyclisation of linear triynes; however, triynes bearing a Br substituent at the terminal positions underwent the cyclisation more efficiently in the presence of [RhCl(CO)₂]₂.

The Choice of Rhodium Catalysts in [2+2+2] Cycloaddition Reaction: A Personal Account

[2+2+2] Cycloaddition reaction is a captivating process that assembles six-membered rings from three unsaturations with complete atom economy. Of the multiple transition metals that can be used to catalyze this reaction, rhodium offers many advantages. These include high activity and versatility, but especially the ability to easily tune the reactivity and selectivity by the modification of the ligands around the metal. In this personal account, we summarize our endeavours in the development of efficient and sustainable [2+2+2] cycloaddition reactions to prepare products of interest, develop conditions in which the catalyst can be recovered and reused, and understand the mechanistic details that govern the selectivity of the processes.

Mechanism of Rhodium(III)-Catalyzed C-H Activation/Annulation of Aromatic Amide with α-Allenol: A Computational Study

With the help of DFT calculations, the reaction mechanisms of the rhodium(III)-catalyzed C-H activation/annulation between aromatic amide and α-allenol leading to the formation of isoindolinone have been theoretically investigated. Our calculated results show that the catalytic cycle consists of four stages: N-H deprotonation and C-H activation (Stage I), allene insertion, rearrangement and isomerization (Stage II), β-H elimination and enol-keto tautomerism (Stage III), and catalyst regeneration resulting in the five-membered ring product (Stage IV). For stage IV, besides the reaction paths proposed by the experimentalists, i.e., the insertion and reductive elimination (labeled as path a) and the reductive elimination and hydroamination (labeled as path b), an alternative path which involves C-N and C-H reductive eliminations (labeled as path c) was proposed and examined. The computational results show that the newly established path c is more energetically favorable than the reaction paths proposed by the experimentalists (paths a and b). The allene (non-terminal double bond) insertion step contributes to the rate-determining step with an overall activation free energy of 24.6 kcal/mol. Our study is beneficial for a better comprehension of the reaction mechanisms and provides a significant suggestion for further development of similar reactions.

A Computational Study of the Intermolecular [2+2+2] Cycloaddition of Acetylene and C60 Catalyzed by Wilkinson's Catalyst

The functionalization of fullerenes helps to modulate their electronic and physicochemical properties, generating fullerene derivatives with promising features for practical applications. Herein, DFT is used to explore the attachment of a cyclohexadiene ring to C₆₀ through a rhodium-catalyzed intermolecular [2+2+2] cycloaddition of C₆₀ and acetylene. All potential reaction paths are analyzed and it can be concluded that the [2+2+2] cycloaddition of C₆₀ and two acetylene molecules catalyzed by [RhCl(PPh₃ )₃ ], yielding a cyclohexadiene ring fused to a [6,6] bond of C₆₀ , is energetically feasible.

Computational Studies on Reaction Mechanism and Origins of Selectivities in Nickel-Catalyzed (2 + 2 + 2) Cycloadditions and Alkenylative Cyclizations of 1,6-Ene-Allenes and Alkenes

The reaction mechanism and origins of ligand-controlled selectivity, regioselectivity, and stereoselectivity of Ni-catalyzed (2 + 2 + 2) cycloadditions and alkenylative cyclizations of 1,6-ene-allenes and alkenes were studied by using density functional theory. The catalytic cycle involves intermolecular oxidative coupling and an intramolecular concerted 1,4-addition step to afford a stable metallacycloheptane intermediate; these steps determine both the regioselectivity and stereoselectivity. Subsequent C-C reductive elimination leads to the cyclohexane product, whereas the β-hydride elimination leads to the trans-diene product. The selectivity between (2 + 2 + 2) cycloadditions and alkenylative cyclizations is controlled by the ligand. Irrespective of the nature of the terminal substituents on the ene-allene and alkene, the P(o-tol)₃ ligand always favors the C-C reductive elimination, resulting in the cyclohexane product. On the other hand, the flexibility of the PBu₃ ligand means that electronic and steric factors play important roles. Electron-withdrawing groups such as CO₂Me in the ene-allene terminal substituent induce obvious substrate-ligand repulsion and destabilize the C-C reductive elimination, giving rise to the trans-diene product.

Chemo- and Regioselective Rhodium(I)-Catalyzed [2+2+2] Cycloaddition of Allenynes with Alkynes

A highly chemo- and regioselective partially intramolecular rhodium(I)-catalyzed [2+2+2] cycloaddition of allenynes with alkynes is described. A range of diverse polysubstituted benzene derivatives could be synthesized in good to excellent yields, in which the allenynes served as synthetic equivalent to the diynes. A high regioselectivity could be observed when allenynes were treated with unsymmetrical alkynes.

Allenes, versatile unsaturated motifs in transition-metal-catalysed [2+2+2] cycloaddition reactions

ThisTutorial Reviewhighlights the versatility of allenes as unsaturated substrates in transition-metal-catalysed [2+2+2] cycloaddition reactions in generating complex architectures.