Rhodium‐Catalyzed [3+2+2] and [2+2+2] Cycloadditions of Two Alkynes with Cyclopropylideneacetamides

It has been established that a cationic rhodium(I)/H₈‐binap complex catalyzes the [3+2+2] cycloaddition of 1,6‐diynes with cyclopropylideneacetamides to produce cycloheptadiene derivatives through cleavage of cyclopropane rings. In contrast, a cationic rhodium(I)/(S)‐binap complex catalyzes the enantioselective [2+2+2] cycloaddition of terminal alkynes, acetylenedicarboxylates, and cyclopropylideneacetamides to produce spiro‐cyclohexadiene derivatives which retain the cyclopropane rings.     

Beyond Reppe: Building Substituted Arenes by [2+2+2] Cycloadditions of Alkynes

Synthetic sequel : The transition‐metal‐catalyzed [2+2+2] cycloaddition is an established method for the construction of carbocyclic frameworks but is often plagued by poor selectivity. Recent literature paints a promising picture—a more general metal‐catalyzed [2+2+2] cycloaddition can be accomplished intermolecularly using three separate alkynes to furnish highly substituted arenas (see scheme).

     

Intramolecular Benzyne–Phenolate [4+2] Cycloadditions

An intramolecular benzyne–phenolate [4+2] cycloaddition is reported. Benzyne precursors, having vicinal halogen‐sulfonate functionalities, linked with a phenol(ate) by various tether groups undergo efficient intramolecular [4+2] cycloaddition by treatment with either Ph₃MgLi or nBuLi for halogen–metal exchange to form various benzobarrelenes.     

In Situ Generation of Difluoromethyl Diazomethane for [3+2] Cycloadditions with Alkynes

A novel approach to agrochemically important difluoromethyl‐substituted pyrazoles has been developed based on the elusive reagent CF₂HCHN₂, which was synthesized (generated in situ) for the first time and employed in [3+2] cycloaddition reactions with alkynes. The reaction is extremely practical as it is a one‐pot process, does not require a catalyst or the isolation of the potentially toxic and explosive gaseous intermediate, and proceeds in a common solvent, namely chloroform, in air. The reaction is also scalable and allows for the preparation of the target pyrazoles on gram scale.     

Diversity in Gold‐Catalyzed Formal Cycloadditions of Ynamides with Azidoalkenes or 2H‐Azirines: [3+2] versus [4+3] Cycloadditions

Gold‐catalyzed cycloadditions of ynamides with azidoalkenes or 2H‐azirines give [3+2] or [4+3] formal cycloadducts of three classes. Cycloadditions of ynamides with 2H‐azirine species afford pyrrole products with two regioselectivities when the C_β‐substituted 2H‐azirine is replaced from an alkyl (or hydrogen) with an ester group. For ynamides substituted with an electron‐rich phenyl group, their reactions with azidoalkenes proceed through novel [4+3] cycloadditions to deliver 1H‐benzo[d]azepine products instead.     

Transition‐Metal‐Catalyzed Enantioselective [2+2+2] Cycloadditions for the Synthesis of Axially Chiral Biaryls

Tying up loose ends : Recent advances towards a development of novel transition‐metal‐catalyzed enantioselective [2+2+2] cycloadditions for the synthesis of biaryls are summarized in this Focus Review. Additionally, the enantioselective synthesis of axially chiral biaryls possessing non‐biaryl axial chirality is also presented. These novel asymmetric aromatization reactions allow the production of various axially chiral biaryl compounds with high enantioselectivity.

     

Catalytic Asymmetric Synthesis of 8‐Oxabicyclooctanes by Intermolecular [5+2] Pyrylium Cycloadditions

Highly enantioselective intermolecular [5+2] cycloadditions of pyrylium ion intermediates with electron‐rich alkenes are promoted by a dual catalyst system composed of an achiral thiourea and a chiral primary aminothiourea. The observed enantioselectivity is highly dependent on the substitution pattern of the 5π component, and the basis for this effect is analyzed using experimental and computational evidence. The resultant 8‐oxabicyclo[3.2.1]octane derivatives possess a scaffold common in natural products and medicinally active compounds and are also versatile chiral building blocks for further manipulations. Several stereoselective complexity‐generating transformations of the 8‐oxabicyclooctane products are presented.     

钌催化二环烯烃与炔烃的[2＋2]环加成反应的研究进展

综述了近年来钌催化二环烯烃与炔烃发生[2＋2]环加成反应的研究进展, 重点讨论了钌催化二环烯烃和炔烃的 [2＋2]环加成反应中反应物结构、反应物比例、溶剂及温度等因素对反应的影响.     

Concerted and Stepwise Mechanisms in Metal‐Free and Metal‐Assisted [4+3] Cycloadditions Involving Allyl Cations

The thermal [4+3] cycloaddition reaction between allenes and tethered dienes (1,3‐butadiene and furan) assisted by transition metals (Au^I, Au^III, Pd^II, and Pt^II) was studied computationally within the density functional theory framework and compared to the analogous non‐organometallic process in terms of activation barriers, synchronicity and aromaticity of the corresponding transition states. It was found that the metal‐mediated cycloaddition reaction is concerted and takes place via transition structures that can be even more synchronous and more aromatic than their non‐organometallic analogues. However, the processes exhibit slightly to moderately higher activation barriers than the parent cycloaddition involving the hydroxyallylic cation. The bond polarization induced by the metal moiety is clearly related to the interaction of the transition metal with the allylic π* molecular orbital, which constitutes the LUMO of the initial reactant. Finally, replacement of the 1,3‐butadiene by furan caused the transformation to occur stepwise in both the non‐organometallic and metal‐assisted processes.