Transannular Enantioselective (3 + 2) Cycloaddition of Cycloalkenone Hydrazones under Brønsted Acid Catalysis

Formation of Radical-like NH Ligand from NH3 at Ambient Conditions Mediated by Dialkyl Rare-Earth Complexes

Although intensive work on ammonia activation has been carried out in recent decades, generating nitrogen-centered radicals from NH3 under ambient conditions remains quite challenging. In the presented research, the conversion of NH3 to radical-like NH ligand has been achieved by the reactions of a series of dialkyl rare-earth (RE) complexes (1-RE, RE = Tb, Dy, Y, Ho, Er, Yb, and Lu) supported by β-diketiminate ligands with NH3 in n-hexane at room temperature, resulting in the formations of the radical-like μ3-NH ligands containing trinuclear RE complexes (2-RE). The radical-like feature of the μ3-NH ligand was revealed by electron paramagnetic resonance and magnetic measurements, radical trapping experiments, and computational spin density analysis. In addition, H2 was detected to form during the reaction of 1-RE with NH3, indicating that the radical-like μ3-NH ligand was likely to be generated via N-H bond homolysis. Moreover, the solvents and coordination pattern of β-diketiminate ligands are crucial for the formation of the radical-like μ3-NH ligand from NH3. When toluene instead of n-hexane was used in the reaction of 1-RE with NH3, an array of octaamido tetranuclear RE complexes (3-RE) was obtained. The reaction of the dialkyl yttrium complex (4-Y) bearing a modified β-diketiminate ligand, in which the two mesityl substituents are replaced by a 2,6-diisopropylphenyl group and a 2-(dimethylamino)ethyl group, with NH3 in both n-hexane and toluene only yielded a tetranuclear yttrium complex carrying the dianionic closed-shell μ3-NH ligands (5-Y).

Umpolung Reactivity of in Situ Derived Aryl Hydrazones: An Asymmetric Brønsted Acid Catalyzed Strategy to Access Fused Pyrazolidines

Herein, we report an intriguing cascade strategy for synthesizing optically active fused pyrazolidines featuring three contiguous stereogenic centers. The formyl-tethered enones are templates for the developed umpolung reactivity, showcasing diverse substrate adaptability with various arylhydrazines. The chiral phosphoric acid catalyst offers stereochemical guidance, forming the fused pyrazolidines with commendable to excellent stereoselectivities. Additionally, the scalability, postsynthetic transformations, and instability of unacylated pyrazolidines have been successfully demonstrated.

Hydrazine Hydrate in Asymmetric Synthesis: A Bifunctional Squaramide Catalytic Approach toward Fused Pyrazolines

A unified strategy has been developed to utilize hydrazine hydrate in asymmetric organic synthesis by overcoming the rapid background reaction with dienone. The H-bond donor ability of the cinchona-alkaloid-derived squaramide catalyst unlocks this previously deemed infeasibility. The dissymmetric hydrazine addition to symmetrical dienones tolerates various substitutions, resulting in the formation of optically pure fused pyrazoline derivatives under mild reaction conditions. Furthermore, the scalability of this methodology and a successful demonstration of postsynthetic transformations have been accomplished.

Computational studies of Brønsted acid-catalyzed transannular cycloadditions of cycloalkenone hydrazones

The contribution to the energy barrier of a series of tethers in transannular cycloadditions of cycloalkenes with hydrazones has been computationally studied by using DFT. The Houk's distortion model has been employed to evaluate the influence of the tether in the cycloaddition reaction. That model has been extended to determine the contribution of each tether and, more importantly, the effect exerted between them. In addition to the distortion induced by the tethers, the entropy effects caused by them has also been studied. The analysis of the evolution of the electron localization function along the reaction revealed the highly concerted character of the reaction.

Diastereoselective Synthesis of Bicyclo[3.3.0]octenones by Copper-Catalyzed Transannular Ring-Closing Reaction

A novel and efficient copper-catalyzed transannular ring-closing reaction of eight-membered rings has been developed that provides a straightforward way to synthesize bicyclo[3.3.0]octane derivatives in good yields. Mechanistic studies revealed that the reaction pathway might involve chlorination followed by the Kornblum reaction. Readily accessible starting materials and good functional group tolerance make this procedure attractive.

Catalytic Enantioselective Construction of Decalin Derivatives by Dynamic Kinetic Desymmetrization of C2-Symmetric Derivatives through Aldol-Aldol Annulation

We have developed and investigated a catalytic desymmetrization reaction strategy that affords functionalized decalin derivatives with high enantioselectivities from C2-symmetric derivatives through aldol-aldol annulation. We identified the structural moieties of the catalyst necessary for the formation of the decalin derivative with high enantioselectivity. We elucidated the mechanisms of the catalyzed reactions: the first aldol reaction step was reversible, and the second aldol step was rate-limiting and stereochemistry-determining and was enantioselective. Using theoretical calculations guided by the experimental results, we identified the interactions between the catalyst and the transition state that led to the major enantiomer. The information obtained in this study will be useful for the development of catalysts and chemical transformations.

Enantioselective transannular reactions by palladium-catalysed conjugate addition of aryl boronic acids

A palladium-catalyzed asymmeric conjugate addition of aryl boronic acids to medium-sized cycloalkenones followed by intramolecular aldol trapping is reported. The use of in situ formed [Pd/(QuinoxP*)] as the catalyst enables the synthesis of arylbicyclic scaffolds in good yields and with excellent stereocontrol (up to 7 : 1 dr, up to 99% ee). The reaction is applicable to a range of medium size ketoenone substrates and funcionalized aryl boronic acids, including heterocyclic compounds.

Recent Developments in Transannular Reactions

Transannular reactions have shown a remarkable performance for the construction of polycyclic scaffolds from medium- or large sized cyclic molecules in an unconventional manner. Recent examples of transannular reactions reported from 2011 have been reviewed, emphasizing the excellent performance of this approach when accessing the target compounds. This review also highlights how this methodology provides an alternative approach to other commonly used methodologies for the construction of cyclic entities such as cyclization or cycloaddition reactions