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The search for efficient and selective methods for the divergent synthesis of multi‐substituted aminotetralins is of much interest and importance. We report herein for the first time the diastereoselective [4+2] annulation of 2‐methyl aromatic aldimines with alkenes via benzylic C(sp3)−H activation by half‐sandwich rare‐earth catalysts, which constitutes an efficient route for the divergent synthesis of both trans and cis diastereoisomers of multi‐substituted 1‐aminotetralin derivatives from readily accessible aldimines and alkenes. The use of a scandium catalyst bearing a sterically demanding cyclopentadienyl ligand such as C5Me4SiMe3 or C5Me5 exclusively afforded the trans‐selective annulation products in the reaction of aldimines with styrenes and aliphatic alkenes. In contrast, the analogous yttrium catalyst, whose metal ion size is larger than that of scandium, yielded the cis‐selective annulation products. This protocol features 100 % atom‐efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The reaction mechanisms have been elucidated by kinetic isotope effect (KIE) experiments and the isolation and transformations of some key reaction intermediates. [ABSTRACT FROM AUTHOR]

Here we report for the first time the regio‐ and diastereoselective [3+2] annulation of a wide range of aliphatic aldimines with alkenes via the activation of an unactivated β‐C(sp3)−H bond by half‐sandwich scandium catalysts. This protocol offers a straightforward and atom‐efficient route for the synthesis of a new family of multi‐substituted aminocyclopentane derivatives from easily accessible aliphatic aldimines and alkenes. The annulation of aldimines with styrenes exclusively afforded the 5‐aryl‐trans‐substituted 1‐aminocyclopentane derivatives with excellent diastereoselectivity through the 2,1‐insertion of a styrene unit. The annulation of aldimines with aliphatic alkenes selectively gave the 4‐alkyl‐trans‐substituted 1‐aminocyclopentane products in a 1,2‐insertion fashion. A catalytic amount of an appropriate amine such as adamantylamine (AdNH2) or dibenzylamine (Bn2NH) showed significant effects on the catalyst activity and stereoselectivity. [ABSTRACT FROM AUTHOR]

Abstract: Isolation of the simplest 4π three‐membered heterocycles (1H‐azirine, oxirene, thiirene, and selenirene) remains a big challenge due to their π‐antiaromaticity and significant ring strain. Here we demonstrate that the incorporation of a transition‐metal fragment could stabilize the antiaromatic selenirene and pentalene frameworks simultaneously by density functional theory (DFT) calculations. Experimental verification leads to the Se‐containing metallapolycycles, osmapentaloselenirenes, with remarkable thermal stability. The osmaselenirene unit in the metallapolycycle is determined to be the first example of σ‐aromaticity dominating in an unsaturated Se‐containing ring. Our results not only highlight a remarkable stabilization by the transition‐metal but also widen the scope of σ‐aromaticity in unsaturated rings, which is traditionally reserved for the domain of π‐aromaticity. [ABSTRACT FROM AUTHOR]

The [3 + 2] or [4 + 2] annulation of α,β-unsaturated aldimines with alkenes via β'- or γ-allylic C(sp 3 )-H activation is, in principle, an atom-efficient route for the synthesis of five- or six-membered-ring cycloalkylamines, which are important structural motifs in numerous natural products, bioactive molecules, and pharmaceuticals. However, such a transformation has remained undeveloped to date probably due to the lack of suitable catalysts. We report herein for the first time the regio- and diastereoselective [3 + 2] and [4 + 2] annulations of α,β-unsaturated imines with alkenes via allylic C(sp 3 )-H activation by half-sandwich rare-earth catalysts having different metal ion sizes. The reaction of α-methyl-substituted α,β-unsaturated aldimines with alkenes by a C 5 Me 4 SiMe 3 -ligated scandium catalyst took place in a trans -diastereoselective [3 + 2] annulation fashion via C(sp 3 )-H activation at the α-methyl group (β'-position), exclusively affording alkylidene-functionalized cyclopentylamines with excellent trans -diastereoselectivity. In contrast, the reaction of β-methyl-substituted α,β-unsaturated aldimines with alkenes by a C 5 Me 5 -ligated cerium catalyst proceeded in a cis -diastereoselective [4 + 2] annulation fashion via γ-allylic C(sp 3 )-H activation, selectively yielding multisubstituted 2-cyclohexenylamines with excellent cis -diastereoselectivity. The mechanistic details of these transformations have been elucidated by deuterium-labeling experiments, kinetic isotope effect studies, and the isolation and transformations of key reaction intermediates. This work offers an efficient and selective protocol for the synthesis of a new family of cycloalkylamine derivatives, featuring 100% atom efficiency, high regio- and diastereoselectivity, broad substrate scope, and an unprecedented reaction mechanism.

The search for efficient and selective methods for the divergent synthesis of multi-substituted aminotetralins is of much interest and importance. We report herein for the first time the diastereoselective [4+2] annulation of 2-methyl aromatic aldimines with alkenes via benzylic C(sp 3 )-H activation by half-sandwich rare-earth catalysts, which constitutes an efficient route for the divergent synthesis of both trans and cis diastereoisomers of multi-substituted 1-aminotetralin derivatives from readily accessible aldimines and alkenes. The use of a scandium catalyst bearing a sterically demanding cyclopentadienyl ligand such as C 5 Me 4 SiMe 3 or C 5 Me 5 exclusively afforded the trans-selective annulation products in the reaction of aldimines with styrenes and aliphatic alkenes. In contrast, the analogous yttrium catalyst, whose metal ion size is larger than that of scandium, yielded the cis-selective annulation products. This protocol features 100 % atom-efficiency, excellent diastereoselectivity, broad substrate scope, and good functional group compatibility. The reaction mechanisms have been elucidated by kinetic isotope effect (KIE) experiments and the isolation and transformations of some key reaction intermediates., (© 2024 Wiley-VCH GmbH.)

Dinitrogen (N 2 ) activation and functionalization is of fundamental interest and practical importance. This review focuses on N 2 activation and addition to unsaturated substrates, including carbon monoxide, carbon dioxide, heteroallenes, aldehydes, ketones, acid halides, nitriles, alkynes, and allenes, mediated by transition metal complexes, which afforded a variety of N-C bond formation products. Emphases are placed on the reaction modes and mechanisms. We hope that this work would stimulate further explorations in this challenging field., (© 2023 Wiley-VCH GmbH.)

Here we report for the first time the regio- and diastereoselective [3+2] annulation of a wide range of aliphatic aldimines with alkenes via the activation of an unactivated β-C(sp 3 )-H bond by half-sandwich scandium catalysts. This protocol offers a straightforward and atom-efficient route for the synthesis of a new family of multi-substituted aminocyclopentane derivatives from easily accessible aliphatic aldimines and alkenes. The annulation of aldimines with styrenes exclusively afforded the 5-aryl-trans-substituted 1-aminocyclopentane derivatives with excellent diastereoselectivity through the 2,1-insertion of a styrene unit. The annulation of aldimines with aliphatic alkenes selectively gave the 4-alkyl-trans-substituted 1-aminocyclopentane products in a 1,2-insertion fashion. A catalytic amount of an appropriate amine such as adamantylamine (AdNH 2 ) or dibenzylamine (Bn 2 NH) showed significant effects on the catalyst activity and stereoselectivity., (© 2021 Wiley-VCH GmbH.)

Isolation of the simplest 4π three-membered heterocycles (1H-azirine, oxirene, thiirene, and selenirene) remains a big challenge due to their π-antiaromaticity and significant ring strain. Here we demonstrate that the incorporation of a transition-metal fragment could stabilize the antiaromatic selenirene and pentalene frameworks simultaneously by density functional theory (DFT) calculations. Experimental verification leads to the Se-containing metallapolycycles, osmapentaloselenirenes, with remarkable thermal stability. The osmaselenirene unit in the metallapolycycle is determined to be the first example of σ-aromaticity dominating in an unsaturated Se-containing ring. Our results not only highlight a remarkable stabilization by the transition-metal but also widen the scope of σ-aromaticity in unsaturated rings, which is traditionally reserved for the domain of π-aromaticity., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)