Scandium-Catalyzed Benzylic C(sp3)-H Alkenylation of Tertiary Anilines with Alkynes

Overlooked Ligand Role of Pyridine Substrate in Site Selectivity of Rare-Earth-Catalyzed C-H Alkylation with Alkenes

The rare-earth-catalyzed C-H alkylation of heteroatom-containing substrates with alkenes has been extensively studied over the past decade. Traditionally, those substrates have been regarded primarily as reactants in these reactions. In this study, the mechanism of rare-earth-catalyzed C-H alkylation of 2-ethylpyridine with styrene was investigated by DFT calculations, revealing the often-overlooked ligand effect of pyridine substrates. Our findings demonstrate that pyridine substrates, acting as ligands, play a pivotal role in modulating site selectivity during C-H activation. These results enhance our understanding of the rare-earth catalysis mechanism and provide valuable insights into its versatile reactivity, offering a novel perspective on the dual roles (both reactant and ligand) of heteroatom-containing substrates, which are widely used in C-H functionalization reactions.

Enantioselective ortho-C-H Addition of Aromatic Amines to Alkenes by Bulky Chiral Anilido-Oxazoline Scandium Complexes

The enantioselective C-H addition of anilines to alkenes represents an ideal protocol for the synthesis of chiral aromatic amines in terms of step- and atom-economy. However, this field remains predominantly unexplored. Herein, a series of newly designed bulky chiral anilido-oxazoline ligand precursors were synthesized, and the corresponding rare-earth metal alkyl complexes were obtained successfully. The resultant scandium complexes exhibit high regioselectivity for the ortho-C-H addition of tertiary anilines to unactivated alkenes, providing a wide range of chiral alkylated anilines in high yields (up to 98% yield) with excellent enantioselectivity (up to 98% ee). Moreover, the addition products can be easily converted into biorelevant derivatives and pharmacophore-containing skeletons.

Mechanisms and Origins of Regioselectivity in Rare-Earth-Catalyzed C-H Functionalization of Anisoles and Thioanisoles

The direct catalytic C-H functionalization of aromatic compounds such as anisoles and thioanisoles is of great interest and significance. However, achieving precise regioselectivity remains a major challenge. In this study, we conducted comprehensive density functional theory calculations to explore the mechanisms of rare-earth-catalyzed regioselective C-H alkylation, borylation, and silylation of anisole and thioanisole. The results reveal that in cationic C-H alkylation systems, the alkene insertion step follows a substrate-assisted mechanism, in which an additional substrate molecule acts as a ligand to facilitate the transformation. In neutral C-H borylation and silylation systems, although mononuclear hydride species readily dimerize into binuclear hydride species due to thermodynamic stability, the catalytic process predominantly proceeds via a mononuclear pathway. Furthermore, the origins of regioselectivity were thoroughly elucidated. A detailed analysis of electronic and steric effects in related transition states reveals that, for anisole, regioselectivity is primarily governed by ring strain. Since α-C(sp3)-H activation involves the formation of a highly strained three-membered ring, the reaction preferentially occurs at the ortho-C(sp2)-H site, forming a less strained four-membered ring. In contrast, for thioanisole, electronic effects play a decisive role, driving C-H activation at the more negatively charged α-C(sp3) site due to stronger metal-carbon interactions.

Theoretical Insights into Rare-Earth-Catalyst-Controlled Diastereo- and Enantioselective [3 + 2] Annulation of Aromatic Aldimines with Styrenes

Rare-earth-catalyzed annulation reactions using alkenes via C-H activation offer an atom-efficient approach to constructing cyclic compounds. However, the mechanisms underlying these reactions remain poorly understood, limiting the rational design of related catalytic systems. Recently, Hou and Cong reported an unprecedented example of rare-earth-catalyst-controlled diastereodivergent asymmetric [3 + 2] annulation of aromatic aldimines with alkenes. To elucidate the mechanisms and the origins of diastereo- and enantioselectivity, density functional theory calculations were performed. The results revealed that the styrene insertion step determines the stereoselectivity. Styrene insertion follows a similar metal-styrene interaction pattern across different catalysts. Specifically, during cis-insertion, styrene interacts strongly with the metal center, exhibiting significant Sc···Ph interactions, whereas such interactions are absent during trans-insertion. Thus, when the catalyst is employed with a small ligand, stereoselectivity is primarily governed by electronic factors, favoring the cis-insertion mode. In contrast, for the more sterically hindered catalyst, the Sc···Ph interactions in cis-insertion are insufficient to overcome the steric effects, leading to a preference for the trans-insertion mode, which minimizes steric hindrance. These findings offer deeper insights into the origins of catalyst-controlled diastereo- and enantioselectivity and will also contribute to the rational design of stereospecific annulation reactions in rare-earth catalysis.

Enantioselective Synthesis of 1-Aminoindanes via [3 + 2] Annulation of Aldimines with Alkenes by Scandium-Catalyzed C-H Activation

Multisubstituted chiral 1-aminoindanes are important components in many pharmaceuticals and bioactive molecules. Therefore, the development of efficient and selective methods for the synthesis of chiral 1-aminoindanes is of great interest and importance. In principle, the asymmetric [3 + 2] annulation of aldimines with alkenes through C-H activation is the most atom-efficient and straightforward route for the construction of chiral 1-aminoindanes, but such a transformation has remained undeveloped to date probably due to the lack of suitable catalysts. Herein, we report for the first time the enantioselective [3 + 2] annulation of a wide range of aromatic aldimines and alkenes via ortho-C(sp2)-H activation by chiral half-sandwich scandium catalysts, which provides a straightforward route for the synthesis of multisubstituted chiral 1-aminoindanes. This protocol features 100% atom-efficiency, broad functional group compatibility, and high regio-, diastereo-, and enantioselectivity (up to >19:1 dr and 99:1 er). Remarkably, by fine-tuning the sterics of the chiral ligand around the catalyst metal center, the diastereodivergent asymmetric [3 + 2] annulation of aldimines and styrenes has been achieved with a high level of diastereo- and enantioselectivity, offering an efficient method for the synthesis of both the trans and cis diastereomers of a novel class of chiral 1-aminoindane derivatives containing two contiguous stereocenters from the same set of starting materials. Moreover, the asymmetric [3 + 2] annulation of aldimines with aliphatic α-olefins, norbornene, and 1,3-dienes has also been achieved.

Mechanism and Origin of Site Selectivity and Regioselectivity of Scandium-Catalyzed Benzylic C-H Alkylation of Tertiary Anilines with Alkenes

Benzylic C(sp³)-H alkylation of tertiary anilines with alkenes by an anilido-oxazoline-ligated scandium alkyl catalyst was recently reported with C-H site selectivity and alkene-dependent regioselectivity. Revealing the mechanism and origin of selectivity is undoubtedly of great importance for understanding experimental observations and developing new reactions. Herein, density functional theory (DFT) calculations have been carried out on the model reaction of Sc-catalyzed benzylic C(sp³)-H alkylation of N,N-dimethyl-o-toluidine with allylbenzene. The reaction generally undergoes the generation of active species, alkene insertion, and protonation steps. The difference of the distortion energy of the aniline moiety in transition states, which is related to the ring size of the forming metallacycles, accounts for the site selectivity of C-H activation. Benzylic C(sp³)-H activation possessing less strained five-membered metallacycle compared to the ortho-C(sp²)-H and α-methyl C(sp³)-H activation results in benzylic C(sp³)-H alkylation observed experimentally. Both steric and electronic factors are responsible for the 1,2-insertion regioselectivity for alkyl-substituted alkenes, while electronic factors control the 2,1-insertion manner for vinylsilanes. The analysis of original alkene substrates further strengthens the understanding of the alkene-dependent regioselectivity. These results help us to obtain the mechanistic understanding and are expected to be conducive to the development of new C-H functionalization reactions.

Substrate Facilitating Roles in Rare-Earth-Catalyzed C-H Alkenylation of Pyridines with Allenes: Mechanism and Origins of Regio- and Stereoselectivity

Although considerable progress has been achieved in C-H functionalization by cationic rare-earth alkyl complexes, the potential facilitating roles of heteroatom-containing substrates during the catalytic cycle remain highly underestimated. Herein, theoretical studies on the model reaction of C(sp²)-H addition of pyridines to allenes by scandium catalyst were carefully carried out to reveal the detailed mechanism. A coordinating pyridine substrate as a ligand can effectively stabilize some key structures. An obvious facilitating role delivered by the coordinating pyridine was found for allene insertion, while the pyridine-free mechanism prefers to occur for C(sp²)-H activation processes. Importantly, the elusive role of heteroatom-containing substrates was systematically revealed for the C-H activation event by designing a metal/ligand combination of catalysts and substrates. We found that the pyridyl C(sp²)-H activation would be switched to the pyridine-coordinated mechanism in the cases of the designed Y and La catalysts. To date, this is the first time to realize the potential substrate-facilitating role in cationic rare-earth-catalyzed C-H activation processes. Moreover, theoretical predictions show that similar switchable mechanisms also work for other types of C-H bonds and other heteroatom-involved substrates by fine-adjusting the steric surroundings of catalysts. The two C-H activation mechanisms are mainly the result of the delicate balance between electronic and steric factors. In general, the catalytic system with less steric hindrance prefers to undergo the substrate-coordinated mechanism. In contrast, the substrate-free mechanism is favorable due to steric repulsion. These results are helpful for us to better understand the variant mechanisms in rare-earth-catalyzed C-H functionalization at the atomistic level and may help guide the rational design of new catalytic reactions. In addition, the origins of the regio- and stereoselectivity were discussed through geometric parameters and distortion/interaction analysis.