The Origin of the Idea

Organo rare-earth catalysis have been emerging as a unique platform for various chemical transformations including the regio- and stereoselective C−H functionalization of heteroatom-containing compounds with alkenes and alkynes. In many of these transformations, the strong heteroatom-affinity of rare-earth metal ions and high reactivity of RE‒C bond towards unsaturated carbon-carbon bonds were essential to achieve reactivity and selectivity. In this context, auxiliary ligand is the core and key of rare-earth metal organic complex catalysis, which significantly affects the reactivity and selectivity of catalytic reactions. At present, cyclopentadiene (Cp)-based ligands occupied a dominated position.¹ Except Cp and its derivatives, the diamine ligand by Mashima,² the b-diimide ligand by Xu,³ and the phosphonamine ligand by Chen⁴ displayed good activity towards the hydroalkylation of pyridine derivatives by coordinating with rare-earth metals. Tamm, Inoue, Eisen, and others found that imidazolin-2-imine was a versatile class of ligands, which can coordinate with main group metals, transition metals and rare-earth metals to form the well-defined metal complexes.⁵ The theoretical calculations and experimental studies implied that imidazolin-2-iminato (NHI) anions can be served as Cp-like ligands. As shown in Figure 1, the two mesomeric structures of the imidazolin-2-iminato groups indicated that they can serve as 2σ,4π-type N-donor ligands. Thanks to their strong electron donation and steric tunability, the related imidazolin-2-iminato rare-earth alkyl complexes exhibit high activity toward several reactions, including hydroamination, hydrosilylation, nucleophilic addition, and polymerization. Nevertheless, it remains unclear whether imidazolin-2-iminato rare-earth alkyl complexes could be used in C–H activation. Motivated by the distinct selectivity and functional group tolerance frequently shown in rare-earth mediated C–H functionalization and elegant works from Tamm's group, we envisaged that the judicious choice of rare-earth ions and basic ligands, as well as modification of imidazolin-2-iminato supporting ligands, may have the potential to achieve C–H alkylation with olefins. An array of imidazolin-2-iminato rare-earth alkyl complexes were synthesized and structurally characterized by X-ray diffraction analysis. Recently, the cationic imidazolin-2-iminato scandium(III) alkyl complex was identified as an efficient catalyst for highly regioselective C–H alkylation of pyridine, anisole and their derivatives.⁶

Figure 1. The structural features of N-heterocyclic iminato ligands.

A New Strategy: RE...π Interaction Promote Catalytic C−H Functionalization

When we carried out the reaction of 2-methylanisoles with β-methylstyrene in the presence of scandium(III) alkyl complexes, dimerization product of β-methylstyrene was isolated in ca. 5% yield rather than the expected C–H alkylation of 2-methylanisole (Figure 2a). The interesting mechanism of the above reaction tempers us to further explore the related reaction, wherein the interaction between rare-earth metal ions and π-bonds of alkene triggers the C–H activation of allylic position (Figure 2b). Notably, compared with rare-earth (RE)…heteroatom interaction, RE...π interaction, frequently used in facilitating regio- and stereoselectivity of olefin polymerizations, is seldomly used to trigger catalytic C−H functionalization.

Figure 2. The allylic C-H functionalization of simple alkenes. a preliminary results on the reaction of 2-methylanisoles with β-methylstyrenes. b Rare-earth mediated direct hydroallylation of styrene derivatives with 1-aryl-2-alkyl alkenes and a-alkenes. Rare-earth (RE).

Challenges and Insights

There are several challenges associated with the above transformation: (1) the selective activation of allylic position through relatively weak RE…alkene interaction; (2) the influence of alkene configuration and highly fluxional behavior of allylic rare-earth metal intermediate on the reactivity and stereoselectivity; and (3) the control of hydroallylation of two different alkenes instead of dimerization or polymerization. In this work, cationic imidazolin-2-iminato (NHI) scandium(III) alkyl complexes were identified to be efficient promoters for the anti-Markovnikov-type hydroallylation of styrenes with alkenes as well as dimerization of internal and terminal alkenes.

Impact and Practical Implication

Taking use of the interplays of rare-earth metal with C=C double bond, we successfully achieved hydroallylation of styrene derivatives with internal and terminal alkenes as well as dimerization of alkenes by cationic imidazolin-2-iminato scandium alkyl complexes. A variety of chain elongated internal alkenes were obtained in an atom-economical and efficient manner with moderate to good yields and high E/Z ratio. A possible catalytic cycle involving the assistance of Lewis base was proposed to understand the reaction mechanism and E/Z ratio of hydroallylation reaction. This work provides a new protocol for the direct hydroallylation of alkenes with alkenes. The development of chiral imidazolin-2-iminato rare-earth alkyl complexes and their further utility in other related C‒H functionalization reactions are in progress.

References

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