A Well‐Defined Aluminum‐Based Lewis Acid as an Effective Catalyst for Diels–Alder Transformations

Lewis acid catalysed polymerisation of cyclopentenone

A modest structural change of a β-diketiminate-supported aluminium complex leads to dramatic differences in the reactivity towards cyclopentenone. While the bulkier complex efficiently executes Diels Alder transformations the smaller analogue performs unique polymerisation of this substrate. This observation appears to be unprecedented in the chemistry of Lewis acids and cyclic dienophiles as it represents a unique way to polymerise a functionalised olefin.

Synthesis, characterisation and reactivity studies of chiral β-diketiminate-like supported aluminium Lewis acid complexes towards difficult Diels Alder cycloadditions

Synthesis and characterisation of several chiral, oxazoline containing β-diketiminate type ligand supported-aluminium compounds are reported. Together with 1 equiv. of Na(BAr^Cl₄) (Ar^Cl = 3,5-Cl₂-C₆H₃), these chiral Lewis acid complexes, which possess an "achiral end" and "chiral end" have been successfully utilised as catalysts in asymmetric Diels-Alder reactions of 1,3-cyclohexadiene and several different chalcones. Systematic increase in the steric demand of the ligand's "achiral end" of these complexes resulted in enhanced enantioinduction for the cyclisation of 1,3-cyclohexadiene and chalcone. Further structural modifications of the "chiral end" clearly established that having a tert-butyl group connected to the stereogenic centre of the oxazoline fragment yielded the highest enantioselectivity value for the examined cyclisation. A substrate scope was then expanded using several different dienophiles (i.e. chalcones) generating an enantiomeric excess range of 24-68%.

Alkali Metal Adducts of an Iron(0) Complex and Their Synergistic FLP-Type Activation of Aliphatic C-X Bonds

We report the formation and full characterization of weak adducts between Li⁺ and Na⁺ cations and a neutral iron(0) complex, [Fe(CO)₃(PMe₃)₂] (1), supported by weakly coordinating [BAr^F₂₀] anions, [1·M][BAr^F₂₀] (M = Li, Na). The adducts are found to synergistically activate aliphatic C-X bonds (X = F, Cl, Br, I, OMs, OTf), leading to the formation of iron(II) organyl compounds of the type [FeR(CO)₃(PMe₃)₂][BAr^F₂₀], of which several were isolated and fully characterized. Stoichiometric reactions with the resulting iron(II) organyl compounds show that this system can be utilized for homocoupling and cross-coupling reactions and the formation of new C-E bonds (E = C, H, O, N, S). Further, we utilize [1·M][BAr^F₂₀] as a catalyst in a simple hydrodehalogenation reaction under mild conditions to showcase its potential use in catalytic reactions. Finally, the mechanism of activation is probed using DFT and kinetic experiments that reveal that the alkali metal and iron(0) center cooperate to cleave C-X via a mechanism closely related to intramolecular FLP activation.

Dinuclear Aluminum Halide Complexes Based on Bis(β-diketiminate) Ligands: Synthesis, Structures, and Electrochemical Characterization

Twenty-three dinuclear aluminum halide complexes based on bis(β-diketiminate) ligands with various flexible and rigid bridging groups have been synthesized and fully characterized by NMR and IR spectroscopy, cyclic voltammetry, and elemental analysis. In addition, nine complexes were structurally characterized by means of single-crystal X-ray diffraction. Attempts to reduce the dinuclear species using C₈K remained unsuccessful, but the reduction potentials investigated by cyclic voltammetry indicate the principle feasibility.

Pursuing the active species in an aluminium-based Lewis acid system for catalytic Diels-Alder cycloadditions

Several Al-based complexes supported by a bis(imino)aryl NCN pincer ligand have been prepared. Systematic structural and experimental evidence identified a cationic species, supported by a THF molecule, as the best Lewis acid precatalyst for a range of Diels-Alder cycloadditions. This highlighted the importance of fully isolating and characterizing the active species in any catalytic process especially when Lewis acids are used due to the presence of hidden Brønsted acids (HBAs). Finally, the control experiments conducted in order to eliminate HBA activity that involve a bulky pyridine base should be performed with caution as the corresponding protonated pyridinium salt could also serve as a proton source.

Unexpected reactivity of an alkylaluminum complex of a non-innocent 1,2-bis[(2,6-diisopropylphenyl)imino]acenaphthene ligand (dpp-bian)

The non-innocence of the bis-amido dpp-bian ligand has been documented by the reactions of compound 1 with acidic substrates, e.g. water.